Synergistic Catalytic Mechanism of Acidic Silanol and Basic Alkylamine Bifunctional Groups Over SBA-15 Zeolite toward Aldol Condensation

It is still not clear at the molecular level how the acidic silanol and basic alkylamine bifunctional groups over SBA-15 zeolite exert catalytic functions in aldol condensation of 5-hydroxymethylfurfural (HMF) with acetone, which plays pivotal roles in the synthesis of renewable fuels from biomass. Here, the catalytic mechanisms of the aldol condensation (1) of 5-hydroxymethylfurfural (HMF) with acetone to produce 4-[5-(hydroxymethyl)-2-furanyl]-3-buten-2-one (C9H10O3) over acidic [−SiOH], basic [−RNH2], and cooperative acidic–basic [−SiOH/–RNH2] active sites on the alkylamine-grafted SBA-15 zeolite (SBA-15–RNH2) have been theoretically investigated, combining quantum mechanical and molecular mechanical calculations, which involves aldol condensation (2) and dehydration (3). Based on the plausible reaction pathways, the revised mechanism is proposed. Toward the aldol condensation (2), the cooperative [−SiOH/–RNH2] displays better catalytic activity than the single [−RNH2] or [−SiOH], in which the rate-determining step is associated with the formation of alkylenamine from the alkylamine with enol–acetone. Toward the dehydration (3), [−RNH2] shows better catalytic activity than [−SiOH], in which the rate-determining step is concerned with the [1,3]-H shift for yielding the product C9H10O3. Toward the gross reaction (1), over [−SiOH/–RNH2], the optimal reaction pathway includes the enolization of acetone, aldol condensation of HMF with enol–acetone, and dehydration. Herein, [−SiOH] is answerable for the activation of acetone to enol–acetone, where it serves as a bridge of H shift. Additionally, [−RNH2] is responsible for the aldol condensation of HMF with enol–acetone and the subsequent dehydration. These findings may advance the rational design of cooperative acidic–basic catalyst toward the aldol condensation reaction at low temperature.