**2. Location, chemical state, and environment of the incorporated metals**

Catalytic properties of the incorporated transition metals in OMSs supports were mainly attributed to their location, chemical state, and environment. The location of metals in mesoporous silica network is the result of the synthesis method, the intrinsic properties of the incorporated metal, and silica support. The location, the loading, and the properties of the incorporated metals can also influence the support mesoporous structure. Thus, the variation of (100) peak in X-ray diffractograms obtained at a lower angle has been detected in many Me-MCM-41 patterns, indicating the effect of metal species on the ordered mesoporous structure of the support [3, 24, 25]. The higher concentrations of metal species (Me[(OH)n(H2O) m]) at the interface affect the electrostatic interaction between surfactant and silica precursor and the polymerization processes of the silica system in alkaline media (pH 11) during synthesis. In such conditions, the free energy of the mesostructure formation decreases and a mixture of nonstructurated oxides (SiO2–Co3O4) was exhibited [25]. Changing the Si-O-Si bond angle due to incorporation of metal in the mesoporous silica support increases the number of local defects within the mesoporous structure. IR spectroscopy is one of the first techniques that has been used for

**15**

Pt-amino-SBA-15 sample [11].

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

the characterization of materials with metal incorporated in mesoporous silica to obtain information on changes of the Si-OH from surface or regarding appearance of the new vibrations as (Si-O-Me). The change in the Si-O-Si bond angle due to incorporation of Me increases the number of local defects within the mesoporous structure. A much disputed is the change in intensity of the band at 960 cm<sup>−</sup><sup>1</sup>

cating the structural changes in the Si-OH surface due to the presence of metal oxide or to the evolution of new vibrations (Si-O-Me) that appear in the same region [26]. For TiMCM-41 synthesized by surfactant-assisted direct hydrothermal (DHT) method [7, 16, 21], the absence of the characteristic bands for crystalline TiO2 phase in the wide angle XRD patterns reveal that the metal ions were either atomically dispersed in MCM-41 framework or may exist in an amorphous dispersed form on the outside surface of mesoporous support. Diffuse reflectance UV–Vis spectra also revealed distorted tetrahedral environments for titanium inside the MCM-41 matrix or octahedrally coordinated titanium sites, due to the possible hydration effects. The presence of titanium in various mesoporous silica supports in +4 oxidation state was confirmed by ESR and XPS analysis [7, 9, 23]. XPS analysis was used as an additional tool of UV–VIS data since the dispersion of Ti species depends strongly on the synthesis method, properties of the support, and other metals [7, 9, 16]. XRD and spectroscopic results reveal that the titanium was dispersed as titanium ions on SBA-15 silica wall surfaces at low titanium loading, whereas a titanium dioxide anatase film was formed at high titanium loading [16, 23]. Thus, the nature of Ti species on TiMCM-41 s surface, prepared by three different methods, i.e., isomorphous substitution, wet impregnation, and mechanical mixing, was analyzed by means of Raman spectra [27]. The obtained results indicated the presence of anatase for TiO2MCM-41 sample obtained by mechanical mixing. In contrast, no Ramanactive bands were observed on TiMCM-41, obtained by direct synthesis, indicating the absence of surface anatase phase. For TiMCM-41, obtained by impregnation method, no surface anatase phase was detected at a low Ti/Si ratio of 0.3, while surface anatase was detected at high Ti/Si ratios such as 0.6 and 0.9. UV–Vis spectra showed for TiMCM-41 a strong absorbance band at 210 nm and a shoulder band at 260 nm. The first was attributed to isolated Ti atoms in tetrahedral coordination, while the band at 260 nm was attributed to isolated Ti atoms in pentahedral or octahedral coordination. Therefore, it is believed most Ti atoms should substitute Si in the framework or surface in TiMCM-41 with the formation of Ti-O-Si-O-Ti bands. For TiO2MCM-41, adsorption bands at 220, 260, and 320 nm were clearly observed in the UV–vis spectra. The band at 320 nm was typical for bulk titania, indicating the existence of bulk TiO2. The band at 220 was attributed to isolated Ti atoms with distorted tetrahedral environment. These Ti species, dispersed on OMSs support, were supported along with other cations (Ce, V, Nb) or was used as support for another active metal as Ce, Pt, Fe [7, 9, 16, 21, 28]. The second or third [16] metal was evidenced by SEM backscattering and TEM microscopy, as extra framework nanoparticles (**Figure 1**). A good correlation between TEM results (**Figure 1C**) and

H2 chemisorption on Pt nanoparticles' diameter was observed.

[29] and together on the third metal [16]. The presence of Pt0

XPS spectroscopy sustained the interaction of the second metal with titanium

effects of titanium loading and of cerium on its percent were explained by metalsupport interaction considering TiKIT-6 and CeTiKIT-6 samples as supports for Pt (**Figure 2**). Due to Ti and Ce redox properties and strong interaction with noble metals, these metal oxides influence Pt/PtO molar ratio on the catalyst surface. The extended X-ray absorption fine structure measurements evidenced for Pt immobilized on SBA-15, in absence of Ti and Ce species, the presence of Pt–oxygen chemical bonds at the surface. The concentration of these Pt species increased for

on surface and the

indi-

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

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

the characterization of materials with metal incorporated in mesoporous silica to obtain information on changes of the Si-OH from surface or regarding appearance of the new vibrations as (Si-O-Me). The change in the Si-O-Si bond angle due to incorporation of Me increases the number of local defects within the mesoporous structure. A much disputed is the change in intensity of the band at 960 cm<sup>−</sup><sup>1</sup> indicating the structural changes in the Si-OH surface due to the presence of metal oxide or to the evolution of new vibrations (Si-O-Me) that appear in the same region [26].

For TiMCM-41 synthesized by surfactant-assisted direct hydrothermal (DHT) method [7, 16, 21], the absence of the characteristic bands for crystalline TiO2 phase in the wide angle XRD patterns reveal that the metal ions were either atomically dispersed in MCM-41 framework or may exist in an amorphous dispersed form on the outside surface of mesoporous support. Diffuse reflectance UV–Vis spectra also revealed distorted tetrahedral environments for titanium inside the MCM-41 matrix or octahedrally coordinated titanium sites, due to the possible hydration effects. The presence of titanium in various mesoporous silica supports in +4 oxidation state was confirmed by ESR and XPS analysis [7, 9, 23]. XPS analysis was used as an additional tool of UV–VIS data since the dispersion of Ti species depends strongly on the synthesis method, properties of the support, and other metals [7, 9, 16]. XRD and spectroscopic results reveal that the titanium was dispersed as titanium ions on SBA-15 silica wall surfaces at low titanium loading, whereas a titanium dioxide anatase film was formed at high titanium loading [16, 23]. Thus, the nature of Ti species on TiMCM-41 s surface, prepared by three different methods, i.e., isomorphous substitution, wet impregnation, and mechanical mixing, was analyzed by means of Raman spectra [27]. The obtained results indicated the presence of anatase for TiO2MCM-41 sample obtained by mechanical mixing. In contrast, no Ramanactive bands were observed on TiMCM-41, obtained by direct synthesis, indicating the absence of surface anatase phase. For TiMCM-41, obtained by impregnation method, no surface anatase phase was detected at a low Ti/Si ratio of 0.3, while surface anatase was detected at high Ti/Si ratios such as 0.6 and 0.9. UV–Vis spectra showed for TiMCM-41 a strong absorbance band at 210 nm and a shoulder band at 260 nm. The first was attributed to isolated Ti atoms in tetrahedral coordination, while the band at 260 nm was attributed to isolated Ti atoms in pentahedral or octahedral coordination. Therefore, it is believed most Ti atoms should substitute Si in the framework or surface in TiMCM-41 with the formation of Ti-O-Si-O-Ti bands. For TiO2MCM-41, adsorption bands at 220, 260, and 320 nm were clearly observed in the UV–vis spectra. The band at 320 nm was typical for bulk titania, indicating the existence of bulk TiO2. The band at 220 was attributed to isolated Ti atoms with distorted tetrahedral environment. These Ti species, dispersed on OMSs support, were supported along with other cations (Ce, V, Nb) or was used as support for another active metal as Ce, Pt, Fe [7, 9, 16, 21, 28]. The second or third [16] metal was evidenced by SEM backscattering and TEM microscopy, as extra framework nanoparticles (**Figure 1**). A good correlation between TEM results (**Figure 1C**) and H2 chemisorption on Pt nanoparticles' diameter was observed.

XPS spectroscopy sustained the interaction of the second metal with titanium [29] and together on the third metal [16]. The presence of Pt0 on surface and the effects of titanium loading and of cerium on its percent were explained by metalsupport interaction considering TiKIT-6 and CeTiKIT-6 samples as supports for Pt (**Figure 2**). Due to Ti and Ce redox properties and strong interaction with noble metals, these metal oxides influence Pt/PtO molar ratio on the catalyst surface. The extended X-ray absorption fine structure measurements evidenced for Pt immobilized on SBA-15, in absence of Ti and Ce species, the presence of Pt–oxygen chemical bonds at the surface. The concentration of these Pt species increased for Pt-amino-SBA-15 sample [11].

*Redox*

immobilize the transition metal complexes and their heterogenization [19, 20]. These catalysts have attracted much interest due to the desirable characteristics of the silica supports such as narrow pore size, high surface area and large pore volume, tunable mesoporous channels with well-defined pore-size distribution, controllable wall composition, and modifiable surface properties. Pore diameter of mesoporous silicas (2–50 nm) and porous structure are usually tailored by the choice of the template surfactant or the incorporation of swelling agents to expand the surfactant micelles during synthesis [21, 22]. In condition of typical synthesis environment of the mesoporous molecular supports, the incorporation of metal

In order to obtain active catalysts, different active redox metal sites have been introduced into specific locations (mesoporous channels and framework) of the OMSs supports by direct synthesis methods (framework substitution) or post-

coordination geometries and positions (surface, lattice) [16, 23]. In a directsynthesis preparation, the condensations of silicon and metal species around the organic micelles occur simultaneously, and it is likely that some of the metal species are trapped in the silica walls during the formation of OMSs supports. This may influence the unit cell parameters, the wall thickness, and the long-range ordering of the material. By contrast, metal species introduced by a postsynthesis treatment (template ion exchange, impregnation, grafting, chemical vapor deposition methods) are mostly located at the surface of the mesopores and do not modify the internal composition of the silica walls, mainly when the samples are prepared in alcohol. The synthesis method offers the advantage that the dispersion and location of metal species are easily controlled. This may be a great advantage with respect to conventional synthesis methods to prepare materials with specific applications in catalysis. The activity of the obtained materials was demonstrated in various reactions, mostly oxidation reactions of the organic compounds in the liquid or gaseous phase. All the reported results show that the localization of the metal ion, morphology, particle and pore channel sizes and their interaction with the support, and other metal (bimetallic catalysts) influence the oxidation state of the catalytic sites, respectively their redox properties. Therefore, the redox properties of these

materials are the result of the support and metal cation synergistic effect.

**2. Location, chemical state, and environment of the incorporated metals**

Catalytic properties of the incorporated transition metals in OMSs supports were mainly attributed to their location, chemical state, and environment. The location of metals in mesoporous silica network is the result of the synthesis method, the intrinsic properties of the incorporated metal, and silica support. The location, the loading, and the properties of the incorporated metals can also influence the support mesoporous structure. Thus, the variation of (100) peak in X-ray diffractograms obtained at a lower angle has been detected in many Me-MCM-41 patterns, indicating the effect of metal species on the ordered mesoporous structure of the support [3, 24, 25]. The higher concentrations of metal species (Me[(OH)n(H2O) m]) at the interface affect the electrostatic interaction between surfactant and silica precursor and the polymerization processes of the silica system in alkaline media (pH 11) during synthesis. In such conditions, the free energy of the mesostructure formation decreases and a mixture of nonstructurated oxides (SiO2–Co3O4) was exhibited [25]. Changing the Si-O-Si bond angle due to incorporation of metal in the mesoporous silica support increases the number of local defects within the mesoporous structure. IR spectroscopy is one of the first techniques that has been used for

can be simultaneously present in different

methods varies with properties of their precursors.

synthetic methods. In any case, Me*n*<sup>+</sup>

**14**

#### **Figure 1.**

*SEM backscattering (A) of PtTi-SBA-15 (unpublished) and TEM images of TiKIT-6 (B) and PtTiKIT-6 (C) samples (with permission from Ref. [16]).*

**Figure 2.** *XPS spectra Pt-modified KIT-6 mesoporous silica and Pt species atomic percent (with permission from Ref. [16]).*

Another transition metal that is present as active component in OMSs supports was vanadium. V-MCM-41 has received many applications in oxidation reactions [3, 30–32]. For ordered mesoporous V-MCM-41 materials synthesized by DHT method [3, 25, 31], vanadium occurs mainly as isolated tetrahedrally coordinated V5+ species incorporated in the pore wall or anchored to the pore wall. UV–Vis spectra reveal that all the samples prepared with low V contents present well-dispersed V species in the silica network as V5+ species. At high surface vanadium coverage, the species are substantially polymerized. The oxidation of V4+ species in the precursor has also been observed. The change in the UV–Vis spectra after calcination was due to the modification in the oxidation state of vanadium (V5+) from the isolated tetrahedral coordination to its distorted octahedral coordination by coming into contact with the water molecules in the atmosphere [31]. Shylesh et al. [32] reported that UV–Vis spectra of VMCM-41-DHT materials showed vanadium incorporated into the framework positions for VMCM-41 samples, while the greater percentage of active species resides on the surface of VMCM-41, enhancing the formation of higher coordinated vanadium species after calcination. Treating MCM-41 with an aqueous or alcoholic solution of vanadyl acetylacetonate can lead either to a grafting of vanadium entities on the silica surface or to an ion exchange between surfactant molecules and vanadium cations in solution. The UV–Vis spectra of the samples prepared in water or alcohol with low V contents (V/Si < 0.1) showed essentially two absorption bands at 275 and 345 nm. The first was assigned to V5+ species inside the silica walls, whereas those corresponding to the band at 345 nm located on the surface of the mesopores. The presence of internal sites is due to the reorganization of the hexagonal tubular structure of MCM-41 upon hydrothermal treatment, during which vanadium species are allowed to penetrate the silica walls. Thus, vanadium species in the samples obtained by impregnation are dispersed on the wall surface while in the samples obtained by direct synthesis they were fixed in

**17**

the Ta concentration.

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

the mesoporous framework. Therefore, the surface vanadium species supported on silica are well known to possess an isolated and distorted VO4 structure with a single V〓O terminal bond and three V–O–Si bridging bonds anchored on silica support. The distorted V5+ species with the bridging V–O–Si could be found in different silica environments. The formation of vanadium oxide nanodomains has also been evidenced by ESR spectroscopy. A quantitative measurement of the ESR signal intensity shows that the corresponding V4+ species represent only 0.1% of the total V species. The majority of the species are V5+. The others species are VO2+ which are very well dispersed and isolated inside the pore channels of MCM materials. Raman and UV–Vis spectroscopic characterization of V-MCM-41 materials were used [33] to obtain more definitive information about the possible presence of XRDamorphous crystalline V2O5 nanoparticles and surface VO*x* species for samples possessing with higher vanadium content (up to 5.3 wt. %V). DR UV–Vis spectra of Me-MCM-41 (Me = Ti, V, Cr) samples obtained by direct synthesis [34] sustained the framework incorporation of Ti4+ ions in the inorganic silica matrix with tetrahedral or octahedral coordination, vanadium (V5+) isolated species with tetrahedral environments and monochromates, with minor amounts of dichromates as well as polychromate species, respectively. The calcination treatments had changed all the Cr(III) ions to Cr(VI). A large part of these species resided on the surface of silica mesoporous support. These results indicated that the major species formed on the Cr-MCM-41 sample were monochromates, with minor amounts of dichromates as well as polychromate species. The ESR spectrum of as-synthesized chromiumcontaining mesoporous silica indicated, for a large part of chromium, the presence

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

of trivalent chromium (Cr3+) in octahedral coordination.

them. Octahedral coordination dominated in bulk SbVOx.

The surface vanadium species supported on silica were well known to possess an isolated and distorted VO4 structure with a single V〓O terminal bond and three V–O–Si bridging bonds anchored on silica support. The distorted V5+ species with the bridging V–O–Si can be found in different silica environments. Similar studies on the surface Nb5+ species present in Nb–MCM-41 have revealed that the Nb atoms were predominately isolated NbO4 species under dehydration conditions, and surface polymeric niobium species and/or bulk Nb2O5 are formed at high niobium loading on silica [35]. Raman spectra of Ta-MCM-41 mesoporous materials indicated [6] that the incorporation of Ta atom into the MCM-41 structure forms a distorted and isolated [TaO4] surrounded by the [SiO4] tetrahedrons with the presence of Ta–O–Si bridging bonds, and three types of tantalum oxide species: an isolated TaO4 species in the MCM-41 framework, an isolated surface TaO4 species on the MCM-41 surface, and bulk Ta2O5, can be present individually or coexist on the Ta–MCM-41 catalysts, and it's relative intensity was dependent on

The idea of the MCM-41 impregnation with vanadium and antimony sources was to locate Sb-V-Ox species on the high surface area of mesoporous materials with various compositions [36]. Vanadium species in the prepared samples have been estimated by UV–Vis and ESR spectroscopic study. All mesoporous matrices modified with vanadium and antimony gave rise to well-resolve signals in the hyperfine structure of ESR spectra characteristic for isolated VO2+ species. Such a structure was not registered in the case of V/SiO2 sample suggesting that mesoporous support was important for the isolation of oxovanadium species. Tetrahedrally coordinated vanadium (IV) species were deduced from UV–Vis spectra on all prepared samples. They were the only registered species on SbV/NbMCM-41 and SbV/AlMCM-41, whereas on SbV/MCM-41 and SbV/SiO2, octahedral ones were also present besides

There are many studies on cobalt incorporation in mesoporous silica supports. The information about the nature, the co-ordination, and the location of the metal *Catalytic Behavior of Metal Active Sites From Modified Mesoporous Silicas in Oxidation... DOI: http://dx.doi.org/10.5772/intechopen.90209*

the mesoporous framework. Therefore, the surface vanadium species supported on silica are well known to possess an isolated and distorted VO4 structure with a single V〓O terminal bond and three V–O–Si bridging bonds anchored on silica support. The distorted V5+ species with the bridging V–O–Si could be found in different silica environments. The formation of vanadium oxide nanodomains has also been evidenced by ESR spectroscopy. A quantitative measurement of the ESR signal intensity shows that the corresponding V4+ species represent only 0.1% of the total V species. The majority of the species are V5+. The others species are VO2+ which are very well dispersed and isolated inside the pore channels of MCM materials. Raman and UV–Vis spectroscopic characterization of V-MCM-41 materials were used [33] to obtain more definitive information about the possible presence of XRDamorphous crystalline V2O5 nanoparticles and surface VO*x* species for samples possessing with higher vanadium content (up to 5.3 wt. %V). DR UV–Vis spectra of Me-MCM-41 (Me = Ti, V, Cr) samples obtained by direct synthesis [34] sustained the framework incorporation of Ti4+ ions in the inorganic silica matrix with tetrahedral or octahedral coordination, vanadium (V5+) isolated species with tetrahedral environments and monochromates, with minor amounts of dichromates as well as polychromate species, respectively. The calcination treatments had changed all the Cr(III) ions to Cr(VI). A large part of these species resided on the surface of silica mesoporous support. These results indicated that the major species formed on the Cr-MCM-41 sample were monochromates, with minor amounts of dichromates as well as polychromate species. The ESR spectrum of as-synthesized chromiumcontaining mesoporous silica indicated, for a large part of chromium, the presence of trivalent chromium (Cr3+) in octahedral coordination.

The surface vanadium species supported on silica were well known to possess an isolated and distorted VO4 structure with a single V〓O terminal bond and three V–O–Si bridging bonds anchored on silica support. The distorted V5+ species with the bridging V–O–Si can be found in different silica environments. Similar studies on the surface Nb5+ species present in Nb–MCM-41 have revealed that the Nb atoms were predominately isolated NbO4 species under dehydration conditions, and surface polymeric niobium species and/or bulk Nb2O5 are formed at high niobium loading on silica [35]. Raman spectra of Ta-MCM-41 mesoporous materials indicated [6] that the incorporation of Ta atom into the MCM-41 structure forms a distorted and isolated [TaO4] surrounded by the [SiO4] tetrahedrons with the presence of Ta–O–Si bridging bonds, and three types of tantalum oxide species: an isolated TaO4 species in the MCM-41 framework, an isolated surface TaO4 species on the MCM-41 surface, and bulk Ta2O5, can be present individually or coexist on the Ta–MCM-41 catalysts, and it's relative intensity was dependent on the Ta concentration.

The idea of the MCM-41 impregnation with vanadium and antimony sources was to locate Sb-V-Ox species on the high surface area of mesoporous materials with various compositions [36]. Vanadium species in the prepared samples have been estimated by UV–Vis and ESR spectroscopic study. All mesoporous matrices modified with vanadium and antimony gave rise to well-resolve signals in the hyperfine structure of ESR spectra characteristic for isolated VO2+ species. Such a structure was not registered in the case of V/SiO2 sample suggesting that mesoporous support was important for the isolation of oxovanadium species. Tetrahedrally coordinated vanadium (IV) species were deduced from UV–Vis spectra on all prepared samples. They were the only registered species on SbV/NbMCM-41 and SbV/AlMCM-41, whereas on SbV/MCM-41 and SbV/SiO2, octahedral ones were also present besides them. Octahedral coordination dominated in bulk SbVOx.

There are many studies on cobalt incorporation in mesoporous silica supports. The information about the nature, the co-ordination, and the location of the metal

*Redox*

**Figure 1.**

**Figure 2.**

*Ref. [16]).*

*samples (with permission from Ref. [16]).*

Another transition metal that is present as active component in OMSs supports was vanadium. V-MCM-41 has received many applications in oxidation reactions [3, 30–32]. For ordered mesoporous V-MCM-41 materials synthesized by DHT method [3, 25, 31], vanadium occurs mainly as isolated tetrahedrally coordinated V5+ species incorporated in the pore wall or anchored to the pore wall. UV–Vis spectra reveal that all the samples prepared with low V contents present well-dispersed V species in the silica network as V5+ species. At high surface vanadium coverage, the species are substantially polymerized. The oxidation of V4+ species in the precursor has also been observed. The change in the UV–Vis spectra after calcination was due to the modification in the oxidation state of vanadium (V5+) from the isolated tetrahedral coordination to its distorted octahedral coordination by coming into contact with the water molecules in the atmosphere [31]. Shylesh et al. [32] reported that UV–Vis spectra of VMCM-41-DHT materials showed vanadium incorporated into the framework positions for VMCM-41 samples, while the greater percentage of active species resides on the surface of VMCM-41, enhancing the formation of higher coordinated vanadium species after calcination. Treating MCM-41 with an aqueous or alcoholic solution of vanadyl acetylacetonate can lead either to a grafting of vanadium entities on the silica surface or to an ion exchange between surfactant molecules and vanadium cations in solution. The UV–Vis spectra of the samples prepared in water or alcohol with low V contents (V/Si < 0.1) showed essentially two absorption bands at 275 and 345 nm. The first was assigned to V5+ species inside the silica walls, whereas those corresponding to the band at 345 nm located on the surface of the mesopores. The presence of internal sites is due to the reorganization of the hexagonal tubular structure of MCM-41 upon hydrothermal treatment, during which vanadium species are allowed to penetrate the silica walls. Thus, vanadium species in the samples obtained by impregnation are dispersed on the wall surface while in the samples obtained by direct synthesis they were fixed in

*XPS spectra Pt-modified KIT-6 mesoporous silica and Pt species atomic percent (with permission from* 

*SEM backscattering (A) of PtTi-SBA-15 (unpublished) and TEM images of TiKIT-6 (B) and PtTiKIT-6 (C)* 

**16**

species for cobalt- and cobalt-vanadium-modified MCM-41 materials obtained by direct synthesis were obtained by TPR, DR-UV–Vis, and XPS analysis [25]. These methods indicated different localization of the cations in extra-framework positions or in the framework of MCM-41molecular sieves. DR-UV–Vis spectra from **Figure 3A** show two different types of V5+ species. The first one was assigned to isolated tetrahedrally coordinated V5+ species and the second originates from polymeric tetrahedral V5+ species grafted on the walls. According to these results, H2-TPR measurements (**Figure 3B**) suggested that the vanadium interaction with MCM-41 was predominant in VCoMCM-41 samples and pointed to the presence of monomeric or low oligomeric dispersed tetrahedral vanadium species obtained by direct synthesis and the formation of less reducible "polymeric-like" vanadium species by postsynthesis. In the low-loaded cobalt catalysts, Co2+ in tetrahedral position was observed. The increase in metal content led to the appearance of Co3+ in Oh symmetry. In both cases, the cobalt ions were placed outside of the silica framework. In the bimetallic samples, vanadium was incorporated inside the framework of the molecular sieves and on the channel walls. V5+ was in tetrahedral symmetry. In the bimetallic samples, cobalt was presented as Co2+ in Td symmetry. When Co and V were introduced together in the starting gel, a lower quantity of vanadium was incorporated into the mesoporous sieve. At a low vanadium concentration, the essential part of cobalt gives rise to the cobalt silicate phase. The latter was reduced at higher temperature. The rest of cobalt forms CoO particles interacting weakly with the siliceous framework reduced at lower temperature. The peak at 710 K for VCo3 sample was most likely the composite one from the reduction of both cobalt and vanadium species.

Impregnation of MCM-41 and SBA-15 materials using aqueous solutions of cobalt nitrate has a significantly different impact on their ordered mesoporous structures. Thus, aqueous impregnation of MCM-41 followed in the surface area and pore volume. By comparison, SBA-15 mesoporous structure remained almost intact after the introduction of significant amounts of cobalt (up to 20%). The different behavior of these two mesoporous silicas was principally attributed to the different pore wall thickness in MCM-41 and SBA-15. Cobalt oxide-modified SBA-15,

**Figure 3.**

*DR-UV–Vis spectra (A) TPR profiles (B) of VCo-MCM-41 and V-MCM-41 samples (with permission from Ref. [25]).*

**19**

which iron substitutes for aluminum.

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

calcination steps; hence, still retained the ordered structure.

Iron is an interesting metal regarding its properties and applications in catalytic oxidation. In case of its immobilization in mesoporous silica by direct synthesis and hydrothermal treatment [4, 32, 38, 39], most of iron species exist in the tetrahedral coordination located in the support framework. The results obtained from DRUV–Vis, ESR, and XANES showed iron in a mixed environment, indicating that some iron was tetrahedrally coordinated, being sited in the framework, and some iron is present as an extra-framework atom, being octahedrally coordinated [38]. XANES results also suggested that copper was present in the Cu–Al-MCM-41 samples both in the framework and in the extra-framework sites as hydroxide and oxide, respectively. ESR spectra of hierarchical silica structures confirmed the presence of Fe3+ ions with tetrahedral coordination both in framework and extraframework support. For samples with higher loading, the presence of interstitial oxide phase and iron oxide clusters was displayed [28]. Similar results were obtained for copper. XANES results also suggested that copper is present in the Cu–Al-MCM-41 samples both in framework and extra-framework sites as hydroxide and oxide, respectively. The presence of Al3+-sites on the surface of the support provides considerably better dispersion of copper [26, 40]. When comparing ZnAl-MCM-41 with FeAl-MCM-41 samples, the interaction between the metal and the framework atoms (Zn…Si) was different. 27Al-MASNMR results have indicated that zinc is not substituted for aluminum, which means taking the EXAFS results into account that zinc (II) substitutes for silicon (IV) in the framework. The presence of several, and probably different, silicon-sites in the mesoporous framework explains the higher disorder in Zn–Al-MCM-41 compared to Fe–Al-MCM-41, in

KIT-5, and KIT-6 mesoporous silicas with different pore size/pore entrances have been synthesized by a conventional wet impregnation method using cobalt nitrate as the precursor. UV–Vis spectra indicated the formation of Co3O4 particles with different degree of dispersion, which are in different interaction with the support [2]. XPS spectra showed the variation of surface Co dispersion with aging temperature that facilitated cobalt species migration and agglomeration through the larger pores of the silica matrix. The effect of the pore size was less pronounced for the SBA-15 materials, where the straight cylindrical pores with 2-D arrangement probably leads to homogeneous distribution of the loaded cobalt oxide particles along the pore surface. For the 3D structures with interpenetrated cylindrical mesopores (KIT-6) or cage-like mesopores (KIT-5), the formation of homogeneously dispersed spinel Co3O4 species seems to be facilitated in mesoporous silicas with pores larger than 6 nm. TPR-DTG results evidenced the co-existence of three types of Co3O4 particles for all cobalt-modified SBA-15, KIT-5, and KIT-6 materials. The first type was easily reducible, relatively larger species, loosely interacting with the support; the second type represented hardly reducible and well-dispersed fraction in a moderate interaction with the silica, and the third type was very finely dispersed, strongly interacted with the support, which could not be reduced up to 873 K. The pronounced differences were observed for Co/KIT-6 materials [2]. Therefore, Co/KIT-6 samples presented a significant portion of crystalline species that weakly interacted with the support. For the KIT-6 aging to higher temperature, the presence of more inhomogeneously dispersed cobalt oxide particles, which were not completely reduced to metallic cobalt in the temperature interval 500–750 K, was revealed. For these materials, TPR-DTG analysis in correlation with the FTIR measurements supported formation of spinel-type Co3O4 species in the case of silicas with larger mesopores. The incorporation of Co and/or Fe in HSM and SBA-15 was evidenced by association of XRD, TEM, and TPR techniques [37]. Low-angle XRD patterns indicated that the mesoporous supports remained unchanged after metal impregnation and

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

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

KIT-5, and KIT-6 mesoporous silicas with different pore size/pore entrances have been synthesized by a conventional wet impregnation method using cobalt nitrate as the precursor. UV–Vis spectra indicated the formation of Co3O4 particles with different degree of dispersion, which are in different interaction with the support [2]. XPS spectra showed the variation of surface Co dispersion with aging temperature that facilitated cobalt species migration and agglomeration through the larger pores of the silica matrix. The effect of the pore size was less pronounced for the SBA-15 materials, where the straight cylindrical pores with 2-D arrangement probably leads to homogeneous distribution of the loaded cobalt oxide particles along the pore surface. For the 3D structures with interpenetrated cylindrical mesopores (KIT-6) or cage-like mesopores (KIT-5), the formation of homogeneously dispersed spinel Co3O4 species seems to be facilitated in mesoporous silicas with pores larger than 6 nm. TPR-DTG results evidenced the co-existence of three types of Co3O4 particles for all cobalt-modified SBA-15, KIT-5, and KIT-6 materials. The first type was easily reducible, relatively larger species, loosely interacting with the support; the second type represented hardly reducible and well-dispersed fraction in a moderate interaction with the silica, and the third type was very finely dispersed, strongly interacted with the support, which could not be reduced up to 873 K. The pronounced differences were observed for Co/KIT-6 materials [2]. Therefore, Co/KIT-6 samples presented a significant portion of crystalline species that weakly interacted with the support. For the KIT-6 aging to higher temperature, the presence of more inhomogeneously dispersed cobalt oxide particles, which were not completely reduced to metallic cobalt in the temperature interval 500–750 K, was revealed. For these materials, TPR-DTG analysis in correlation with the FTIR measurements supported formation of spinel-type Co3O4 species in the case of silicas with larger mesopores. The incorporation of Co and/or Fe in HSM and SBA-15 was evidenced by association of XRD, TEM, and TPR techniques [37]. Low-angle XRD patterns indicated that the mesoporous supports remained unchanged after metal impregnation and calcination steps; hence, still retained the ordered structure.

Iron is an interesting metal regarding its properties and applications in catalytic oxidation. In case of its immobilization in mesoporous silica by direct synthesis and hydrothermal treatment [4, 32, 38, 39], most of iron species exist in the tetrahedral coordination located in the support framework. The results obtained from DRUV–Vis, ESR, and XANES showed iron in a mixed environment, indicating that some iron was tetrahedrally coordinated, being sited in the framework, and some iron is present as an extra-framework atom, being octahedrally coordinated [38]. XANES results also suggested that copper was present in the Cu–Al-MCM-41 samples both in the framework and in the extra-framework sites as hydroxide and oxide, respectively. ESR spectra of hierarchical silica structures confirmed the presence of Fe3+ ions with tetrahedral coordination both in framework and extraframework support. For samples with higher loading, the presence of interstitial oxide phase and iron oxide clusters was displayed [28]. Similar results were obtained for copper. XANES results also suggested that copper is present in the Cu–Al-MCM-41 samples both in framework and extra-framework sites as hydroxide and oxide, respectively. The presence of Al3+-sites on the surface of the support provides considerably better dispersion of copper [26, 40]. When comparing ZnAl-MCM-41 with FeAl-MCM-41 samples, the interaction between the metal and the framework atoms (Zn…Si) was different. 27Al-MASNMR results have indicated that zinc is not substituted for aluminum, which means taking the EXAFS results into account that zinc (II) substitutes for silicon (IV) in the framework. The presence of several, and probably different, silicon-sites in the mesoporous framework explains the higher disorder in Zn–Al-MCM-41 compared to Fe–Al-MCM-41, in which iron substitutes for aluminum.

*Redox*

and vanadium species.

species for cobalt- and cobalt-vanadium-modified MCM-41 materials obtained by direct synthesis were obtained by TPR, DR-UV–Vis, and XPS analysis [25]. These methods indicated different localization of the cations in extra-framework positions or in the framework of MCM-41molecular sieves. DR-UV–Vis spectra from **Figure 3A** show two different types of V5+ species. The first one was assigned to isolated tetrahedrally coordinated V5+ species and the second originates from polymeric tetrahedral V5+ species grafted on the walls. According to these results, H2-TPR measurements (**Figure 3B**) suggested that the vanadium interaction with MCM-41 was predominant in VCoMCM-41 samples and pointed to the presence of monomeric or low oligomeric dispersed tetrahedral vanadium species obtained by direct synthesis and the formation of less reducible "polymeric-like" vanadium species by postsynthesis. In the low-loaded cobalt catalysts, Co2+ in tetrahedral position was observed. The increase in metal content led to the appearance of Co3+ in Oh symmetry. In both cases, the cobalt ions were placed outside of the silica framework. In the bimetallic samples, vanadium was incorporated inside the framework of the molecular sieves and on the channel walls. V5+ was in tetrahedral symmetry. In the bimetallic samples, cobalt was presented as Co2+ in Td symmetry. When Co and V were introduced together in the starting gel, a lower quantity of vanadium was incorporated into the mesoporous sieve. At a low vanadium concentration, the essential part of cobalt gives rise to the cobalt silicate phase. The latter was reduced at higher temperature. The rest of cobalt forms CoO particles interacting weakly with the siliceous framework reduced at lower temperature. The peak at 710 K for VCo3 sample was most likely the composite one from the reduction of both cobalt

Impregnation of MCM-41 and SBA-15 materials using aqueous solutions of cobalt nitrate has a significantly different impact on their ordered mesoporous structures. Thus, aqueous impregnation of MCM-41 followed in the surface area and pore volume. By comparison, SBA-15 mesoporous structure remained almost intact after the introduction of significant amounts of cobalt (up to 20%). The different behavior of these two mesoporous silicas was principally attributed to the different pore wall thickness in MCM-41 and SBA-15. Cobalt oxide-modified SBA-15,

*DR-UV–Vis spectra (A) TPR profiles (B) of VCo-MCM-41 and V-MCM-41 samples (with permission from* 

**18**

**Figure 3.**

*Ref. [25]).*

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

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 with silica support.

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 on HMS silica [45]. Thus, through Raman spectroscopy, isolated [WO4] <sup>2</sup><sup>−</sup> or low 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

**21**

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

the external surface, which blocked the pores partially, thereby reducing the surface area. By adjusting the nH2O/nHCl molar ratio, Sn was incorporated into the lattice of SBA-15 at a low Sn concentration [46]. The Sn4+ ions exhibited both tetrahedral and octahedral coordination depending upon the location of these ions either on the walls of the silica or in the corona region of the structure, respectively. The existence of isolated oxide species that have degraded the ordered structure of the silica support and especially the formation of the oxide agglomerations in the pores or on the external surface has been highlighted for other metals with large diameter

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 brought a new aspect of creating metal-functionalized OMSs [51]. 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. The synergistic effect between loccation, its dispersion, and mesoporous ordered silica structure on the metal electronic properties and catalytic needed development of the advanced characterization techniques.

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,

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

(Ru, La) but very active in oxidation reactions [24, 47–50].

**3. Catalytic oxidation of organic compounds**

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

the external surface, which blocked the pores partially, thereby reducing the surface area. By adjusting the nH2O/nHCl molar ratio, Sn was incorporated into the lattice of SBA-15 at a low Sn concentration [46]. The Sn4+ ions exhibited both tetrahedral and octahedral coordination depending upon the location of these ions either on the walls of the silica or in the corona region of the structure, respectively. The existence of isolated oxide species that have degraded the ordered structure of the silica support and especially the formation of the oxide agglomerations in the pores or on the external surface has been highlighted for other metals with large diameter (Ru, La) but very active in oxidation reactions [24, 47–50].

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 brought a new aspect of creating metal-functionalized OMSs [51]. 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. The synergistic effect between loccation, its dispersion, and mesoporous ordered silica structure on the metal electronic properties and catalytic needed development of the advanced characterization techniques.
