Ionosilica-based anion exchangers for low-temperature thermochemical storage of energy under mild conditions of adsorbent regeneration and saturation

Abstract

Three ionosilica materials have been tested for low-temperature thermochemical storage of energy under mild conditions of adsorbent regeneration and saturation. The previous synthesis procedures were adapted to prepare materials possessing an organosilica framework with silylated cationic blocs and Cl-, HSO4-, SO42- anions in various proportions as extra-framework compensating counter-ions. Transmission and Scanning Electron Microscopy, Wavelength Dispersive X-Ray Fluorescence, Energy Dispersive X-ray Spectroscopy, Thermogravimetric analysis, adsorption of gaseous nitrogen at 77 K, 29Si Solid-State Nuclear Magnetic Resonance spectroscopy were employed to establish the key characteristics of the three ionosilica samples in terms of elemental composition, particle morphology and textural properties, thermal stability and regenerability, or variability of surface activity during repeated hydration-dehydration cycles. Their capacity to adsorb water vapor at cool and moderate ambient temperatures after incomplete surface drying was demonstrated by means of the scanning microscopy operating in low-vacuum mode at 275 K, or more precisely by measuring the water adsorption isotherms at 313 K and the related differential heats under static conditions. Finally, flow calorimetry operating in the moist-gas flow mode was used to measure the integral heat accompanying the adsorption of water vapor at its partial pressure of 2.8 kPa and 296 K as well as the kinetics of heat release in three drying-saturation cycles. After a drying procedure carried out under helium flow at 353 K, the thermal performance of two ionosilica samples containing both HSO4- and SO42- counter-ions was sufficient enough to consider them as potential adsorbents for auxiliary space heating in homes, small businesses, or public buildings.