Quantum Chemical Modeling of Benzene Ethylation over H-ZSM-5 Approaching Chemical Accuracy: A Hybrid MP2:DFT Study

Abstract

The alkylation of benzene by ethene over H-ZSM-5 is analyzed by means of a hybrid MP2:DFT scheme. Density functional calculations applying periodic boundary conditions (PBE functional) are combined with MP2 energy calculations on a series of cluster models of increasing size which allows extrapolation to the periodic MP2 limit. Basis set truncation errors are estimated by extrapolation of the MP2 energy to the complete basis set limit. Contributions from higher-order correlation effects are accounted for by CCSD(T) coupled cluster calculations. The sum of all contributions provides the "final estimates" for adsorption energies and energy barriers. Dispersion contributes significantly to the potential energy surface. As a result, the MP2:DFT potential energy profile is shifted downward compared to the PBE profile. More importantly, this shift is not the same for reactants and transition structures due to different self-interaction correction errors. The final enthalpies for ethene, benzene, and ethylbenzene adsorption on the Brønsted acid site at 298 K are -46, -78, and -110 kJ/mol, respectively. The intrinsic enthalpy barriers at 653 K are 117 and 119/94 kJ/mol for the one- and two-step alkylation, respectively. Intrinsic rate coefficients calculated by means of transition state theory are converted to apparent Arrhenius parameters by means of the multicomponent adsorption equilibrium. The simulated apparent activation energy (66 kJ/mol) agrees with experimental data (58-76 kJ/mol) within the uncertainty limit of the calculations. Adsorption energies obtained by adding a damped dispersion term to the PBE energies (PBE+D), agree within +/-7 kJ/mol, with the "final estimates", except for physisorption (pi-complex formation) and chemisorption of ethene (ethoxide formation) for which the PBE+D energies are 12.4 and 26.0 kJ/mol, respectively larger than the "final estimates". For intrinsic energy barriers, the PBE+D approach does not improve pure PBE results.

Theoretical Simulation of n-Alkane Cracking on Zeolites

“…Increasing the cluster size leads to a significant reduction in the activation barrier, consistent with the recent findings of Hansen et al 31 for benzene alkylation by ethene. Improving the level of theory results in smaller changes in the activation barrier, which may be positive or negative depending on the specifics of the reaction considered and the size of the cluster.…”

“…75 For the alkylation of benzene it has been shown recently that this underestimation can be as large as 30 ( 10 kJ/mol. 31 As a result, our barriers calculated on the T5 cluster are reasonable approximations although an uncertainty of at least (10 kJ/mol will be present. Since the calculation of all reaction pathways considered in this study using a T23 cluster at the highest level of theory or periodic DFT is prohibitively expensive, calculations of the apparent rate coefficient were carried out using the intrinsic rate coefficients determined using the T5 cluster at the DFT/B3LYP/ SV(P) level.…”

Stability and reactivity of active sites for direct benzene oxidation to phenol in Fe/ZSM-5: A comprehensive periodic DFT study

“…The reaction mechanism for benzene oxidation to phenol considered over extraframework iron species (Fe EF ) is schematically shown in Fig. 1c [66][67][68][69]. This presents an interesting possibility to determine the preferred location of the suspected active sites and to investigate the influence of the zeolitic micropores on their reactivity and ultimately to create a molecular picture of the role of different iron species in Fe/ZSM-5 catalyst in transformations of benzene to phenol.…”

The structural properties of boron and nitrogen adsorption on benzene molecule: a density functional study

“…In chemical studies, benzene is used as a useful substance including as a solvent or also as a precursor of many synthetic materials, and so on [1]. While some works studied mono-benzene or multibenzene molecules solely [2,3], yet others considered benzene rings with hetero-atoms or hetero-molecules [4,5]. Among all hetero-atoms, we have selected boron and nitrogen as two atoms on both sides of carbon atom in periodic table with difference of one electron less and more than the number of valence electrons in carbon, respectively.…”