EPOXIDATION OF BETA-ISOPHORONE OVER A TITANIA-SILICA AEROGEL - EFFECTOF CATALYST PRETREATMENTS WITH BASES

The epoxidation of beta-isophorone with TBHP and an amorphous mesoporo us titania-silica aerogel, containing 20 wt% TiO2, has been investigat ed. The nature of side reactions and the possibility of suppressing th em by modification of the catalyst with bases was studied. The as-synt hesized (unmodified but calcined) aerogel afforded only moderate selec tivity to epoxide (36-87%) in the temperature range of 343-373 K. 4-Hy droxy-isophorone and alpha-isophorone were the major by-products, form ed in isomerization reactions of product and reactant, respectively, a nd catalyzed by acidic sites present on titania-silica. The selectivit y to epoxide could be improved by pretreatment of the aerogel with neu tral or weakly basic alkali or alkaline earth metal salts. Treatment o f the aerogel with aqueous NaOAc followed by recalcination at 873 K af forded 94% selectivity at 90% peroxide conversion. Application of stro nger bases was detrimental to the epoxidation activity and selectivity . We propose that the improved selectivity is due to ion exchange betw een the weakly basic additive and the surface Bronsted sites (silanol groups). Neutral salts are preferentially anchored to the polar (acidi c) surface sites and reduce their accessibility for the more bulky and less polar reactant. The detrimental effect of strong bases is partly due to excess basicity of the catalyst after pretreatment, as the sid e reactions are also catalyzed by bases. Besides, prolonged treatment with NaOH partially hydrolyzed the Ti-O-Si bonds and restructured the material (titania microdomains in the silica matrix), as evidenced by FTIR and Uv-vis spectroscopy. The study of the epoxide ring opening re action demonstrated the limited relevance of ex situ acidity measureme nts for predicting the role of acid sites in complex epoxidation react ions. (C) 1997 Academic Press.