Catalytic upgrading of bio-oil in hydrothermal liquefaction of algae major model components over liquid acids

We selected acetic acid and sulfuric acid as liquid acid catalysts to investigate their performance on hydrothermal liquefaction (HTL) of algae major model components, including polysaccharides, proteins, lipids, and polysaccharides-proteins mixture. The aim of this study was to understand the catalytic effects of liquid acid catalysts as well as the formation process mechanisms and properties of the bio-oils from HTL of algae. We also used the two acids in HTL process of Chlorella vulgaris to estimate their catalytic performances in algae and compare this performance with the results from HTL of model components. Product distributions from different catalytic conditions were compared with blank results. Bio-oils were analyzed by elemental analysis, gas chromatography-mass spectrometry (GC-MS), and thermal gravimetric analysis (TGA). The results showed that the addition of acetic acid or sulfuric acid had no positive effect on the enhancement of bio-oil yield from HTL of individual algae major model components; however, the use of liquid acid catalysts could prevent the detrimental effect of interaction between polysaccharides and proteins on bio-oil yield. H/C ratios and higher heating values (HHVs) of bio-oils obtained from HTL of algae major model components increased significantly in the presence of acid catalysts. Sulfuric acid favored the oxygen removal process, and acetic acid reduced the nitrogen content in bio-oil from HTL of proteins. GC-MS results showed that bio-oil composition was greatly altered when adding acids. The light component proportion of catalytic upgraded bio-oils was significantly higher than that of bio-oils obtained with no catalyst. The results of HTL of Chlorella vulgaris with acid catalysts indicate that both acids favor to the hydrolysis of cellulose; therefore, the use of acid catalysts could accelerate the formation of bio-oil from cellular components. The difference between the HTL of algae major model components and real algae using acid catalysts was probably due to the complexity of algae structure.