Highly enhanced and weakened adsorption properties of two MOFs by water vapor for separation of CO2/CH4 and CO2/N2 binary mixtures

• Positive and negative effects of H2O vapor on CO2 adsorption of MOFs were reported.
• Water vapor adsorbed on MIL-100(Fe) promoted formation of more CO2 adsorptive sites.
• Adsorbed H2O made CO2 capacity and selectivity of MIL-100(Fe) increase by 150% and 200%.
• Competitive adsorption of H2O badly weakened CO2 capacity and selectivity of MIL-101.

It is well-known that water vapor is omnipresent. It always has strong influence on adsorption performances of various applied porous materials in realistic situations. Adsorption behaviors of CO2/N2 and CO2/CH4 binary mixtures over MIL-100(Fe) and MIL-101(Cr) were investigated in presence and absence of water vapor in feed stream by static adsorption, fixed bed experiments, CO2-TPD technique and in situ FTIR analysis. Interesting and unexpected results were obtained. It was found that the presence of water vapor significantly enhanced the CO2 working adsorption capacity and CO2/CH4 selectivity of MIL-100(Fe) due to formation of more adsorptive sites toward CO2, but it badly weakened those of MIL-101(Cr) due to H2O competitive adsorption. When relative humidity of feed stream increased from 0% to 50%, the CO2 capacity and CO2/CH4 selectivity of MIL-100(Fe) increased by 150% and 200%, respectively, and in contrast to that, those of MIL-101(Cr) decreased by 44% and 18% respectively. CO2-TPD curve of MIL-100(Fe) exhibited only one desorption peak in dry atmosphere, while two peaks in humid atmosphere, suggesting that new type of alkaline adsorptive sites were formed due to H2O dissociation on MIL-100(Fe), which was confirmed by in situ FTIR analysis. This water vapor-enhanced mechanism is interesting, and is worthy of further exploiting for obtaining novel adsorbents.

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