Selective catalytic oxidation titanium silicate

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

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. 

The photodegradation of hexabromocyclododecane in aqueous solution has been obtained by Fe -oxalate and Fe -citrate complexes, in the presence of H2O2 and simulated sunlight. " Fe-g-C3N4 (Fe-CN) and titanium silicate zeolite (TS-1) hybrid materials have been produced from dicyandiamide, metal chloride as precursors and TS-1 as a support. The material has been applied as heterogeneous catalyst for the photo-catalytic selective oxidation of benzene to phenol, in the presence of H2O2 as benign oxidant under ambient conditions. Noticeably, the hybrid... 

Inclusion of these cations does impart new catalytic activities, but in many cases the active site results from a metal ion that has left the framework and entered the pore space upon heating, especially in the presence of water vapour. This is thought to be the case for zinc- and gallium-containing solids used in the dehydrocyclisation of butane and propane to aromatics in the Cyclar process (Chapter 9). Boron, iron, chromium and vanadium all appear to leave the framework under harsh conditions. The incorporation of titanium and more recently tin into framework sites within silicates have become very important substitutions, because both titanosilicates and stannosilicates have been shown to contain stable Lewis acid sites of importance in selective oxidation catalysis. The metal atom can coordinate additional water molecules in the as-prepared material, but these can be removed by heating. In the synthesis of titanosilicates, titanium is usually added to the gel as the alkoxide, and synthesis performed in the absence of sodium hydroxide to avoid precipitation of sodium titanate or nanoparticulate titanium oxides. 

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