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

Abstract

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

Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCl3

“…42 For the application of these equations, rate coefficients (per Al tot atom) were normalized by the fraction of [GaH] 2+ cations per Al tot atom determined via NH 3 -TPD measurements (details described in the SI). 41 2.3. Theoretical Methods.…”

“…Detailed characterization of these samples shows that for Ga/Al ratios below 0.3, and upon reduction under anhydrous conditions, all of the Ga is present as isolated [GaH] 2+ cations or as [GaH 2 ] + -H + cation pairs; neutral GaO x agglomerates are undetectable in these samples. 41…”

Multiple Methane Activation Pathways on Ga‐modified ZSM‐5 Zeolites Revealed by Solid‐State NMR Spectroscopy

“…In a similar way, it was reported that [Ga(OH)H] + cations on zeolite undergo condensation with the proximate Brønsted acid protons to form [GaH] 2 + cations and one equivalent of H 2 O. [14] The divalent [GaH] 2 + cations show high activity for CÀ H activation of light alkane. [6c,19] This lead us to deduce that the [GaCH 3 ] 2 + may further react with CH 4 to form [Ga(CH 3 ) 2 ] + ( 13 C signal at ca.…”

“…The formation of Ga-OH groups is reflected by the two weak signals at 0.9 and 1.4 ppm on Ga/ ZSM-5-redox ( Figure 4b), probably due to different types of Ga species. [8,14] It is interesting to note that the signal intensity of Ga-OH (0.9 ppm) is remarkably increased on Ga/ZSM-5-redox- 13 CH 4 , accompanied with a new signal at 0.1 ppm (Figure 4c). According to the 1 H chemical shift for metal methyl groups on zeolites, [11b,15] the Ga-CH 3 species is responsible for the 0.1 ppm signal.…”

Quantification and Statistical Analysis of Errors Related to the Approximate Description of Active Site Models in Metal‐Exchanged Zeolites

“…[10] Hsieh and coworkers have shown that exchanging 8 % of the Brønsted acid sites of H-ZSM-5 increases the selectivity to aromatics from~15 % to~60 % during DHA of ethylene. [11] Cu-, Zn-, Ga-and Fe-exchanged zeolites are known to catalyze reactions such as the partial oxidation of methane to methanol, [12,13] dehydrogenation of light alkanes, [14][15][16][17] and decomposition of N 2 O to N 2 and O 2 , [18][19][20] respectively. Overall, these selected examples demonstrate that metal-exchanged zeolites offer the potential to convert readily available methane to value-added products.…”