HF-assisted synthesis of ultra-microporous [Mg3(OOCH)6] frameworks for selective adsorption of CH4 over N2

• The uniform [Mg3(OOCH)6] crystals were synthesized with the modulation of HF.
• By varying the amount of HF different crystals size and morphology were obtained.
• The Mg2+-induced sites and ultra-micropore can effect the selectivity of CH4/N2.

Ultra-microporous [Mg3(OOCH)6] frameworks with different uniform shapes, were synthesized via a facile coordination modulation method, in which HF was used as a modulator to promote the growth of [Mg3(OOCH)6] crystals. The as-prepared [Mg3(OOCH)6] frameworks were scrutinized and evaluated the CH4 adsorption capacity and selectivity over N2 by pure gas adsorption and breakthrough experiments. The [Mg3(OOCH)6] frameworks exhibit preferential adsorption of CH4 over N2, and much higher CH4 adsorption capacity (up to 0.74 mmol/g) by comparison with conventional zeolites. It has been confirmed that the equilibrium selectivities of [Mg3(OOCH)6] frameworks are shape dependent, and the uniform prism-like framework with size of 100 μm has the highest selectivity up to 5.5 at 298 K. These results suggest that achieving the optimal coupling of polarizability and structure of [Mg3(OOCH)6] framework is key factor to obtain a high selectivity for the separation of CH4/N2 mixture.

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