Quantitatively Probing the Al Distribution in Zeolites

• Published in: Journal of the American Chemical Society Vol. 136; no. 23; pp. 8296 - 8306
• Main Authors: Vjunov, Aleksei, Fulton, John L, Huthwelker, Thomas, Pin, Sonia, Mei, Donghai, Schenter, Gregory K, Govind, Niranjan, Camaioni, Donald M, Hu, Jian Zhi, Lercher, Johannes A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 11.06.2014
• Subjects: absorption; aluminum; catalysts; geometry; ion exchange; molecular dynamics; nuclear magnetic resonance spectroscopy; quantitative analysis; stable isotopes; X-radiation; zeolites
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja501361v

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 Brønsted 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 3–4 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.