Reaction kinetics of the liquid-phase hydrogenation of succinic anhydride on CuZnO-catalysts with varying copper-to-zinc ratios in a three-phase slurry reactor

The liquid-phase hydrogenation of maleic anhydride is a promising alternative to already established processes (e.g. Reppe process) for the production of 1,4-butanediol. The attractivity of the maleic anhydride hydrogenation results from its efficiency and flexibility since gamma-butyrolactone, 1,4-butanediol and tetrahydrofuran can be obtained in a single-stage process. The fact that the partial oxidation of low value n-butane replaced benzene as a feedstock for maleic anhydride production over the last decades is another major advantage of the maleic anhydride route, The liquid-phase hydrogenation of maleic anhydride proceeds via succinic anhydride to gamma-butyrolactone and 1.4-butanediol in consecutive hydrogenation steps. However, the formation of 1,4-butanediol takes place exclusively on CuZnO-catalysts having a suitable composition, whereas no 1,4-butanediol is formed on zinc-free copper catalysts. An excessively high surface coverage of the copper sites with succinic anhydride inhibits the adsorption and thus hydrogenation of gamma-butyrolactone to 1,4-butanediol on zinc-free copper catalysts. An experimentally observed reversible carbon balance deficit in the hydrogenation of succinic anhydride on CuZnO-catalysts led to the assumption, that the catalytically inactive zinc oxide provides additional sorption sites for succinic anhydride whereby the surface coverage of copper is reduced. Furthermore, succinic anhydride is a highly undesirable intermediate since it reacts with LA-butanediol to form various oligo- and polyesters. Thus, the occurrence of succinic anhydride in the reactor has to be suppressed. According to the mechanistical approach., the copper-to-zinc ratio of CuZnO-catalysts is expected to have an elementary influence on the hydrogenation of succinic anhydride. Therefore, a set of binary CuZnO-catalysts with copper-to-zinc ratios from 0.1 to 9.0 was prepared and applied in kinetic measurements performed in a stir-red three-phase slurry reactor. A model discrimination led to a detailed kinetic model for the hydrogenation of succinic anhydride on CuZnO-catalysts, considering the experimentally observed reversible carbon balance deficit. The rate constants of the kinetic model were determined significantly for all copper-to-zinc ratios by performing a numerical parameter estimation. Thus. a kinetic model performing a good agreement between experimental and calculated results was achieved. Moreover, the experimentally observed carbon balance deficit could be predicted by the derived kinetic model. (C) 2002 Elsevier Science B.V. All rights reserved.