Transition-metal-free reverse water-gas-shift catalysts for sustainable liquid-fuel production
Keywords: Humanities and Sciences.
Chemistry.

Renewable hydrocarbon liquid fuels are needed to displace fossil-based liquid fuels from hard to abate sectors such as aviation and heavy shipping. The most advanced existing renewable liquid fuel options rely on biomass pathways that are limited by photosynthetic efficiency and compete with food production for arable land. There is a critical need for scalable synthetic methods that produce liquid fuels from H₂O and CO₂ emissions. Existing synthetic pathways for liquid fuel generation start with syngas, a gas mixture containing H₂ and CO, which can be converted to short chain alcohols via gas fermentation or to various length hydrocarbons through Fischer-Tropsch. There are various upscaling methods that can turn these products into fuels suitable for hard-to-abate sectors. However, syngas is currently produced from coal or natural gas via steam reforming. Reverse water-gas shift (RWGS), which thermochemically converts CO₂ and H₂ into CO and H₂O, provides the critical link between renewable power and liquid fuels by generating syngas from CO₂ and electrochemically derived H₂. Current RWGS technologies use Ni-based catalysts that must be operated at very high (> 900 °C) temperatures to minimize the production of methane through the competing Sabatier reaction. Operating at these high temperatures requires specialized and expensive reactor materials and complicates heat integration with downstream syngas-to-liquids conversions. My work has developed transition metal-free, alkali carbonate-based RWGS catalysts for renewable liquid fuel generation at intermediate temperatures ≤750 °C. These catalysts consist of an alkali carbonate salt dispersed on a mesoporous support material. Because it lacks a transition metal, the catalyst has very little affinity for the RWGS product CO, which precludes its further reduction to methane. The catalysts were evaluated in a custom-built flow reactor suitable for operating at temperatures up to 525 °C and pressures up to 10 bar. Experimental results demonstrate high, equilibrium-limited conversion of CO₂ to CO with nearly 100% selectivity. The catalysts were also stable in the presence of 50 ppm H₂S impurity for more than 40 hours, which poisons typical transition metal-based catalysts. Based on these results, a larger set of dispersed carbonate catalysts were evaluated in high-throughput-experimentation over an expanded range of temperatures up to 750 °C and pressures up to 30 bar. The catalysts were stable over the course of 200+ hours of continuous operation at industrially relevant space velocities > 24,000 h⁻¹. Stable catalyst performance in the presence of methane and propane was also demonstrated, which is important for integration with Fischer-Tropsch syngas-to-liquids processing because recycle-loops are expected to have significant short hydrocarbon content. Additional screening was used to explore broader catalyst loading and preparation techniques. Dispersed carbonate catalysts are highly active, selective, and low-cost RWGS catalysts. Their robust performance in the presence of common gas impurities makes them suitable for combination with downstream liquid fuel production pathways involving recycle loops. Catalyst preparation is very simple, and the scalable manufacture of these catalysts has been validated by industrial collaborators. This catalyst technology has the potential to simplify and accelerate the deployment of syngas-based renewable liquid fuel production to meet the ongoing demand for liquid fuels. The work presented in this dissertation provides an overview of the evaluation and development of these catalysts. In the first chapter, an overview of existing sustainable liquid fuel is provided and the pros and cons of each method is discussed. It explains why dispersed carbonate are a good target for catalysts of RWGS based on some of the previous work done with them. Chapter 2 discusses the significant development in terms of hardware and software required to make a lab-scale flow reactor in order to evaluate these catalysts. The reactor was designed to access industrially relevant conditions while maintaining strict safe operating procedures as well as software controls. Discussion about why certain decisions were made and a brief tutorial is provided to assist future users in operating the system. Chapter 3 discusses the preliminary but promising results obtained from evaluating the dispersed carbonate catalysts in the lab-scale reactor. Key results are discussed and catalyst performance is benchmarked against a transition-metal based catalyst which was highly-active for RWGS. The stable behavior of the dispersed-carbonate catalyst in the presence of H₂S impurity is a key result that differentiates it from transition-metal based ones. Finally, Chapter 4 discusses ongoing work to validate the performance of the catalyst for industrial application. The use of high-throughput experimentation is highlighted to provide a large volume of results and allow for rapid screening of potential catalyst formulations. The performance of catalysts prepared at the lab scale is compared against that of catalysts prepared by commercial catalyst manufacturers. Additional metrics for study are identified for the goal of a pilot-scale validation experiment