Computational studies of water adsorption in the zeolite H-ZSM-5

被引:85
作者
Zygmunt, SA
Curtiss, LA
Iton, LE
Erhardt, MK
机构
[1] ARGONNE NATL LAB, ARGONNE, IL 60439 USA
[2] VALPARAISO UNIV, DEPT PHYS & ASTRON, VALPARAISO, IN 46383 USA
关键词
D O I
10.1021/jp952913z
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Ab initio molecular orbital calculations using Hartree-Fock theory and Moller-Plesset perturbation theory have been used to study the interaction of H2O with the Bronsted 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.
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页码:6663 / 6671
页数:9
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