As a new class of organic-inorganic hybrid materials, metal-organic frameworks (MOFs) have received tremendous interest for gas storage and separation. In practice, a gas or gas mixture usually contains a small amount of moisture. The presence of H2O may be beneficial or adverse to gas adsorption in porous media. Several studies have reported the effect of moisture on the adsorption in zeolitic materials; however, few exist in MOFs.
We have systematically examined the effect of water on adsorption and separation in several MOFs including charged and neutral MOFs. In charged MOF, rho-ZMOFs, upon the addition of H2O into a CO2/CH4 mixture, the interaction between CO2 and Na+ is substantially reduced. Consequently, CO2 adsorption drops and the selectivity decreases by one order of magnitude. In the hydrated charged Li-MOF, the selectivity of CO2/H2 and CO2/N2 decreases compared to that in the dehydrated Li-MOF due to the reduced free volume. In other cases, the presence of H2O may be beneficial to the gas separation. For instance, with a trace amount of H2O, the selectivity of CO2/H2 in soc-MOF increases at low pressures due to the promoted adsorption of CO2 by H2O bound onto the indium atoms. Similarly, the adsorption of CO2 and CH4 and the selectivity of CO2/CH4 in mesoporous MIL-101 are enhanced by terminal H2O molecules. However, the addition of H2O has a marginal effect on the adsorption of CO2 and CH4 and subsequently the selectivity in highly hydrophobic Zn(BDC)-(TED)0.5, which has very weak affinity for H2O. This study reveals that a prewater treatment is probably required for gas separation depending on the nature of MOFs used.
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