Phase Transitions and CO2 Adsorption Properties of Polymeric Magnesium Formate

The solvothermal reaction of magnesium(II) salts with formic acid in N,N -dimethylformamide/H 2 O at 140 °C for 24 h afforded two different polymeric magnesium(II) formates, depending on the relative DMF/H 2 O content. A trigonal (space group R 3̅ c ) anhydrous phase Mg(HCOO) 2 (HCOOH) ⊃ (CH 3 ) 2 NH ( 1 ) is formed under “dilute” conditions, while the orthorhombic (space group Pbca ) dihydrate Mg(HCOO) 2 ·2H 2 O ( 2 ) is the main product when the water content is higher. When 2 is dehydrated at 300 °C and subsequently exposed to water vapors, a quasi-reversible phase transition was observed, leading to a different polymorph of the same species ( 2a, monoclinic, space group P 2 1 /c ). The process was entirely followed by X-ray powder diffraction (XRPD) at variable temperatures, and the same XRPD technique was used for structure solution and refinement of compound 2a . Finally, CO 2 adsorption isotherms at ambient temperature were recorded for (preactivated) 2, in order to test it as greenhouse gas storage material. A (promising) maximum value of 1.32 wt% CO 2 is stored under these conditions, but the plateau in the adsorption curve was not reached because of the higher carbon dioxide pressures required.