Progress in chemical looping-based transformations of small molecules

Chemical looping is a process in which one reaction is split into two or more separate reactions that occur sequentially in one reactor or simultaneously in different reactors. The transport of matter and energy between reactants is achieved via a medium, and the valence of one element of the medium swings between two values. Compared to co-feed mode in conventional reaction technologies, alternative or separate feeding has the advantages of high exergy efficiency, excellent process safety, and flexible operation. It was ever commercialized to produce oxygen, hydrogen, and carbon dioxide before viable processes were developed. Since the 21st century, the application of chemical looping technology in transformations of small molecules like light hydrocarbons and nitrogen has attracted extensive attention all over the world. In this paper, the recent progress in the field is reviewed. The paper mainly focuses on solar CO2 and H2O splitting, methane reforming, selective hydrogen combustion coupled with dehydrogenation, and ammonia synthesis. The main challenges of the chemical looping-based transformations of small molecules are also briefly discussed.