Stabilising Ni catalysts for the dehydration-decarboxylation-hydrogenation of citric acid to methylsuccinic acid

A new reaction sequence of dehydration-decarboxylation-hydrogenation to transform citric acid into methylsuccinic acid has recently been developed using Pd as a noble metal catalyst in water. In this work Ni catalysts were investigated as low cost, non-noble metal alternatives. Several home-made and commercial catalysts were screened for this reaction. Citric acid was very reactive and full conversions were readily obtained in all cases. However, the selectivity to methylsuccinic acid was initially low, since typical Ni catalysts were not stable and therefore not able to hydrogenate the formed C=C double bonds. Due to the lower hydrogenation activity of Ni compared to Pd, new side products appeared. Particularly, hydration of the C=C double bonds made the reaction network more complex in this case. Fortunately, the formation of all hydration products - even the rather stable lactone, beta-carboxy-gamma-butyrolactone - was eventually shown to be completely reversible. Three routes were then studied to stabilise Ni catalysts and to enable the Ni catalyzed conversion of citric acid to methylsuccinic acid; partial neutralisation of the acid reactant, adding Fe to Ni/ZrO2 or to the reaction mixture and coating Ni particles with carbon, all proved to stabilise Ni and all resulted in high to very high methylsuccinic acid yields. Furthermore, the role of Fe was unravelled by performing reference reactions with different Fe compounds and by in depth characterisation of the NiFe/ZrO2 catalyst. Finally, the reaction conditions were optimised using the carbon-coated Ni nanoparticles and kinetic profiles were recorded to confirm the extended reaction network.