Understanding the Concept of Basicity in Zeolites. A DFT Study of the Methylation of Al-O-Si Bridging Oxygen Atoms
详细信息 查看全文
- 作者：Pierre Mignon ; Paul Geerlings ; Robert Schoonheydt
- 刊名：Journal of Physical Chemistry B
- 出版年：2006
- 出版时间：December 14, 2006
- 年：2006
- 卷：110
- 期：49
- 页码：24947 - 24954
- 全文大小：199K
- 年卷期：v.110,no.49(December 14, 2006)
- ISSN：1520-5207
文摘
DFT calculations on a 4-ring cluster and on ONIOM models of faujasite were carried out to assess the conceptof basicity in zeolites, exchanged with alkali cations. The considered reaction is the methylation of the Si-O-Al bridging oxygen by methanol and methyl iodide. The reaction involves both the dissociation of theH3C-OH or H3C-I bonds and the formation of the C-O-zeolite bond. The former involves the hardnessof the alkaline cation. The latter reflects the charge density of the basic oxygen, well described by the "hard"descriptor: the molecular electrostatic potential. The harder is the alkali metal, the easier is the H3C-OH orH3C-I bond dissociation, and the lower is the basicity of the bridging oxygen, and thus the more difficult isthe C-O-zeolite bond formation. The fact that these two processes compete has been established by comparingthe energy profiles for the methylation with methyl iodide and methanol. For methanol the role of the alkalinemetal on the bond dissociation prevails because of the larger hardness of the OH group as compared to thatof the iodine atom. For methyl iodide the oxygen basicity prevails over the interaction of I with metal. Thisstudy clearly shows that in both experimental and theoretical studies the role of the Lewis acidity or hardnessof the alkali metal ion and the role of the basicity of the framework oxygen have to be separated from eachother for a good interpretation of zeolite basicity. Also, the hardness of the probe molecule is particularlyimportant when considering the interaction with the alkali metal ion.