Scandium/Alkaline Metal–Organic Frameworks: Adsorptive Properties and Ionic Conductivity

Several synthetic approaches have been employed to obtain novel {[ScM(μ4-pmdc)2(H2O)2]·solv}n [EHU1(Sc,M)] (where M = Li, Na; pmdc = pyrimidine-4,6-dicarboxylate; solv = corresponding solvent) compounds. The synthesis method is crucial to determine the type of alkaline that could be hosted in the structure as well as the crystallinity, adsorption performance, and ionic conductivity of the resulting materials. Compared with other synthetic methods, a heat-assisted solvent-free procedure has proven to be the most effective route, giving materials with adsorption capacities close to those expected from GCMC (Grand Canonical Monte Carlo) calculations. Despite the presence of alkaline ions in the framework, the pristine materials exhibit rather low conductivity values of ca. 10–7 S cm–1. The concentration of charge carriers has been increased by means of a doping approach that incorporates divalent transition metal ions to the structure and forces an increase of the alkaline ions, thus raising the ionic conductivity by 1 order of magnitude. Additionally, soaking the samples in solutions containing alkaline salts led to materials possessing an even higher number of carriers achieving conductivity values among the best results reported for MOFs at room temperature, i.e., 4.2 × 10–4 and 9.2 × 10–5 S cm–1 for EHU1(Sc,Li) and EHU1(Sc,Na) obtained by the solvent-free procedure, respectively.