Mechanism and Kinetics of Light Alkane Dehydrogenation and Cracking over Isolated Ga Species in Ga/H-MFI,ACS Catalysis

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Mechanism and Kinetics of Light Alkane Dehydrogenation and Cracking over Isolated Ga Species in Ga/H-MFI
ACS Catalysis ( IF 11.3 ) Pub Date : 2021-02-02 , DOI: 10.1021/acscatal.0c04906
Neelay M. Phadke ₁ , Erum Mansoor ₁ , Martin Head-Gordon ₂ , Alexis T. Bell ₁

Affiliation

The objective of this study is to examine the mechanisms and kinetics of C₂H₆ dehydrogenation and n-C₄H₁₀ dehydrogenation and cracking over isolated Ga species in Ga/H-MFI and to compare these results to those reported previously for C₃H₈ dehydrogenation and cracking. C₂H₆ dehydrogenation is found to be catalyzed by both [GaH]²⁺ and [GaH₂]⁺ cations at similar turnover frequencies. Rate measurements over Ga/H-MFI containing predominantly [GaH₂]⁺ cations reveal that C₂H₆ dehydrogenation rates exhibit a Langmuir–Hinshelwood dependence on C₂H₆ partial pressure at elevated temperatures (>730 K), consistent with the involvement of chemisorbed [C₂H₅–GaH]⁺ species. The reaction kinetics suggest that C₂H₆ dehydrogenation proceeds via heterolytic C–H cleavage of adsorbed C₂H₆ by [GaH₂]⁺ cations to form H₂ and [C₂H₅–GaH]⁺ species, which further decompose via β-hydride elimination to form C₂H₄. By contrast, C₄H₁₀ dehydrogenation and both terminal and central cracking are catalyzed exclusively by [GaH]²⁺ cations. All three reactions exhibit a Langmuir–Hinshelwood dependence on C₄H₁₀ partial pressure and are inhibited by H₂. Ratios of dehydrogenation to cracking (total) and terminal to central cracking are approximately independent of C₄H₁₀ partial pressure consistent with the involvement of a common C₄H₁₀-derived surface intermediate. The observed reaction kinetics are consistent with an alkyl-mediated mechanism occurring over [GaH]²⁺, analogous to that reported previously for C₃H₈ dehydrogenation/cracking over Ga/H-MFI (Phadke, N. M.; J. Am. Chem. Soc. 2019, 141, 1614−1627). The mechanism proceeds via facile, heterolytic dissociation of adsorbed C₄H₁₀ to form [C₄H₉–GaH]⁺–H⁺ cation pairs via methyl C–H-activated pathways. Dehydrogenation then proceeds via β-hydride elimination, respectively, forming butene, while terminal and central cracking proceed via C–H-activated H⁺ attack. Methylene activation was also considered but found to occur at a significantly lower rate. Theoretical analysis of the proposed reaction pathways leads to apparent activation enthalpies in good agreement with values extracted from the measured kinetics, thereby supporting the proposed pathways and the roles of [GaH]²⁺ and [GaH₂]⁺ cations in the dehydrogenation and cracking of light alkanes on Ga/H-MFI.

中文翻译：

Ga / H-MFI中孤立Ga物种的轻链烷烃脱氢裂化的机理和动力学

这项研究的目的是检查在Ga / H-MFI中分离的Ga物种上C ₂ H ₆脱氢和n -C ₄ H ₁₀脱氢和裂解的机理和动力学，并将这些结果与先前报道的C _{3进行比较。} H ₈脱氢裂化。发现C ₂ H ₆脱氢由[GaH] ²⁺和[GaH ₂ ] ⁺阳离子以相似的转换频率催化。在主要包含[GaH ₂ ] ⁺阳离子的Ga / H-MFI上的速率测量表明，C₂ H ₆脱氢速率在升高的温度（> 730 K）下表现出Langmuir–Hinshelwood对C ₂ H ₆分压的依赖性，这与化学吸附的[C ₂ H ₅ –GaH] ⁺物质的参与一致。反应动力学表明，C ₂ H ₆脱氢是通过[GaH ₂ ] ⁺阳离子将吸附的C ₂ H ₆进行杂合的C–H裂解，形成H ₂和[C ₂ H ₅ –GaH] ^{+来进行的。}种类，其通过β-氢化物消除进一步分解形成C ₂ H ₄。相反，C ₄ H ₁₀脱氢以及末端和中心裂化都仅由[GaH] ²⁺阳离子催化。这三个反应均表现出Langmuir-Hinshelwood对C ₄ H ₁₀分压的依赖性，并被H ₂抑制。脱氢与裂化的比例（总）与末端裂化与中心裂化的比率与C ₄ H _10的分压近似无关，而C ₄ H _10的分压与常见的C ₄ H ₁₀的参与一致衍生的表面中间体。观察到的反应动力学与在[GaH] ^2+上发生的烷基介导机理相一致，类似于先前报道的在Ga / H-MFI上进行C ₃ H ₈脱氢/裂解的机理（新墨西哥法克; J.上午化学 Soc。 2019，141，1614年至1627年）。该机制通过吸附的C ₄ H ₁₀通过甲基C–H激活的途径容易地，杂化解离形成[C ₄ H ₉ –GaH] ⁺ –H ⁺阳离子对而进行。脱氢然后分别通过β-氢化物消除进行，形成丁烯，而末端和中心裂化通过C–H活化的H ^+进行。攻击。还考虑了亚甲基活化，但是发现其发生率显着降低。对拟议的反应途径的理论分析导致表观活化焓与从测得的动力学中提取的值高度吻合，从而支持了拟议的途径以及[GaH] ²⁺和[GaH ₂ ] ⁺阳离子在脱氢和裂解中的作用。 Ga / H-MFI上的轻烷烃。

更新日期：2021-02-19

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