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The separation of xylene isomers still remains an industrially challenging task. Here, porous purine-based metal–organic frameworks (MOFs) have been synthesized and studied for their potential in xylene separations. In particular, Zn(purine)I showed excellent para-xylene/ortho-xylene separation capability with a diffusion selectivity of 6 and high equilibrium adsorption selectivity as indicated by coadsorption experiments. This high selectivity is attributed to the shape and size of the channel aperture within the rigid framework of Zn(purine)I. [ABSTRACT FROM AUTHOR]

Molecular diffusion calculations of metal–organic frameworks (MOFs) including structure flexibility are considerably more computationally expensive than more approximate calculations based on rigid frameworks. For this reason, efforts to probe diffusion in MOFs in a high-throughput manner have relied on rigid framework simulations. It is unclear, however, when this simplification is justified. We compare the self-diffusion coefficients of 12 adsorbates in a diverse set of MOFs with flexible and rigid frameworks to systematically study the influence of framework flexibility on molecular diffusion. In many cases framework flexibility has a strong impact on molecular diffusion. Descriptors based only on the properties of empty MOFs were not able to capture the effects of framework flexibility even qualitatively. In contrast, the average window size in the presence of a constrained molecule was found to be useful for judging the importance of framework flexibility. A transferable descriptor of adsorbate size in diffusion was introduced to efficiently estimate loaded window sizes for diverse collections of molecules. These results provide a practical approach to qualitatively establishing whether framework flexibility strongly influences molecular diffusion in MOFs, which in turn points to more reliable approaches to high throughput calculations of molecular diffusivities in these materials. [ABSTRACT FROM AUTHOR]

Molecular modeling of mixture adsorption in nanoporous materials can provide insight into the molecular-level details that underlie adsorptive separations. Modeling of adsorption often employs a rigid framework approximation for computational convenience. All real materials, however, have intrinsic flexibility due to thermal vibrations of their atoms. In this article, we examine quantitative predictions of the adsorption selectivity for a dilute concentration of a chemical warfare agent, sarin, from bulk mixtures with aqueous and non-aqueous (methanol, isopropyl alcohol) solvents using metal–organic frameworks (MOFs). These predictions were made in MOFs approximated as rigid and also in MOFs allowed to have intrinsic flexibility. Including framework flexibility appears to have important consequences for quantitative predictions of adsorption selectivity, particularly for sarin/water mixtures. Our observations suggest the intrinsic flexibility of MOFs can have a nontrivial impact on adsorption modeling of molecular mixtures, especially for mixtures containing polar species and molecules of different sizes. [ABSTRACT FROM AUTHOR]

Swing capacity is a key performance metric for processes designed to capture CO2 by pressure swing adsorption (PSA). Sub-ambient operation of PSA units enables large changes in CO2 swing capacity, and can be economically viable when coupled with heat integration and power recovery. Here, we examine what upper bounds on CO2 swing capacity exist via molecular simulation of a large collection of metal–organic frameworks (MOFs). As has been observed previously for zeolites, the materials with the largest swing capacity at a given temperature have large pore volumes and heats of adsorption within a narrow range of optimal values. A number of materials are identified with swing capacities up to 40 mol kg−1 using a pressure swing from 0.1 bar to 2.0 bar. [ABSTRACT FROM AUTHOR]

Density Functional Theory (DFT) based force fields (FFs) for Ar and Xe adsorption in six metal–organic frameworks were developed using three DFT functionals (PBE-D2, vdW-DF, vdW-DF2) in periodic systems. These force fields include van der Waals (vdW) and polarization terms, and the effect of the latter was shown to be small. Using our DFT-derived and standard (UFF) FFs in grand canonical Monte Carlo simulations, adsorption isotherms and heats of adsorption were calculated and compared with experiment. In most of the cases, it was possible to accurately predict adsorption isotherms using one of the DFT-derived FFs. Still, among the DFT functionals investigated, no single DFT functional could accurately describe all of the adsorbate-framework interactions. On average, performance of UFF and PBE-D2 based FFs to predict experimental isotherms were at a similar quality, still, UFF was slightly superior. Although vdW-DF2 based FFs predicted experimental isotherms almost perfectly for ZIF-8 and HKUST-1 up to 20 bar, their average performance was less than that of PBE-D2 based FFs. Nevertheless, the overall performance of UFF, PBE-D2 and vDW-DF2 FFs was similar. Lastly, vdW-DF based FFs always over-predicted experiments. [ABSTRACT FROM AUTHOR]

A test set of chemically and topologically diverse Metal–Organic Frameworks (MOFs) with high accuracy experimentally derived crystallographic structure data was compiled. The test set was used to benchmark the performance of Density Functional Theory (DFT) functionals (M06L, PBE, PW91, PBE-D2, PBE-D3, and vdW-DF2) for predicting lattice parameters, unit cell volume, bonded parameters and pore descriptors. On average PBE-D2, PBE-D3, and vdW-DF2 predict more accurate structures, but all functionals predicted pore diameters within 0.5 Å of the experimental diameter for every MOF in the test set. The test set was also used to assess the variance in performance of DFT functionals for elastic properties and atomic partial charges. The DFT predicted elastic properties such as minimum shear modulus and Young's modulus can differ by an average of 3 and 9 GPa for rigid MOFs such as those in the test set. The partial charges calculated by vdW-DF2 deviate the most from other functionals while there is no significant difference between the partial charges calculated by M06L, PBE, PW91, PBE-D2 and PBE-D3 for the MOFs in the test set. We find that while there are differences in the magnitude of the properties predicted by the various functionals, these discrepancies are small compared to the accuracy necessary for most practical applications. [ABSTRACT FROM AUTHOR]

There is a great need to identify high-performance nanoporous materials for gas storage, separation and catalysis to solve a wide range of industrially relevant challenges. Molecular simulations have become an important complement to experiments in the screening and selection of suitable porous materials. Successful implementation of molecular simulations requires accurate force fields (FFs) to describe the interactions of guest molecules with porous frameworks. However, developing accurate and transferable FFs for porous materials can be extremely challenging. While generic FFs and experimentally-derived FFs work well for many simple systems, they often fail to describe interactions in more complex porous materials. Since first-principles quantum mechanical (QM) approaches are capable of accurately predicting intermolecular interactions, deriving FFs from QM data without experimental input is a promising solution. In this paper, we review recent developments in deriving first-principles FFs for molecules in a variety of nanoporous materials. We summarize and classify the methodologies used in these studies, and discuss the factors that are responsible for the accuracy and reliability of FFs. Finally, we conclude the review with a discussion of current challenges and future directions for this rapidly growing field. [ABSTRACT FROM AUTHOR]

The development of accurate force fields is vital for predicting adsorption in porous materials. Previously, we introduced a first principles-based transferable force field for CO2 adsorption in siliceous zeolites (Fang et al., J. Phys. Chem. C, 2012, 116, 10692). In this study, we extend our approach to CO2 adsorption in cationic zeolites which possess more complex structures. Na-exchanged zeolites are chosen for demonstrating the approach. These methods account for several structural complexities including Al distribution, cation positions and cation mobility, all of which are important for predicting adsorption. The simulation results are validated with high-resolution experimental measurements of isotherms and microcalorimetric heats of adsorption on well-characterized materials. The choice of first-principles method has a significant influence on the ability of force fields to accurately describe CO2–zeolite interactions. The PBE-D2 derived force field, which performed well for CO2 adsorption in siliceous zeolites, does not do so for Na-exchanged zeolites; the PBE-D2 method overestimates CO2 adsorption energies on multi-cation sites that are common in cationic zeolites with low Si/Al ratios. In contrast, a force field derived from the DFT/CC method performed well. Agreement was obtained between simulation and experiment not only for LTA-4A on which the force field fitting is based, but for other two common adsorbents, NaX and NaY. [ABSTRACT FROM AUTHOR]

First principles calculations have played a useful role in screening mixtures of complex metal hydrides to find systems suitable for H2 storage applications. Standard methods for this task efficiently identify the lowest energy reaction mechanisms among all possible reactions involving collections of materials for which DFT calculations have been performed. The resulting mechanism can potentially differ from physical reality due to inaccuracies in the DFT functionals used, or due to other approximations made in estimating reaction free energies. We introduce an efficient method to probe the robustness of DFT-based predictions that relies on identifying reactions that are metastable relative to the lowest energy reaction path predicted with DFT. An important conclusion of our calculations is that in many examples DFT cannot unambiguously predict a single reaction mechanism for a well defined metal hydride mixture because two or more mechanisms have reaction energies that differ by a small amount. Our approach is illustrated by analyzing a series of single step reactions identified in our recent work that examined reactions with a large database of solids [Kim et al., Phys. Chem. Chem. Phys. 2011, 13, 7218]. [ABSTRACT FROM AUTHOR]

Systematic thermodynamics calculations based on density functional theory-calculated energies for crystalline solids have been a useful complement to experimental studies of hydrogen storage in metal hydrides. We report the most comprehensive set of thermodynamics calculations for mixtures of light metal hydrides to date by performing grand canonical linear programming screening on a database of 359 compounds, including 147 compounds not previously examined by us. This database is used to categorize the reaction thermodynamics of all mixtures containing any four non-H elements among Al, B, C, Ca, K, Li, Mg, N, Na, Sc, Si, Ti, and V. Reactions are categorized according to the amount of H2that is released and the reaction's enthalpy. This approach identifies 74 distinct single step reactions having that a storage capacity >6 wt.% and zero temperature heats of reaction 15 ≤ ΔU0≤ 75 kJ mol−1H2. Many of these reactions, however, are likely to be problematic experimentally because of the role of refractory compounds, B12H12-containing compounds, or carbon. The single most promising reaction identified in this way involves LiNH2/LiH/KBH4, storing 7.48 wt.% H2and having ΔU0= 43.6 kJ mol−1H2. We also examined the complete range of reaction mixtures to identify multi-step reactions with useful properties; this yielded 23 multi-step reactions of potential interest. [ABSTRACT FROM AUTHOR]

Computational methods have been used in the past to generate large libraries of hypothetical zeolite structures, but to date analysis of these structures has typically been limited to relatively simple physical properties such as density. We use efficient methods to analyze the adsorption and diffusion properties of simple adsorbate molecules in a library of >250 000 hypothetical silica zeolites that was generated previously by Deem and co-workers (J. Phys. Chem. C, 2009, 113, 21353). The properties of this library of materials are compared to the complete set of ∼190 zeolites that have been identified experimentally. Our calculations provide information on the largest cavities available in each material for adsorption, and the size of the largest molecules that can diffuse through each material. For a subset of ∼8000 materials, we computed the Henry's constant and diffusion activation energy for adsorbed CH4and H2. We show that these calculations provide a useful screening tool for considering large collections of nanocrystalline materials and choosing materials with particular promise for more detailed modeling. [ABSTRACT FROM AUTHOR]

First-principles calculations represent a potent tool for screening metal hydride mixtures that can reversibly store hydrogen. A number of promising new hydride systems with high hydrogen capacity and favorable thermodynamics have been predicted this way. An important limitation of these studies, however, is the assumption that H2is the only gas-phase product of the reaction, which is not always the case. This paper summarizes new theoretical and numerical approaches that can be used to predict thermodynamic equilibria in complex metal hydride systems with competing reaction pathways. We report thermochemical equilibrium calculations using data obtained from density functional theory (DFT) computations to describe the possible occurrence of gas-phase products other than H2in three complex hydrides, LiNH2, LiBH4, and Mg(BH4)2, and mixtures of these with the destabilizing compounds LiH, MgH2, and C. The systems under investigation contain N, C, and/or B and thus have the potential to evolve N2, NH3, hydrocarbons, and/or boranes as well as H2. Equilibria as a function of both temperature and total pressure are predicted. The results indicate that significant amounts of these species can form under some conditions. In particular, the thermodynamic model predicts formation of N2and NH3as products of LiNH2decomposition. Comparison with published experimental data indicates that N2formation must be kinetically limited. Our examination of C-containing systems indicates that methane is the stable gas-phase species at low temperatures, not H2. On the other hand, very low amounts of boranes (primarily BH3) are predicted to form in B-containing systems. [ABSTRACT FROM AUTHOR]

Density functional theory calculations have been used to study the adsorption of glycine, alanine, serine, cysteine, aspartic acid and asparagine on the hydroxylated (1010) surface of α-quartz. We found measurable energy differences between the two enantiomers of serine, aspartic acid and asparagine in their most stable states on this surface, while negligible differences in adsorption energies for the most stable minima of enantiomers of alanine and cysteine were observed. Our results provide fundamental information on how amino acids can exhibit enantiospecific adsorption on hydroxylated quartz surfaces. [ABSTRACT FROM AUTHOR]

The identification of membrane materials with high selectivity for CO2/CH4mixtures could revolutionize this industrially important separation. We predict using computational methods that a metal organic framework (MOF), Cu(hfipbb)(H2hfipbb)0.5, has unprecedented selectivity for membrane-based separation of CO2/CH4mixtures. Our calculations combine molecular dynamics, transition state theory, and plane wave DFT calculations to assess the importance of framework flexibility in the MOF during molecular diffusion. This combination of methods should also make it possible to identify other MOFs with attractive properties for kinetic separations. [ABSTRACT FROM AUTHOR]

First principles calculations suggest that mobility of H in solid borohydrides is dominated by neutral interstitial H2, not charged defects. [ABSTRACT FROM AUTHOR]

The development of accurate force fields is vital for predicting adsorption in porous materials. Previously, we introduced a first principles-based transferable force field for CO2 adsorption in siliceous zeolites (Fang et al., J. Phys. Chem. C, 2012, 116, 10692). In this study, we extend our approach to CO2 adsorption in cationic zeolites which possess more complex structures. Na-exchanged zeolites are chosen for demonstrating the approach. These methods account for several structural complexities including Al distribution, cation positions and cation mobility, all of which are important for predicting adsorption. The simulation results are validated with high-resolution experimental measurements of isotherms and microcalorimetric heats of adsorption on well-characterized materials. The choice of first-principles method has a significant influence on the ability of force fields to accurately describe CO2-zeolite interactions. The PBE-D2 derived force field, which performed well for CO2 adsorption in siliceous zeolites, does not do so for Na-exchanged zeolites; the PBE-D2 method overestimates CO2 adsorption energies on multi-cation sites that are common in cationic zeolites with low Si/Al ratios. In contrast, a force field derived from the DFT/CC method performed well. Agreement was obtained between simulation and experiment not only for LTA-4A on which the force field fitting is based, but for other two common adsorbents, NaX and NaY.

Computational methods have been used in the past to generate large libraries of hypothetical zeolite structures, but to date analysis of these structures has typically been limited to relatively simple physical properties such as density. We use efficient methods to analyze the adsorption and diffusion properties of simple adsorbate molecules in a library of >250,000 hypothetical silica zeolites that was generated previously by Deem and co-workers (J. Phys. Chem. C, 2009, 113, 21353). The properties of this library of materials are compared to the complete set of ∼190 zeolites that have been identified experimentally. Our calculations provide information on the largest cavities available in each material for adsorption, and the size of the largest molecules that can diffuse through each material. For a subset of ∼8000 materials, we computed the Henry's constant and diffusion activation energy for adsorbed CH(4) and H(2). We show that these calculations provide a useful screening tool for considering large collections of nanocrystalline materials and choosing materials with particular promise for more detailed modeling.

Density functional theory calculations have been used to study the adsorption of glycine, alanine, serine, cysteine, aspartic acid and asparagine on the hydroxylated (10 1 0) surface of alpha-quartz. We found measurable energy differences between the two enantiomers of serine, aspartic acid and asparagine in their most stable states on this surface, while negligible differences in adsorption energies for the most stable minima of enantiomers of alanine and cysteine were observed. Our results provide fundamental information on how amino acids can exhibit enantiospecific adsorption on hydroxylated quartz surfaces.

The identification of membrane materials with high selectivity for CO(2)/CH(4) mixtures could revolutionize this industrially important separation. We predict using computational methods that a metal organic framework (MOF), Cu(hfipbb)(H(2)hfipbb)(0.5), has unprecedented selectivity for membrane-based separation of CO(2)/CH(4) mixtures. Our calculations combine molecular dynamics, transition state theory, and plane wave DFT calculations to assess the importance of framework flexibility in the MOF during molecular diffusion. This combination of methods should also make it possible to identify other MOFs with attractive properties for kinetic separations.

First principles calculations suggest that mobility of H in solid borohydrides is dominated by neutral interstitial H(2), not charged defects.

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage, but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through mixing metal hydrides with other compounds. A very large number of possible destabilized metal hydride reaction schemes exist, but the thermodynamic data required to assess the enthalpies of these reactions are not available in many cases. We have used density functional theory calculations to predict the reaction enthalpies for more than 300 destabilization reactions that have not previously been reported. The large majority of these reactions are predicted not to be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low, and hence these reactions need not be investigated experimentally. Our calculations also identify multiple promising reactions that have large enough hydrogen storage capacities to be useful in practical applications and have reaction thermodynamics that appear to be suitable for use in fuel cell vehicles and are therefore promising candidates for experimental work.