Krishna M. Gupta

Metal–organic framework supported ionic liquid membranes for CO2 capture: anion effects

IRMOF-1 supported ionic liquid (IL) membranes are investigated for CO(2) capture by atomistic simulation. The ILs consist of identical cation 1-n-butyl-3-methylimidazolium [BMIM](+), but four different anions, namely hexafluorophosphate [PF(6)](-), tetrafluoroborate [BF(4)](-), bis(trifluoromethylsulfonyl)imide [Tf(2)N](-), and thiocyanate [SCN](-). As compared with the cation, the anion has a stronger interaction with IRMOF-1 and a more ordered structure in IRMOF-1. The small anions [PF(6)](-), [BF(4)](-), and [SCN](-) prefer to locate near to the metal-cluster, particularly the quasi-spherical [PF(6)](-) and [BF(4)](-). In contrast, the bulky and chain-like [BMIM](+) and [Tf(2)N](-) reside near the phenyl ring. Among the four anions, [Tf(2)N](-) has the weakest interaction with IRMOF-1 and thus the strongest interaction with [BMIM](+). With increasing the weight ratio of IL to IRMOF-1 (W(IL/IRMOF-1)), the selectivity of CO(2)/N(2) at infinite dilution is enhanced. At a given W(IL/IRMOF-1), the selectivity increases as [Tf(2)N](-) < [PF(6)](-) < [BF(4)](-) < [SCN](-). This hierarchy is predicted by the COSMO-RS method, and largely follows the order of binding energy between CO(2) and anion estimated by ab initio calculation. In the [BMIM][SCN]/IRMOF-1 membrane with W(IL/IRMOF-1) = 1, [SCN](-) is identified to be the most favorable site for CO(2) adsorption. [BMIM][SCN]/IRMOF-1 outperforms polymer membranes and polymer-supported ILs in CO(2) permeability, and its performance surpasses Robesons upper bound. This simulation study reveals that the anion has strong effects on the microscopic properties of ILs and suggests that MOF-supported ILs are potentially intriguing for CO(2) capture.

Reversed thermo-switchable molecular sieving membranes composed of two-dimensional metal-organic nanosheets for gas separation

It is highly desirable to reduce the membrane thickness in order to maximize the throughput and break the trade-off limitation for membrane-based gas separation. Two-dimensional membranes composed of atomic-thick graphene or graphene oxide nanosheets have gas transport pathways that are at least three orders of magnitude higher than the membrane thickness, leading to reduced gas permeation flux and impaired separation throughput. Here we present nm-thick molecular sieving membranes composed of porous two-dimensional metal-organic nanosheets. These membranes possess pore openings parallel to gas concentration gradient allowing high gas permeation flux and high selectivity, which are proven by both experiment and molecular dynamics simulation. Furthermore, the gas transport pathways of these membranes exhibit a reversed thermo-switchable feature, which is attributed to the molecular flexibility of the building metal-organic nanosheets.

Molecular insight into cellulose regeneration from a cellulose/ionic liquid mixture: effects of water concentration and temperature

Cellulose regeneration from a cellulose/ionic liquid (IL) mixture is investigated using molecular simulation. The IL considered is 1-n-butyl-3-methylimidazolium acetate ([BMIM][Ac]). Water is added as an anti-solvent into the cellulose/[BMIM][Ac] mixture to regenerate cellulose. The simulated structural properties demonstrate that cellulose interacts more strongly with the anion [Ac]− than with the cation [BMIM]+. With increasing water concentration, the cellulose–[Ac]− interaction strength diminishes. The addition of water leads to the destruction of the cellulose–[Ac]− hydrogen-bonds (H-bonds), and the subsequent formation of cellulose–cellulose and [Ac]−–water H-bonds. On this basis, a mechanism for cellulose regeneration is proposed. The torsional angle distributions of hydroxymethyl groups in regenerated cellulose chains are substantially different from those in cellulose crystals, implying that the regenerated cellulose is amorphous, as is also observed in the experiment. Furthermore, the effect of temperature on regeneration is investigated. At a higher temperature, the cellulose–cellulose interaction is enhanced and regeneration is increased. On a microscopic level, this simulation study provides a useful insight into the structural and energetic properties in cellulose/[BMIM][Ac]/water mixtures, and reveals that H-bonding is the key factor governing cellulose regeneration.

Water Desalination through Zeolitic Imidazolate Framework Membranes: Significant Role of Functional Groups

A molecular simulation study is reported for water desalination through five zeolitic imidazolate framework (ZIF) membranes, namely ZIF-25, -71, -93, -96, and -97. The five ZIFs possess identical rho-topology but differ in functional groups. The rejection of salt (NaCl) is found to be around 97% in ZIF-25, and 100% in the other four ZIFs. The permeance ranges from 27 to 710 kg/(m(2)·h·bar), about one∼two orders of magnitude higher compared with commercial reverse osmosis membranes. Due to a larger aperture size da, ZIF-25, -71, and -96 exhibit a much higher water flux than ZIF-93 and -97; however, the flux in ZIF-25, -71, and -96 is governed by the polarity of functional group rather than da. With the hydrophobic CH3 group, ZIF-25 has the highest flux despite the smallest da among ZIF-25, -71, and -96. The lifetime of hydrogen bonding in ZIF-25 is shorter than that in ZIF-71 and -96. Furthermore, water molecules undergo a fast flushing motion in ZIF-25, but frequent jumping in ZIF-96 and particularly in ZIF-97. An Arrhenius-type relationship is found between water flux in ZIF-25 and temperature, and the activation energy is predicted to be 6.5 kJ/mol. This simulation study provides a microscopic insight into water desalination in a series of ZIFs, reveals the key factors (aperture size and polarity of functional group) governing water flux, and suggests that ZIF-25 might be an interesting reverse osmosis membrane for high-performance water desalination.

Cellulose regeneration from a cellulose/ionic liquid mixture: the role of anti-solvents

A molecular simulation study is reported to investigate the role of anti-solvents (water, ethanol, and acetone) in cellulose regeneration from a cellulose/1-n-butyl-3-methylimidazolium acetate ([BMIM][Ac]) mixture. Structural analysis based on radial distribution functions reveals that the interaction of cellulose–[BMIM][Ac] decreases in the order acetone > ethanol > water, with cellulose–[Ac]− forming the smallest number of H-bonds in water. However, the interaction of cellulose–cellulose increases in the reverse order (acetone < ethanol < water), with the largest number of H-bonds between cellulose chains being observed in water. Among the three solvents, water is identified to be the most effective at breaking the cellulose–[Ac]− H-bonds and leading to the subsequent formation of cellulose–cellulose H-bonds. Furthermore, the dynamic analysis based on survival time-correlation functions and mean-squared displacements demonstrates that [Ac]− in water has the shortest residence time near cellulose and the highest mobility compared to [Ac]− in ethanol and acetone. This simulation study suggests that water outperforms ethanol and acetone for cellulose regeneration, and provides a microscopic insight into the mechanism of cellulose regeneration.

Enhancement of CO2 uptake in iso-reticular Co based zeolitic imidazolate frameworks via metal replacement

Three Co based Zeolitic Imidazolate Frameworks (Co-ZIF-68, -69 and -81) which adopt a GME topology with high porosity have been synthesized. These Co-ZIFs show high CO2 (273 K/298 K) uptake compared to their isostructural Zn based analogues, which has been proved experimentally as well as by ab initio calculations.

Seawater Pervaporation through Zeolitic Imidazolate Framework Membranes: Atomistic Simulation Study.

An atomistic simulation study is reported for seawater pervaporation through five zeolitic imidazolate framework (ZIF) membranes including ZIF-8, -93, -95, -97, and -100. Salt rejection in the five ZIFs is predicted to be 100%. With the largest aperture, ZIF-100 possesses the highest water permeability of 5 × 10(-4) kg m/(m(2) h bar), which is substantially higher compared to commercial reverse osmosis membranes, as well as zeolite and graphene oxide pervaporation membranes. In ZIF-8, -93, -95, and -97 with similar aperture size, water flux is governed by framework hydrophobicity/hydrophilicity; in hydrophobic ZIF-8 and -95, water flux is higher than in hydrophilic ZIF-93 and -97. Furthermore, water molecules in ZIF-93 move slowly and remain in the membrane for a long time but undergo to-and-fro motion in ZIF-100. The lifetime of hydrogen bonds in ZIF-93 is found to be longer than in ZIF-100. This simulation study quantitatively elucidates the dynamic and structural properties of water in ZIF membranes, identifies the key governing factors (aperture size and framework hydrophobicity/hydrophilicity), and suggests that ZIF-100 is an intriguing membrane for seawater pervaporation.

Efficient ethanol/water separation via functionalized nanoporous graphene membranes: insights from molecular dynamics study

Systematic molecular dynamics simulations are conducted to study the separation of ethanol/water mixture through single-layer graphene with designed nanoscale pores. The effects of pore size, chemical functionalization, and applied pressure were investigated. It was found that the diameter of pore plays a key role for efficient separation of ethanol from water. With appropriate diameter, water molecules can pass through but flow of ethanol is essentially blocked. Compared to hydrophobic, hydrophilic functionalization is found to be more efficient for ethanol/water separation as energy barrier for water molecule is less than ethanol in case of hydrophilic porous graphene membrane. Overall, our results indicate that the flux through hydrophilic functionalized (P2_OH) graphene membrane is nearly four times higher than conventional reverse osmosis membranes with a good selectivity for ethanol/water separation. This simulation study provides molecular-level understanding of ethanol/water separation through functionalized nonporous graphene and reveals the key governing factors that are essential for designing novel graphene membranes for bioethanol purification.

Characterization of water vapor permeable membranes

Air permeability and water vapor permeability of naphion membranes were measured using specially built instruments. Air permeability was almost zero. Water vapor permeability was zero during an incubation period. After the incubation period, the permeability became large and then gradually decreased with time. The behavior has been attributed to the contributions of chemical and mechanical forces to the net flux through the membrane. A model consistent with these results has been presented to explain absorption and transport of water vapor through naphion membranes.

Catalytic amino acid production from biomass-derived intermediates

Significance Today, amino acids are primarily manufactured via microbial cultivation processes, which are costly, are time consuming, and require extensive separations processes. As an alternative, chemocatalytic approaches to produce amino acids from renewable feedstocks such as bio-based sugars could offer a rapid and potentially more efficient means of amino acid synthesis, but efforts to date have been limited by the development of facile chemistry and associated catalyst materials to selectively produce α-amino acids. In this work, various α-amino acids, including alanine, leucine, aspartic acid, and phenylalanine, were obtained from both biomass-derived α-hydroxyl acids and glucose. The route bridges plant-based biomass and proteinogenic α-amino acids, offering a chemical approach that is potentially superior to microbial cultivation processes. Amino acids are the building blocks for protein biosynthesis and find use in myriad industrial applications including in food for humans, in animal feed, and as precursors for bio-based plastics, among others. However, the development of efficient chemical methods to convert abundant and renewable feedstocks into amino acids has been largely unsuccessful to date. To that end, here we report a heterogeneous catalyst that directly transforms lignocellulosic biomass-derived α-hydroxyl acids into α-amino acids, including alanine, leucine, valine, aspartic acid, and phenylalanine in high yields. The reaction follows a dehydrogenation-reductive amination pathway, with dehydrogenation as the rate-determining step. Ruthenium nanoparticles supported on carbon nanotubes (Ru/CNT) exhibit exceptional efficiency compared with catalysts based on other metals, due to the unique, reversible enhancement effect of NH3 on Ru in dehydrogenation. Based on the catalytic system, a two-step chemical process was designed to convert glucose into alanine in 43% yield, comparable with the well-established microbial cultivation process, and therefore, the present strategy enables a route for the production of amino acids from renewable feedstocks. Moreover, a conceptual process design employing membrane distillation to facilitate product purification is proposed and validated. Overall, this study offers a rapid and potentially more efficient chemical method to produce amino acids from woody biomass components.