A comparative investigation of non-catalysed versus catalysed microwave-assisted hydrolysis of common North and South European seaweeds to produce biochemicals

Using a circular economy approach, this study investigated microwave assisted non-catalysed (water only), acid catalysed (H2SO4), and metal cation catalysed (PbII and AlIII) hydrolysis of brown and red seaweed biomass prior to and following extraction of high value products. Results show a wide variety of organic acid products is attained following each hydrolytic method, which are heavily depending on initial quantity and type of carbohydrates in seaweed as well as process conditions, such microwave reactor temperature and catalyst properties. The best results were achieved with extracted Gracilaria gracilis residues, which resulted in 20.1% w.t. levulinic acid, produced at 180 °C after 10 min with an overall carbohydrates conversion rate over 95%. Metal ion catalysed hydrolysis of the same extracted seaweed also yielded interesting quantities of lactic acid (5% w.t. at 240 °C), 5-HMF (6% w.t. at 220 °C) with Pb(II) and propionic acid (5% w.t. at 220–240 °C) with Al(III), bringing seaweed residues closer to full bioresource valorisation.