Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3
Published Web Locationhttps://doi.org/10.1021/acscatal.8b01254
Ga/H-MFI was prepared by vapor-phase reaction of GaCl3 with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl2]+ cations in the as-exchanged zeolite are treated in H2 at 823 K to stoichiometrically remove Cl ligands and form [GaH2]+ cations. Subsequent oxidation in O2 and 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 GaOx species. 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 H2 and 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 105 Pa H2 and in the absence of water vapor.