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.

On the rational design of zeolite clusters for converging reaction barriers: Quantum study of aldol kinetics confined in HZSM-5

2019 Spring National Meeting

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