Separation of CH4/N2 mixtures in metal–organic frameworks with 1D micro-channels

Abstract

This work aims to study the features of CH₄ and N₂ adsorption inside 1D micro-channels and develop the best suitable MOF candidates for the adsorptive separation of CH₄ against N₂. For this purpose, four MOFs ([Ni₃(HCOO)₆], [Cu(INA)₂], Al-BDC and Ni-MOF-74) with similar network topology and single 1D micro-channel have been systematically investigated via structure characterization and selective gas adsorption and separation. The selected MOFs are classified into three groups. Thereinto, Ni-MOF-74, with coordinatively unsaturated metal sites, is considered as strong polar adsorbent, whilst Al-BDC is treated as moderate polar adsorbent owing to the polar linkers. However, [Ni₃(HCOO)₆] and [Cu(INA)₂] are regarded as apolar (weak polarity) adsorbents because of lack of any polar functional groups inside the frameworks. The adsorption potential of CH₄ follows the trend Ni-MOF-74 > [Ni₃(HCOO)₆] > [Cu(INA)₂] > Al-BDC, while Ni-MOF-74 > Al-BDC > [Ni₃(HCOO)₆] > [Cu(INA)₂] for the potential of N₂. This implies that pore size and electrostatic interactions have different contributions to the CH₄ or N₂–MOFs interactions, resulting in excellent CH₄/N₂ selectivity more than 6 on [Ni₃(HCOO)₆] and [Cu(INA)₂].