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.

Theoretical and Experimental Study on the 7‐Hydroxy‐4‐Methylcoumarin Synthesis with H‐Beta Zeolite

“…The more stable IN2 compared to AD2 indicated the thermodynamic favour toward the product complex. The activation barrier for the C−C bond formation was compared with the activation energies of the Aldol condensation of acetone and formaldehyde on H‐ZSM‐5 and H‐FAU that were in range of 0.4‐2.0 kcal⋅mol −1 …”

Cascade Reactions in Tunable Lamellar Micro‐ and Mesopores for C=C Bond Coupling and Hydrocarbon Synthesis

“…We hypothesize that steric constraints imposed by the micropores of MFI (5.4×5.6 Å) destabilize the large, bimolecular transition state formed during intermolecular coupling of butanals (kinetic diameter ca. 5.0 Å) . The partially confined sites are more reactive than both the confined and the unconfined sites ( k inter, pc > k inter, uc, HSiW > k inter, uc, MCM > k inter, c ), apparently because the partially confined environment of these sites promotes the intermolecular C=C bond coupling reactions by additional stabilization of the bimolecular transition state through van der Waals interactions with the mouths of the zeolite pores .…”

DFT investigations of the unusual reactivity of 2‐pyridinecarboxaldehyde in base‐catalyzed aldol reactions with acetophenone

“…[11] B3LYP functional was chosen as this functional has been consistently found to provide reliable results in the theoretical study of aldol condensation reactions involving organic molecules. [12][13][14][15][16][17] Correlation consistent cc-pVDZ basis set [18] was used for all the atoms. All the structures were optimized in water solvent, modeled by using SMD implicit solvation.…”