Ethylene Dehydroaromatization over Ga‐ZSM‐5 Catalysts: Nature and Role of Gallium Speciation

• Published in: Angewandte Chemie (International ed.) 2020-10, Vol.59 (44), p.19592-19601
• Main Authors: Zhou, Yunwen, Thirumalai, Hari, Smith, Scott K., Whitmire, Kenton H., Liu, Jing, Frenkel, Anatoly I., Grabow, Lars C., Rimer, Jeffrey D.
• Format: Article
• Language: English
• Subjects: acid siting; Acids; Aromatic compounds; aromatics; bifunctional catalysis; Catalysis; Catalysts; Chemical synthesis; Density functional theory; Gallium; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; Lewis acid; Mathematical analysis; Selectivity; Speciation; zeolite synthesis; Zeolites
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202007147

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. Direct synthesis of Ga‐ZSM‐5 catalysts leads to dramatic enhancement in the production of aromatics from ethylene compared to post‐synthesis preparation methods. Combined experiments and modeling reveal an energetic preference for Ga extra‐framework sites to associate with select framework sites, suggesting the possibility of tailoring Lewis acid siting in zeolites by controlled heteroatom substitution in framework tetrahedral sites.