Ga/H-MFI was prepared by vapor-phase reaction of GaCl with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl ] cations in the as-exchanged zeolite are treated in H at 823 K to stoichiometrically remove Cl ligands and form [GaH ] cations. Subsequent oxidation in O and characterization by IR spectroscopy and NH -temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga exists predominantly as [Ga(OH) ] -H cation pairs and to a lesser degree as [Ga(OH)] 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)] 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 Ga is present predominantly as [Ga(OH) ] cations (or [Ga(OH) ] -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 GaO species. Upon reduction in H , the oxidized Ga species produce [Ga(OH)H] -H cation pairs, [GaH ] -H cation pairs, and [GaH] cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga species depends sensitively on temperature, Al-Al interatomic distance, and H and H O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH ] -H cation pairs and [GaH] cations are the predominant species present in Ga/H-MFI reduced above 673 K in 10 Pa H and in the absence of water vapor. 3 2 2 2 2 3 2 2 2 x 2 2 2 2 2 2 + + 3+ + + 2+ 2+ 3+ + + + 3+ + + + + 2+ 3+ + + 2+
