Temperature and Loading-Dependent Diffusion of Light Hydrocarbons in ZIF-8 as Predicted Through Fully Flexible Molecular Simulations

Abstract

Accurate and efficient predictions of hydrocarbon diffusivities in zeolitic imidazolate frameworks (ZIFs) are challenging, due to the small pore size of materials such as ZIF-8 and the wide range of diffusion time scales of hydrocarbon molecules in ZIFs. Here we have computationally measured the hopping rates of 15 different molecules (kinetic diameters of 2.66-5.10 Å) in ZIF-8 via dynamically corrected transition state theory (dcTST). Umbrella sampling combined with the one-dimensional weighted histogram analysis method (WHAM) was used to calculate the diffusion free energy barriers. Both the umbrella sampling and dynamical correction calculations included ZIF-8 flexibility, which is found to be critical in accurately describing molecular diffusion in this material. Comparison of the computed diffusivities to extant experimental results shows remarkable agreement within an order of magnitude for all the molecules. The dcTST method was also applied to study the effect of hydrocarbon loadings. Self and transport diffusion coefficients of methane, ethane, ethylene, propane, propylene, n-butane, and 1-butene in ZIF-8 are reported over a temperature range of 0-150 °C and loadings from infinite dilution to liquid-like loadings.

On the molecular mechanisms for the H2/CO2 separation performance of zeolite imidazolate framework two-layered membranes

“…For this particular structure more than four years were needed, starting in 2011 with the first attempts to understand the adsorption isotherms of light gasses at low temperatures, 30,52 and ending in 2015 with the development of the fully flexible models and the complex specific methods required to understand the mechanisms involved in the gas diffusivity within the ZIF-8 pores. 34,62 In any event, the molecular simulations carried out justify the experimental implementation of our strategy based on double-layered ZIF membranes inside polymeric HF supports fabricated stepwise by microfluidics.…”

“…33 Some recent molecular simulation studies already include flexible bonding conditions that allow the prediction of worse gas separation efficiencies than those expected from simulations when working with ZIFs. 34,35 Zhao et al observed the influence of the CO 2 pressure on its adsorption on benzimidazole ZIF-7 and ZIF-9 and their phase transition response. 36–38 …”

Rational Tuning of Zirconium Metal–Organic Framework Membranes for Hydrogen Purification

“…[41] Diffusion through MOFs or other microporous materials requires gas molecules to hop from one cage to an adjacent one by passing through the gates. [42] In UiO-66 and Anth, the gas molecules diffuse from the center of the tetrahedral cage (TetraC) and move through the center of the octahedral site (OctC;F igure 4a,e). In Naph, the gas molecules begin from the center of the tetrahedral cage,a nd once the gas passes through Gate 1, the pathway is slightly bent towards the Gate 2 ( Figure 4c).…”

Improved Hill–Sauer Force Field for Accurate Description of Pores in 8-Ring Zeolites

“…Molecular diffusion provides an example of this kind of problem. Although approximate methods exist that neglect coupling between diffusing molecules and flexible frameworks 11, 54-55 , quantitatively predicting the diffusion rates of molecules that are complex or large relative to pore openings intrinsically involves an interplay between molecules near pore mouths and deformations of the pores induced by these molecules 12,53 . Calculations based on classical force fields are well suited to analyze this situation.…”