Reactivity of AlPO4-5 and the origin of its hydrophilic property

Abstract

The reactivity of AlPO₄-5 was measured through the isotope exchange reaction of ¹⁸O between C¹⁸O₂ and the framework oxygen, and its hydrophilicity was investigated by measuring the heat of adsorption of water. About 25% of the framework oxygen atoms were exchanged, and one quarter of them (ca. 6% of the total oxygen atoms) was highly reactive. The differential molar heat of adsorption, calorimetrically determined, was ca. 86 kJ mol^–1 at the initial stage and suddenly dropped to ca. 50 kJ mol^–1. This tendency is characteristic of hydrophobic zeolites, and indicates the existence of special hydrophilic sites. It is concluded, through quantitative analyses, that the highly reactive oxygen atoms and hydrophilic sites originate from defects, and that the concentrations of the defects, estimated from each of the above two, are in good agreement.