(382ad) Copper-Based Catalysts for Ester Hydrogenation and Plastics Upcycling.

My research interests have been greatly influenced by my upbringing in Nigeria – a crude oil producing nation plagued with many of the environmental challenges of oil exploration and refining. This has motivated me to seek ways to use the knowledge of chemistry and chemical engineering to solve these challenges by combining experimental approaches to valorize waste into more useful chemicals and fuels. Throughout my PhD, I have worked on the design of copper-based catalysts to achieve these goals. Copper is of interest because it is a cheap metal that is very selective in many important reactions for waste valorization – one major example is the hydrogenation of C-O bond containing molecules like ethers, esters, carboxylic acids, and CO₂ derived from bio-based sources. These reactions have been studied to avoid the dependence on fossil fuels for essential chemicals. Carboxylic acids and their esters, for example, can be hydrogenated to the respective alcohols and diols, which have several industrial applications. These C-O bonds are also important when addressing the upcycling of polyesters into more valuable products compared to the recycled plastics. The hydrogenation of methyl acetate is of particular interest because of the ester’s simple structure and the numerous applications of the ethanol product. Copper catalysts supported on silica are usually utilized for the selective production of methanol and ethanol from methyl acetate. The state-of-the-art copper catalysts for this reaction are made using the ammonia evaporation method (AE). These catalysts made by AE have been shown to significantly improve activity compared to those made by the conventional incipient wetness method. This has been attributed to the high ratio of Cu⁺/Cu⁰ in the reduced catalyst sample. The increased amount of Cu⁺ is attributed to the incomplete reduction of the Cu-O-Si units present in copper phyllosilicates formed during the AE synthesis. My work during my PhD has sought to investigate the fundamental role of these Cu-O-Si units. By combining experimental heterogeneous catalysis techniques with microkinetic modelling from collaborators, we were able to elucidate that the Cu⁺ content in the AE catalysts plays a significant role in the rates and product selectivity for methyl acetate hydrogenation due to differences in rate-controlling steps compared to a Cu (111) model. I have also used advanced synthesis techniques to coat silica layers on the already synthesized A.E catalyst to explore the role of increasing the Cu-O-Si units on reactivity. Our findings reveal that these units are indeed important for improved methyl acetate hydrogenation activity. The importance of this Cu⁺ content has also been demonstrated in a plastic upcycling study where polylactic acid (PLA) was converted to propanediol (a valuable downstream product) in a one-pot system with high rates and selectivity. Even though PLA is biodegradable, the degradation process takes several years, and CO₂ is released into the atmosphere during this process. Our study combines the ethanolysis of PLA, and the hydrogenation of the ethanolysis product - ethyl lactate - to yield the propanediol product. Catalyst characterization techniques such as CO - DRIFTS and NEXAFS were used to demonstrate the abundance of Cu⁺ in the synthesized A.E catalyst compared to other Cu catalysts synthesized on silica and alumina, which showed low rates for PLA ethanolysis and in turn, for the overall one-pot reaction. The valuable skills I have picked from my PhD work namely: experimental design & execution, catalyst design and characterization, chemical reactor design and optimization as well as the analysis and interpretation of scientific data have positioned me for several industry roles that pertain to the application of catalysis design and characterization techniques to chemical products manufacturing, biomass upgrading and plastics upcycling.