Tuning the Ultra-Micropore Size of Fluorinated MOFs (M′F6-Ni-L) for CO2 Capture from Flue Gases by Advanced Computational Methods

The CO2 capture capability of ultra-microporous pillared square grid fluorinated metal-organic framework (MOF), i.e., [(M'F-6)M(L)(2)](n), (where M', M, and L are pillared metal ion, divalent cation, and ligand, respectively), is exercised with a combined advanced density functional theory (DFT) and grand canonical Monte Carlo (GCMC)-based simulation through tuning of its structural skeleton by several possible metal ions and various ligands. As an initial step, M'FSIX-Ni-pyr MOF series were made by altering various metal ions possessing octahedral coordination in M'F-6 moieties (M' = Si, Ga, Zr, Ge, Sn, Ti, V, and Nb) but keeping nickel and pyrazine as divalent cation (M = Ni) and ligand, respectively. M'FSIX-Ni-pyr MOF series were considered based on the prototype RM'F-6)Ni(1,4-pyrazine)(2)](n) porous structure having ultra-micropore size as 4.0 angstrom (when M' = Si, the MOF is denoted as SIFSIX-Ni-pyr). The replacement of pillared metal ion possessing octahedral coordination in M'F-6 moieties does not provide any significance in CO2 adsorption capability than the parent structure, SIFSIX-Ni-pyr. Further, we adopted another criterion for tuning the structural dimension in [(M'F-6)Ni(L)(2)](n) by choosing various ligands (L) such as 4,4'-bipyridine (byp), 4,4'-azopyridine (apy), 4,4'-bipyridylacetylene (bpa), and 3,-di(4-pyridyl)-1,2,4,5-tetrazine (dpt), i.e., SIFSIX-Ni-L MOF series. Coadsorption studies of CO2 and N-2 with molar compositions of 15 and 85%, respectively, reveal that the CO2/N-2 selectivity decreased drastically on increasing the length of ligands, even though the adsorption capacity is much better than SIFSIX-Ni-pyr. However, for keeping better selectivity without compromising the adsorption capacity, a double-folded interpenetration is carried out in all SIFSIX-Ni-L MOFs to generate another SIFSIX-Ni-L-i MOF series. Eventually, SIFSIX-Ni-L-i MOF series showed better adsorption capacity toward CO2 without compromising the optimal selectivity, which is validated by pore size distribution (PSD), working capacity (WC), and adsorption performance indicator (API). Among the three possibilities of changing the structural configuration of M'FSIX-Ni-L, [(M'F-6)M(L)(2)](n) fluorinated MOFs such as pillared metal-ion variants, length of ligand, and its structural interpenetration, the last one stands out for CO2 capture performance. From screening of entire fluorinated MOFs, both SIFSIX-Ni-bpa-i and SIFSIX-Ni-dpt-i showed better CO2 capture performance from industrial flue gas composition, which suggested that the optimal adsorbent should possess a threshold pore size, i.e., 4.0-4.5 angstrom, with better CO2 adsorption sites in the pore wall.