How Impurities Affect CO2 Capture in Metal-Organic Frameworks Modified with Different Functional Groups

The introduction of functional groups in metal organic frameworks (MOFs) has been found to be a successful strategy for improving CO2 selective separation from N-2. However, little is known regarding how impurities such as H2O and SO2 may interfere with CO2 capture as a function of the properties of the functional groups in MOF adsorbent materials. Here, the effects of water and SO2 on CO2 capture in UiO-66(Zr) MOFs are systematically explored. The basic structure of UiO-66(Zr) is modified in each case with -NH2, -OH, or -Br functional groups, and CO2 capture is investigated using molecular simulations. It is found that for UiO-66(Zr) with -NH2 and -OH groups, due to strong interactions between water and the framework, the presence of water lowers CO2 adsorption significantly. In contrast, due to the water-phobic effects of -Br, and subsequent low binding strength between water and UiO-66-Br, water has much smaller effect on CO2 capture in this MOF. Regarding SO2 effects, the presence of SO2 in the mixtures decreases water adsorption in both UiO-66-NH2 and UiO-66-Br. The lower water adsorption for the CO2/N-2/H2O/SO2 mixture in UiO-66-NH2 can be mainly attributed to the strong binding strength between SO2 and the framework. On the other hand, in UiO-66-Br, the lower water adsorption is mainly ascribed to the stronger affinity of water toward SO2 rather than to the framework. The lower water adsorption makes more sites available for CO2 adsorption, and therefore, the CO2 adsorptions are enhanced accordingly in UiO-66-Br.