Dehydration of cyclohexanol, derived from lignocellulosic biomass, within zeolite nanopores is a crucial biofuel production process, yet the impact of aqueous environments in different zeolite pore geometries towards dehydration reaction mechanisms remains inadequately understood. This study focuses on developing an explicit solvation scheme for porous zeolites (called eSZS), which allows for a deeper understanding about the impact of pore geometry on solvent effects and facilitates the investigation of dehydration reaction mechanisms under aqueous environments. We employed density functional theory (DFT) in a hybrid quantum mechanical and molecular mechanical (QM/MM) model to accurately capture the effects of long-range dispersive and electrostatic interactions between the reaction intermediates, aqueous solvent, and zeolite framework. In this QM/MM approach, active site and reactant are described at the DFT level of theory while the water and the zeolite framework far away from the active site are described at the force field level of theory. Cyclohexanol dehydration proceeds via stepwise (E1) or concerted (E2) reaction mechanism, in both dry and wet conditions. Under dry conditions, the cyclohexyl cation (C6H11+), formed in an E1 mechanism, is found unstable, causing the dehydration to follow the E2 route, with activation barriers of 0.85eV in H-MFI and 0.75eV in H-BEA. Under wet conditions, the proton from the zeolite framework delocalizes in water, forming a mobile acid site (H5O2+). However, the E1 mechanism remains absent, as lower water concentrations in the frameworks are unable to stabilize C6H11+. The activation barrier for the E2 mechanism is lower in H-BEA (1.02 eV) than H-MFI (1.31 eV), which is consistent with experimental observations. This absence of an E1 mechanism can be attributed to a higher Si/Al ratio and lower water concentrations, however, we anticipate a shift from E2 to E1 mechanism when applying our eSZS methodology for calculating solvent effects.
