Regulation on Topological Architectures and Gas Adsorption for Cadmium-Azolate-Carboxylate Frameworks by the Ligand Flexibility

Metal-azolate-carboxylate frameworks have great potential for gas adsorption applications because azole ligands with a small size easily construct small pore MOFs with bare −N­(H) donor sites. However, most of the currently reported metal-azolate-carboxylate frameworks were constructed by individual triazole or tetrazole and carboxylic acid; there are still few MOFs based on polytetrazole ligands. Herein, two novel mixed-linker Cd-tetrazolate-carboxylate frameworks, [Cd4­(BDC)3­(HBTT)­(H2O)2] n (SNNU-17, BDC = 1,4-terephthalic acid, H3BTT = 1,3,5-tris­(2H-tetrazol-5-yl)­benzene) and {[Cd2­(HDC)­(HBTT)­(H2O)]} n (SNNU-18) (BDC = 1,4-terephthalic acid and HDC = trans-1,4-cyclohexanedicarboxylic acid), have been synthesized. Because BDC is a rigid ligand, whereas HDC is flexible, the resulting two MOFs have completely different structures, and their percentage of uncoordinated N-donor sites increased from 33% to 50% for SNNU-17 and SNNU-18. It is worth noting that, although SNNU-18 has almost no N2 adsorption at 77 K, the adsorption capacity of CO2 and C1/C2 hydrocarbons at 298 K is capable with SNNU-17 which illustrated the potential application of a flexible MOF in gas adsorption and separation. In addition, because the pore size for SNNU-17 is 6.0 × 6.0 Å, smaller than that for SNNU-18 (11.0 × 11.0 Å), the breakthrough curves indicated that SNNU-17 has good C2H2/CO2 and C2 hydrocarbon or CO2 over CH4 dynamic separation performance. The promising gas separation performance was further supported by the GCMC simulations.