Selective transformation of fructose and high fructose content biomass into lactic acid in supercritical water

• Fructose was converted into pyruvaldehyde with a yield of 89% in supercritical water.
• Aldoses produce C-2 molecules while ketoses produce C-3 molecules in SCW reactions.
• Natural biomass was transformed into lactic acid in a SCW medium.
• Sodium hydroxide was a promising catalyst for the production of lactic acid.

The reactions of fructose in sub- and supercritical water were analyzed changing the chemical properties of the reaction medium (Kw, ɛ, pH and free radical kidnapers). The reactions were performed in a continuous reactor at 260 °C, 330 °C and 400 °C, at 23 MPa and 27 MPa using water as reaction medium. The pH of the medium was modified using oxalic acid and sodium hydroxide. Also, scavengers (TEMPO and BHT) were tested in order to determine its influences in the radical reactions. The main product of fructose hydrolysis in supercritical water was pyruvaldehyde (>80%, w·w−1) at 400 °C and 23 MPa with a reaction time of 0.7 s. Furthermore, the reactions of fructose were analyzed in combination with glucose. It was determined that different retro-aldol condensation products can be obtained depending on the starting material. Fructose produced mainly C-3 molecules (pyruvaldehyde) and glucose produced mainly C-2 molecules (glycolaldehyde). The isomerization of fructose to glucose was negligible and so was the production of C-2 when the starting material was fructose. The yield of 5-HMF was negligible when the starting material was glucose. Three different biomass enriched in fructose (sugar cane molasses, beet molasses and inulin) were tested as starting material for the hydrolysis process. It was determined that lactic acid (50%, w·w−1) was the main product of molasses and inulin hydrolysis in a supercritical water medium modified with basic catalysts.

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