ABSTRACT
Defect engineering has arisen as a promising approach to tune and optimise the adsorptive performance of metal-organic frameworks. However, the balance between enhanced adsorption and structural stability remains an open question. Here both CO2 adsorption capacity and mechanical stability are calculated for the zirconium-based UiO-66, which is subject to systematic variations of defect scenarios. Modulator-dependence, defect concentration and heterogeneity are explored in isolation. Mechanical stability is shown to be compromised at high pressures where uptake is enhanced with an increase in defect concentration. Nonetheless this reduction in stability is minimised for reo type defects and defects with trifluoroacetate substitution. Finally, heterogeneity and auxeticity may also play a role in overcoming the compromise between adsorption and stability.
ABSTRACT
The storage and separation of H2 and CO2 are investigated in a highly porous ionic rht metal-organic framework (rht-MOF) using molecular simulation. The rht-MOF possesses a cationic framework and charge-balancing extraframework NO3(-) ions. Three types of unique open cages exist in the framework: rhombicuboctahedral, tetrahedral, and cuboctahedral cages. The NO3(-) ions exhibit small mobility and are located at the windows connecting the tetrahedral and cuboctahedral cages. At low pressures, H2 adsorption occurs near the NO3(-) ions that act as preferential sites. With increasing pressure, H2 molecules occupy the tetrahedral and cuboctahedral cages and the intersection regions. The predicted isotherm of H2 at 77 K agrees well with the experimental data. The H2 capacity is estimated to be 2.4 wt % at 1 bar and 6.2 wt % at 50 bar, among the highest in reported MOFs. In a four-component mixture (15:75:5:5 CO2/H2/CO/CH4) representing a typical effluent gas of H2 production, the selectivity of CO2/H2 in rht-MOF decreases slightly with increasing pressure, then increases because of cooperative interactions, and finally decreases as a consequence of entropy effect. By comparing three ionic MOFs (rht-MOF, soc-MOF, and rho-ZMOF), we find that the selectivity increases with increasing charge density or decreasing free volume. In the presence of a trace amount of H2O, the interactions between CO2 and NO3(-) ions are significantly shielded by H2O; consequently, the selectivity of CO2/H2 decreases substantially.
ABSTRACT
MIL-101 is a chromium terephthalate-based mesoscopic metal-organic framework and one of the most porous materials reported to date. In this study, we investigate the adsorption of CO(2) and CH(4) in dehydrated and hydrated MIL-101 and the effect of terminal water molecules on adsorption. The atomistic structures of MIL-101 are constructed from experimental crystallographic data, energy minimization, and quantum mechanical optimization. The adsorption isotherm of CO(2) predicted from molecular simulation agrees well with experiment and is relatively insensitive to the method (Merz-Kollman or Mulliken) used to estimate the framework charges. Both the united-atom and five-site models of CH(4) predict the isotherm fairly well, though the former overestimates and the latter underestimates. Adsorption first occurs in the microporous supertetrahedra at low pressures and then in the mesoscopic cages with increasing pressure. In the dehydrated MIL-101, more adsorbate molecules are located near the exposed Cr(2) sites than the fluorine saturated Cr(1) sites. The terminal water molecules in the hydrated MIL-101 act as additional interaction sites and enhance adsorption at low pressures. This enhancement is more pronounced for CO(2) than for CH(4), because CO(2) is quadrapolar and interacts more strongly with the terminal water molecules. At high pressures, however, the reverse is observed, as the presence of terminal water molecules reduces free volume and adsorption. For the adsorption of CO(2)/CH(4) mixture, a higher selectivity is found in the hydrated MIL-101.
ABSTRACT
The adsorption, mobility, and vibration of water in ion-exchanged rho-zeolite-like metal-organic frameworks (ZMOFs) are investigated using atomistic simulations. Because of the high affinity for the ionic framework and nonframework ions, water is strongly adsorbed in rho-ZMOFs with a three-step adsorption mechanism. At low pressures, water is preferentially adsorbed onto Na(+) ions, particularly at site II; with increasing pressure, adsorption occurs near the framework and finally in the large cage. Upon water adsorption, Na(+) ions are observed to redistribute from site I to site II and gradually hydrated with increasing pressure. In Li-, Na-, and Cs-exchanged rho-ZMOFs, the adsorption capacity and isosteric heat decrease with increasing ionic radius attributed to the reduced electrostatic interaction and free volume. The mobility of water in Na-rho-ZMOF increases at low pressures but decreases upon approaching saturation. With sufficient amount of water present, the mobility of Na(+) ions is promoted. The vibrational spectra of water in Na-rho-ZMOF exhibit distinct bands for librational motion, bending, and stretching. The librational motion has a frequency higher than bulk water due to confinement. With increasing loading and hence stronger coordinative attraction, the bending frequency shows a blue shift. Symmetric and asymmetric modes are observed in the stretching as a consequence of the strong water-ion interaction. This study provides a fundamental microscopic insight into the static and dynamic properties of water in charged ZMOFs and reveals the subtle interplay between water and nonframework ions.
