Molecular Mechanism of Si/Al Ratios and Pore Structures Governing Efficient H ₂ O Removal in CO ₂ Conversion Reactions via ZSM-5 Membranes

In CO2 catalytic conversion reactions (such as methanol synthesis and methanation), the accumulation of water byproduct limits reaction equilibrium, making efficient water removal critical for enhancing yields. Na-ZSM-5 zeolite membranes have gained attention for enabling in situ dehydration during these processes. Nevertheless, the molecular transport mechanism within these membranes remains inadequately understood, particularly regarding how the Si/Al ratio (Na+ content) and pore topology jointly regulate transport behavior. This study combines density functional theory (DFT), grand canonical Monte Carlo (GCMC), and molecular dynamics (MD) simulations to systematically examine the adsorption–diffusion behavior of H2O and CO2 in Na-ZSM-5. Results show that Na+ enhances H2O adsorption over CO2. Although higher Na+ content reduces diffusivity for both molecules, H2O retains higher mobility. Moreover, straight channels enable more efficient transport than sinusoidal channels, where structural tortuosity traps molecules and hinders diffusion. Permeation simulations confirm that the synergy between high Na+ site density and straight-channel geometry greatly enhances H2O/CO2 selectivity. This improvement stems from the lower diffusion energy barrier in straight channels, in which Na+ ions promote water cluster formation, facilitating efficient transport via hydrogen-bond-mediated cooperative diffusion and osmotic effects. In contrast, sinusoidal channels exhibit high diffusion resistance and strong Na+-molecule adsorption, substantially limiting permeation of both CO2 and H2O. Under mixed-gas conditions, preferential water adsorption and clustering further block CO2 diffusion pathways, leading to high separation selectivity.