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Abstract: The three-point model and molecular simulation are used to understand the enantioselective adsorption of a homologous series of chiral compounds on a common chiral selector that is incorporated on a solid surface. We demonstrate how the enantioselectivity of homologous chiral compounds may go down, go up, remain the same, or even reverse. Conditions that lead to consistent enantioselectivity for homologous chiral compounds are also proposed. [Copyright &y& Elsevier]

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: 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]

Diffusion of methane, ethane, propane and n-butane was studied within the micropores of several metal-organic frameworks (MOFs) of varying topologies, including the MOFs PCN-14, NU-125, NU-1100 and DUT-49. Diffusion coefficients of the pure components, as well as methane/ethane, methane/propane and methane/butane binary mixtures, were calculated using molecular dynamics simulations to understand the effect of the longer alkanes on uptake of natural gas in MOFs. The calculated self-diffusion coefficients of all four components are on the order of 10 −8 m 2 /s. The diffusion coefficients of the pure components decrease as a function of chain length in all of the MOFs studied and show different behaviour as a function of loading in different MOFs. The self-diffusivities follow the trend D PCN-14 NU-125≈NU-1100>DUT-49. By comparing the diffusion of pure methane and methane mixtures with the higher alkanes, it is observed that the diffusivity of methane is unaffected by the presence of the higher alkanes in the MOFs considered, indicating that the diffusion path of methane is not blocked by the higher alkanes present in natural gas. [ABSTRACT FROM AUTHOR]

Highlights: [•] Calculated hydrogen Maxwell–Stefan diffusivity in Mg-functionalized MOFs. [•] Highlighted relationship between hydrogen heat of adsorption and M–S diffusivity. [•] Showed M–S diffusivity decreases exponentially with increasing heat of adsorption. [•] Lessons from this study can be applied to diffusion in other host/guest systems. [ABSTRACT FROM AUTHOR]

Abstract: Molecular simulations were used to screen a diverse collection of eight homochiral metal–organic frameworks (MOFs) for their ability to separate 19 chiral compounds by enantioselective adsorption. The simulation model was validated by comparison with available experimental data. It was found that high enantioselectivity is strongly correlated with a close match between the size of the pore and the size of the chiral sorbate molecule. However, there is also a possibility of no enantioselectivity even when the size of the pore matches with the size of the chiral sorbate molecule. A four-point model was used to explain this observation, and a solution to promote high enantioselectivity has been proposed. [Copyright &y& Elsevier]

Abstract: Quantum chemical calculations were employed to develop a better understanding of the adsorption properties of BaNaY interacting with molecules relevant to deNO x catalysis. First, various basis sets and levels of theory were tested for barium-containing species and gas-phase reactions, and it was shown that the choice of the basis set for barium is critical. Density functional theory (DFT) with the B3LYP functional and SDD as the basis set was selected based on its combination of relative accuracy and speed. This level of theory was then used to calculate energies, geometries, and frequencies for acetaldehyde, acetic acid, nitromethane, and water adsorbed and in the gas phase. The predicted properties were compared to experiment where available and reasonable agreement was found. To study the effect of the zeolite framework on the adsorption properties, the size of the zeolite cluster was increased from 6T to 36T to 96T using the embedded ONIOM method. Inclusion of van der Waals interactions with increasing cluster size only changed the adsorption enthalpy by a small amount for all of the adsorbates studied. It was found that the interaction between the empty 6s orbital of Ba and the lone pair orbital of the oxygen atom in the adsorbed state, revealed by natural bond orbital analysis, correlates with the adsorption enthalpy and the gas-phase charge of the oxygen atom that interacts with Ba. [Copyright &y& Elsevier]

The methane deliverable capacity of adsorbent materials is a critical performance metric that will determine the viability of using adsorbed natural gas (ANG) technology in vehicular applications. ARPA-E recently set a target deliverable capacity of 315 cc(STP)/cc that a viable adsorbent material should achieve to yield a driving range competitive with incumbent fuels. However, recent computational screening of hundreds of thousands of materials suggests that the target is unattainable. In this work, we aim to determine whether the observed limits in deliverable capacity (~200 cc(STP)/cc) are fundamental limits arising from thermodynamic or material design constraints. Our efforts focus on simulating methane adsorption isotherms in a large number of systems, resulting in a broad exploration of different combinations of spatial distributions and energetics of adsorption sites. All systems were classified into five adsorption scenarios with varying degrees of realism in the manner that adsorption sites are created and endowed with energetics. The scenarios range from methane adsorption on discrete idealized lattice sites to adsorption in metal–organic frameworks with coordinatively unsaturated sites (CUS) provided by metalated catechol groups. Our findings strongly suggest that the ARPA-E target is unattainable, although not due to thermodynamic constraints but due to material design constraints. On the other hand, we also find that the currently observed deliverable capacity limits may be moderately surpassed. For instance, incorporation of CUS in IRMOF-10 is predicted to yield a 217 cc(STP)/cc deliverable capacity. The modified material has a ~0.85 void fraction and a heat of adsorption of ~15 kJ/mol. This suggests that similar, moderate improvements over existing materials could be achieved as long as CUS incorporation still maintains a relatively large void fraction. Nonetheless, we conclude that more significant improvements in deliverable capacity will require changes in the currently proposed operation conditions. [ABSTRACT FROM AUTHOR]

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: Molecular simulations are known to be able to reproduce many experimental isotherms of adsorbates in the metal-organic framework IRMOF-1. Here, we show that also the positions and occupations of the adsorption sites of argon and nitrogen match well with experiments. At 30 K the molecules are localized around their crystallographic sites, while at room temperature the molecules are spread throughout the pore volume. The orientations found for individual nitrogen molecules differ from those found experimentally, although the average orientations agree well. We elucidate the pore filling mechanism for both argon and nitrogen. In addition we compute the binding energy at sites for individual argon, nitrogen, carbon dioxide, hydrogen, methane, ethane, and propane molecules and show that for all molecules the preferred site is near the zinc–oxygen cluster in the cavities where the linkers point outward. The second site, higher in energy, is near the zinc–oxygen cluster in the cavities where the linkers point inward, and the least energetically favorable site is above and beneath the linker molecule. We explain why the site in the smaller cavities is only filled up at high 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]