Ligand‐Functional Groups Induced Tuning MOFs’ 2D into 1D Pore Channels for Pipeline Natural Gas Purification

The solvothermal reactions of CoCl2 ⋅ 6H2 O, 3,5-pyridinedicarboxylic acid (H2 L) and isonicotinic acid (HL1 )/3-amino isonicotinic acid (HL2 )/3-chloro isonicotinic acid (HL3 ) successfully led to three tfz-d topological pillar-layer [Co4 (μ-F)2 (COO)6 (NC5 H4 )4 ] cluster-based MOFs, namely, [Co4 (μ-F)2 (L)2 (L1 )2 ⋅ 2DMA] ⋅ DMA ⋅ 2H2 O (SNNU-Bai76, SNNU-Bai=Shaanxi Normal University Bai's group), [Co4 (μ-F)2 (L)2 (L2 )2 ⋅ 2H2 O] ⋅ 2DMA ⋅ 2H2 O (SNNU-Bai77) and [Co4 (μ-F)2 (L)2 (L3 )2 ⋅ 2H2 O] ⋅ 2DMF ⋅ 2H2 O (SNNU-Bai78). With the 2D pore channels in SNNU-Bai76 and SNNU-Bai77 being tuned to the 1D pore channel in SNNU-Bai78, C3 H8 and C2 H6 adsorption uptakes are apparently improved and the IAST selectivities of C3 H8 /CH4 and C2 H6 /CH4 almost remain, which indicate that SNNU-Bai78 may be one potential separation material for the pipeline natural gas purification. These were further confirmed by the breakthrough experiments for the simulated pipeline natural gas (C3 H8 /C2 H6 /CH4 : 5/10/85 gas mixture) of three isostructural MOFs. Furthermore, GCMC simulations revealed that due to one of the pore channels blocked by Cl atoms in a couple of 3-chloro isonicotinic acid with the changed conformation as the pillar, the pore wall of the formed 1D pore channel in SNNU-Bai78 may interact with the adsorbed C3 H8 or C2 H6 molecule more strongly, for which more atoms of framework at the new adsorption site will interact with the adsorbed gas molecule by more intermolecular interactions. This was also evidenced by the increased binding energies, being consistent with the tuning of adsorption enthalpies for C3 H8 and C2 H6 gas molecules, and the reduced C3 H8 and C2 H6 gas diffusion coefficients in SNNU-Bai78. Very interestingly, this work is the first example of finely tuning the pore connectivity of MOFs toward strengthened host-guest interactions for the gas adsorption and separation.