(111d) Aldol Condensation of Cyclopentanone and Acetone on MgO: A Mechanism Study By DFT

Aldol condensation is an effective reaction to elongate the carbon chain for biomass conversion. Here we report a mechanistic study of the self- and cross-aldol condensation of cyclopentanone [C] and acetone [A] using density functional theory (DFT) calculations, though which we provide an explanation for the experimental finding of aldol condensation catalyzed by MgO, a solid base catalyst. Experimental results suggest that the [C]-activated products play a dominant role, as indicated by the initial rates with various A-to-C feed ratios. When more acetone is added to the system, the TOF of [C]-activated products remains considerably higher than that of [A]-activated ones. Calculations of the activation barriers show that α-H abstraction is the rate-determining step, and that the selectivity of aldol condensation (self vs cross) is controlled by the sequential C-C bond forming step. The selectivity to [C]C over [C]A is observed in experiment, while calculations show that formation of the [C]A product is more kinetically favorable. Both adsorption measurement and calculations suggest that cyclopentanone competes strongly for adsorption on MgO(100) and dominates the catalytic surface. The higher coverage of cyclopentanone than acetone on MgO may thus lead to higher selectivity toward the self-aldol condensation products. This conclusion is further supported by results from zeolite-catalyzed reactions.