**Vermiculite: Structural Properties and Examples of the Use**

Marta Valášková and Gražyna Simha Martynková

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51237

## **1. Introduction**

208 Clay Minerals in Nature – Their Characterization, Modification and Application

edition. Tokyo:Gihodo. pp. 28−35 (in Japanese).

Tokyo: Tokyo Kagaku Dojin, 546p.

[61] Vote D, Vote J. G (1990) Biochemistry New York: John Wiley & Sons. (Translated into Japanese by Tamiya N, Muramatsu M, Yagi T, Yoshida H (1992) Vote Seikagaku (Jo),

[62] The Clay Science Society of Japan (2009) Handbook of Clays and Clay Minerals, 3rd

The effort to clarify the meanings of the terms 'clay', 'clays', and 'clay minerals' was the subject of the joint nomenclature committees (JNCs) of the Association Internationale pour l'Etude des Argiles (AIPEA) and the Clay Minerals Society (CMS). The JNCs have proposed the term a class of hydrated phyllosilicates forming the fine-grained fraction of rocks, sediments, and soils and have defined 'clay' as ''a naturally occurring material composed primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden when dried or fired'' [1]. According to this definition synthetic clays and clay-like materials are not regarded as clay even though they may be fine grained, and display the attributes of plasticity and hardening on drying and firing.

For phyllosilicates, the terms ''planes'', ''sheet'' and ''layer'' refer to specific parts of the structure, with atomic arrangements that increase in thickness and they cannot be used interchangeably [2,3].

A '*plane*' can occur consisting of one or more types of atoms (e.g. a plane of Si and Al atoms, a plane of basal oxygen atoms).

A '*tetrahedral sheet*' is composed of continuous corner-sharing tetrahedra like ´octahedral sheet´ is composed of the edge-sharing octahedra.

A '*layer*' contains one or more tetrahedral sheets and an octahedral sheet.

*Interlayer material'* separates the layers and generally may consist of cations, hydrated cations, organic materials, and/or hydroxide octahedral sheets. In certain cases (e.g. talc, pyrophyllite) there is no interlayer material, and thus an empty interlayer separates the layers.

*Layer charge (x) per half unit cell (p.f.u*.*)*, is the *net negative charge per layer*, expressed as a positive number (eq/(Si,Al)4010). The net negative layer charge arises from substitution of Al3+ for Si4+ in tetrahedra and substitution for lower charge cations in octahedra and from the presence of vacancies. The negative layer charge is balanced by the positively charged interlayer material.

Vermiculite: Structural Properties and Examples of the Use 211

Vermiculites are generally trioctahedral and are termed (according to the joint nomenclature committees AIPEA and CMS) on the basis of a negative layer charge, which is between 0.6 to 0.9 p.f.u. [4,7,8]. The negative layer charge of vermiculites results from the substitution of Si4+ by trivalent cations in tetrahedral positions [9,10]. Vermiculites were mostly formed by removal of potassium from biotite, phlogopite or muscovite [5,6] and therefore vermiculite chemistry from this perspective is closely linked to that of mica. The thickness of the structural unit (2:1 layer and interlayer space) is about 1.4 nm, depending on the water interlamellar layers and the interlayer cations. Bailey [9] and Lagaly [10] described a method to determine the layer charge based on the measurement of basal spacings after exchange with alkylammonium cations of varying chain lengths. Alkylammonium ions in the interlayer spaces of vermiculite acquire distinct arrangements: monolayer and two-layer (bilayer) structures with the alkyl chains parallel to the surface, pseudotrimolecular layers and paraffin type structures. Vermiculites have paraffin-type interlayers if the layer charge

The identification of trioctahedral and dioctahedral vermiculites is based on the position of reflection (060) on their XRD patterns. Dioctahedral vermiculites show an interlayer space value *d*(060) between 0.149 nm and 0.150 nm, while trioctahedral vermiculites have *d*(060)

*The cation exchange capacity (CEC)* of clay minerals is defined as "the quantity of cations available for exchange at a given pH expressed in meq/100g which is equivalent to cmol(+)/kg [13]. The CEC varies between the 120 and 200 cmol+/kg (air dried vermiculites) or 140 and 240 cmol(+)/kg (dehydrated vermiculites) [14,15]. Water molecules associated with internal surfaces evokes hydration of interlayer cations. The total amount of alkylammonium ions bound by 2:1 clay minerals is often slightly higher than the total CEC determined by other methods [16]. This is a consequence of the charge regulation at the edges. Density of the charges at the crystal edges depend on the pH of the dispersion and arises from adsorption or dissociation of protons. In an acidic medium an excess of protons creates positive edge charges, the density of which decreases with rising pH. Negative

charges are produced by the dissociation of silanol and aluminol groups [17,18].

regular arrangement of the layers in vermiculites occurs rarely [30-33].

Vermiculite structures contain water interlamellar layers which are subjected to the hydration and dehydration processes [19-26]. The hydration properties are controlled by the interlayer cations Mg2+ and minor amounts of Ca2+, Na+, and K+. The cation radius and charge influence the degree of hydration state in the interlayer and the stacking layer sequences [9,15,19,22,28]. The hydration state of vermiculite was defined by the number of water layers in the interlayer space. The basal space of Mg-vermiculite was declared 0.902 nm for zero-water layer, 1.150 nm for one-water layer and 1.440 nm for two water layer

Ordering in vermiculite layered structure occurs when the pseudohexagonal cavities (made of six-membered tetrahedra rings) are facing each other in adjacent tetrahedral layers. In most cases the vermiculites have various possible layer-stacking sequences and therefore the

is at least 0.75 p.f.u.

hydration state [29].

between 0.151 nm and 0.153 nm [11].

A '*unit structure*' is the total assembly and includes the layer and interlayer material. Therefore it is inappropriate to refer to a 'tetrahedral layer' or an 'octahedral layer', although these incorrect terms are commonly found in the literature.

Guggenheim et al [4] give the revised classification scheme for planar hydrous phyllosilicates [5].

The phyllosilicates are divided by layer type, and within the layer type, and by groups based on charge p.f.u. Further subdivisions by subgroups is based on dioctahedral or trioctahedral character, and finally by species based on chemical composition. Two types of layers, depending on the component sheets are a '1:1 layer' consisting of one tetrahedral sheet and one octahedral sheet, and a '2:1 layer' containing an octahedral sheet between two opposing tetrahedral sheets.

The structure of 2:1 phyllosilicates is composed of tetrahedral (T) and octahedral (O) sheets (Fig. 1). According to the AIPEA Nomenclature Committee [4], tetrahedral sheet is composed of continuous two-dimensional corner-sharing tetrahedra [TO4]4- involving three basal oxygens and the apical oxygen. The tetrahedral sheet has a composition of [T4O10]4- where T = Si4+, Al3+, Fe3+. The apical oxygens form a corner of the octahedral coordination unit around larger octahedral cations. The octahedral sheet consists of two planes of closely packed O2-, OH anions of octahedra with the central cations Mg2+ or Al3+. The smallest structural unit contains three octahedral sites. The *trioctahedral* structures of phyllosilicates have all three sites occupied with cations (e.g. hydroxide sheet Mg6(OH) <sup>12</sup> <sup>12</sup> ). The *dioctahedral* phyllosilicates have two octahedral sites occupied with cations (e.g. hydroxide sheet, Al4(OH) <sup>12</sup> <sup>12</sup> ) and one site is vacant.

Smectites are 2:1 phyllosilicates with a total (negative) layer charge between 0.2 and 0.6 p.f.u.. The octahedral sheet may either be dominantly occupied by trivalent cations (dioctahedral smectites) or divalent cations (trioctahedral smectites).

**Figure 1.** Model of the structure of vermiculite from Brazil; (with courtesy of dr. Jonáš Tokarský).

Vermiculites are generally trioctahedral and are termed (according to the joint nomenclature committees AIPEA and CMS) on the basis of a negative layer charge, which is between 0.6 to 0.9 p.f.u. [4,7,8]. The negative layer charge of vermiculites results from the substitution of Si4+ by trivalent cations in tetrahedral positions [9,10]. Vermiculites were mostly formed by removal of potassium from biotite, phlogopite or muscovite [5,6] and therefore vermiculite chemistry from this perspective is closely linked to that of mica. The thickness of the structural unit (2:1 layer and interlayer space) is about 1.4 nm, depending on the water interlamellar layers and the interlayer cations. Bailey [9] and Lagaly [10] described a method to determine the layer charge based on the measurement of basal spacings after exchange with alkylammonium cations of varying chain lengths. Alkylammonium ions in the interlayer spaces of vermiculite acquire distinct arrangements: monolayer and two-layer (bilayer) structures with the alkyl chains parallel to the surface, pseudotrimolecular layers and paraffin type structures. Vermiculites have paraffin-type interlayers if the layer charge is at least 0.75 p.f.u.

210 Clay Minerals in Nature – Their Characterization, Modification and Application

these incorrect terms are commonly found in the literature.

[5].

opposing tetrahedral sheets.

cations (e.g. hydroxide sheet Mg6(OH) <sup>12</sup>

sites occupied with cations (e.g. hydroxide sheet, Al4(OH) <sup>12</sup>

(dioctahedral smectites) or divalent cations (trioctahedral smectites).

*Layer charge (x) per half unit cell (p.f.u*.*)*, is the *net negative charge per layer*, expressed as a positive number (eq/(Si,Al)4010). The net negative layer charge arises from substitution of Al3+ for Si4+ in tetrahedra and substitution for lower charge cations in octahedra and from the presence of vacancies. The negative layer charge is balanced by the positively charged interlayer material. A '*unit structure*' is the total assembly and includes the layer and interlayer material. Therefore it is inappropriate to refer to a 'tetrahedral layer' or an 'octahedral layer', although

Guggenheim et al [4] give the revised classification scheme for planar hydrous phyllosilicates

The phyllosilicates are divided by layer type, and within the layer type, and by groups based on charge p.f.u. Further subdivisions by subgroups is based on dioctahedral or trioctahedral character, and finally by species based on chemical composition. Two types of layers, depending on the component sheets are a '1:1 layer' consisting of one tetrahedral sheet and one octahedral sheet, and a '2:1 layer' containing an octahedral sheet between two

The structure of 2:1 phyllosilicates is composed of tetrahedral (T) and octahedral (O) sheets (Fig. 1). According to the AIPEA Nomenclature Committee [4], tetrahedral sheet is composed of continuous two-dimensional corner-sharing tetrahedra [TO4]4- involving three basal oxygens and the apical oxygen. The tetrahedral sheet has a composition of [T4O10]4- where T = Si4+, Al3+, Fe3+. The apical oxygens form a corner of the octahedral coordination unit around larger

of octahedra with the central cations Mg2+ or Al3+. The smallest structural unit contains three octahedral sites. The *trioctahedral* structures of phyllosilicates have all three sites occupied with

Smectites are 2:1 phyllosilicates with a total (negative) layer charge between 0.2 and 0.6 p.f.u.. The octahedral sheet may either be dominantly occupied by trivalent cations

**Figure 1.** Model of the structure of vermiculite from Brazil; (with courtesy of dr. Jonáš Tokarský).

<sup>12</sup> ). The *dioctahedral* phyllosilicates have two octahedral

<sup>12</sup> ) and one site is vacant.

anions

octahedral cations. The octahedral sheet consists of two planes of closely packed O2-, OH-

The identification of trioctahedral and dioctahedral vermiculites is based on the position of reflection (060) on their XRD patterns. Dioctahedral vermiculites show an interlayer space value *d*(060) between 0.149 nm and 0.150 nm, while trioctahedral vermiculites have *d*(060) between 0.151 nm and 0.153 nm [11].

*The cation exchange capacity (CEC)* of clay minerals is defined as "the quantity of cations available for exchange at a given pH expressed in meq/100g which is equivalent to cmol(+)/kg [13]. The CEC varies between the 120 and 200 cmol+/kg (air dried vermiculites) or 140 and 240 cmol(+)/kg (dehydrated vermiculites) [14,15]. Water molecules associated with internal surfaces evokes hydration of interlayer cations. The total amount of alkylammonium ions bound by 2:1 clay minerals is often slightly higher than the total CEC determined by other methods [16]. This is a consequence of the charge regulation at the edges. Density of the charges at the crystal edges depend on the pH of the dispersion and arises from adsorption or dissociation of protons. In an acidic medium an excess of protons creates positive edge charges, the density of which decreases with rising pH. Negative charges are produced by the dissociation of silanol and aluminol groups [17,18].

Vermiculite structures contain water interlamellar layers which are subjected to the hydration and dehydration processes [19-26]. The hydration properties are controlled by the interlayer cations Mg2+ and minor amounts of Ca2+, Na+, and K+. The cation radius and charge influence the degree of hydration state in the interlayer and the stacking layer sequences [9,15,19,22,28]. The hydration state of vermiculite was defined by the number of water layers in the interlayer space. The basal space of Mg-vermiculite was declared 0.902 nm for zero-water layer, 1.150 nm for one-water layer and 1.440 nm for two water layer hydration state [29].

Ordering in vermiculite layered structure occurs when the pseudohexagonal cavities (made of six-membered tetrahedra rings) are facing each other in adjacent tetrahedral layers. In most cases the vermiculites have various possible layer-stacking sequences and therefore the regular arrangement of the layers in vermiculites occurs rarely [30-33].

Criteria for defining *the degree of regularity of alternation of different layer types* (e.g. 1:1 or 2:1 layers) were recommended in criteria for interstratification nomenclatures [4,12].

Vermiculite: Structural Properties and Examples of the Use 213

(Si2.83Al1.17) (Mg2.01Al0.2Fe2+0.16 Fe3+0.40 Ti0.14)O10(OH)2Mg0.235 (Ojen, Spain), [35]

(Si2.83Al1.17)(Mg2.01Fe3+0.4Fe2+0.16Al0.2Ti0.12) O10(OH)2Mg0.275 (Ojen, Spain), [57]

(Si3.43 Al0.57)(Al0.26 Fe0.32 Mg2.34)O10(OH)2 Ca0.064Na0.016K0.047 (Brazil), [38].

"*delamination/exfoliation*" should be use to describe the process [40].

**2. Obtaining vermiculite small particles** 

or "delamination/exfoliation" should be use [40].

"*intercalated structure*" [40].

Republic), [28].

[37].

(Si2.64 Al1.36)(Mg2.38Fe2+0.02Al0.06Fe3+0.51Ti0.03)O10(OH)2 Mg0.35 Ca0.01Na0.01· 4.9 H2O (Letovice, Czech

(Si3.02Al0.79Ti0.05Fe3+0.14)(Mg2.50Fe2+0.38Fe3+0.09)O10(OH,F)2Ba0.29K0.14Ca0.08 (Palabora, South Africa),

Natural vermiculite flakes are characterized by high values of aspect ratio. In clay science, *exfoliation* involves a degree of separation of the layers of a host structure where units, either individual layers or stacking of several layers, are dispersed (freely oriented and independent) in a solvent or polymer matrix [39]. This may be achieved by intercalation, by mechanical procedures, or by other methods. Exfoliation implies that the orientation between the layers of the host structure is lost, and that interlayer cohesive forces are overcome. If delamination or exfoliation cannot be distinguished, the terms "intercalation"

*Delamination* is a term used to describe a layer-separation process between the planar faces of adjacent layers of a particle. Delamination describes a process where intercalation occurs: guest material introduces between the layers while the stacking of layers remains. When delamination cannot be distinguished from exfoliation, the terms "*intercalation*" or

*Intercalation* is a general term to describe the movement of atoms, ions or molecules into a layered host structure, often a swelling clay mineral. The resulting structure is an

Exfoliated vermiculite can be produced either by a thermal or a chemical treatment. Heating a particle of vermiculite rapidly above about 200 °C results in the transformation of the interlayer water into steam. *The thermal shock* is the best-known procedure for dissociating the macroscopic packet of vermiculite [41]. The pressure of the steam separates silicate layers to the several orders thicker than were the fundamental layers. Such exfoliated particles exhibit the "accordion" type *morphology.* The large thermal expansion of the vermiculite after explosive dehydration of interlayer water causes cracking of vermiculite flakes. This exfoliation generates a 10 to 20 times volume expansion but the basal dimensions of the particles are unchanged [42]. This exfoliation is associated with a sudden release of water molecules between the silicate layers and also the hydroxyl water gradually releasing on heating from about 500 to 850°C [44]. The water rapidly vaporized cause a disruptive effect upon the particles, in industry called exfoliation. Several authors have

(Si3.02Al0.98) (Mg2.27Al0.12Fe3+ 0.28 Fe2+0.05Ti0.07)O10(OH)2Ca0.09Na0.21K0.50 (West China), [36].

The structure of vermiculite is called semi-ordered when the transition from a layer to the next layer can be obtained in two or more different ways. For semi-ordered stacks the reciprocal space cannot be described by a set of *hkl* indexes but rather by modulated reciprocal *hk* with the variable intensity along them [15]. The regular alterations of 1.150 and 0.902 nm domains can create the regular interstratification with *d* = 2.060 nm. Collins et al. [21] studied the variability in *d* values of the basal spacings of vermiculite from Llano (Texas). Authors assigned interlayer values *d* = 1.04–1.03 nm to the interstratified one-zero layer hydrate and the value *d* = 1.28 nm as random interstratified phase of two- and onelayer hydrates. The X-ray diffraction patterns of the South African raw vermiculite with high potassium content in the interlayer space (K0.53) showed the value *d* = 1.45 nm corresponding to the vermiculite and additional peaks with *d* = 2.52 nm, *d* = 1.26 and 1.205 nm, which were attributed to a biotite-vermiculite mixed-layer mineral [34]. Reichenbach and Beyer [22] evaluated two superstructures formed by a regular 1:1 interstratification. One of them with *d* = 2.541 nm as a result of the altering layers with *d* = 1.376 and 1.165 nm. Other superstructure with the value *d* = 2.153 nm was assigned to the altering layers 1.151nm and 1.002 nm. The hydrated states and interstratified phases in the vermiculites from Sta. Olalla (Spain), Paulistana (Brazil), Palabora (South Africa) and West China described Marcos et al. [25, 26]. Authors came to the founding that vermiculite from China consists of alternating mica layers (*d* = 1.02 nm, zero-water layer hydration) with vermiculite layers (*d* = 1.47 nm, two-water layer hydration) and two–one layer hydration interstratification with *d* = 1.21 nm.

The structural formula of vermiculites is often reported on the basis of the structure unit (half unit-cell content). The general formula can be written as:

X4 (Y2-3 ) O10 (OH)2 M . *n* H2O,

where M is exchangeable (Mg2+, Ca2+, Ba2+, Na+, K+) cations positioned in the interlayer space, that compensate negative layer charge, Y is octahedral Mg2+,Fe2+ or Fe3+, Al3+, and X is tetrahedral Si, Al.

The half unit cell compositions of vermiculites given in literature are listed below for comparison.

(Si2.86 Al1.14)(Mg2.83Al0.15Fe3+0.02 )O10(OH)2 Mg0.41 · 3.72 H2O (Llano), [9]

(Si2.72 Al1.28)(Mg2.36Al0.16Fe 0.58)O10(OH)2 Mg0.32 · 4.32 H2O (Kenya), [19]

(Si2.72 Al1.28)(Mg2.59Fe2+0.03 Al0.06Fe3+0.24 Ti0.08)O10(OH)2 Mg0.39 Ca0.02 ·4.7 H2O (Santa Olalla, Spain), [15]

(Si2.64Al1.36)(Mg2.48 Fe2+0.04 Al0.14 Fe3+0.32Ti0.01Mn0.01)O10(OH)2 Mg0.44 (Santa Olalla, Spain), [35]

(Si2.69Al1.31)(Mg2.48Fe3+0.324Fe2+0.036Al0.14 Ti0.01 Mn0.01)O10(OH)2Mg0.39, (Santa Olalla, Spain), [57]

(Si2.83Al1.17) (Mg2.01Al0.2Fe2+0.16 Fe3+0.40 Ti0.14)O10(OH)2Mg0.235 (Ojen, Spain), [35]

(Si2.83Al1.17)(Mg2.01Fe3+0.4Fe2+0.16Al0.2Ti0.12) O10(OH)2Mg0.275 (Ojen, Spain), [57]

(Si2.64 Al1.36)(Mg2.38Fe2+0.02Al0.06Fe3+0.51Ti0.03)O10(OH)2 Mg0.35 Ca0.01Na0.01· 4.9 H2O (Letovice, Czech Republic), [28].

(Si3.02Al0.98) (Mg2.27Al0.12Fe3+ 0.28 Fe2+0.05Ti0.07)O10(OH)2Ca0.09Na0.21K0.50 (West China), [36].

(Si3.02Al0.79Ti0.05Fe3+0.14)(Mg2.50Fe2+0.38Fe3+0.09)O10(OH,F)2Ba0.29K0.14Ca0.08 (Palabora, South Africa), [37].

(Si3.43 Al0.57)(Al0.26 Fe0.32 Mg2.34)O10(OH)2 Ca0.064Na0.016K0.047 (Brazil), [38].

## **2. Obtaining vermiculite small particles**

212 Clay Minerals in Nature – Their Characterization, Modification and Application

interstratification with *d* = 1.21 nm.

X4 (Y2-3 ) O10 (OH)2 M . *n* H2O,

tetrahedral Si, Al.

comparison.

[15]

(half unit-cell content). The general formula can be written as:

(Si2.86 Al1.14)(Mg2.83Al0.15Fe3+0.02 )O10(OH)2 Mg0.41 · 3.72 H2O (Llano), [9] (Si2.72 Al1.28)(Mg2.36Al0.16Fe 0.58)O10(OH)2 Mg0.32 · 4.32 H2O (Kenya), [19]

Criteria for defining *the degree of regularity of alternation of different layer types* (e.g. 1:1 or 2:1

The structure of vermiculite is called semi-ordered when the transition from a layer to the next layer can be obtained in two or more different ways. For semi-ordered stacks the reciprocal space cannot be described by a set of *hkl* indexes but rather by modulated reciprocal *hk* with the variable intensity along them [15]. The regular alterations of 1.150 and 0.902 nm domains can create the regular interstratification with *d* = 2.060 nm. Collins et al. [21] studied the variability in *d* values of the basal spacings of vermiculite from Llano (Texas). Authors assigned interlayer values *d* = 1.04–1.03 nm to the interstratified one-zero layer hydrate and the value *d* = 1.28 nm as random interstratified phase of two- and onelayer hydrates. The X-ray diffraction patterns of the South African raw vermiculite with high potassium content in the interlayer space (K0.53) showed the value *d* = 1.45 nm corresponding to the vermiculite and additional peaks with *d* = 2.52 nm, *d* = 1.26 and 1.205 nm, which were attributed to a biotite-vermiculite mixed-layer mineral [34]. Reichenbach and Beyer [22] evaluated two superstructures formed by a regular 1:1 interstratification. One of them with *d* = 2.541 nm as a result of the altering layers with *d* = 1.376 and 1.165 nm. Other superstructure with the value *d* = 2.153 nm was assigned to the altering layers 1.151nm and 1.002 nm. The hydrated states and interstratified phases in the vermiculites from Sta. Olalla (Spain), Paulistana (Brazil), Palabora (South Africa) and West China described Marcos et al. [25, 26]. Authors came to the founding that vermiculite from China consists of alternating mica layers (*d* = 1.02 nm, zero-water layer hydration) with vermiculite layers (*d* = 1.47 nm, two-water layer hydration) and two–one layer hydration

The structural formula of vermiculites is often reported on the basis of the structure unit

where M is exchangeable (Mg2+, Ca2+, Ba2+, Na+, K+) cations positioned in the interlayer space, that compensate negative layer charge, Y is octahedral Mg2+,Fe2+ or Fe3+, Al3+, and X is

The half unit cell compositions of vermiculites given in literature are listed below for

(Si2.72 Al1.28)(Mg2.59Fe2+0.03 Al0.06Fe3+0.24 Ti0.08)O10(OH)2 Mg0.39 Ca0.02 ·4.7 H2O (Santa Olalla, Spain),

(Si2.64Al1.36)(Mg2.48 Fe2+0.04 Al0.14 Fe3+0.32Ti0.01Mn0.01)O10(OH)2 Mg0.44 (Santa Olalla, Spain), [35] (Si2.69Al1.31)(Mg2.48Fe3+0.324Fe2+0.036Al0.14 Ti0.01 Mn0.01)O10(OH)2Mg0.39, (Santa Olalla, Spain), [57]

layers) were recommended in criteria for interstratification nomenclatures [4,12].

Natural vermiculite flakes are characterized by high values of aspect ratio. In clay science, *exfoliation* involves a degree of separation of the layers of a host structure where units, either individual layers or stacking of several layers, are dispersed (freely oriented and independent) in a solvent or polymer matrix [39]. This may be achieved by intercalation, by mechanical procedures, or by other methods. Exfoliation implies that the orientation between the layers of the host structure is lost, and that interlayer cohesive forces are overcome. If delamination or exfoliation cannot be distinguished, the terms "intercalation" or "delamination/exfoliation" should be use [40].

*Delamination* is a term used to describe a layer-separation process between the planar faces of adjacent layers of a particle. Delamination describes a process where intercalation occurs: guest material introduces between the layers while the stacking of layers remains. When delamination cannot be distinguished from exfoliation, the terms "*intercalation*" or "*delamination/exfoliation*" should be use to describe the process [40].

*Intercalation* is a general term to describe the movement of atoms, ions or molecules into a layered host structure, often a swelling clay mineral. The resulting structure is an "*intercalated structure*" [40].

Exfoliated vermiculite can be produced either by a thermal or a chemical treatment. Heating a particle of vermiculite rapidly above about 200 °C results in the transformation of the interlayer water into steam. *The thermal shock* is the best-known procedure for dissociating the macroscopic packet of vermiculite [41]. The pressure of the steam separates silicate layers to the several orders thicker than were the fundamental layers. Such exfoliated particles exhibit the "accordion" type *morphology.* The large thermal expansion of the vermiculite after explosive dehydration of interlayer water causes cracking of vermiculite flakes. This exfoliation generates a 10 to 20 times volume expansion but the basal dimensions of the particles are unchanged [42]. This exfoliation is associated with a sudden release of water molecules between the silicate layers and also the hydroxyl water gradually releasing on heating from about 500 to 850°C [44]. The water rapidly vaporized cause a disruptive effect upon the particles, in industry called exfoliation. Several authors have

found that the presence of the mixed-layer vermiculite–mica or mixed-layer minerals phases containing vermiculite in different hydration states contributed to the exfoliation of vermiculites [26,45,46].

Vermiculite: Structural Properties and Examples of the Use 215

water and after the same numer of sonication cycles in an aqueous 15% H2O2 to 54 m2g-1. The surface area of the Santa Olalla vermiculite after six cycles of sonication was smaller and decreased to the 36 m2g-1 after sonication in hydrogen peroxide and 38 m2g-1 in water. In both vermiculites, one cycle of sonication provided by half greater value increase in the surface area in water in comparison with sonication in hydrogen peroxide. This difference disappeared after three cycles of sonication. Authors stated that delamination by sonication plays a significant role in particle size reduction and in the increase of the surface areas only after a short time. The pH during sonication in water 6.5 increased after three cycles to 7.5–8 and in hydrogen peroxide from 3.7 to 6.5. After sonication no structural change in Santa Olalla vermiculite was observed in comparison with the Ojén-vermiculite. Whereas the ratio Fe3+/Fetotal was found almost constant in the Santa Olalla vermiculite, it increased from 0.79

*A strong hydrogen peroxide concentration* (up to 50 %) for exfoliation vermiculite was used by Obut and Girgin [58] and Üçgül and Girgin [59]. Authors interpreted the exfoliation mechanism by a hydrogen peroxide penetration into the interlayer spaces and its decomposition with evolution of atomic oxygen. These interactions disrupt the electrostatic equilibrium between the layers and the interlayer cations. Weiss et al. [60] studied exfoliation of Mg-vermiculite after thermal, microwave and/or hydrogen peroxide treatments. A stronger than hydrogen peroxide oxidation agent potassium persulfate (K2S2O8) used Matějka et al. [61] to delaminate/exfoliate Mg-vermiculite. The vermiculite treatment with potassium persulfate solution caused a collapse of the layered structure at considerably lower molar

Kehal et al. [62] modified vermiculite from Palabora by the combination of the thermal shock (700°C), chemical exfoliation (80°C in the presence of H2O2) and ultrasonic treatments (20 kHz, H2O or H2O2) to improve the adsorption of boron. Authors found that only 1 h treatment of vermiculite in ultrasound and 35 wt.% H2O2 produced small particles of drastically decreased density from 1.026 g cm−3 for raw vermiculite to about 0.23 g cm−3. Furthermore, it was found by acido–basic titrations, that breaking of the particles by 20 kHz sonication induced the generation of OH groups on the edges of the layers which acted as

The negative surface charge that results from the ion substitution or from the site vacancies at the tetrahedral and/or octahedral sheets predetermined the use of layered 2:1 phyllosilicates as substrates for the growth of metallic nanoparticles. Uncompensated charges occur at the broken edges of the clay mineral layers, predominantly at the hydroxyl groups. The cations like Al3+ or Fe3+, which typically occupy the octahedral positions, remain at the crystal edges, i.e. at the Lewis acid sites, where they coordinate water molecules. The exchangeable cations between the layers (at the Brønsted acid sites) compensate the negative charge and may be easily exchanged by other metal cations [63,64]. The reduction of the metal cations on the clay minerals matrix initiates the growth of nanoparticles preferably on

to 0.85 in case of the Ojén sample together with a decrease of the layer charge.

concentration in comparison to treatment using hydrogen peroxide solution.

**3. Vermiculite particles as carrier of silver nanoparticles** 

active adsorption sites.

Preparation of submicron-sized vermiculites is usually accomplished by applying *wet and dry grinding* [47].

Progressive amorphization and agglomeration of vermiculite particles takes place when grinding time increases [48].

Vermiculite can be also delaminated using the mechanical shearing force [49,50].

Balek et al.[51] found that vermiculite from Santa Olalla after grinding for 2 min increased specific surface area from 1 m2g-1 to 39 m2g-1. Prolongation of grinding time to 10 min led to the formation of amorphous phase and the surface area decrease to 20 m2g-1. The thermogravimetry measurement recorded different thermal behavior of original and milled vermiculite samples. Original vermiculite showed two dehydration steps after heating up to 250ºC and one dehydroxylation step at 900ºC. The grounded sample showed no such steps but dehydration was observed as continuous mass loss from 50 to 350ºC and dehydroxylation was completed at 800ºC.

*Ultrasound* is an alternative method for reduction of the particle size using treatment by high power. The ultrasound caused delamination and particle size reduction not only along the basal planes but also in different directions [52-55]. Péréz-Maqueda et al. [56] compared vermiculites after the grinding and ultrasonic treatments. According to authors, the sonication resulted in particle size reduction and particles retained the plate-like morphology of the original vermiculite. The prolonged grinding caused the loss of longrange order, crystallinity and agglomeration of the particles. The edges of vermiculite particles were changed and allowed easier access nitrogen during measurements and therefore yielding surface areas larger than those expected only from the particle size reduction. The ultrasound on the high-and low-charge vermiculites studied also Wiewiora et al. [35]. A high-charge vermiculite from Santa Olalla (0.88 p.f.u.) preserved the flake shape, whereas the low-charge vermiculite from Ojén (0.47 p.f.u.) displayed scrolling of the flakes into tubes. Sonication differently influenced the specific surface area and increased from the 1 m2g-1 at the raw samples to 36 and 54 m2 g-1 at high-charged and low-charged vermiculites, respectively.

The particle size reduction by sonication may be accompanied by a change of the redox state and the layer charge of the material. Sonication in a 1:1 mixture (volume ratio) of water and hydrogen peroxide (30% H2O2) is a soft method for particle size reduction of phyllosilicate minerals like vermiculites [57]. The oxidation state of the iron in the high-charge vermiculite from Santa Olalla and the low-charge vermiculite from Ojén was found different. In spite that the chemical composition of vermiculites is similar (see unit half cell composition formulas), the layer charge is different 0.78 and 0.52 p.f.u. for the Santa Olalla and Ojén vermiculite, respectively. Both of the original vermiculites had the specific surface area 1 m2g-1. The area in Ojén-vermiculite increased to 51 m2g-1 after eight cycles of sonication in water and after the same numer of sonication cycles in an aqueous 15% H2O2 to 54 m2g-1. The surface area of the Santa Olalla vermiculite after six cycles of sonication was smaller and decreased to the 36 m2g-1 after sonication in hydrogen peroxide and 38 m2g-1 in water. In both vermiculites, one cycle of sonication provided by half greater value increase in the surface area in water in comparison with sonication in hydrogen peroxide. This difference disappeared after three cycles of sonication. Authors stated that delamination by sonication plays a significant role in particle size reduction and in the increase of the surface areas only after a short time. The pH during sonication in water 6.5 increased after three cycles to 7.5–8 and in hydrogen peroxide from 3.7 to 6.5. After sonication no structural change in Santa Olalla vermiculite was observed in comparison with the Ojén-vermiculite. Whereas the ratio Fe3+/Fetotal was found almost constant in the Santa Olalla vermiculite, it increased from 0.79 to 0.85 in case of the Ojén sample together with a decrease of the layer charge.

214 Clay Minerals in Nature – Their Characterization, Modification and Application

vermiculites [26,45,46].

grinding time increases [48].

vermiculites, respectively.

dehydroxylation was completed at 800ºC.

*dry grinding* [47].

found that the presence of the mixed-layer vermiculite–mica or mixed-layer minerals phases containing vermiculite in different hydration states contributed to the exfoliation of

Preparation of submicron-sized vermiculites is usually accomplished by applying *wet and* 

Progressive amorphization and agglomeration of vermiculite particles takes place when

Balek et al.[51] found that vermiculite from Santa Olalla after grinding for 2 min increased specific surface area from 1 m2g-1 to 39 m2g-1. Prolongation of grinding time to 10 min led to the formation of amorphous phase and the surface area decrease to 20 m2g-1. The thermogravimetry measurement recorded different thermal behavior of original and milled vermiculite samples. Original vermiculite showed two dehydration steps after heating up to 250ºC and one dehydroxylation step at 900ºC. The grounded sample showed no such steps but dehydration was observed as continuous mass loss from 50 to 350ºC and

*Ultrasound* is an alternative method for reduction of the particle size using treatment by high power. The ultrasound caused delamination and particle size reduction not only along the basal planes but also in different directions [52-55]. Péréz-Maqueda et al. [56] compared vermiculites after the grinding and ultrasonic treatments. According to authors, the sonication resulted in particle size reduction and particles retained the plate-like morphology of the original vermiculite. The prolonged grinding caused the loss of longrange order, crystallinity and agglomeration of the particles. The edges of vermiculite particles were changed and allowed easier access nitrogen during measurements and therefore yielding surface areas larger than those expected only from the particle size reduction. The ultrasound on the high-and low-charge vermiculites studied also Wiewiora et al. [35]. A high-charge vermiculite from Santa Olalla (0.88 p.f.u.) preserved the flake shape, whereas the low-charge vermiculite from Ojén (0.47 p.f.u.) displayed scrolling of the flakes into tubes. Sonication differently influenced the specific surface area and increased from the 1 m2g-1 at the raw samples to 36 and 54 m2 g-1 at high-charged and low-charged

The particle size reduction by sonication may be accompanied by a change of the redox state and the layer charge of the material. Sonication in a 1:1 mixture (volume ratio) of water and hydrogen peroxide (30% H2O2) is a soft method for particle size reduction of phyllosilicate minerals like vermiculites [57]. The oxidation state of the iron in the high-charge vermiculite from Santa Olalla and the low-charge vermiculite from Ojén was found different. In spite that the chemical composition of vermiculites is similar (see unit half cell composition formulas), the layer charge is different 0.78 and 0.52 p.f.u. for the Santa Olalla and Ojén vermiculite, respectively. Both of the original vermiculites had the specific surface area 1 m2g-1. The area in Ojén-vermiculite increased to 51 m2g-1 after eight cycles of sonication in

Vermiculite can be also delaminated using the mechanical shearing force [49,50].

*A strong hydrogen peroxide concentration* (up to 50 %) for exfoliation vermiculite was used by Obut and Girgin [58] and Üçgül and Girgin [59]. Authors interpreted the exfoliation mechanism by a hydrogen peroxide penetration into the interlayer spaces and its decomposition with evolution of atomic oxygen. These interactions disrupt the electrostatic equilibrium between the layers and the interlayer cations. Weiss et al. [60] studied exfoliation of Mg-vermiculite after thermal, microwave and/or hydrogen peroxide treatments. A stronger than hydrogen peroxide oxidation agent potassium persulfate (K2S2O8) used Matějka et al. [61] to delaminate/exfoliate Mg-vermiculite. The vermiculite treatment with potassium persulfate solution caused a collapse of the layered structure at considerably lower molar concentration in comparison to treatment using hydrogen peroxide solution.

Kehal et al. [62] modified vermiculite from Palabora by the combination of the thermal shock (700°C), chemical exfoliation (80°C in the presence of H2O2) and ultrasonic treatments (20 kHz, H2O or H2O2) to improve the adsorption of boron. Authors found that only 1 h treatment of vermiculite in ultrasound and 35 wt.% H2O2 produced small particles of drastically decreased density from 1.026 g cm−3 for raw vermiculite to about 0.23 g cm−3. Furthermore, it was found by acido–basic titrations, that breaking of the particles by 20 kHz sonication induced the generation of OH groups on the edges of the layers which acted as active adsorption sites.

## **3. Vermiculite particles as carrier of silver nanoparticles**

The negative surface charge that results from the ion substitution or from the site vacancies at the tetrahedral and/or octahedral sheets predetermined the use of layered 2:1 phyllosilicates as substrates for the growth of metallic nanoparticles. Uncompensated charges occur at the broken edges of the clay mineral layers, predominantly at the hydroxyl groups. The cations like Al3+ or Fe3+, which typically occupy the octahedral positions, remain at the crystal edges, i.e. at the Lewis acid sites, where they coordinate water molecules. The exchangeable cations between the layers (at the Brønsted acid sites) compensate the negative charge and may be easily exchanged by other metal cations [63,64]. The reduction of the metal cations on the clay minerals matrix initiates the growth of nanoparticles preferably on

the surface, because the interlamellar space of clay minerals limits the particle growth [65- 70].

Vermiculite: Structural Properties and Examples of the Use 217

CCM 4224). *K. pneumoniae* resides in the normal flora of the mouth, skin and intestines. *P. aeruginosa* is the most common pathogen isolated from patients, who have been hospitalized longer than one week. *S. aureus* is a constituent of the skin flora that is frequently found in the nose and on the skin. *E. faecalis* inhabits the gastrointestinal tracts of humans and other

**Figure 2.** TEM images of silver nanoparticles on montmorillonite (a) and vermiculite substrates (b). HRTEM images of nanoparticles grew together (c) and microstructure of the Ag nanoparticles (d); (with

Nanocrystalline particles of silver reduced from the solution of silver nitrate on the clay mineral substrates were characterized using TEM and HRTEM. Transmission electron microscopy (TEM) revealed silver nanoparticles grown on the surface of both clay minerals of the mean size between 40 and 50 nm. Small Ag particles were substantially smaller than 20 nm. Furthermore, TEM/HRTEM found essential differences in the size distribution of the Ag particles grown on the surface of the montmorillonite and on the surface of the vermiculite. On montmorillonite, all Ag particles grew with a similar size and were well distributed on the surface (Fig. 2a). Huge Ag particles with the size much larger than 50 nm were observed only on the edges of the montmorillonite flakes. The size of the Ag particles grown on the surface of vermiculite was very heterogeneous (Fig. 2b). Some of nanoparticles were agglomerated or grew together (Fig. 2c). A lot of small particles contained smaller domains with high defect density. Microstructure of the Ag nanoparticles showed crystallites disorientation with a lot of defects, especially different planar defects (Fig. 2d).

Based on the results of the microstructure analysis of silver nanoparticles and clay mineral substrates authors [36, 70] assumed the manner of growth of nanoparticles on the surface of

The particle growth process began after the previous docking of silver (precipitated from the solution of AgNO3) on the clay mineral surface. During their further growth, the Ag nanoparticles separated the clay mineral crystallites into several domains. Single nanocrystalline particles of Ag grow next to each other and then growing to larger nanoparticles with many microstructure defects. Still, the atomic ordering of the Ag nanoparticles at the silver/clay mineral interface is controlled by the local orientation of the clay mineral matrix. During the further growth, larger Ag nanoparticles re-crystallize by reducing the energy of their internal defects through the rearrangement and through the conservation or formation of the low-energy boundaries (as it was observed by HRTEM).

mammals.

courtesy of dr. Valter Klemm)

vermiculite and montmorillonite.

The metal nanoparticles are investigated in the function of biosensors, label for cell and cancer therapeutics. The silver nanoparticles have shown to be a promising antiviral material. The strong toxicity of several silver compounds to a wide range of microorganisms is well known. Silver acts as catalyst of the oxidation of the microorganisms, which leads to the disruption of electron transfer in bacteria [71,72]. Silver offers sufficiently small repulsion to oxygen, thus only a small amount of thermal energy is required to move the atomic oxygen readily through its crystal lattice [73]. Monovalent silver ions have a high affinity to sulfhydryl (–S–H) groups in bacteria cells. The resulting stable –S–Ag bonds inhibit the hydrogen transfer and prevent the respiration and the electron transfer. After exposing silver containing molecular oxygen to aqueous media, the molecular oxygen reacts quickly with sulfhydryl (–S–H) groups on the surface of bacteria and replaces hydrogen [74]. Silver nanoparticles exhibit cytoprotectivity towards HIV-1 infected cells [75]. The biological distribution as well as the potential toxicity of silver nanoparticles on the montmorillonite substrate was studied using Swiss mice [76]. The animal study demonstrated that the Ag-montmorillonite was nontoxic, showed no immune response, exhibited increased blood half-life and neurotransmission. Based on the study authors noted that Ag-montmorillonite enables diverse applications in life sciences such as drug development, protein detection and gene delivery for any organs, lungs and brain in particular.

The catalytic activity of silver particles depends on their size, shape and the size distribution as well as chemical–physical environment [77, 78].

Silver nanoparticles, size about 2 nm, were usually prepared by chemical reduction from silver nitrate under ultraviolet irradiation [79]. Another method employed reduction with sodium borohydride [80]. During this reaction, irregular silver nanoparticles with the size between 1.5 and 2.5 nm nucleate first. This rapid nucleation was followed by the growth of these nuclei into aggregates of circular particles with the size of approximately 12–16 nm. Several approaches have been investigated to prepare silver nanoparticles with well-defined size and morphology. The available free-network spaces between hydrogel networks helped to grow and to stabilize the nanoparticles [81-86].

The antibacterial activity of silver-montmorillonite and copper-montmorillonite studied Magaňa et al. [87]. The authors stated that the overall antibacterial effect was related to the surface characteristics of the sample and to the quantity of silver. Similarly, the antibacterial behaviour of silver grown attached to copper-palygorskite characterized Zhao et al. [88]. Valášková et al. [89] have prepared and characterized Ag nanoparticles on vermiculite and compared them with Ag nanoparticles on montmorillonite. The antibacterial activity of both nanocomposite types was tested on two bacteria strains. The Gram negative (G-) strain was represented with two bacteria *Klebsiella pneumoniae* (*K. pneumoniae*, 9CCM 4415) and *Pseudomonas aeruginosa* (*P. aeruginosa*, CCM 1960), and the Gram positive (G+) strain using two bacteria *Staphylococcus aureus* (*S. aureus*, CCM 3953) and *Enterococcus faecalis* (*E. faecalis*, CCM 4224). *K. pneumoniae* resides in the normal flora of the mouth, skin and intestines. *P. aeruginosa* is the most common pathogen isolated from patients, who have been hospitalized longer than one week. *S. aureus* is a constituent of the skin flora that is frequently found in the nose and on the skin. *E. faecalis* inhabits the gastrointestinal tracts of humans and other mammals.

216 Clay Minerals in Nature – Their Characterization, Modification and Application

as well as chemical–physical environment [77, 78].

to grow and to stabilize the nanoparticles [81-86].

70].

particular.

the surface, because the interlamellar space of clay minerals limits the particle growth [65-

The metal nanoparticles are investigated in the function of biosensors, label for cell and cancer therapeutics. The silver nanoparticles have shown to be a promising antiviral material. The strong toxicity of several silver compounds to a wide range of microorganisms is well known. Silver acts as catalyst of the oxidation of the microorganisms, which leads to the disruption of electron transfer in bacteria [71,72]. Silver offers sufficiently small repulsion to oxygen, thus only a small amount of thermal energy is required to move the atomic oxygen readily through its crystal lattice [73]. Monovalent silver ions have a high affinity to sulfhydryl (–S–H) groups in bacteria cells. The resulting stable –S–Ag bonds inhibit the hydrogen transfer and prevent the respiration and the electron transfer. After exposing silver containing molecular oxygen to aqueous media, the molecular oxygen reacts quickly with sulfhydryl (–S–H) groups on the surface of bacteria and replaces hydrogen [74]. Silver nanoparticles exhibit cytoprotectivity towards HIV-1 infected cells [75]. The biological distribution as well as the potential toxicity of silver nanoparticles on the montmorillonite substrate was studied using Swiss mice [76]. The animal study demonstrated that the Ag-montmorillonite was nontoxic, showed no immune response, exhibited increased blood half-life and neurotransmission. Based on the study authors noted that Ag-montmorillonite enables diverse applications in life sciences such as drug development, protein detection and gene delivery for any organs, lungs and brain in

The catalytic activity of silver particles depends on their size, shape and the size distribution

Silver nanoparticles, size about 2 nm, were usually prepared by chemical reduction from silver nitrate under ultraviolet irradiation [79]. Another method employed reduction with sodium borohydride [80]. During this reaction, irregular silver nanoparticles with the size between 1.5 and 2.5 nm nucleate first. This rapid nucleation was followed by the growth of these nuclei into aggregates of circular particles with the size of approximately 12–16 nm. Several approaches have been investigated to prepare silver nanoparticles with well-defined size and morphology. The available free-network spaces between hydrogel networks helped

The antibacterial activity of silver-montmorillonite and copper-montmorillonite studied Magaňa et al. [87]. The authors stated that the overall antibacterial effect was related to the surface characteristics of the sample and to the quantity of silver. Similarly, the antibacterial behaviour of silver grown attached to copper-palygorskite characterized Zhao et al. [88]. Valášková et al. [89] have prepared and characterized Ag nanoparticles on vermiculite and compared them with Ag nanoparticles on montmorillonite. The antibacterial activity of both nanocomposite types was tested on two bacteria strains. The Gram negative (G-) strain was represented with two bacteria *Klebsiella pneumoniae* (*K. pneumoniae*, 9CCM 4415) and *Pseudomonas aeruginosa* (*P. aeruginosa*, CCM 1960), and the Gram positive (G+) strain using two bacteria *Staphylococcus aureus* (*S. aureus*, CCM 3953) and *Enterococcus faecalis* (*E. faecalis*,

**Figure 2.** TEM images of silver nanoparticles on montmorillonite (a) and vermiculite substrates (b). HRTEM images of nanoparticles grew together (c) and microstructure of the Ag nanoparticles (d); (with courtesy of dr. Valter Klemm)

Nanocrystalline particles of silver reduced from the solution of silver nitrate on the clay mineral substrates were characterized using TEM and HRTEM. Transmission electron microscopy (TEM) revealed silver nanoparticles grown on the surface of both clay minerals of the mean size between 40 and 50 nm. Small Ag particles were substantially smaller than 20 nm. Furthermore, TEM/HRTEM found essential differences in the size distribution of the Ag particles grown on the surface of the montmorillonite and on the surface of the vermiculite. On montmorillonite, all Ag particles grew with a similar size and were well distributed on the surface (Fig. 2a). Huge Ag particles with the size much larger than 50 nm were observed only on the edges of the montmorillonite flakes. The size of the Ag particles grown on the surface of vermiculite was very heterogeneous (Fig. 2b). Some of nanoparticles were agglomerated or grew together (Fig. 2c). A lot of small particles contained smaller domains with high defect density. Microstructure of the Ag nanoparticles showed crystallites disorientation with a lot of defects, especially different planar defects (Fig. 2d).

Based on the results of the microstructure analysis of silver nanoparticles and clay mineral substrates authors [36, 70] assumed the manner of growth of nanoparticles on the surface of vermiculite and montmorillonite.

The particle growth process began after the previous docking of silver (precipitated from the solution of AgNO3) on the clay mineral surface. During their further growth, the Ag nanoparticles separated the clay mineral crystallites into several domains. Single nanocrystalline particles of Ag grow next to each other and then growing to larger nanoparticles with many microstructure defects. Still, the atomic ordering of the Ag nanoparticles at the silver/clay mineral interface is controlled by the local orientation of the clay mineral matrix. During the further growth, larger Ag nanoparticles re-crystallize by reducing the energy of their internal defects through the rearrangement and through the conservation or formation of the low-energy boundaries (as it was observed by HRTEM).

Differences between silver nanoparticles on the vermiculite and montmorillonite substrates authors confronted with the different sources of negative layer charge. When at dioctahedral montmorillonite negative layer charge results from an octahedral substitution of magnesium for iron and the substitution of Si4+ in tetrahedra is negligible [90] the trioctahedral vermiculite has a negative charge resulting mainly from the substitution of trivalent cations for Si4+ in tetrahedra [9]. Further it was found that montmorillonite and vermiculite after treatment with the silver nitrate solution released the Na+, K+, Ca2+ and Mg2+ cations from the interlayer space of both clay minerals and from the octahedral positions in the montmorillonite. Such structural change allowed the Ag+ to be incorporated in the host structure of the clay minerals through their surface and edges. The total amount of silver on vermiculite was higher than on the montmorillonite. The size distribution of the Ag nanoparticles was much more homogeneous on the montmorillonite substrate than on the vermiculite. The lower negative layer charge of montmorillonite results from the substitution of cations, which are located at the octahedral positions, is responsible for a uniform size of the Ag nanoparticles. The Ag particles with the size much larger than 50 nm were only on the edges. The negative layer charge of vermiculite as a consequence of the charge on tetrahedra can hold higher content of silver than montmorillonite.

Vermiculite: Structural Properties and Examples of the Use 219

The hydrophilic clay minerals are commonly treated with ammonium cations with long alkyl chains to improve the compatibility between the silicate layers and the polymer matrix. The methods used for the preparation of clay–polymer nanocomposites, include solvent intercalation, in situ polymerization, and melt-compounding [98-102]. Both solvent intercalation and in situ polymerization allow polymer chains to enter into the galleries of silicate clays. Melt blending is more attractive because the nanocomposites can be processed by conventional methods such as extrusion and injection molding. The influences of the addition of various types of clays on the non-isothermal crystallization process of thermoplastics have been studied by several researchers. Fornes and Paul [103] found that depending on the dispersion of clays in thermoplastic matrices the clay nanofillers can either promote or retard the crystallization of polymers and observed that the degree of

crystallinity of nanocomposites showed a strong dependence on the cooling rates.

dipole moments in PE [106-109].

intercalated with ODA.

Polyethylene (PE) is one of the most widely used polyolefins. The molecular structure is generally simply written by the formula (CH2)n, where n is very large. The PE specimen may contain chains of different lengths [104]. Low-density PE (LDPE) contains many statistically placed paraffinic branches. The defects in the arrangement of chains reduce the structure crystallinity [105]. When mixing PE with hydrophilic clay minerals the additives can play a role of a polymeric surfactant, or they may act as a compatibilizer when mixed with organophilized clay minerals. The polar additives or charge of carriers introduce

Clay mineral vermiculite (VER) was used as the clay mineral nanofiller into polymer matrix and was intercalated with the maleic anhydride into polyamide [110], polyethylene [109], and polypropylene [89, 111]. It should be noted that nanocomposites of polypropylene with nanofiller of VER were prepared without any compatibilizer by solid-state shear compounding (S3C) using pan-mill equipment [112]. Vermiculite particles were only partly

Organovermiculite nanofiller octadecylamine/vermiculite (ODA/VER) was exfoliated in polypropylene (PP) [89]. The organovermiculite nanofillers were prepared in three ways. Sample 1 was original VER (Fig. 3a) intercalated using melt intercalation with octadecylamine (ODA). Sample 2 was VER milled in jet mill and intercalated with ODA (Fig. 3b). Sample 3 was VER exfoliated using an oxidizing agent potassium persulfate [61] and subsequently

The thermal compounding of the organovermiculites (4 wt. %) with maleated PP caused the partial deintercalation of organovermiculites. As the jet milled original VER flakes have corrugated edges [113] in PP remained intercalated only in their central parts. The frayed edges and small particles showed somewhat higher interlayer distances and the penetration of polymeric segments from the softened polymer matrix was possible. The sample 2 was very well dispersed within PP matrix. The layered vermiculite structure after exfoliation with potassium persulfate was destroyed [61] and therefore intercalation of ODA molecules into the interlayer was limited (sample 3). On the X-ray diffraction patterns of composite sample 2 in PP authors identified new reflection of the orthorhombic γ-PP in addition to the

intercalated with polypropylene and exfoliated in PP matrix (Shao et al., 2006).

Both sample series of silver/vermiculites and silver/montmorillonites, showed good inhibitory action against the Gram negative bacteria strains. The higher content of silver reduced on vermiculite predestines silver/vermiculite to be stronger antibacterial agent than silver/montmorillonite.

## **4. Vermiculite nanofillers to polymer/clay nanocomposites**

Nanocomposites have at least one ultrafine phase dimension, typically in the range of 1–100 nm, and exhibit other properties with comparison to the micro- and macro-composites. The clay–polymer nanocomposite could be considered as "one-nano-dimensional" because the clay filler has one dimension at the nanometer scale [91]. The high aspect ratio of layered silicate nanoparticles is ideal to modify the properties of the polymer, but the hydrophilic nature of silicate surfaces impedes their homogeneous dispersion in the organic polymer phase [92]. On the other hand it was found that polymers can interact with the external surface of clay minerals and also penetrate into the nano- structural spaces, holes and tunnels [93].

The polymer nanocomposites exhibit new and sometimes improved properties that are not displayed by the individual phases or by their conventional composite counterparts. Significant improvement of mechanical properties, thermal stability, resistance to solvent swelling and suppression of flammability have been achieved with the only up to 5 wt.% exfoliated layered silicates nanoparticles in polymer matrix [94-97]. The development of clay–polymer nanocomposites at low clay loadings (3–7 wt %) as advanced structural materials brings significant improvements in mechanical strength and stiffness, enhanced gas barrier behavior, reduced linear thermal expansion coefficients, and increased solvent resistance in comparison with pristine polymers.

The hydrophilic clay minerals are commonly treated with ammonium cations with long alkyl chains to improve the compatibility between the silicate layers and the polymer matrix. The methods used for the preparation of clay–polymer nanocomposites, include solvent intercalation, in situ polymerization, and melt-compounding [98-102]. Both solvent intercalation and in situ polymerization allow polymer chains to enter into the galleries of silicate clays. Melt blending is more attractive because the nanocomposites can be processed by conventional methods such as extrusion and injection molding. The influences of the addition of various types of clays on the non-isothermal crystallization process of thermoplastics have been studied by several researchers. Fornes and Paul [103] found that depending on the dispersion of clays in thermoplastic matrices the clay nanofillers can either promote or retard the crystallization of polymers and observed that the degree of crystallinity of nanocomposites showed a strong dependence on the cooling rates.

218 Clay Minerals in Nature – Their Characterization, Modification and Application

Differences between silver nanoparticles on the vermiculite and montmorillonite substrates authors confronted with the different sources of negative layer charge. When at dioctahedral montmorillonite negative layer charge results from an octahedral substitution of magnesium for iron and the substitution of Si4+ in tetrahedra is negligible [90] the trioctahedral vermiculite has a negative charge resulting mainly from the substitution of trivalent cations for Si4+ in tetrahedra [9]. Further it was found that montmorillonite and vermiculite after treatment with the silver nitrate solution released the Na+, K+, Ca2+ and Mg2+ cations from the interlayer space of both clay minerals and from the octahedral positions in the montmorillonite. Such structural change allowed the Ag+ to be incorporated in the host structure of the clay minerals through their surface and edges. The total amount of silver on vermiculite was higher than on the montmorillonite. The size distribution of the Ag nanoparticles was much more homogeneous on the montmorillonite substrate than on the vermiculite. The lower negative layer charge of montmorillonite results from the substitution of cations, which are located at the octahedral positions, is responsible for a uniform size of the Ag nanoparticles. The Ag particles with the size much larger than 50 nm were only on the edges. The negative layer charge of vermiculite as a consequence of the

charge on tetrahedra can hold higher content of silver than montmorillonite.

**4. Vermiculite nanofillers to polymer/clay nanocomposites** 

silver/montmorillonite.

tunnels [93].

resistance in comparison with pristine polymers.

Both sample series of silver/vermiculites and silver/montmorillonites, showed good inhibitory action against the Gram negative bacteria strains. The higher content of silver reduced on vermiculite predestines silver/vermiculite to be stronger antibacterial agent than

Nanocomposites have at least one ultrafine phase dimension, typically in the range of 1–100 nm, and exhibit other properties with comparison to the micro- and macro-composites. The clay–polymer nanocomposite could be considered as "one-nano-dimensional" because the clay filler has one dimension at the nanometer scale [91]. The high aspect ratio of layered silicate nanoparticles is ideal to modify the properties of the polymer, but the hydrophilic nature of silicate surfaces impedes their homogeneous dispersion in the organic polymer phase [92]. On the other hand it was found that polymers can interact with the external surface of clay minerals and also penetrate into the nano- structural spaces, holes and

The polymer nanocomposites exhibit new and sometimes improved properties that are not displayed by the individual phases or by their conventional composite counterparts. Significant improvement of mechanical properties, thermal stability, resistance to solvent swelling and suppression of flammability have been achieved with the only up to 5 wt.% exfoliated layered silicates nanoparticles in polymer matrix [94-97]. The development of clay–polymer nanocomposites at low clay loadings (3–7 wt %) as advanced structural materials brings significant improvements in mechanical strength and stiffness, enhanced gas barrier behavior, reduced linear thermal expansion coefficients, and increased solvent Polyethylene (PE) is one of the most widely used polyolefins. The molecular structure is generally simply written by the formula (CH2)n, where n is very large. The PE specimen may contain chains of different lengths [104]. Low-density PE (LDPE) contains many statistically placed paraffinic branches. The defects in the arrangement of chains reduce the structure crystallinity [105]. When mixing PE with hydrophilic clay minerals the additives can play a role of a polymeric surfactant, or they may act as a compatibilizer when mixed with organophilized clay minerals. The polar additives or charge of carriers introduce dipole moments in PE [106-109].

Clay mineral vermiculite (VER) was used as the clay mineral nanofiller into polymer matrix and was intercalated with the maleic anhydride into polyamide [110], polyethylene [109], and polypropylene [89, 111]. It should be noted that nanocomposites of polypropylene with nanofiller of VER were prepared without any compatibilizer by solid-state shear compounding (S3C) using pan-mill equipment [112]. Vermiculite particles were only partly intercalated with polypropylene and exfoliated in PP matrix (Shao et al., 2006).

Organovermiculite nanofiller octadecylamine/vermiculite (ODA/VER) was exfoliated in polypropylene (PP) [89]. The organovermiculite nanofillers were prepared in three ways. Sample 1 was original VER (Fig. 3a) intercalated using melt intercalation with octadecylamine (ODA). Sample 2 was VER milled in jet mill and intercalated with ODA (Fig. 3b). Sample 3 was VER exfoliated using an oxidizing agent potassium persulfate [61] and subsequently intercalated with ODA.

The thermal compounding of the organovermiculites (4 wt. %) with maleated PP caused the partial deintercalation of organovermiculites. As the jet milled original VER flakes have corrugated edges [113] in PP remained intercalated only in their central parts. The frayed edges and small particles showed somewhat higher interlayer distances and the penetration of polymeric segments from the softened polymer matrix was possible. The sample 2 was very well dispersed within PP matrix. The layered vermiculite structure after exfoliation with potassium persulfate was destroyed [61] and therefore intercalation of ODA molecules into the interlayer was limited (sample 3). On the X-ray diffraction patterns of composite sample 2 in PP authors identified new reflection of the orthorhombic γ-PP in addition to the

α-PP. According to the literature it is known that the formation of γ-PP in α-PP takes place when the VER particles reduce the PP chain mobility within the narrow space surrounded by the dispersed clay mineral particles [114].

Vermiculite: Structural Properties and Examples of the Use 221

This agrees with observations of Gilman [112], who reported that intercalated clay mineral particles in polymer matrix make composite a much more resistant to temperatures than

*Nanoclays* are clay minerals optimized for use in clay nanocomposites– multi-functional material systems with several property enhancements targeted for a particular application. The polymer–clay nanocomposites have been reviewed in several literature sources [112-

Depending on the structure of dispersed clay-filler in the polymeric matrix, the composites can be classified as intercalated or exfoliated nanocomposites. Polymer nanocomposites can be prepared by two main processes, that is, in situ polymerization and melt compounding. Use of nano-sized filler particles to form polymer composites has attracted much attention in recent years because of the potential performance advantages that could create new technological opportunities. Potential benefits include increased mechanical strength, decreased gas permeability, superior flame-resistance, and even enhanced transparency

The key issue is to obtain an effective dispersion and exfoliation of the platelets into the polymer matrix to yield well-aligned, high-aspect ratio particles for mechanical reinforcement or a tortuous diffusion pathway for improved barrier properties [124]. In situ polymerization process consists of intercalation of monomer as precursor species, followed by their polymerization inside the interlayer of clay mineral. Numerous nanocomposite materials were prepared by method with clay having inorganic metallic interlayer cation, which can promote formation of monomer radical inducing its polymerization, or modifying agent for nonpolar polymers. Both thermosets and thermoplastics have been

Simulation techniques become an integral part of experimental techniques, since the information about the spatial arrangement of molecules within the interlayer is hard to obtain without the aid of computer simulation. It offers a range of modeling and simulation methods, covering the length range from the subatomic quantum scale, through the molecular level, to the micrometer scale. Methods to model the behavior of systems on each of these scales are combined with analytical instrument simulation and statistical correlation techniques, allowing detailed study of structure, properties, and processes. Molecular modeling using empirical force field represents a way to preliminary estimation of the hostguest complementarity and prediction of structure and properties for nanomaterial design

The group of vermiculite–polymeric nanocomposites, where vermiculite or its silicate layers in the final composite stage is altering properties of full system, represents economically and technologically unpretentious materials. Vermiculite here enters the composite system unmodified; it means that it keeps its original characteristics. Usually, polymer and/or

when dispersed nanoclay plates suppress polymer crystallization [119-123].

exfoliated particles.

incorporated into nanocomposites.

in molecular nanotechnology.

**5. Industrial vermiculite nanocomposites** 

118].

**Figure 3.** SEM images of original particle VER (a) and jet milled VER intercalated with ODA (sample 2). Bar shows 10 µm.

Thermal stability of PP and PP/VER nanocomposites was evaluated by thermogravimetric analysis (Fig. 4).

**Figure 4.** TG and DTA of ODA/VER nanofillers in PP.

The thermal resistance of the nanocomposites was evaluated by comparing temperatures at the certain weight loss points. Decomposition of pure PP started at 247°C. The degradation point characterized by onset temperature followed the sequence: sample 3 (286°C) → sample 2 (273°C) → sample 1 (267°C).

At 5 % mass loss, the dispersed exfoliated organovermiculite nanofillers sample 2 showed about 51°C, and sample 3 (weekly exfoliated in PP) even about 65°C higher thermal resistance than pure PP.

This agrees with observations of Gilman [112], who reported that intercalated clay mineral particles in polymer matrix make composite a much more resistant to temperatures than exfoliated particles.

220 Clay Minerals in Nature – Their Characterization, Modification and Application

by the dispersed clay mineral particles [114].

**Figure 4.** TG and DTA of ODA/VER nanofillers in PP.

sample 2 (273°C) → sample 1 (267°C).

resistance than pure PP.

Bar shows 10 µm.

analysis (Fig. 4).

α-PP. According to the literature it is known that the formation of γ-PP in α-PP takes place when the VER particles reduce the PP chain mobility within the narrow space surrounded

**Figure 3.** SEM images of original particle VER (a) and jet milled VER intercalated with ODA (sample 2).

Thermal stability of PP and PP/VER nanocomposites was evaluated by thermogravimetric

The thermal resistance of the nanocomposites was evaluated by comparing temperatures at the certain weight loss points. Decomposition of pure PP started at 247°C. The degradation point characterized by onset temperature followed the sequence: sample 3 (286°C) →

At 5 % mass loss, the dispersed exfoliated organovermiculite nanofillers sample 2 showed about 51°C, and sample 3 (weekly exfoliated in PP) even about 65°C higher thermal *Nanoclays* are clay minerals optimized for use in clay nanocomposites– multi-functional material systems with several property enhancements targeted for a particular application. The polymer–clay nanocomposites have been reviewed in several literature sources [112- 118].

Depending on the structure of dispersed clay-filler in the polymeric matrix, the composites can be classified as intercalated or exfoliated nanocomposites. Polymer nanocomposites can be prepared by two main processes, that is, in situ polymerization and melt compounding.

Use of nano-sized filler particles to form polymer composites has attracted much attention in recent years because of the potential performance advantages that could create new technological opportunities. Potential benefits include increased mechanical strength, decreased gas permeability, superior flame-resistance, and even enhanced transparency when dispersed nanoclay plates suppress polymer crystallization [119-123].

The key issue is to obtain an effective dispersion and exfoliation of the platelets into the polymer matrix to yield well-aligned, high-aspect ratio particles for mechanical reinforcement or a tortuous diffusion pathway for improved barrier properties [124]. In situ polymerization process consists of intercalation of monomer as precursor species, followed by their polymerization inside the interlayer of clay mineral. Numerous nanocomposite materials were prepared by method with clay having inorganic metallic interlayer cation, which can promote formation of monomer radical inducing its polymerization, or modifying agent for nonpolar polymers. Both thermosets and thermoplastics have been incorporated into nanocomposites.

Simulation techniques become an integral part of experimental techniques, since the information about the spatial arrangement of molecules within the interlayer is hard to obtain without the aid of computer simulation. It offers a range of modeling and simulation methods, covering the length range from the subatomic quantum scale, through the molecular level, to the micrometer scale. Methods to model the behavior of systems on each of these scales are combined with analytical instrument simulation and statistical correlation techniques, allowing detailed study of structure, properties, and processes. Molecular modeling using empirical force field represents a way to preliminary estimation of the hostguest complementarity and prediction of structure and properties for nanomaterial design in molecular nanotechnology.

## **5. Industrial vermiculite nanocomposites**

The group of vermiculite–polymeric nanocomposites, where vermiculite or its silicate layers in the final composite stage is altering properties of full system, represents economically and technologically unpretentious materials. Vermiculite here enters the composite system unmodified; it means that it keeps its original characteristics. Usually, polymer and/or

layered silicate nanocomposites are synthesized from polymeric macromolecules directly or from monomers followed by in situ polymerization to intercalate into and then enlarge the interlayer of the used clay.

Vermiculite: Structural Properties and Examples of the Use 223

solutions (NaCl, Na2SO4, and Na3PO4), and pH solutions were observed. Equilibrium water absorbency for the PAA–vermiculite superabsorbent composite was significantly affected by the content of vermiculite. The highest water absorbency was obtained when 20 wt.% of vermiculite was incorporated. Water absorbency of these composite materials was also significantly dependent on properties of external saline solutions, including valence of ions, ionic strength, and pH values. FTIR indicated that the reactions occur between –COO– groups and –OH groups on the surface of vermiculite. SEM and TEM studies illustrated more finely dispersion of the clay particle in the polymer matrix. In addition, XRD analysis showed that the polymerization reaction is performed on the surface of vermiculite and the *d*-space is not changed. TGA indicated that introduction of vermiculite into the polymer network leads to an increase in thermal stability of the composites. The reaction between organics and clay is related to the structure and properties of clay. The equilibrium water absorbency decreases with increasing clay content owing to the increasing crosslinking points and the decreasing percentage of hydrophilic groups in polymeric network. The vermiculite based nanocomposite acquired high- water absorbency in CaCl2 aqueous

Simple mixing technique of vermiculite into polymer matrix could be applied in case of preparation of *membranes*. Several minerals, including vermiculite 10–100 µm particle size, were tested for having desirable features to be used in membrane. Many membranes are designed to be selectively permeable, commonly intended for use in membrane separations or to be impermeable barriers to protect a surface or a product. Paint, food wrap, and electronic packaging are examples. For these barrier membranes, we seek polymers through which solutes such as water, oxygen, and chloride permeate slowly, rather than rapidly. Developing barrier membranes containing aligned mineral flakes could be solution for those requirements. Since mineral flakes are generally crystalline, they have very low permeability, and can reduce the permeability of the composite film. The permeability of polymer films can be reduced dramatically with many layers of thin mineral flakes aligned

As the *gas barrier membrane* could be utilized of *butyl rubber–vermiculite nanocomposite* coating in form of thing layer. The coating formulation consisting of a butyl rubber (polyisobutylene (C4H8)*n*) latex, which is well known among elastomers for its superior gas barrier characteristics, to which dispersion of exfoliated vermiculite was added. The gas permeability and diffusion coefficients of the nanocoating compare to pure polymer were altered remarkably by the presence of the high loadings of vermiculite. The gas permeability was reduced by 20–30-fold by the vermiculite. Diffusion coefficients computed from time lag data were reduced by two orders of magnitude. Solubility coefficient obtained from the time lag observation increased significantly with vermiculite content in contradiction to that expected by theory (the solubility coefficient should decrease with filler loading since the volume of available polymer is decreased). The excess sorption appears to be the effect of

Mixing dilution of *polyvinyl alcohol (PVA) (C2H4O)n with vermiculite suspension* both in water was prepared membrane – precursor, which was later cast on block heated to 75°C. For a

solution [133].

parallel to the film's surface.

gas adsorption by the vermiculite [127].

The struggle to prepare nanocomposites is affected by many factors such as the blending or reaction time, temperature and the affinity between the host and the matrix. Moreover, the intercalated nanocomposites instead of the exfoliated nanocomposites are commonly gained in most cases. Generally, dispersion as well as delamination of vermiculite in polymer matrices is observed using electron microscopy and XRD techniques.

Three main approaches have been used in the preparation of clay–polymer composites: (1) melt blending, (2) solution blending, and (3) in situ polymerization. However, homogeneity at nanoscale level may not be fully achieved using these methods. One way of approaching this problem is complete delamination of the clay particles to give colloidal dispersions of single layers in a suitable solvent and restacking the layers in presence of guest species.

Utilization of vermiculite in nanocomposite is not as extensive as for montmorillonite. Most of the applications limits low swell ability of vermiculite compare to montmorillonite. However, several main streams of applications were introduced.

*Superabsorbents* are a type of loosely crosslinked hydrophilic polymer that can swell, absorb, and retain a large volume of aqueous or other biological fluid. The superabsorbents have found extensive applications in many fields such as agriculture, hygienic products, wastewater treatment, drug-delivery systems, etc.

The conventional superabsorbents are based on expensive fully petroleum-based polymers. Their production consumes lots of petroleum and their usage can also cause a nonnegligible environment problem. New types of superabsorbents by introducing naturally available raw materials as additives were desired. The incorporation of clays reduces production cost, and also improves the properties (such as swelling ability, gel strength, mechanical, and thermal stability) of superabsorbents and accelerates the generation of new materials for special applications. The properties of traditional superabsorbent could be enhanced by incorporating vermiculite [125]. It is expected that organomodification of vermiculite can further improve dispersion and performance of the resultant nanocomposite. The superabsorbent nanocomposites were prepared from natural guar gum and organovermiculite by solution polymerization and analyses indicated that organovermiculite was exfoliated during polymerization and uniformly dispersed in the polymeric matrix [126].

In the superabsorbent field recently, much attention has been paid especially to layered silicate as favorable compound of absorbent composites. *Polyacrylate* (PAA)–*vermiculite superabsorbent* composites were tested for their absorption abilities. Acrylamide and acrylic acid mixed with vermiculite were polymerized. Final nanopowders had the particle size in range 40–80 mesh. Nanocomposite was immersed in examined medium and let to reach swelling equilibrium for 4 h, which resulted in absorption of water into network of composite and the formation of hydrogel. Swelling behavior of the superabsorbent composites of various cations from salt solutions (NaCl, CaCl2, and FeCl3), anions salt solutions (NaCl, Na2SO4, and Na3PO4), and pH solutions were observed. Equilibrium water absorbency for the PAA–vermiculite superabsorbent composite was significantly affected by the content of vermiculite. The highest water absorbency was obtained when 20 wt.% of vermiculite was incorporated. Water absorbency of these composite materials was also significantly dependent on properties of external saline solutions, including valence of ions, ionic strength, and pH values. FTIR indicated that the reactions occur between –COO– groups and –OH groups on the surface of vermiculite. SEM and TEM studies illustrated more finely dispersion of the clay particle in the polymer matrix. In addition, XRD analysis showed that the polymerization reaction is performed on the surface of vermiculite and the *d*-space is not changed. TGA indicated that introduction of vermiculite into the polymer network leads to an increase in thermal stability of the composites. The reaction between organics and clay is related to the structure and properties of clay. The equilibrium water absorbency decreases with increasing clay content owing to the increasing crosslinking points and the decreasing percentage of hydrophilic groups in polymeric network. The vermiculite based nanocomposite acquired high- water absorbency in CaCl2 aqueous solution [133].

222 Clay Minerals in Nature – Their Characterization, Modification and Application

matrices is observed using electron microscopy and XRD techniques.

However, several main streams of applications were introduced.

wastewater treatment, drug-delivery systems, etc.

interlayer of the used clay.

layered silicate nanocomposites are synthesized from polymeric macromolecules directly or from monomers followed by in situ polymerization to intercalate into and then enlarge the

The struggle to prepare nanocomposites is affected by many factors such as the blending or reaction time, temperature and the affinity between the host and the matrix. Moreover, the intercalated nanocomposites instead of the exfoliated nanocomposites are commonly gained in most cases. Generally, dispersion as well as delamination of vermiculite in polymer

Three main approaches have been used in the preparation of clay–polymer composites: (1) melt blending, (2) solution blending, and (3) in situ polymerization. However, homogeneity at nanoscale level may not be fully achieved using these methods. One way of approaching this problem is complete delamination of the clay particles to give colloidal dispersions of single layers in a suitable solvent and restacking the layers in presence of guest species.

Utilization of vermiculite in nanocomposite is not as extensive as for montmorillonite. Most of the applications limits low swell ability of vermiculite compare to montmorillonite.

*Superabsorbents* are a type of loosely crosslinked hydrophilic polymer that can swell, absorb, and retain a large volume of aqueous or other biological fluid. The superabsorbents have found extensive applications in many fields such as agriculture, hygienic products,

The conventional superabsorbents are based on expensive fully petroleum-based polymers. Their production consumes lots of petroleum and their usage can also cause a nonnegligible environment problem. New types of superabsorbents by introducing naturally available raw materials as additives were desired. The incorporation of clays reduces production cost, and also improves the properties (such as swelling ability, gel strength, mechanical, and thermal stability) of superabsorbents and accelerates the generation of new materials for special applications. The properties of traditional superabsorbent could be enhanced by incorporating vermiculite [125]. It is expected that organomodification of vermiculite can further improve dispersion and performance of the resultant nanocomposite. The superabsorbent nanocomposites were prepared from natural guar gum and organovermiculite by solution polymerization and analyses indicated that organovermiculite was exfoliated during polymerization and uniformly dispersed in the polymeric matrix [126]. In the superabsorbent field recently, much attention has been paid especially to layered silicate as favorable compound of absorbent composites. *Polyacrylate* (PAA)–*vermiculite superabsorbent* composites were tested for their absorption abilities. Acrylamide and acrylic acid mixed with vermiculite were polymerized. Final nanopowders had the particle size in range 40–80 mesh. Nanocomposite was immersed in examined medium and let to reach swelling equilibrium for 4 h, which resulted in absorption of water into network of composite and the formation of hydrogel. Swelling behavior of the superabsorbent composites of various cations from salt solutions (NaCl, CaCl2, and FeCl3), anions salt Simple mixing technique of vermiculite into polymer matrix could be applied in case of preparation of *membranes*. Several minerals, including vermiculite 10–100 µm particle size, were tested for having desirable features to be used in membrane. Many membranes are designed to be selectively permeable, commonly intended for use in membrane separations or to be impermeable barriers to protect a surface or a product. Paint, food wrap, and electronic packaging are examples. For these barrier membranes, we seek polymers through which solutes such as water, oxygen, and chloride permeate slowly, rather than rapidly. Developing barrier membranes containing aligned mineral flakes could be solution for those requirements. Since mineral flakes are generally crystalline, they have very low permeability, and can reduce the permeability of the composite film. The permeability of polymer films can be reduced dramatically with many layers of thin mineral flakes aligned parallel to the film's surface.

As the *gas barrier membrane* could be utilized of *butyl rubber–vermiculite nanocomposite* coating in form of thing layer. The coating formulation consisting of a butyl rubber (polyisobutylene (C4H8)*n*) latex, which is well known among elastomers for its superior gas barrier characteristics, to which dispersion of exfoliated vermiculite was added. The gas permeability and diffusion coefficients of the nanocoating compare to pure polymer were altered remarkably by the presence of the high loadings of vermiculite. The gas permeability was reduced by 20–30-fold by the vermiculite. Diffusion coefficients computed from time lag data were reduced by two orders of magnitude. Solubility coefficient obtained from the time lag observation increased significantly with vermiculite content in contradiction to that expected by theory (the solubility coefficient should decrease with filler loading since the volume of available polymer is decreased). The excess sorption appears to be the effect of gas adsorption by the vermiculite [127].

Mixing dilution of *polyvinyl alcohol (PVA) (C2H4O)n with vermiculite suspension* both in water was prepared membrane – precursor, which was later cast on block heated to 75°C. For a

film with a volume fraction φ of flakes of aspect ratio α, the permeability reduction is expected to be proportional to αφ for the dilute limit (φ << 1 and αφ < 1) but proportional to (αφ) 2 in the semidilute limit (φ << 1 but αφ > 1). Permeability of hydrochloric acid and sodium hydroxide across films containing mica or vermiculite in PVA agree with the second, semidilute prediction. These improvements in barrier properties are independent on the flake size, permeate, and polymer chemistry [128].

Vermiculite: Structural Properties and Examples of the Use 225

The nanocomposites consisting of the emeraldine salt of polyaniline (PANI) and layered vermiculite were synthesized to improve *thermal stability of nanocomposite* [134]. Emeraldine salt (base) is regarded as the most useful form of PANI owing to its high stability at room temperature. *PANI–vermiculite nanocomposites* were prepared via in situ polymerization of monomer compounded with vermiculite. Vermiculite silicate layers could be highly dispersed within PANI bulk thanks to pretreatment of vermiculite with hydrochloric acid. The introduction of nanolayers of vermiculite improves greatly the thermal stability of nanocomposite, but its electrical conductivity decrease slightly only, as demonstrated by

An increased understanding, on the micro- and nanolevel, of polymers at surfaces and in confined geometries assists in the development and improvement of new technologies. Of special interest is to understand the nature of the glass transition – not fully solved problem of condensed matter physics. By confining molecules in very small spaces (e.g., clay gallery), the existence or not of a length scale associated with molecular motions responsible for the glass transition can be established. The increment of its relaxation strength in the clay has been related to the 2D geometry of the confinement. These polymers have a dipole moment component parallel to the chain and, therefore, the total dipole vector is proportional to the end-to-end vector and so the overall chain dynamics can be measured by dielectric

Vermiculite was selected for study the molecular dynamics of oligomeric poly(propylene glycol (PG)) (PPG) liquids (Mw = 1200, 2000, and 4000 g mol–1). The thickness of the liquid layers was 0.55 nm in the case of PG and 0.37 nm for 7-PG and PPG. The PG oligomers form a flat monolayer in vermiculite gallery with their methyl groups pointing in the direction of the clay surfaces. In contrast, the monomers are probable orientated either perpendicular to the clay layers or parallel to the layers, but with the methyl groups pointing toward a clay surface. The dynamics generally become slower with increasing chain length but the dynamics of the 7-mers was established to be faster than for the single monomers. A possible explanation for the fact may be that the OH end groups of the monomers are linked together forming a network, and, as a result, slow down the diffusion at temperatures low enough to keep the network structure intact [135]. The PPG confined in Na-vermiculite has been studied by broadband dielectric spectroscopy. In addition to the temperature dependence of the main (α-) relaxation process and the related high-temperature translational or segmental diffusion, the normal mode relaxation process was studied for all samples in both bulk and confinement. For the normal mode process the relaxation rate and the temperature dependence of the relaxation time in the clay is drastically shifted to lower frequencies compared to that of the bulk material. The α-process relaxation time is only

The fact that interactions of PPG with the clay surfaces are very weak was implied based on similar temperature dependence of the relaxation time the α-relaxation in both bulk and confinement [136]. Relaxation process corresponds to the molecular motions of translational character and that it is almost unaffected by the present true 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the

TGA and electrical conductivity measurements.

spectroscopy.

slightly affected by the confinement.

However, it is very difficult to prepare an entirely exfoliated nanocomposite using natural vermiculite as nanofiller by conventional techniques. Therefore, vermiculite has been successfully delaminated with acid treatment and used to mix with engineering polymer directly to synthesize polymer–vermiculite nanocomposites [129, 130]. Acid delaminated vermiculite was successfully utilized for preparation of PVA nanocomposite. The properties of composite significantly depend on the preparation procedure for the reason that chemical reactions and physical interactions involved. Two steps preparation, delamination with hydrochloric acid and then addition to the PVA solution using various mixing times. The positive effect of the vermiculite content on the thermal behavior of the PVA–vermiculite blends was observed [131].

The utility of the *PVP–vermiculite nanocomposite materials* can be found in area of *wastewater cleaning* as the remover of color from dye wastewater, since vermiculite is an excellent adsorbent for basic blue dye, requiring moderately short contact times. The phenols and iodine form a molecular complex with pyridine, as well as with PVP. Thus, common impurities from the industrial waste stream such as iodine, weak acids, and phenol can be removed using the PVP–vermiculite nanocomposite. It can also be used as a reusable mild acid scavenger, which can be easily removed from the reaction medium by filtration. The cost of vermiculite is 10% of activated carbon and it is possible to regenerate it by simple heating, so it offers an attractive alternative for waste removal and recovery.

Nanocomposites poly(4-vinylpyridine(VP)) (PVP) and poly(N-vinyl-2-pyrrolidinone(NVP)) (PNVP) both combined with vermiculite have been synthesized by the intercalative redox polymerization of monomer in the gallery of Cu2+ ion-exchanged vermiculite (Cu2+ serves as polymerization agent). The formation of a single filament of the PVP polymer in the vermiculite gallery is confirmed by the increase in gallery spacing of 0.47 nm as indicated by XRD analysis. XRD analysis following intercalative polymerization of PNVP indicates the presence of two prominent peaks with the corresponding basal spacing *d*002 = 1.43 nm (intercalated) and 0.99 nm (not intercalated), suggesting the formation of a partially intercalated hybrid material. The amount of polymers present in the gallery is found to be around 20 wt.% by TG analysis. Presence of the polymer in vermiculite gallery results in enhanced thermal stability that is evident from the increase of initial decomposition temperature by 300°C. Differential scanning calorimetry of the nanocomposite indicates that the polymer is confined to a restricted geometry because of the absence of a glass-transition temperature, which confirms the XRD finding. The IR absorption peaks corresponding to PVP and the expected PVP of UV –\* transition at 275 nm, along with the XRD, and thermal data confirms that the gallery expansion is owing to the PVP filament [132].

The nanocomposites consisting of the emeraldine salt of polyaniline (PANI) and layered vermiculite were synthesized to improve *thermal stability of nanocomposite* [134]. Emeraldine salt (base) is regarded as the most useful form of PANI owing to its high stability at room temperature. *PANI–vermiculite nanocomposites* were prepared via in situ polymerization of monomer compounded with vermiculite. Vermiculite silicate layers could be highly dispersed within PANI bulk thanks to pretreatment of vermiculite with hydrochloric acid. The introduction of nanolayers of vermiculite improves greatly the thermal stability of nanocomposite, but its electrical conductivity decrease slightly only, as demonstrated by TGA and electrical conductivity measurements.

224 Clay Minerals in Nature – Their Characterization, Modification and Application

the flake size, permeate, and polymer chemistry [128].

blends was observed [131].

film with a volume fraction φ of flakes of aspect ratio α, the permeability reduction is expected to be proportional to αφ for the dilute limit (φ << 1 and αφ < 1) but proportional to (αφ) 2 in the semidilute limit (φ << 1 but αφ > 1). Permeability of hydrochloric acid and sodium hydroxide across films containing mica or vermiculite in PVA agree with the second, semidilute prediction. These improvements in barrier properties are independent on

However, it is very difficult to prepare an entirely exfoliated nanocomposite using natural vermiculite as nanofiller by conventional techniques. Therefore, vermiculite has been successfully delaminated with acid treatment and used to mix with engineering polymer directly to synthesize polymer–vermiculite nanocomposites [129, 130]. Acid delaminated vermiculite was successfully utilized for preparation of PVA nanocomposite. The properties of composite significantly depend on the preparation procedure for the reason that chemical reactions and physical interactions involved. Two steps preparation, delamination with hydrochloric acid and then addition to the PVA solution using various mixing times. The positive effect of the vermiculite content on the thermal behavior of the PVA–vermiculite

The utility of the *PVP–vermiculite nanocomposite materials* can be found in area of *wastewater cleaning* as the remover of color from dye wastewater, since vermiculite is an excellent adsorbent for basic blue dye, requiring moderately short contact times. The phenols and iodine form a molecular complex with pyridine, as well as with PVP. Thus, common impurities from the industrial waste stream such as iodine, weak acids, and phenol can be removed using the PVP–vermiculite nanocomposite. It can also be used as a reusable mild acid scavenger, which can be easily removed from the reaction medium by filtration. The cost of vermiculite is 10% of activated carbon and it is possible to regenerate it by simple

Nanocomposites poly(4-vinylpyridine(VP)) (PVP) and poly(N-vinyl-2-pyrrolidinone(NVP)) (PNVP) both combined with vermiculite have been synthesized by the intercalative redox polymerization of monomer in the gallery of Cu2+ ion-exchanged vermiculite (Cu2+ serves as polymerization agent). The formation of a single filament of the PVP polymer in the vermiculite gallery is confirmed by the increase in gallery spacing of 0.47 nm as indicated by XRD analysis. XRD analysis following intercalative polymerization of PNVP indicates the presence of two prominent peaks with the corresponding basal spacing *d*002 = 1.43 nm (intercalated) and 0.99 nm (not intercalated), suggesting the formation of a partially intercalated hybrid material. The amount of polymers present in the gallery is found to be around 20 wt.% by TG analysis. Presence of the polymer in vermiculite gallery results in enhanced thermal stability that is evident from the increase of initial decomposition temperature by 300°C. Differential scanning calorimetry of the nanocomposite indicates that the polymer is confined to a restricted geometry because of the absence of a glass-transition temperature, which confirms the XRD finding. The IR absorption peaks corresponding to PVP and the expected PVP of UV –\* transition at 275 nm, along with the XRD, and

thermal data confirms that the gallery expansion is owing to the PVP filament [132].

heating, so it offers an attractive alternative for waste removal and recovery.

An increased understanding, on the micro- and nanolevel, of polymers at surfaces and in confined geometries assists in the development and improvement of new technologies. Of special interest is to understand the nature of the glass transition – not fully solved problem of condensed matter physics. By confining molecules in very small spaces (e.g., clay gallery), the existence or not of a length scale associated with molecular motions responsible for the glass transition can be established. The increment of its relaxation strength in the clay has been related to the 2D geometry of the confinement. These polymers have a dipole moment component parallel to the chain and, therefore, the total dipole vector is proportional to the end-to-end vector and so the overall chain dynamics can be measured by dielectric spectroscopy.

Vermiculite was selected for study the molecular dynamics of oligomeric poly(propylene glycol (PG)) (PPG) liquids (Mw = 1200, 2000, and 4000 g mol–1). The thickness of the liquid layers was 0.55 nm in the case of PG and 0.37 nm for 7-PG and PPG. The PG oligomers form a flat monolayer in vermiculite gallery with their methyl groups pointing in the direction of the clay surfaces. In contrast, the monomers are probable orientated either perpendicular to the clay layers or parallel to the layers, but with the methyl groups pointing toward a clay surface. The dynamics generally become slower with increasing chain length but the dynamics of the 7-mers was established to be faster than for the single monomers. A possible explanation for the fact may be that the OH end groups of the monomers are linked together forming a network, and, as a result, slow down the diffusion at temperatures low enough to keep the network structure intact [135]. The PPG confined in Na-vermiculite has been studied by broadband dielectric spectroscopy. In addition to the temperature dependence of the main (α-) relaxation process and the related high-temperature translational or segmental diffusion, the normal mode relaxation process was studied for all samples in both bulk and confinement. For the normal mode process the relaxation rate and the temperature dependence of the relaxation time in the clay is drastically shifted to lower frequencies compared to that of the bulk material. The α-process relaxation time is only slightly affected by the confinement.

The fact that interactions of PPG with the clay surfaces are very weak was implied based on similar temperature dependence of the relaxation time the α-relaxation in both bulk and confinement [136]. Relaxation process corresponds to the molecular motions of translational character and that it is almost unaffected by the present true 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the

confinement. Thus, there is no indication, for none of the confined liquids, that the OH-end groups should form strong hydrogen bonds to the clay surfaces. In fact, the rather small effect of the present confinement on the diffusive dynamics and the main relaxation time suggest that the surface interactions are considerably weaker than in many other model systems [135].

Vermiculite: Structural Properties and Examples of the Use 227

Fracture fixation, interference screws, suture anchors, meniscus repair

non-woven fibrous fabrics;suture anchors, meniscus repair, medical devices, drug delivery

Suture coating, dental orthopaedic implants, bone tissue implantant

material for waste management strategies; biocompatibility in the medical devices

well maintained after the coating process under polymerization conditions and exhibit semicrystalline behavior. Thermogravimetric analysis showed that the thermal stability of PPy/VMT nanocomposites was enhanced and these can be attributed to the retardation effect of amine-functionalized VMT as barriers for the degradation of PPy. The morphology of PPy/VMT nanocomposites showed the layered structure and encapsulated morphology. The composites possess high electrical conductivity at room temperature, weakly temperature

Intercalated nanocomposites comprised of poly(propylene carbonate) (PPC) and organovermiculite (OVMT) was *first prepared* via *direct melt compounding of the alkali-vermiculite intercalated host with PPC in a twin rotary mixer*. The dispersion and morphologies of OVMT within PPC were investigated by X-ray diffraction and transmission electron microscopic techniques. The results revealed the formation of intercalated-exfoliated vermiculite sheets in the PPC matrix. Because of the thermally sensitive nature of PPC, thermal degradation occurred during the melt compounding. The degradation led to a deterioration of the mechanical properties of the nanocomposites. Tensile test showed that the yield strength and modulus of the nanocomposites decrease with increasing vermiculite content [141].

*Polymeric bio-related nanocomposites* can be classified into two classes: (1) natural-based materials, including polysaccharides (starch, alginate, chitin/chitosan, hyaluronic acid derivatives) or proteins (soy, collagen, fibrin gels, silk); (2) synthetic polymers, such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(3-caprolactone) (PCL), poly(hydroxyl

Polymer Type Utilization

Poly(lactic acid) (PLA) aliphatic polyester

poly(glycolic acid) (PLG) aliphatic polyester

poly(ε-caprolactone) (PCL) aliphatic polyester

poly(hydroxylbutyrate) (PHB) Polyester

**Table 1.** Biopolymers for composites

dependence of the conductivity.

butyrate) (PHB) [142].

**6. Polymeric bio-related nanocomposites** 

*Nanocomposites based on magnetic nanoparticles supported within thermally-expanded vermiculite* are extremely interesting for many applications, including its use in removing oil from water after oil spills. Two distinct magnetic nanocomposites based on thermally-expanded vermiculite pellets were treated with two different magnetic fluid samples. One of them was an ionic magnetic fluid (IMF), and the second a surfacted magnetic fluid (SMF) was consisting of oleic acid-coated nanoparticles suspended in organic medium. Magnetic fluids (MFs) are highly stable colloids consisting of nanosized magnetic particles (mainly iron oxides) suspended in a hydrophilic or hydrophobic liquid carrier. Besides numerous industrial and biomedical applications MFs can be successfully used to introduce nanosized magnetic particles into a variety of hosting structures and templates [137].

Recently, *layered silicates have been used to modify bitumen*. It has been found that physical properties, rheological behaviors and aging resistance of bitumen and polymer modified bitumen could be obviously improved due to barrier properties of MMT. Expanded vermiculite (EVMT) clays modified bitumen was prepared by using EVMT, cetyltrimethyl ammonium bromide (CTAB)–EVMT and octadecyl dimethyl benzyl ammonium chloride (ODBA)–EVMT as modifiers. The morphology characteristic of the modified bitumen was investigated by X-ray diffraction (XRD). The binders were aged by pressure aging vessel (PAV), in situ thermal aging and ultraviolet (UV) radiation. The phase-separated structure was observed in EVMT modified bitumen, while CTAB–EVMT and ODBA–EVMT modified bitumen formed the intercalated and exfoliated nanostructures, respectively [138].

In situ ring-opening polymerization of the *cyclic oligomers in the gallery of vermiculite* can form high performance polymer nanocomposites, in the absence of any catalyst and without any volatile by-products formed. The new approach allows to intercalate the silicate layers with cyclic(arylene disulfide) oligomers via direct melt compounding, and in turn to prepare in situ the corresponding nanocomposites. The cyclic arylene disulfide oligomer acts both as a precursor of polymeric matrix and as a swelling agent of the vermiculite layers. High molecular weight polymer can be formed in a few minutes [139].

*A conducting polymer/inorganic host hybrid nanocomposite* was created from the conductive polypyrrole (PPy) self-assembled monolayer (SAM) coated on expanded vermiculite (VMT) [140]. The conducting polymer/inorganic host hybrid composites provide the new synergistic properties, which cannot attain from individual materials. Their conductivity is more easily controlled, and the mechanical or thermal stability is improved through the synthesis of the composites. resulting in PPy/VMT nanocomposites after VMT particles surface modification.

X-ray diffraction (XRD) analysis confirmed that the main peaks of PPy/VMT nanocomposites are similar to the SAM–VMT particles, which reveal that the crystal structure of SAM–VMT is well maintained after the coating process under polymerization conditions and exhibit semicrystalline behavior. Thermogravimetric analysis showed that the thermal stability of PPy/VMT nanocomposites was enhanced and these can be attributed to the retardation effect of amine-functionalized VMT as barriers for the degradation of PPy. The morphology of PPy/VMT nanocomposites showed the layered structure and encapsulated morphology. The composites possess high electrical conductivity at room temperature, weakly temperature dependence of the conductivity.

Intercalated nanocomposites comprised of poly(propylene carbonate) (PPC) and organovermiculite (OVMT) was *first prepared* via *direct melt compounding of the alkali-vermiculite intercalated host with PPC in a twin rotary mixer*. The dispersion and morphologies of OVMT within PPC were investigated by X-ray diffraction and transmission electron microscopic techniques. The results revealed the formation of intercalated-exfoliated vermiculite sheets in the PPC matrix. Because of the thermally sensitive nature of PPC, thermal degradation occurred during the melt compounding. The degradation led to a deterioration of the mechanical properties of the nanocomposites. Tensile test showed that the yield strength and modulus of the nanocomposites decrease with increasing vermiculite content [141].

## **6. Polymeric bio-related nanocomposites**

226 Clay Minerals in Nature – Their Characterization, Modification and Application

systems [135].

surface modification.

confinement. Thus, there is no indication, for none of the confined liquids, that the OH-end groups should form strong hydrogen bonds to the clay surfaces. In fact, the rather small effect of the present confinement on the diffusive dynamics and the main relaxation time suggest that the surface interactions are considerably weaker than in many other model

*Nanocomposites based on magnetic nanoparticles supported within thermally-expanded vermiculite* are extremely interesting for many applications, including its use in removing oil from water after oil spills. Two distinct magnetic nanocomposites based on thermally-expanded vermiculite pellets were treated with two different magnetic fluid samples. One of them was an ionic magnetic fluid (IMF), and the second a surfacted magnetic fluid (SMF) was consisting of oleic acid-coated nanoparticles suspended in organic medium. Magnetic fluids (MFs) are highly stable colloids consisting of nanosized magnetic particles (mainly iron oxides) suspended in a hydrophilic or hydrophobic liquid carrier. Besides numerous industrial and biomedical applications MFs can be successfully used to introduce nanosized

Recently, *layered silicates have been used to modify bitumen*. It has been found that physical properties, rheological behaviors and aging resistance of bitumen and polymer modified bitumen could be obviously improved due to barrier properties of MMT. Expanded vermiculite (EVMT) clays modified bitumen was prepared by using EVMT, cetyltrimethyl ammonium bromide (CTAB)–EVMT and octadecyl dimethyl benzyl ammonium chloride (ODBA)–EVMT as modifiers. The morphology characteristic of the modified bitumen was investigated by X-ray diffraction (XRD). The binders were aged by pressure aging vessel (PAV), in situ thermal aging and ultraviolet (UV) radiation. The phase-separated structure was observed in EVMT modified bitumen, while CTAB–EVMT and ODBA–EVMT modified

magnetic particles into a variety of hosting structures and templates [137].

bitumen formed the intercalated and exfoliated nanostructures, respectively [138].

molecular weight polymer can be formed in a few minutes [139].

In situ ring-opening polymerization of the *cyclic oligomers in the gallery of vermiculite* can form high performance polymer nanocomposites, in the absence of any catalyst and without any volatile by-products formed. The new approach allows to intercalate the silicate layers with cyclic(arylene disulfide) oligomers via direct melt compounding, and in turn to prepare in situ the corresponding nanocomposites. The cyclic arylene disulfide oligomer acts both as a precursor of polymeric matrix and as a swelling agent of the vermiculite layers. High

*A conducting polymer/inorganic host hybrid nanocomposite* was created from the conductive polypyrrole (PPy) self-assembled monolayer (SAM) coated on expanded vermiculite (VMT) [140]. The conducting polymer/inorganic host hybrid composites provide the new synergistic properties, which cannot attain from individual materials. Their conductivity is more easily controlled, and the mechanical or thermal stability is improved through the synthesis of the composites. resulting in PPy/VMT nanocomposites after VMT particles

X-ray diffraction (XRD) analysis confirmed that the main peaks of PPy/VMT nanocomposites are similar to the SAM–VMT particles, which reveal that the crystal structure of SAM–VMT is *Polymeric bio-related nanocomposites* can be classified into two classes: (1) natural-based materials, including polysaccharides (starch, alginate, chitin/chitosan, hyaluronic acid derivatives) or proteins (soy, collagen, fibrin gels, silk); (2) synthetic polymers, such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(3-caprolactone) (PCL), poly(hydroxyl butyrate) (PHB) [142].


**Table 1.** Biopolymers for composites

Many advantages and disadvantages characterize these two different classes of biomaterials. Synthetic polymers have relatively good mechanical strength and their shape and degradation rate can be easily modified, but their surfaces are hydrophobic and lack of cellrecognition signals. Naturally derived polymers have the potential advantage of biological recognition that may positively support cell adhesion and function, but they have poor mechanical properties. Many of them are also limited in supply and can therefore be costly [142].

Vermiculite: Structural Properties and Examples of the Use 229

materials are extraordinarily versatile as they could be formed from a large variety of biopolymers and also from different inorganic solid particles such as layered silicates, hydroxyapatite, cellulose and other metal oxides [148]. Among them, chitosan/layered silicate nanocomposites have received much more attention. These nanocomposites are of interest for advanced biomedical materials, as for instance tissue engineering, artificial bones or gene therapy. Other possible fields of applications are related to their mechanical, thermal and barrier properties, making this class of materials attractive for potential uses in controlled drug and pesticides delivery, membranes for food processing, drinking water

Chitosan is the deacetylated product of chitin, a natural polymer found in the cell wall of fungi and microorganisms. The active groups in the chitosan structure are the free amine groups, located in the C2 position of the glucose residue in the polysaccharide chain, and the hydroxyl groups [148]. *The chitosan/vermiculite nanocomposites* have been successfully prepared with different modified vermiculite, which was treated by acid, sodium and CTAB cations. The modification and the nano-scale dispersion of the modified vermiculites were

*1Nanotechnology Centre, VŠB – Technical University of Ostrava, Ostrava-Poruba, Czech Republic 2IT4Innovations Centre of Excellence, VŠB-Technical University of Ostrava, Ostrava-Poruba, Czech* 

Financial support of the Czech Grant Agency (projects GA ČR 210/11/2215) and the IT4Innovations Centre of Excellence (project reg. no. cz.1.05/1.1.00/02.0070) are gratefully

[1] Guggenheim S, Martin RT (1995) Definition of Clay and Clay Mineral. Joint Report of the AIPEA Nomenclature and CMS Nomenclature Committees. Clays clay miner. 43:

[2] Brindley GW, Bailey SW, Faust GT, Forman SA, Rich CI (1968) Report of the Nomenclature Committee (1966-67) of The Clay Minerals Society. Clays clay miner. 16:

[3] Brindley GW, Pedro G (1972) Report of the AIPEA Nomenclature Committee. AIPEA

purification, oxygen barrier films and food package.

Marta Valášková1,2, \* and Gražyna Simha Martynková1,2

confirmed.

*Republic* 

**Author details** 

**Acknowledgement** 

acknowledged.

**7. References** 

255-256.

322-324.

Corresponding Author

 \*

Newsletter 7: 8-13.

During the last decade, significant attention has been focused on biodegradable polymers. Among all these polymers, poly(lactic acid) (PLA) is one of the most promising because it is thermoplastic, biocompatible and has a high strength, a high modulus, and good processability [143]. PLA also has been revealed an inefficient crystallization process for both the lower crystallization rate and crystallinity as compared with other polymers [144- 147].

To maximize therapeutic activity while minimizing negative side effects is the main driven force for the continuous development of new controlled drug delivery systems. Because the release of drugs in drug-intercalated layered materials is potentially controllable (Fig.5), these new materials have a great potential as a delivery host in the pharmaceutical field. Calcium clay has been used extensively in the treatment of pain, open wounds, colitis, diarrhea, hemorrhoids, stomach ulcers, intestinal problems, acne, anemia, and a variety of other health issues.

**Figure 5.** Schema of drugs delivery.

*Bionanocomposites* belong to group of materials, being the result of the combination of biopolymers and inorganic solids at the nanometer scale. These hybrid organic–inorganic materials are extraordinarily versatile as they could be formed from a large variety of biopolymers and also from different inorganic solid particles such as layered silicates, hydroxyapatite, cellulose and other metal oxides [148]. Among them, chitosan/layered silicate nanocomposites have received much more attention. These nanocomposites are of interest for advanced biomedical materials, as for instance tissue engineering, artificial bones or gene therapy. Other possible fields of applications are related to their mechanical, thermal and barrier properties, making this class of materials attractive for potential uses in controlled drug and pesticides delivery, membranes for food processing, drinking water purification, oxygen barrier films and food package.

Chitosan is the deacetylated product of chitin, a natural polymer found in the cell wall of fungi and microorganisms. The active groups in the chitosan structure are the free amine groups, located in the C2 position of the glucose residue in the polysaccharide chain, and the hydroxyl groups [148]. *The chitosan/vermiculite nanocomposites* have been successfully prepared with different modified vermiculite, which was treated by acid, sodium and CTAB cations. The modification and the nano-scale dispersion of the modified vermiculites were confirmed.

## **Author details**

228 Clay Minerals in Nature – Their Characterization, Modification and Application

[142].

147].

other health issues.

**Figure 5.** Schema of drugs delivery.

Many advantages and disadvantages characterize these two different classes of biomaterials. Synthetic polymers have relatively good mechanical strength and their shape and degradation rate can be easily modified, but their surfaces are hydrophobic and lack of cellrecognition signals. Naturally derived polymers have the potential advantage of biological recognition that may positively support cell adhesion and function, but they have poor mechanical properties. Many of them are also limited in supply and can therefore be costly

During the last decade, significant attention has been focused on biodegradable polymers. Among all these polymers, poly(lactic acid) (PLA) is one of the most promising because it is thermoplastic, biocompatible and has a high strength, a high modulus, and good processability [143]. PLA also has been revealed an inefficient crystallization process for both the lower crystallization rate and crystallinity as compared with other polymers [144-

To maximize therapeutic activity while minimizing negative side effects is the main driven force for the continuous development of new controlled drug delivery systems. Because the release of drugs in drug-intercalated layered materials is potentially controllable (Fig.5), these new materials have a great potential as a delivery host in the pharmaceutical field. Calcium clay has been used extensively in the treatment of pain, open wounds, colitis, diarrhea, hemorrhoids, stomach ulcers, intestinal problems, acne, anemia, and a variety of

*Bionanocomposites* belong to group of materials, being the result of the combination of biopolymers and inorganic solids at the nanometer scale. These hybrid organic–inorganic Marta Valášková1,2, \* and Gražyna Simha Martynková1,2 *1Nanotechnology Centre, VŠB – Technical University of Ostrava, Ostrava-Poruba, Czech Republic 2IT4Innovations Centre of Excellence, VŠB-Technical University of Ostrava, Ostrava-Poruba, Czech Republic* 

## **Acknowledgement**

Financial support of the Czech Grant Agency (projects GA ČR 210/11/2215) and the IT4Innovations Centre of Excellence (project reg. no. cz.1.05/1.1.00/02.0070) are gratefully acknowledged.

## **7. References**


<sup>\*</sup> Corresponding Author

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**Chapter 12** 

© 2012 Ookawa, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Ookawa, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The tubular structure of imogolite was proposed by Cradwick et al. [2] based on results from electron diffraction observation and is shown in figure 2. The tube wall consists of a

**Synthesis and Characterization** 

Additional information is available at the end of the chapter

Masashi Ookawa

**1. Introduction** 

http://dx.doi.org/10.5772/48171

composition is (OH)3Al2O3Si(OH).

**Figure 1.** Photograph of natural imogolite films in soil.

**of Fe-Imogolite as an Oxidation Catalyst** 

Imogolite is a hydrated aluminosilicate with a unique tubular structure, which is found in volcanic ash soil and is shown in Figure 1. It was first discovered in glassy volcanic ash soil in Japan and was named after the soil in Hitoyoshi, Kumamoto Prefecture [1]. Its chemical


## **Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst**

Masashi Ookawa

238 Clay Minerals in Nature – Their Characterization, Modification and Application

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http://dx.doi.org/10.5772/48171

## **1. Introduction**

Imogolite is a hydrated aluminosilicate with a unique tubular structure, which is found in volcanic ash soil and is shown in Figure 1. It was first discovered in glassy volcanic ash soil in Japan and was named after the soil in Hitoyoshi, Kumamoto Prefecture [1]. Its chemical composition is (OH)3Al2O3Si(OH).

**Figure 1.** Photograph of natural imogolite films in soil.

The tubular structure of imogolite was proposed by Cradwick et al. [2] based on results from electron diffraction observation and is shown in figure 2. The tube wall consists of a

© 2012 Ookawa, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Ookawa, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

single continuous Al(OH)3 (gibbsite) sheet and orthosilicate anions (O3SiOH groups) associated with each vacant octahedral site of the gibbsite sheet. The imogolite has an outer diameter of ca. 2 nm and an inner diameter of ca. 1 nm.

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 241

(as sodium hydroxide) ratio to synthesizing

In the latter half of the 1970's, synthesis of imogolite was succeeded from a dilute solution. Recently, various synthesis methods have been investigated and many characterizations have been carried out. In this section, synthesis methods, structural characterization and

Farmer et al. reported [24] the synthesis method of imogolite in 1977. Imogolite was synthesized from a dilute solution containing of hydroxyaluminum cations (2.4 mmol L-1) and orthosilicic acid (1.4 mmol L-1). Afterward, the solution was adjusted to pH 5 with sodium hydroxide, 1 mmol L-1 of hydrochloric acid solution and 2 mmol L-1 of acetic acid. Imogolite was obtained in this solution by heating it near the boiling point. Wada et al. [25]

A diluted inorganic solution is necessary to form imogolite in these methods, because preventing the condensation of orthosilicic acid and the formation inhibition of it by anions is important to form the nanotube structure. However, it is difficult to obtain a large sample. Suzuki et al. [6] developed a synthetic method of producing imogolite using a concentrated inorganic solution. Sodium orthosilicate was used as a starting material to prevent the condensation of orthosilicic acid. Furthermore, a desalination process is carried out by centrifugation. We have synthesized Fe containing imogolite based on improving Suzuki's

More recently, new synthesis methods of imogolite have been reported. Levard et al. [8] synthesized it from a decimolar concentration solution at 95 °C for 60 days. Abidin et al. [9] proposed a new supplying method of a silicon source using colloidal silica for the synthesis

Transmission electron microscopy (TEM) or scanning electron microscopy (SEM) is used mostly in order to observe the morphology and structure of imogolite nanotubes. Figure 3

Bursill et al. [2] have observed the various aggregations of them such as randomly oriented

On the other hand, atomic force microscopy (AFM) is a powerful tool to investigate them under ambient conditions. The tapping-mode AFM especially shows the morphological features of the synthetic imogolite clearly [26, 27]. Figure 4 shows the tapping mode AFM image of synthetic imogolite. Fibrous materials with a length of 100 - 1000 nm can be

single tube, close-packed arrays or fiber bundles by high resolution TEM.

**2. Synthetic imogolite** 

catalytic properties of synthetic imogolite are described.

**2.1. Synthesis method of imogolite** 

later also investigated the effects of Al-to-OH-

method which will be described below in detail.

**2.2. Characterization of imogolite** 

shows an SEM image of synthetic imogolite.

of imogolite.

*2.2.1. Morphology* 

observed in this image.

imogolite and allophane using a dilute inorganic solution.

**Figure 2.** A cross-section of the structural model of an imogolite tube.

Recently, imogolite has drawn a new attention as a new nano-material because of its unique nano-scale tubular structure similar to that of a single-walled carbon nanotube [3]. Studies on synthesis [4-9], mechanisms of formation [10], structural evolution [11, 12], stability [13, 14], electronic states [14, 15] and application have been carried out. Proposed applications, such as a polymer composite [16,17], a fuel gas storage [18], an absorbent [19], an exchange material for heat pump system [20], a humidity-controlling material [6] and an antideweling material [6] have been discussed.

Application as a catalyst or a catalyst support also attracts attention because it is expected to have a shape-selective characteristic property as molecular sieving zeolites due to its unique tubular structure. However, few investigations [21, 22] have been reported using natural imogolite as a catalyst because the extraction of pure imogolite from the soil is difficult and time consuming [21]. Therefore, synthesis of imogolite has become necessary in order to utilize it as a functional material.

We synthesized imogolite containing Fe3+ ions (Fe-imogolite) using NaSiO4, FeCl3 and AlCl3 to investigate its catalytic properties [23]. Because of its chemical stability, an incorporation of another element is necessary to generate a chemical function. We found that it served as a catalyst of liquid-phase oxidation reactions of some hydrocarbons.

In this chapter, the synthetic methods, characterization and the general properties of imogolite is described at first. And then the characterization and catalytic properties of Feimogolite obtained from our researches is described.

## **2. Synthetic imogolite**

240 Clay Minerals in Nature – Their Characterization, Modification and Application

**Figure 2.** A cross-section of the structural model of an imogolite tube.

catalyst of liquid-phase oxidation reactions of some hydrocarbons.

imogolite obtained from our researches is described.

deweling material [6] have been discussed.

utilize it as a functional material.

diameter of ca. 2 nm and an inner diameter of ca. 1 nm.

single continuous Al(OH)3 (gibbsite) sheet and orthosilicate anions (O3SiOH groups) associated with each vacant octahedral site of the gibbsite sheet. The imogolite has an outer

Recently, imogolite has drawn a new attention as a new nano-material because of its unique nano-scale tubular structure similar to that of a single-walled carbon nanotube [3]. Studies on synthesis [4-9], mechanisms of formation [10], structural evolution [11, 12], stability [13, 14], electronic states [14, 15] and application have been carried out. Proposed applications, such as a polymer composite [16,17], a fuel gas storage [18], an absorbent [19], an exchange material for heat pump system [20], a humidity-controlling material [6] and an anti-

Application as a catalyst or a catalyst support also attracts attention because it is expected to have a shape-selective characteristic property as molecular sieving zeolites due to its unique tubular structure. However, few investigations [21, 22] have been reported using natural imogolite as a catalyst because the extraction of pure imogolite from the soil is difficult and time consuming [21]. Therefore, synthesis of imogolite has become necessary in order to

We synthesized imogolite containing Fe3+ ions (Fe-imogolite) using NaSiO4, FeCl3 and AlCl3 to investigate its catalytic properties [23]. Because of its chemical stability, an incorporation of another element is necessary to generate a chemical function. We found that it served as a

In this chapter, the synthetic methods, characterization and the general properties of imogolite is described at first. And then the characterization and catalytic properties of FeIn the latter half of the 1970's, synthesis of imogolite was succeeded from a dilute solution. Recently, various synthesis methods have been investigated and many characterizations have been carried out. In this section, synthesis methods, structural characterization and catalytic properties of synthetic imogolite are described.

## **2.1. Synthesis method of imogolite**

Farmer et al. reported [24] the synthesis method of imogolite in 1977. Imogolite was synthesized from a dilute solution containing of hydroxyaluminum cations (2.4 mmol L-1) and orthosilicic acid (1.4 mmol L-1). Afterward, the solution was adjusted to pH 5 with sodium hydroxide, 1 mmol L-1 of hydrochloric acid solution and 2 mmol L-1 of acetic acid. Imogolite was obtained in this solution by heating it near the boiling point. Wada et al. [25] later also investigated the effects of Al-to-OH- (as sodium hydroxide) ratio to synthesizing imogolite and allophane using a dilute inorganic solution.

A diluted inorganic solution is necessary to form imogolite in these methods, because preventing the condensation of orthosilicic acid and the formation inhibition of it by anions is important to form the nanotube structure. However, it is difficult to obtain a large sample. Suzuki et al. [6] developed a synthetic method of producing imogolite using a concentrated inorganic solution. Sodium orthosilicate was used as a starting material to prevent the condensation of orthosilicic acid. Furthermore, a desalination process is carried out by centrifugation. We have synthesized Fe containing imogolite based on improving Suzuki's method which will be described below in detail.

More recently, new synthesis methods of imogolite have been reported. Levard et al. [8] synthesized it from a decimolar concentration solution at 95 °C for 60 days. Abidin et al. [9] proposed a new supplying method of a silicon source using colloidal silica for the synthesis of imogolite.

## **2.2. Characterization of imogolite**

#### *2.2.1. Morphology*

Transmission electron microscopy (TEM) or scanning electron microscopy (SEM) is used mostly in order to observe the morphology and structure of imogolite nanotubes. Figure 3 shows an SEM image of synthetic imogolite.

Bursill et al. [2] have observed the various aggregations of them such as randomly oriented single tube, close-packed arrays or fiber bundles by high resolution TEM.

On the other hand, atomic force microscopy (AFM) is a powerful tool to investigate them under ambient conditions. The tapping-mode AFM especially shows the morphological features of the synthetic imogolite clearly [26, 27]. Figure 4 shows the tapping mode AFM image of synthetic imogolite. Fibrous materials with a length of 100 - 1000 nm can be observed in this image.

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 243

**Figure 4.** The tapping mode AFM image of synthetic imogolite.

**Intensity**

**Figure 5.** XRD profiles of synthetic imogolite (a) and natural imogolite (b).

**10 20 30**

**(a)** 

500 nm

**(b)**

**2 / ° (Cu K)**

**Figure 3.** The FE-SEM image of synthetic imogolite.

### *2.2.2. Structural characterization*

The analytical methods, such as X-ray diffraction (XRD), infrared (IR) spectroscopy and solid state nuclear magnetic resonance (NMR) are used in order to identify or characterize imogolite. In this section, the features of imogolite obtained using these analytical methods are mentioned.

The XRD profiles which were obtained by Cu K irradiation are given in Figure 5. Imogolite is characterized by three broad peaks in the low angle region. There are three peaks at 2= 5.1°, 11° and 15.6° in the XRD profile of natural imogolite and at 2= 4.6°, 9.6° and 14.3° in that of synthetic imogolite. The difference in peak position is due to the difference in the diameter of a tube of a natural and synthetic imogolite [7, 24]. The diameter of 1.8 - 2.2 nm in natural imogolite [1] and 2.7 - 3.2 nm in synthetic imogolite [25] were reported.

FT-IR spectra of Natural and synthetic imogolite, which are shown in figure 6, have a characteristic absorption that appears as a doublet at around 1000 cm-1. These absorptions are attributed to Si-O (higher frequency) and Si-O-Al (lower frequency) stretching [28]. These samples also have some absorption bands in the region from 400 cm-1 to 750 cm-1. The absorption bands at 685 cm-1, 563 cm-1 and 427 cm-1 in natural imogolite arise from various Al-O vibrations [29].

Imogolite is also characterized by 29Si and 27Al solid state NMR [30-34]. Magic angle spinning (MAS) techniques are used in general in order to obtain a high resolution NMR spectrum of sold state. Goodman et al. [34] have carried out 29Si and 27Al MAS NMR measurements of synthetic imogolite. The peak is observed at -78.8 ppm in 29Si MAS NMR spectrum of it as well as natural imogolite. Barron et al. [30] have shown that the observed

**Figure 4.** The tapping mode AFM image of synthetic imogolite.

**Figure 3.** The FE-SEM image of synthetic imogolite.

The analytical methods, such as X-ray diffraction (XRD), infrared (IR) spectroscopy and solid state nuclear magnetic resonance (NMR) are used in order to identify or characterize imogolite. In this section, the features of imogolite obtained using these analytical methods

The XRD profiles which were obtained by Cu K irradiation are given in Figure 5. Imogolite is characterized by three broad peaks in the low angle region. There are three peaks at 2= 5.1°, 11° and 15.6° in the XRD profile of natural imogolite and at 2= 4.6°, 9.6° and 14.3° in that of synthetic imogolite. The difference in peak position is due to the difference in the diameter of a tube of a natural and synthetic imogolite [7, 24]. The diameter of 1.8 - 2.2 nm in

FT-IR spectra of Natural and synthetic imogolite, which are shown in figure 6, have a characteristic absorption that appears as a doublet at around 1000 cm-1. These absorptions are attributed to Si-O (higher frequency) and Si-O-Al (lower frequency) stretching [28]. These samples also have some absorption bands in the region from 400 cm-1 to 750 cm-1. The absorption bands at 685 cm-1, 563 cm-1 and 427 cm-1 in natural imogolite arise from various

Imogolite is also characterized by 29Si and 27Al solid state NMR [30-34]. Magic angle spinning (MAS) techniques are used in general in order to obtain a high resolution NMR spectrum of sold state. Goodman et al. [34] have carried out 29Si and 27Al MAS NMR measurements of synthetic imogolite. The peak is observed at -78.8 ppm in 29Si MAS NMR spectrum of it as well as natural imogolite. Barron et al. [30] have shown that the observed

natural imogolite [1] and 2.7 - 3.2 nm in synthetic imogolite [25] were reported.

*2.2.2. Structural characterization* 

are mentioned.

Al-O vibrations [29].

**Figure 5.** XRD profiles of synthetic imogolite (a) and natural imogolite (b).

chemical shift of natural imogolite is consistent with silicon tetrahedra which are isolated by coordination through oxygen with three aluminum atoms and one proton. On the other hand, 27Al MAS NMR spectrum of imogolite has one peak at 0 ppm and it is attributed to the six coordinated octahedral Al3+ species [31-34].

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 245

imogolite are explained based on their literature. The endothermic peaks at 110 °C and 400 °C were attributed to the loss of adsorbed water and dehydroxylation, respectively. Amorphism of imogolite occurred by these dehydroxylations. The exothermic peak at 950 °C was attributed to crystallization to mullite (Al6Si2O13). Donkai *et al.* [11] investigated thermal transformation of natural imogolite up to 1600 °C using XRD and IR and reported the formation of tridymite (SiO2) with mullite crystals above 1200 °C. Hatakeyama et al. [34] investigated the transformation heat-treated synthetic imogolite by using 27Al MAS NMR and 27Al multiple-quantum magic-angle-spinning (MQMAS) NMR. These results show five- and four-coordinated Al is formed above 350 °C in amorphous materials

It has been known that imogolite has surface acidity [35]. The acid strength of it is increased by heat treatment. Natural imogolite calcined at various temperatures were used as catalysts for the isomerization of 1-butene [21]. This reaction proceeded effectively over samples calcined at 400 °C. The decomposition reaction of organic peroxides also was investigated using Cu2+ loaded imogolite calcined at 500 °C. Furthermore, it was mentioned that imogolite calcined at temperature up to 750 °C exhibited the shape selective adsorption. Bonelli et al. [36] have studied *in situ* IR spectroscopy of synthetic imogolite which adsorbed CO, ammonia, methanol or phenol and catalytic tests using gas-phase phenol reactions with methanol. They showed that a small amount of Al3+ Lewis acid sites to adsorb CO existed, the adsorbed ammonia on imogolite evacuated at 150 °C was observed as NH4+ species and, in addition, a probe such as CO, ammonia, methanol or phenol could interact with inner silanols. The reaction of phenol with methanol was performed over imogolite after thermal treatment at 300 °C and at 500 °C. The activity was shown over samples heated at 500 °C

We have investigated the acidic property of imogolite without heat treatment [37]. At first, we attempted an isomerization reaction of -pinene on synthetic imogolite. It is known that the isomerization products of -pinene depend on the acid or base property of the catalyst. In the case of acid catalysts, -pinene isomerize to limonene, camphene and tricyclene [38]. Prior to the reaction imogolite was dried at 120 °C for 12 hours. The isomerization reaction of -pinene was carried out at 80 °C for 3 hours or 24 hours using an evacuated batch reactor. In a typical experiment, the reactor was loaded with 12.6 mmol of -pinene and 50 mg of catalyst. The -pinene did not react on it. It was reported that imogolite had a weak

Although isomerization of -pinene did not occurre, some oxidation products of it were detected slightly. So the oxidation reaction of cyclohexene using hydrogen peroxide was

In a typical catalytic experiment for oxidation [38], the reaction was carried out by using 25 mmol of cyclohexene, 25 mmol of H2O2 (30 wt%) and 30 ml of acetic acid or acetonitrile as a

clearly.

*2.2.4. Catalytic properties* 

and o-cresol and anisole were obtained as products.

acid property, however it was not detected in this reaction.

carried out to test the possibility as an oxidation catalyst.

**Figure 6.** FT-IR spectra of synthetic imogolite (a) and natural imogolite (b).

#### *2.2.3. Thermal transformation of synthetic imogolite*

The differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) traces of synthetic imogolite are shown in Figure 7. Two broad endothermic peaks with a weight loss and an exothermic peak without a weight change were observed.

**Figure 7.** DTA-TGA curves of synthetic imogolite.

MacKenzie et al. [31] investigated thermal transformation of natural imogolite by using DTA-TGA, 29Si MAS NMR and 27Al MAS NMR and proposed the structural models which were changed by heating. The results of DTA-TGA showing thermal transformation of imogolite are explained based on their literature. The endothermic peaks at 110 °C and 400 °C were attributed to the loss of adsorbed water and dehydroxylation, respectively. Amorphism of imogolite occurred by these dehydroxylations. The exothermic peak at 950 °C was attributed to crystallization to mullite (Al6Si2O13). Donkai *et al.* [11] investigated thermal transformation of natural imogolite up to 1600 °C using XRD and IR and reported the formation of tridymite (SiO2) with mullite crystals above 1200 °C. Hatakeyama et al. [34] investigated the transformation heat-treated synthetic imogolite by using 27Al MAS NMR and 27Al multiple-quantum magic-angle-spinning (MQMAS) NMR. These results show five- and four-coordinated Al is formed above 350 °C in amorphous materials clearly.

#### *2.2.4. Catalytic properties*

244 Clay Minerals in Nature – Their Characterization, Modification and Application

**Figure 6.** FT-IR spectra of synthetic imogolite (a) and natural imogolite (b).

and an exothermic peak without a weight change were observed.

*2.2.3. Thermal transformation of synthetic imogolite* 

**(b)**

**(a)**

**Figure 7.** DTA-TGA curves of synthetic imogolite.

the six coordinated octahedral Al3+ species [31-34].

**Absorbance**

chemical shift of natural imogolite is consistent with silicon tetrahedra which are isolated by coordination through oxygen with three aluminum atoms and one proton. On the other hand, 27Al MAS NMR spectrum of imogolite has one peak at 0 ppm and it is attributed to

The differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) traces of synthetic imogolite are shown in Figure 7. Two broad endothermic peaks with a weight loss

**1400 1200 1000 800 600 400**

**-1**

**Wavenumber / cm**

MacKenzie et al. [31] investigated thermal transformation of natural imogolite by using DTA-TGA, 29Si MAS NMR and 27Al MAS NMR and proposed the structural models which were changed by heating. The results of DTA-TGA showing thermal transformation of It has been known that imogolite has surface acidity [35]. The acid strength of it is increased by heat treatment. Natural imogolite calcined at various temperatures were used as catalysts for the isomerization of 1-butene [21]. This reaction proceeded effectively over samples calcined at 400 °C. The decomposition reaction of organic peroxides also was investigated using Cu2+ loaded imogolite calcined at 500 °C. Furthermore, it was mentioned that imogolite calcined at temperature up to 750 °C exhibited the shape selective adsorption. Bonelli et al. [36] have studied *in situ* IR spectroscopy of synthetic imogolite which adsorbed CO, ammonia, methanol or phenol and catalytic tests using gas-phase phenol reactions with methanol. They showed that a small amount of Al3+ Lewis acid sites to adsorb CO existed, the adsorbed ammonia on imogolite evacuated at 150 °C was observed as NH4+ species and, in addition, a probe such as CO, ammonia, methanol or phenol could interact with inner silanols. The reaction of phenol with methanol was performed over imogolite after thermal treatment at 300 °C and at 500 °C. The activity was shown over samples heated at 500 °C and o-cresol and anisole were obtained as products.

We have investigated the acidic property of imogolite without heat treatment [37]. At first, we attempted an isomerization reaction of -pinene on synthetic imogolite. It is known that the isomerization products of -pinene depend on the acid or base property of the catalyst. In the case of acid catalysts, -pinene isomerize to limonene, camphene and tricyclene [38]. Prior to the reaction imogolite was dried at 120 °C for 12 hours. The isomerization reaction of -pinene was carried out at 80 °C for 3 hours or 24 hours using an evacuated batch reactor. In a typical experiment, the reactor was loaded with 12.6 mmol of -pinene and 50 mg of catalyst. The -pinene did not react on it. It was reported that imogolite had a weak acid property, however it was not detected in this reaction.

Although isomerization of -pinene did not occurre, some oxidation products of it were detected slightly. So the oxidation reaction of cyclohexene using hydrogen peroxide was carried out to test the possibility as an oxidation catalyst.

In a typical catalytic experiment for oxidation [38], the reaction was carried out by using 25 mmol of cyclohexene, 25 mmol of H2O2 (30 wt%) and 30 ml of acetic acid or acetonitrile as a

solvent with 100 mg of catalyst under being stirred at 50 °C. The results of the oxidation reaction of cyclohexene with H2O2 are listed in Table 1. Without a catalyst the oxidation products such as trans-1,2-cyclohexanediol, cis-1,2-cyclohexanediol and 2-cyclohexene-1-ol were observed using acetic acid as solvent. With imogolite the yield of these oxidation products was increased, but the selectivity was almost the same as the control experiment. This result indicates that imogolite has the potential of concentration and acts as a field of reaction to increase collision frequency.

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 247

5. The salt-free precursor was obtained from thick solution by centrifugal separation three

Fe containing samples with x = 0, 0.05, 0.1 were prepared in order to survey the effect of Fe3+ contents in starting solutions to formation of imogolite nanotubes,. The XRD profiles and IR

As mentioned above, imogolite (x = 0) was characterized by three broad peaks in the low angle region of the XRD profile and a doublet at around 1000 cm-1 in the FT-IR spectrum. In the case of x = 0.1, it lacks the three broad peaks in XRD and a doublet in FT-IR, meaning the sample does not have the imogolite structure. In the case of x=0.05, the XRD profile and FT-IR spectrum were similar to the imogolite. The intensity of the three broad peaks is weaker

6. It was dispersed in 2 L of water and 40 mLof 0.1 mol L-1 HCl was added.

9. This aging solution was dialyzed and then dried at 100 °C for two days.

7. The solution was stirred for two hours at room temperature.

times.

8. It was aged at 100 °C for 40 hours.

10. The film-like Fe-imogolite was obtained.

**Scheme 1.** Flowchart of a synthetic method of Fe-imogolite.

**3.2. Characterization of Fe-imogolite** 

spectra of these samples are shown in Figure 8.


**Table 1.** Oxidation of cyclohexene with H2O2 over imogolite in acetic acid as a solvent. [38] Reaction condition: temperature 50 °C, time 6h, imogolite 100 mg

1-ol : 2-cyclohexene-1-ol, trans-diol : trans-1,2-cyclohexanediol, cis-diol: 1,2-cyclohexanediol

In the case of using acetonitrile as the solvent, the oxidation products were not detected for 27 hours of reaction time. In the case of imogolite, 2-cyclohexene-1-ol and 1,2 epoxycyclohexene were produced with 0.5 % and 0.8 % yields, respectively. This results shows that imogolite has the possibility as an oxidation catalyst.

## **3. Fe-imogolite**

Although we found the new possibility of synthetic imogolite as an oxidation catalyst, a chemical modification of imogolite like an introduction of Fe3+ ion was necessary to promote the reaction because imogolite is chemically stable. In this section, our results of the synthetic method, characterization and catalytic test of Fe-imogolite are described.

## **3.1. Synthesis of Fe-imogolite**

Fe-imogolite was synthesized based on improving Suzuki's method [6, 7]. The typical method [23] is described here and its flowchart is shown in Scheme 1.


Reaction condition: temperature 50 °C, time 6h, imogolite 100 mg

shows that imogolite has the possibility as an oxidation catalyst.

reaction to increase collision frequency.

Yield / %

Selectivity / %

**3. Fe-imogolite** 

**3.1. Synthesis of Fe-imogolite** 

stirred for 90 minutes.

5.5.

solvent with 100 mg of catalyst under being stirred at 50 °C. The results of the oxidation reaction of cyclohexene with H2O2 are listed in Table 1. Without a catalyst the oxidation products such as trans-1,2-cyclohexanediol, cis-1,2-cyclohexanediol and 2-cyclohexene-1-ol were observed using acetic acid as solvent. With imogolite the yield of these oxidation products was increased, but the selectivity was almost the same as the control experiment. This result indicates that imogolite has the potential of concentration and acts as a field of

Product 1-ol trans-diol cis-diol

**Table 1.** Oxidation of cyclohexene with H2O2 over imogolite in acetic acid as a solvent. [38]

1-ol : 2-cyclohexene-1-ol, trans-diol : trans-1,2-cyclohexanediol, cis-diol: 1,2-cyclohexanediol

In the case of using acetonitrile as the solvent, the oxidation products were not detected for 27 hours of reaction time. In the case of imogolite, 2-cyclohexene-1-ol and 1,2 epoxycyclohexene were produced with 0.5 % and 0.8 % yields, respectively. This results

Although we found the new possibility of synthetic imogolite as an oxidation catalyst, a chemical modification of imogolite like an introduction of Fe3+ ion was necessary to promote the reaction because imogolite is chemically stable. In this section, our results of the

Fe-imogolite was synthesized based on improving Suzuki's method [6, 7]. The typical

1. We prepared 100 mL of 0.15 mol L-1 aqueous solutions consist of FeCl3 and AlCl3 with x = 0.05 (x = Fe/Al+Fe, atomic ratio). Another way of saying, 0.00075 mol FeCl3・6H2O

3. Na4SiO4 aqueous solution was added to AlCl3 and FeCl3 mixture and the solution was

4. 0.1 mol L-1 NaOH aqueous solution was added to the solution consist of Na4SiO4, FeCl3 and AlCl3 at the rate of 1.0 mL min-1 under stirring until the pH of the mixture become

synthetic method, characterization and catalytic test of Fe-imogolite are described.

method [23] is described here and its flowchart is shown in Scheme 1.

2. 100 mL of 0.1 mol L-1 Na4SiO4 aqueous solution was prepared.

and 0.01425 mol AlCl3・6H2O were dissolved in 100 mL of water.

imogolite 32.3 21.5 2.9 no catalyst 14.6 12.0 1.3

imogolite 56.9 38.0 5.1 no catalyst 52.3 43.2 4.5


**Scheme 1.** Flowchart of a synthetic method of Fe-imogolite.

#### **3.2. Characterization of Fe-imogolite**

Fe containing samples with x = 0, 0.05, 0.1 were prepared in order to survey the effect of Fe3+ contents in starting solutions to formation of imogolite nanotubes,. The XRD profiles and IR spectra of these samples are shown in Figure 8.

As mentioned above, imogolite (x = 0) was characterized by three broad peaks in the low angle region of the XRD profile and a doublet at around 1000 cm-1 in the FT-IR spectrum. In the case of x = 0.1, it lacks the three broad peaks in XRD and a doublet in FT-IR, meaning the sample does not have the imogolite structure. In the case of x=0.05, the XRD profile and FT-IR spectrum were similar to the imogolite. The intensity of the three broad peaks is weaker than that of imogolite because XRD measurements were carried out using Cu K irradiation. The color and the profiles of XRD and FT-IR indicate the sample with x=0.05 is considered Fe containing imogolite and is thus called Fe-imogolite.

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 249

**Figure 9.** Diffuse reflectance UV-VIS spectra of (a) Fe-imogolite, (b) FeCl3/imogolite and

**Figure 10.** Fe K-edge XANES spectra of (a) Fe-imogolite, (b) FeCl3/imogolite and

(c) imogolite [23].

(c) Fe2O3 (hematite) [23].

**Figure 8.** Characterization of Fe containing samples with x (= Fe/Al+Fe, atomic ratio) =0, 0.05, 0.1. XRD profiles (a) and FT-IR spectra (b) [23].

In order to investigate the state of iron ions in Fe-imogolite, it was compared with the Fe3+ ion adsorbed on imogolite The adsorbed Fe3+ ions sample was prepared by adsorbing FeCl3 onto imogolite from aqueous solution of FeCl3 and is called FeCl3/imogolite. These samples are both reddish brown and the absorption bands of them were observed at the region above 15,000 cm-1 in diffuse reflectance ultraviolet visible (UV-VIS) spectra (Figure 9). The absorption bands of Fe3+ in many minerals are observed in this region.

It has been known that when the tetrahedral species of Fe3+ exist, the pre-edge peak appears strongly in X-ray absorption near edge structure (XANES) spectra. XANES spectra of Fe Kedge (7.111 keV) are shown in Figure 10. The pre-edge peak was not observed in the spectrum of Fe-imogolite nor FeCl3/imogolite or Fe2O3. Thus it is clear that the state of iron in Fe-imogolite is octahedral Fe3+ ion from the results of UV-VIS and XANES spectra.

Figure 11 shows their Fourier transforms (FT) spectra of Fe-imogolite, FeCl3/imogolite and Fe2O3. FT spectrum as radial structure function was obtained by Fourier transformation of *k*3-weighted extended X-ray absorption fine structure (EXAFS) function. The FT spectrum of Fe-imogolite is different to that of FeCl3/imogolite or Fe2O3. It can be concluded that the state of Fe3+ in Fe-imogolite is different to the state of Fe3+ ions adsorbed on imogolite. We speculate that Fe3+ replaced the Al3+ sites in imogolite from these results.

The tapping mode AFM image of Fe-imogolite is shown in figure 12 [39]. The fibrous morphology was observed similar to the synthetic imogolite. It was found that the tube diameter was almost uniform and estimated to be 2.2-2.4 nm from section analysis.

**Figure 9.** Diffuse reflectance UV-VIS spectra of (a) Fe-imogolite, (b) FeCl3/imogolite and (c) imogolite [23].

profiles (a) and FT-IR spectra (b) [23].

considered Fe containing imogolite and is thus called Fe-imogolite.

than that of imogolite because XRD measurements were carried out using Cu K irradiation. The color and the profiles of XRD and FT-IR indicate the sample with x=0.05 is

**Figure 8.** Characterization of Fe containing samples with x (= Fe/Al+Fe, atomic ratio) =0, 0.05, 0.1. XRD

In order to investigate the state of iron ions in Fe-imogolite, it was compared with the Fe3+ ion adsorbed on imogolite The adsorbed Fe3+ ions sample was prepared by adsorbing FeCl3 onto imogolite from aqueous solution of FeCl3 and is called FeCl3/imogolite. These samples are both reddish brown and the absorption bands of them were observed at the region above 15,000 cm-1 in diffuse reflectance ultraviolet visible (UV-VIS) spectra (Figure 9). The

It has been known that when the tetrahedral species of Fe3+ exist, the pre-edge peak appears strongly in X-ray absorption near edge structure (XANES) spectra. XANES spectra of Fe Kedge (7.111 keV) are shown in Figure 10. The pre-edge peak was not observed in the spectrum of Fe-imogolite nor FeCl3/imogolite or Fe2O3. Thus it is clear that the state of iron in

Figure 11 shows their Fourier transforms (FT) spectra of Fe-imogolite, FeCl3/imogolite and Fe2O3. FT spectrum as radial structure function was obtained by Fourier transformation of *k*3-weighted extended X-ray absorption fine structure (EXAFS) function. The FT spectrum of Fe-imogolite is different to that of FeCl3/imogolite or Fe2O3. It can be concluded that the state of Fe3+ in Fe-imogolite is different to the state of Fe3+ ions adsorbed on imogolite. We

The tapping mode AFM image of Fe-imogolite is shown in figure 12 [39]. The fibrous morphology was observed similar to the synthetic imogolite. It was found that the tube

diameter was almost uniform and estimated to be 2.2-2.4 nm from section analysis.

Fe-imogolite is octahedral Fe3+ ion from the results of UV-VIS and XANES spectra.

absorption bands of Fe3+ in many minerals are observed in this region.

speculate that Fe3+ replaced the Al3+ sites in imogolite from these results.

**Figure 10.** Fe K-edge XANES spectra of (a) Fe-imogolite, (b) FeCl3/imogolite and (c) Fe2O3 (hematite) [23].

250 Clay Minerals in Nature – Their Characterization, Modification and Application

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 251

were produced slightly as stated above. EP was also detected in separate experiments using gibbsite, boehmite and Al2O3 as catalysts. Mandelli et al. [40] have reported the epoxidation of cyclohexene using H2O2 over Al2O3. It was attributed that these oxidation compounds may be produced on the outer surface of the imogolite. The oxidation reaction was promoted by Feimogolite and not only alcohol and epoxy compounds but trans-1,2-cyclohexanediol, cis-1,2-

Phenol is one of the most important chemicals in the fields of fiber and medicine manufacturing. More than 90% of phenol is produced by the cumen process, which is a three-step process and produces acetone as a by-product. The development of a one-step process for phenol synthesis by the direct oxidation of benzene is important when concerned with green chemistry as an environment-friendly technique. It was found that benzene or other aromatic hydrocarbons were oxidized by H2O2 over Fe-imogolite. These

The reaction of benzene was carried out using 2 mmol of benzene, 11 mmol of H2O2 and 10 mL of acetonitrile as a solvent with 100 mg of catalyst under being stirred at 60 °C. The products were analyzed by GC-MASS. The conversion of H2O2 was determined by a

Figure 13 shows the results of oxidation reactions of benzene using H2O2 and compounds containing Fe. None of the oxidation products were detected without catalysts and with FeCl3/imogolite, Fe2O3 (hematite) and -FeOOH (goethite) as a catalyst. It is an interesting finding that only phenol was obtained as an oxidation product by GC-Mass using Fe-

The oxidation reactions of benzene using four solvents were examined. The results are summarized in Table 2. None of oxidation products were detected in the case of acetic acid and propionic acid as a solvent. It was found that acetonitrile was the most effective for this reaction among the examined solvents. Although the solution turned palish yellow using acetonitrile as a solvent, after the reaction the absorption due to the Fe3+ ion was not detected in UV-VIS absorption spectrum. It was suggested that the origin of the coloring could be phenolic tars that could not detected by GC-Mass. A conversion of H2O2 was high with all solvent and H2O2 efficiency was 2 % using acetonitrile as a solvent. Thus, the decomposition

The results of the oxidation reaction of aromatic hydrocarbons using Fe-imogolite as a catalyst are summarized in Table 3 [39]. Hydroquinone and catechol were produced as products from phenol by the oxidation with H2O2. *o-*Chlorophenol and *p-*chlorophenol were produced from chlorobenzene. When the side chain is OH group or Cl group, only the benzene ring was oxidized and the *ortho* and *para* isomers were obtained. Benzaldehyde, *o*cresol and *p-*cresol were produced from toluene. In the case of benzaldehyde, most of the

of H2O2 was only caused by using other solvents except for acetonitrile.

cyclohexanediol and 2-cyclohexene-1-one were also obtained as products.

*3.3.2. Oxidation of aromatic hydrocarbon [23, 39]* 

results are described.

imogolite [23].

volumetric analysis with KMnO4.

oxidation product was benzoic acid.

**Figure 11.** FT spectra of Fe K-edge k3-weighted EXAFS functions. (a) Fe-imogolite, (b) FeCl3/imogolite and (c) Fe2O3 (hematite) [23].

**Figure 12.** AFM image of Fe-imogolite (a) and section analysis (b). The height profile (right bottom) shows the height on black line in AFM picture (right top) [39].

## **3.3. Oxidation of hydrocarbons using Fe-imogolite catalyst**

## *3.3.1. Oxidation of cyclohexene [23]*

The oxidation of cyclohexene was carried out using cyclohexene, H2O2 and acetonitrile as a solvent with catalysts under being stirred at 50 °C for 24 hours. No product was detected without a catalyst. In the case of imogolite, 2-cyclohexene-1-ol and 1,2-epoxycyclohexane (EP) were produced slightly as stated above. EP was also detected in separate experiments using gibbsite, boehmite and Al2O3 as catalysts. Mandelli et al. [40] have reported the epoxidation of cyclohexene using H2O2 over Al2O3. It was attributed that these oxidation compounds may be produced on the outer surface of the imogolite. The oxidation reaction was promoted by Feimogolite and not only alcohol and epoxy compounds but trans-1,2-cyclohexanediol, cis-1,2 cyclohexanediol and 2-cyclohexene-1-one were also obtained as products.

## *3.3.2. Oxidation of aromatic hydrocarbon [23, 39]*

250 Clay Minerals in Nature – Their Characterization, Modification and Application

and (c) Fe2O3 (hematite) [23].

**Figure 11.** FT spectra of Fe K-edge k3-weighted EXAFS functions. (a) Fe-imogolite, (b) FeCl3/imogolite

**Figure 12.** AFM image of Fe-imogolite (a) and section analysis (b). The height profile (right bottom)

The oxidation of cyclohexene was carried out using cyclohexene, H2O2 and acetonitrile as a solvent with catalysts under being stirred at 50 °C for 24 hours. No product was detected without a catalyst. In the case of imogolite, 2-cyclohexene-1-ol and 1,2-epoxycyclohexane (EP)

shows the height on black line in AFM picture (right top) [39].

*3.3.1. Oxidation of cyclohexene [23]* 

**3.3. Oxidation of hydrocarbons using Fe-imogolite catalyst** 

Phenol is one of the most important chemicals in the fields of fiber and medicine manufacturing. More than 90% of phenol is produced by the cumen process, which is a three-step process and produces acetone as a by-product. The development of a one-step process for phenol synthesis by the direct oxidation of benzene is important when concerned with green chemistry as an environment-friendly technique. It was found that benzene or other aromatic hydrocarbons were oxidized by H2O2 over Fe-imogolite. These results are described.

The reaction of benzene was carried out using 2 mmol of benzene, 11 mmol of H2O2 and 10 mL of acetonitrile as a solvent with 100 mg of catalyst under being stirred at 60 °C. The products were analyzed by GC-MASS. The conversion of H2O2 was determined by a volumetric analysis with KMnO4.

Figure 13 shows the results of oxidation reactions of benzene using H2O2 and compounds containing Fe. None of the oxidation products were detected without catalysts and with FeCl3/imogolite, Fe2O3 (hematite) and -FeOOH (goethite) as a catalyst. It is an interesting finding that only phenol was obtained as an oxidation product by GC-Mass using Feimogolite [23].

The oxidation reactions of benzene using four solvents were examined. The results are summarized in Table 2. None of oxidation products were detected in the case of acetic acid and propionic acid as a solvent. It was found that acetonitrile was the most effective for this reaction among the examined solvents. Although the solution turned palish yellow using acetonitrile as a solvent, after the reaction the absorption due to the Fe3+ ion was not detected in UV-VIS absorption spectrum. It was suggested that the origin of the coloring could be phenolic tars that could not detected by GC-Mass. A conversion of H2O2 was high with all solvent and H2O2 efficiency was 2 % using acetonitrile as a solvent. Thus, the decomposition of H2O2 was only caused by using other solvents except for acetonitrile.

The results of the oxidation reaction of aromatic hydrocarbons using Fe-imogolite as a catalyst are summarized in Table 3 [39]. Hydroquinone and catechol were produced as products from phenol by the oxidation with H2O2. *o-*Chlorophenol and *p-*chlorophenol were produced from chlorobenzene. When the side chain is OH group or Cl group, only the benzene ring was oxidized and the *ortho* and *para* isomers were obtained. Benzaldehyde, *o*cresol and *p-*cresol were produced from toluene. In the case of benzaldehyde, most of the oxidation product was benzoic acid.

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 253

**HO**

**CH3**

OH

**Cl**

**OH**

**OH**

**CH3**

**OH**

**CHO**

**OH**

methyl groups by H2O2 in acetonitrile over Fe-imogolite catalysts, other organic compounds such as saturated hydrocarbons could be oxidized under this reaction condition. We

phenol catechol (13.7) hydroquinone (15.9)

chlorobenzene *p-* chlorophenol (9.7) *o-* chlorophenol (4.8)

toluene benzaldehyde (5.8) *p-* cresol (2.2) *o-* cresol (1.3)

benzaldehyde benzoic acid (71.7) 2-hydroxy-benzaldehyde (1.7)

**Table 3.** Oxidation reactions of aromatic hydrocarbons with H2O2 over Fe-imogolite [39].

**COOH**

\*The yield of product is estimated using the ratio of peak area of GC-Mass. Temperature = 60°C, time = 6 h, reactant = 2 mmol, H2O2 = 11.5 mmol, solvent = 10 mL

examined the oxidation of cyclohexane using this catalyst.

**OH**

**CH3**

**CHO**

Reactant Products (Yield\* / %)

**OH**

**OH**

**OH**

**CHO**

**Cl Cl**

**Figure 13.** Results of the oxidation reactions of benzene using Fe-imogolite, FeCl3/imogolite, Fe2O3 (hematite) or FeOOH (geothite) as a catalyst [23].


**Table 2.** The effects of solvents on the oxidation of benzene [39].

Catalyst Fe-imogolite, temperature = 60 °C, time = 6 h, benzene = 2 mmol,

H2O2 = 11.5 mmol, solvent = 10 mL

When the side chain is a hydrocarbon group such as methyl, both the benzene ring and the side chain group were oxidized. It was found that the side chain group was more easily oxidized.

Monfared and Amouei [41] have reported direct oxidation reactions of benzene or some aromatic hydrocarbon compounds over Fe3+ loaded Al2O3 (Fe3+-Al2O3) with H2O2 in acetonitrile. In their system, however, *o-*cresol and *m-*cresol as main products were produced from toluene and no oxidation compounds were produced from phenol. It has been shown that the oxidation property of a Fe-imogolite catalyst is different from that of a Fe3+ - Al2O3 catalyst.

#### *3.3.3. Oxidation of cyclohexane [42, 43]*

Oxidation of cycloxane under mild conditions have been very interesting and have attempted widely [44]. Since the oxidation takes place not only on aromatic rings but on methyl groups by H2O2 in acetonitrile over Fe-imogolite catalysts, other organic compounds such as saturated hydrocarbons could be oxidized under this reaction condition. We examined the oxidation of cyclohexane using this catalyst.

252 Clay Minerals in Nature – Their Characterization, Modification and Application

(hematite) or FeOOH (geothite) as a catalyst [23].

H2O2 = 11.5 mmol, solvent = 10 mL

oxidized.

Fe3+ - Al2O3 catalyst.

*3.3.3. Oxidation of cyclohexane [42, 43]* 

**Table 2.** The effects of solvents on the oxidation of benzene [39].

Catalyst Fe-imogolite, temperature = 60 °C, time = 6 h, benzene = 2 mmol,

**Figure 13.** Results of the oxidation reactions of benzene using Fe-imogolite, FeCl3/imogolite, Fe2O3

Solvent Conversion / % Yield of

Acetonitrile 10.6 93 10.6 2-Propanol tr. 99 tr. Acetic acid 0.0 95 0.0 Propionic acid 0.0 95 0.0

When the side chain is a hydrocarbon group such as methyl, both the benzene ring and the side chain group were oxidized. It was found that the side chain group was more easily

Monfared and Amouei [41] have reported direct oxidation reactions of benzene or some aromatic hydrocarbon compounds over Fe3+ loaded Al2O3 (Fe3+-Al2O3) with H2O2 in acetonitrile. In their system, however, *o-*cresol and *m-*cresol as main products were produced from toluene and no oxidation compounds were produced from phenol. It has been shown that the oxidation property of a Fe-imogolite catalyst is different from that of a

Oxidation of cycloxane under mild conditions have been very interesting and have attempted widely [44]. Since the oxidation takes place not only on aromatic rings but on

benzene H2O2 Phenol / %

**Table 3.** Oxidation reactions of aromatic hydrocarbons with H2O2 over Fe-imogolite [39]. \*The yield of product is estimated using the ratio of peak area of GC-Mass. Temperature = 60°C, time = 6 h, reactant = 2 mmol, H2O2 = 11.5 mmol, solvent = 10 mL

The reaction of cyclohexane was carried out using 2 mmol of cyclohexane, 10 mmol of H2O2 and 10 ml of acetonitrile as a solvent with 50 mg of catalyst under being stirred at 60 °C for 3 hours. The products were analyzed by GC-Mass and GC with FID detector. The oxidation products were hardly detected without catalysts. With Fe-containing imogolite as a catalyst, this reaction was promoted and three oxidation products were created. Two compounds were easily identified by retention time of standard reagents such as cyclohexanone and cyclohexanol among these products. Another is speculated as cyclohexyl hydroperoxide from fragmentation patterns in the mass spectrum. It was reported [45] that cyclohexyl hydroperoxide was prepared efficiently and selectively using cyclohexane and H2O2 over Fe3+ ion-changed montmorillonite. We identified one of the oxidation products as cyclohexyl hydroperoxide by the compound obtained following this examination. The conversion of cyclohexane was ca. 25%.

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst 255

produced via cyclohexyl hydroperoxide. The possibility of Fe-imogolite as an oxidation catalyst is shown. However, its reaction mechanism has not been clarified yet. It is necessary

*Department of Chemistry and Biochemistry, Numazu National College of Technology, Numazu,* 

[1] Yoshinaga N., Aomine S. (1962) Imogolite in some Ando soils, Soil Sci. Plant Nutr. 8:

[2] Cradwick P. D. G., Farmer V. C., Rucell J. D., Masson C. R., Wada K., Yoshinaga N., (1972) Imogolite, a Hydrated Aluminum Silicate of Tubular Structure Nature Phys. Sci.

[3] Bursill L. A., Peng J. L., Burgeois L. N. (2000) Imogolite: an aluminosilicate nanotube

[4] Koenderink G.H., Kluijtmans S. G J M, Philipse A. P,(1999) On the Synthesis of

[5] Hu J., Kannangara G.S. K., Wilson M.A., Reddy N., (2004) The fused silicate rote to

[6] Suzuki M., Ohashi F., Inukai K., Maeda M., Tomura S. (2000) Synthesis of Allophane and Imogolite from Inorganic Solution - Influence of Co-Existing lon Concentration and Titration Rate on Forming Precursor -, Nendo Kagaku (J. Clay Sci. Soc. Japan), 40:1-14. [7] Suzuki M., Inukai K., (2010) Synthesis and Applications of Imogolite Nanotubes, in: Kijima T. editors, Inorganic and Metallic Nanotubular Materials, Topics in Applied

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to investigation the catalytic properties and the structure of Fe-imogolite further.

**Author details** 

Masashi Ookawa

**5. References** 

114−121.

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*Japan* 

The cyclohexyl hydroperoxide was produced immediately as soon as the reaction started. Additionally, it was detected only in oxidation reactions with Fe2O3 or -FeOOH as a catalyst. It was found that cyclohexanone and cyclohexanol were produced via cyclohexyl hydroperoxide as an intermediate which was obtained by reacting cyclohexane and H2O2 (Figure 14).

**Figure 14.** Oxidation reactions of cyclohexane using H2O2 over Fe-imogolite [42].

#### **4. Conclusion**

Fe-imogolite was synthesized using Na4SiO4, AlCl3 and FeCl3 with the atomic ratio Fe / (Al+Fe) = 0.05 and applied as a liquid-phase oxidation catalyst with hydrogen peroxide.

The XRD profile and FT-IR spectrum of this material were similar to the synthetic imogolite. AFM images showed fibrous morphology with ca. 2 nm of diameter. UV-VIS and X-ray absorption spectra revealed the state of Fe3+ to be in the octahedral coordination. It was found that Fe-imogolite played as an oxidation catalyst of some hydrocarbons such as cyclohexane, benzene, phenol, toluene and cyclohexane with hydrogen peroxide. The oxidation reaction of cyclohexene was promoted by using Fe-imogolite instead of imogolite as a catalyst. It gave 2-cyclohexene-1-ol, 1,2-epoxycyclohexane, 1,2-cyclohexanediol and 2 cyclohexene-1-one as products. Phenol was produced by the oxidation reaction of benzene. The benzene ring in the aromatic hydrocarbons such as phenol, chlorobenzene, toluene and benzaldehyde was oxidized. Moreover, when the side-chain is a hydrocarbon group, sidechain group was also oxidized. It could be more easily oxidized than the benzene ring. Cyclohexyl hydroperoxide, Cyclohexanone and cyclohexanol were obtained as oxidation products of cyclohexane. It was clarified found that cyclohexanone and cyclohexanol were produced via cyclohexyl hydroperoxide. The possibility of Fe-imogolite as an oxidation catalyst is shown. However, its reaction mechanism has not been clarified yet. It is necessary to investigation the catalytic properties and the structure of Fe-imogolite further.

#### **Author details**

254 Clay Minerals in Nature – Their Characterization, Modification and Application

cyclohexane was ca. 25%.

(Figure 14).

**4. Conclusion** 

The reaction of cyclohexane was carried out using 2 mmol of cyclohexane, 10 mmol of H2O2 and 10 ml of acetonitrile as a solvent with 50 mg of catalyst under being stirred at 60 °C for 3 hours. The products were analyzed by GC-Mass and GC with FID detector. The oxidation products were hardly detected without catalysts. With Fe-containing imogolite as a catalyst, this reaction was promoted and three oxidation products were created. Two compounds were easily identified by retention time of standard reagents such as cyclohexanone and cyclohexanol among these products. Another is speculated as cyclohexyl hydroperoxide from fragmentation patterns in the mass spectrum. It was reported [45] that cyclohexyl hydroperoxide was prepared efficiently and selectively using cyclohexane and H2O2 over Fe3+ ion-changed montmorillonite. We identified one of the oxidation products as cyclohexyl hydroperoxide by the compound obtained following this examination. The conversion of

The cyclohexyl hydroperoxide was produced immediately as soon as the reaction started. Additionally, it was detected only in oxidation reactions with Fe2O3 or -FeOOH as a catalyst. It was found that cyclohexanone and cyclohexanol were produced via cyclohexyl hydroperoxide as an intermediate which was obtained by reacting cyclohexane and H2O2

Fe-imogolite was synthesized using Na4SiO4, AlCl3 and FeCl3 with the atomic ratio Fe / (Al+Fe) = 0.05 and applied as a liquid-phase oxidation catalyst with hydrogen peroxide.

The XRD profile and FT-IR spectrum of this material were similar to the synthetic imogolite. AFM images showed fibrous morphology with ca. 2 nm of diameter. UV-VIS and X-ray absorption spectra revealed the state of Fe3+ to be in the octahedral coordination. It was found that Fe-imogolite played as an oxidation catalyst of some hydrocarbons such as cyclohexane, benzene, phenol, toluene and cyclohexane with hydrogen peroxide. The oxidation reaction of cyclohexene was promoted by using Fe-imogolite instead of imogolite as a catalyst. It gave 2-cyclohexene-1-ol, 1,2-epoxycyclohexane, 1,2-cyclohexanediol and 2 cyclohexene-1-one as products. Phenol was produced by the oxidation reaction of benzene. The benzene ring in the aromatic hydrocarbons such as phenol, chlorobenzene, toluene and benzaldehyde was oxidized. Moreover, when the side-chain is a hydrocarbon group, sidechain group was also oxidized. It could be more easily oxidized than the benzene ring. Cyclohexyl hydroperoxide, Cyclohexanone and cyclohexanol were obtained as oxidation products of cyclohexane. It was clarified found that cyclohexanone and cyclohexanol were

**Figure 14.** Oxidation reactions of cyclohexane using H2O2 over Fe-imogolite [42].

#### Masashi Ookawa

*Department of Chemistry and Biochemistry, Numazu National College of Technology, Numazu, Japan* 

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**Chapter 13** 

© 2012 Sato et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Sato et licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Application of Clay** 

http://dx.doi.org/10.5772/47793

**1. Introduction** 

**Mineral-Iridium(III) Complexes Hybrid** 

Hisako Sato, Kenji Tamura and Akihiko Yamagishi

Additional information is available at the end of the chapter

oxygen molecule in the triplet ground state.

**Langmuir-Blodgett Films for Photosensing** 

There has been an extensive interest in developing photo-responsive devices based on luminescent transition metal complexes (Sato & Yamagishi, 2007). As a promising applicant for emitting composites, cyclometalated iridium(III) complexes are attracting a wide attention due to their highly emitting properties in a visible region (Lo et al., 2011, Ulbricht et al., 2009). The lifetime of the excited triplet states is very long (ca. 1 s) and the quantum yield attains a value as high as 10 ~ 100 %. These iridium(III) complexes are used for photoresponsive molecular devices such as photo-diodes and oxygen sensors (Lowry & Benhard, 2006, Sajoto et al., 2009 ). The attempts are based on the fact that energy transfer takes place efficiently from the triplet excited state of an iridium(III) complex to semiconductors or an

Clay is an environmentally-friendly ubiquitous material. They are characterized by layered structures with cation-exchange properties (Ogawa & Kuroda, 1995). Cationic molecules are intercalated in the narrow galleries between aluminosilicate layers. The materials are used as a host for various types of photochemical reactions. We recently studied the interactions of cationic iridium(III) complexes with a colloidally dispersed clay (Sato et al., 2009, 2011a). The adsorption of iridium(III) complexes by a clay was found to result in the drastic enhancement of emission intensity in an aqueous solution. The attempts demonstrate that emission behavior

Recently the application of clay minerals for photochemical reactions was further extended to thin-film systems. For such purposes, luminescent Langmuir-Blodgett (LB) films were prepared by depositing the monolayers of amphiphilic iridium(III) complexes onto a glass substrate (Sato et al., 2010). The emission properties of a single layered film were studied under vacuum or

often provide a key to monitoring the delicate change of adsorption structures.

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