Adsorption characteristics and surface free energy of AlPO4-5

The micropores and surface characteristics of aluminophosphate-type zeolite, AlPO₄-5, were analyzed by examining the adsorption behavior of water and other adsorbates. Water adsorption on AlPO₄-5 occurred on both structural defects and nonpolar surfaces. Adsorption on structural defects, accompanied by high heats of adsorption, is attributed to adsorption to surface hydroxyls. Water adsorption increased steeply at a certain relative pressure depending on the adsorption temperature, and this was considered attributable to capillary condensation. The contact angle of water on AlPO₄-5 micropore surfaces can be determined quantitatively by applying the Kelvin equation. The surface free energy of AlPO₄-5 calculated on the basis of the contact angle was revealed to be about 120 mJ/m², in agreement with accepted values of the dispersion component of the surface free energy of metal oxides. Adsorption heat values of adsorbates with different polarities indicate that the AlPO₄-5 surface is essentially nonpolar and interacts only with dispersion interaction. In the case of n-hexane the contact angle was assumed to be zero, showing high affinity with the result of enhanced adsorption due to pore filling. Received: 21 May 1998 Accepted: 28 July 1998