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[Display omitted] • Atomistic modeling techniques for microporous feature analysis. • In silico predictions of adsorption with flexible frameworks. • Data-driven methods are positioned to accelerate PIM discovery and design. Polymers of intrinsic microporosity (PIMs) are a family of materials with potential to be effective and scalable solutions for challenging adsorbent and membrane applications. The broad range of repeat unit chemistry, microporous structural features, and polymer processing makes exploration of the expansive PIM design space inefficient via chemical and materials intuition alone. Computational techniques such as molecular simulations and machine learning can provide a leap in capabilities to address this polymer design challenge and will be central to the future development of PIMs. We highlight recent microporous material studies that arrived at key results by employing computational techniques and provide our perspective on the prospects for in silico design and development of PIMs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Big data approaches are needed to efficiently explore MOF catalyst space. • High-throughput protocols have been developed to screen large MOF datasets. • Machine learning is poised to accelerate MOF catalyst discovery. • While challenges remain, there are many recent developments and new opportunities. Metal-organic frameworks (MOFs) have been widely investigated for challenging catalytic transformations due to their well-defined structures and high degree of synthetic tunability. These features, at least in principle, make MOFs ideally suited for a computational approach towards catalyst design and discovery. Nonetheless, the widespread use of data science and machine learning to accelerate the discovery of MOF catalysts has yet to be substantially realized. In this review, we provide an overview of recent work that sets the stage for future high-throughput computational screening and machine learning studies involving MOF catalysts. This is followed by a discussion of several challenges currently facing the broad adoption of data-centric approaches in MOF computational catalysis, and we share possible solutions that can help propel the field forward. [ABSTRACT FROM AUTHOR]

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: 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: A cesium-exchanged mordenite zeolite was tested as a hydrocarbon trap for cold-start automotive applications using a modified temperature programmed desorption (TPD) procedure. In the modified TPD procedure, a step increase in feed concentration was performed simultaneously with the start of the temperature ramp to more closely mimic cold start conditions. The feed stream contained mixtures of toluene, propane, and water. The cesium-exchanged mordenite trapped propane under these dynamic conditions, demonstrated improved propane trapping when toluene was coadsorbed, and retained its trapping capacity after nearly 40h of hydrothermal treatment. [Copyright &y& Elsevier]

Abstract: The adsorption of xylene isomers in AlPO4-11 (AEL network) was investigated using biased grand canonical Monte Carlo (GCMC) simulations. Preferential o-xylene adsorption was predicted by the simulations, in agreement with previously reported experimental data. In AlPO4-11 the selective adsorption behavior comes from the smaller length of the o-xylene molecule along the crystallographic c-axis compared to p-xylene. This is in contrast to AlPO4-5 and AlPO4-8, where the ortho-selectivity is caused by the characteristic face-to-face positioning of o-xylene. Energy minimization studies were also performed in a flexible AlPO4-11 lattice to study the structural changes upon xylene adsorption. The energy minimization study showed that the AlPO4-11 crystal distorts upon p- and o-xylene adsorption. The distortion mechanism is related to the strong interaction between xylene methyl groups and the sieve oxygen atoms in the O3 position in the wide region of the pore. [Copyright &y& Elsevier]

The extraframework locations of many metal cations in pentasil zeolites, such as ferrierite, are not known, but new infrared bands in the 800–1000 cm−1 region are experimentally observed upon exchange. To investigate the origin of these bands and to better understand cation siting, density functional theory calculations were performed on ferrierite cluster models for seven divalent metals: magnesium, manganese, iron, cobalt, nickel, copper, and zinc. The calculated properties, such as the metal–zeolite binding energy, follow a U-shaped trend with respect to the number of metal d electrons. The calculated infrared bands are consistent with the occupation of the B and G ferrierite sites by the metal cations. However, a definitive assignment of the published experimental infrared bands to particular extraframework sites was not possible. Despite this limitation, all seven metals were shown to perturb the ferrierite framework similarly. All of the cations coordinate to four framework oxygen atoms and the framework T–O bonds associated with these oxygen atoms are elongated by the metal. This elongation is shown to be proportional to the wavenumber of the infrared bands. [Copyright &y& Elsevier]

Highlights from the article: With this special issue on the frontiers of Molecular Simulation in Chemical Engineering, we set out to highlight the broad impact that molecular modeling and simulation are making across our field. The various contributions show a plethora of methods being used in the field of molecular simulation within chemical engineering, including molecular dynamics methods, machine learning, group contribution methods and many others. Indeed, these contributions provide an excellent snapshot of the subfield of molecular simulation within chemical engineering in 2019 and should serve as a guide to newcomers and practitioners for years to come.

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]

Knudsen dynamics simulations were performed in cylindrical pores with multiple sections of different diameters. The number and location of hits between particles and the pore walls can be related to the known distributions in the uniform diameter cases. Both sweep-gas and pass-through modes of operation were examined. The contact of the particles traveling inside the pores with the pore walls can be regulated by adjusting the aspect ratio (section length/section diameter) of the pore sections and the ratio of their cross-sectional areas. The results were found to be independent of the transition angle between sections except for very smooth transitions. Analytical expressions were developed to relate the transmission probability in pores of multiple sections to the transmission probabilities of the constituent sections. [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: 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]

Highlights: [•] The geometric properties of over 137,000 hypothetical MOFs were analyzed. [•] The hypothetical MOFs have diverse geometrical properties. [•] Only 6 different framework nets appear in this database of hypothetical MOFs. [•] Energy minimization provides insights to MOF construction algorithms. [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: Incorporation of Li cations into MOFs by either of two methods, chemical reduction or cation exchange, significantly improves the CO2/CH4 selectivity. While the selectivity enhancement by the chemical reduction seems to come from favorable displacement of catenated frameworks, the selectivity improvement by the Li cation exchange comes from enhanced solid–gas interactions. [Copyright &y& Elsevier]

Abstract: A piston-based particle damper geometry is proposed and investigated using experiments and particle dynamics simulations. In particle damping, energy is dissipated through the inelastic collisions and friction between granular particles. Due to their temperature-independent performance, particle dampers are promising alternatives for use in extreme temperature conditions. Using the appropriate inter-particle force models, the simulations agree well with the experimental results. Using simulations, many parameters are investigated in this work for their effects on the damping performance, including material properties, particle size, device geometry, and excitation level. These results provide new understanding of particle damping and may help in the design of next generation particle dampers. [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]

Several zeolites with varying pore dimensionality have been investigated as hydrocarbon (HC) traps by studying the temperature programmed desorption of propane and toluene mixtures. In one-dimensional (1-D) zeolites, the diffusive motion of propane is blocked by the more-strongly adsorbed toluene thus raising the desorption temperature of propane above its single-component desorption temperature. In the 1-D zeolite EUO, propane is trapped within the pores by the toluene and is only able to desorb to the gas phase once toluene begins to desorb. This leads to an increase in the propane desorption temperature of 35 °C over the single-component value. In the 1-D zeolite mordenite, propane desorbs in the presence of toluene at an intermediate temperature that is 100 °C larger than the single component case. Toluene then desorbs at an even higher temperature. It is suggested that propane undergoes an activated process of passing toluene within the 1-D pores that requires the intermediate temperatures. This HC trapping is not observed in the three-dimensional pore zeolites ZSM-5 or Y. Propane in the binary mixture with toluene in these zeolites desorbs at the same temperature as in the single component cases, suggesting that propane is able to diffuse around the more-strongly adsorbed toluene due to the connectivity of the pores. Therefore, it is the one-dimensionality of the zeolites that allows the trapping of propane to higher temperatures in the presence of the more-strongly adsorbing toluene. This phenomenon may find application in reducing cold start HC emissions from automotive exhaust. [Copyright &y& Elsevier]