Phase transitions and CO2 adsorption properties of polymeric magnesium formate

The solvothermal reaction of magnesium(II) salts with formic acid in N,N-dimethylformamide/H2O at 140 degrees C for 24 h afforded two different polymeric magnesium(H) formates, depending on the relative DMF/H2O content. A trigonal (space group R3c) anhydrous phase Mg(HCOO)(2)(HCOOH) superset of (CH3)(2)NH (1) is formed under "dilute" conditions, while the orthorhombic (space group Pbca) dihydrate Mg(HCOO)(2)center dot 2H(2)O (2) is the main product when the water content is higher. When 2 is dehydrated at 300 degrees 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 P2(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, CO2 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% CO2 is stored under these conditions, but the plateau in the adsorption curve was not reached because of the higher carbon dioxide pressures required.