A Critical Review of the Synthesis and Applications of Spinel‐Derived Catalysts to Bio‐Oil Upgrading

The transformation of renewable bio-oil into value-added chemicals and bio-oil through catalytic processes embodies an efficient approach within the realm of advancing sustainable energy. Spinel-based catalysts have garnered significant attention owing to their ability to precisely tune metals within the framework, thereby facilitating adjustments to structural, physical, and electronic properties, coupled with their remarkable thermal stability. This review aims to provide a comprehensive overview of recent advancements in spinel-based catalysts tailored specifically for upgrading bio-oil. Its objective is to shed light on their potential to address the limitations of conventional catalysts, thereby advancing sustainable biofuel production. Initially, a comprehensive analysis is conducted on different metal oxide composites in terms of their similarity and dissimilarity on properties. Subsequently, the synthesis methodologies of spinels are scrutinised and potential avenues for their modification are explored. Following this, an in-depth discussion ensues regarding the utilisation of spinels as catalysts or catalyst precursors for catalytic cracking, ketonisation, catalytic hydrodeoxygenation, steam and aqueous-phase reforming, as well as electrocatalytic upgrading of bio-oil, with a specific emphasis on elucidating their catalytic reactivity, and underlying structure-activity correlation and catalysis mechanisms. Finally, the challenges and potential prospects in utilising spinels for the catalytic valorisation of renewable biofuel are addressed, with a specific focus on the use of machine learning - based approaches to optimise the structure and activity of spinel catalysts. This review aims to provide specific directions for further exploration and maximisation of the spinel catalysts in the bio-oil upgrading field.