The production of biorenewable chemicals has significantly evolved over the past years and numerous processes either have reached commercial stage or are close to commercialization.¹ Recently, the biological synthesis of diacids has been receiving greater attention for the manufacture of green polyesters and polyamides.² Muconic acid, an unsaturated C6 diacid, is quickly evolving as a high interest intermediate for the production of a wide variety of renewable chemicals from biomass (Fig. 1). The increased interest is due to the increasing market size of adipic acid and the need for greener synthesis.

Electrocatalytic hydrogenation (ECH) represents a promising approach to combine the fermentative synthesis of muconic acid with its chemical catalytic diversification to produce these value-added chemicals. ECH typically proceed by applying a -0.3 to -1.8 V potential vs. Ag/AgCl on a transition metal electrode. In contrast to high pressure hydrogenations using noble metals, the electrocatalytic hydrogenation of muconic acid is facilitated through the reduction of protons on the surface of base metal catalysts, yielding interesting selectivities and conversion rates. We demonstrate that the use of Earth-abundant catalysts such as Pb also mitigate the necessity to separate amino acids from the fermentation broth that have been shown to readily poison archetypical hydrogenation catalysts such as Ni, Pd, and Pt³.

Herein we screen a wide variety of early, late, and post transition metal catalysts in an attempt to identify key parameters that govern ECH selectivity and faradaic efficiency. In particular, we differentiate the selected transition metals based on the free energy of hydrogen absorption ΔG_H on ECH performance (Fig. 2). Our studies detail the synthesis of trans,trans-muconic acid and trans-3-hexenedoic acid, two bio-based intermediates for the production of polyamides and polyesters, using low hydrogen overpotential metal catalysts. In contrast, high hydrogen overpotential metals produced trans-3-hexenedioic acid with high yield and faradaic efficiency. Conversion and selectivities after 1 h reaction durations for two particular electrocatalysts are displayed in table 1. In all cases, the need to identify binding strengths of the organic reactant will necessitate further correlations that determine hydrogenation product selectivity.

 

References

 

1. Choi, S.; Song, C. W.; Shin, J. H.; Lee, S. Y. Metab. Eng. 2015,28, 223-239.

2. Becker, J.; Lange, A.; Fabarius, J.; Wittmann, C. Curr. Opin. Biotechnol. 2015,36, 168-175.

3. Schwartz, T.; Brentzel, Z.; Dumesic, J. Catal Lett 2015,145, 15-22.

4. Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Norskov, J. K. Nat. Mater. 2006,5, 909-913.

5. Trasatti, S. J. Electroanal. Chem. Interfacial Electrochem. 1972, 39, 163-184.