The abundance of shale-derived hydrocarbon and non-food-based biomass feedstocks presents a momentous opportunity in catalysis science to rationally design new materials for transforming small organic molecules, such as aldehydes and sugars, into a diverse range of value-added chemicals by combining carbon-carbon (C-C) bonds with high selectivity. Molecular sieve zeotypes containing tetravalent Lewis acid sites and basic oxygen atoms within connecting oxide tetrahedra are promising catalysts to activate these carbonyl-containing oxygenates and promote C-C coupling through aldol condensation pathways. For this study, the Claisen-Schmidt condensation of benzaldehyde (BA) and propionaldehyde (PA) to α-methyl-cinnamaldehyde is used as a probe reaction to examine the influence of acid-base cooperative interactions between framework Lewis sites (M) and lattice oxygens in Si-O-M ensembles on apparent rate constants, site time yields (STY), and rate-controlling steps during cross-aldol condensation events between enolizable substrates (PA) and aldehydes lacking α-hydrogens (BA). Experimental evidence from kinetic studies performed between 363-393 K in a batch reactor at contact times ranging from 0.5-24 h suggest that cross-aldol condensation occurs with ~10-20× higher STY when Lewis acid sites are confined within large- (12-membered ring, MR) and extra-large pore (14-MR) Ti-*BEA and Ti-UTL zeotypes, respectively, compared to anatase TiO₂. Additionally, STY that are systematically higher for Zr-*BEA by ~8× relative to titanosilicate materials, indicate that either differences in the functional Lewis acid strength of the metal center, or changes in the dominant active site structure (i.e., defect-open (OH)_x-M-(OSi)_3-_x vs. closed (SiO)₄M) can lower intrinsic activation barriers for C-C coupling between BA and PA. The implications of these findings and the role of acid-base cooperativity in possible cross-aldol condensation mechanisms will be discussed.

Viktor Cybulskis

Syracuse University

Appearances