Activated carbons for the hydrothermal upgrading of crude duckweed bio-oil

• ACs exhibited activity similar to Ru/C toward the denitrogenation and deoxygenation.
• Bamboo stem AC provided the highest performance.
• Leach ions and weak acids into the reaction solution catalyze the upgrading reaction.
• ACs showed different roles from the noble metal for the upgrading of bio-oil.
• All of the upgraded bio-oils had a large fraction (>80%) boiling below 350 °C.

This study examined a two-stage (noncatalytic pretreatment followed by catalytic upgrading) hydrothermal processing of crude bio-oil produced from the hydrothermal liquefaction of duckweed. The activities of six activated carbons (ACs)-pine wood AC, coconut shell AC, bamboo stem AC, apricot pit AC, peach pit AC, and coal AC-toward the deoxygenation and denitrogenation of the pretreated duckweed bio-oil were determined in supercritical water at 400 °C for 1 h with the addition of 6 MPa of H2 and 10 wt% AC. All of the ACs exhibited activity similar to Ru/C toward the denitrogenation and deoxygenation of the pretreated duckweed bio-oil. Of the ACs tested, bamboo stem AC produced an upgraded bio-oil with the highest yield (76.3 wt%), the highest fraction (90.13%) of material boiling below 350 °C, and the highest energy density (44.1 MJ/kg). Decreased ash and acidic groups in the pre-treated AC disfavored the production of upgraded bio-oil but aided denitrogenation and desulfurization. The ACs are suspected to leach ions and weak acids into the reaction solution, which would catalyze denitrogenation and desulfurization. The gases mainly consisted of unreacted H2, CO2 and CH4 together with small amounts of CxHy (x ≤ 5, y ≤ 12) hydrocarbon gases produced from the cracking of the upgraded bio-oil.

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