Electrocatalytic Reduction of Aldonic Acids to Aldoses on Gold Electrodes

Abstract

Spent sulfite liquor, a side-stream from the pulp and paper industry, is an abundantly available carbon source for bio-based platform chemicals. The biotechnological valorization of side streams in biorefineries is hampered by the inability of many microorganisms to metabolize and deal with aldonic acids. Based on the principles of Green Chemistry, the electrochemical reduction of aldonic acids into the corresponding biomass sugars appears as a prospective process for the conversion of these acids into fermentable carbohydrates. In our paper, the investigation of electrochemical reduction of gluconic and xylonic acids into glucose and xylose, respectively, is presented. The proposed mechanism on a gold-coated silver electrode was determined via ReaxFF molecular dynamics simulations and quantum chemistry calculations. Model solutions with an aldonic acid concentration of 2.5 wt % were used for the experiments. Compared to a two-electrode compartment cell, the amounts of glucose and xylose produced in the undivided cell were more than 4 and 5.5 times higher, respectively. The electrode surface was analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Despite the relatively low conversion rate, our results show that electrochemical reduction of aldonic acids into their corresponding aldoses in model solutions is possible, which represents an important step toward side-stream valorization.

Potential dependence of gluconic acid to glucose electroreduction on silver

“…Therefore, it can be assumed that there is no inhibition of the reaction over this period. This is in line with our previous study, 24 where the inhibition of the reaction by the Pt CE could be excluded. To further prove this, a reaction was carried out with carbon paper as a CE in the same reaction system.…”

“…To further prove this, a reaction was carried out with carbon paper as a CE in the same reaction system. 24 For the less cathodic reaction conditions, at −0.66 and −0.86 V vs. RHE, only a slight increase of the conversion of gluconic acid to glucose could be observed. At −0.66 V vs. RHE 0.008 mmol L −1 glucose was produced, and at −0.86 V vs. RHE 0.015 mmol L −1 was produced.…”

“…The curve representing the concentration of produced glucose as a function of the potential exhibits a distribution similar to a Gaussian bell curve, with the peak concentration occurring near the maximum. 24 With higher and lower potentials, a decrease in conversion takes place. The conversion rate increased by twofold from a reaction potential of −1.26 V vs. RHE to −1.56 V vs. RHE.…”

“…For comparison of the Ag WE with the previously reported Au WE, 24 the glucose concentration of a reaction with a 3.5 cm 2 Au WE in an H cell was determined. The reaction potential and reaction time were the ones previously established (−1.06 V vs. RHE and 8 h).…”

“…24 At a potential of −1.04 V vs. reversible hydrogen electrode (RHE), using gold as the active electrocatalytic material, a conversion of 0.43% of gluconic acid to glucose could be achieved in the H cell. 24 Gold is known to be one of the most expensive metals (USD 1964 per ounce in June 2023), whereas silver, in comparison, is approximately 100 times cheaper (USD 23.89 per ounce in June 2023). 25,26 The cost difference is one of the reasons why silver was considered as an alternative electrode material in this study, making it a more economically viable while also a benign approach.…”