Tuning the Transport Properties of Gases in Porous Graphene Membranes with Controlled Pore Size and Thickness

Porous graphene membranes emerged as promising alternatives for gas separation applications due to their atomic thickness enabling ultra-high permeance, but suffer from low gas selectivity. Whereas decreasing the pore size below 3 nm is expected to increase the gas selectivity due to molecular sieving, it is rather challenging to generate large number of uniform small pores on the graphene surface. Here, we introduce a pore narrowing approach via gold deposition onto porous graphene surface to tune the pore size and thickness of the membrane to achieve large number of small pores. Through our systematic approach, we determined the ideal combination as pore size below 3 nm obtained at the thickness of 100 nm to attain high selectivity and high permeance. The resulting membrane showed a H2 /CO2 separation factor of 31.3 at H2 permeance of 2.23 × 105 GPU (1 GPU = 3.35 × 10-10  mol s-1 m-2 Pa-1 ), which is the highest value reported to date in the 105 GPU permeance range. This result is explained by comparing the predicted binding energies of gas molecules with the Au surface, -5.3 versus -21 kJ mol-1 for H2 and CO2 , respectively, increased surface-gas interactions and molecular sieving effect with decreasing pore size. This article is protected by copyright. All rights reserved

The role of hydrogen bonding in the dehydration of bioalcohols in hydrophobic pervaporation membranes

The dehydration of bioalcohols is considered one of the major factors contributing to the cost of biofuel production. In this study, liquid phase separation of water from methanol and ethanol in a siliceous MFI pervaporation membrane was studied by performing concentration gradient driven molecular dynamic (CGD-MD) simulations. CGD-MD simulations work by imposing a higher concentration in the feed side and a lower concentration in the permeate side of the membrane. This creates a concentration gradient across the membrane that facilitates the diffusion of molecules from the feed to the permeate side, mimicking the experimental pervaporation membrane set up. Fluxes of methanol, ethanol and water were calculated in single component permeation simulations and in equimolar methanol–water and ethanol–water mixture separation simulations. It was found that water formed hydrogen bonds with the silanol (Si-OH) groups on the external surface of the MFI and did not enter the membrane in the single component permeation simulation. While this may suggest that MFI can be used to effectively dehydrate bioalcohols, our simulations showed that water permeated through the MFI membrane when it was in a mixture with either methanol or ethanol. Furthermore, in the alcohol-water mixture simulations, the fluxes of methanol and ethanol were significantly lower than that of expected based on their single component fluxes. A detailed analysis of hydrogen bonding in the alcohol-water mixture separation simulations revealed that water preferred making hydrogen bonds with methanol and ethanol rather than with the silanol groups. This resulted in drifting of water molecules along with permeating alcohol molecules in to the MFI membrane in mixture simulations, while slowing the permeation of methanol and ethanol fluxes. The hydrogen bonding between water and alcohol molecules indicates that it may not be possible to achieve complete alcohol selectivity even if defect-free membranes were manufactured; however, our findings also hint at the possibility of functionalizing membrane surfaces with chemical groups that will overcome water-alcohol hydrogen bonding and retain water molecules in order to approach complete selectivity

Functionalization of Metal-Organic Framework Nanochannels for Water Transport and Purification

Artificial water nanochannels (AWCs) have drawn great attention due to their potential use in water purification. Herein, we propose an AWC design, which is based on coordinatively functionalizing unsaturated metal sites found in metal-organic frameworks (MOFs) with one-dimensional nanochannels. As a computational demonstration, we consider two MOFs, namely, Ni-CPO-27 and Ni-CPO-54, and graft proline, imidazolecarboxylic acid, imidazolecarboxaldehyde, pyrazolecarboxylic acid, and pyrazole carbaldehyde molecules into the MOF nanochannels. To assess the strength of the molecule-metal binding, binding energies were calculated using density functional theory. The results indicate that the MOFs containing either proline or 2-imidazolecarboxylic acid form water-stable AWCs with binding energies twice that of the binding energy of water. To shed light on the water diffusion mechanism in the proline-Ni-CPO-27/54 and 2-imidazolecarboxylic-Ni-CPO-27/54 AWCs, molecular dynamics simulations were performed to calculate the mean-squared displacement of water molecules and nonbonded interaction energies between select pairs of atoms in water and coordinated molecules were analyzed. It was found that the fastest water diffusion occurs in proline-Ni-CPO-54 with a self-diffusion coefficient of 7.2 ± 0.5 × 10-8 cm2/s. In comparison, the fastest water self-diffusion coefficient reported in a carbon nanotube-based AWC is 9 × 10-6 cm2/s. Nonbonded interactions between specific atom pairs regulate water diffusion in the functionalized MOF nanochannels. In particular, the change in water mean-squared displacement with changing water loading correlates well with the nonbonded energies between the partially positively charged hydrogen atoms in water and the partially negatively charged oxygen and nitrogen atoms in the proline and 2-imidazolecarboxylic acid molecules. The results presented herein indicate that water-stable MOFs could perform well as AWCs, thereby lending support to the further design and synthesis of MOF-based AWCs for water purification

Modelling of Gas Transport through Polymer/MOF Interfaces: A Microsecond-Scale Concentration Gradient-Driven Molecular Dynamics Study

Membrane-based separation technologies offer a cost-effective alternative to many energy-intensive gas separation processes, such as distillation. Mixed matrix membranes (MMMs) composed of polymers and metal–organic frameworks (MOFs) have attracted a great deal of attention for being promising systems to manufacture durable and highly selective membranes with high gas fluxes and high selectivities. Therefore, understanding gas transport through these MMMs is of significant importance. There has been longstanding speculation that the gas diffusion behavior at the interface formed between the polymer matrix and MOF particles would strongly affect the global performance of the MMMs due to the potential presence of nonselective voids or other defects. To shed more light on this paradigm, we have performed microsecond long concentration gradient-driven molecular dynamics (CGD-MD) simulations that deliver an unprecedented microscopic picture of the transport of H2 and CH4 as single components and as a mixture in all regions of the PIM-1/ZIF-8 membrane, including the polymer/MOF interface. The fluxes of the permeating gases are computed and the impact of the polymer/MOF interface on the H2/CH4 permselectivity of the composite membrane is clearly revealed. Specifically, we show that the poor compatibility between PIM-1 and ZIF-8, which manifests itself by the presence of nonselective void spaces at their interface, results in a decrease of the H2/CH4 permselectivity for the corresponding composite membrane as compared to the performances simulated for PIM-1 and ZIF-8 individually. We demonstrate that CGD-MD simulations based on an accurate atomistic description of the polymer/MOF composite is a powerful tool for characterization and understanding of gas transport and separation mechanisms in MMMs

Tuning the Hydrophobicity of Layer-Structure Silicates To Promote Adsorption of Nonaqueous Fluids: Effects of F– for OH– Substitution on CO2 Partitioning into Smectite Interlayers

The intercalation of non-aqueous fluids in the nanopores of organic and inorganic materials is of significant interest, particularly in the energy science community. Recently, XRD and computational modeling results have shown that structural F- for OH- substitution in layered silicates makes them more hydrophobic. Here, we use Grand Canonical Molecular Dynamics (GCMD) calculations to investigate how increasing the F-/(F-+OH-) ratio of a prototypical layered silicate (the smectite Na-hectorite) impacts the intercalation behavior of CO2 and H2O at elevated temperature and pressure. At the conditions of this study (T = 323 K, P = 90 bar, water-saturated CO2), increasing F- for OH- substitution causes decreasing total CO2+H2O intercalation, increasing CO2/(CO2+H2O) ratios in the interlayer galleries, and an increasing energy barrier to CO2 and H2O intercalation. CO2 intercalation is greatest at monolayer basal spacings, and the results support the idea that with Na+ as the exchangeable cation the interlayers must be propped open by some H2O molecules to allow CO2 to enter the interlayer galleries. The computed immersion energies suggest that the bilayer or a more expanded structure is the stable state under these conditions, in agreement with experimental results, and that the basal spacings of the minimum energy 2L structures increase with increasing F- for OH- substitution. These results are consistent with a wide range of experimental data for smectites at ambient conditions and elevated pressures and temperatures and suggest that F- for OH- substitution in conjunction with reduced structural charge and exchange with large, low charge cations may increase the ability of smectite minerals to incorporate hydrophobic species such as CH4, CO2, H2, and other organic compounds

Adsorption of Atrazine from Water in Metal-Organic Framework Materials

The adsorptive removal of atrazine, an agricultural herbicide, from water by three water stable MOFs, ZIF-8, UiO-66, and UiO-67, and a commercial activated carbon, F400, was investigated. UiO-67, ZIF-8, and F400 were found to remove up to 98% of the atrazine from water, whereas UiO-66 is found to be ineffective. In an exceptional performance compared to the other adsorbents considered in our study, UiO-67 removed 98% of atrazine from water within only 2 min, whereas ZIF-8 and F400 took over 40 and 50 min, respectively, to remove the same amount of atrazine. Upon regeneration of UiO-67, minimal loss of adsorption capacity was observed, affirming its effective use for atrazine removal from water

Electric field induced rotation of halogenated organic linkers in isoreticular metal-organic frameworks for nanofluidic applications

We present a systematic computational study which provides a plausible route to control the rotation of organic linkers in isoreticular metal-organic frameworks (IRMOF) by using an external electric field in order to manipulate the diffusion of molecules in nanopores. We achieve this by halogenating the organic linkers of IRMOF-1 and IRMOF-7 to create permanent dipole moments on the linkers, hence making them responsive to changes in the strength and direction of an electric field. More importantly we show that by varying the ligand size and the halogen type, number and substitution positions, the strength of the electric field required to control the rotation of linkers can be reduced significantly. Cl substitution is most effective in making the organic linkers electric field responsive since a greater dipole moment is created compared to those obtained by F or Br substitution. Cl substitution of a larger organic linker, i.e. 1,4-naphthalenedicarboxylate (IRMOF-7) rather than 1,4-benzenedicarboxylate (IRMOF-1), results in a greater dipole moment and reduces the electric field strength required for the rotation of the ligand. Furthermore, double Cl substitution and the optimization of the Cl substitution positions enable controlled rotation of the IRMOF-7 linkers with an electric field strength as low as 0.5 V nm−1. Finally, using the electric field induced rotation of organic linkers we show that it is possible to enhance the diffusion of methane molecules in a chosen direction while limiting their mobility in other directions. Our study hints at the potential of using MOFs for flow control in nanofluidic systems

Computational Investigation of Structure-Function Relationship in Fluorine-Functionalized MOFs for PFOA Capture from Water

A strategy that can be used to develop metal-organic frameworks (MOFs) to capture per- and poly-fluoroalkyl substances (PFAS) from water is functionalizing them with fluorine moieties. We investigated different fluorine-functionalization strategies and their performance in removing PFAS from water using molecular simulations. Perfluorooctanoic acid (PFOA), one of the most widely encountered PFAS in water sources, was used as the probe molecule. Our simulations show that fluorine functionalization by incorporating fluorinated anions as bridging ligands in MOFs creates additional binding sites for PFOA; however, the same sites also attract water molecules, which casts doubt on their potential use. In contrast, trifluoromethyl or fluorine substitution of the MOF ligands results in higher hydrophobicity. However, the pores fluorinated with this method should have the optimum size to accommodate PFOA. Likewise, post-synthetic fluorine functionalization of MOFs through grafting of perfluorinated alkanes showed increased PFOA affinity. Fluorine-functionalized MOFs with high hydrophobicity and optimized pore sizes can effectively capture PFOA from water at very low concentrations of PFOA

Cation and Water Structure, Dynamics, and Energetics in Smectite Clays: A Molecular Dynamics Study of Ca-Hectorite

The incorporation of Ca2+ into smectite minerals is well-known to have a significant effect on the swelling behavior and mechanical properties of this environmentally and technologically important group of materials. Relative to common alkali cations such as Na+, K+, and Cs+, Ca2+ has a larger charge/ionic radius ratio and thus interacts very differently with interlayer water molecules and the oxygens of the clay basal surface. Recent 2H and 43Ca NMR studies of the smectite mineral, hectorite, show that the molecular scale interlayer dynamics is quite different with Ca2+ than with alkali cations. Classical molecular dynamics (MD) simulations presented here use a newly developed hectorite model with a disordered distribution of Li+/Mg2+ substitutions in the octahedral sheet and provide new insight into the origin of the effects of Ca2+ on the structure, dynamics, and energetics of smectite interlayers. The computed basal spacings and thermodynamic properties suggest the potential for formation of stable monolayer hydrates that have partial and complete water contents, a bilayer hydrate, and possible expansion to higher hydration states. The system hydration energies are comparable to those previously calculated for Ca–montmorillonite and are more negative than for Cs– and Na–hectorite due to the higher hydration energy of Ca2+. The coordination environments of Ca2+ change significantly with increasing interlayer hydration, with the extent of coordination to basal oxygens decreasing as the number of interlayer molecules increases. On external (001) surfaces, the H2O molecules closest to the surface are adsorbed at the centers of ditrigonal cavities and bridge Ca2+ to the surface. The Ca2+ ions on the external surface are all in outer-sphere coordination with the basal oxygens of the surface, and the proximity-restricted region with a significant number of Ca2+ is approximately 6 Å thick. Quantification of these interactions provides a basis for understanding intercalation of Ca2+ by organic species and smectite minerals

Diffusion Behavior of Methane in 3D Kerogen Models

As global energy demand increases, natural gas recovery from source rocks is attracting considerable attention since recent development in shale extraction techniques has made the recovery process economically viable. Kerogens are thought to play an important role in gas recovery; however, the interactions between trapped shale gas and kerogens remain poorly understood due to the complex, heterogeneous microporous structure of kerogens. This study examines the diffusive behavior of methane molecules in kerogen matrices of different types (Type I, II, and II) and maturity levels (A to D for Type II kerogens) on a molecular scale. Models of each kerogen type were developed using simulated annealing. We employed grand canonical Monte Carlo simulations to predict the methane loadings of the kerogen models and then used equilibrium molecular dynamics simulations to compute the mean square displacement of methane molecules within the kerogen matrices under reservoir-relevant conditions, that is, 365 K and 275 bar. Our results show that methane self-diffusivity exhibits some degree of anisotropy in all kerogen types examined here except for Type I-A kerogens, where diffusion is the fastest and isotropic diffusion is observed. Self-diffusivity appears to correlate positively with pore volume for Type II kerogens, where an increase in diffusivity is observed with increasing maturity. Swelling of the kerogen matrix up to a 3% volume change is also observed upon methane adsorption. The findings contribute to a better understanding of hydrocarbon transport mechanisms in shale and may lead to further development of extraction techniques, fracturing fluids, and recovery predictions