Impact of Loading-Dependent Intrinsic Framework Flexibility on Adsorption in UiO-66
JK Johnson and PB Shukla, JOURNAL OF PHYSICAL CHEMISTRY C, 126, 17699-17711 (2022).
The vast majority of molecular simulations of adsorption in porous materials make use of the assumption that the framework of the porous material may be held rigid without significantly impacting the accuracy of the computed adsorption isotherm. One reason for this approximation is that explicit inclusion of framework flexibility in adsorption simulations dramatically increases the computational cost of the simulation. Approximate methods for including framework flexibility have been developed that are computationally efficient and have been applied to adsorption of gases in metal-organic frameworks (MOFs) by generating an ensemble of snapshots from a molecular dynamics calculation of an empty MOF and employing these snapshots to generate an isotherm averaged over all the snapshots, holding each snapshot rigid during the adsorption calculation. This method assumes that the adsorbate has negligible impact on the dynamic configurations of the MOF. We demonstrate an efficient method for including adsorbate-induced changes to the MOF framework and show that this assumption is not always valid. Specifically, for acetone and isopropyl alcohol adsorbed in UiO-66, the saturation loading and degree of hydrogen bonding with the framework significantly increased through the inclusion of adsorbate-induced framework flexibility. We show that adsorption simulations that include adsorbate-induced flexibility match the experimental data for these systems much better than simulations using either the rigid approximation or including the framework flexibility of the empty MOF. We expect that adsorbate-induced flexibility may be important for adsorption in many similar systems.
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