Engineering aspects of the aqueous noble metal catalysed alcohol oxidation

The aqueous noble metal catalysed alcohol oxidation is a reaction which can profitably be applied in fine-chemistry and for carbohydrate conversion. In this paper engineering aspects of this reaction are treated, i.e. the reaction kinetics, oxygen mass transfer restrictions, catalyst deactivation and reactivation, and implications for reactor design and operation. First a reaction mechanism is proposed, which is very helpful for understanding the observed phenomena. Also a short summary is given on catalyst deactivation mechanisms. Two different reaction regimes can clearly be distinguished: the oxygen mass transfer limited regime and the intrinsic kinetic regime, which are treated separately. Oxidations using noble metal catalysts promoted with less noble metals, like Pb, Bi, generally fall in the first regime, those using unpromoted noble metals in the second. Reaction rate data are evaluated for the Pd/Bi catalysed oxidation of glucose and the Pt catalysed oxidation of methyl-glucoside, respectively, illustrating the typical kinetic behaviour in both regimes. From oxidation kinetics in the mass transfer limited regime, it is concluded that adherence of catalyst particles to the gas¿liquid interface, is a major factor determining reaction kinetics. Oxygen transfer, direct from the gas to the catalyst particle, is likely. For the Pt catalysed oxidation, a kinetic model is presented for catalyst deactivation by over-oxidation and for catalyst reactivation. Finally specific reactor options and suggestions for future engineering research are given: slurry catalyst versus fixed bed catalyst operation, avoidance of explosion risks, redox cycle reactors, electrochemical reactors, and multi-functional reactors