Fine Control of Water-Adsorption Properties of Crystalline Microporous Aluminophosphates by Changing Porous Structure and Framework Composition for Application in Water Adsorption Chiller

We studied the water-adsorption properties of crystalline aluminophosphates (in brief, "AlPO4") containing ordered micropores, focusing on the effects of porous structure and heteroatoms incorporated in AlPO4 frameworks. For this investigation, AlPO4 with AFI-and CHA-type structures were hydrothermally synthesized. Also, the AFI-AlPO4 had Si, Fe, Mg, Co, or Cr incorporated at controlled amounts (0-5 mol%). Water adsorption isotherms of the AlPO4 samples were taken at 308 K, showing S-shape curve which is typical characterization of nanomaterial containing uniform nanopores. The amount of water adsorbed in AlPO4 was dependent on the specific surface area. The CHA-AlPO4 exhibited much higher water adsorption capacity than the AFI-AlPO4. The inflection-point humidity of the S-curve was also dependent on the microporous structure. In the case of AFI-AlPO4, the inflection point differed markedly depending on the amount and type of the heteroatoms incorporated in the frameworks. In a group with same micropore structure, the S-curve inflection point decreased in the order of "solely aluminophosphate"> "Co-doped" approximate to "Crdoped" > "Fe-doped"> "Mg-doped"> "Si-doped." The decrease in the inflection point humidity was found to be positively correlated to the amount of the heteroatom incorporated. Different inflection point humidities might be attributed to changes in the affinity between the AlPO4 framework and water molecules as a result of incorporation of heteroatoms. The results of this study show that it is possible to control the water adsorption properties of AlPO4 precisely, which would be very helpful in developing suitable water adsorbents for use in water adsorption chilling systems under specific working conditions.