t,t- muconic acid hydrogenation for bio-adipic acid production

t,t-muconic acid (MA) is a metabolic intermediate of the catechol ortho-cleavage pathway. This pathway allows the degradation of many aromatic compounds, such as benzoate, toluene, benzene, phenol, aniline, and salicylate. All these intermediates require the synthesis of catechol as central aromatic intermediate (Harwood, 1996). In nature, microorganisms displaying this pathway are quite common and they can accumulate at least 0.5 g/l of MA (Mizuno, 1988). The so produced MA is then converted to adipic acid (AA) with a heterogeneous hydrogenation chemical reaction; this particular reaction is only partially reported and discussed in literature, in fact articles often describe mainly the bio-process for the production and recovery of the muconic acid (Vardon, 2015). Currently there are not detailed studies of kinetic hydrogenation of muconic acid to adipic acid in water, because all the research groups analyzed only few catalysts in limited pressure and temperature ranges. Draths and Frost (1994) obtain high yield (90%) at low pressure and temperature (3.5 bar, 25 °C) with Pt/C using alcoholic solvents as reaction medium. Due to the low solubility of the muconic acid in this type of solvents, we will take into account to lead the hydrogenation reaction directly on the muconate salt recovered from the biological conversion reactor. The sodium muconate salt is widely more soluble in water (the green solvent par excellence) than in alcohols and the possibility to work at higher muconic acid concentration open the possibility to increase a lot the productivity of the plants. Different hydrogenation reactions monitoring the conversion of MA and the selectivity to AA on Pt/C catalyst 5% were conducted. The reaction on the sodium muconate salt took place at 40°C and 4 bar of hydrogen. The main results are reported in Fig. 1. Figure 1: (◊) MA conversion and (■) AA selectivity selectivity at 40°C, 4 bar, 7 10-2M of sodium muconate.