Zhongqiao Hu

Storage of pure CO2 and CH4 and separation of their binary mixture in three different classes of nanostructured adsorbents--silicalite, C168 schwarzite, and IRMOF-1--have been compared at room temperature using atomistic simulation. CH4 is represented as a spherical Lennard-Jones molecule, and CO2 is represented as a rigid linear molecule with a quadrupole moment. For pure component adsorption, CO2 is preferentially adsorbed than CH4 in all the three adsorbents over the pressure range under this study, except in C168 schwarzite at high pressures. The simulated adsorption isotherms and isosteric heats match closely with available experimental data. A dual-site Langmuir-Freundlich equation is used to fit the isotherms satisfactorily. Compared to silicalite and C168 schwarzite, the gravimetric adsorption capacity of pure CH4 and CO2 separately in IRMOF-1 is substantially larger. This implies that IRMOF-1 might be a potential storage medium for CH4 and CO2. For adsorption from an equimolar binary mixture, CO2 is preferentially adsorbed in all three adsorbents. Predictions of mixture adsorption with the ideal-adsorbed solution theory on the basis of only pure component adsorption agree well with simulation results. Though IRMOF-1 has a significantly higher adsorption capacity than silicalite and C168 schwarzite, the adsorption selectivity of CO2 over CH4 is found to be similar in all three adsorbents.

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.

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).

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.

For the first time, a homochiral metal-organic framework membrane was prepared for the enantioselective separation of important chiral compounds, especially chiral drug intermediates, which will allow for the potential development of a new, sustainable and highly efficient chiral separation technique.