ZIF-8 is a zeolitic imidazolate framework with very good thermal and chemical stability that opens up many applications that are not feasible by other metal-organic frameowrks (MOFs) and zeolites. Several works report the adsorption properties of ZIF-8 for strategic gases. However, despite the vast experimental corpus of data reported, there seems yet to be a dearth in the understanding of the gas adsorption properties. In this work we provide insights at a molecular level on the mechanisms governing the ZIF-8 structural deformation during molecular adsorption. We demonstrate that the ZIF-8 structural deformation during the adsorption of different molecules at cryogenic temperature goes beyond the gas-induced rotation of the imidazolate linkers. We combine experimental and simulation studies to demonstrate that this deformation is governed by the polarizability and molecular size and shape of the gases, and that the stepped adsorption behavior is defined by the packing arrangement of the guest inside the host.
We use molecular simulations to analyze the preferential adsorption sites of molecules that differ in size, shape, and polarizability in Cu-BTC metal organic framework. The cage system of the framework can be exploited to enhance adsorption of small gases. We find that nonpolar molecules adsorb preferentially in the small tetrahedral cages, whereas alcohols and water molecules adsorb close to the copper atoms in one of the big cages. Blocking potentially enhances selective adsorption and separation and we therefore investigate how to block these cages in a practical manner. We propose to use ionic liquids for it and we find that the addition of these components reduces the adsorption of polar molecules near the open metal centers. For this reason, the presence of ionic liquids reduces the attack of the molecules of water to the metallic centers improving the framework stability.
The adsorption of polar water and methanol vapors on the meso-and microporous metal−organic frameworks (MOFs) MIL-100(Cr) and MIL-101(Cr) has been studied by a combined experimental and simulation approach. The computational effort for these MOFs with large unit cells was reduced by using primitive unit cells that were 4 times smaller. Our results demonstrate that both adsorbate− adsorbent and adsorbate−adsorbate interactions control the adsorption process. At low loadings, before all coordinatively unsaturated chromium sites are occupied, the MOF structure determines the shape of the isotherm, and the molecular model for the polar sorbate is less important. A clear difference was found between fully fluorinated and hydroxylated MIL-101 structures for both methanol and water, demonstrating that partial charges on Cr drive the initial shape of the isotherm. At higher loadings, adsorbate−adsorbate interactions become much more important, and the choice of the water model, in particular, is crucial for the agreement between experimental and simulation results. The simplest SPC/E model reproduces the experimental results with the best accuracy, in contrast to more advanced models such as TIP5PEw, which can be explained by the slightly stronger Coulombic interactions predicted by the former. For methanol, the general TraPPE force field performs well. A composite type IV isotherm for methanol and a composite type V isotherm for water, according to the IUPAC classification, were found. The heats of adsorption are in line with these conclusions. To the best of our knowledge, this effect of the sorbate model has not been observed in adsorption in microporous materials, and it highlights the complexity behind molecular simulations in periodic mesostructured materials.
This work discusses the adsorption of polar and apolar molecules on the copper–benzene-1,3,5-tricarboxylate (Cu–BTC) metal–organic framework. Vapor phase adsorption isotherms of various polar adsorbates such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 1-hexanol, water, acetone, acetonitrile, tetrahydrofuran, and N,N-dimethylformamide, as well as some apolar adsorbates such as n-hexane, n-heptane, m-xylene, and cyclohexane, on the Cu–BTC framework are presented. We show that water exposure of the Cu–BTC framework has an adverse effect on the uptake capacity. However, with minimized water exposure, we find high adsorption capacities, exceeding 0.65 cm3/g for all adsorbates with the exception of water, and we show that small polar adsorbates exhibit a two-step adsorption behavior. This behavior is further studied using molecular simulation and proposed to occur due to the presence of the various Cu–BTC cages. The cages containing the exposed coordinatively unsaturated copper sites have a more polar character, while the other cages behave in a more apolar way, causing a two-step adsorption behavior depending on the character of the adsorbate.
We report a molecular simulation study aimed to ascertain the effect exerted in gas adsorption when room-temperature ionic liquids (RTILs) are added into the pores of the Cu-BTC metal-organic framework (MOF). Carbon dioxide, methane, nitrogen, and their mixtures are studied. We take into account the influence of the type of anion and the relative amount of RTILs used. It is observed that the presence of RTILs in the MOF pores enhances significantly CO2 adsorption at low pressures, whereas methane and nitrogen adsorption is unaffected.
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