2024 Theses Doctoral

Controlling the Product Selectivity of Oxygenate Transformations on Metal-Based Catalysts

Porter, William Neil

The design of heterogeneous catalysts for selective chemical conversions is a critical factor in developing a more sustainable and efficient chemical industry. In particular, there is significant interest in developing catalysts for the production and valorization of C₂‒C₄ oxygenates, which are versatile platform chemicals, especially from alternative sources of carbon. Promising catalysts for such transformations have been identified, but fundamental understanding of the reaction mechanisms and active sites on these catalytic materials is still lacking.

This work utilized three representative reactions to develop this fundamental understanding through the use of model surfaces, probe molecules, in-situ characterization, and reactor evaluation. The three classes of reactions that were investigated are alcohol dehydration and dehydrogenation, ethylene hydroformylation, and olefin epoxidation. This work elucidates how interactions between active species, surface intermediates, and catalyst/support interfaces influence the catalytic performance of catalysts based on bimetallic and transition metal nitride materials.

The first part of this dissertation used ethanol and isopropanol as biomass model compounds to probe the active sites of metal-modified molybdenum nitride catalysts. The non-oxidative dehydrogenation of alcohols is a route to synthesize aldehydes from biomass-derived alcohols while simultaneously producing hydrogen. Comparing the reaction pathways of ethanol, the simplest molecule containing O−H, C−H, C−O and C−C bonds that are present in biomass-derived molecules, with isopropanol, the simplest secondary alcohol, provided useful insights into the upgrading of more complex biomass. Chapter 3 compared the two alcohols on Cu-modified molybdenum nitride, and Chapter 4 focused solely on the reaction of isopropanol over Fe- and Pt-modified molybdenum nitride. This work showed how the orientation of intermediates, chemical state of active centers, and metal d-band structures influenced the bond scission preference. In addition, this work demonstrated effective strategies for promoting dehydrogenation over molybdenum nitride-based catalysts, as well as the feasibility of using model surface experiments to guide the design of practical powder catalysts.

Following the investigations of the selective bond scission of oxygenates, Chapter 5 of the dissertation was focused on the production of C₃ oxygenate molecules through ethylene hydroformylation, a C−C coupling reaction. The influence of a mesoporous silica support on bimetallic interactions between Rh and Co for ethylene hydroformylation was elucidated through a systematic study of monometallic and bimetallic catalysts. In-situ vibrational studies suggested that the mesoporous silica-supported bimetallic catalyst facilitated moderate binding of important gem-dicarbonyl species that enabled facile co-adsorption of CO and ethylene, ultimately leading to improved hydroformylation performance. Kinetic measurements revealed a lower hydroformylation barrier for the Rh-Co bimetallic compared to the Rh monometallic catalyst.

Then, Chapter 6 investigated another class of reaction, olefin epoxidation, focusing on the direct epoxidation of propylene with oxygen. The critical challenge of this reaction is facilitating the formation of the oxametallacycle intermediate and minimizing the abstraction of allylic hydrogen atoms. In this work, propylene oxide and 1-epoxy-3-butene were used to study the interaction between the epoxide ring and Ag(111) and Pt(111) model surfaces. Cu modification of Ag(111) was shown to lead to improved stabilization of the oxametallacycle. Following this, Pt(111) was used to identify the factors that influence the undesirable complete oxidation pathway. Chapter 7 outlined potential future avenues of research, which include the use molybdenum nitride-based catalysts for reactions of CO₂ and ethane, and propylene epoxidation with in-situ generated H₂O₂ as the oxidant.