Effective conversion of the carbohydrate-rich macroalgae (Saccharina japonica) into bio-oil using low-temperature supercritical methanol

The use of supercritical methanol (scMeOH) for the liquefaction of the carbohydrate-rich macroalgae Saccharina japonica was investigated at low temperature (250–300 °C). At 300 °C, almost complete conversion (98.1 wt%) and a high bio-oil yield (66.0 wt%) were achieved. These values are higher than those achieved with supercritical ethanol (scEtOH, 87.8 wt% conversion, 60.5 wt% bio-oil yield) and subcritical water (subH₂O, 91.9 wt% conversion, 40.3 wt% bio-oil yield) under identical reaction conditions. The superior liquefaction in scMeOH is attributed to the beneficial physical properties of scMeOH, including its higher polarity, superior reactivity, and higher acidity. The superior reactivity of scMeOH was evident from the larger amount of esters (54.6 area%) produced in scMeOH as compared to that in scEtOH (47.2 area%), and the larger amount of methyl/methoxy-containing compounds (78.6 area%) produced in scMeOH than that of ethyl/ethoxy-containing compounds (58.2 area%) produced in scEtOH. The higher bio-oil yield combined with its higher calorific value (29.2 MJ kg⁻¹) resulted in a higher energy recovery of 135% for scMeOH as compared to those of scEtOH (118%) and subH₂O (96%). When considering the amount of alcohol consumed during the liquefactions and the production of light bio-oil fractions that evaporate during bio-oil recovery, the higher methanol consumption (5.3 wt%) than that of ethanol (2.3 wt%) leads to similar bio-oil yields (∼51 wt%).