Integrated Electrocatalytic Processing of Levulinicacid and Formic Acid to Produce Biofuelintermediate Valeric Acid

With the world population rapidly increasing and major crude oil reserves diminishing, seeking renewable energy resources is becoming of critical importance, and thus has attracted enormous R&D attention worldwide. Abundant and renewable ligno-cellulosic biomass is expected to occupy a significant position in our future energy landscape, and one of the most important components derived from ligno-cellulosic biomass is highly polymerized cellulose (DP 7000–15 000), which can be employed as a sustainable source to produce levulinic acid (LA) through dilute acid-catalyzed hydrolysis processes. Levulinic acid can be further upgraded to a wide range of value-added chemicals and fuel additives, and has been identified as one of the top abundant, renewable building-block biomass compounds by the US-DOE. The hydrolysis of waste cellulose to levulinic acid is carried out in 0.1–0.5 M sulfuric acid aqueous solution; equal molar levulinic acid and formic acid(FA) (mol_FA : mol_LA = 1 : 1) can be cheaply produced at yields of 70% and 50%, respectively.

Unfortunately, both the by-product formic acid and the residual sulfuric acid that remained in the hydrolysis downstream will bring some complexities to the subsequent transformation of levulinic acid to biodegradable chemicals or fuel additives during heterogeneous catalytic processes, such as the rapidly deactivation effect of formic acid on Ru/C for de-oxygenation of levulinic acid to gamma-valerolactone (gVL). Thus, novel processing routes and advanced catalysts have recently been explored to remove or utilize the by-product formic acid

Herein, we report integrated electrocatalytic processing of simulated acid-catalyzed cellulose hydrolysis downstream (levulinic acid + formic acid) to the biofuel intermediate valeric acid (VA). This green electrobiorefining process does not require complex steps to separate levulinic acid and formic acid (FA) from H₂SO₄; instead it couples electrocatalytic hydrogenation (ECH) of levulinic acid in a single electrocatalytic flow cell reactor and electrocatalytic oxidation of formic acid in a proton exchange membrane-direct formic acid fuel cell (DFAFC). The presence of formic acid has shown no negative effect on the ECH of levulinic acid and a high valeric acid selectivity of >90% can be achieved on a non-precious Pb electrode while the Faradaic efficiency remains >47% during 8 hours of reaction in the single electrocatalytic flow cell reactor. This stream is fed directly to the DFAFC with a Pd/C anode catalyst to self-sustainably remove formic acid where 47% conversion of formic acid can be reached in 6 hours. However, electro-oxidation of formic acid over Pd/C appears to be reversibly inhibited by the product valeric acid produced during ECH of levulinic acid. The electro-oxidation of FA + C2–C5 alkyl carboxylic acid in the half cell study shows that such an inhibition effect could have originated from the –COOH adsorption on the Pd surface. Higher carboxylic acid concentration and longer carbon chain lead to more serious loss of the electrocatalytic surface area (ECSA) of Pd/C.