Constructing metal-π sites in MOF to enhance CO2/CH4 separation performance within mixed matrix membranes☆

Mixed matrix membranes (MMMs) are used to separate CO2 from biogas, with the objective of striking a satisfactory balance between permeability and selectivity. The precise design of metal-pi sites for MOFs in MMMs helps to address this challenge by utilizing the distinct molecular orbital differences between CO2 and CH4. In this work, a range of MOF-808 (Mx-MOF-808) fillers with unsaturated Zr sites were synthesized using a method involving competitive coordination substitution, and they mixed with Pebax to fabricate MMMs for CO2/CH4 separation. The Pebax/Mx-MOF-808 MMMs exhibited an excellent CO2 recognition function, which is attributed to the metal-pi interactions between the Zr sites within the MOF and CO2 molecules. This selective CO2 recognition is achieved by pi backdonation and pi complexation forces, which enable the Zr sites to interact selectively with CO2. In contrast, CH4 molecules are difficult to recognize due to their lack of unsaturated pi orbitals, which disfavors selective interactions with the Zr sites. Compared to pure Pebax membrane, the CO2 permeability of Pebax/M30-MOF-808-4 MMM increased to 987 Barrer (approximate to 140 %), and the CO2/CH4 selectivity was increased to 28.9 (approximate to 30 %). The findings suggest that the development of metal-pi sites with CO2 recognition in MOFs for MMMs is of great significance in the purification of biogas.