Rationally tuning the separation performances of [M3(HCOO)6] frameworks for CH4/N2 mixtures via metal substitution

• A series of isostructural ultra-microporous MOFs are readily synthesized.
• The pore size and internal surface properties are tuned by using different metal ions.
• The adsorption behaviour of gas molecule in [M3(HCOO)6] is revealed by NH3-TPD.

A series of isostructural ultra-microporous metal-organic framework (MOF) compounds [M3(HCOO)6] (M = Mg, Mn, Co and Ni) have been readily synthesized in large-scale, characterized, and evaluated for the separation of CH4 and N2. Results indicate that the metallic formates exhibit different CH4 adsorption capacities and distinct CH4/N2 selectivity in a sequence of Ni > Co > Mg > Mn analogue owing to their varied CH4 affinities. Thereinto, [Ni3(HCOO)6] shows the highest CH4 adsorption capacity of 1.09 mmol/g and CH4/N2 selectivity up to 6.5 at 0.4 MPa and 298 K in the dynamic experiments, which suggests the most suitable synergistic effect between constricted pores and surface properties among [M3(HCOO)6] frameworks. At the same time, the adsorption behaviour in [M3(HCOO)6] is investigated by NH3-TPD, revealing that the adsorbed NH3 molecules should have two different states. One is that gas molecules stay inside the pores, the other is that gas molecules are adsorbed on the adsorption sites induced by the coordinated metal ions or exposed oxygen of the [M3(HCOO)6], which directly affects the adsorption capacity, the ratio of two states of molecules and the final selectivity. These results confirm that alteration of metal ion plays an important role in the tuning of pore size and internal surface properties, thus providing new clues to design MOFs with different pore characteristics for enhanced gas sorption and separation.

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