Cooperative acidity and hydricity: Design of electrocatalysts for the synthesis of bio-derived propylene products

Manufacturing high-volume, commodity chemicals relies heavily on catalytic processes that require feedstocks derived from natural gas or petroleum, together with energy from the combustion of natural gas – both contributing to atmospheric carbon dioxide and methane. A promising path to net-zero carbon emissions involves the transition to bio-based feedstocks combined with electrochemical technology powered by sustainable electricity generated, for example, from wind or solar energy. Electrochemical processing of bio-feedstocks to high-value chemical products is challenging, however. This project will prove a novel concept for electrocatalytically converting bio-resourced carboxylic acids to alcohols, creating an opportunity for reduced carbon emissions relative to current technology. The project will provide research opportunities for undergraduate students and educational initiatives for students, scientists, and non-expert community members. A research experience for local community college instructors will be offered to help them shape future curricula and guide students in continuing from two-year to four-year degree programs. The project explores the hypothesis that electrochemically-generated surface hydride can cooperate with Lewis acid catalysts to selectively reduce carboxylic acids to alcohols, imitating the mechanism of harsh chemical reagents such as aluminum hydride. The proposed systems bear similarity to so-called “frustrated” Lewis pairs, which can activate hydrogen heterolytically and reduce challenging bonds. These concepts will be investigated in the context of synthesizing 1,2-propanediol (propylene glycol), a major commodity and outlet for propylene, using bio-derived lactic acid. To that end, the project bridges gaps between homogeneous and heterogeneous catalysis with the design of molecular-catalyst-like surfaces, and will delineate fundamental requirements for harnessing strong acidity and hydricity to activate difficult bonds. Controlling these properties simultaneously is a grand challenge as they are intrinsically at odds and require sophisticated schemes to avoid reactivity quenching. The project also will explore new mechanisms to mimic the activity of strong reagents with electrochemical driving forces. It will further expand basic understanding of phenomena such as proton transfer and pH in non-aqueous environments and develop approaches to rigorous benchmarking of novel materials that lack established protocols. Fundamental requirements will be examined in non-aqueous electrolytes with hydrogen as the dominant hydride precursor, while further work will aim to translate the chemistry to more desirable aqueous conditions. In a similar vein, initial studies will combine heterogeneous hydride generation with homogeneous Lewis-acid electrolytes and then move to full “heterogenization” with solid acid/metal interfacial catalysts. 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.