The ketonic decarboxylation of carboxylic acids has been carried out experimentally and studied theoretically by DFT calculations. In the experiments, monoclinic zirconia was identified as a good catalyst, giving high activity and high selectivity when compared with other potential catalysts, such as silica, alumina, or ceria. It was also shown that it could be used for a wide range of substrates, namely, for carboxylic acids with two to eighteen carbon atoms. The reaction mechanism for the ketonic decarboxylation of acetic acid over monoclinic zirconia was investigated by using a periodic DFT slab model. A reaction pathway with the formation of a β-keto acid intermediate was considered, as well as a concerted mechanism, involving simultaneous carbon-carbon bond formation and carbon dioxide elimination. DFT results showed that the mechanism with the β-keto acid was the kinetically favored one and this was further supported by an experiment employing a mixture of isomeric (linear and branched) pentanoic acids.
In the transformation of lignocellulosic biomass into fuels and chemicals carbon-carbon bond formations and rising hydrophobicity are highly desired. The ketonic decarboxylation fits these requirements perfectly as it converts carboxylic acids into ketones forming one carbon-carbon bond and eliminates three oxygen atoms as carbon dioxide and water. This reaction is used, in a cascade process, together with a hydrogenation and dehydration catalyst to obtain hydrocarbons in the kerosene range from hexose-derived valeric acid. It is shown that zirconium oxide is a very selective and stable catalyst for this process and when combined with platinum supported on alumina, the oxygen content was reduced to almost zero. Furthermore, it is demonstrated that alumina is superior to active carbon, silica or zirconium oxide as support for the hydrogenation/dehydration/hydrogenation sequence and a palladiumbased catalyst deactivated more rapidly than the platinum catalyst. Hence, under optimized reaction conditions valeric acid is converted into n-nonane with 80% selectivity (together with a 10% of C10 to C15 hydrocarbons) in the organic liquid phase up to a 100 : 1 feed to catalyst ratio [wt/wt]. The oxygen free hydrocarbon product mixture (85% yield) meets well the boiling point range of kerosene as evidenced by a simulated distillation. In the gas phase butane was detected together with mainly carbon dioxide.
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