Zeolite-Supported Iron Oxides as Durable and Selective Oxygen Carriers for Chemical Looping Combustion

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO₂ separation from flue gases, circumventing high cost for CO₂ capture and NO_x elimination as in conventional combustion processes. Metal oxides are commonly used as oxygen carriers (OCs) in CLC. However, sintering and coking of OCs and the consequent degradation in their activity and durability during multiple cycles inhibit the practical applications of CLC technology. In the present study, we employed a silicalite-1 zeolite support to achieve OCs with high resistance against sintering and coking in CLC. Iron oxides (Fe₂O₃) with methane fuel were employed to demonstrate the approach and to quantify the influence of silicalite-1 support on conversion efficiency, durability, and selectivity of these OCs in CLC cycles. Two iron oxide-zeolite geometrical structures, a core-shell Fe₂O₃@silicalite-1 and a Fe₂O₃-impregnated silicalite-1 (Fe₂O₃/silicalite-1), were created to improve Fe₂O₃ stability. The CLC tests showed that both structures led to less aggregation of Fe₂O₃ OCs at 1223 K. A comparison between Fe₂O₃/silicalite-1 and Fe₂O₃@silicalite-1 in CLC tests illustrates that Fe₂O₃ impregnated in zeolite had higher durability than in the core-shell structure. The selectivity of CH₄ to CO₂ followed the order of Fe₂O₃/silicalite-1 > Fe₂O₃@silicalite-1 ≫ bare Fe₂O_3. The high selectivity of Fe₂O₃/silicalite-1 to CO₂ in CLC tests can be attributed to the encapsulation of Fe₂O₃ inside channels of silicalite-1 that provides physical barriers for aggregation of OCs in CLC cycles as well as coke deposition on OCs. In conclusion, our study of the structure-function relation for silicalite-1-supported Fe₂O₃ OCs can form the basis for the development of silicalite-1 as an efficient support in chemical looping applications.