Control of acidity and selectivity of titania-silica aerogel for the epoxidation of beta-isophorone

The epoxidation of P-isophorone (3,5,5-trimethyl-3-cyclohexene-1-one) with an amorphous mesoporous titania-silica aerogel and t-butyl hydroperoxide ha s been investigated. Dominant side reactions were the acid/base catalyzed i somerizations of beta-isophorone to alpha-isophorone and of the product epo xide to 4-hydroxy-isophorone. The acidity of the aerogel was characterized by the epoxide ring opening reaction (to 4-hydroxy-isophorone). Two methods were shown to be efficient in tuning the acidity and the hydrophilic-hydro phobic character of the aerogel, and controlling the product distribution. A catalyst pretreatment in ethyl benzene afforded 98.5 and 93% epoxide sele ctivities at 75 and 90% peroxide conversions, respectively, without any inf luence on the oxidation rate. A simple addition of a weak inorganic base (s uch as NaHCO3 or Na2HPO4) to the reaction mixture and carrying out the reac tion in the presence of two solids in an apolar solvent was even more effic ient: epoxide selectivities up to 95% were achieved at 90% conversion, with only small changes in reaction time and peroxide selectivity. Moreover, ap plying other bases the product composition could be varied in a very broad range. 4-Hydroxy-isophorone was the major product with Na2CO3 (65% selectiv ity at 90% conversion), while alpha-isophorone was formed almost exclusivel y in the presence of BaCO3 or NaOH. (C) 1999 Elsevier Science B.V. All righ ts reserved.