Study On Structures And Phenolation Modification Of γ-Valerolactone Lignin

Lignocellulosic biomass is the most abundant renewable resource in the world.In recent years,in order to make better use of this material,many new biomass pretreatment or biomass refining methods have been proposed.These new technologies are different from the traditional chemical pulping methods,and have good application prospects in the extraction and conversion of cellulose and hemicellulose.γ-Valerolactone（GVL）is a kind of renewable natural compound.Even at high temperature,the pressure produced by GVL is relatively low.In recent years,it has been applied to biomass pretreatment and achieved good results.However,there are few reports about the structure changes or reaction mechanisms of lignins in GVL pretreatment processes.Lignin is one of main components of lignocellulosic biomass,the study on the structural changes of lignin during GVL/H₂SO₄/water pretreatment system（lignin obtained was abbreviated as GVL lignin）is beneficial to optimization of pretreatment process and study on the further application of GVL lignin.In this work,the structural changes of lignin model compounds,wood biomass and isolated cellulolytic enzyme lignin（CEL）in the pretreatment of GVL/H₂SO₄/water system were investigated.The obtained GVL lignins were phenolated,and the structural changes of GVL lignins during phenolation processes were also studied,in order to provide some basic research support to the application of GVL lignins.Firstly,the structural changes of lignin model compounds（β-O-4,β-5 andβ-β）in GVL/H₂SO₄/water pretreatment system were studied.The synthesizedβ-O-4,β-5 andβ-βlignin model compounds were used as the starting material,the reaction product mixtures were fractionated by silica-gel liquid chromatography and TLC（thin layer chromatography）.The products were characterized by ¹H,¹³C,and two-dimension NMR（2D NMR）experiments and Gas Chromatography-Mass Spectrometrometry（GC-MS）.Some compounds were synthesized in order to further confirm the structures of the products.The main products produced from theβ-O-4 model compound were 1,3-dioxane structures,phenyl-dihydrobenzofuran,enol ether,diphenylmethane structures,guaiacol,Hibbert’s Ketones,homovanillin and Cαcondensation trimer.The quantitative analysis of the main products ofβ-O-4 were carried out.Theβ-5 lignin model compounds（with saturated side chain or unsaturated side chain）produced coumarin and stilbene products in GVL/H₂SO₄/water pretreatment system.The unsaturated side chain ofβ-5 was converted to a benzaldehyde group,and the saturated side chain ofβ-5 was kept intact.The major compounds detected fromβ-βmodel compound syringaresinol were isomers of syringaresinol（epi-syringaresinol and dia-syringaresinol）,which were slowly degraded or condensed,in the GVL/H₂SO₄/water pretreatment system.Secondly,the structural changes of different lignins materials in GVL/H₂SO₄/water pretreatment system were studied.In this work,the structural changes of pinus khasys powder,pinus khasys CEL and Eucalyptus CEL under GVL/H₂SO₄/water system were studied using2D HSQC（two-dimension heteronuclear single quantum coherence）NMR spectra.It was found that the structures of GVL lignins include Hibbert’s Ketones,1,3-dioxane structures,phenyl-dihydrobenzofuran,enol ether,trans stilbene structure SB5 which obtained fromβ-5and trans stilbene structure SB1 which obtained fromβ-1.At the same time,it was confirmed that theβ-βstructure of lignin would be isomerized after GVL treatment.The structures of GVL lignins in the presence or absence of H₂SO₄were compared,the results showed that the structures of GVL lignins without H₂SO₄were closer to the original lignins structures,but the GVL lignins yields were very low.Furthermore,the structural changes of enol ether,phenolic and nonphenolicβ-O-4 with or withoutγ-CH₂OH,β-O-4 withγ-CD₂OH andβ-βlignin model compounds treated with H₂SO₄/phenol were studied.The results mentioned above showed that there was a certain amount of enol ether in GVL lignins,the phenolation of enol ether was studied in order to investigated the phenolation of GVL lignins.It was found that main phenolated products of enol ether were Cαcondensation trimer and triphenyl ethane structures.Main phenolated products ofβ-O-4 were also Cαcondensation trimer and triphenyl ethane structures.In addition,β-O-4 withγ-CH₂OH produced diphenylmethane structures.It was proved that diphenylmethane structures main obtained from the condensation reaction of formaldehyde with phenol and guaiacol usingβ-O-4 withγ-CD₂OH lignin model compound.Based on results mentioned above,the phenolation pathways ofβ-O-4 were proposed.The main phenolated products of syringaresinol（β-βlignin model compound）include2,6-dimethoxyphenol,1,3-dimethoxy-2-naphthol,diphenylmethanes,naphthols with a methylene（-CH₂-）group connecting to one phenol,annulated homosyringaldehyde and naphthols with two methylene（-CH₂-）group each connecting to one phenol.The GG typeβ-βlignin model compound also produced the products similar to syringaresinol,even though the products yields were low.And then the phenolation pathways ofβ-βlignin model compounds were also proposed.Finally,the structural changes of pinus khasys GVL lignin,alkali lignin and CEL lignin treated with H₂SO₄/phenol system were studied.The 2D HSQC NMR spectra of phenolated GVL lignins were obtained,compared with that of phenolated products obtained from lignin model compounds mentioned above,it was found that Cαcondensation trimer and triphenyl ethane structures were produced.A certain amount of naphthols with a methylene（-CH₂-）group connecting to one phenol,and naphthols with two methylene（-CH₂-）group each connecting to one phenol were generated in the phenolated Eucalyptus alkali lignin.The phenolation rates of GVL lignins were higher than that of alkali lignins.It was also found that the phenolation degrees of three kinds of lignins were as follows:CEL>GVL lignin>alkali lignin.Molecular weights of phenolated lignins were decreased rapidly after 30 min,and then molecular weights of lignins were decreased in minor speed with reaction time.The polydispersity of phenolated lignins changed small compared with lignins raw materials.The core of this work were the study on pretreatment and phenolation of lignins.In this work,the structures of GVL lignins and phenolated lignins were characterized using lignocellulosic biomass and lignins as raw materials based on the pretreatment and phenolation of lignin model compounds.The work provides useful information to further understand the degradation,derivatization and condensation reaction pathways of lignins during GVL pretreatment and phenolation process helping to develop better processes for utilizing the GVL lignins.