Lewis acidity and substituent effects influence aldehyde enolization and C–C coupling in beta zeolites

The Claisen-Schmidt condensation between propanal and benzaldehyde was used a model aldol reaction to assess the reactivity of tetravalent framework Lewis acid sites (Ti, Zr, Sn, Hf) in hydrophobic beta zeolites (M-*BEA) and determine rates of enolization and carbon–carbon (C–C) bond formation along with selectivities among cross- and self-condensation products. Rates of enolate formation during liquid phase reactions (393 K) span nearly 100-fold, depending on the identity of the metal heteroatom, and correlate with enthalpies for adsorption of pyridine in liquid-filled pores of M-*BEA, as measured by isothermal titration calorimetry. Comparisons among adsorption enthalpies suggest that tetrahedrally substituted Zr and Hf are more effective Lewis acids than Sn and Ti for coordinating aldehydes within these environments. Ratios of initial rates of C–C coupling between propanal and benzaldehyde compared to propanal self-condensation were less than unity for all M-*BEA catalysts. These selectivity patterns reflect the combination of electronic and steric effects imposed by the larger aromatic substituent at the carbonyl carbon atom and its concerted interactions with the Lewis acid site, which causes rates for nucleophilic attack of enolates to benzaldehyde to be far less than those to propanal.