Stability of Zn(II) Cations in Chabazite Studied by Periodical Density Functional Theory
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- 作者：Luis Antonio M. M. Barbosa ; Rutger A. van Santen ; and Jü ; rgen Hafner
- 刊名：Journal of the American Chemical Society
- 出版年：2001
- 出版时间：May 16, 2001
- 年：2001
- 卷：123
- 期：19
- 页码：4530 - 4540
- 全文大小：248K
- 年卷期：v.123,no.19(May 16, 2001)
- ISSN：1520-5126
文摘
The location of the Zn2+ cation in Zn-exchanged chabazite has been studied by the periodical densityfunctional method. Chabazite was chosen as a zeolite model, because it contains three different types of ringscommonly found in the zeolite structures: four-, six-, and eight-membered rings. Two aluminum atoms havebeen employed to substitute the silicon atoms in the same D6R unit cell of the zeolite framework. This leadsto different arrangements for the Br
nsted site pair and the Zn(II) cation. The two Brnsted sites are found tobe more stable when placed in the small ring (4T ring) than in the other rings. This suggests that the mostreactive Brnsted sites are located in the large rings. Two Brnsted sites are most stable when the O(H)-Al-O-Si-O(H)-Al sequence is followed in the same ring instead of being located in two different rings.This resembles the aluminum distribution in the small four-membered ring and agrees with bond orderconservation rules. The cation stability is markedly influenced by the distortions of the framework. Otherfactors that also contribute to the stabilization are the aluminum content near the cation and the stability of theoriginal Brnsted sites. The Zn2+ cation is more stable in the large rings than in the small ones, the six-membered one being the most stable configuration. In the small rings, the cation is, therefore, more reactive.Two different probe molecules have been used to study the interaction with the Zn(II) cation: water andmethane. These probe molecules can extract the active center from its original position. For the water molecule,this effect is large and leads to a high framework relaxation. The value of the binding energy of this moleculeto the active sites is influenced by these framework relaxations as well as by the cationic position environment.For weakly interacting methane, these effects are significantly less.
nsted site pair and the Zn(II) cation. The two Brnsted sites are found tobe more stable when placed in the small ring (4T ring) than in the other rings. This suggests that the mostreactive Brnsted sites are located in the large rings. Two Brnsted sites are most stable when the O(H)-Al-O-Si-O(H)-Al sequence is followed in the same ring instead of being located in two different rings.This resembles the aluminum distribution in the small four-membered ring and agrees with bond orderconservation rules. The cation stability is markedly influenced by the distortions of the framework. Otherfactors that also contribute to the stabilization are the aluminum content near the cation and the stability of theoriginal Brnsted sites. The Zn2+ cation is more stable in the large rings than in the small ones, the six-membered one being the most stable configuration. In the small rings, the cation is, therefore, more reactive.Two different probe molecules have been used to study the interaction with the Zn(II) cation: water andmethane. These probe molecules can extract the active center from its original position. For the water molecule,this effect is large and leads to a high framework relaxation. The value of the binding energy of this moleculeto the active sites is influenced by these framework relaxations as well as by the cationic position environment.For weakly interacting methane, these effects are significantly less.