Oxide-based catalysts titanium silicalite

An issue which deserves further mention is the environmentally fiiendly nature of TS-l/H Oj system. It involves the use of a safe silica based catalyst, titanium silicalite, and a reagent, hydrogen peroxide, which yields water as the coproduct. This holds for the in situ route illustrated in Scheme I and also for the epoxidation of propylene with preformed hydrogen peroxide, either used as an aqueous solution (72) or extracted by means of die epoxidation solvent (Scheme 11). Hazardous chemicals, such as chlorine, performic or other organic peracids, are not required in the process. The disposal of chlorinated salts or the recycle of brine (chloroydrin process) and any possible burden resulting from the coproduction of odier chemicals (styrene and r-butanol in the hydroperoxide route) are eliminated. The liquid phase oxidation of isobutane and ethylbenzene with air under pressure and at high temperature, to produce... 

Indeed, several interesting procedures based on three families of active catalysts organometallic complexes, phase-transfer compounds and titanium silicalite (TS-1), and peroxides have been settled and used also in industrial processes in the last decades of the 20th century. The most impressive breakthrough in this field was achieved by Katsuki and Sharpless, who obtained the enantioselective oxidation of prochiral allylic alcohols with alkyl hydroperoxides catalyzed by titanium tetra-alkoxides in the presence of chiral nonracemic tartrates. In fact Sharpless was awarded the Nobel Prize in 2001. 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

The advantage of the above two methods are high yields of epoxides, and the titanium silicalite catalyst is not deactivated or poisoned by the contaminants in the crude oxidation mixture. Hence, the processes are commercially attractive. The in situ hydrogen peroxide generation based on the AO process from either the anthraquinone/anthrahydroquinone or ketone/alcohol redox couples has also been used for the following synthetic reactions ... 

Zr- and Ti-based POMs have been suggested as molecular models for the well-known heterogeneous titanium silicalite catalyst, TS-1, with major industrial appeal in the field of H2O2 activation and selective oxidation. In some cases, POMs... 

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