Highly enhanced selectivity for the separation of CH4 over N2 on two ultra-microporous frameworks with multiple coordination modes

• Two ultra-microporous frameworks were synthesized in a feasible route to scale up.
• The CH4/N2 separation performances for MOFs and zeolites were evaluated.
• The highest selectivities ever reported for the separation of CH4 against N2 were achieved.
• The mechanism explains the good separation performances for the two adsorbents.

Two typical ultra-microporous adsorbents namely [Ni3(HCOO)6] and [Co3(HCOO)6] were successfully prepared in a feasible route to scale up using non-corrosive methyl methanoate instead of formic acid. Pure gas and binary gas mixture adsorption equilibria of CH4 and N2 were conducted on the two compounds to evaluate their CH4 adsorption capacities and selectivities against N2. The two compounds exhibit preferential adsorption of CH4 over N2 at 298 K in the pressure range 0.1–1.0 MPa, and the adsorption selectivities of CH4/N2 mixture are determined to be 6.0–6.5 for [Ni3(HCOO)6] and 5.1–5.8 for [Co3(HCOO)6], respectively. The uniform ultra-micropores and optimal polarizability resulted from multiple coordination modes play the essential roles in their preferential adsorption of CH4 with the highest selectivities up to date, making the two compounds as promising candidates of existing adsorbents for unconventional natural gas upgrading.

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