Quantitatively Probing the Al Distribution in Zeolites

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• 作者：Aleksei Vjunov ; John L. Fulton ; Thomas Huthwelker ; Sonia Pin ; Donghai Mei ; Gregory K. Schenter ; Niranjan Govind ; Donald M. Camaioni ; Jian Zhi Hu ; Johannes A. Lercher
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：June 11, 2014
• 年：2014
• 卷：136
• 期：23
• 页码：8296-8306
• 全文大小：693K
• 年卷期：v.136,no.23(June 11, 2014)
• ISSN：1520-5126

文摘

The degree of substitution of Si⁴⁺ by Al³⁺ in the oxygen-terminated tetrahedra (Al T-sites) of zeolites determines the concentration of ion-exchange and Br酶nsted acid sites. Because the location of the tetrahedra and the associated subtle variations in bond angles influence the acid strength, quantitative information about Al T-sites in the framework is critical to rationalize catalytic properties and to design new catalysts. A quantitative analysis is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) analysis and ²⁷Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based molecular dynamics simulations, allowing quantitative treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3鈥? atom shells surrounding the Al absorption center. It is observed that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been determined using this analysis.