Tandem Reactions and Reactors for Sustainable Chemical Synthesis
Samay Garg, Chemical Engineering
Abstract: As the dire consequences of climate change become ever more apparent, it is clear that broad changes to the status quo are required across all aspects of modern society, from land use to energy consumption to industrial processes. In order to limit global warming to 1.5 oC above pre-industrial levels and avoid the worst effects of climate change, it is necessary to bring anthropogenic emissions into balance with carbon removal through a combination of decarbonization, carbon utilization, and carbon sequestration. This will restore a net-zero carbon flux and close the carbon cycle. As the world transitions to renewable energy and embraces electrification, decarbonizing the chemical industry will be a key challenge, because this sector relies on fossil fuels for both energy and material feedstocks. Harnessing renewable energy to power chemical reactors while simultaneously replacing fossil fuel feedstocks with widely available and sustainably sourced precursors is an import part of the clean energy transition and a promising strategy for closing the carbon cycle.
This dissertation investigates tandem catalytic systems as a pragmatic and versatile strategy for the sustainable synthesis of high-demand commodity chemicals and carbon nanomaterials from CO2. Tandem reactor systems, which combine two or more reactor types into a unified system, offer several compelling advantages over conventional single-reactor approaches. By coupling well-established thermocatalytic processes with upstream electrochemical reactors, tandem systems can circumvent the selectivity limitations and product separation challenges inherent to direct electrochemical CO2 reduction, while reimagining competing electrochemical products (forexample, H2) as useful co-feedstocks. The modular architecture of tandem systems also enables independent optimization of each reactor's operating conditions and facilitates straightforward technology upgrades as individual aspects of the system advance. Chapter 3 presents a tandem electrocatalytic-thermocatalytic reactor system for producing aromatic hydrocarbons directly from CO2. In this system, CO2 is first electrochemically reduced to ethylene (C2H4) over a copper catalyst, after which C2H4 undergoes thermocatalytic aromatization over a Ga/ZSM-5/P zeolite catalyst to yield BTEX. In-situ X-ray diffraction and X-ray absorption spectroscopy were employed to characterize the catalyst under operating conditions, revealing the critical role of water management in the tandem system. This work establishes a new pathway for the direct valorization of CO2 into complex aromatic molecules, demonstrating that tandem catalysis can unlock products not accessible through direct CO2 reduction alone.
Chapter 4 investigates Earth-abundant transition metal carbide and nitrides as catalytically active supports for Pd and Cu catalysts in membrane electrode assembly devices for the co-electrolysis of CO2 reduction and H2O. Electrochemical experiments demonstrate that the combination of metal and carbide or nitride support can both enhance overall catalytic activity and serve as a means by which to adjust the CO:H2 ratio in syngas production, enabling the output to be optimized as a feedstock for downstream thermocatalytic reactions. This work also provides a promising avenue for reducing the reliance on scarce platinum group metals for catalysis. Chapter 5 provides a comprehensive review of tandem catalysis for CO2 conversion, including multifunctional catalysts, single-reactor tandem catalysis, and tandem reactor systems, with an emphasis on electrocatalytic and thermocatalytic CO2 conversion. This chapter also examines burgeoning subfields of catalysis including plasma catalysis and biocatalysis, and presents an outlook for the field, identifying key opportunities and hurdles for scaling tandem systems toward industrial deployment.
Chapter 6 details a tandem plasma-thermocatalytic reactor for ethane (C2H6)-assisted CO2 sequestration as carbon nanotubes (CNTs) and carbon nanofibers (CNFs). A thermodynamic analysis and energy consumption calculations are presented and benchmarked against conventional CNT and CNF production methods, demonstrating that tandem approaches are a viable option for producing value-added solid carbon nanomaterials. The results presented in this dissertation establish tandem reactor systems as a versatile platform for both carbon-neutral and carbon-negative chemical synthesis. By integrating electrochemical, thermochemical, and plasma-based reactors in modular configurations, this work demonstrates pathways to sustainable production of BTEX aromatics, syngas, and CNFs, & CNTs from CO2 and renewable feedstocks. These findings contribute foundational knowledge for the design of next-generation sustainable chemical processes and position tandem catalysis as a promising strategy for the decarbonization of chemical manufacturing.
