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

Abstract

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.