Liquefaction of Macroalgae Enteromorpha prolifera in Sub-/Supercritical Alcohols: Direct Production of Ester Compounds

The liquefaction of “green tide” macroalgae Enteromorpha prolifera in sub-/supercritical alcohols in a batch reactor had been investigated. Effects of the temperature and algae/solvent ratio on the liquefaction yields in methanol and ethanol were studied. The results showed that, under the conditions of the reaction time of 15 min and algae/solvent ratio set at 1:10, the macroalgae in methanol at 280 °C produced a bio-oil yield at 31.1 wt % of dry weight and the ethanol at 300 °C yielded bio-oil at 35.3 wt %. Different from bio-oils obtained by hydrothermal liquefaction of microalgae as well as macroalgae in our previous work, the bio-oils obtained by liquefaction of macroalgae in alcohols are mainly composed of ester compounds. A variety of fatty acid (C 3 –C 22 ) esters (methyl or ethyl) in the bio-oils obtained in methanol and ethanol, respectively, were qualified by gas chromatography–mass spectrometry, and their relative contents are above 60% of the total area for each bio-oil. In addition, some N-containing compounds, sugars, fatty alcohols/ketones, and very few hydrocarbons were also qualified. Overall, bio-oils obtained in two alcohols are much similar to biodiesel on the composition. The elemental analysis of bio-oils indicated that bio-oils still have high oxygen contents. Moreover, the bio-oils are found to contain a considerable fraction of light components using thermogravimetric analysis (TGA), and the contents of low-boiling-point (bp < 350 °C) compounds are up to 70% of the weight for both bio-oils; therefore, it might help for the further separation and refining of bio-oils to produce fuels and chemicals.