The combination of biological catalysis and chemical catalysis has emerged as a promising strategy to convert renewable feedstocks to chemicals.^1,2 One facet of this approach is to ferment carbohydrates to platform molecules and then chemocatalytically convert these intermediates to biorenewable chemicals. Although a cascade between these conversion steps utilizes water as a green solvent and minimizes work-up steps, impurities that are inherent to the fermentation broth readily deactivate Pt group metal catalysts.³ To mitigate catalysts deactivation substantial purification and catalyst engineering is required before sequential conversion steps can take place.⁴An alternative approach to mitigate catalyst deactivation would be to utilize novel conversion schemes using robust base metals that are stable in the presence of biogenic impurities.

Electrocatalytic hydrogenation (ECH) represents a promising approach to cascade fermentation and chemical catalytic conversion. In this approach, a cascade would entail the use of water, salts, and acidic impurities in the fermentation broth as an electrolyte. Protons in the solution would then be electro-reduced by applying a potential of -0.8 to -1.8 V vs. Ag/AgCl on a transition metal electrode (Fig. 1). Adsorbed hydrogen is then available to perform a hydrogenation reaction on an important biorenewable platform molecule.

In our studies, lead was found to catalyze the hydrogenation of muconic acid to 3-hexenedioic acid, an important C6 diacids for the production of polyamides and polyesters, directly in the fermentation broth at -1.5 V vs. Ag/AgCl. Significantly, after 5 successive batch reactions no catalyst deactivation was observed (Fig. 2). In an attempt to integrate the two processes further, an ECH was attempted during the fermentation. As shown in figure 3, glucose was fermented to muconic acid with S. Cerevisiae. After 72 h, ECH proceeded with a Pb catalyst for 2 h, converting most of the muconic acid to 3‑hexenedioic acid. Fermentation was carried out for an additional 24 h and, as observed in Fig. 3, the production of muconic acid was unaltered by the ECH step. These experiments confirm that a fully integrated process combining fermentation and electrocatalytic conversion is possible. This work was extended to other industrially relevant base metals (Fe, Ni, Sn, Zn). Similar performance was obtained for Fe, Sn, and Zn, thus demonstrating the potential of this integrated approach for the production of bio-based chemicals in biorefineries.

 

References

1. Vennestrøm, P. N. R.; Christensen, C. H.; Pedersen, S.; Grunwaldt, J.-D.; Woodley, J. M. ChemCatChem 2010,2, 249-258.

2. Gröger, H.; Hummel, W. Curr. Opin. Chem. Biol. 2014,19, 171-179.

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

4. Schwartz, T. J.; Johnson, R. L.; Cardenas, J.; Okerlund, A.; Da Silva, N. A.; Schmidt-Rohr, K.; Dumesic, J. A. Angew. Chem. Int. Ed. 2014, 53, 12718-12722.