Water adsorption performance of UiO-66 modified by MgCl_(2) 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(~1293 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-66more suitable for AHT applications.