Computational prediction of promising pyrazine and bipyridine analogues of a fluorinated MOF platform, MFN-Ni-L (M = SI/AL; N = SIX/FIVE; L = pyr/bipyr), for CO capture under pre-humidified conditions.

The CO2 separation performance has been explored under dry and pre-humidified conditions for the fluorinated metal organic frameworks SIFSIX-Ni-pyr, ALFFIVE-Ni-pyr, SIFSIX-Ni-bipyr and ALFFIVE-Ni-bipyr, which are incorporated with/without a coordinatively unsaturated site (CUS), Al, and organic linkers 1,4-pyrazine and 4,4'-bipyridine. The ultra-small pore size (∼4 Å) of the pyrazine analogues of fluorinated-MOFs, i.e. SIFSIX-Ni-pyr and ALFFIVE-Ni-pyr, showed infinite selectivity towards CO2 with low uptake capability under dry conditions, which drastically reduced to 0 wt% under very low pre-humidification conditions (∼10-15 wt% adsorbed moisture). In marked contrast, the bipyridine analogues SIFSIX-Ni-bipyr and ALFFIVE-Ni-bipyr showed significant CO2 capture performance of up to 30-40 wt% with pre-humidification. Further, both SIFSIX-Ni-bipyr and ALFFIVE-Ni-bipyr showed high CO2 selectivity as well as good working capacity at 1-10 bar while poor selectivity is observed for the pyrazine analogues with pre-humidification. The incorporation of Al as a CUS provides stability to the ALFFIVE-Ni-bipyr fluorinated-MOF under pre-humidified conditions due to the ability of Al to co-ordinate with water molecules in hydrogen bonded networks. Therefore, to develop physisorption-based CO2 capturing processes under pre-humidified conditions, the use of Al incorporated ALFFIVE-Ni-bipyr is highly suitable, which may outperform most of the fluorinated-MOFs and other classes of porous solids reported so far.