Mechanistic insights into hydrodeoxygenation for bio-oil upgrading: A comprehensive review of catalytic pathways and material innovations

Hydrodeoxygenation (HDO) has emerged as a key approach for upgrading oxygen-dense bio-oils to hydrocarbons that can be used as biofuels. Catalytic systems used for upgrading bio-oils have been widely investigated, however, the reaction mechanisms that explains oxygen removal via HDO is still fragmented. This review explores the fundamentals of the steps involved in HDO, such as hydrogenolysis, hydrogenation, decarboxylation, and decarbonylation routes for a spectrum of varied catalysts with different catalytic properties. Model compounds were used to explain the fundamental steps that are to be observed when bio-oil is converted. Interest is placed on catalytic properties that enhance catalytic activity and how they mechanistically benefit the reaction. Experimental data is supplemented with computational data gathered from previous reports for a better understanding of the HDO mechanism. Catalyst deactivation such as coking, sintering, and poisoning during hydrodeoxygenation processes often explored and measured to prevent or limit catalyst deactivation are herein discussed. At this current stage, the need to upgrade bio-oil outweighs proof of concept often achieved by using bio-oil derived model compounds. Therefore, this review looks at catalytic systems that are used in upgrading raw bio-oil to viable biofuel while looking at catalyst type and its properties and employed reaction conditions. This review aims to provide a conceptual bridge between molecular-scale HDO and industrial-scale HDO, offering valuable insight in the design and development of catalysts. Furthermore, this review ties together the trends observed in elementary surface-mediated reaction mechanisms that are influenced by catalysts’ properties and sustainable drop-in fuel production.