(684g) Monte Carlo Simulations to Understand Solvent Adsorption in MOF Uio-66

Understanding solvent adsorption in metal-organic frameworks (MOFs) can provide insights into the pore environment and its solvophilicity that can affect separation processes, reactivities in solvent-phase heterogeneous catalysis, and diffusion behavior. In order to understand the adsorption behavior, Monte Carlo simulations utilizing generic force fields for MOFs are often used to obtain the adsorption isotherm of different molecules. In this work, we utilize a combination of TraPPE and other force-fields to understand ethanol and water adsorption in UiO-66.

NpT-Gibbs Ensemble Monte Carlo (GEMC) simulations are carried out to obtain the simulated unary adsorption isotherms with ethanol and water as the sorbates. We show that specific hydrogen-bonding interactions of solvent molecules with UiO-66 sites affect their adsorption behavior and solvent structuring. In particular, UiO-66 has multiple sites in the form of carboxylic oxygens from the linkers, μ₃-O, and μ₃-OH groups to which water and ethanol preferentially bind at low-to-intermediate relative pressures. These sites impart some degree of hydrophilicity, and the observed hydrogen-bonding interactions between sorbate and sorbent explain the loadings at low-pressure, pore-filling, and saturation regions of the isotherms. Further, desorption isotherms obtained using the simulations show only a small hysteresis loop for the solvent adsorption-desorption, in qualitative agreement with experiments.

Our results show that generic force-fields for UiO-66 do not capture the experimental adsorption behavior of these hydrogen-bonding molecules in defect-free UiO-66. We show that the use of TraPPE-based force-field parameters for the MOF sites can better capture the MOF-solvent hydrogen bonding interactions. These findings are now being applied to understand solvent adsorption in defective UiO-66 frameworks, which have shown promise as a Lewis acid catalyst for solvent-phase reactions.