Metal-organic frameworks chelated by zinc fluorides for ultra-high affinity to acetylene during C2/C1 separations

Separations of C-2 hydrocarbons from methane (C-1) are very important but challenging processes in petrochemical industry. Metal-organic frameworks (MOFs) are considered as energy-saving materials owing to the weak interactions with gas molecules. In this work, the selective C-2 adsorptions over C-1 hydrocarbon in the ZnF2 chelated MOFs were investigated by grand canonical Monte Carlo (GCMC) methods and compared with those in the parent MOFs. The effects of many factors including pore diameters, isosteric heat differences, gas composition ratios, dispersion and electrostatic interactions were deeply analyzed as well. Besides, the effects of former adsorbed molecules on the later adsorptions of guest molecules were evaluated through comparing the binding energies with density functional theory (DFT) method. The results show that the incorporation of ZnF2 groups can significantly enhance the C-2/C-1 separation selectivity of MOFs. The C2H2/CH4 selectivity of Zn-2(tmbdc)(2)(ZnF2-bpy) is 5263.05 which is much higher than that of the parent Zn-2(tmbdc)(2)(bpy) (11.25) and close to the record of C2H2/CH4 selectivity in MOFs (NKMOF-1-Ni) at 298 K. In addition, smaller pore sizes and larger isosteric heat differences tend to result in higher C-2/C-1 separation selectivities. For ZnF2-chelated MOFs, the former adsorbed molecules mainly show competitive effects on the later adsorptions of guest molecules. This is different from the collaborative effects in the case of non-chelated MOFs.