(6d) Insight into the Origins of Different Catalytic Activity Observed in Hydrophilic Versus Hydrophobic Zeolite

Solvents play a critical role in the catalysis of biomass process. In confined spaces, such as zeolite, this solvent effect is more profound. For example, studies have found that hydrophilicity influences zeolite catalyst performance, but with different reactions responding to varying extents of hydrophilicity. Specifically, aldol addition, hydrogenation, and glucose isomerization reactions exhibit higher rates in hydrophobic zeolites, whereas alkene epoxidation exhibits higher conversions in hydrophilic zeolites. The difference in performance between hydrophobic and hydrophilic zeolites has been suggested to result from the larger density of water molecules and the restricted mobility of water and intermediates within hydrophilic pore.

Thus, we aim to investigate these hypotheses computationally. Specifically, we compute the free energies of solvation of oxygenates in Ti-FAU zeolite in pure water solvent using the method of multiscale sampling (MSS), combining density functional theory and molecular dynamics. We find that solvent orientational and translational mobilities are indeed smaller in hydrophilic pores compared to those in hydrophobic pores, but that water molecule densities are the same in both pores. We further find that solvation free energies are more negative in hydrophobic pores and that the phenomena that control solvation free energies are different in hydrophilic versus hydrophobic pores. Specifically, we find that solvation entropy is related to the adsorbate-water hydrogen bonds and adsorbate solvent accessible surface area in hydrophobic pores, while a less correlation was observed in hydrophilic pore. This suggests that solvents strongly interact with adsorbates than with hydrophobic pore surface. This finding is in agreement with the less mobility of solvent molecules around adsorbates. Furthermore, adsorbates interacting with hydrophilic sites reduce their probability of contacting with solvent molecules, resulting in the higher solvent mobility. To investigate this further, we extend this analysis to methanol/water mixtures in order to learn how solvent polarity influences solvation thermodynamics in confinement.