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

Abstract

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.