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 aw. For α-alumina, kw,1=ca. 0·001 g.–1 hr.–1 at 115°. For κ-alumina, kw,1= 0·018 g.–1 hr–1. at 85°, 0·053 at 100°, and 0·180 at 115°. For γ-alumina kw,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.