On the Rational Design of Zeolite Clusters for Converging Reaction Barriers: Quantum Study of Aldol Kinetics Confined in HZSM‑5

• Published in: Journal of physical chemistry. C Vol. 122; no. 40; pp. 23230 - 23241
• Main Authors: Migues, Angela N, Sun, Qinfang, Vaitheeswaran, S, Sherman, Woody, Auerbach, Scott M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 11.10.2018
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.8b08684

We have performed density functional theory calculations to investigate the convergence of reaction barriers with respect to zeolite cluster size, for multistep reactions catalyzed in HZSM-5. We constructed cluster models of HZSM-5 using the delta-cluster approach reported previously by us. We then computed barriers for different reaction types to determine the cluster sizes and neighbor-list radii needed to fully treat zeolite confinement effects. In particular, we studied the acid-zeolite-catalyzed aldol reactions of acetone with formaldehyde, furfural, and hydroxymethyl-furfural, in three steps: keto/enol tautomerization of acetone, combination between each aldehyde and the enol, and aldol dehydration. We found that the delta-cluster radius of 4.0 Å consistently converges barriers with respect to cluster size, yielding complete treatments of confinement in HZSM-5 with clusters containing up to 99 atoms. For comparison, periodic density functional theory (DFT) on HZSM-5 includes 288 atoms, requiring 19 times more CPU time in head-to-head comparisons. Our converged acetone–formaldehyde dehydration barrier agrees quantitatively with a comparable barrier obtained with periodic DFT, showing that cluster calculations can converge properties at a fraction of the cost of periodic DFT. Interestingly, we found that the bulkier, furan-containing aldehydes exhibit faster reactivity because of charge delocalization from aromatic rings, which significantly speeds up aldol dehydration.