A DFT Study of Methanol Adsorption in 8T Rings of Chabazite

Abstract

Hybrid B3LYP and gradient-corrected PW91 functionals were used for studying methanol adsorption on a zeolite cluster consisting of an 8T ring of chabazite. The comparison of the results obtained with PW91 with periodic calculations has shown that the adopted ring is an adequate approximation for the Brønsted sides in chabazite. Both physisorbed and chemisorbed methanol were found to be a minimum on the potential energy surface, with an energy difference up to 10 kJ/mol in favor of the hydrogen-bonded complex. It has been shown that compared to B3LYP, the PW91 functional overestimates the hydroxyl bond distance and underestimates the hydrogen bond distance. In the physisorbed mode, the methanol oxygen atom is strongly bonded to the zeolite proton, whereas the distance between the methanol proton and the framework oxygen atoms is 1.912−2.090 Å. We have calculated for hydrogen bonded methanol hydroxyl stretch frequencies in the intervals 3677−3582 and 2358−2187 cm-1 for the methanol and the zeolite OH bonds, respectively.

The heterogeneity of the hydroxyl groups in chabazite

“…To assess the basis set superposition error ͑BSSE͒ we have carried out counterpoise calculations. 31,32 We have used a PW91 and DZVP2 basis set on all atoms in the 8T ring 18 and we have not found significant differences in the geometry and the harmonic frequencies when the STO-3G and DZVP2 basis sets were used on the dangling H atoms. Cluster calculations as described above have been shown to give good results for the interaction of molecules with zeolites.…”

“…2͒, represented in the same way as described in Ref. 18. To assess the basis set superposition error ͑BSSE͒ we have carried out counterpoise calculations.…”

“…Cluster calculations as described above have been shown to give good results for the interaction of molecules with zeolites. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In the calculations of the shifts of the frequency of the zeolite OH stretch vibration the anharmonicity has been included. This has been done by representing the potential energy in the 1-dimensional vibrational Hamiltonian by a polynomial:…”

“…The adsorption of water and methanol has been studied both on clusters [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] representing a small part of the zeolite and in periodic calculations [19][20][21][22][23][24][25][26] including the zeolite crystal structure. The role of the zeolite structure on the activation of methanol has been studied in periodic approach but the conclusions are somewhat contradictory.…”

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…[50][51][52][53][54][55][56][57][58][59][60][61] Water and methanol are two crucial molecules during the MTO process. In some cases water is added to the methanol feed in the form of process condensate or steam to tune the product selectivity.…”