Abstract
We investigated the activity, epoxide selectivity, H 2O 2 efficiency, and recyclability of heterogeneous alkene epoxidation catalysts prepared by encapsulation of tripodal Ti silsesquioxane complexes in polydimethylsiloxane (PDMS) membrane. We found that [Ti(NMe 2){( c-C 6H 11) 7Si 7O 12}]/PDMS ( 3) and [Ti(NMe 2){( i-C 4H 9) 7Si 7O 12}]/PDMS ( 4) displayed high activity, epoxide selectivity (⩾97%), and H 2O 2 efficiency (⩾97%) in cyclohexene- and 1-octene epoxidation with aqueous H 2O 2. Moreover, these catalysts were highly recyclable. The high H 2O 2 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]:[H 2O 2] 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.
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