Titanium silicate molecular sieves epoxidation

V.C. Epoxidation on Titanium Silicate Molecular Sieves V.C.l. General Features of Epoxidations... 

Highly regioselective cyclizations of 3,4-, 4,5- and 5,6-unsaturated alcohols to yield tetrahydrofuranols and tetrahydropyranols have been carried out with the TS-I-H2O2 system (this is a titanium silicate molecular sieve-H202 complex.) The reactions involve the intermediate formation of epoxides and their Ni ring opening. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

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 insertion of Ti in the zeolite framework was accompanied by a significant decrease in A1 content (Table 1). However, there was no stoichiometric process between A1 removal and Ti insertion. Moreover, it was found that the treatment of Ig of an aluminum containing beta zeolite with a 75 ml of 3 x 10 M oxalic acid solution decreased the Si/Al ratio from its original value of 30 to 85 due to A1 extraction. Attempts to incorporate Ti into other zeolites like ZSM-12 and mordenite were not successful. Interestingly, the extraction of A1 from these zeolite structures was also unsuccessful with oxalic acid solutions with comparable concentrations. However, preliminary data show that siliceous mesoporous molecular sieves (MCM-41 and HMS) treated similarly with ammonium titanyl oxalate solutions exhibit good epoxidation activity. It is inferred that the presence of framework cations that can be extracted by oxalate species and/or the presence of defect sites in the parent zeolite is a requisite for the subsequent incorporation of titanium. 

Microporous titanium silicate (e.g., TS-1, Ti-(3, Ti-ZSM-12, Ti-mordenite) is an effective molecular-sieve catalyst for the selective oxidation of alkanes, the hydroxyla-tion of phenol, and the epoxidation of alkenes with aqueous H202. The range of organic compounds that can be oxidized is greatly limited, however, by the relatively small pore size (about 0.6 nm) of the host framework. 

The porous titanium silicate TS-1 represents one of the great commercial successes of recent years. Despite only being reported for the first time in the last decade, it is already established as an oxidation catalyst in the manufacture of hydroquinone, and processes based on its use as a catalyst in the epoxidation of propene and the ammoxidation of cyclohexanone are near the production stage.14 The use of the increasingly diverse range of molecular sieve solid catalysts is also described in Chapter 2. 

---