Importance of pore and chemical structures of activated biochar on bio-oil deoxygenation during online catalytic cracking of volatiles over activated biochars obtained from different activation methods

Activated biochar (ABC) is a promising catalyst for bio-oil deoxygenation, its catalytic performance is closely related to the physicochemical structures. In this study, the online catalytic cracking of pine sawdust volatiles over ABCs obtained from different activation methods was conducted in a fixed-bed reactor at 500 °C. ABC participated in reactions with reduced O-containing surface groups and porosity. The ultra-micropores (<0.68 nm) in biochar were fully blocked by coke, an appropriate micropore and mesopore distribution (e.g., 1–3 nm) was preferable for retaining its catalytic activity. Bio-oil was deoxygenated over char surface in the form of CO (predominately), CO2 and H2O, along with dehydrogenation and dealkylation reactions. The O-containing surface groups of ABCs acted as active sites, but their porous structures played a dominated role during bio-oil deoxygenation, the depth/degree of bio-oil cracking/deoxygenation was determined by the BET surface areas obtained from different activation methods. KOH-ABC and K2CO3-ABC with high porosities deeply deoxygenated bio-oil to selectively form phenols and aromatics. H2O-ABC with medium BET surface area showed moderate deoxygenation ability, resulting in more carbonyls formation. In contrast, ZnCl2-ABC, H3PO4-ABC, KMnO4-ABC, and CO2-ABC with less porosities exhibited low cracking/deoxygenation abilities. This study provides insight into optimizing biochar activation strategies for bio-oil catalytic deoxygenation.