Impact of urea-based deep eutectic solvents on Mg-MOF-74 morphology and sorption properties

Deep eutectic solvents (DES) based on different urea derivatives have been demonstrated to be efficient green alternatives for the ionothermal synthesis of the prototypical Mg-MOF-74 with a strong impact on the morphology and sorption properties of the material. While the synthesis of the material is rather straightforward in the reline (choline chloride:urea 1:2) DES, higher temperatures and longer reaction times are necessary in the e-urea (2-imidazolidinone, ethylene-urea) based analogous system. Interestingly, in the latter, a variety of intermediate crystalline phases could be observed and characterized by single-crystal X-ray diffraction. In these compounds, coordination of the DES components – the chloride anion and e-urea derivative – to the Mg(II) cation was found to compete with the carboxylate linker. It was rationalized that the difference in the synthesis conditions and in the isolation of intermediate systems originate from the varying decomposition kinetics of the DES and hence from the basicity of the solvent. Although the same material is obtained as ascertained by powder X-ray diffraction and elemental analysis, the final morphology characterized by SEM and TEM is dependent on the nature of the solvent. Whereas the classical rod-like shape is observed in reline, an unusual morphology showing slices perpendicular to the main growth axis is present in the e-urea based DES. For the material featuring this unusual morphology, a higher specific surface area and CO2 uptake were found, which were associated with a higher degree of microporosity.