Influence of surface area and additives on the thermal stability of transition alumina catalyst supports. II: Kinetic model and interpretation

Abstract

The common features of the gamma, delta and theta forms of transition aluminas found in their structural and chemical properties allow them to be described by a unique formula, Al 2□O 3-v/2(OH) v < > 1-v/2 in which Al represents the aluminium ions in the trivalent sites, □ the cationic vacancies in the divalent sites, OH the hydroxyl groups substituting the oxygen in the normal positions of the anionic sublattice and < > the anionic vacancies of a spinel structure. This formula satisfies the conditions of electrical neutrality of the crystal, accounts for the required ratio of the number of sites in the structure and describes the range of stoichiometries from boehmite (v = 2) to alpha alumina (v = 0). Thus the incorporation of a foreign cation MZ +, either in an Al 3+ site or in a divalent vacancy, leads to a modification of the stoichiometry of different structural elements. The evolution of transition aluminas during thermal treatment consists of dehydroxylation by removal of water followed by an exothermic structural transformation to the alpha phase, which can be described in terms of an annihilation reaction between anionic and cationic vacancies. A theoretical model for spherical particeles based on the annihilation of vacancies as the rate-determining step leads to a kinetic law involving the initial specific surface area, the valency of the foreign cation, and a parameter which depends on its ionic radius. Kinetic data, previously reported for pure and various doped aluminas, agree satisfactorily with the theoretical law. The interest in and limitations of this model are discussed in relation to the experimental results.