Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation.
Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation.
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DOI：
10.1002/anie.202007147
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发表时间：
2020-08
影响因子：
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通讯作者：
Yunwen Zhou;Hari Thirumalai;Scott K. Smith;K. Whitmire;Jing Liu;A. Frenkel;L. Grabow;J. Rimer
中科院分区：
文献类型：
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作者：
Yunwen Zhou;Hari Thirumalai;Scott K. Smith;K. Whitmire;Jing Liu;A. Frenkel;L. Grabow;J. Rimer
具有布朗斯特德和路易斯酸位点的沸石中的双功能催化为定制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物种对 ZSM-5 沸石中富集芳烃生产的影响。在本研究中，我们比较了三种不同的 Ga-ZSM-5 制备方法，并揭示了与后合成方法相比，直接（单步）合成可产生最佳催化剂。通过结合最先进的表征、催化剂测试和密度泛函理论计算，我们表明 Ga 路易斯酸位点强烈有利于芳构化。我们的研究结果还表明，Ga（骨架）-Ga（骨架外）配对只能在直接合成制备的材料中实现，是产生芳烃的反应途径最有利的位点。计算出的由 Al 或 Ga 组成的骨架位点上的骨架外 Ga 之间的酸位点交换能揭示了稳定路易斯酸的位点特异性偏好，这在质量上与实验测量一致。这些发现表明可以通过将Ga杂原子放置在沸石骨架中不同的四面体位点来调整路易斯酸位点，相对于传统的H-ZSM-5，这可以对催化剂性能产生显着影响。
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. In this study 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.