Ti-containing silicas

Shiraishi, Y., Morishita, M., and Hirai, T. (2005) Acetonitrile-assisted highly selective photocatalytic epoxidation of olefins on Ti-containing silica with molecular oxygen. Chemical Communications (48), 5977—5979. 

Broensted and Lewis acid sites in crystalline phosphates, silicates and in gels with moleculeu sieve properties were studied by IR-spectroscopy. Two types of bridged hydroxyls were fovmd in SAPO-5 which were accessible to adsorbed molecules and were able to interact with ethylene. Lewis sites in metallophosphates, zirconosilicate and in Ti-containing silica gel were observed which did not interact with weak bases (CO, hydrogen) but formed strong complexes with acetonitrile. They were supposed to be framework metal ions in tetrahedral coordination. 

LEWIS ACID SITES IN CRYSTALLINE PHOSPHATES, ZIRCONOSILICATE AND IN Ti-CONTAINING SILICA GELS. 

P-12 - Nb- and Ti-containing silica-based mesoporous molecular sieves as catalysts for photocatalytic oxidation of methane... 

Nb- and Ti-containing silica-based mesoporous molecular sieves were used as catalysts for photocatalytic oxidation of methane. It is found that methanol was formed at 323 K and water pre-adsorbed samples exhibited higher catalytic activity than their pure metal oxides. By comparison with Nb-MCM-41, Ti-MCM-41 gave better methane conversion and methanol yield although traces of formaldehyde were observed over Nb-MCM-41 catalyst. Interestingly, OH radical was detected by the in-situ ESR spectroscopy using DMPO as trapper. 

Ikeue, K., Nozaki, S., Ogawa, M., and Anpo, M. (2002) Characterization of self standing Ti-containing porous silica thin films and their reactivity for the photocatalytic reduction of C02 with H20. Catalysis Today, 74 (3-4), 241—248. 

Ti-SBA-15 mesoporous, titanium-containing silica self-assembly-15 (with uniform, hexagonal, tubular channels) synthesized by using a triblock organic copolymer as a template... 

Gas-phase epoxidation of propylene with 02/H2 mixtures was accomplished over Ag1267 or Au1268 catalysts dispersed on TS-1 or other Ti-containing supports and Ti-modified high-silica zeolites.1269 Sodium ions were shown to be beneficial on the selectivity of propylene epoxidation with H202 over titanium silicalite.1270 A chromia-silica catalyst is active in the visible light-induced photoepoxidation of propylene by molecular oxygen.1271... 

However, when small size molecules, which can freely diffuse either in Beta zeolite and MCM-41, are used a lower turnover is observed on Ti-MCM-41 catalysts compared to that obtained on Ti containing Beta zeolite [13]. This low activity can be attributed to that some of the Ti sites in MCM-41 type of catalysts are buried on the silica walls, being non-accessible to the reactants and also, to the very different adsorptive properties of Ti-MCM-41 and Ti-... 

The interpretation of the results obtained is still under debate. The changes in UCV of the Ti-containing materials with respect to the pure silica ones are not... 

Surface properties of Ti-containing adsorbents such as Ti-silica gel, Ti-Al beta zeolite, two samples of TS-1 zeolite prepared in different ways, as well as of pure silicalite were studied by means of IR spectroscopy of CO adsorbed at low temperatures Surface silanol groups, acidic bridged hydroxyls, coordinatively unsaturated Ti and A1 atoms were characterized as adsorption sites. The capability of IR spectroscopy of adsorbed CO to distinguish between TS-1 samples of different origin is demonstrated. 

The oxidation of aniline was carried out in the liquid phase over a series of transition metal - substituted molecular sieves. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, the reaction was limited by diffusion for medium pore zeolites like TS-l and mesoporous silicas were preferred as they permitted the use of both H2O2 and tert-butyl hydroperoxide as oxidants. Higher oxidant/aniline ratios (>2) led to the formation of nitrobenzene (NB), whose selectivity was proportional to the catalyst concentration. In contrast, vanadium containing molecular sieves were only active with TBHP and aniline was converted very selectively into nitrobenzene for all oxidant concentrations. 

Ti containing mesoporous silicas (Ti-HMS) were synthesized following a method recently reported by Tanev et al. [7]. A similar recipe was used to prepare V-HMS but vanadyl acetylacetonate was used instead of titanium isopropoxide in the gel. 

We choose Ti-containing materials as an example to discuss heteratom-containing silica material and nonsilica mesoporous materials, because Ti-containing materials are useful catalysts and were studied extensively. 

By the interaction of silica with phosphorus oxychloride the phosphorus-containing silica were synthesized, with the increase of phosphorus content from 0 to 0.95 mmol/g the amount of OH-groups in the samples decreases from 3.62 to 1.42 mmol/g. Obtained P-silica were treated by titanium and vanadium chlorides. The content of bonded at the expense of reaction with silanols of titanium- and vanadium-containing groups decreases from 1.08 to 0.35 (Ti, Table 2) and from 1.11 to 0.31 mmol/g (V, Table 3) accordingly [62,63]. 

Noteworthy, a remarkable increase in the activity of Ti-containing mesoporous catalysts was obtained by anchoring trimethylsilyl groups on the silanols, which led to an increase of the hydrophobicity of the surface [15,16]. In this paper we report on peculiar aspects of the preparation and characterisation of mesoporous Ti-fimctionalised MCM-48 silicas. The spectroscopic and catalytic properties of Ti-MCM-48 catalysts upon a post treatment of silylation with hexamethyldisilazane (HDMS) as silylating agent, are also reported. 

Micro- and mesoporous crystalline titanium silicas, such as Ti silicalite TS-1 and the Ti-containing MCM-41 zeolite, represent another important family of modem materials and find versatile application as catalysts for oxidation reactions and in absorption technologies. These materials are prepared under hydrothermal conditions using tetraalkylammonium hydroxide, titanium tetraalkoxide, and tetraethyl orthosilicate as preeursors. Only limited amounts of Ti can enter the Si02 framework and the actual bonding situation at Ti is not yet fully understood. Several data suggest that Ti substitution for Si in the lattice positions retaining the tetrahedral coordination is important for catalysis. 

Shan, Z. Jansen, J.C. Marchese, L. Maschmeyer, Th. Synthesis, Characterization, and Catalytic Testing of a 3-D Mesoporous Titanosilica, Ti-TUD-1 , Mesoporous and Mesoporous Materials 48 (2001) 181-187. Shan, Z. Gianotti, E. Jansen, J.C. Peters, J.A. Marchese, L. Maschmeyer, Th. One-Step Synthesis of a Highly Active, Mesoporous, Titanium-Containing Silica by Using Bifimctional Templating , Chem. Eur. J. 2001,7, no.7, 1437-1443... 

Here, we give an example for the synthesis of Ti-containing mesoporous silicas. This kind of catalyst has abilities for the selective oxidation of olefin and other unsaturated compounds, such as the epoxidation of 1-hexene, cyclohexene, and styrene [61,62]. The ratio of Si/Ti, structure, and hydrophobic nature of the material are the three most important factors in the catalytic activity. Therefore, a full understanding of the synthesis is necessary. 

To improve the hydrophobicity of Ti-containing mesoporous silicas, cocondensation and grafting method with the organosilanes can be used... 

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