New family of solid sorbents for adsorptive cooling: Material scientist approach

This paper summarizes literature data concerning a new family of materials for adsorptive cooling. They are composites of a type of “salt confined to a porous host matrix” (so called selective water sorbents or SWSs). These materials demonstrate an intermediate behavior between solid adsorbents, salt hydrates, and liquid absorbents. The thermodynamic equilibrium with water vapor has been measured for more than thirty SWSs based on halides, sulphates, and nitrates of alkaline and alkaline earth metals that are confined to various matrices (silica, alumina, porous carbons, clays, MCM-41, etc.). The important advantage of the SWSs is the possibility to controllably modify their water sorption properties in a wide range by varying (a) the chemical nature of the impregnated salt, (b) the porous structure of the host matrix, (c) the amount of the confined salt, and (d) the preparation conditions. This, in principle, allows purposeful synthesis of new solid sorbents with predetermined properties that fit the demands of particular adsorptive cycles. Appropriate examples are presented in the paper. A recent study of adsorption chillers with granulated and compact layers of SWS-1L (CaCl2 in mesoporous silica) showed an experimental COP as high as 0.6, which is larger than that reported for silica gel/water and zeolite/water systems under the same cycle conditions.The concept of matrix modification with an active salt can also be used for synthesis of efficient sorbents of carbon dioxide, methanol, and ammonia. Other practical applications of SWSs are briefly discussed, such as heat protection, gas drying, and fresh water production from the atmosphere.