Enhanced gas-sorption properties of a high surface area, ultramicroporous magnesium formate

Abstract

The direct solvothermal reaction of Mg(NO₃)₂·6H₂O in dimethylformamide at 130 °C leads to the formation of highly crystalline α-[Mg₃(O₂CH)₆] with an expanded unit cell. Successful pore activation was performed after heating the material at 240 °C under dynamic vacuum. In contrast to literature reports, the pores in the activated solid are accessible to nitrogen molecules and the corresponding BET surface area is 496 m² g⁻¹, very close to the geometric area calculated from the single crystal structure. A detailed gas-sorption study (N₂, Ar, H₂, CO₂, CH₄ and NH₃) is reported. The material is ammonia stable showing a reversible uptake of 5.4 mmol g⁻¹ at 298 K, while the corresponding CO₂ uptake is 2.2 mmol g⁻¹ with moderate CO₂/CH₄ (4.6) selectivity but high CO₂/N₂ (23) and CH₄/N₂ (5.2) selectivity. The latter is among the highest values reported to date in the entire family of porous materials. The H₂ uptake at 77 K is 1.2 wt% which is the highest among the reported values for various magnesium formates.