**3. Catalytic oxidation of organic compounds**

The introduction of the metal cations in the mesoporous silica generated both acid and redox centers depending on their charge and their chemical properties. In oxidation reactions, these properties determine both the activity and the selectivity of the catalyst. To introduce the redox active sites in the OMSs, various transition metals have been chosen. The supports like M41S, SBA-n, and KIT-n families modified by incorporation of one, two, or more transitional metals such as Ti, V, Cr, Fe, Co, Ni, Mn, Cu, La, Ru, Ni–Ru, Cr–Ni, V–Cu, and V–Co created materials with new redox and acidic properties. The introduction of active transition metal into the framework of molecular sieves creates isolated metal sites and these centers are believed for their exceptional catalytic activity. Their catalytic properties were influenced by localization and surroundings of the metal ions. The high dispersion of metals on a support with high surface area, large pore diameter, and uniform pore size distribution determined the formation of new active centers with redox properties different from those of the oxide in the agglomerated form. This explained the increased interest in them and their applications as catalysts.

**Table 1** shows a wide range of metals incorporating in mesoporous silica supports with catalytic applications in liquid phase oxidation of organic compounds, with H2O2 or *tert*-butyl hydroperoxide, and gas phase with O2 from air. Various publications have shown the effects of metals and their associations with silica support and other metal on catalytic activity and selectivity. Thus, a high variety of transition-metals incorporated in mesoporous silica showed interesting catalytic properties in oxidation of organic compounds. Among them, vanadium and titanium were mostly used both single as well as associated with other metals. Vanadium-containing mesoporous materials are found to be active in liquid-phase oxidation reactions as oxidation of cyclohexane to cyclohexanone and cyclohexanol [31], oxidation of aromatic hydrocarbons and alcohols [3] using H2O2 as oxidant. V-MCM-41 catalysts exhibited low activity in the oxidation of alcohols but higher activity and selectivity in oxidation of cyclohexene and aromatic hydrocarbons. This suggested the association of vanadium with another metal more suitable for other oxidation reactions [21, 24]. V-TiMCM-41, V-CoMCM-41 catalysts were used in oxidation of aromatic hydrocarbons and alcohols [21, 24]. In these reactions, FeMCM-41, CoMCM-41, NiMCM-41 [3], NbMCM-41, Nb-TiMCM-41, Co-(Nb,

*Redox*

The incorporation of Ce, another trivalent metal, within the MCM-41 was favored by the greater flexibility of the silica network. However, the size incompatibility between Ce3+ and Si4+ ions led to longer Si\O\bonds and caused the strain bond angle in the substituted silica network. Also, the incorporation of Ce induced a drastic reduction in mesopore ordination. These results were probably due to partial substitution of the structural Si4+ for the Ce3+ ion, resulting a substantial change in the textural properties of the hexagonal structure of MCM-41 [41]. The Si/Ce molar ratio is a key factor influencing the textural properties and structural regularity of CeMCM-41 mesoporous molecular sieves. As well, XRD, UV–Vis, and XPS spectra evidenced the presence of cerium species as tetra-coordinated Ce4+/ Ce3+ and the formation of CeO2. This was in accord with results obtained on cerium incorporated in SBA-15 [42] and KIT-6 mesoporous silica [16]. The effect of pH on SBA-15 ordered hexagonal structure and incorporation of Ce species was evidenced [42]. For the samples synthesized at pH = 10.0, the position of Ce species was

High metal dispersion and incorporation of Ni in MCM-41 framework was evidenced for lower metal loading. Typical XRD diffractograms for ordered hexagonal mesoporous structure, obtained at small angle, evidenced decreasing of structural regularity with metal loading. Considering UV–Vis of NiO as reference, a distorted tetrahedral environment was observed for the most of Ni species in these MCM-41 materials. The effect of Zr4+ on Ni2+ local symmetry and the presence of distorted tetrahedral Ti species were evidenced by UV–Vis for Ni-ZrMCM-41 and Ni-TiMCM-41 bimetallic samples. Thus, a small shoulder at around 293 nm was assigned to penta- or octahedral coordinated Ti species, resulting from the interaction of Ti species with Ni species [27]. For Ni–MnMCM-41 sample, it was assumed that both tetrahedral and octahedral Mn3+ species co-exist. Mn3+ was evidenced both in tetrahedral and octahedral coordination. The results obtained for bimetal samples were compared with them with single metal. Such for Mn-MCM-41 sample, XRD patterns showed the absence of the diffraction peaks of the MnOx species suggesting that a strong interaction between MnOx and silica matrix exists because most of the Mn3+ or Mn2+ cations were either incorporated into the silica framework or highly dispersed on the silica walls. The TPR results on Mn-MCM-41 samples [43] indicated the coexistence of different manganese species. In the samples of different pore dimensions and manganese loadings prepared by impregnation, the nature of the species, identified as well dispersed, strongly interacting with silica surface was similar. In the case of samples prepared by the hydrothermal method, the effect of pore dimensions was more complex. Narrow pores of silica materials caused the formation of small species strongly interacting with silica surface or incorporated into the framework. An increase in Mn loading and pore diameter favored formation of larger particles weakly interacting with silica support. It was observed that the presence of small oxide species of the size partially controlled by pore dimension or preparation method, and simultaneously not strongly interacting

The incorporation of larger species into the silica framework was hindered and the formation of extra-framework oxide species was favored. Regarding the incorporation of tungsten species into the MCM-41 framework, there is a critical value for the Si/W ratio of about 30. In the case of smaller Si/W ratio, the formation of extra-framework tungsten oxide species was observed [44]. Variation of the Si/W ratio and the synthesis method has led to various species of W immobilized

<sup>2</sup><sup>−</sup> or low

on HMS silica [45]. Thus, through Raman spectroscopy, isolated [WO4]

condensed oligomeric framework species were displayed. Tin is another metal with redox properties and large size which forms SnO2 clusters distributed on the external pore structure. SnO2 agglomerates were highlighted in the channels or on

evidenced as deposits only on the surface of SBA-15.

**20**

with silica support.


#### **Table 1.**

*Catalytic applications of modified mesoporous silica ordered networks with transitional metal.*

La)MCM-41 [21, 24, 50] and Ru-(Cr, Ni, Cu)MCM-41, La-(Co, Mn)MCM-41 [49], and WHMS were also used as catalysts [45]. It was interesting to note that samples which are active in the oxidation of styrene to benzaldehyde would be less active in the oxidation of benzene to phenol and vice versa, suggesting that active centers for oxidation of styrene might probably be different to those for oxidation of benzene. Reaction data showed that the oxidation activity is higher when H2O2 is used as an oxidant, acetonitrile as solvent, and V-MCM-41 as catalyst. However, the selectivity toward the desired keto derivatives (ethyl benzene to acetophenone and diphenyl methane to benzophenone) follows the order, Ti-MCM-41 > V-MCM-41 > Cr-MCM-41 [34]. The vanadium content in catalysts was evidenced as a key factor in the oxidation of styrene and the conversion increases with metal loading. However, for hydroxylation of benzene, catalysts with an ordered mesostructure presented higher catalytic activity and the V content only serves as secondary role.

**23**

**Figure 4.**

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation...*

It can be concluded that high vanadium content favored styrene conversion and higher ordering of mesostructure led to high hydroxylation of benzene. The higher activity of vanadium-incorporated MCM-41 compared to vanadium-grafted MCM-41 may be due to the presence of active isolated tetrahedral-coordinated vanadium ions in the framework positions. The lower activity was a result of the V–O–V bond formed for vanadium-grafted MCM-41. The difference in the selectivity of as-synthesized and calcined vanadium-grafted MCM-41 showed that apart from the active redox sites, the nature of hydrophilic–hydrophobic interactions still play an important role in selective oxidation reactions. **Figure 4** depicts the variation in styrene conversion and reaction rate as a function of reaction time. The conversion and reaction rate in oxidation of styrene were modified by the introduction mode of the H2O2 into reaction medium in order to find the optimal reaction conditions. The higher conversion and reaction rates were obtained in the first period of reaction when H2O2 was introduced after adsorption the step (samples VTi-2 and NbTi-2). These results sustained the effect of the adsorption step on oxidation reaction confirmed, over time, by photocatalytic reactions, other oxidation processes in which both Ti and V catalysts or other transition metals with redox properties were used immobilized on mesoporous silica [7–10, 28]. An induction period was needed (VTi-1, NbTi-1catalyst) in oxidation of aromatic hydrocarbons when the oxidant, reagent, catalyst, and solvent were mixed together at the beginning of the reaction and effect of the second metal was significant. The effect of the second metal on the properties of catalysts was evidenced for others materials. Firstly, the introduction

The increasing of Co/V molar ratio (decreasing V amount) led to increasing of the catalytic activity. The highest conversion was obtained for Co/V molar ratio of 3.0. On the other hand, the effect of the second metal on the reactivity was different for styrene and benzene. The introduction of V or Nb into CoMCM-41 molecular sieves decreased or increased the conversion of styrene. The effect of La on CoMCM-41 was quite surprising since the activity for oxidation of styrene and benzene decreased, implying the inhibitory effect of La for oxidation of aromatics [21, 24, 50]. The main products of reaction were benzaldehyde, for oxidation of styrene, and phenol for oxidation of benzene. After the first utilization, the separated and dried catalysts were reused. The catalytic activity of these reused catalysts increased in the second cycle of reaction and decreased after the third. It was observed that the selectivity decreased gradually with reaction time and in the second cycle of reaction. Some

*Variation of the styrene conversion (A) and of the reaction rate (B) as a function of reaction time (h). VTi-1 and NbTi-1: the defined amount of H2O2 was introduced at the beginning of the reaction and VTi-2 and NbTi-2: the defined amount of H2O2 was divided in different portions and introduced step by step ((×) VTi-1,* 

*(*●*) VTi-2, (*■*) NbTi-1, (*▲*) NbTi-2) (with permission from Ref. [21]).*

*DOI: http://dx.doi.org/10.5772/intechopen.90209*

of V and La reduced sharply the conversion of styrene.

#### *Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation... DOI: http://dx.doi.org/10.5772/intechopen.90209*

It can be concluded that high vanadium content favored styrene conversion and higher ordering of mesostructure led to high hydroxylation of benzene. The higher activity of vanadium-incorporated MCM-41 compared to vanadium-grafted MCM-41 may be due to the presence of active isolated tetrahedral-coordinated vanadium ions in the framework positions. The lower activity was a result of the V–O–V bond formed for vanadium-grafted MCM-41. The difference in the selectivity of as-synthesized and calcined vanadium-grafted MCM-41 showed that apart from the active redox sites, the nature of hydrophilic–hydrophobic interactions still play an important role in selective oxidation reactions. **Figure 4** depicts the variation in styrene conversion and reaction rate as a function of reaction time. The conversion and reaction rate in oxidation of styrene were modified by the introduction mode of the H2O2 into reaction medium in order to find the optimal reaction conditions.

The higher conversion and reaction rates were obtained in the first period of reaction when H2O2 was introduced after adsorption the step (samples VTi-2 and NbTi-2). These results sustained the effect of the adsorption step on oxidation reaction confirmed, over time, by photocatalytic reactions, other oxidation processes in which both Ti and V catalysts or other transition metals with redox properties were used immobilized on mesoporous silica [7–10, 28]. An induction period was needed (VTi-1, NbTi-1catalyst) in oxidation of aromatic hydrocarbons when the oxidant, reagent, catalyst, and solvent were mixed together at the beginning of the reaction and effect of the second metal was significant. The effect of the second metal on the properties of catalysts was evidenced for others materials. Firstly, the introduction of V and La reduced sharply the conversion of styrene.

The increasing of Co/V molar ratio (decreasing V amount) led to increasing of the catalytic activity. The highest conversion was obtained for Co/V molar ratio of 3.0. On the other hand, the effect of the second metal on the reactivity was different for styrene and benzene. The introduction of V or Nb into CoMCM-41 molecular sieves decreased or increased the conversion of styrene. The effect of La on CoMCM-41 was quite surprising since the activity for oxidation of styrene and benzene decreased, implying the inhibitory effect of La for oxidation of aromatics [21, 24, 50]. The main products of reaction were benzaldehyde, for oxidation of styrene, and phenol for oxidation of benzene. After the first utilization, the separated and dried catalysts were reused. The catalytic activity of these reused catalysts increased in the second cycle of reaction and decreased after the third. It was observed that the selectivity decreased gradually with reaction time and in the second cycle of reaction. Some

#### **Figure 4.**

*Redox*

**Catalyst Reaction**

VMCM-41, FeMCM-41, CoMCM-41,

VMCM-41, NbMCM-41, V-TiMCM-41, Nb-TiMCM-41, Co-(V,Nb,La)MCM-41,

Ru-(Cr, Ni, or Cu) MCM-41,La-(Co or

Pt-SBA-15, Pt-NH2SBA-15, Pt-CoSBA-15,

TiKIT-6, Pt-TiKIT-6, Ce-TiKIT-6,

([Cu(acac)(phen) (H2O)](ClO4), [Cu(acac)(Me2bipy)](ClO4)) on HSM or

CuImph (Imph = bis(4-imidazolyl methyl)

NiMCM-41

V-CoMCM-41

Mn) MCM-41

Pt-Co-NH2SBA-15

Pt-Ce-TiKIT-6,

NH2HMS

**Table 1.**

benzylamine) on MSNs

VMCM-41 Oxidation of cyclohexane to cyclohexanone and

V-MoMCM-41, Cu-FeMCM-41 Oxidation of o-xylene to phthalic anhydride in air [1],

MeMCM-41 (Me = V, Fe, Co, Ni), WHMS Oxidation of styrene and benzene with H2O2 [4, 45]

CeMCM-41, CeKIT-6 Hydroxylation of 1-naphthol with H2O2 or *tert*-butyl

M (M = Al, Zr, W, B, or P) MCM-41 Oxidation of ethane [40]

PtSBA-15/PtSiO2 Oxidation of toluene with O2 [12] CoSBA-15, CoKIT-5, CoKIT-6 Ethyl acetate total oxidation [2] LaKIT-6, La-BKIT-6 Oxidation of styrene with H2O2 [15]

M/KIT-6 (M = Mn, Cu, Fe, Cr, Sn) Catalytic combustion of chlorobenzene [18]

[3, 4, 30, 32]

MMCM-41 (M = Ti, V, Cr) Oxidation of ethyl benzene and diphenyl methane with H2O2 [34] TaMCM-41 Oxidative dehydrogenation of propane and oxidation of methanol [6]

with H2O2 [11, 30]

Methane in air [16]

Oxidation of toluene in air [19]

and N2O as oxidant) [33]

oxidation of adamantane with H2O2 [39]

cyclohexanol [31], oxidative dehydrogenation of propane (O2

Oxidation of aromatic hydrocarbons and alcohols with H2O2

Oxidation of aromatic hydrocarbons and alcohols [21, 24]

Oxidation of aromatic hydrocarbons with H2O2 [49, 50]

hydroperoxide [41], oxidation of cyclohexene [52]

3-Butene-1-ol, *cis*-2-butene-1, 4-diol, cyclohexene oxidation

Oxidation of aromatic compounds (anisole, phenol) with

**22**

La)MCM-41 [21, 24, 50] and Ru-(Cr, Ni, Cu)MCM-41, La-(Co, Mn)MCM-41 [49], and WHMS were also used as catalysts [45]. It was interesting to note that samples which are active in the oxidation of styrene to benzaldehyde would be less active in the oxidation of benzene to phenol and vice versa, suggesting that active centers for oxidation of styrene might probably be different to those for oxidation of benzene. Reaction data showed that the oxidation activity is higher when H2O2 is used as an oxidant, acetonitrile as solvent, and V-MCM-41 as catalyst. However, the selectivity toward the desired keto derivatives (ethyl benzene to acetophenone and diphenyl methane to benzophenone) follows the order, Ti-MCM-41 > V-MCM-41 > Cr-MCM-41 [34]. The vanadium content in catalysts was evidenced as a key factor in the oxidation of styrene and the conversion increases with metal loading. However, for hydroxylation of benzene, catalysts with an ordered mesostructure presented higher catalytic activity and the V content only serves as secondary role.

*Catalytic applications of modified mesoporous silica ordered networks with transitional metal.*

H2O2 [20]

*Variation of the styrene conversion (A) and of the reaction rate (B) as a function of reaction time (h). VTi-1 and NbTi-1: the defined amount of H2O2 was introduced at the beginning of the reaction and VTi-2 and NbTi-2: the defined amount of H2O2 was divided in different portions and introduced step by step ((×) VTi-1, (*●*) VTi-2, (*■*) NbTi-1, (*▲*) NbTi-2) (with permission from Ref. [21]).*

products of polycondensation resulted by polymerization of formaldehyde and/ or benzaldehyde were detected after long time of reaction, a possible reason for the decrease in selectivity with reaction time. The characterization of the used and reused materials confirmed, in many cases, the good stability of bimetallic molecular sieves. TEM images of the used catalysts showed a well-ordered structure with a hexagonal arrangement of channels, indicating no effect of the reaction on the support structure after first and second reaction [24]. The IR study of adsorbed phase in discharged catalysts after first cycle reaction of styrene and benzene upon desorption at a series of temperatures (293, 373, 623, and 723 K) was made [50]. The presence of different aromatic species such as benzaldehyde and styrene glycol was observed. These species adsorbed very strongly in the catalyst since a complete desorption of these species can be made only after desorption at 723 K. The strong adsorption of these species was confirmed by thermal analysis.

The association of trivalent cations Ru or La with other transition metals modifies the activity and selectivity of the monometallic molecular sieves [24, 49]. The results evidenced that RuMCM-41 and LaMCM-41 presented good conversion in oxidation of styrene but very low activity for benzene hydroxylation. While all the bimetallic catalysts, except Ru-NiMCM-41 and La-CoMCM-41, have a higher activity and efficiency of the H2O2 (H2O2 quantity used for oxidation/H2O2 quantity transformed) in the benzene hydroxylation. In the styrene oxidation, only RuCr-MCM-41 gives a good conversion. Generally, excepting LaCo-MCM-41, the conversion of catalysts in oxidation of benzene was higher than that in oxidation of styrene. This behavior was specific for these bimetallic samples since all the monometallic modified MCM-41 catalysts by incorporation of the same transition metals have a higher activity in oxidation of styrene and lower conversion in hydroxylation of benzene. LaCo-MCM-41 catalyst has a very low activity and RuCr-MCM-41 catalyst has a high activity both in the styrene and benzene oxidation. Under all investigated experimental conditions for oxidation of styrene, the main reaction products detected by GC analysis were epoxy ethyl benzene (styrene oxide), phenyl ethanediol (styrene glycol), and benzaldehyde.

More transition ions were incorporated into MCM-41, HMS, SBA-15 materials, and tested in liquid-phase oxidation reactions (**Table 1**). The best activity in oxidation of benzene was obtained for Ti-MCM-41 and in oxidation of styrene for Cr-MCM-41 and CrNi-MCM-41 samples. Their activity decreases with increasing of number of 3d electrons of the metal ions. After immobilization and interaction with support and another metal, it was highlighted that increasing of their oxidation state leads to the growth of their activity. Ti, V, Cr, and Mn ions were more active. After immobilization and interaction with support and another metal, an increase of their oxidation state has been observed. The obtained redox molecular sieves by direct synthesis incorporation of tungsten into hexagonal mesoporous silica (HMS) were tested in oxidation of styrene with hydrogen peroxide. The influence of the synthesis parameters, such as, chemical composition of the gels, surfactant, precursors, and time of the hydrothermal treatment, on the structure, morphology, nature of metal species, and catalytic properties has been examined. The best results were obtained for the catalysts synthesized by oxo-polyoxo mode: ([WO(O2)2(H2O)2]/ H2O/H3O+ /surfactant/Si(OR)4 in which surfactant was cetylpyridine chloride [45]. The catalysts with more ordered structure, higher surface area, and [WO4] <sup>2</sup><sup>−</sup> species strongly bounded or high dispersed on silica have a higher activity. Conversion and selectivity to benzaldehyde decreased with Si/W molar ratio (**Figure 5**). The higest conversion of styrene to benzaldehyde has been evidenced the sample with the highest amount of isolated W sites and higher surface area.

In addition to applications in the oxidation reactions [11, 24, 31], cobalt-based catalysts were considered as a suitable alternative to the high cost of catalysts based

**25**

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation...*

on noble metals. The catalysts obtained by immobilization of Co on mesoporous silica were used for degradation of toxic compounds from the exhausted automobile and industrial emissions via combustion. Cobalt-modified SBA-15, KIT-5, and KIT-6 mesoporous silicas with different pore sizes/pore entrances have been synthesized and their catalytic activities in total oxidation of ethyl acetate were evaluated [2]. The optimal size of Co3O4 particles and their homogeneous distribution in the porous support are of primary importance for the catalytic activity. Silicas with larger pores facilitate the mass transfer during the sample preparation procedure and lead to more homogeneous distribution of cobalt oxide particles inside the pore system. The Co3O4 particle growth is facilitated by 3D structures with interpenetrating-(KIT-6) or cage-like (KIT-5) pores and much less promoted by 2D arranged

*Variation of conversion and selectivity with Si/W molar ratio (with permission from Ref. [45]).*

Furthermore, more open pore structures could facilitate the catalytic process due to enhanced mass transfer, as the catalytic activity of CoKIT-6 materials is

Cerium-containing materials have been used as catalysts for selective oxidation

of organic compounds. The liquid-phase oxidation of cyclohexane was carried out over Ce-KIT-6 with various Si/Ce molecular sieves at temperature between 70 and 90°C. Although the catalyst contains both Ce3+ and Ce4+, as evidenced from DR-UV analysis, the main active sites that activate H2O2 were considered to be Ce4+ [52]. Ce4+ was suggested to activate hydrogen peroxide by coordination to oxidize cyclohexane, and cyclohexanol was found to be the major product (74%). Cyclohexyl acetate, as the major secondary product, was considered result of Bronsted acidity generated by Ce3+. Cerium and titanium oxides, immobilized on KIT-6 silica, were used as supports for Pt. The supported Pt nanoparticles on mesoporous silica possess the ability to strongly dissociate toluene to benzene and hydrocarbon fragments (CHx) [12]. The metal–support interaction was evaluated in terms of TiO2 loading and ceria effect on titanium oxide under condition of their dispersion on silica [16]. The highest activity was obtained in oxidation of CH4 for all the catalyst samples containing Pt respectively for PT5K6 sample with lower dispersion of Pt and higher diameter of particles (**Figure 6**). Two opposite effects determined in this case a very small decrease in the CH4 conversion degree

*DOI: http://dx.doi.org/10.5772/intechopen.90209*

straight pores of SBA-15 support.

**Figure 5.**

generally lower than that of CoSBA-15 and CoKIT-5.

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation... DOI: http://dx.doi.org/10.5772/intechopen.90209*

**Figure 5.** *Variation of conversion and selectivity with Si/W molar ratio (with permission from Ref. [45]).*

on noble metals. The catalysts obtained by immobilization of Co on mesoporous silica were used for degradation of toxic compounds from the exhausted automobile and industrial emissions via combustion. Cobalt-modified SBA-15, KIT-5, and KIT-6 mesoporous silicas with different pore sizes/pore entrances have been synthesized and their catalytic activities in total oxidation of ethyl acetate were evaluated [2]. The optimal size of Co3O4 particles and their homogeneous distribution in the porous support are of primary importance for the catalytic activity. Silicas with larger pores facilitate the mass transfer during the sample preparation procedure and lead to more homogeneous distribution of cobalt oxide particles inside the pore system. The Co3O4 particle growth is facilitated by 3D structures with interpenetrating-(KIT-6) or cage-like (KIT-5) pores and much less promoted by 2D arranged straight pores of SBA-15 support.

Furthermore, more open pore structures could facilitate the catalytic process due to enhanced mass transfer, as the catalytic activity of CoKIT-6 materials is generally lower than that of CoSBA-15 and CoKIT-5.

Cerium-containing materials have been used as catalysts for selective oxidation of organic compounds. The liquid-phase oxidation of cyclohexane was carried out over Ce-KIT-6 with various Si/Ce molecular sieves at temperature between 70 and 90°C. Although the catalyst contains both Ce3+ and Ce4+, as evidenced from DR-UV analysis, the main active sites that activate H2O2 were considered to be Ce4+ [52]. Ce4+ was suggested to activate hydrogen peroxide by coordination to oxidize cyclohexane, and cyclohexanol was found to be the major product (74%). Cyclohexyl acetate, as the major secondary product, was considered result of Bronsted acidity generated by Ce3+. Cerium and titanium oxides, immobilized on KIT-6 silica, were used as supports for Pt. The supported Pt nanoparticles on mesoporous silica possess the ability to strongly dissociate toluene to benzene and hydrocarbon fragments (CHx) [12]. The metal–support interaction was evaluated in terms of TiO2 loading and ceria effect on titanium oxide under condition of their dispersion on silica [16]. The highest activity was obtained in oxidation of CH4 for all the catalyst samples containing Pt respectively for PT5K6 sample with lower dispersion of Pt and higher diameter of particles (**Figure 6**). Two opposite effects determined in this case a very small decrease in the CH4 conversion degree

*Redox*

confirmed by thermal analysis.

ethanediol (styrene glycol), and benzaldehyde.

products of polycondensation resulted by polymerization of formaldehyde and/ or benzaldehyde were detected after long time of reaction, a possible reason for the decrease in selectivity with reaction time. The characterization of the used and reused materials confirmed, in many cases, the good stability of bimetallic molecular sieves. TEM images of the used catalysts showed a well-ordered structure with a hexagonal arrangement of channels, indicating no effect of the reaction on the support structure after first and second reaction [24]. The IR study of adsorbed phase in discharged catalysts after first cycle reaction of styrene and benzene upon desorption at a series of temperatures (293, 373, 623, and 723 K) was made [50]. The presence of different aromatic species such as benzaldehyde and styrene glycol was observed. These species adsorbed very strongly in the catalyst since a complete desorption of these species can be made only after desorption at 723 K. The strong adsorption of these species was

The association of trivalent cations Ru or La with other transition metals modifies the activity and selectivity of the monometallic molecular sieves [24, 49]. The results evidenced that RuMCM-41 and LaMCM-41 presented good conversion in oxidation of styrene but very low activity for benzene hydroxylation. While all the bimetallic catalysts, except Ru-NiMCM-41 and La-CoMCM-41, have a higher activity and efficiency of the H2O2 (H2O2 quantity used for oxidation/H2O2 quantity transformed) in the benzene hydroxylation. In the styrene oxidation, only RuCr-MCM-41 gives a good conversion. Generally, excepting LaCo-MCM-41, the conversion of catalysts in oxidation of benzene was higher than that in oxidation of styrene. This behavior was specific for these bimetallic samples since all the monometallic modified MCM-41 catalysts by incorporation of the same transition metals have a higher activity in oxidation of styrene and lower conversion in hydroxylation of benzene. LaCo-MCM-41 catalyst has a very low activity and RuCr-MCM-41 catalyst has a high activity both in the styrene and benzene oxidation. Under all investigated experimental conditions for oxidation of styrene, the main reaction products detected by GC analysis were epoxy ethyl benzene (styrene oxide), phenyl

More transition ions were incorporated into MCM-41, HMS, SBA-15 materials, and tested in liquid-phase oxidation reactions (**Table 1**). The best activity in oxidation of benzene was obtained for Ti-MCM-41 and in oxidation of styrene for Cr-MCM-41 and CrNi-MCM-41 samples. Their activity decreases with increasing of number of 3d electrons of the metal ions. After immobilization and interaction with support and another metal, it was highlighted that increasing of their oxidation state leads to the growth of their activity. Ti, V, Cr, and Mn ions were more active. After immobilization and interaction with support and another metal, an increase of their oxidation state has been observed. The obtained redox molecular sieves by direct synthesis incorporation of tungsten into hexagonal mesoporous silica (HMS) were tested in oxidation of styrene with hydrogen peroxide. The influence of the synthesis parameters, such as, chemical composition of the gels, surfactant, precursors, and time of the hydrothermal treatment, on the structure, morphology, nature of metal species, and catalytic properties has been examined. The best results were obtained for the catalysts synthesized by oxo-polyoxo mode: ([WO(O2)2(H2O)2]/

/surfactant/Si(OR)4 in which surfactant was cetylpyridine chloride [45].

<sup>2</sup><sup>−</sup> spe-

The catalysts with more ordered structure, higher surface area, and [WO4]

the highest amount of isolated W sites and higher surface area.

cies strongly bounded or high dispersed on silica have a higher activity. Conversion and selectivity to benzaldehyde decreased with Si/W molar ratio (**Figure 5**). The higest conversion of styrene to benzaldehyde has been evidenced the sample with

In addition to applications in the oxidation reactions [11, 24, 31], cobalt-based catalysts were considered as a suitable alternative to the high cost of catalysts based

**24**

H2O/H3O+

#### **Figure 6.**

*CH4 conversion and its variation with temperature for catalysts with different composition: Supported on T5K6 (a), CPTnK6 (B), and TnK6 (C) (with permission from Ref. [16]).*

compared to PT5K6 catalyst. The higher concentration of Pt2+on the catalyst surface (CPT4K6 samples) and absence of noble metal (CT5K6, T5K6 samples) decreased the catalytic activity and a slow growth of conversion with temperature was observed. A good correlation was obtained between catalytic activity and reducibility of the catalysts for samples with Ti and Ce. The higher conversion degree of CH4 was obtained for PtTi-KIT-6 samples between 250 and 400°C. KIT-6 was an interesting support for other transition metals such as Mn, Cu, Fe, Cr, Sn, Ln, and the obtained catalysts were used in oxidative degradation of various organic pollutants from air (toluene, chlorobenzene) [18].

### **4. Incorporated metal active sites with redox properties**

It can be considered that the interest on catalytic performances obtained by incorporating titanium into the zeolite (TS-1 catalyst) and the advantages of silica mesoporous molecular sieves have generated research on the metal-functionalized ordered mesoporous silicas. These catalysts have attracted much attention in the past few years for selective oxidation and oxidative photodegradation of large organic molecules. This explains the particular interest in the synthesis and use of catalysts based on titanium immobilized on various mesoporous silica substrates. The active center of TS-1 catalyst in the selective oxidation reactions was considered the titanium-isolated sites. A systematic study on the function of the different titanium species in TS-1 [53] concluded that the framework Ti species in TS-1 were the active sites for propylene epoxidation, while the nonframework Ti species were responsible for the further conversion of propylene oxide to propylene glycol and methoxy-2-propanol. Similar to TS-1, the remarkably catalytic properties of the metal-functionalized ordered mesoporous silicas result from the isolation of the metal centers in the framework. The high surface area of mesoporous sieves and the presence of ordered arrays of mesopores provide new opportunities for transition metal incorporation. It is possible to obtain metal heteroatoms as framework tetrahedral T atoms, bound in defect sites of the framework, anchored to the surface, extra-framework counter ions or extraframework oxides.

**27**

**Figure 7.**

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation...*

The isolated heterogeneous single-site catalysts have been attracted great interest in diverse catalytic reactions because of their uniform and distinct geometric and electronic structure [54]. They have great potential to integrate the distinct advantages of homogeneous and heterogeneous catalysts into a single counterpart. The imobilization of active metals in the specific locations of ordered mesoporous silicas by direct synthesis routes with the help of organic groups of surfactants opened a new path in creating of metal-functionalized OMSs. Although the strong interactions between active metals and support were obtained, the controllable morphology and structure of OMSs synthesized by these direct synthesis routes have not been well developed. It is still difficult to understand the relationship between the structural morphology of OMSs and the electronic structure of active species, thus the development of advanced characterization techniques was necesary. Similar to other supported single sites [54], it can be considered that their catalytic activity is due to the following which allows: the ordered arrangement of silica support mesoporous structure; the high dispersion of metal sites, located in framework and extra-framework support, or covalently bounded on the pore surface functionalized with different ligands; the better contact of single site with the reactants thus generating catalysts with high activity and excellent selectivity; the ordered porosity can provide a special environment for the substrate interaction with the catalytic active sites which can

Considering Ti (IV) species to be the active sites in TiMCM-41 catalyst in the oxidation of cyclohexene to cyclohexene epoxide, a mechanism in three steps was proposed for surface reaction [55]. The main reaction step involved the reaction of Ti single sites with H2O2 to form titanium hydroperoxide, which further reacts with cyclohexene molecules to form cyclohexene epoxide in a concerted manner. It was evidenced that titanium-isolated species from silica framework or surface were the active species. According to titanium location, two Ti-peroxide (η<sup>1</sup>

intermediates wherein Ti binds 1, respectively both oxygen atoms of the peroxide

mainly responsible for the further conversion of propane oxide.

and η<sup>2</sup>

The stability and reactivity of Ti-peroxide intermediates was affected by solvent coordination. The best results were obtained in acetonitrile. The extra-framework Ti species were amorphous or crystalline TiO2. These species had a negative effect on the yield of propane oxide. The amorphous Ti species were more acidic and

The mechanisms proposed for oxidation of organic compounds with H2O2 on vanadium-modified mesoporous silica supports proposed also the formation of

activity of other immobilized metals. Thus, the specific catalytic activity (per one Cu2+-active site accessible to the reactants) depended strongly on the structure of

*The oxidation mechanism proposed for possible isolated Ti4+ active sites immobilized on mesoporous silica.*

). These results were also generalized for the

 and η<sup>2</sup> )

*DOI: http://dx.doi.org/10.5772/intechopen.90209*

further enhance the activity and selectivity.

(**Figure 7**), have been identified.

V-peroxide intermediates (η<sup>1</sup>

#### *Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation... DOI: http://dx.doi.org/10.5772/intechopen.90209*

The isolated heterogeneous single-site catalysts have been attracted great interest in diverse catalytic reactions because of their uniform and distinct geometric and electronic structure [54]. They have great potential to integrate the distinct advantages of homogeneous and heterogeneous catalysts into a single counterpart. The imobilization of active metals in the specific locations of ordered mesoporous silicas by direct synthesis routes with the help of organic groups of surfactants opened a new path in creating of metal-functionalized OMSs. Although the strong interactions between active metals and support were obtained, the controllable morphology and structure of OMSs synthesized by these direct synthesis routes have not been well developed. It is still difficult to understand the relationship between the structural morphology of OMSs and the electronic structure of active species, thus the development of advanced characterization techniques was necesary. Similar to other supported single sites [54], it can be considered that their catalytic activity is due to the following which allows: the ordered arrangement of silica support mesoporous structure; the high dispersion of metal sites, located in framework and extra-framework support, or covalently bounded on the pore surface functionalized with different ligands; the better contact of single site with the reactants thus generating catalysts with high activity and excellent selectivity; the ordered porosity can provide a special environment for the substrate interaction with the catalytic active sites which can further enhance the activity and selectivity.

Considering Ti (IV) species to be the active sites in TiMCM-41 catalyst in the oxidation of cyclohexene to cyclohexene epoxide, a mechanism in three steps was proposed for surface reaction [55]. The main reaction step involved the reaction of Ti single sites with H2O2 to form titanium hydroperoxide, which further reacts with cyclohexene molecules to form cyclohexene epoxide in a concerted manner. It was evidenced that titanium-isolated species from silica framework or surface were the active species. According to titanium location, two Ti-peroxide (η<sup>1</sup> and η<sup>2</sup> ) intermediates wherein Ti binds 1, respectively both oxygen atoms of the peroxide (**Figure 7**), have been identified.

The stability and reactivity of Ti-peroxide intermediates was affected by solvent coordination. The best results were obtained in acetonitrile. The extra-framework Ti species were amorphous or crystalline TiO2. These species had a negative effect on the yield of propane oxide. The amorphous Ti species were more acidic and mainly responsible for the further conversion of propane oxide.

The mechanisms proposed for oxidation of organic compounds with H2O2 on vanadium-modified mesoporous silica supports proposed also the formation of V-peroxide intermediates (η<sup>1</sup> and η<sup>2</sup> ). These results were also generalized for the activity of other immobilized metals. Thus, the specific catalytic activity (per one Cu2+-active site accessible to the reactants) depended strongly on the structure of

*Redox*

**Figure 6.**

compared to PT5K6 catalyst. The higher concentration of Pt2+on the catalyst surface (CPT4K6 samples) and absence of noble metal (CT5K6, T5K6 samples) decreased the catalytic activity and a slow growth of conversion with temperature was observed. A good correlation was obtained between catalytic activity and reducibility of the catalysts for samples with Ti and Ce. The higher conversion degree of CH4 was obtained for PtTi-KIT-6 samples between 250 and 400°C. KIT-6 was an interesting support for other transition metals such as Mn, Cu, Fe, Cr, Sn, Ln, and the obtained catalysts were used in oxidative degradation of various

*CH4 conversion and its variation with temperature for catalysts with different composition: Supported on T5K6* 

It can be considered that the interest on catalytic performances obtained by incorporating titanium into the zeolite (TS-1 catalyst) and the advantages of silica mesoporous molecular sieves have generated research on the metal-functionalized ordered mesoporous silicas. These catalysts have attracted much attention in the past few years for selective oxidation and oxidative photodegradation of large organic molecules. This explains the particular interest in the synthesis and use of catalysts based on titanium immobilized on various mesoporous silica substrates. The active center of TS-1 catalyst in the selective oxidation reactions was considered the titanium-isolated sites. A systematic study on the function of the different titanium species in TS-1 [53] concluded that the framework Ti species in TS-1 were the active sites for propylene epoxidation, while the nonframework Ti species were responsible for the further conversion of propylene oxide to propylene glycol and methoxy-2-propanol. Similar to TS-1, the remarkably catalytic properties of the metal-functionalized ordered mesoporous silicas result from the isolation of the metal centers in the framework. The high surface area of mesoporous sieves and the presence of ordered arrays of mesopores provide new opportunities for transition metal incorporation. It is possible to obtain metal heteroatoms as framework tetrahedral T atoms, bound in defect sites of the framework, anchored to the surface, extra-framework counter ions or extra-

organic pollutants from air (toluene, chlorobenzene) [18].

*(a), CPTnK6 (B), and TnK6 (C) (with permission from Ref. [16]).*

**4. Incorporated metal active sites with redox properties**

**26**

framework oxides.

the localized site [40]. Isolated Cu2+-sites grafted to Al-MCM-41 showed relatively high activity for the sample calcined at lower temperature. Thermal treatment at higher temperatures caused a sharp loss of specific Cu2+ catalytic activity of CuAl-MCM-41 as result of copper oxide agglomeration.

If the organic substrate was introduced in the first step of the reaction, it was adsorbed on the surface and reacted more rapidly with the Me-peroxide intermediate formed by the addition of H2O2 in the second step [21]. The presence of the organic compound on the surface prevented H2O2 decomposition more in the presence of extra-framework oxides that favor its decomposition. Depending of their loading and dispersion, the activity of these metal extra-framework species was similar with that of clusters or bulk oxides. For Ru- and La-incorporated MCM-41 molecular silica, the ratio of metal and oxygen ion radii (*R*M*(n*<sup>+</sup> *)/R*O*(*2<sup>−</sup>*) >* 0*.*5), an important structural parameter, showed a little possibility to be incorporated metals in the silica framework [49]. For many of these catalysts, the efficiency of the H2O2 (H2O2 quantity used for oxidation/H2O2 quantity transformed) was very low. These catalysts had a good conversion in oxidation of styrene but very low for benzene hydroxylation. Their association with metals with smaller diameter as Co or Mn led to a higher activity and efficiency of the H2O2.

The hydrophobicity of metal-functionalized ordered mesoporous silicas was improved, in many cases, by functionalization of surface with organic groups in direct (co-condensation) or postsynthetic silylation treatment. The coexistence of metal cations with other organic or inorganic species favored the distribution on the pore surface of catalytic active species, thus favoring the access of the reactants during the chemical reaction. However, in this case, the metal species formed oxide agglomerations, which led to the decreasing of the conversion. Thus, for LaKIT-6 catalyst, the coexistence of B hindered the dispersion or incorporation of La into the framework [15]. La3+ species in the framework of LaKIT-6 were favorable to improve the catalytic performance of LaKIT-6 for the oxidation of styrene. When H2O2 was used as an oxidant, LaBKIT-6 showed much lower styrene conversion than LaKIT-6 catalyst with similar product selectivity, which was ascribed to the formation on surface of La2O3 nanocrystals.

The controllable dispersion of metal single sites and a higher hydrophobicity were obtained by immobilization of the metal complexes on mesoporous silica supports. A number of mono- and binuclear metal complexes have been investigated as biomimetic catalysts for organic compound oxidation. New biomimetic catalysts were obtained by immobilization of [Cu(AcAc)(Phen)(H2O)]ClO4, [Cu(AcAc) (Me2bipy)] ClO4 complexes on HMS or NH2-functionalized mesoporous silica [20]. The copper-substituted mesoporous silica was a good catalyst for oxidation of aromatic compounds or a very good support for biomimetic oxidation catalyst due to the possible interactions between the metallic ions of the biomimetic complex and the stabilized cuprous species in the silica framework. The immobilization on different mesoporous silica supports of (Schiff base) copper(II) or Mn(II) complexes was synthesized and applied in oxidation of alcohols in acetonitrile and H2O2. Comparing the effect of silica supports on catalytic activity, the higher performances of the metal complexes supported on MCM-41 were evidenced. A higher catalytic activity was obtained for Mn complexes, especially in oxidation of cyclohexene in conditions of significant lower M/L ratio and metal content. The most probable mechanism for attachment of the aminopropyl groups to the surface of mesoporous silicas was proposed through siloxane linkages (Si–O–Si) between the silicon of the aminopropylsilane group and the surface silicon atoms. It has been assumed the possibility that each aminopropylsilane silicon attaches to the internal surface of the silica via one, as well two and/or three siloxane linkages. **Figure 8** proposes the metal complex structure immobilized on SBA-15 mesoporous support.

**29**

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation...*

The stability and heterogeneous nature of these catalysts in the oxidation of different substrates have been investigated. The oxidation in the liquid phase with organic hydroperoxides or even H2O2 was often the subject of leaching phenomena and the question about the true nature of the catalytic reaction (homogeneous or

*Copper single site supported by complexation with Schiff-base ligands obtained from 2-hydroxyacetopheneone* 

The photocatalytic activity of supported transition metals (Ti, V, Ce, Fe, Cu, Au, Ag, Ta, Nb) on mesoporous silica supports in mono- or bimetallic photocatalysts was evaluated in degradation of various organic pollutants from water or air. Among the various mesoporous silica materials, MCM-48 and KIT-6 were several advantages in photocatalysis due to their cubic arrangement of the threedimensional mesopores and their considerable amount of surface silanol groups. The photocatalytic properties of CeTi-MCM-48 photocatalysts were evaluated by phenol photodegradation under UV irradiation at two different wavelengths and the photocatalytic activity was correlated with the active metallic distribution, speciation, and their immobilization method [7, 8]. The incorporation of Ce in the Ti-MCM-48 framework allowed activation by UV light, generating more electrons and holes in the photocatalytic redox reactions and a better photoactivity for phenol. The presence of redox couple Ce4+/Ce3+ exchaged the surface oxygen vacancies that could be transformed in oxygen radicals with formation of superoxide radicals. Many other photocatalysts were obtained by immobilization of the photoactive transition metals on various mesoporous silica supports, and their activity was evaluated in oxidation of organic polutants from water as dyes, functionalized 8-hydroxyquinolinate, antibiotics [9, 10, 28, 56]. In all of these, the active photoactive metallic sites were Ti4+, Co2+, Fe3+, Al3+, Zn2+, Eu3+, Tb3+, Er3+, Nd3+ with various

In case of the catalysts used in gas-phase reactions, dispersion, specific surface area, and reducibility was observed as the main factors influencing the catalytic activity. In combustion of chlorobenzene (CB), catalytic activity of metalmodified KIT-6 (M = Mn, Cu, Fe, Cr, Sn) mesoporous catalysts was basically in line with their redox properties, with the exception of CrKIT-6, which differs from this trend due to the aggregation of Cr species in the pore structure and surface of the KIT-6 [18]. The proposed mechanism was follows: CB was adsorbed to the catalyst by physical adsorption, then the adsorbed CB molecule dissociated

dispersion on MCM-41, SBA-15 or hierarchical silica supports.

*DOI: http://dx.doi.org/10.5772/intechopen.90209*

heterogeneous) was a serious one.

*covalently attached to amino-functionalized SBA-15 (unpublished).*

**Figure 8.**

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation... DOI: http://dx.doi.org/10.5772/intechopen.90209*

**Figure 8.**

*Redox*

the localized site [40]. Isolated Cu2+-sites grafted to Al-MCM-41 showed relatively high activity for the sample calcined at lower temperature. Thermal treatment at higher temperatures caused a sharp loss of specific Cu2+ catalytic activity of CuAl-

If the organic substrate was introduced in the first step of the reaction, it was adsorbed on the surface and reacted more rapidly with the Me-peroxide intermediate formed by the addition of H2O2 in the second step [21]. The presence of the organic compound on the surface prevented H2O2 decomposition more in the presence of extra-framework oxides that favor its decomposition. Depending of their loading and dispersion, the activity of these metal extra-framework species was similar with that of clusters or bulk oxides. For Ru- and La-incorporated MCM-41

important structural parameter, showed a little possibility to be incorporated metals in the silica framework [49]. For many of these catalysts, the efficiency of the H2O2 (H2O2 quantity used for oxidation/H2O2 quantity transformed) was very low. These catalysts had a good conversion in oxidation of styrene but very low for benzene hydroxylation. Their association with metals with smaller diameter as Co or Mn led

The hydrophobicity of metal-functionalized ordered mesoporous silicas was improved, in many cases, by functionalization of surface with organic groups in direct (co-condensation) or postsynthetic silylation treatment. The coexistence of metal cations with other organic or inorganic species favored the distribution on the pore surface of catalytic active species, thus favoring the access of the reactants during the chemical reaction. However, in this case, the metal species formed oxide agglomerations, which led to the decreasing of the conversion. Thus, for LaKIT-6 catalyst, the coexistence of B hindered the dispersion or incorporation of La into the framework [15]. La3+ species in the framework of LaKIT-6 were favorable to improve the catalytic performance of LaKIT-6 for the oxidation of styrene. When H2O2 was used as an oxidant, LaBKIT-6 showed much lower styrene conversion than LaKIT-6 catalyst with similar product selectivity, which was ascribed to the forma-

The controllable dispersion of metal single sites and a higher hydrophobicity were obtained by immobilization of the metal complexes on mesoporous silica supports. A number of mono- and binuclear metal complexes have been investigated as biomimetic catalysts for organic compound oxidation. New biomimetic catalysts were obtained by immobilization of [Cu(AcAc)(Phen)(H2O)]ClO4, [Cu(AcAc) (Me2bipy)] ClO4 complexes on HMS or NH2-functionalized mesoporous silica [20]. The copper-substituted mesoporous silica was a good catalyst for oxidation of aromatic compounds or a very good support for biomimetic oxidation catalyst due to the possible interactions between the metallic ions of the biomimetic complex and the stabilized cuprous species in the silica framework. The immobilization on different mesoporous silica supports of (Schiff base) copper(II) or Mn(II) complexes was synthesized and applied in oxidation of alcohols in acetonitrile and H2O2. Comparing the effect of silica supports on catalytic activity, the higher performances of the metal complexes supported on MCM-41 were evidenced. A higher catalytic activity was obtained for Mn complexes, especially in oxidation of cyclohexene in conditions of significant lower M/L ratio and metal content. The most probable mechanism for attachment of the aminopropyl groups to the surface of mesoporous silicas was proposed through siloxane linkages (Si–O–Si) between the silicon of the aminopropylsilane group and the surface silicon atoms. It has been assumed the possibility that each aminopropylsilane silicon attaches to the internal surface of the silica via one, as well two and/or three siloxane linkages. **Figure 8** proposes the metal complex structure immobilized on SBA-15 mesoporous support.

*)/R*O*(*2<sup>−</sup>*) >* 0*.*5), an

MCM-41 as result of copper oxide agglomeration.

to a higher activity and efficiency of the H2O2.

tion on surface of La2O3 nanocrystals.

molecular silica, the ratio of metal and oxygen ion radii (*R*M*(n*<sup>+</sup>

**28**

*Copper single site supported by complexation with Schiff-base ligands obtained from 2-hydroxyacetopheneone covalently attached to amino-functionalized SBA-15 (unpublished).*

The stability and heterogeneous nature of these catalysts in the oxidation of different substrates have been investigated. The oxidation in the liquid phase with organic hydroperoxides or even H2O2 was often the subject of leaching phenomena and the question about the true nature of the catalytic reaction (homogeneous or heterogeneous) was a serious one.

The photocatalytic activity of supported transition metals (Ti, V, Ce, Fe, Cu, Au, Ag, Ta, Nb) on mesoporous silica supports in mono- or bimetallic photocatalysts was evaluated in degradation of various organic pollutants from water or air. Among the various mesoporous silica materials, MCM-48 and KIT-6 were several advantages in photocatalysis due to their cubic arrangement of the threedimensional mesopores and their considerable amount of surface silanol groups. The photocatalytic properties of CeTi-MCM-48 photocatalysts were evaluated by phenol photodegradation under UV irradiation at two different wavelengths and the photocatalytic activity was correlated with the active metallic distribution, speciation, and their immobilization method [7, 8]. The incorporation of Ce in the Ti-MCM-48 framework allowed activation by UV light, generating more electrons and holes in the photocatalytic redox reactions and a better photoactivity for phenol. The presence of redox couple Ce4+/Ce3+ exchaged the surface oxygen vacancies that could be transformed in oxygen radicals with formation of superoxide radicals. Many other photocatalysts were obtained by immobilization of the photoactive transition metals on various mesoporous silica supports, and their activity was evaluated in oxidation of organic polutants from water as dyes, functionalized 8-hydroxyquinolinate, antibiotics [9, 10, 28, 56]. In all of these, the active photoactive metallic sites were Ti4+, Co2+, Fe3+, Al3+, Zn2+, Eu3+, Tb3+, Er3+, Nd3+ with various dispersion on MCM-41, SBA-15 or hierarchical silica supports.

In case of the catalysts used in gas-phase reactions, dispersion, specific surface area, and reducibility was observed as the main factors influencing the catalytic activity. In combustion of chlorobenzene (CB), catalytic activity of metalmodified KIT-6 (M = Mn, Cu, Fe, Cr, Sn) mesoporous catalysts was basically in line with their redox properties, with the exception of CrKIT-6, which differs from this trend due to the aggregation of Cr species in the pore structure and surface of the KIT-6 [18]. The proposed mechanism was follows: CB was adsorbed to the catalyst by physical adsorption, then the adsorbed CB molecule dissociated

on the active sites (mainly metal oxides) via nucleophilic attacks on C–Cl bond. The results obtained showed decreasing catalytic activity in the following order: MnKIT-6 > CrKIT-6 > CuKIT-6 > FeKIT-6 > Sn/KIT-6, in correlation with redox properties of the catalysts. Therefore, the best catalytic combustion activity of MnKIT-6 catalysts benefited from their large specific surface area, good Mn dispersion, better reducibility, and large amount of chemisorbed oxygen species. However, as the content of Mn species increasing, due to increased agglomeration of Mn, more MnOx clusters gave rise to pore plugging and cover adsorption sites, leading to weaker catalytic activity. Mesoporous silica KIT-6 with 3D interconnected mesopores offered a confined environment, a better dispersion of the oxide phase, and a faster diffusion of the reactants and products. The Mo/KIT-6, Fe/KIT-6, and Mo–Fe/KIT-6 were tested as potential alternatives to noble metals in the conversion of MCP [57]. Only the isolated tetrahedral Fe ions and highly dispersed small FeOx nanoclusters were responsible for the endocyclic C-C bond rupture at substituted C-C. The synergy between Fe and Mo was not observed at low temperatures and for total conversion but only for ring-opening reactions at higher temperature.

In particular, the good performance of the Ti(IV)-doped SBA-15-supported catalysts in CH4 oxidation was due to the combination of Ti(IV) structurally incorporated into the silica lattice and present as surface-dispersed TiO2 particles. The negative effect of the Ti(IV) over the HMS-supported catalysts was related to the high acidity induced by the more homogeneous incorporation of Ti(IV) into the silica structure. The loss of catalyst activity during CH4 oxidation reaction was not necessarily related to the sintering of the active sites but rather to the variation of the oxygen storage capacity and oxygen mobility strictly related to the support properties. For PdTi-SBA-15 catalysts, combustion of methane occurs through a redox or Mars van Krevelen mechanism was accepted [58]. Accordingly, during this reaction, PdO was locally reduced to Pd by methane, producing H2O and CO2, and then Pd was reoxidized by oxygen. Although the active species was considered PdO, it was widely recognized that metallic Pd plays the important role in decomposing and activating the methane molecule. Similar conclusions were obtained also for PtTiKIT-6 catalysts [16]. Therefore, the efficiency of all process was related to the redox properties of the catalyst and to the oxygen mobility. The maintenance of the activity, in spite of the significant palladium oxide sintering, was attributed to the good interaction between surface titania and silica. Such interaction favored the formation of Ti–O–Si linkages with an increase of the oxidizing potential of the Ti(IV) cations. In the presence of a support metal interaction, the increase of the PdO particle size [58], respectively PtO [16] after the long reaction is not detrimental. Another effect of metal–support interaction was observed for Ce-TiKIT-6 samples. In this case, two factors were influencing the Pt activity: dispersion and size of the platinum species and their concentration on the surface. Two opposite effects determined a decreasing in the CH4 conversion. The higher concentration of Pt2+ and lower Pt0 concentration on the catalyst surface. A good correlation was obtained between catalytic activity, and reducibility of the catalysts was also obtained for samples with Ti and Ce supported on KIT-6.

### **5. Conclusions**

It can be concluded that OMSs containing transition metals showed extremely promising redox properties in the oxidation of organic compounds with larger molecules thanks to the mesoporous support. The applications and performances of the redox couple diversity resulted by association of transition metals on an increasing

**31**

**Author details**

Viorica Parvulescu

Bucharest, Romania

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation...*

Institute of Physical Chemistry "Ilie Murgulescu" of the Romanian Academy,

© 2019 The Author(s). Licensee IntechOpen. This chapter is 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,

\*Address all correspondence to: vpirvulescu@icf.ro

provided the original work is properly cited.

diversity of mesoporous or hierarchical ordered structures based on silica offer new research directions in this field. However, the decoration of multiple metal active species and their synergistic interactions on the surface of mesoporous silica remain

*DOI: http://dx.doi.org/10.5772/intechopen.90209*

to be explored in the future.

*Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation... DOI: http://dx.doi.org/10.5772/intechopen.90209*

diversity of mesoporous or hierarchical ordered structures based on silica offer new research directions in this field. However, the decoration of multiple metal active species and their synergistic interactions on the surface of mesoporous silica remain to be explored in the future.
