Enhanced selectivity of water-alcohol with cross-linked polybenzimidazole (PBI) membrane: Insight from molecular dynamics simulation

Designing advanced cross-linked membranes with selective solute rejection and high water permeability remains challenging due to complex molecular-level diffusion. In this study, we leverage systematic molecular dynamics (MD) simulations to engineer polybenzimidazole (PBI) membranes through controlled cross-linking using α, α'-Dichloro-p-xylene (DCX) at four distinct cross-linking densities. We also investigate the diffusion and dissolution processes of water-alcohol molecules within these cross-linked PBI membranes. The influence of the target structure on separation performance is estimated using parameters such as fractional free volume (FFV), end-to-end distance, pore size distribution (PSD), and radial distribution function (RDF). As the degree of cross-linking increased, the fractional free volume dropped from 16.33 % to 10.76 %, and the pore size distribution decreased from 3.42 Å to 2.48 Å. These changes reduced defects, potentially enhancing molecular screening performance compared to the pristine PBI. Water-ethanol selectivity increases from 8.82 to 57.34, while water-isopropanol selectivity surges by two orders of magnitude. The study also investigated the solubility coefficients of water-alcohol in the membrane. The results indicated that the cross-linked PBI membrane has a significantly higher affinity for water molecules compared to alcohol molecules. Cross-linked PBI membranes preferentially sorb water by occupying hydrogen-bonding sites, sterically restricting alcohol permeation and enabling high water/alcohol selectivity. This provides theoretical evidence that cross-linked PBI membranes can enhance selective water-alcohol separation. • Cross-linked PBI membrane redistributes free volume, reducing membrane defects. • The -NH group enables water to better occupy advantageous positions. • Solubility and diffusion rate of water are about twice those of alcohol. • Solution stage dominates selectivity enhancement in cross-linked PBI membrane.