Adsorption-desorption characteristics of water on silico-alumino-phosphate- 34 molecular sieve for cooling and air conditioning

SAPO (silico-alumino-phosphate) molecular sieves might be promising for adsorption refrigeration driven by low-grade heat source due to their weak polar frameworks. The SAPO-34 molecular sieve was synthesized using a hydrothermal method. Its crystal phase, morphology, and microstructure were characterized by X-ray diffraction, scanning electron microscopy, and N₂ sorption isotherms, respectively. Vacuum gravimetric method and thermo-balance were applied to analyze the water adsorption and desorption performance, respectively. Results show that the synthesized molecular sieve has characteristic diffraction peaks of SAPO-34, regular crystal grains, a microporous structure with size of 0.4-0.6 nm, and a specific surface area of 545.8 m²/g. Water adsorption strength on SAPO-34 is between that on 13X molecular sieve and A type silica gel. During 100-400 Pa, the water uptake on SAPO-34 increases sensitively to pressure, and equilibrium water uptake reaches 0.35 kg/kg, 25% higher than that on 13X. Dubinin-Astakhov equation is able to well fit SAPO-34 water uptake isotherm to some extent, with a correlation coefficient R of 96.3%. Water is completely desorbed from SAPO-34 at 155°C, which is lower than that of 13X by 145°C. Correlation of SAPO-34 structure to its adsorption/desorption behavior would provide fundamentals in developing high efficiency adsorbent for adsorption refrigeration system.