Structure and Gas Sorption Behavior of a New Three Dimensional Porous Magnesium Formate

A new three-dimensional magnesium formate polymorph, namely, γ-[Mg3(O2CH)6] has been synthesized via in situ formate anion generation method. γ-Mg-formate crystallizes in space group Pbcn, and structural determination by X-ray single crystal diffraction reveals a three-dimensional network of Mg2+ linked by formate anions. All formate anions possess similar binding mode to the metal center with one oxygen of a particular formate anion binds to one metal center (μ1 oxygen) and other oxygen binds to two metal centers (μ2 oxygen). N2 adsorption studies indicate that the framework displays permanent porosity. The specific surface area of γ-Mg-formate (BET, 120 m2 gm−1) is lower than the α- polymorph (BET, 150 m2 gm−1). However, the initial hydrogen uptake of γ-Mg-formate reached almost 1.0 wt % when the adsorbate pressure approached 760 Torr at 77 K. This is higher than the hydrogen uptake of α-Mg-formate (0.6 wt %). γ-Mg-formate, shows a moderate affinity and capacity for CO2 (3.4 Å kinetic diameter) at 298 K. The CO2 uptake at 760 Torr is 2.01 mmol gm−1(47.0 cc gm−1). Although this CO2 uptake is somewhat modest, it compares well with the CO2 uptake of several Mg-MOFs and ZIFs reported in the literature.