Catalytic hydrothermal liquefaction of nutrient-stressed microalgae for production of high-quality bio-oil over Zr-doped HZSM-5 catalyst

The Hydrothermal liquefaction (HTL) of a nutrient-stressed microalgae ( Scenedesmus obliquus ) (NSM) with and without the use of zirconium-doped HZSM-5 catalyst was investigated under temperature conditions ranging from 250 to 350 ° C. The wet impregnation method was used to prepare the catalyst, and HTL experiments were conducted on the unstressed microalgae (CM) for comparison. Under the stressed condition, the protein content of the microalgae was reduced from 42.35% to 22.08% while the carbohydrate and lipid contents were increased from 25.36% to 42.55% and 17.16%–21.62% respectively. The maximum HTL bio-oil yield of 52.8 wt% and 24.27 wt% were found for NSM and CM respectively at 350 ° C with addition of Zr-HZSM-5 catalyst. Higher denitrogenation and deoxygenation was achieved with NSM compared to CM. At high temperature of 350 ° C, the most abundant fatty acid in NSM was found to be cis -vaccenic acid (omega-7- fatty acid), and this could be explored for the possibility of extracting products of great value from the bio-oil for applications other than biofuels. Mainly, the use of Zr-doped HZSM-5 catalyst on nutrient-stressed S. obliquus microalgae resulted in enhanced bio-oil yield and characteristics which compared well with petroleum crude. • Nutrient stressing altered the microalgae compositions especially reduction of protein by ∼50%. • The presence of Zr/HZSM-5 catalyst was beneficial and improved the HTL bio-oil yield and quality. • The nitrogen content in HTL bio-oil from stressed algae was within the range for petroleum crude and the most abundant fatty acid was found to be omega-7 fatty acid. • High heating value (43.56 MJ kg −1 ) within the range for petroleum crude (42–48 MJ kg −1 ) was achieved. • Suitability of nutrient stressed algae in enhancing the quality of HTL bio-oil was established.