Synthesis and water sorption properties of a new composite “CaCl2 confined into SBA-15 pores”

The composite CaCl2@SBA-15 with the mass ratio Ca/Si = 0.58 has been synthesized via impregnation of nano-structured mesoporous silica SBA-15 with saturated aqueous solution of calcium chloride. The material was studied by methods of X-ray diffraction, differential scanning calorimetry, infrared spectroscopy, nitrogen adsorption and thermal analysis. It was shown that calcium chloride is located basically in the pores of silica. The isotherms and kinetics curves of water ad/desorption were measured at temperature 50 °C and vapor pressure range from 0 to 50 mbar. A small hysteresis was observed for the formation di- and tetra-hydrates of CaCl2 in the pores. The equilibrium pressure over the dispersed CaCl2 dihydrate was found to be 2.5–4 times lower relative to the bulk one, allowing a significant improvement of the salt desiccative properties. The possible reasons of this pressure decrease might be the higher mobility of water in the dispersed dihydrate and/or the increase in the surface energy of the salt (hydrate) confined to the SBA pores. The latter effect is a reason of the melting point depression for dispersed hydrates CaCl2·nH2O with n = 2, 4 and 6 that was revealed by DSC tests. The effective water diffusivity in SBA pores has been determined. The Knudsen diffusion in the SBA mesopores was found to be a rate-limiting stage of water sorption on CaCl2@SBA-15. The equilibrium sorption curves for the new composite were compared with other composite sorbents “CaCl2 in porous matrix”. Several possible applications of the new sorbent, among which are the gas drying, heat storage and adsorptive heat pumps, were preliminary evaluated.