Tuning solid catalysts to control regioselectivity in cross aldol condensations with unsymmetrical ketones for biomass conversion

Several families of catalysts characterized by differenttypes of acid and base sites were developed to steer regioselectivity in the aldol condensation of unsymmetrical ketones. The catalysts were propyl sulfonicacid (PrSO3H)functionalized SBA-15 with various acid loadings and with or without hydrophobization by octyl groups (Oc); isomorphously substituted BEA zeotypes; and sulfated zirconias with various sulfate loadings. Benzaldehyde and 2-butanone served as test reactants at a temperature of 140 ◦C. Sulfonic acidfunctionalized catalysts preferentially formed branched condensation product and metal-substituted BEA zeotypes selectively formed linear condensation product. Maximum condensation product yields were 65% branched at 98% conversion with the highest site density Oc/PrSO3H/SBA-15 and 76% linear at 94% conversion with Sn-BEA. Sulfated zirconia catalysts were less selective. Product distributions analyzed at different conversions revealed how kinetics and equilibria of addition and dehydration govern
regioselectivity. Further observations were: 1. Increasing the acid site density significantly increased the turnover frequency for PrSO3H/SBA-15, suggesting a cooperative effect between adjacent acid sites. 2. Brønsted acid sites catalyzed an undesired side reaction, the cleavage of the branched addition product to
-methyl styrene and acetic acid. This reaction was suppressed by hydrophobizing the surface through octyl co-functionalization. 3. Deactivation upon re-use was generally slight, and Sn-BEA could be fully regenerated by calcination.