Aldol condensation reaction in hierarchical ZSM-5 zeolite: A molecular dynamics simulation

Abstract

Aldol condensation reaction is considered as a typical process to form C–C single bonds and result in long-length chain, however, the selectivity of aldol reaction is too complicated to understand. Among the kinetic steps of typical catalytic reactions, diffusion affects the behavior of both reactants and products, subsequently plays a decisive role in the selectivity of reaction path. Herein, molecular dynamics simulation was used to study the adsorption and diffusion behavior of the key components (i.e., reactants, intermediates, aldehydes and α,β-unsaturated ketones product) in the aldol condensation reaction of benzaldehyde with 2-butanone catalyzed by hierarchical ZSM-5 zeolite at 373 K. The results show that as small products, 2-hydroxy-1-butene, 3-hydroxy-2-butene will tend to form via 2-butanone in the micropores; while as intermediates, 2-hydroxy-1-butene, 3-hydroxy-2-butene will continue to react with benzaldehyde to form large products 1-phenyl-pentan-1-ol-3-one, 4-phenyl-3-methyl-butan-4-ol-one, 1-phenyl-1-pentan-3-one and 3-methyl-4-phenyl-3-butan-2-one in the mesopores. Additionally, the presence of water will slow down the diffusion of reactants, which might be the main reason why aldol condensation reaction is commonly inhibited with the presence of water. This work can not only provide a basis understanding for mechanism of aldol condensation reaction from the perspective of diffusion, but also guide the design of hierarchical zeolite catalysts.