Skip to main content
eScholarship
Open Access Publications from the University of California

Characterization of Isolated Ga3+Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3

  • Author(s): Phadke, NM
  • Van Der Mynsbrugge, J
  • Mansoor, E
  • Getsoian, AB
  • Head-Gordon, M
  • Bell, AT
  • et al.
Abstract

© 2018 American Chemical Society. Ga/H-MFI was prepared by vapor-phase reaction of GaCl3with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl2]+cations in the as-exchanged zeolite are treated in H2at 823 K to stoichiometrically remove Cl ligands and form [GaH2]+cations. Subsequent oxidation in O2and characterization by IR spectroscopy and NH3-temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga3+exists predominantly as [Ga(OH)2]+-H+cation pairs and to a lesser degree as [Ga(OH)]2+cations at low Ga/Al ratios (∼0.1); while both species are associated with proximate cation-exchange sites, calculated free energies of formation suggest that [Ga(OH)]2+cations are more stable on cation-exchange sites associated with NNN (next-nearest neighbor) framework Al atoms than on those associated with NNNN (next-next-nearest neighbor) framework Al atoms. Ga K-edge X-ray Absorption Near Edge Spectroscopy (XANES) measurements indicate that, under oxidizing conditions and for all Ga/Al ratios, all Ga species are in the +3 oxidation state and are tetrahedrally coordinated to 4 O atoms. Fourier analysis of Ga K-edge Extended X-ray Absorption Fine Structure (EXAFS) data supports the conclusion that Ga3+is present predominantly as [Ga(OH)2]+cations (or [Ga(OH)2]+-H+cation pairs). For Ga/Al ratios ≤0.3, wavelet analysis of EXAFS data provide evidence for backscattering from nearest neighboring O atoms and from next-nearest neighboring framework Al atoms. For Ga/Al > 0.3, backscattering from next-nearest neighboring Ga atoms is also evident, characteristic of GaOxspecies. Upon reduction in H2, the oxidized Ga3+species produce [Ga(OH)H]+-H+cation pairs, [GaH2]+-H+cation pairs, and [GaH]2+cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga3+species depends sensitively on temperature, Al-Al interatomic distance, and H2and H2O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH2]+-H+cation pairs and [GaH]2+cations are the predominant species present in Ga/H-MFI reduced above 673 K in 105Pa H2and in the absence of water vapor.

Many UC-authored scholarly publications are freely available on this site because of the UC Academic Senate's Open Access Policy. Let us know how this access is important for you.

Main Content
Current View