Study On Isolation Of Willow Lignin By Deep Eutectic Solvent

The separation of relatively well-structured lignin molecules by suitable isolation method is a major problem to be solved for the utilization of lignin.Deep eutectic solvents?DESs?are solvents that are synthesized in liquid state at room temperature by a quaternary ammonium salt and hydrogen bond donors.The deep eutectic solvents has the advantages of simple preparation process,low costs,environment friendly and easy degradation,etc.Moreover,it also has similar dissolution properties as conventional ionic liquids.It has important significance as a substitute for traditional ionic liquids for the isolation and extraction of lignin.In this thesis,the deep eutectic solvents with low cost,environmental friendliness and simple preparation process are taken as the object of study.The types of deep eutectic solvents,the mole ratio of choline chloride to hydrogen bond donors,the treatment temperature and treatment time on the yield of lignin separation were studied.Meanwhile,the effect of deep eutectic solvents on the lignin components were also investigated.Fourier infrared spectroscopy?FT-IR?,carbon nuclear magnetic resonance spectroscopy(¹³C-NMR),phosphor nuclear magnetic resonance spectroscopy(³¹P-NMR)and two-dimensional nuclear magnetic resonance spectroscopy?2D-HSQC?were used to analyze the structure of lignin.The characterization of the lignin was carried out by means of thermogravimetric analysis?TGA?and pyrolysis-gas chromatography/mass spectrometry?Py-GC/MS?.The optimal conditions for isolation of lignin by deep eutectic solvent were:the molar ratio of choline chloride to lactic acid was 1:10,the treatment temperature and time were 120°C and 12 hours,respectively.Under this condition,the yield of crude lignin reached 91.8%,the purity of lignin reached 94.5%.Based on the acid hydrolysis monosaccharides of raw materials,82.1%of glucose and 10.77%of xylose were not dissolved by the deep eutectic solvent,while arabinose,mannose,and galactose were not detected in the acid hydrolysate of the solid residue.Only 6.2%of the lignin remained in the solid residue.With the increased treatment time of deep eutectic solvent,the yield of lignin increased slowly,and more carbohydrates were dissolved in the solvent.The purity of lignin extracted under different treatment conditions were higher than 90%.The weight-average molecular weight and number-average molecular weight of lignin gradually decreased with the extension of treatment time.The longer the time,the narrower the molecular weight interval and the more uniform the distribution,the lower the polydispersity coefficient from 1.3402 to 1.1329.The number average/weight average molecular weight of the lignin extracted under the optimum isolation conditions were1071.1 g·mol^-1/1261.1 g·mol^-1,and the polydispersity coefficient was 1.1774.the particle size of the extracted lignin was 494.2 nm.Elemental analysis showed that lignin was mainly composed of four elements:C,H,N,and O.With the increased treatment time of the deep eutectic solvent,the H content in the lignin gradually increased,and the H/C molar ratio increased from 0.86 to 1.42.Fourier infrared spectroscopy analysis of raw material,solid residue and lignin extracted from willow showed that most of the lignin was removed after treatment of willow raw materials by deep eutectic solvent,and almost no carbohydrates were present in the lignin sample.Combined analysis of ¹³C-NMR and ³¹P-NMR spectrum showed that lignin extracted by deep eutectic solvent was mainly composed of guaiacyl and syringyl structural units,and the total phenolic hydroxyl group content and total hydroxyl group content were 0.915 and 2.06 mmol·g-1,respectively.The 2D-HSQC characterization results showed that the?-O-4 associated with guaiacyl and syringyl structures disappeared in lignin.The degradation of lignin extracted by deep eutectic solvent can be classified as three stages:the water loss at the stage of 120¹85°C;lignin molecular depolymerization at the stage of 200³00°C and lignin degradation at the stage of300⁶00°C.The DTG loss-derivative peaks of lignin with stronger isolation conditions at this stage of 300⁶00°C were smaller.The final remaining residue accounts for 40%of the total weight of the raw lignin samples.The contents of G-phenols?Guaiacyl type-phenols?and S-phenols?Syringyl type phenols?in lignin pyrolysis products extracted from deep eutectic solvents at 120°C for 12 h reached their respective highest at 450°C and 550°C,which were 47.14%and 32.36%,respectively.H-phenols?P-hydroxyphenyl type-phenols?was mainly composed of phenol,and it starts to appear in large quantities from 450°C,and it was relatively stable with increasing temperature.At650°C and 800°C,S-phenols and H-phenols successively occupied the highest values in G:S:H,and at 800°C,27.22%of non-phenolic aromatic compounds such as benzene,toluene,etc.occured in large amounts.The distribution of pyrolysis products of lignin extracted by deep eutectic solvents under different conditions was different.When the extraction time was higher than 12 h,the pyrolysis products of the extracted lignin produced a large amount of G-phenols,while S-phenol and H-Phenols decreased with the extraction time.Phenol as the main component of H-phenols was gradually reduced with the extension of the lignin extraction time,when the extraction time was higher than 24h,S-phenols mainly consists of syringaldehyde.