Metal-organic frameworks of general composition [M(OH)(O)(PDC)(Cl)(HO)] with M = Zr, Ce, Hf; PDC = 2,5-pyridinedicarboxylate and 0 ≤ x ≤ 2 were obtained under reflux using formic, nitric or acetic acid as an additive. Rietveld refinements carried out using a fixed occupancy of the linker molecules according to the results of thermogravimetric measurements confirmed that the MOFs crystallize in the UiO-66 type structure and demonstrate that the structural models describe the data well. Further characterization was carried out by NMR spectroscopy, thermogravimetric analysis, Zr K-edge EXAFS- and Ce L-edge XANES measurements. To highlight the influence of the additional nitrogen atom of the pyridine ring, luminescence and vapour sorption measurements were carried out. The hydrophilisation of the MOFs was shown by the adsorption of water at lower p/p (<0.2) values compared to the corresponding BDC-MOFs (0.3). For water and methanol stability cycling adsorption experiments were carried out to evaluate the MOFs as potential adsorbents in heat transformation applications.

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Closed adsorption storages have been investigated in several projects for heat storage in building applications with focus on energy density and performance. This study complements this research with the assessment of the environmental impacts over the life cycle. Global warming potential (GWP) was chosen as the assessment criterion. Selected sorption materials in combination with water as the refrigerant were analyzed first by themselves and then embedded in a generic storage configuration. Sensible storage in water served as the reference benchmark. Results on material and component level showed that the relative storage capacity compared to water under realistic operating conditions reached values of below 4 and 2.5, respectively, in the best cases. Since the effort for producing the sorbents as well as the auxiliary material demand for assembling storage components was significantly higher than in the reference case, the specific environmental impact per storage capacity also turned out to be ~2.5 to ~100 times higher. We therefore suggest focusing sorption storage research on applications that (a) maximize the utilization of the uptake of sorbents, (b) do not compete with water storages, and (c) require minimal auxiliary parts.