Zhongqiao Hu

A highly permeable and selective zeolitic imidazolate framework ZIF-95 membrane for H2/CO2 separation

Making use of the preferred adsorption affinity and capacity to CO(2) as well as the highly porous structure with huge cavities of 2.4 nm, a highly permeable and selective ZIF-95 molecular sieve membrane was developed for the separation of H(2) from CO(2).

Sorption-Induced Structural Transition of Zeolitic Imidazolate Framework-8: A Hybrid Molecular Simulation Study

A new force field and a hybrid Monte Carlo/molecular dynamics simulation method are developed to investigate the structural transition of zeolitic imidazolate framework-8 (ZIF-8) induced by N2 sorption. At a high loading (approximately 50 N2 molecules per unit cell), ZIF-8 shifts from low-loading (LL) to high-loading (HL) structure. A stepped sorption isotherm is predicted with three distinct regions, which agrees well with experimental data. The orientation of imidazolate rings and the motion of framework atoms exhibit sharp changes upon structural transition. Furthermore, pronounced changes are observed in various contributions to potential energies (including stretching, bending, torsional, van der Waals, and coulombic). The analysis of radial distribution functions between N2 and framework atoms suggests N2 interacts strongly with the imidazolate rings in ZIF-8. The simulation reveals that the structural transition of ZIF-8 is largely related to the reorientation of imidazolate rings, as attributed to the enhanced van der Waals interaction between N2 and imidazolate rings as well as the reduced torsional interaction of framework in the HL structure. This is the first molecular simulation study to describe the continuous structural transition of ZIF-8 and, it provides microscopic insight into the underlying mechanism.

Zeolitic imidazolate framework-8 as a reverse osmosis membrane for water desalination: Insight from molecular simulation

A molecular simulation study is reported for water desalination in zeolitic imidazolate framework-8 (ZIF-8) membrane. The simulation demonstrates that water desalination occurs under external pressure, and Na(+) and Cl(-) ions cannot transport across the membrane due to the sieving effect of small apertures in ZIF-8. The flux of water permeating the membrane scales linearly with the external pressure, and exhibits an Arrhenius-type relation with temperature (activation energy of 24.4 kJ∕mol). Compared with bulk phase, water molecules in ZIF-8 membrane are less hydrogen-bonded and the lifetime of hydrogen-bonding is considerably longer, as attributed to the surface interactions and geometrical confinement. This simulation study suggests that ZIF-8 might be potentially used as a reverse osmosis membrane for water purification.

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.

Development of a force field for zeolitic imidazolate framework-8 with structural flexibility.

A force field is developed for zeolitic imidazolate framework-8 (ZIF-8) with structural flexibility by combining quantum chemical calculations and classical Amber force field. The predicted crystalline properties of ZIF-8 (lattice constants, bond lengths, angles, dihedrals, and x-ray diffraction patterns) agree well with experimental results. A structural transition from crystalline to amorphous as found in experiment is observed. The mechanical properties of ZIF-8 are also described fairly well by the force field, particularly the Youngs modulus predicted matches perfectly with measured value. Furthermore, the heat capacity of ZIF-8 as a typical thermophysical property is predicted and close to experimental data available for other metal-organic frameworks. It is revealed the structural flexibility of ZIF-8 exerts a significant effect on gas diffusion. In rigid ZIF-8, no diffusive behavior is observed for CH(4) within the simulation time scale of current study. With the structural flexibility, however, the predicted diffusivities of CH(4) and CO(2) are close to reported data in the literature. The density distributions and free energy profiles of CH(4) and CO(2) in the pore of ZIF-8 are estimated to analyze the mechanism of gas diffusion.

Assessment of biomolecular force fields for molecular dynamics simulations in a protein crystal

Different biomolecular force fields (OPLS‐AA, AMBER03, and GROMOS96) in conjunction with SPC, SPC/E and TIP3P water models are assessed for molecular dynamics simulations in a tetragonal lysozyme crystal. The root mean square deviations for the Ca atoms of lysozymes are about 0.1 to 0.2 nm from OPLS‐AA and AMBER03, smaller than 0.4 nm from GROMOS96. All force fields exhibit similar pattern in B‐factors, whereas OPLS‐AA and AMBER03 accurately reproduce experimental measurements. Despite slight variations, the primary secondary structures are well conserved using different force fields. Water diffusion in the crystal is approximately ten‐fold slower than in bulk phase. The directional and average water diffusivities from OPLS‐AA and AMBER03 along with SPC/E model match fairly well with experimental data. Compared to GROMOS96, OPLS‐AA and AMBER03 predict larger hydrophilic solvent‐accessible surface area of lysozyme, more hydrogen bonds between lysozyme and water, and higher percentage of water in hydration shell. SPC, SPC/E and TIP3P water models have similar performance in most energetic and structural properties, but SPC/E outperforms in water diffusion. While all force fields overestimate the mobility and electrical conductivity of NaCl, a combination of OPLS‐AA for lysozyme and the Kirkwood‐Buff model for ions is superior to others. As attributed to the steric restraints and surface interactions, the mobility and conductivity in the crystal are reduced by one to two orders of magnitude from aqueous solution.

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.

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.

Self-Assembly of Amphiphilic Peptide (AF)6H5K15: Coarse-Grained Molecular Dynamics Simulation

Amphiphilic peptides are receiving considerable interest for drug delivery because of their self-assembly nature. A molecular dynamics simulation study is reported here to investigate the self-assembly of FA32 peptide composed of 32 amino acid (AF)6H5K15. The peptide, as well as water and counterions, are represented by the MARTINI coarse-grained model. Within 5 μs simulation duration, the peptide is observed to form micelles. Ala and Phe stay in the hydrophobic core, Lys in the hydrophilic shell, and amphiphilic His at the interface. The assembly process and microscopic structures are analyzed in terms of the number of clusters, the radii of micelle, core and shell, and the density profiles of residues. A three-step process is proposed for the assembly: small clusters are initially aggregated and then merged into large clusters, eventually micelles are formed. The effects of simulation box size and peptide concentration are examined in detail. It is found that the micellar structures and microscopic properties are essentially independent of box size. With increasing concentration, quasi-spherical micelles change to elongated shape and micelle size generally increases. The simulation study provides microscopic insight into the assembly process of FA32 peptide and the microscopic structures.

Molecular Dynamics Simulations for Water and Ions in Protein Crystals

The spatial and temporal properties of water and ions in bionanoporous materials-protein crystals-have been investigated using molecular dynamics simulations. Three protein crystals are considered systematically with different morphologies and chemical topologies: tetragonal lysozyme, orthorhombic lysozyme, and tetragonal thermolysin. It is found that the thermal fluctuations of C(alpha) atoms in the secondary structures of protein molecules are relatively weak due to hydrogen bonding. The solvent-accessible surface area per residue is nearly identical in the three protein crystals; the hydrophobic and hydrophilic residues in each crystal possess approximately the same solvent-accessible surface area. Water distributes heterogeneously and has different local structures within the biological nanopores of the three protein crystals. The mobility of water and ions in the crystals is enhanced as the porosity increases and also by the fluctuations of protein atoms particularly in the two lysozyme crystals. Anisotropic diffusion is found preferentially along the pore axis, as experimentally observed. The anisotropy of the three crystals increases in the order: tetragonal thermolysin < tetragonal lysozyme < orthorhombic lysozyme.