Organometallic Model Complexes Elucidate the Active Gallium Species in Alkane Dehydrogenation Catalysts Based on Ligand Effects in Ga K-Edge XANES

Singfoong Cheah, Joshua Schaidle, Daniel Ruddy, Jesse Hensley, Andrew Getsoian, Ujjal Das, Jeffrey Camacho-Bunquin, Guanghui Zhang, James Gallagher, Bo Hu, Theodore Krause, Larry Curtiss, Jeffrey Miller, Adam Hock

Research output: Contribution to journalArticlepeer-review

88 Scopus Citations

Abstract

Gallium-modified zeolites are known catalysts for the dehydrogenation of alkanes, reactivity that finds industrial application in the aromatization of light alkanes by Ga-ZSM5. While the role of gallium cations in alkane activation is well known, the oxidation state and coordination environment of gallium under reaction conditions has been the subject of debate. Edge shifts in Ga K-edge XANES spectra acquired under reaction conditions have long been interpreted as evidence for reduction of Ga(iii) to Ga(i). However, a change in oxidation state is not the only factor that can give rise to a change in the XANES spectrum. In order to better understand the XANES spectra of working catalysts, we have synthesized a series of molecular model compounds and grafted surface organometallic Ga species and compared their XANES spectra to those of gallium-based catalysts acquired under reducing conditions. We demonstrate that changes in the identity and number of gallium nearest neighbors can give rise to changes in XANES spectra similar to those attributed in literature to changes in oxidation state. Specifically, spectral features previously attributed to Ga(i) may be equally well interpreted as evidence for low-coordinate Ga(iii) alkyl or hydride species. These findings apply both to gallium-impregnated zeolite catalysts and to silica-supported single site gallium catalysts, the latter of which is found to be active and selective for dehydrogenation of propane and hydrogenation of propylene.

Original languageAmerican English
Pages (from-to)6339-6353
Number of pages15
JournalCatalysis Science and Technology
Volume6
Issue number16
DOIs
StatePublished - 2016

Bibliographical note

Publisher Copyright:
© 2016 The Royal Society of Chemistry.

NREL Publication Number

  • NREL/JA-5100-67032

Keywords

  • gallium
  • ligand effects
  • XANES

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