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

This work aims to study the features of CH4 and N-2 adsorption inside 1D micro-channels and develop the best suitable MOF candidates for the adsorptive separation of CH4 against N-2. For this purpose, four MOFs ([Ni-3(HCOO)(6)], [Cu(INA)(2)], Al-BDC and Ni-MOF-74) with similar network topology and single 1D microchannel 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, [Ni-3(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 > [Ni-3(HCOO)(6)] > [Cu(INA)(2)] > Al-BDC, while Ni-MOF-74 > Al-BDC > [Ni-3(HCOO)(6)] > [Cu(INA)(2)] for the potential of N-2. This implies that pore size and electrostatic interactions have different contributions to the CH4 or N-2-MOFs interactions, resulting in excellent CH4/N-2 selectivity more than 6 on [Ni-3(HCOO)(6)] and [Cu(INA)(2)].