A green approach to CO2 capture using fly ash-based catalysts: Performance and mechanistic insights

Fly ash (FA), an abundant industrial by-product, has emerged as a promising catalyst for regenerating liquid sorbents used in CO2 capture, significantly reducing the overall energy consumption of the process. Considering that the primary metal oxides in FA − Fe2O3 and Al2O3 − provide Brønsted and Lewis acid sites critical for enhancing CO2 release, this study explores the development of modified catalysts by loading additional amounts of these metal oxides onto FA (resulting in Fe2O3-FA and Al2O3-FA), aiming to further improve FA-catalyzed CO2 desorption. The performance of these catalysts was examined in desorption of a CO2-loaded MEA solution, focusing on key metrics such as CO2 desorption rate, cyclic capacity and heat duty. Our findings indicate that modifying FA with metal oxides increases its surface acidity (both Brønsted and Lewis) and optimizes acid/alkali strength. Among the catalysts tested, Al2O3-FA exhibited superior performance, achieving a higher CO2 desorption rate, greater cyclic capacity, and lower heat duty compared to Fe2O3-FA, unmodified Al2O3 and FA, and the uncatalyzed system. Additionally, the stability of Al2O3-FA was confirmed over 20 continuous repetitions of CO2 capture and desorption, finding no significant alteration in catalytic activity or material structure (as confirmed by FT-IR and XRD characterization) after prolonged use. The machine learning also was used to correlate the catalysts features and performance while the importance of each feature was identified. Finally, a potential catalytic reaction mechanism is proposed, involving the deprotonation of MEAH+ and decomposition of MEACOO−, both of which synergistically enhance CO2 desorption during sorbent regeneration process.