Interpenetrated metal-organic frameworks and their uptake of CO2 at relatively low pressures

Adsorption-driven separation of CO₂ from flue gas has the potential to cut the cost for carbon capture and storage. Among the porous physisorbents, metal-organic frameworks (MOFs) are a class of promising candidates for gas separation and storage owing to their extraordinarily high specific surface areas and pore volumes, and predesigned pore structures. Here, we report three interpenetrated MOFs composed of Zn₄O clusters and rigid dicarboxylate anions, namely SUMOF-n (SU = Stockholm University; n = 2, 3, 4). All the interpenetrated MOFs possess small pores of two different types and high pore volumes. SUMOF-2 had a structure similar to interpenetrated MOF-5, but with an extra-framework cation present in one of the two types of pores. SUMOF-3 was an interpenetrated version of IRMOF-8 while SUMOF-4 crystallized with mixed linkers, biphenyl-4,4′-dicarboxylic acid and benzene-1,4-dicarboxylic acid. Among the three SUMOFs, SUMOF-4 had the largest specific surface area (1612 m² g^-1) and pore volume. Single component adsorption of CO₂ and N₂ was determined at 273 K. We showed that the interpenetrated SUMOF-2 adsorbed more CO ₂ than non-interpenetrated MOF-5 under 273 K and 1 bar. This may be explained by the increased electric field gradients due to the interpenetration in the MOF. The uptake of CO₂ for SUMOF-2 and SUMOF-4 was significant at somewhat higher pressure. Their CO₂ isotherms were close to linear, which could be beneficial for separation of CO₂via pressure swing adsorption from biogas or natural gas. On the other hand, SUMOF-3 adsorbed most CO₂ at pressures relevant for CO₂ capture from flue gas.