Controlled flexibility of porous coordination polymers by shifting the position of the -CH₃ group around coordination sites and their highly efficient gas separation

As an emerging class of crystalline materials, porous coordination polymers (PCPs) with regular and flexible nanopores have become particularly promising for adsorption applications. Here, we report a new method to synthesize PCPs with varied flexibility by shifting the position of the methyl group, the shortest alkyl chain, around the coordination sites of T-shaped ligands of H₂NL¹ (5-(2′-methyl-imidazol-1-yl)-isophthalic acid) and H₂NL² (5-(4′-methyl- imidazol-1-yl)-isophthalic acid). The two generated PCPs (NTU-40 and NTU-41) showed a significant change in gate opening pressure (P/P₀: 0.25 to 0.0001) under the stimulus of N₂ at 77 K. In addition, the square window (5 × 5 Ų) of the one-dimensional (1D) zigzag channel was divided into two small triangular and straight channels in NTU-41. More importantly, the synergistic effect of structural flexibility, channel type and the micro-pores enabled highly efficient CO₂/CH₄ and C₂H₄/CH₄ separation under both equilibrium state and dynamic conditions, as well as having good potential for challenging C₂H₄/C₂H₆ separation.