The Study Of The Catalytic Conversion Of Chitin Biomass Into Furan Derivatives

With the increasing environmental pollution and the diminishing of petrochemical resources,biomass as a green renewable resource,has attracted more and more attention.Chitin,as a marine biomass,is the second most abundant class of biomass resource only next to cellulose,and it is a natural nitrogen-containing carbohydrate polymer on earth.The conversion of chitin biomass to oxygen-containing platform compounds or nitrogen-containing fine chemicals is an effective way to utilize such biomass.Exploring the reaction mechanism and pathway of the biomass conversion and the interactions between the substrates is the key to find the appropriate catalytic system based on the characteristics of the biomass in the biological refining process.In this paper,we investigated some scientific problems about the interaction relationship between catalysts and substrates and the catalytic reaction mechanism of the preparing oxygen-containing platform compound 5-hydroxymethylfurfural?5-HMF?and nitrogen-containing fine chemical 3-acetamido-5-acetylfuran?3A5AF?from the monomer of chitin biomass,D-glucosamine?GlcNH2?and N-acetyl-D-glucosamine?GlcNAc?.Firstly,the effects of the addition of co-catalyst Lewis acid tin chloride?SnCl4?on the yiled of 5-HMF for the conversion of GlcNH2 to 5-HMF in 65 wt%zinc chloride?ZnCl₂?aqueous solution was studied.The strong interaction between SnCl4and GlcNH2 in 65 wt%ZnCl₂ aqueous solution was explored.Likewise,the effects of SnCl4 on the conversion of Glucose to 5-HMF under the same conditions was examined by comparing the conversion of GlcNH2 in 65 wt%ZnCl₂ aqueous solution.In addition,the liquid nuclear magnetic resonance?NMR?techniques were employed to explain this interaction between the amino group of GlcNH2 and SnCl4 at molecular level for the first time,which changed the reactivity of substrate.Secondly,both the neutral ionic liquid 1-allyl-3-methylimidazolium chloride?[AMim]Cl?and boric acid?B?OH?3?were used as catalysts for the preparation of3A5AF from GlcNAc.The effects of catalysts[AMim]Cl or B?OH?3 on the yield of3A5AF were investigated,respectively.Additionally,the interaction between[AMim]Cl or B?OH?3 and substrate GlcNAc was systematically studied via NMR methods.The ¹³C isotope labeled N-acetyl-D-glucosamine([¹³C-1]GlcNAc)was utilized as substrate to monitor the reaction process by in situ NMR,and the signals of intermediates.At the same time,one-dimensional?1D?and two-dimensional?2D?NMR techniques as well as mass spectrometry were employed to identify the structure of the reaction intermediates.The proposed reaction pathway on the basis of intermediates was also further illustrated by density functional theory?DFT?calculation.According to the above research,the main results could be obtained as follows:?1?In the 65 wt%ZnCl₂ aqueous solution,the addition of SnCl4 inhibited the conversion of GlcNH2 at the beginning of the reaction when the SnCl4 was 0.1equivalent GlcNH2.As the reaction time increased,the conversion of GlcNH2 was promoted instead.When the amount of SnCl4 was increased to 0.5 equivalent GlcNH2,the conversion of GlcNH2 to 5-HMF was completely inhibited.The reason for this results is that Zn²⁺,which is incompletely coordinated in 65 wt%ZnCl₂ aqueous solution,forms a complex with GlcNH2 to promote this reaction.While the addition of SnCl4 replaces the coordination of ZnCl₂ and form a stronger and more stable complex,thus the conversion of GlcNH2 was hindered.?2?The catalysts[AMim]Cl and B?OH?3,especially[AMim]Cl,can promote the conversion of GlcNAc to 3A5AF.Moreover,the synergic catalytic effect between these two catalysts can promote this reaction more efficiently.The NMR titration experiments results indicated that Cl^-ions play the most important role in the interaction between[AMim]Cl and GlcNAc,and B?OH?₃ promoted the isomerization of GlcNAc from?to?,and form a complex with GlcNAc to facilitate this reaction in the solution with[AMim]Cl.?3?The results of in situ NMR experiments of tracing[¹³C-1]GlcNAc showed that there were stable intermediates appeared during the conversion of GlcNAc to 3A5AF.A variety of 1D/2D NMR spectra and mass spectrometry confirmed that the intermediates were mainly composed of the Chromogen I and the Chromogen III.On the basis of these conclusions,we proposed two reaction pathways for GlcNAc into3A5AF,and illustrated the most suitable pathway under our experimental conditions by DFT calculations.