Catalytic hydrothermal liquefaction of Euglena sp. microalgae over zeolite catalysts for the production of bio-oil

Abstract

In this paper, Euglena sp. microalgae with low lipid and high ash contents were successfully converted into bio-oil with/without catalysts through hydrothermal liquefaction (HTL) at 280 °C and a reaction time of 30 min. The introduction of acidic microporous zeolite catalysts (HZSM-22, HZSM-5, H beta, MCM-22, and SAPO-11) with high hydrothermal stability further improved the bio-oil quality in situ. Various methods, including elemental analysis, high heat value (HHV), and gas chromatography (GC)-mass spectrometry (MS), were used to analyze the physicochemical properties of the obtained bio-oil. Results indicated that catalyst addition could enhance C and H contents, reduce O and N contents, and also improve HHV (the maximum value of 37.08 MJ kg⁻¹ was obtained for the H beta catalyst). GC-MS revealed that the bio-oil obtained by direct HTL contained relatively high amounts of N-containing compounds (30.87%) and acid compounds (16.44%). Meanwhile, the catalysts introduced in situ not only lowered the contents of nitrogen and acids to some extent, but also simultaneously increased the hydrocarbon content. This result was most pronounced over the H beta catalyst, which reduced the nitrogen content to 16.68% and decreased the acid content to 9.50%. The hydrocarbon content increased to 43.43%. Ultimately, a reasonable reaction network for Euglena sp. HTL was proposed and provides a basis for the process's further industrialization.