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

Abstract

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.