Influence of thermal, hydrothermal, and acid-base treatments on structural stability and surface and catalytic properties of AlPO₄^5-

Abstract

Effects of thermal treatment (at 813-1433 K), hydrothermal treatment [low-pressure water vapor (35 Torr) at 773 K for 0-168 h, high-pressure steam (16 atm), and water (at autogenous pressure) at 473 K for 12 h], and acid-base (pH 1-12.5) treatment (at 303 K for 20 h) on the structural stability, physical properties (viz., size and morphology of crystals, relative surface concentrations of Al and P, and N₂ sorption capacity at 78 K), acidity, and catalytic activity (in the cumene cracking and o-xylene isomerization reactions at 673 K) of AlPO₄^5- have been investigated. Although AIPO^5-₄ has good thermal stability, calcination at higher temperatures (≥973 K) causes a small decrease in its crystallinity and N₂ sorption capacity and a large decrease in its acidity and catalytic activity. The crystallinity, surface properties, acidity, and catalytic properties of AlPO₄^5- are affected significantly by hydrothermal treatment, particularly treatment with high-pressure steam or water at autogenous pressure at 473 K. These treatments lead to a complete collapse of the channel structure. AlPO₄^5- dissolves completely in both HCl and NaOH solutions at concentrations of 0.1 mol cm-3 or greater. The pH of the acid or alkali solutions has a pronounced effect on its structural stability, as well as its surface and catalytic properties and acidity.