Investigating the Influence of Catalyst Structure on the Electrochemical Hydrogenation of Cis,Cis-Muconic Acid to Yield Adipic Acid

The conversion of plant biomass to synthesize sustainable and innovative chemicals is the focus of intense research efforts in the midwestern United States due to the abundance of crops and agricultural waste products (e.g., corn stover). In this context, cis,cis-muconic acid (ccMA) is emerging as a versatile compound derived from biomass through fermentation that can be transformed into large volume drop-in chemicals like adipic acid and caprolactam, as well as novel species like trans-3-hexenedioic acid and trans-2-hexenedioic acid. There is particular interest in the production of adipic acid (AA), as it serves as a precursor for nylon-6,6, a plastic that finds applications in apparel, food packaging, house goods, and transportation. Early studies showed that ccMA can be hydrogenated to AA in the presence of a catalyst using H₂ gas. However, H₂ is typically derived from fossil hydrocarbons using processes that are energy intensive and emit large amounts of greenhouse gases. To overcome this limitation, we hypothesized that ccMA could be electrochemically hydrogenated to AA using water (H₂O) as a source of hydrogen. Exploratory work performed in our group demonstrated the feasibility of this novel approach. This project built on these early findings and focused on understanding the pathway of this reaction in order to guide the design of high-performance catalysts. Understanding the fundamental reaction mechanism for converting ccMA to AA will also help us design a system for possible scale-up. Utilizing an electrolysis flow cell, two noble metals acting as the reaction catalyst, palladium, and platinum, were used to convert ccMA to AA. Samples of the electrolyte were collected at 15-minute intervals and analyzed through quantitative ¹H nuclear magnetic resonance. Nanostructured metal catalysts (Pd/C and Pt/C) were active for the desired conversion. In contrast, the corresponding bulk metals (Pd and Pt foils) only transformed trans-3-hexenedioic acid. These results are supported by theoretical calculations and demonstrate that the electrochemical hydrogenation of ccMA to AA is structure sensitive. This research has provided valuable insights into determining the ideal conditions involved in the electrochemical hydrogenation of cis,cis-muconic acid into novel and promising bio-derived compounds.