Density Functional Theory Study of Mixed Aldol Condensation Catalyzed by Acidic Zeolites HZSM-5 and HY

Abstract

We applied density functional theory to investigate the mixed aldol condensation of acetone and formaldehyde in acid zeolites HZSM-5 and HY, as a prototypical bond-forming reaction in biofuel production. We modeled the acid-catalyzed reaction in HZSM-5 and HY in two steps: keto–enol tautomerization of acetone and bimolecular condensation between formaldehyde and the acetone enol. For both acid zeolites, the keto–enol tautomerization of acetone was found to be the rate-determining step, consistent with the accepted mechanism in homogeneous acid-catalysis. Convergence studies of the activation energy for keto–enol tautomerization, with respect to cluster sizes of HZSM-5 and HY, exhibit rather different convergence properties for the two zeolites. The keto–enol activation energy was found to converge in HY to ∼20 kcal/mol for a cluster with 11 tetrahedral atoms (11T cluster), which does not complete the HY supercage. In contrast, the activation energy for HZSM-5 reaches an initial plateau at a value of ∼28 kcal/mol for clusters smaller than 20T and then converges to ∼20 kcal/mol for clusters of size 26T or greater, well beyond the completion of the HZSM-5 pore. As such, completing a zeolite pore surrounding a Brønsted acid site may be insufficient to converge activation energies; instead, we recommend an approach based on converging active-site charge.