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

DFT-based evaluation of porous metal formates for the storage and separation of small molecules

“…For methane, no experimental isotherms that approach saturation have been reported; however, it is worth noting that maximal uptakes obtained from the extrapolation of low-pressure isotherms in Co(fa) 2 and Ni(fa) 2 range between 1.36 and 2.20 mmol g -1 . [20,21] When the higher weight of the metal centres is taken into account, the highest of these values corresponds almost exactly to the equivalent of four molecules/cell (one per segment).…”

“…In this context, it is regrettable that the lack of lattice parameters determined for the fully desolvated system prevented an inclusion of Ni(fa) 2 in this study, as this system has been studied experimentally for the separation of CH 4 /N 2 mixtures. [20,21] As a final point regarding the selective adsorption of methane, it is worth noting that the difference in affinity observed in Mg(fa) 2 is virtually as high as for Zn(fa) 2 for CH 4 /N 2 mixtures, and only one kJ mol -1 smaller for CH 4 /H 2 mixtures. Due to its lower weight, resulting in higher possible uptakes, magnesium formate might thus be an attractive alternative to the previously investigated nickel-and cobalt-containing systems.…”

“…Previous experimental work has shown that methane can diffuse through the structure of Co(fa) 2 , the system with the narrowest portals, at room temperature. [20] In the light of the smaller critical dimensions of the other molecules, it is reasonable to assume that diffusion limitations do not play a role for the M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 7 guest species considered in this work. The initial structures used in the DFT calculations were taken from the following references: Mg(fa) 2 : ref.…”

“…In the field of gas separation, recent efforts have addressed the separation of Xe/Kr mixtures using Co(fa) 2 and Ni(fa) 2 , [19] and the selective adsorption of methane over N 2 in the same two systems. [20,21] Relatively high M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4 selectivities were observed for these challenging separations. A very recent experimental study has revealed promising selectivities towards CO 2 /N 2 and CH 4 /N 2 mixtures, and a moderate selectivity towards a CO 2 /CH 4 mixture, in a magnesium formate system that has an expanded unit cell when compared to α-Mg(fa) 2 .…”

“… [20]. For the case of hydrogen, an experimental adsorption study of Mg(fa) 2 delivered a heat of adsorption of approximately 6.5 kJ mol -1 [18].…”