Proton exchange reactions of C2–C4 alkanes sorbed in ZSM-5 zeolite

Abstract

An extensive theoretical study has been carried out to determine barriers for the proton exchange reactions of C2-C4 alkanes in ZSM-5. It was found that cluster size and cavity structure are very important for predicting this barrier. A decrement of up to 20 kcal/mol was observed when employing the periodic model instead of using the small cluster model. Effects of basis set quality and electron correlation to the activation energy are positive and in combination could contribute up to 8 kcal/mol. An extrapolation scheme for estimating the reaction barrier that takes into account effects of cluster size, basis set quality, and electron correlation has been proposed. The regioselectivity and the chain length were discussed.

Ab Initio Prediction of Proton Exchange Barriers for Alkanes at Brønsted Sites of Zeolite H-MFI

“…The adsorption energies of ethane, propane and i -butane calculated as periodic MP2 estimate by Sukrat et al, −30.5, −42.6, and −48.5, respectively, are much less binding than our best hybrid QM:QM estimate, −44.9, −58.2, and −63.8 kJ/mol, respectively. Their apparent energy barriers (56, 55, 58, and 46 kJ/mol for ethane, propane, n -butane, and i -butane, respectively) are much lower than our final estimates and show no dependence on the number of carbon atoms.…”

“…31 A study of ethane, propane, and butane in H-MFI considered cluster models of increasing size up to 96 T atoms (T = Si, Al) and proposed an extrapolation scheme that included effects of the cluster size, basis set, and electron correlation. 32 The calculated barriers were substantially underestimated with respect to the experiment, and it remained unclear why. The most recent application of DFT to the H/D exchange of ethane, propane, i-butane and i-hexane over the H-MFI used the ONIOM scheme, 33 another hybrid QM:QM implementation.…”

“…A study of ethane, propane, and butane in H-MFI considered cluster models of increasing size up to 96 T atoms (T = Si, Al) and proposed an extrapolation scheme that included effects of the cluster size, basis set, and electron correlation . The calculated barriers were substantially underestimated with respect to the experiment, and it remained unclear why.…”

Predicting adsorption enthalpies on silicalite and HZSM‐5: A benchmark study on DFT strategies addressing dispersion interactions

“…The goal of this study is to evaluate the accuracy of periodic density functional (DF) calculations using a gradient‐corrected density functional with empirical dispersion corrections (PBE‐D) as well as a nonlocal correlation functional (vdW‐DF2) for determining the adsorption enthalpies of water, alkanes, and alcohols in silicalite and proton form of Zeolite Socony Mobil‐5 (HZSM‐5). Similar benchmark studies, addressing Density Functional Theory based methods with empirical dispersion corrections (DFT‐D) with or without periodic boundary conditions, have recently been reported for the (co‐)adsorption of water, alcohols, nitriles, olefins, aromatics, and alkanes in these or other zeolites. However, for alkanes in MFI‐type zeolites (structure type ZS M‐five ), only a subset of possible adsorption geometries had previously been considered or adsorption at the OH group at the Al12 site, mainly discussed in the literature, was not included at all .…”

“…Similar benchmark studies, addressing Density Functional Theory based methods with empirical dispersion corrections (DFT‐D) with or without periodic boundary conditions, have recently been reported for the (co‐)adsorption of water, alcohols, nitriles, olefins, aromatics, and alkanes in these or other zeolites. However, for alkanes in MFI‐type zeolites (structure type ZS M‐five ), only a subset of possible adsorption geometries had previously been considered or adsorption at the OH group at the Al12 site, mainly discussed in the literature, was not included at all . After initial optimization of the zeolite lattice, we explored computationally the sorption of the guest molecules at various positions and compared the calculated enthalpies with experimental values reported in the literature .…”