Upgrading technologies and catalytic mechanisms for heteroatomic compounds from bio-oil – A review

•Wash pretreatment can remove alkaline earth metals from biomass to improve bio-oil quality.•The different mechanisms of catalysts in catalytic cracking and hydrotreating summarized.•Analyzed the technical difficulties of bio-oil upgrading technology towards industrialization.•Hydrotreating processing can remove many heteroatomic compounds.•The removal mechanisms of sulfur and chlorine from bio-oil reviewed.