Tetraneopentyltitanium derived supported catalysts - Part 1. Synthesis and catalytic properties for the epoxidation of cyclohexene with aqueous hydrogen peroxide

The reaction between tetraneopentyltitanium and the isolated hydroxyl groups of the surface of a silica partially dehydroxylated at 753 K leads to the formation of isolated surface complexes (=SiO)TiNp3, Np = neopentyl. A mild hydrolysis at 298 K or a calcination at 673 K of these silica-bonded trisneopentyl titanium complexes both lead to surface species, H and C types, catalytically active for the epoxidation of cyclohexene by organic hydroperoxides or aqueous hydrogen peroxide. The origin and, thus, the purity of the silica support, the treatment performed on the precursor anchored trisalkyl complex, and the nature of the oxidant are all important parameters. Thus, with (BuOOH)-Bu-t, the selectivity of these Ti/silica catalysts for the epoxide is very good (ca 95%); but with aqueous H2O2, the epoxide yields are only moderate and partial decomposition of the oxidant is occurring. All solids could be recycled by simple filtration and only very minor leaching of titanium into the solution is detected. The related H-type Zr catalyst, prepared from ZrNp4 and silica, shows an unexpected activity, similar to that observed with the analog titanium catalysts. The low epoxide selectivity is correlated to the fact that the same Zr centers catalyze both the formation and the decomposition of the epoxide. The greater stability towards hydrolysis of the =SiO-Zr bond when compared to the =SiO-Ti bond may explain the catalytic properties observed, which must be at least partially correlated to a higher concentration of isolated, mononuclear Zr sites than Ti sites, the latter coexisting with nanosized particles of TiO2. (C) 1998 Academic Press.