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

Ga/H-MFI was prepared by vapor-phase reaction of GaCl<sub>3</sub> with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl<sub>2</sub>]<sup>+</sup> cations in the as-exchanged zeolite are treated in H<sub>2</sub> at 823 K to stoichiometrically remove Cl ligands and form [GaH<sub>2</sub>]<sup>+</sup> cations. Subsequent oxidation in O<sub>2</sub> and characterization by IR spectroscopy and NH<sub>3</sub>-temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga<sup>3+</sup> exists predominantly as [Ga(OH)<sub>2</sub>]<sup>+</sup>-H<sup>+</sup> cation pairs and to a lesser degree as [Ga(OH)]<sup>2+</sup> 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)]<sup>2+</sup> 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<sup>3+</sup> is present predominantly as [Ga(OH)<sub>2</sub>]<sup>+</sup> cations (or [Ga(OH)<sub>2</sub>]<sup>+</sup>-H<sup>+</sup> 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<sub><em>x</em></sub> species. Here, upon reduction in H<sub>2</sub>, the oxidized Ga<sup>3+</sup> species produce [Ga(OH)H]<sup>+</sup>-H<sup>+</sup> cation pairs, [GaH<sub>2</sub>]<sup>+</sup>-H<sup>+</sup> cation pairs, and [GaH]<sup>2+</sup> cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga<sup>3+</sup> species depends sensitively on temperature, Al-Al interatomic distance, and H<sub>2</sub> and H<sub>2</sub>O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH<sub>2</sub>]<sup>+</sup>-H<sup>+</sup> cation pairs and [GaH]<sup>2+</sup> cations are the predominant species present in Ga/H-MFI reduced above 673 K in 10<sup>5</sup>Pa H<sub>2</sub> and in the absence of water vapor.