Abstract

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.

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