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The influence of zinc oxide on the kinetics and mechanism of the liquid-phase hydrogenation of maleic anhydride (MA) and intermediates was investigated on copper-based catalysts. No influence of zinc oxide on the hydrogenation of maleic anhydride was observed in previous experiments. The discontinuous hydrogenation of succinic anhydride (SA) resulted in the formation of γ-butyrolactone (γ-BL) and 1,4-butanediol (1,4-BD) on a copper/zinc catalyst. On a zinc-free copper catalyst only γ-butyrolactone was formed while the hydrogenation of γ-butyrolactone to 1,4-butanediol was inhibited. It was observed that succinic anhydride which is adsorbed on the copper surface of the catalyst prevents the adsorption of γ-butyrolactone. On copper/zinc catalysts the reversible adsorption of succinic anhydride on the inactive zinc oxide crystallites, which led to a reversible decrease of the carbon balance, is responsible for a decrease of the succinic anhydride coverage of the copper sites. It appears that the decrease of the succinic anhydride coverage of the copper surface is proceeding by surface diffusion of succinic anhydride to the adjacent zinc oxide crystallites. On this basis two different reaction pathways via succinic anhydride adsorbed on the copper surface and via succinic anhydride adsorbed on the zinc oxide crystallites were proposed for the hydrogenation of maleic anhydride and intermediates. Kinetic modeling of the reaction pathway taking into account both reaction pathways led to good agreement of calculated and experimental results.

The liquid phase hydrogenation of maleic anhydride was investigated in a packed bubble column reactor using different copper-based catalysts. A copper-zinc catalyst was found to be active in the formation of 1,4-butanediol, whereas on zinc-free copper catalysts, mainly succinic anhydride and γ-butyrolactone were formed. At suitable reaction conditions, maleic anhydride hydrogenation over a copper-zinc catalyst gave valuable products with high yield and selectivity whereas succinic anhydride was absent in the reactor outlet. Based on a three-phase reactor model and a kinetic model of the reaction mechanism, the influence of reaction conditions on reactor performance was determined. It was observed that the use of large particles and a high axial dispersion of liquid phase is a necessary condition for the feasibility of a “one-step-hydrogenation” of highly concentrated maleic anhydride feed solutions because of a significant decrease of succinic anhydride formation rate.

The liquid phase hydrogenation of maleic anhydride and intermediates was investigated using copper-based and noble metal catalysts. The experiments were performed in a stirred tank slurry reactor in discontinuous as well as continuous operation. Copper chromites and noble metal catalysts were found to be suitable for the hydrogenation of maleic anhydride. However, the hydrogenation of succinic anhydride proceeded with high selectivity to γ-butyrolactone and 1,4-butanediol on copper zinc catalysts, whereas other copper catalysts revealed no activity in the formation of 1,4-butanediol. Selective sorption interactions of succinic anhydride with the zinc surface were assumed to be responsible for this effect. Starting from γ-butyrolactone all copper catalysts were active in the formation of 1,4-butanediol while noble metal catalysts showed no or little activity. Kinetic models have been proposed for the hydrogenation of maleic anhydride and intermediates on the basis of experimental data obtained in a continu...