Water adsorption behaviour of CAU-10-H: a thorough investigation of its structure–property relationships†
Aluminium isophthalate CAU-10-H [Al(OH)(benzene-1,3-dicarboxylate)]·nH2O exhibits water adsorption characteristics which make it a promising adsorbent for application in heat-exchange processes. Herein we prepared a stable coating of this MOF and evaluated its long-term stability under closed-cycle conditions for 10 000 water adsorption and desorption cycles, which are typical lifetimes for adsorption heat storage (AHS) applications. No degradation of the adsorption capacity could be observed which makes CAU-10-H the most stable MOF under these humid cycling conditions reported until now. Moreover, thermophysical properties like thermal conductivity and heat of adsorption were directly measured. In order to identify the structural features associated with the adsorption behaviour, the structural differences between the dry and the water loaded CAU-10-H were studied by Rietveld refinements and second harmonic generation (SHG) microscopy. The observed transition of space group symmetry from I41 to I41/amd between the humid and dry forms is induced by the adsorption/desorption of water into/out of the MOF channels. This originates from a torsional motion around the C–C bond between the carboxylate groups and the aromatic ring in half of the linker molecules. These observations are in excellent agreement with molecular dynamics simulations which confirm the energetic benefit of this transition.