Mesoporous titanium silicates

DEVELOPMENT OF THE WET METHODS FOR SYNTHESIS OF MESOPOROUS TITANIUM-SILICATE BASED COMPOSITES... 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

In green oxidation reactions, zeolite TS-1 is the typical catalyst. Since the size of its channels ranges from 5 to 6 A, TS-1 can be used as the catalyst only for benzene and phenol conversion. However, ordered mesoporous titanium silicate materials have pores large enough for the catalytic reactions of bulkier molecules, and this is very important for the production of fine chemicals. For example, for the oxidation reaction of terpineol, Ti-MCM-41 performs much better than do microporous titanium silicate molecular sieves as a catalyst.1-291... 

Y. A. Kalvachev, T. Hayashi, S. Tsubota, M. Haruta, Vapor-phase selective oxidation of aliphatic hydrocarbons over gold deposited on mesoporous titanium silicates in the co-presence of oxygen and hydrogen, J. Catal. 186 (1999) 228. 

Figure 14.8 PO yields as a function of time-on-stream for Au/ Ti-Si02 (sponge-like mesoporous titanium silicate) catalysts with different modifications Catalyst, 0.35 wt%Au/ Ti-SiOj 0.15 g feed gas, CjHs/Oj/Hj/Ar = 10/10/10/70 space velocity, 4000h mlgc. ...

The oxidation of 2-allylphenol and phenols bearing alcohol functional groups over a mesoporous titanium-silicate catalyst produces BQ with good to moderate yields keeping the other oxidizable sites intact [133], 2-Hydroxybenzyl alcohol, which can alternatively be converted to salicylalde-hyde, gave 2-hydroxymethyl-p-benzoquinone in a 73% yield. 

Kholdeeva, O., Trukhan, N., Vanina, M., et al. (2002). A New Mesoporous Titanium-silicate Ti-MMM-2 A Highly Active and Hydrothermally Stable Catalyst for H202-based Selective Oxidations, Catal. Today, 75, pp. 203—209. 

Effective use of microwave heating in the oxidation of alkenes and alcohols under the action of hydrogen peroxide was described in [177]. The catalyst was MCM l-type mesoporous titanium silicate modified by organic compounds. It was recycled several times without activity loss. 

The review of Notari (33) covers the synthesis methodologies of titanium silicate molecular sieves available up to 1996. The reviews of Corma (279) and subsequently of Biz and Occelli (280) describe the synthesis of mesoporous molecular sieves. An informative article on the preparation of TS-1 was reported recently by Perego et al. (68). In this section we list some of the recent developments in the synthesis of micro and mesoporous titanosilicate molecular sieves. 

For such reasons, the following section considers in more detail some of the most significant results obtained by our team on the epoxidation with TBHP of unsaturated FAMEs over mesoporous titanium-grafted silicates. In these examples, the epoxidation tests were carried out either in ethyl acetate, which could be even obtained, in principle, from renewable sources and which is relatively less harmful than other polar non-protic solvents, or under solvent-free conditions. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

It is interesting to indicate that the dehinnidification efficiency increased in the same order of the development of the micropore and mesopores. Therefore, it was concluded that the superior dehumidification behavior of the aluminum silicate and titanium silicate-impregnated ceramic sheets should mainly originate from the well developed micropores and mesopores, especially the micropores with diameter less than 3tim. 

Further interesting developments in this field are the discovery of the large pore titanium silicate molecular sieve ETS-10 [77] and the synthesis of mesoporous materids containing titanium and vanadium [78-81], Much systematic work will be required to elucidate how useful these new materials are as catalysts in selective oxidation reactions. 

The recent examination of titanium containing Au/SBA-15 catalysts was carried out comparing titanium incorporated hydrothermally and by posts)mthesis grafting using titanium (IV) oxyacetylacetonate monohydrate [84]. Titanium addition by grafting resulted in more active and selective catalysts, though overall the catalysts had lower activity relative to others prepared using mesoporous titano-silicates. This is in direct contrast to the results found for Au/Ti-MCM-41... 

Several framework titanium-substituted mesoporous silicates, including Ti-MCM-41 (42,43), Ti-HMS (198), Ti-MCM-36 (180), Ti-MCM-48 (199), and Ti-SBA-15 (200), have shown promising activity for the epoxidation of bulky olefins with alkyl hydroperoxides as oxidants. Unfortunately, compared with the microporous MFI-type titanium silicates, the mesoporous materials exhibit low activity for epoxidation reactions. The hydrophilic nature of mesoporous silica catalysts with isomorphous titanium substitution is considered to be one of the major reasons for the low activity (179). Various attempts have been made to improve the activity. Using a different synthetic procedure, titanium species have been grafted onto... 

There are two main techniques for the incorporation of atomically dispersed titanium into a mesoporous silica framework. In the cocondensation method a titanium source is added during the preparation of the silica material framework and formation proceeds via simultaneous condensation of both titanium and silicon precursors, resulting in titanium incorporation throughout the material. Conversely, postmodification involves the grafting of a titanium-alkoxide precursor to the pore surface by condensation with surface hydroxyls of a preformed silica material. This forms a surface-modified titanium silicate. 

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