Probing the Structural and Electronic Factors Affecting the Adsorption and Reactivity of Alkenes in Acidic Zeolites Using DFT Calculations and Multivariate Statistical Methods

Abstract

Quantum mechanical (QM) cluster calculations have been performed on a model of ZSM-5 at DFT and MP2 levels. We investigated how the adsorption energies and the energetics of alkoxide intermediate formation of six different alkene substrates, ethene, propene, 1-butene, cis/trans butene, and isobutene, vary in this zeolite model. An analysis of the DFT geometric, electronic, and energetic parameters of the zeolite-substrate complexes, transition states, and alkoxide intermediates is performed using principal components analysis (PCA) and partial least squares (PLS). These deliver an insight into the correlated changes that occur between molecular structure and energy along the reaction coordinate between the physisorbed and chemisorbed species within the zeolite. To the best of our knowledge, this is the first occasion multivariate techniques such as PCA or PLS have been employed to profile the changes in electronics, distances, and angles in QM calculations of catalytic systems such as zeolites. We find the calculated adsorption and the alkoxide intermediate energies correlate strongly with the absolute charge on the substrate and the length of the substrate double bond. The transition states' energies are not affected by the zeolite framework as modeled, which explains why they correlate strongly with the gas-phase substrate protonation energy. Our cluster results show that for ethene, propene, 1-butene, and isobutene, the relative energetics associated with the formation of the alkoxide intermediate in ZSM-5 follow the same trends as calculations where the effects of the framework are included.

Application of QM simulations and multivariate analysis in the study of alkene reactivity in the zeolite H‐ZSM5

“…This effect has also been reported by Tantanak et al [9] when analyzing the correlation between the same 3T energetics reported here and the comparable energies of Boronat et al's [11] from QM/periodic simulations of theta-1. There the cluster model The line of unity is denoted by the black dot-dash line.…”

“…Alkoxide intermediate formation involves the transfer of the Brønsted proton on the zeolite O1 atom to the C1 of the alkene with the subsequent formation of a bond between the C2 and O2 atoms of the substrate and zeolite, respectively (Figure 1). Theoretical studies on alkoxide intermediates have often been restricted to relatively small cluster models [9][10][11] Studies comparing gas phase cluster models to those that encode information about the active site are generally restricted to reactions (chemisorption) of relatively small numbers of stationary points, or restricted to the adsorption (physisorption) process only [16]. A comparison of cluster calculations to more complete 50T ONIOM, or 20T periodic models, using reactions associated with ketones and small alkenes, respectively, reveals the same energetic trends irrespective of the model-type, however the absolute values may not be identical [9,17].…”

“…Theoretical studies on alkoxide intermediates have often been restricted to relatively small cluster models [9][10][11] Studies comparing gas phase cluster models to those that encode information about the active site are generally restricted to reactions (chemisorption) of relatively small numbers of stationary points, or restricted to the adsorption (physisorption) process only [16]. A comparison of cluster calculations to more complete 50T ONIOM, or 20T periodic models, using reactions associated with ketones and small alkenes, respectively, reveals the same energetic trends irrespective of the model-type, however the absolute values may not be identical [9,17]. Additionally, Jansang et al [18] report that 3T or 10T cluster models significantly underpredict the adsorption energies of benzene in mordenite when compared to 120T ONIOM simulations, and Solans-Monfort et al [16] report that a 5T cluster model study of NH 3 adsorption to mordenite was unable to reproduce the structure or adsorption energy when compared to ONIOM or periodic density functional theory (DFT) simulation.…”

“…The results have been systematically assessed using statistical methods in order to accurately quantify the impact of the extended zeolite framework. Principal components analysis (PCA) [19][20][21][22][23] is employed here, a widely used statistical technique in cheminformatics [24][25][26], which has also recently been applied to rationalize the results of zeolite simulations [9,27]. …”

“…Descriptors consisted of the key bond distances and angles and the Mulliken charges on the heavy atoms of the system. The PCA model building process is the same as that reported elsewhere [9].Linear regression is used to assess the concordance between the three different energies. The Spearman's correlation coefficient is used in preference to the Pearson's correlation coefficient as the data are not normally distributed making the latter statistic unreliable.…”