Recently, there has been growing interest in hafnium (Hf) metal-organic frameworks (MOFs). These MOFs may perform better as gas adsorbents than zirconium (Zr) MOFs due to the presence of Brønsted acid sites with high affinity toward adsorbates, together with the outstanding chemical and hydrothermal stabilities similar to their Zr analogues. However, Hf-MOFs have been rarely reported due to the lack of effective synthetic methods. We herein report a modulated hydrothermal synthesis of UiO-66(Hf)-type MOFs. Among these MOFs, UiO-66(Hf)-(OH)2 possesses a very high CO2 gravimetric uptake of 1.81 mmol g(-1) at 0.15 bar and 298 K, which is 400% higher than that of UiO-66(Hf) (0.36 mmol g(-1)). It also exhibits a record-high volumetric CO2 uptake of 167 v/v at 1 bar and 298 K. Ideal adsorbed solution theory calculations showed a CO2/N2 (molar ratio 15:85) selectivity of 93 and CO2/H2 (molar ratio 30:70) selectivity above 1700. Breakthrough simulations also confirmed its optimal CO2 separation attribute. Our results have demonstrated for the first time the strong potential of Hf-MOFs for advanced adsorbents for high-performance CO2-related separations.

A molecular simulation study is reported for the adsorption of water and alcohols (methanol and ethanol) in two metal−organic frameworks (MOFs) topologically similar to rho-zeolite; one is a hydrophilic Na+-exchanged rho-zeolite-like MOF (Na-rho-ZMOF), and the other is a hydrophobic zeolitic−imidazolate framework-71 (ZIF-71). The adsorption isotherms in Na-rho-ZMOF are type I as a consequence of the high affinity of the nonframework Na+ ions and ionic framework. The adsorption capacity decreases in the order of water > methanol > ethanol. Water is adsorbed more closely in the window region, whereas methanol and ethanol are populated in the α cage due to steric effect. In water/methanol and water/ethanol mixtures, water adsorption increases continuously with increasing pressure and replaces alcohols competitively at high pressures. In ZIF-71, the framework−adsorbate affinity is relatively weaker and type V adsorption is observed. Water has vanishingly small adsorption at low pressures and a sharp increase in adsorption at 22 kPa due to capillary condensation. Methanol and ethanol exhibit cluster-growth adsorption, followed by continuous pore filling. The adsorption in ZIF-71 increases in the order of water < methanol < ethanol at low pressures; however, the opposite order is observed at high pressures because of entropy effect. In water/alcohol mixtures, alcohols are selectively more adsorbed at low pressures but surpassed by water with increasing pressure. The framework charges have a substantial effect on adsorption in Na-rho-ZMOF, but not in ZIF-71. This study provides a molecular understanding for the adsorption of water and alcohols in two zeolitic MOFs with the identical topology and reveals the significantly different adsorption mechanisms.

A novel covalent organic framework (COF) IISERP-COOH-COF1 membrane with both high water permeance and high ions rejection was developed for desalination through post-modification, which shows superior ions rejection based on size exclusion.

A molecular simulation study is reported for biofuel purification in six zeolitic imidazolate frameworks (ZIF-8, -25, -71, -90, -96 and -97) with different functional groups. For pure ethanol and water, the predicted adsorption isotherms agree fairly well with experimental data. Hydrogen bonding has an important effect on the adsorption of ethanol and water. In hydrophilic ZIFs (ZIF-90, -96 and -97) with polar groups, adsorption capacities are higher than in hydrophobic counterparts (ZIF-8, -25 and -71). The atomic charges in symmetrically functionalized ZIF-8, -25, and -71 are found to have an indiscernible effect on adsorption, in remarkable contrast to asymmetrically functionalized ZIF-90, -96 and -97. For ethanol-water mixtures representing the biofuel, the selectivity of ethanol-water drops with increasing ethanol in mixtures. It is revealed that the selectivity is determined primarily by framework hydrophobicity as well as the cage size. Among the six ZIFs, ZIF-8 exhibits the highest selectivity. This simulation study provides a microscopic insight into the adsorption of ethanol and water in various ZIFs, reveals the significant role of functional groups in governing biofuel purification, and would facilitate the development of new nanoporous materials for high-efficacy liquid separation.

A molecular simulation study is reported for ion exchange in a rho zeolite-like metal–organic framework (ZMOF). The nonframework Na+ ions in rho-ZMOF are observed to exchange with Pb2+ ions in PbCl2 solution. At equilibrium, all Pb2+ ions are exchanged and reside in rho-ZMOF, while Na+ ions are in a dynamic equilibrium with solution. By umbrella sampling, the potential of mean force for Pb2+ moving from solution into rho-ZMOF is estimated to be −10k B T, which is more favorable than −5k B T for Na+ and contributes to the observed ion exchange. The residence-time distributions and mean-squared displacements reveal that all the exchanged Pb2+ ions stay continuously in rho-ZMOF without exchanging with other ions in solution due to strong interaction with rho-ZMOF; however, Na+ ions have a shorter residence time and a larger mobility than Pb2+ ions. The exchanged Pb2+ ions in rho-ZMOF are located at eight-, six-, and four-membered rings. As attributed to the confinement effect, distinctly different dynamic properties are found for Pb2+ ions at the three locations. Pb2+ ions at 8MR have the highest mobility due to the largest ring size, while those at 4MR have a negligible mobility. This simulation study provides microscopic insight into the ion-exchange process in ionic MOF and suggests that rho-ZMOF might be an intriguing candidate for water purification.