Kinetics and mechanism of the heterogeneous reactions of γ-, χ-, and α- aluminas with aqueous sodium hydroxide solutions

Abstract

γ-, κ-, and α- Alumina powders, prepared from gibbsite (S₀= 0·4 m.²/g.), react with excess of well stirred hydroxide solutions (1–16M) according to a second-order reaction with respect to powder weight (that is, third order in powder surface area).

The rate constants vary with (a_±)^½ according to a Langmuir-type adsorption isotherm and also with a_w. For α-alumina, k_w,1=ca. 0·001 g.^–1 hr.^–1 at 115°. For κ-alumina, k_w,1= 0·018 g.^–1 hr^–1. at 85°, 0·053 at 100°, and 0·180 at 115°. For γ-alumina k_w,1 0·05 g^–1. hr.^–1 at 85°, 0·20 at 100°, and 0·46 at 115°. The energy of activation for the reactions is for γ-alumina, 20,200; for κ-alumina, 22,000; and for α-alumina ca. 27,000 cal./mole. The kinetics suggest that the rate-determining step for dissolution of aluminas is a reaction between a hydrated defect region on the alumina surface and two hydroxyl ions, simultaneously weakly chemisorbed (near this region): additional water molecules may also participate in the reaction. Reaction mechanisms are discussed.