A. Tuel

Modification of mesoporous silicas by incorporation of heteroelements in the framework

Mesoporous silicas (HMS) prepared at room temperature using primary alkylamines as surfactant molecules have been modified by incorporation of heteroelements in the silica framework. As for zeolites, the modification of the framework provides materials with interesting properties in catalysis. The extent of incorporation as well as the stability of the products towards calcination depends not only on the nature of the incorporated element, but also strongly on the synthesis conditions. Materials with high substitution levels can be relatively easily obtained in their as-synthesized form, but direct calcination in air often leads to a partial degradation of the mesoporous structure with extraction of some of the metal species. This can be circumvented by removing the organic phase by solvent extraction, which prevents undesirable framework modifications usually observed upon thermal treatment. The determination of the nature, location and coordination of metal species in mesoporous silicas is complicated due to the non-crystalline nature of these materials and by the fact that they can readily change with the degree of hydration of the sample. Moreover, in contrast to zeolites, the discrimination between species located on the internal surface of the mesopores and inside the silica walls is not always trivial by means of conventional spectroscopic techniques. This paper presents a piece of the work that has been achieved over the last years in the field of synthesis and characterization of mesoporous silicas modified by different metals. For each metal, examples of specific catalytic reactions are given and briefly discussed.

Metallophthalocyanine functionalized silicas : catalysts for the selective oxidation of aromatic compounds

Abstract Metallophthalocyanines of iron, manganese and cobalt have been successfully anchored onto the surface of mesoporous and amorphous silicas and used as catalysts in the liquid phase oxidation of 2-methylnaphthalene and 2,3,6-trimethylphenol with hydrogen peroxide and t -butylhydroperoxide. The iron tetrasulfophthalocyanine showed an activity from far superior to that of the other metallic complexes and to the corresponding homogeneous catalysts. The activity and selectivity depend on the nature of the solvent, oxidizing agent and the catalyst itself. In particular, iron tetrasulfophthalocyanine anchored in the dimeric form yielded catalyst much more active and selective that those containing monomeric species, which was unexpected since dimeric species are usually considered as inactive in homogeneous systems. The catalytic properties of these materials were compared with those of Ti containing silica-based molecular sieves. The latter were active in the oxidation of 2,3,6-trimethylphenol but not in that of 2-methylnaphthalene. Iron (III) peroxo species, formed by reaction of the peroxide with iron phthalocyanine were proposed to undergo a homolytic or heterolytic cleavage of O–O bond in case of the monomeric and dimeric complexes, respectively, to explain the difference in their catalytic properties.

Hydroxylation of phenol over TS-1: Surface and solvent effects

Abstract Titano-Silicalite 1 (TS-1) has been used as a catalyst for the hydroxylation of phenol with hydrogen peroxide. The reaction was carried out in different organic solvents in order to investigate the solvent effect and its influence on the product distribution during the oxidation. Furthermore, the contribution of the external surface sites of TS-1 to the catalytic activity is discussed. It is suggested that the surface sites — outside the zeolite channels — as well as the gradual poisoning of these sites are responsible for the differences observed at low conversions.

Synthesis and characterization of Ti-containing mesoporous silicas

Ti-containing mesoporous silicas have been synthesized at ambient temperature using amines with alkyl chains of 10–16 carbon atoms as templating surfactant. Following this synthesis route, it was possible to prepare materials with more than 2 wt% Ti without forming extrawall TiO2. Physicochemical characterization of the samples showed that Ti atoms were probably tetrahedrally coordinated, in a configuration similar to that observed on TiSi mixed oxides. In contrast to data reported previously, these materials could be obtained within less than 15 min and were thermally stable at 650°C in air. The pore dimensions of Ti-HMS could be varied by changing the amine chain length. Materials with very high surface areas were obtained using relatively low surfactant/SiO2 ratios. Increasing this ratio in the synthesis gel did not modify the symmetry of the mesoporous silica as reported for M41S materials but drastically decreased its surface area.

Synthesis, characterization, and catalytic properties of the new TiZSM-12 zeolite

A novel titanium-containing zeolite TS-12 with the MTW-type structure has been synthesized under hydrothermal conditions in the presence of hexamethylene bis(diethylmethylammonium) cation (Et2MeN+C3H6)2 as templating agent. Physicochemical characterization of the samples showed that Ti atoms were part of the framework, probably in a tetrahedral coordination for low Ti contents. These zeolites have been shown to be active catalysts in the epoxidation of cyclohexene with various peroxides as oxidant.

Comparison between TS-1 and TS-2 in the hydroxylation of phenol with hydrogen peroxide

Abstract The hydroxylation of phenol with hydrogen peroxide has been carried out over a series of titanium silicalites. Results have shown that the activity of the catalyst strongly depended on the synthesis route and on the crystal size. For TS-1 and TS-2 samples with similar titanium contents and crystal sizes, the influence of the zeolite structure was not observed.

EPR evidence for the isomorphous substitution of titanium in silicalite structure

TS-1 zeolite has been synthesized following the literature procedure and characterized via X-ray diffraction, IR spectrometry and 29Si NMR and shown to match previous findings. In addition titanium-exchanged HZSM-5 was prepared. Both solids were examined by EPR spectroscopy following CO reduction. Whereas the titanium-exchanged ZSM-5 exhibited the usual EPR signal consistent with trivalent titanium in tetragonally distorted octahedral symmetry with g⊥ = 1.97 and g∥ = 1.94, the Ti-substituted silicalite exhibited an axially symmetric signal with a split g∥ component (g∥)1 = 1.98, (g∥)2 = 1.97 and g⊥ = 1.913 best interpreted as arising from a trivalent titanium in a tetrahedral coordination, indicating that the precursor tetravalent ion was indeed in a T site. Reactivity towards O2 was also monitored and differences in the gzz values for O2− adsorbed on titanium confirm the different crystal fields prevailing at each titanium siting. It is also concluded that reduction of framework titanium with CO does not result in its removal from the lattice.

Synthesis, characterization, and catalytic properties of titanium silicoaluminophosphate TAPSO-5

Abstract Titanium-containing silicoaluminophosphates with the AFI structure type (TAPSO-5) have been synthesized hydrothermally. Titanum could be incorporated into the SAPO-5 structure without formation of TiO2 up to Ti Si = 0.05 in the precursor gel. Under the synthesis conditions, Ti4+ ions were most likely isolated and tetrahedrally coordinated in the siliceous domains of the crystals, as evidenced by u.v.-vis and i.r. spectroscopies. The materials were found to be active as catalysts in epoxidation reactions at low temperatures. In contrast to pure aluminophosphate molecular sieves, the presence of large amounts of SiO2 in TAPSO-5 increased the hydrophobic character of the material, and both H2O2 and tert-butyl hydroperoxide could be used as oxidants. The activity of the catalyst in the epoxidation of cyclohexane as well as the nature of the products obtained strongly depended on the peroxide used.

Crystallization of titanium silicalite-1 (TS-1) from gels containing hexanediamine and tetrapropylammonium bromide

Abstract Titanium substituted silicalite-1 (TS-1) samples have been synthesized using hexanediamine (C 6 DN) and tetrapropylammonium bromide (TPABr). Following this method, titanium could be incorporated up to a level of about 2 Ti/unit cell in the framework of silicalite-1. Crystals had a typical habit, very different from those obtained with standard syntheses, which led to exceptionnally high para/ortho ratios in the hydroxylation of phenol with hydrogen peroxide.

Aerobic hydrocarbon oxidation catalyzed by the vanadomolybdophosphate polyoxometalate, H5PV2Mo10O40, supported on mesoporous MCM-41

The impregnation of H5PV2Mo10O40 polyoxometalate onto MCM‐41 and amino‐modified MCM‐41 materials provided mesoporous active catalysts with large surface areas for aerobic hydrocarbon oxidation using isobutyraldehyde as a reducing agent. The results of the oxidation of alkenes and alkanes gave product selectivities similar to those observed in the corresponding homogeneous reaction although catalytic activity was somewhat reduced. Under appropriate experimental conditions there was no leaching and the solid catalyst could be recovered and reused without loss in activity.