Effects of Intrinsic Flexibility on Adsorption Properties of Metal–Organic Frameworks at Dilute and Nondilute Loadings

Abstract

Molecular simulation of adsorption in nanoporous materials has become a valuable complement to experimental studies of these materials. In almost all cases, these simulations treat the adsorbing material as rigid. We use molecular simulations to examine the validity of this approximation for the adsorption in metal–organic frameworks (MOFs) that have framework flexibility without change in their unit cells because of thermal vibrations. All nanoporous materials are subject to this kind of framework flexibility. We examine the adsorption of nine molecules (CO2, CH4, ethane, ethene, propane, propene, butane, Xe, and Kr) and four molecular mixtures (CO2/CH4, ethane/ethene, propane/propene/butane, and Xe/Kr) in 100 MOFs at dilute and nondilute adsorption conditions. Our results show that single-component adsorption uptakes at nondilute conditions are only weakly affected by framework flexibility, but adsorption selectivities at both dilute and nondilute conditions can be significantly affected by flexibility. The most dramatic impacts of framework flexibility occur for adsorption uptake in the limit of dilute adsorption. These results suggest that the importance of including framework flexibility when attempting to make quantitative predictions of adsorption selectivity in MOFs and similar materials may have been underestimated in the past.

Design principles for the ultimate gas deliverable capacity material: nonporous to porous deformations without volume change

“…Fully flexible simulations are required to evaluate their potential, 33,34 and therefore would have remained undiscovered from rigid screening studies, for which there are very few exceptions. 35,36 Third, one must find an adsorbent that mimics the idealized flexible slit pore model as closely as possible in order to maximize deliverable capacity.…”

“…29,30 It should be noted that such materials could only ever be computationally identified with a fully flexible treatment coupling framework dynamics and adsorption; 16,[31][32][33][34] hence, they will have remained hidden from almost any high-throughput adsorbent screening study to date. 35,36 Finally, we show that the isoreticular tunability 37,38 of metal-organic frameworks (MOFs) can be used to design an improved version of the M(NDC) adsorbent. Ultimately, this work outlines a path forward to discover deliverable capacity materials through concrete design strategies motivated by the models and simulations developed herein.…”

Performance-Based Screening of Porous Materials for Carbon Capture

“…Recent studies have shown that the importance of intrinsic flexibility arising from thermal vibrations varies considerably for MOFs depending on the adsorption property of interest. 206 , 207 For example, it was shown that intrinsic flexibility can be more important where pore sizes are comparable to the kinetic diameter of adsorbate molecules. 206 , 208 For zeolites, however, the effect of thermal framework vibrations on molecular adsorption was shown to be negligible and the rigid framework assumption can be reliably used in GCMC simulation for these materials.…”

“… 206 , 207 For example, it was shown that intrinsic flexibility can be more important where pore sizes are comparable to the kinetic diameter of adsorbate molecules. 206 , 208 For zeolites, however, the effect of thermal framework vibrations on molecular adsorption was shown to be negligible and the rigid framework assumption can be reliably used in GCMC simulation for these materials. 209 For simulation of flexible materials that undergo significant changes in unit cell volume, more advanced simulation methods such as the osmotic ensemble and Gibbs ensemble Monte Carlo should be used.…”

Design Principles for the Ultimate Gas Deliverable Capacity Material: Nonporous to Porous Deformations Without Volume Change

“…29,30 It should be noted that such materials could only ever be computationally identified with a fully flexible treatment coupling framework dynamics and adsorption; 16,[31][32][33][34] hence, they will have remained hidden from almost any high-throughput adsorbent screening study to date. 35,36 Finally, we show that the isoreticular tunability 37,38 of metal-organic frameworks (MOFs) can be used to design an improved version of the M(NDC) adsorbent. Ultimately, this work outlines a path forward to discover deliverable capacity materials through concrete design strategies motivated by the models and simulations developed herein.…”