Experimental and computational study on a dual structural cage-based Ni-MOF for efficient C2H2/CO2 separation

In this study, a porous Ni-MOF formulated as {(H₂NMe₂)[Ni₃(μ₃OH)(TPT)(TZB)₃](H₂O)₆(DMA)₇}_n (TZB = 4-(1H-tetrazol-5-yl)benzoate, TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine, and DMA = N,N-dimethylacetamide) was prepared and its C₂H₂ and CO₂ adsorption performances were systemically studied both experimentally and computationally. The structural analysis by single crystal X-ray diffraction studies reveals that the prepared Ni-MOF crystallizes in the space group P6₃mc and features a porous pacs-type network composed of trigonal bipyramidal and octahedral cages. The activated Ni-MOF (Ni-MOF-a) shows a high C₂H₂ uptake capacity of 157 cm³/g with a moderate C₂H₂/CO₂ IAST selectivity of 3.3 at 298 K and 1 bar. More importantly, Ni-MOF-a shows a low adsorption heat for C₂H₂ ranging from 24.6 to 27.1 kJ/mol at 298 K with a high separation potential of 2.85 mol/kg. In the computational simulation section, six sets of point charge derived from different approaches (Qeq, Mulliken, PACMOF, mCBAC, Mepoml, PACMAN) and two different force-fields (UFF and Dreiding) were used to calculate the C₂H₂ and CO₂ adsorption isotherms and then compared with the experimental values. Our results reveal that great care should be taken in the selection of point charges and force fields, and the unsuitable combinations might result in an inappropriate C₂H₂/CO₂ selectivity which is against the experimental results.