Polysiloxane gels have been prepared in bulk and in non-aqueous suspension using an oligomeric dimethyl silanol, S5 or S10, and tetraethoxysilane (TEOS) as the main reactants, with a tin-based catalyst. Controlled levels of Ti and Zr have been introduced into the polysiloxane networks by replacing part of the TEOS by either Ti tetraisopropoxide (TIPO) or Ti tetrakis(2-ethylhexyl oxide) (TEHO) in the case of Ti, and Zr tetraisopropoxide (ZPO) in the case of Zr. Providing TEOS is maintained as the major crosslinking component, toluene insoluble gels are formed with Ti and Zr covalently incorporated into the network.
The Ti- and Zr-containing elastomeric polysiloxane gels have been used as heterogeneous catalysts in the epoxidation of cyclohexene using tBHP or 30% aqueous H2O2. With the former oxidant the gels are indeed active and selective catalysts; however, essentially no activity was observed when aqueous H2O2 was employed.
The structure of the gels and their role in catalysis is discussed in the context of Ti and Zr-containing zeolite and other mixed oxide catalysts.