Research On The Separation And Modification Of Lignin Based On Deep Eutectic Solvents For Synthesis Of Phenolic Resins

Lignocellulose is becoming favored as a renewable energy because of the increase of energy crisis and environment pollution problem in this century.Lignin,one of the main components in lignocellulose,as the only natural polymer which contains aromatic structure,has the great potential for utilization.If lignin can be separated preferentially and exploited further,the added value of biomass resources will be enhanced greatly.However,common methods cann't dissolve lignin effiently because of its complex structure.So Deep Eutectic Solvents?DESs?were used to pretreat eucalyptus urophylla in this study,and dissolution rates of lignin in different DESs were investigated.Then another DES was used in lignin modification.After that,modified lignin was used to substitute phenol partly to synthesize phenol-furfural resins?PF?.This process not only could achive the selective separation of lignin in biomass,but also enhance the activity in DES,beneficial to synthesis PF,which could make full use of lignin.Different kinds of deep eutectic solvents were synthesized with different quaternary ammonium salts,methyl tributyl ammonium chloride,tetramethylammonium chloride,benzyl triethyl ammonium chloride and choline chloride,and DL-lactic acid with molar ratio of 1:9,respectively,and the changes of their viscosity and Hildebrand solubility parameters???with temperature were investigated,and then used to dissolve the lignin in eucalyptus.It was found that deep eutectic solvents of benzyl triethyl ammonium chloride/DL-lactic acid?TEBA/HL?had best solubility selectivity,as the dissolution ratio of lignin was 92.3% after 10 h under 90°C and the dissolution ratio of total cellulose was only 8.3%,meanwhile,the specific value?K?of solubility selectivity coefficient between lignin and total cellulose reached to 158.5,while the Ks of MTBAC/HL,TMAC/HL and ChCl/HL were 113.8,16.8 and 49.7,respectively,suggesting the best solubility selectivity for lignin of TEBA/HL.FT-IR spectrum analysis showed that the structure of the extraction materials contained characteristic functional groups of lignin,and GPC analysis suggested that the molecular weights of the extraction materials were all reduced.CP/MAS¹³ C NMR analysis suggested that structure characteristics of the extraction materials from TEBA/HL at 90°C were close to lignin of hardwoods.It is found that oxalic acid/choline chloride DES had the best lignin modification efficiency comparing with DESs combined with other organic acids and choline chloride And the modification conditions were determined at 100°C and 6 h,with a molar ratio of oxalic acid to choline chloride 1:1,and 1:10 of solid to liquid ratio,when the content of phenolic –OH of lignin modified increased from 1.80 mmol·g^-1 to 3.15 mmol·g^-1 and the content of –OCH3 decreased from 10.77 wt% to 8.51 wt%,meanning that eucalyptus urophylla lignin's reactivity was enhanced.Then the modified eucalyptus urophylla lignin was partially substituting for phenol under various substitution rates in phenol-furfural resins?PF?synthesis,which compared with PF synthesized by modified enzymatic hydrolysis lignin,and it was found that when the substitutiion rate of lignin to phenol was 50%,the bond strength?1.92 MPa,higher than PF with modified enzymatic hydrolysis lignin?,free phenol content and solid content of PF prepared all reached to Chinese National Standards and exhibited high curing rate and cross-linking degree.