(344af) The Effect of Confinement on Self-Diffusivities and Hydrogen Bonding Network of Organic Liquids inside a Nanoporous Silica Catalyst

Transport properties of molecules confined in nanoporous materials are substantially different from those present in the bulk liquid. This is attributed to altered liquid-liquid interactions and liquid-surface interactions within the porous media. Changes in transport properties and molecular interactions can also significantly alter the reaction kinetics within the nanopore. In the present work, a combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) was used to study the confinement effects of organic liquids within a porous catalyst. Self-diffusivities of propanol, 1,3-propanediol, 1,4-butanediol, glycerol, acetone, propionic acid, and heptane were analysed in the bulk and inside a silica nanopore. In this regard, GCMC was used to obtain the intrapore density of liquids, and MD was used to calculate the self-diffusivities and to analyse the hydrogen bonding (H-bond) structure. The ratio of self-diffusivities in bulk to self-diffusivities inside the pore was taken as an estimate for comparison between the organic compounds. This factor was used for comparison with the published experimental results obtained from pulse-field gradient nuclear magnetic resonance (PFG-NMR) experiments. In this study, alkanes were used as a benchmark, as their lack of functional groups results in minimal interactions with porous media, and intermolecular interactions. Interestingly, polyols (1,3-propanediol, 1,4-butanediol, and glycerol) showed an anomalous enhanced rate of self-diffusion within the nanopore. This anomaly is in accordance with the experimental observations, obtained using PFG-NMR. It is well known that a reduction in the number of H-bonds increases the mobility of molecules, leading to increased self-diffusivities. To attain mechanistic insights into the phenomena behind this anomaly, the H-bonding structure, its population and lifetime, and intermolecular interactions were analysed inside the nanopore. Based on these calculations, the anomalous behaviour of polyols, in contrast to alcohols, carbonyl compounds, and carboxylic acids, was explained.