Deconstructing hydrogen-bond networks in confined nanoporous materials: Implications for alcohol-water separation

Essential topological indices of the hydrogen-bond networks of water, methanol, ethanol, and their binary mixtures adsorbed in microporous silicalite-1 (a hydrophobic zeolite with potential application for biofuel processing) are analyzed and compared to their bulk liquid counterparts. These include the geodesic distribution (the shortest H-bond pathways between molecular vertices), the average length, the geodesic index, the orientation and distance of the adsorbate to the interior of the zeolite, and the sorbate-sorbate and sorbate-sorbent distributions of H-bonds. In combination, they describe how the H-bond networks are altered when going from the bulk to the confined silicalite-1 environment. The speciation of the adsorbed compounds is quantified in terms of their network connectivity, revealing that pure water has a high probability of forming long, contiguous H-bonded chains in silicalite-1 at high loading, while alcohols form small dimeric/trimeric clusters. The extent to which the H-bond network of binary water-alcohol systems is altered relative to either unary system is quantified, demonstrating an enhanced interconnectivity that is reflected in the tendency of individual H₂O molecules to become co-adsorbed with alcohol clusters in the zeolite framework. Selectivity for the alcohol over water diminishes with increasing alcohol loading as the H-bonded clusters serve as favorable adsorption sites for H₂O.