Efficient Conversions of Macroalgae-Derived Anhydrosugars to 5‑Hydroxymethylfurfural and Levulinic Acid: The Remarkable Case of 3,6-Anhydro‑d‑galactose

Macroalgae or seaweed is considered a renewable and sustainable resource to produce biobased fuels, polymers, and chemicals due to its high content of polysaccharides. Various studies have reported the obtained 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) from seaweeds. However, the source of the saccharides that is responsible for HMF formation, accurate yield data (often only HMF concentrations are given instead of yields on feed), and the reaction pathways (including byproducts) is not well understood. We here report a kinetic study on the conversion of 3,6-anhydro-d-galactose (D-AHG), one of the main building blocks of the polysaccharides in seaweed, to HMF and LA in water using sulfuric acid as a catalyst with the aim to rationalize and optimize the production of HMF and LA from seaweeds. The experiments were carried out in batch at temperatures between 160 and 200 °C using various initial concentrations of D-AHG (0.006–0.06 M) and sulfuric acid (0.0025–0.05 M) as the catalyst. The highest experimental yield of HMF within this range of experimental conditions was remarkably high (61 mol %) and obtained at 160 °C, with a low initial D-AHG concentration (0.006 M) and a low acid concentration (0.0025 M). These findings imply that D-AHG is a very good precursor for the HMF synthesis. Additional experiments outside the experimental window gave an even higher HMF yield of 67 mol %. The highest LA yields were 51 mol % [160 °C, low initial D-AHG concentration (0.006 M), and high acid concentration (0.05 M)]. The experimental data were modeled using a power law approach, and the kinetic model was used to determine reactor configurations giving the maximum yield of HMF and LA. The result showed that a plug flow reactor is favorable to achieve the highest yield of HMF, whereas a continuously ideally stirred tank reactor is the preferable reactor configuration to obtain the highest yield of LA