Brønsted Acidity and Cracking Reactivity in Y Zeolites: Effects of Mesoporosity and Extra-Framework Aluminum

Mesoporosity and extra-framework aluminum (EFAL) species often form during post-treatment of Y-type zeolites, such as steaming or dealumination, and can significantly influence catalytic performance in fluid catalytic cracking (FCC). However, their individual effects on Brønsted acidity and intrinsic catalytic activity remain incompletely understood. In this study, we employ density functional theory (DFT) and hybrid quantum mechanical/molecular mechanical (QM/MM) methods to systematically investigate the influence of mesopores and EFAL on acid strength and cracking reactivity in FAU-type zeolites. Three representative models were constructed: a conventional microporous HY, an EFAL-containing HY, and a mesoporous HY. Proton affinity calculations reveal that mesopore formation slightly weakens Brønsted acidity, while the impact of EFAL depends on its type and configuration—Al(OH)₂⁺ enhances acidity, whereas Al(OH)₃ exhibits variable effects. To isolate the effect of mesoporosity, we simulate the cracking of n-hexane and 1,3,5-triisopropylbenzene (1,3,5-TIPB) on HY and meso-HY models. For n-hexane, mesoporosity has minimal influence on reaction energetics. In contrast, for bulky molecules such as 1,3,5-TIPB, higher free energy profiles are observed in meso-HY due to reduced framework interactions. These results provide molecular-level insight into how structural and compositional modifications affect acidity and catalytic function, offering guidance for the design of hierarchical zeolite catalysts.