Hydrodeoxygenation of fast-pyrolysis bio-oils from various feedstocks using carbon-supported catalysts

• Pyrolysis bio-oils from different feedstocks underwent hydrodeoxygenation catalysis.
• Switchgrass bio-oil HDO over Pt/C was best at carbon retention and deoxygenation.
• High syringol content of bio-oil strongly affects hydrogen consumption efficiency.
• Total extent of upgrading was independent of feedstock and catalyst type.

In this paper, we sought to elucidate the relationships between biomass feedstock type and the suitability of their fast-pyrolysis bio-oils for hydrodeoxygenation (HDO) upgrading. Switchgrass, Eucalyptus benthamii, and equine manure feedstocks were pyrolyzed into bio-oil using a continuous fast-pyrolysis system. We also synthesized variations of switchgrass bio-oil using catalytic pyrolysis methods (HZSM-5 catalyst or tail-gas recycle method). Bio-oil samples underwent batch HDO reactions at 320 °C under ~ 2100 psi H2 atmosphere for 4 h, using Pt, Ru, or Pd on carbon supports. Hydrogen consumption was measured and correlated with compositional trends. The resulting organic, aqueous, and gas phases were analyzed for their chemical compositions. Mass balances indicate little coke formation. Switchgrass bio-oil over Pt/C performed the best in terms of hydrogen consumption efficiency, deoxygenation efficiency, and types of upgraded bio-oil compounds. Eucalyptus feedstocks consistently consumed more than twice the normal amount of hydrogen gas per run, primarily due to the elevated syringol content. Catalytically pyrolyzed bio-oils deoxygenated poorly over Pt/C but hydrogenated more extensively than other oils. Although the relative deoxygenation (%DOrel) varied based on feedstock and catalyst, the absolute deoxygenation (%DOabs) depended only on the overall yield. The total extent of upgrading (hydrogenation + deoxygenation) remained independent of feedstock and catalyst.