date:Katholieke Univ Leuven, Centrum Oppervlaktechem & Katalyse, B-3001 Louvain, Belgium
In our preceding paper, we showed that cobalt substituted aluminophosphate molecular sieves can be successfully employed as active and truly heterogeneous autoxidation catalysts [D.L. Vanoppen, D.E. De Vos, M.J. Genet, P.G. Rouxhet, P.A. Jacobs, Angew. Chem., Int. Ed. Eng. 35 (1995) 560]. These solid catalysts show a higher selectivity for cyclohexyl hydroperoxide and an improved overall selectivity for mono-oxygenates when compared to a classic homogeneous cobalt naphthenate catalyst. In this paper, oxygen uptake curves of the homogeneous and heterogeneous catalyzed reactions are fitted to a simplified kinetic expression. The solid catalysts exhibit a markedly different kinetic behavior when compared to the classic homogeneous catalyst. This difference in reaction kinetics may indicate a different reaction mechanism, but may be accounted for equally well by effects of sorption, competitive sorption and diffusion that are inherent to heterogenization of the cobalt ion on a microporous support. Finally, this difference in kinetic behaviour explains the higher peroxide selectivity and overall selectivity that were observed using these heterogeneous catalysts. (C) 1999 Elsevier Science B.V. All rights reserved.