title: Three-coordinate aluminum in zeolites observed with in situ x-ray absorption near-edge spectroscopy at the Al K-edge: flexibility of aluminum coordinations in zeolites.
abstract: Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions. A unique, previously not observed, pre-edge feature is detected in zeolites H-Mordenite and steamed and unsteamed H-Beta at temperatures above 675 K. Spectra simulations using the full multiple scattering code Feff8 identify the unique pre-edge feature as three-coordinate aluminum. The amount of three-fold coordinated aluminum is a function of temperature and pretreatment of a zeolite: a steamed zeolite Beta contains more three-coordinate aluminum than an unsteamed sample. No clear differences between zeolites H-Mordenite and H-Beta were observed. Octahedrally coordinated aluminum forms in zeolites H-Mordenite and H-Beta at room temperature in a stream of wet helium. This octahedrally coordinated aluminum is unstable at temperatures higher than 395 K, where it quantitatively reverts to the tetrahedral coordination.
authors.affiliation: Institute for Chemical and Bioengineering, ETH Hönggerberg, CH-8093 Zurich, Switzerland. j.a.vanbokhoven@tech.chem.ethz.ch
authors.forename: Jeroen A
authors.initials: JA
authors.lastname: van Bokhoven
authors.forename: Ad M J
authors.initials: AM
authors.lastname: van der Eerden
authors.forename: Diek C
authors.initials: DC
authors.lastname: Koningsberger