Strain-Dependent Catalytic Hydrotreatment of Chlorella Microalgae HTL Biocrudes Assessed by FT-ICR Mass Spectrometry

Two microalgae strains, Chlorella sorokiniana and Chlorella vulgaris, were subject to hydrothermal liquefaction (HTL). The resulting HTL biocrudes were then upgraded via hydrotreatment (HDT), using a sulfided NiW/Al2O3 catalyst, with samples collected after 0, 2.5, 5, and 8 h of HDT reaction. The heavy fractions from these HTL biocrudes and HDT bio-oils were subsequently characterized by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). These analyses evidenced thousands of distinct molecular formulas, corresponding to CHON, CHN, CHO, and CH compound classes. The lipids present in microalgae were converted by HTL into long-chain acids and CHN, CHO, and CHNO compounds. A large amount of alkanes and aromatics was obtained through deoxygenation and denitrogenation reactions during HDT. However, a significant population of refractory species was identified, mainly in C. sorokiniana HDT oils. This strain is indeed defined by its higher content of carbohydrates, compared to C. vulgaris, which yields Maillard reaction products, along with algal proteins. These high-mass species comprising HDT bio-oils were therefore better characterized by FT-ICR MS than by gas chromatography (GC). Furthermore, analysis of the bio-oils obtained after a short HDT time exhibited the lowest unsaturation level, likely due to the reaction between the fatty chains and amino acids. In summary, the FT-ICR MS approach combined with multivariate statistical analyses proved to be highly complementary to GC analyses. It enabled to decipher the molecular complexity of the heavy fraction of the HTL biocrudes and HDT bio-oils, clarified the influence of the microalgal strain on the HDT efficiency, and provided meaningful insight into the reactions occurring during the early stages of the upgrading process.