Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation.,Angewandte Chemie International Edition

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Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation.
Angewandte Chemie International Edition ( IF 16.1 ) Pub Date : 2020-08-03 , DOI: 10.1002/anie.202007147
Yunwen Zhou ₁ , Hari Thirumalai ₁ , Scott K Smith ₂ , Kenton H Whitmire ₃ , Jing Liu ₄ , Anatoly I Frenkel _{5,

6} , Lars C Grabow _{1,

2,

7} , Jeffrey D Rimer _{1,

2}

Affiliation

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra‐framework gallium species on enriched aromatics production in zeolite ZSM‐5. We compare three distinct methods of preparing Ga‐ZSM‐5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post‐synthesis methods. Using a combination of state‐of‐the‐art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)–Ga(extra‐framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra‐framework Ga at framework sites comprised of either Al or Ga reveal a site‐specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H‐ZSM‐5.

中文翻译：

Ga-ZSM-5催化剂上的乙烯脱氢芳构化：镓形态的性质和作用。

同时具有布朗斯台德和路易斯酸位点的沸石中的双功能催化为调整形状选择性和增强催化剂性能提供了独特的机会。在这里，我们研究了骨架和骨架外镓种类对ZSM-5沸石中富集芳烃生产的影响。我们比较了制备Ga‐ZSM‐5的三种不同方法，并揭示了与合成后方法相比，直接（一步法）合成可得到最佳催化剂的方法。综合使用最新的表征，催化剂测试和密度泛函理论计算，我们表明Ga Lewis酸位点强烈支持芳构化。我们的研究结果还表明，Ga（骨架）-Ga（额外骨架）配对只能在通过直接合成制备的材料中实现，是产生芳族化合物的反应途径中在能量上最有利的部位。在由Al或Ga组成的构架位点上，构架外的Ga之间的酸位交换能量表明，特定位置偏爱稳定路易斯酸，这在质量上与实验结果一致。这些发现表明，通过将Ga杂原子放置在沸石骨架中不同的四面体位置来调整Lewis酸的位置的可能性，相对于传统的H-ZSM-5，这可能会对催化剂的性能产生显着影响。

更新日期：2020-08-03

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