title: Quantitatively probing the Al distribution in zeolites.
abstract: The degree of substitution of Si(4+) by Al(3+) 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 (27)Al 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.
authors.affiliation: Institute for Integrated Catalysis, §Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States.
authors.forename: Aleksei
authors.initials: A
authors.lastname: Vjunov
authors.forename: John L
authors.initials: JL
authors.lastname: Fulton
authors.forename: Thomas
authors.initials: T
authors.lastname: Huthwelker
authors.forename: Sonia
authors.initials: S
authors.lastname: Pin
authors.forename: Donghai
authors.initials: D
authors.lastname: Mei
authors.forename: Gregory K
authors.initials: GK
authors.lastname: Schenter
authors.forename: Niranjan
authors.initials: N
authors.lastname: Govind
authors.forename: Donald M
authors.initials: DM
authors.lastname: Camaioni
authors.forename: Jian Zhi
authors.initials: JZ
authors.lastname: Hu
authors.forename: Johannes A
authors.initials: JA
authors.lastname: Lercher