Tripodal titanium silsesquioxane complexes immobilized in polydimethylsiloxane (PDMS) membrane: Selective catalysts for epoxidation of cyclohexene and 1-octene with aqueous hydrogen peroxide

We investigated the activity, epoxide selectivity, H2O2 efficiency, and recyclability of heterogeneous alkene epoxidation catalysts prepared by encapsulation of tripodal Ti silsesquioxane complexes in polydimethylsiloxane (PDMS) membrane. We found that [Ti(NMe2){(c-C6H11)7Si7O12}]/PDMS (3) and [Ti(NMe2){(i-C4H9)7Si7O12}]/PDMS (4) displayed high activity, epoxide selectivity (⩾97%), and H2O2 efficiency (⩾97%) in cyclohexene- and 1-octene epoxidation with aqueous H2O2. Moreover, these catalysts were highly recyclable. The high H2O2 efficiency can be attributed to the uniformly non-polar environment provided about the Ti silsesquioxane complex in 3 and 4 by the PDMS membrane, which presumably results in low local water concentrations and higher [alkene]:[H2O2] ratios at the Ti center than in the bulk reaction medium; both of these effects favor the epoxide selectivity and minimal leaching of titanium. Our study of the effect of solvent on catalyst activity revealed that acetonitrile is to be preferred as solvent, since methanol inhibits the epoxidation at long reaction times.