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Many zeolites are deployed in various industrial processes owing to their robust catalytic performance and hydrothermal stability. Reactions in zeolites are catalyzed via framework aluminum. The Si/Al ratio is a metric that describes the relative aluminum content in zeolites. However, several researchers noted that the proximity of aluminum in the framework could impact the catalyst output [1–3]. In this work, the influence of paired acid sites is examined in ZSM-22. The 1-dimensional nature of ZSM-22 allows for direct assessment of aluminum proximity without the influence of channel intersection. Theoretical investigations via static density functional theory (DFT) optimization calculations on isolated and paired BAS in ZSM-22 revealed a potential increase in deprotonation potential energy (DPE), indicating a weaker acid with closer aluminum sites. One specific paired model, however, suggested stronger acid behavior, likely due to unfavorable proton-proton interactions influenced by proximity and orientation. Additionally, ammonia adsorption calculations inferred improved adsorption by isolated models, possibly due to unfavorable ammonium-proton interactions in the paired models. Reaction state calculations of ethylene and propylene oligomerization suggested enhanced stabilization of reactant molecules in paired sites. The synthesis of ZSM-22 showed sensitivity to precursor ratios and conditions, but pure samples were successfully achieved through iterative optimization. Catalytic testing of ethylene oligomerization with these samples, classified by their Si/Al ratios and unique fractions of paired acid sites, showed a correlation between higher fractions of paired BAS and increased catalytic activity and selectivity. Samples with higher fractions of paired BAS displayed a higher activity and selectivity for heavier hydrocarbons, explained by the enhanced adsorption capacity of paired BAS for larger reactant molecules, prompting further oligomerization and enhanced catalytic activity. Our findings demonstrate the impact of BAS proximity in dictating the activity and selectivity in ZSM-22 and provide valuable insights for designing more efficient industrial zeolite-based catalysts. |
