Catalytic upgrading of pretreated algal bio-oil over zeolite catalysts in supercritical water

We report the catalytic hydrothermal upgrading of pretreated algal bio-oil. The reaction was performed at 400 °C for 240 min with the addition of 6 MPa H₂ and 10 wt.% zeolite catalyst in supercritical water (ρ_H2O = 0.025 g/cm³). Nine zeolites (Hβ, HZSM-5 (SiO₂/Al₂O₃ = 25:1), HZSM-5 (SiO₂/Al₂O₃ = 50:1), HZSM-5 (SiO₂/Al₂O₃ = 170:1), HY (5% Na₂O), HY (0.8% Na₂O), SAPO-11, MCM-41 (50% Si), and MCM-41 (100% Si) were screened to investigate their effects on the yields of the product fraction and the properties (e.g., elemental composition and heating value) of the upgraded bio-oil. The catalyst type affected the yields of the product fractions: SAPO-11 produced the lowest upgraded bio-oil yield of 42.4 wt.%, and MCM-41 provided the highest yield of 54.5 wt.%. Compared with non-catalytic upgrading reactions, all of the zeolites promoted the denitrogenation, deoxygenation, and desulfurization of the pretreated bio-oil due to the presence of acid sites. HY (5% Na₂O), HY (0.8% Na₂O), and HZSM-5 (SiO₂/Al₂O₃ = 25:1) showed the highest activity toward denitrogenation, deoxygenation, and desulfurization, respectively. The upgraded bio-oil mainly consisted of hydrocarbons, accounting for 80% in total and as high as 95.6% of the fraction below 400 °C.