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Abstract: Metal-organic frameworks are promising materials for the capture of carbon dioxide. Finding the best metal-organic frameworks from the vast number of possibilities could be greatly accelerated by efficient computational screening techniques. We have previously reported an effective screening protocol for predicting carbon dioxide adsorption performance in metal-organic frameworks that uses grand canonical Monte Carlo simulations of gas adsorption. In the model, molecules interact via van der Waals and electrostatic interactions with each other and the framework. However, the method requires single-point quantum mechanics calculations for the estimation of atomic partial charges. In this study we investigate the feasibility of a modified protocol that bypasses the computationally expensive quantum mechanics calculations by applying instead the charge equilibration method. We compare the results of both protocols directly on fourteen metal-organic frameworks and conclude that the new protocol is sufficiently accurate for screening purposes and is significantly faster. [Copyright &y& Elsevier]

Abstract: Hydrogen adsorption isotherms in 20 metal–organic frameworks (MOFs) were obtained from grand canonical Monte Carlo simulations and used to develop quantitative correlations to quickly estimate the H2 uptake of MOFs at 120bar from the free volume of the adsorbent. These correlations were then verified with a diverse set of 22 MOFs that were not used in developing the correlations. The simulated high-pressure H2 isotherms in five representative MOFs show that extremely high pressures (∼80,000bar) are required to completely fill the MOF pores with H2 at 298K. Nevertheless, much lower pressures (several 100bar) are sufficient to attain the H2 storage targets for MOFs with large free volumes. For IRMOF-16 with a large free volume (4.322cm3/g), only modest pressures (170 and 280bar) are required to achieve the 2010 and 2015 gravimetric targets. However, considerably higher pressures are required for attaining the volumetric targets. In addition, three interesting pressure regions, which are observed in the H2 densities within the MOFs (ρad ) up to very high pressures, are analyzed by radial distribution functions between the adsorbed H2 molecules as well as those between H2 molecules and metal corners. [ABSTRACT FROM AUTHOR]

Abstract: Hydrogen storage and delivery in eight representative metal–organic frameworks (MOFs) was investigated using grand canonical Monte Carlo (GCMC) simulations. The simulations demonstrate that the optimal isosteric heat of adsorption (Q st) for maximum hydrogen delivery using MOFs is approximately 20kJ/mol. The results also suggest that increasing the Q st for MOFs with large surface areas (>4800m2/g) is required to attain current hydrogen storage targets in terms of deliverable capacity. [Copyright &y& Elsevier]

Abstract: In this short communication we present a study of xylene adsorption equilibria in distorted and non-distorted AlPO4-11 (AEL) materials using grand canonical Monte Carlo (GCMC) simulations. A previous energy minimization study showed that the AlPO4-11 crystal distorts upon xylene adsorption, but the effect of this distortion on the adsorption loadings was not known. In this work, we compare predictions from GCMC simulations in the distorted and non-distorted frameworks with experiments. The simulations show that the distorted structure adsorbs smaller amounts of xylenes than the non-distorted structure, and simulations in both structures predict ortho-selectivity, as seen in experiment. The lower adsorption capacity of the distorted structure can be related to changes in the adsorption sites. The results indicate that, besides pore blocking, structural deformations can be an important cause of lower adsorption capacity in molecular sieves. [Copyright &y& Elsevier]

Abstract: Monte Carlo simulations in the grand canonical and Gibbs ensembles were carried out to predict the adsorption isotherms of argon, nitrogen, and n-perfluorohexane in BCR-704, a faujasite-type calcium aluminosilicate with a Si/Al ratio of 1.6. Starting from existing force fields for the sorbate molecules and a transferable force field for all-silica zeolites, attempts were made to develop a force field that would reproduce the experimentally determined argon and nitrogen adsorption isotherms in BCR-704. However, good agreement for the nitrogen adsorption isotherm could only be obtained either for a set of force field parameters with a greatly reduced partial charge on the calcium cation or using a larger partial charge and assuming that co-adsorbed water molecules are present at the temperature used for the argon and nitrogen adsorption isotherms. Predictions of the adsorption isotherm for n-perfluorohexane in BCR-704 using the latter force field parameters and the FAU-X structure with a water/calcium ratio of 1 yield good agreement with the experimental benchmark data. [Copyright &y& Elsevier]

Abstract: Grand canonical Monte Carlo simulations were used to calculate hydrogen adsorption in IRMOF-16, NU-100, and UiO-68 functionalized with Mg or Fe catecholates on the linkers. We examined how altering the number of metal catecholate groups affects H2 uptake and deliverable capacity near ambient temperature. We find that large free volume and an isosteric heat of adsorption (Q st) of 20kJmol−1 at low loading will maximize gravimetric deliverable capacity while a small pore diameter will maximize volumetric deliverable capacity. This suggests a trade-off between the properties that lead to maximal gravimetric and volumetric capacities. For example, our calculations suggest that NU-100 functionalized with six Fe catecholate groups per linker takes up 5.5wt.% deliverable H2 at 243K and 100bar, but only 24.2gL−1 deliverable H2. [Copyright &y& Elsevier]

Abstract: Separation of Xe/Kr mixtures was studied in two copper-paddlewheel metal-organic framework materials, MOF-505 and HKUST-1. For MOF-505, which has small pores with strong adsorption sites, high Xe/Kr selectivities (9–10) are obtained from breakthrough measurements and grand canonical Monte Carlo (GCMC) simulations. The consistent results from both techniques suggest that MOF-505 is a promising candidate for Xe/Kr separation. For HKUST-1, which has small octahedral pores, only modest Xe/Kr selectivities (4.5) are observed from breakthrough measurements, although the GCMC simulations predicted unusually high selectivities at low loadings. [Copyright &y& Elsevier]

Abstract: Grand canonical Monte Carlo simulations have been carried out to investigate the adsorption of water from the vapor phase into the zeolite silicalite. For truly hydrophobic micropores, the simulations predict essentially no adsorption of water at low pressures, followed by rapid pore filling as pressure is increased. The effect of silanol defects in real silicalite samples was explored through simulations using “seeded” water molecules to represent hydrophilic defects. These defects promote adsorption of some water at low pressures, as molecules form hydrogen-bonded clusters around the defects. The defects also shift the pore filling to a lower pressure than in the completely hydrophobic material. [Copyright &y& Elsevier]