Spatial Proximity and Charge Transfer: Two Key Factors Governing Brønsted/Lewis Acid Synergy in Dealuminated H-ZSM-5 Zeolites

Abstract

In dealuminated zeolites, extra-framework Al (EFAL) species released from the zeolite framework can act as Lewis acid sites (LAS) and exhibit a synergistic effect with neighboring Brønsted acid sites (BAS). However, the factors governing this synergy remain elusive. In this study, we elucidate how BAS and EFAL species cooperatively modulate the acidity of dealuminated H-ZSM-5 zeolites and then extend this understanding to the activation of propane. Through a combination of high-level density functional theory calculations, including ab initio thermodynamics (AITD) and ab initio molecular dynamics (AIMD), the charge transfer of EFAL species and their spatial proximity to BAS are identified as two key factors governing both zeolite acid strength and catalytic performance. The results indicate that a BAS–EFAL distance less than 4.5 Å plays a crucial role in enhancing BAS acidity, owing to strong electrostatic interactions. Within this proximity, the extent of charge transfer from cationic EFAL to the zeolite framework significantly increases the intrinsic acid strength of BAS. The presence of trace water slightly reduces the acid strength but preserves the increased acidity of BAS/EFAL pairs compared to isolated BAS. AITD confirms the stability of hydrated BAS/EFAL species under operando conditions, while AIMD reveals that active complexes adopt either penta-coordinated (e.g., BAS/[Al(H₂O)]³⁺ and BAS/Al³⁺) or tetra-coordinated (e.g., BAS/[Al(OH)(H₂O)]²⁺ and BAS/[Al(OH)₂]⁺) configurations, with EFAL autonomically adjusting its coordination to framework oxygens. We further evaluate the catalytic activity of BAS/EFAL sites in propane cracking and dehydrogenation and find that cationic EFAL species significantly lower the activation enthalpy, thereby improving the reaction activity. This work demonstrates that the charge transfer from EFAL species to the framework and the BAS–EFAL distance could serve as design principles for the rational development of highly efficient zeolite catalysts.