Computational Studies of Water Adsorption in the Zeolite H-ZSM-5

Abstract

Ab initio molecular orbital calculations using Hartree−Fock theory and Møller−Plesset perturbation theory have been used to study the interaction of H2O with the Brønsted acid site in the zeolite H-ZSM-5. Aluminosilicate clusters with up to 28 T atoms (T = Si, Al) were used as models for the zeolite framework. Full optimization of a 3 T atom cluster at the MP2/6-31G(d) level indicates that the “ion-pair” structure, Z-···HOH2 +, formed by proton transfer from the acid site of the zeolite (ZH) to the adsorbed H2O molecule, is a transition state, while the “neutral” adsorption structure, ZH···OH2, is a local energy minimum. Partial optimization of a larger 8 T cluster at the HF/6-31G(d) level also gave results suggesting that the ion-pair structure is a transition state. Calculations were carried out to obtain corrections for high levels of theory, zero-point energies, and larger cluster size. The resulting energy difference between the neutral and ion-pair structure is small (less than 5 kcal/mol and possibly close to zero). The interaction energy of ZH···OH2 is 13−14 kcal/mol, in agreement with experiment. We find that addition of a second H2O molecule to Z-···HOH2 + in the 3 T atom cluster stabilizes the ion-pair structure, Z-···H(OH2)2 +, making it a local energy minimum. Finally, calculated vibrational frequencies for a 3 T atom cluster are used to help interpret experimental IR absorption spectra.

The heterogeneity of the hydroxyl groups in chabazite

“…For the open 7T clusters the difference almost disappears. All values for adsorption energies listed in Table II are within the range of previous theoretical studies, 8,10,14,17,19 from Ϫ45.6 to Ϫ92.5 kJ/mol. Geometries.…”

“…With regard to the distance between the water molecule and the zeolite proton and the zeolite hydroxyl bond distance, our results differ from previous theoretical calculations. 7,8,10,17,19 We have found water to be closer to the zeolite proton compared to the adsorption modes found by Ryder et al 17 and Zygmunt et al 10 and further from the zeolite proton in comparison with Krossner et al 8 and Nusterer et al 19 The zeolite hydroxyl bond distance was found to be shorter.…”

“…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.…”

Quantum chemical calculation of infrared spectra of acidic groups in chabazite in the presence of water

“…During the last decade, adsorption of water on zeolites has been extensively studied both experimentally [1][2][3][4][5][6][7][8] and theoretically [9][10][11][12][13][14][15][16][17][18][19][20] in order to understand the nature of the interaction of basic molecules with the Brøn-sted sites in the zeolites. The reaction can be studied by infrared spectroscopy because the frequencies of the vibrations of the zeolite hydroxyl group will change upon interaction with the water molecule.…”