Unraveling Solvent Effects on <i>n</i> -Butane Cracking and Deprotonation in H-MFI Zeolites: A Hybrid QM/MM Approach

Understanding solvent effects on chemical transformations in porous zeolites is crucial for the rational design of aqueous-phase processes relevant to biomass conversion and hydrocarbon upgrading. In this study, a hybrid quantum mechanical/molecular mechanical (QM/MM) approach, combined with mean-field microkinetic modeling, was employed to investigate n-butane cracking at 673 K over H-MFI zeolite under dry, water-vapor, and aqueous-phase conditions. Gas- and water-vapor-phase analyses revealed that central C–C bond cleavage in butane exhibits a lower activation barrier than terminal bond scission. Under water-vapor and condensed, supercritical water conditions, proton delocalization leads to the formation of a mobile acid site (H5O2+). Microkinetic modeling further indicates that this mobile acid site is intrinsically less effective for C–C bond activation than the framework-bound acid site and consequently suppresses n-butane cracking activity. To assess solvent effects at 673 K, with water chemical potential corresponding to a bulk water density of 500 kg/m3, an explicit solvation scheme for zeolites (eSZS) was employed using grand canonical Monte Carlo (GCMC) simulations in a QM/MM-FEP framework. Solvent effects on n-butane adsorption are weakly endergonic for both proton-localized and proton-delocalized scenarios, with ΔΔGgas→liquid = 0.053 ± 0.018 and 0.078 ± 0.029 eV, respectively, while the corresponding ethyl activation barriers in the liquid phase (i.e., condensed water phase) remain essentially unchanged relative to their gas- and water-vapor-phase values. Analysis under subcritical water conditions (298, 473, and 573 K) reveals that increasing local water density enhances solvent effects, as evidenced by microsolvation-shell analysis and a linear correlation between predicted solvent effects (ΔΔG) and local water density. Collectively, these results demonstrate that the framework-bound Brønsted acid site remains the kinetically preferred active site for n-butane cracking under supercritical water conditions, while variations in local water density govern the magnitude of solvent effects in aqueous-phase zeolite catalysis.