NSF-DFG Echem: CAS: Cooperativity Between Immobilized Redox Mediators for Selective Anodic Biomass Valorization

With the support of the Chemical Catalysis program in the Division of Chemistry, Adam Holewinski and Wei Zhang of the University of Colorado-Boulder are studying new designs for catalytic materials. The project also involves collaborators from Rostock University in Germany. Electrocatalysts facilitate the interconversion of electrical energy and chemical bonds.

These transformations are increasingly of interest for storage and extraction of renewable electricity via chemical fuels, as well as for synthesizing high volume chemical products that rely on large energy inputs. Efficient catalysts such as the enzymes found in nature often rely on cooperative interactions among multiple chemical functional groups to steer a reaction toward desired products.

This project is focused on designing cooperative electrocatalysts where two species work together to drive a reaction with less electrical input than would be required from either species alone. Work will center on synthesizing catalysts where each mediator is attached to a polymer such that they remain in close proximity. While the knowledge to be gained is fundamentally applicable for a wide range of important chemistries, the project will progress from model reactions (e.g. selective oxidation of simple alcohols to aldehydes) to reactions involving higher value substrates.

In particular, production of environmentally friendly monomer compounds for green plastics will be targeted using raw materials derived from biomass. From an applied perspective, the development of scalable oxidative processes could alter the economic landscape of electrolysis for chemical production. Educational integration initiatives will include inclusion of diverse undergraduates in the research process and continued development of web-based educational modules.

International collaboration with Rostock University in Germany is a key component of the project and participating graduate students will have the opportunity for visiting research. The project was awarded through the "NSF-DFG Lead Agency Activity in Electrosynthesis and Electrocatalysis (NSF-DFG EChem)" opportunity, a collaborative solicitation that involves NSF and Deutsche Forschungsgemeinschaft (DFG).

Under this collaborative research award, Adam Holewinski and Wei Zhang of the University of Colorado-Boulder will study new designs for catalytic materials. Designing tailored substrate binding environments with multiple participant functional groups is difficult to achieve with homogeneous molecular catalysts, and even more challenging using heterogeneous catalyst materials.

This proposal aims to understand and develop cooperative electrocatalytic sites, comprised of an organic redox mediator and redox-active metal center, both performing electron transfer. The work draws on recent demonstrations of homogeneous redox mediator mechanisms in which a two-electron oxidation is achieved by extraction of one electron into each of two mediators, lowering the necessary applied potentials by avoiding higher mediator oxidation states, normally accessed to achieve two electron oxidation.

The mechanism is fundamentally distinct from more common examples of cooperativity, such as metal-ligand. The collaborative research team seeks to develop the fundamental understanding and methodologies needed to effectively immobilize such cooperative catalysts onto (a) soluble (but easily separated) polymers and (b) heterogeneous electrodes. These two platforms provide complementary means to identify constraints imposed by immobilization and understand how best to retain cooperativity.

Mechanistic studies will be used to understand electro-oxidation cooperativity in probe reactions of variable complexity: (i) selective conversion of primary alcohols to aldehydes; (ii) oxidation of 5-(hydroxymethyl)-furfural (HMF) to diformylfuran (DFF) (a monomer for green plastics along with other chemical applications), and (iii) multi-electron oxidation to generate carboxylic acids—particularly HMF to 2,5-furandicarboxylic acid (FDCA), another monomer requiring both aldehyde and alcohol group oxidation. These reactions are critical to the expanding field of biomass valorization and simultaneously serve as an informative testbed for understanding the key constraints to realize cooperativity.

The work further addresses widespread issues related to benchmarking of electrocatalytic activity on complex materials, aiming to establish rigorous precedents for activity characterization.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.