Renewable Adipic Acid From the Hydrogenation of Trans,Trans-Muconic Acid: Selection of a Three Phases Kinetic Model

Abstract

The hydrogenation of muconic acid to adipic acid is studied in a three phases slurry reactor. This is the last step of a novel biotechnological process which, starting from lignocellulosic biomass, allows the production of renewable PA 6,6. A new Pt/C 5% catalyst was selected, which outperformed all the catalysts mentioned in previous literature, and was considered for a more detailed kinetic study. Following an original approach, trans, trans-muconic acid was preferred for the hydrogenation, even though the biological transformation of biomass yields only the cis,cis isomer. This choice is motivated by the higher stability of trans,trans form, which allows to exclude isomerization reactions when studying the mechanism. Also, this isomer is the most likely to be found after the fermentation broth workout and in presence of noble metal catalysts. Preliminary tests identified in 60 °C and 4 bar of hydrogen pressure the mildest conditions to achieve complete conversion to adipic acid in a reasonably short time (1.5 h). The kinetic data collected were used to suggest a plausible reaction pathway and to derive some preliminary kinetic equations. Different models were proposed to interpret the experimental values, developed according to the LHHW theory to consider different adsorption- desorption equilibria. A dual-step hydrogenation mechanism, characterized by hydrogen dissociation on the metal, was hypothesized and validated. These original results provide a first insight on the kinetics of muconic acid hydrogenation and a first reference to further investigations. The purpose is to pave the way for the industrialization of the concept of renewable adipic acid.