Comparison of a combined quantum mechanics/interatomic potential function approach with its periodic quantum-mechanical limit: Proton siting and ammonia adsorption in zeolite chabazite

Abstract

Comparison is made between a combined quantum mechanics/interatomic potential function approach ͑QM-Pot͒ and its fully quantum-mechanical limit, ab initio calculation applying periodic boundary conditions. The Hartree-Fock ͑HF͒ method is combined with ab initio-parametrized ion pair shell model potential functions. The CRYSTAL code is employed for the periodic Hartree-Fock calculations. The same double-/valence triple-zeta polarization basis sets are used in both the approaches. The proton siting and ammonia adsorption in a high-silica acidic zeolite catalyst, H-chabazite ͑Si/Alϭ11, space group P1, unit cell H-AlO 2 ͓SiO 2 ] 11 ) are examined. The combined QM-Pot relative stabilities and reaction energies deviate from the periodic full QM results by 4-9 kJ/mol only, which demonstrates the power of our combined approach. This conclusion is also supported by comparison of the electrostatic potential inside the zeolite pore, calculated from the periodic wave function and by the QM-Pot approach. Framework oxygen O1 is found to be the preferred proton site and on interaction with NH 3 the proton is predicted to move to NH 3 yielding NH 4ϩ . The NH 4 ϩ surface species is coordinated to two framework oxygen atoms. It is by 30-35 kJ/mol more stable than the neutral adsorption complex of NH 3 . Evidence is produced that the failure of previous periodic HF calculations to predict a stable NH 4 ϩ ion is due to the limitations of the minimum basis set used.

The heterogeneity of the hydroxyl groups in chabazite

“…26,39 and embedded 2T clusters. 44 The corresponding distance for the H 1 proton position in our calculation is longer by 0.025 Å. Greatbanks et al 7 have also found a slight increase of the Al-O͑H͒ and OH distances when embedding a 3T cluster.…”

“…Based on periodic calculation Haase et al 26 have found a value of 1196 kJ/mol for the H 1 proton position. Embedding 2T clusters Brändle et al 44 have reported 1277 kJ/mol and 1258 kJ/mol for H 1 and H 3 proton positions, respectively. Compared to our results, the difference in the proton affinity of the two oxygen atoms is larger, 17 kJ/mol, and in their calculations the oxygen atom O 1 has a higher proton affinity than the oxygen atom O 3 .…”

A theoretical study of cis–trans isomerisation in H-ZSM5: probing the impact of cluster size and zeolite framework on energetics and structure

“…In particular, quantum mechanical (QM) simulations have been extensively employed to study concepts in zeolite catalysis, offering a unique way to rationalise experimental results. Simulations of zeolites have ranged from the use of small QM clusters (3T [2,3] & 5T [4,5]), large ring clusters [4][5][6], or more complete representations of the zeolite lattice using ONIOM [4,[7][8][9], QM/MM [10,11] or alternate embedded/periodic methods [9,[12][13][14][15][16]. These references represent a very small sample of those available in the literature and have been highlighted by the author due to the direct relevance to this study.…”

Finding transition structures in extended systems: A strategy based on a combined quantum mechanics–empirical valence bond approach

“…They accurately describe the reaction center and, at the same time account for structure constraints imposed by, and the long-range interactions with the environment of the site at modest computational expense. 24 We have proposed our own QM-Pot scheme 17,25 and applied it to different problems of zeolite catalysis which involved minima on the potential energy surfaces. 17,[24][25][26][27][28][29][30][31][32] In this work we extend the QM-Pot scheme to studies of transition structures.…”