Phase Transformation of Hydrothermally Stressed Adsorbents

Abstract

Inorganic materials of zeolite type, silica gel or related materials are used as adsorbents in air conditioning machines. The cooling effect is obtained by evaporation of water thereby the adsorbent acts as pump system. After itôs saturation, the storage must be regenerated by heating. The hydrothermal stress may generate decomposition of their structure accompanied by a loss of sorption capacity. This work describes the hydrothermal stability of AlPO‐5, SAPO‐34, and silica gel in comparison with a newly developed dealuminated Y zeolite DAY. DAY (Si/Al = 3.1) and AlPO‐5 zeolites are hydrothermally stable over a wide temperature range even under harsh condition of maximum water loading of their pore system. DAY exhibits a higher water sorption capacity but becomes amorphous with increasing temperature, whereas AlPO‐5 transforms into a tridymite analogous crystalline phase with an intermediate state at 140 °C. But, both phase transformation processes are not relevant for the application of the compounds in low‐temperature driven heat pumps due to less hydrothermal stressing there. SAPO‐34 decomposes already under mild hydrothermal conditions. In analogy to AlPO‐5 it forms the tridymite analogous structure at high temperature. The silica gel looses the sorption capacity systematically because of the healing of its structure by condensation of free silanol groups, whereby Q2 and Q3 groups change into Q4 groups.