Valorization of the Exoskeletons of Crustaceans in Seafood Wastes to Chemicals in Renewable Solvents: A Catalytic and Mechanistic Study

Levulinic acid (LA) and γ-valerolactone (GVL) are considered valuable platform chemicals that can be derived from various types of biomass ranging from food wastes to agricultural residues. Herein, the valorization of the exoskeletons of crustaceans in seafood wastes into LA, GVL, acetic acid (AA), and ammonium (NH₄)⁺ was studied including the catalytic and mechanistic aspects. Chitin was used as a model compound to optimize the conditions for converting the exoskeletons of crustaceans in seafood wastes using acetic acid (AA) and GVL as bio-originated renewable solvents. The same conditions were applied to convert various pretreated seafood wastes, such as the exoskeletons of crabs and lobsters. The decalcification of the crustacean samples using phosphoric acid was also studied. GVL was also used as a solvent to produce formic acid (FA), LA, NH₄⁺, and GVL to simplify the product purification process. The reaction mixture of chitin (0.41 g, equivalent to 2 mmol of N-acetyl-glucosamine) in a mixture of 10 mL of GVL and 1.5 mL of 5 M H₂SO₄ was heated at 150 °C for 4 h followed by neutralization with additional NH₄⁺ (NH₄OH) to result in two phases due to the salting out effect of (NH₄)₂SO₄. Ru-based Shvo’s catalyst was then added to the organic phase for transfer hydrogenation of LA with FA as the hydrogen donor to yield GVL. Uniformly labeled N-acetyl-[¹³C₆]glucosamine (UL-¹³C₆-NAG) was used to confirm the formation of ¹³C₅-GVL in ¹²C₅-GVL via ¹³C₅-LA and ¹³C-FA. Detailed in situ NMR studies revealed the presence of two bicyclic compounds, protonated salt of 1,6-anhydro-2-deoxy-2-ammonio-glucopyranose (AGluNPH⁺) and 1,6-anhydro-2-deoxy-2-ammonio-glucofuranose (AGluNFH⁺), as proposed key intermediates of the of UL-¹³C₆-NAG conversion.