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

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Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3
ACS Catalysis ( IF 11.3 ) Pub Date : 2018-05-21 00:00:00 , DOI: 10.1021/acscatal.8b01254
Neelay M. Phadke ₁ , Jeroen Van der Mynsbrugge ₁ , Erum Mansoor ₁ , Andrew Bean Getsoian ₂ , Martin Head-Gordon ₃ , Alexis T. Bell ₁

Affiliation

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 GaO_x 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.

中文翻译：

H / MFI沸石与GaCl ₃气相交换制备的Ga / H-MFI中孤立的Ga ³⁺阳离子的表征

Ga / H-MFI是通过在干燥的H-MFI沸石中GaCl ₃与布朗斯台德酸OH基团进行气相反应制备的。将交换后的沸石中生成的[GaCl ₂ ] ⁺阳离子在H _2中于823 K处理，以化学计量去除Cl配体并形成[GaH ₂ ] ⁺阳离子。随后在O _2中发生氧化并通过红外光谱和NH _3程序升温脱附（TPD）进行表征，结果表明，对于Ga / Al比率≤0.3，Ga ³⁺主要以[Ga（OH）₂ ] ⁺ –H ⁺阳离子的形式存在对和较小程度的[Ga（OH）] ²⁺低Ga / Al比（〜0.1）的阳离子；虽然这两个物种都与附近的阳离子交换位点相关，但计算的形成自由能表明[Ga（OH）] ²⁺阳离子在与NNN（近邻）骨架Al原子相关的阳离子交换位点上比在位上更稳定。与NNNN（下一个-最近邻）框架A1原子相关的原子。Ga K边缘X射线吸收近边缘光谱（XANES）测量表明，在氧化条件下，对于所有Ga / Al比率，所有Ga物种均处于+3氧化态，并且四面体配位为4 O原子。Ga K边缘扩展X射线吸收精细结构（EXAFS）数据的傅里叶分析支持以下结论：Ga ³⁺主要以[Ga（OH）₂ ] ^{+的形式存在}阳离子（或[Ga（OH）₂ ] ⁺ –H ⁺阳离子对）。对于Ga / Al比率≤0.3，EXAFS数据的小波分析为从最近的相邻O原子和下一个最近的相邻骨架Al原子的反向散射提供了证据。当Ga / Al> 0.3时，GaO _x物种的特征还表明来自最近的邻近Ga原子的反向散射。H ₂还原后，氧化的Ga ^3+会生成[Ga（OH）H] ⁺ -H ⁺阳离子对，[GaH ₂ ] ⁺ -H ⁺阳离子对和[GaH] ²⁺阳离子。计算的相图表明，还原的Ga ³⁺物种的热力学稳定性敏感地取决于温度，Al-Al原子间距离以及H ₂和H ₂ O分压。对于Ga / Al比≤0.2，可以得出结论，[GaH ₂ ] ⁺ –H ⁺阳离子对和[GaH] ²⁺阳离子是Ga / H-MFI中存在的主要种类，在10 ⁵ Pa H _2中还原到673 K以上并且在没有水蒸气的情况下。

更新日期：2018-05-21

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