Impact of Loading-Dependent Intrinsic Framework Flexibility on Adsorption in UiO-66

Abstract

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.

Effects of MOF linker rotation and functionalization on methane uptake and diffusion

“…This approach allows for a less computationally demanding means of accessing the variation in channel environment, but it relies on the assumption that the presence of guest molecules has a negligible impact on the dynamic configurations of the MOF, which does not hold true for all MOFs. 26,27 These computational approaches to incorporate flexibility have shown that framework motion can have strong effects on guest molecule properties, 19,[34][35][36][37][38] such as adsorption [39][40][41] and diffusivity, 25,27,[42][43][44][45][46][47][48]57 in accordance with experiment. 20,47,[58][59][60][61][62][63][64][65] Yang et al 47 studied the diffusion of methane in UiO-66 using flexible force fields and observed a significant improvement in prediction over rigid force fields, although predictions were still lacking in quantitative and qualitative agreement with experiment.…”

“…33 The dynamics of linker rotation have been probed experimentally, both in empty MOFs and in the presence of guest molecules. [21][22][23][24][25] Framework motion can influence guest molecule behavior, and the adsorbed guest molecules can also affect framework motion and pore size, 23,26,27 including through 'gateopening' mechanisms where linkers experience steric effects that lead to larger pores and framework changes. MOF linker functionalization can alternately enhance or suppress linker dynamics, depending on the functional group.…”

“…This approach allows for a less computationally demanding means of accessing the variation in channel environment, but it relies on the assumption that the presence of guest molecules has a negligible impact on the dynamic configurations of the MOF, which does not hold true for all MOFs. 26,27…”

“…28 All of these forms of motion can be altered further by temperature, pressure, light, and mechanical forces. 21,22,26,[29][30][31][32][33] In order to make accurate and reliable computational predictions of properties such as gas adsorption and diffusivity in MOFs, it is important to understand the influence of framework motion.…”

Computational insights into the interaction of water with the UiO-66 metal–organic framework and its functionalized derivatives

“…Recent simulation studies investigating the influence of guest molecule adsorption in the UiO-66 framework have highlighted the importance of considering framework flexibility to accurately reproduce experimental results. 30 These newly developed force fields are specifically designed to be compatible with the MB-pol data-driven many-body potential for water. [31][32][33][34] MB-pol has been shown to accurately predict the properties of water from the gas to the condensed phases.…”