Physisorption and Chemisorption of Hydrocarbons in H-FAU Using QM-Pot(MP2//B3LYP) Calculations

Abstract

The DFT parametrized zeolite force field in the QM-Pot program is extended with carbon−carbon, carbon−hydrogen, and alkoxy bond describing parameters. The extended force field has been combined with B3LYP and with MP2 as the high-level quantum mechanical (QM) method to simulate the physisorption and chemisorption of ethene, isobutene, 1-butene, 1-pentene, and 1-octene in H-FAU (Si/AlF = 95) and for physisorption of 1-pentene, n-pentane, 1-octene, and n-octane in all silica FAU. The new parametrization predicts more stable chemisorption complexes than physisorbed π complexes, but with smaller chemisorption energies which are more reliable as shown by comparison with experimental results and with accurate hybrid MP2:DFT calculations. An embedded cluster size study shows that, due to the importance of the stabilizing van der Waals part in the MM contribution of the cluster, QM-Pot(MP2//B3LYP) calculations yield more reliable physisorption and chemisorption energies of hydrocarbons in zeolites than QM-Pot(B3LYP). The QM-Pot(MP2//B3LYP) results are in good agreement with available experimental data. In H-FAU, the H+···alkane interaction was found to contribute at most 7 kJ/mol to the total physisorption energy of n-alkanes while the H+···π interaction contributes 20−25 kJ/mol to the total physisorption energy of alkenes. For n-alkene physisorption in H-FAU, a linear increase of both the physisorption and chemisorption energies of 8.7 kJ/mol per C-atom is found. The protonation energy of n-alkenes in H-FAU was found to be independent of the C-number and amounts to −50 kJ/mol for the formation of secondary alkoxides. The formation of tertiary alkoxides in H-FAU suffers slightly from steric constraints imposed by the zeolite framework.

Adsorption, reduction and storage of hydrogen within ZSM-5 zeolite exchanged with various ions: A comparative theoretical study

“…Density functional theory (DFT) methods are known to be deficient to describe long-range dispersion interactions [51][52][53][54][55]. The hybrid MP2:DFT scheme was proposed to account for the dispersion energies reproducing the reaction barriers with near chemical accuracy [51,52].…”

“…Besides the default method (MP2:DFT) that has been testified to reproduce reaction barriers with near chemical accuracy [51,52], the adsorption energies on the nH 2 /ZMg II cluster models are calculated by other theoretical methods (0 6 n 6 4). of the MP2 method but reduces significantly the computational costs.…”

Theoretical Chemistry of Zeolite Reactivity

“…In this case, the ab initio part (usually treated by DFT methods) describing the bond rearrangement at the zeolite active site is intentionally limited to a small part of the zeolite, while the van der Waals and electrostatic interactions with the remaining zeolite lattice are described using a computationally less demanding force field methods. This methodology allows fast and rather accurate calculation of the heats of adsorption and reaction energies of various hydrocarbons in zeolites [40,51,52]. However, when using the conventional DFT as the ''high-level'' method, the correct description of the dispersion contribution to the adsorption energy of longer-chain hydrocarbons requires the use of very small, usually containing only 3T atoms, cluster model [40].…”

“…This methodology allows fast and rather accurate calculation of the heats of adsorption and reaction energies of various hydrocarbons in zeolites [40,51,52]. However, when using the conventional DFT as the ''high-level'' method, the correct description of the dispersion contribution to the adsorption energy of longer-chain hydrocarbons requires the use of very small, usually containing only 3T atoms, cluster model [40]. The energetics can be substantially improved by correcting the DFT results by single-point MP2 calculations.…”

A new massively parallel version of CRYSTAL for large systems on high performance computing architectures

“…This means that the quantum mechanical modeling of adsorption and reactions in this system is well within the capabilities of the present code without the need to resort to more approximate quantum mechanics (QM)/molecular mechanics (MM) techniques. [13] Applicability of MPPCRYSTAL to systems of chemical interest is not limited to crystalline systems. Due to the localized nature of the Gaussian basis functions, isolated molecules can be efficiently and rigorously treated, at variance with plane wave-based codes.…”