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

J. Phys. Chem. C 2018, 122, 40, 23230–23241

Authors: A. N. Migues, Q. Sun, S. Vaitheeswaran, W. Sherman and S. M. Auerbach

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.

The Journal of Physical Chemistry C