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

Abstract 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/P0<0.3) limits its application in AHT. We prepared the UiO-66 modified by MgCl2 with the methods of solvothermal and impregnation, and studied their water vapor adsorption performance and heat storage capacity. Attributed to the extremely high saturated water uptake and excellent hydrophilicity of MgCl2, the water adsorption performance of UiO-66 was improved, although the introduction of MgCl2 reduced its specific surface area and pore volume. The water adsorption capacity at (P/P0=0.3) and the saturated water adsorption capacity of the UiO-66 (with MgCl2 content of 0.57 wt.%) modified by the solvothermal method was 0.27 g/g and 0.57 g/g at 298 K, respectively, which is 68.8 % and 32.6 % higher than that of pure UiO-66. Compared with pure UiO-66, the water adsorption capacity of the UiO-66 (with MgCl2 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 showed high heat storage densities (~1293 J/g and 1378 J/g). Therein, the UiO-66 modified by the solvothermal method exhibits excellent cyclic stability. These results suggest that the introduction of an appropriate amount of MgCl2 makes UiO-66 more suitable for AHT applications.