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 le ads to the formation of isolated surface complexes (=SiO)TiNp3, Np = n eopentyl. A mild hydrolysis at 298 K or a calcination at 673 K of thes e silica-bonded trisneopentyl titanium complexes both lead to surface species, H and C types, catalytically active for the epoxidation of cy clohexene by organic hydroperoxides or aqueous hydrogen peroxide. The origin and, thus, the purity of the silica support, the treatment perf ormed on the precursor anchored trisalkyl complex, and the nature of t he 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 coul d be recycled by simple filtration and only very minor leaching of tit anium into the solution is detected. The related H-type Zr catalyst, p repared from ZrNp4 and silica, shows an unexpected activity, similar t o that observed with the analog titanium catalysts. The low epoxide se lectivity is correlated to the fact that the same Zr centers catalyze both the formation and the decomposition of the epoxide. The greater s tability 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 nanos ized particles of TiO2. (C) 1998 Academic Press.