In-situ construction of ionic ultramicroporous metal–organic frameworks for high-efficiency CO2/CH4 separation

The high-efficiency capture of CO2 from biogas by adsorption is highly desirable, which, yet faces the challenges regards of the unsatisfactory adsorption efficiency, selectivity, and regeneration performance. Herein, ionic ultramicroporous metal organic frameworks (IUM-MOFs) with high CO2 affinity are constructed by one-step in-situ coordination strategy and used for CO2/CH4 separation. The phenoxylate anion-functionalized ionic liquid (FIL) serves as competitive ligands in the formation of IUM-MOFs. The oxyl group exhibits high affinity with CO2 as the Lewis basic center, while the rigid counterion occupies the adsorption space of CH4 in the cavities of IUM-MOFs, thus bringing about superior CO2 adsorption capacity of 149 cm3 g−1 (150% higher than that of Cu-BTC) and high IAST predicted selectivity with CH4 of 22 under atmospheric condition. Mechanism analysis demonstrated that the formation of hydrogen bonds between CO2 with coordinated IL and H2O and IL-enhanced polarity of MOFs benefit to the CO2 adsorption. In addition, the IUM-MOFs could be facilely regenerated at 120 °C with favorable recycling stability. This work provides facile strategy for the rational design of IUM-MOFs and lays useful insights for structural tuning to promote their applications.