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.