The Conversion Of Chitosan And Chitin To Levulinic Acid Catalyzed By Ionic Liquids

In this paper,acidic ionic liquids were used to hydrolyze chitosan and chitin to produce levulinic acid?LA?with high selectivity under hydrothermal condition,which provided methodological support to the treatment of shrimp and crab crust biomass.Under hydrothermal conditions,LA was highly selectively produced by catalytic conversion of chitosan using acidic ionic liquid[C₃SO₃Hmim]HSO₄.By optimizing the reaction conditions,the best reaction condition was 170°C,1 g of ionic liquid,4 g of water,50 mg of chitosan,and the highest yield of LA was 64 mol%.Through SEM and FT-IR characterizations,the solid residues during the reaction course were studied in detail.It was found that the reaction went through two stages.The first stage was the depolymerization process of chitosan,and the second stage was glucosamine conversion to LA.A total of six acidic ionic liquids were synthesized and used in the reaction.Through the experimental results,it was concluded that the reaction is related to the acidity of the ionic liquid.The stronger the acidity is,the higher the yield of LA.During the recycle process of the ionic liquid,NH₃ was generated during the reaction due to amino groups in the raw material,therefore the acidity of the ionic liquid was reduced,resulting in a decrease of LA yield.When 0.16 g of sulfuric acid was added to the ionic liquid during the cycle,the LA yield of the reaction could be maintained at 49 mol%,and the ionic liquid could be reused.Under hydrothermal conditions,LA was highly selectively produced by the catalytic conversion of chitin using acidic ionic liquid[C₃SO₃Hmim]HSO₄.By optimizing the reaction conditions,the best reaction condition of the conversion was 180°C,1 g of ionic liquid,6 g of water,50 mg of chitin,and the highest yield of LA was 67 mol%.Six types of acidic ionic liquids were synthesized and used in the reaction.The experimental results showed that the reaction efficiency was related to the acidity of the ionic liquid and the structure of the ionic liquid anion.The more acidic the ionic liquid is,the higher the yield of LA.When the anion of the ionic liquid is Cl^-,it could form hydrogen bonds with the hydroxyl groups in chitin,so that the chitin fibers become loose,which facilitates the depolymerization reaction,so that improving the reaction yield to 54 mol%.When the anion of the ionic liquid contains benzene structures,it could also loosen the chitin fiber structure,thereby making the reaction easier to proceed.For example,[C₃SO₃Hmim]1-NS with lower acidity than[C₃SO₃Hmim]CH₃SO₃,could give 49 mol%of LA surprisingly.During the recycle of the ionic liquid,since the acetamino group in the raw material generates NH₃ during the reaction,the acidity of the ionic liquid will decrease,resulting in a decrease in the yield of LA.When 0.30 g of sulfuric acid?50%?was added to the ionic liquid during the cycle,the LA yield of the reaction could be maintained at 56%,and the ionic liquid could be reused.