Improving CO2 adsorption capacity and CO2/N2 selectivity of UiO-66-NH2 via defect engineering and IL-encapsulation

Metal-organic frameworks (MOFs) has gained great research interest for CO2 capture due to their porous structure, functionality, and versatility. UiO-66 and its derivatives have been of particular interest since they have exhibited superior stability in aqueous, acidic, and thermal conditions. Among the derivatives of UiO-66, UiO-66-NH2 has shown the highest CO2 uptake capacity compared to others due to its CO2-philic amine group. However, there are only a few reports on further improving performances of UiO-66-NH2 through either synthetic or post-synthetic methods. On the other hands, incorporation of CO2-philic ionic liquid (IL) into porous materials such as MOFs and zeolites has shown enhanced CO2 uptake and CO2/N2 adsorption selectivity. Here, we report defective UiO-66-NH2 and their IL-encapsulated counterparts by using hydrochloric acid as a defect modulator followed by post-synthetic IL-encapsulation. The defective UiO-66-NH2 showed enhanced CO2 uptake capacity compared to its non-defective counterpart because of improved microporous structure. It was found that there exists an optimum linker deficiency yielding the highest CO2 capture performance due to enhanced textural properties overcoming the negative effect of reduced CO2-philic amine groups. Furthermore, IL-encapsulated UiO-66-NH2 showed a slight improvement in CO2/N2 selectivity but exhibited a notable reduction in CO2 uptake capacity because of decreased textural properties.