Water adsorption performance of UiO-66 modified by MgCl₂ for heat transformation applications

UiO-66 is a potential material for adsorption heat transformation (AHT) with high specific surface area, and excellent thermal and chemical stability. However, the low water adsorption capacity of UiO-66 in the low relative pressure range (0< P/P_0< 0.3) limits its application in AHT. We prepare the UiO-66 modified by MgCl_2 through using the solvothermal method and impregnation method, and study their water vapor adsorption performances and heat storage capacities. Attributed to the extremely high saturated water uptake and excellent hydrophilicity of MgCl_2, the water adsorption performance of UiO-66 is improved, although the introduction of MgCl_2 reduces its specific surface area and pore volume. The water adsorption capacity at P/P_0=0.3 and the saturated water adsorption capacity of the UiO-66 (with MgCl_2 content of 0.57 wt%) modified by the solvothermal method are 0.27 g/g and 0.57 g/g at 298 K, respectively, which are 68.8% and 32.6% higher than the counterparts of pure UiO-66, respectively. Comparing with pure UiO-66, the water adsorption capacity of the UiO-66 (with MgCl_2 content of 1.02 wt%) modified by the impregnation method is increased by 56.3% and 14.0% at the same pressure, respectively. During 20 water adsorption/desorption cycles, the above two materials show high heat storage densities (\sim1293 J/g and 1378 J/g). Therein, the UiO-66 modified by the solvothermal method exhibits the excellent cyclic stability. These results suggest that the introduction of an appropriate amount of MgCl_2 makes UiO-66 more suitable for AHT applications.