The mechanisms of dehydrogenation reactions as important processes in methanol to aromatics (MTA) have been controversial. Recent work on the active center for dehydrogenation at either Lewis acid site (LAS) or Brønsted−Lewis (B−L) acid synergistic site is a matter. The dehydrogenation processes on L−acid site (GaO) or B−L acid site (HGaO) over GaO/HZSM-5 with different Lewis acid locations for -hexene to 1,5-hexadiene, as well as cyclohexene to benzene have been researched by applying the density functional theory (DFT) method. The results reflect that active center of dehydrogenation reactions is B−L acid synergistic site through B−L acid synergy mechanism. All elementary steps including C−H bond activation, the formation of H, hydrogen transfer as well as the regeneration of B−acid site are easy to proceed. However, the isomerization for carbonium ions from GaO to skeleton oxygen of zeolites is relatively difficult. The analysis shows that isomerization is influenced by the structural and electronic properties of carbonium ions chemisorbed on zeolites. Lewis acid strength of GaO/HZSM-5 and energy gap between HOMO and LUMO on adsorption complexes are appropriate descriptors for the C−H bond activation. The rate constants analysis indicates that increasing temperature is more favorable for C−H bond activation. Further considering that C−H bond activation occurred preferentially than isomerization, it could be a critical initial step to primarily screen catalysts.

中文翻译：

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深入探讨GaO+/HZSM-5上甲醇脱氢制芳烃机理：哪个是活性中心、Lewis酸性位点还是Brønsted−Lewis协同位点？

脱氢反应作为甲醇制芳烃（MTA）的重要过程，其机制一直存在争议。最近关于路易斯酸位点（LAS）或布朗斯特-刘易斯（BL）酸协同位点脱氢活性中心的研究是一个问题。研究了不同Lewis酸位置的GaO/HZSM-5上L−酸位(GaO)或B−L酸位(HGaO)上己烯脱氢成1,5-己二烯以及环己烯脱氢成苯的过程通过应用密度泛函理论（DFT）方法。结果表明，通过BL酸协同机制，脱氢反应的活性中心是BL酸协同位点。所有基本步骤包括C−H键活化、H的形成、氢转移以及B−酸位点的再生都很容易进行。然而，碳正离子从GaO异构化为沸石骨架氧相对困难。分析表明异构化受到化学吸附在沸石上的碳正离子的结构和电子性质的影响。 GaO/HZSM-5 的路易斯酸强度以及吸附配合物上 HOMO 和 LUMO 之间的能隙是 C−H 键激活的适当描述符。速率常数分析表明，升高温度更有利于C−H键活化。进一步考虑到 C−H 键活化比异构化优先发生，这可能是初步筛选催化剂的关键初始步骤。