Programming MOFs for water sorption: amino-functionalized MIL-125 and UiO-66 for heat transformation and heat storage applications

Abstract

Sorption-based heat transformation and storage appliances are very promising for utilizing solar heat and waste heat in cooling or heating applications. The economic and ecological efficiency of sorption-based heat transformation depends on the availability of suitable hydrophilic and hydrothermally stable sorption materials. We investigated the feasibility of using the metal–organic frameworks UiO-66(Zr), UiO-67(Zr), H₂N-UiO-66(Zr) and H₂N-MIL-125(Ti) as sorption materials in heat transformations by means of volumetric water adsorption measurements, determination of the heat of adsorption and a 40-cycle ad/desorption stress test. The amino-modified compounds H₂N-UiO-66 and H₂N-MIL-125 feature high heat of adsorption (89.5 and 56.0 kJ mol⁻¹, respectively) and a very promising H₂O adsorption isotherm due to their enhanced hydrophilicity. For H₂N-MIL-125 the very steep rise of the H₂O adsorption isotherm in the 0.1 < p/p₀ < 0.2 region is especially beneficial for the intended heat pump application.