Research On Structure And Pyrolytic Characteristics Of Organosolv Lignin

Biomass is an important renewable resource,which can be converted into green fuels or platform compounds through certain technologies.It is one of the main guarantees to achieve 'carbon neutralization' and 'carbon peak',and its development and utilization has received extensive attentions.Lignin is one of the three major components of biomass,and is also the main by-product of papermaking and biomass ethanol industry.A large number of lignin has not been fully utilized,which wastes resources and pollutes the environment.It is urgent to develop high-value utilization technologies of lignin.Among the existing lignin utilization methods,pyrolysis is considered to be one of the most promising methods,which can convert lignin into high value-added chemicals such as phenols and aromatic hydrocarbons.However,due to the complex chemical structure of lignin,its pyrolysis behavior is extremely complicated and its process is difficult to control.In order to develop efficient lignin pyrolytic technology,it is necessary to have a clear understanding of the chemical structure and pyrolytic characteristics of lignin as well as the correlation between them.Therefore,organosolv lignins were extracted from different biomass using 1,4-dioxane,and the relationship between chemical structure and pyrolytic characteristics of organosolv lignin was studied by multiple characterization methods.1.Structures and pyrolytic characteristics of organosolv lignins from different speciesThree typical lignins from pine(softwood),poplar(hardwood)and bamboo(herbaceous biomass)were isolated using organosolv 1,4-dioxane.Their chemical structures and the pyrolytic characteristics were comprehensively investigated via Fourier transform infrared spectroscopy(FT-IR),two-dimensional-heteronuclear single-quantum correlation nuclear magnetic resonance(2D HSQC NMR),gel permeation chromatography(GPC),in-situ diffuse reflectance infrared Fourier transform spectroscopy(in-situ DRIFT),thermogravimetry-Fourier transform infrared spectroscopy(TG-FTIR)and pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS).The results indicated that pine lignin possessed the highest content of guaiacyl(G)units(99.4%),the least ?-O-4' aryl ether linkages(46.0%)and the best thermal-stability.A high selectivity of 77.26%for guaiacol-type products from pine lignin was obtained at 400?.Bamboo lignin had the most uniform molecular distribution and the most p-coumarate(pCA)units of 40.3%.The dominant pyrolytic product of bamboo lignin was 4-vinyl phenol(4-VP)with a high content up to 32.64%at 500?.48.2%p-hydroxybenzoates(PB)and abundant syringyl(S)units existed in poplar lignin,and phenol was the predominant pyrolytic product with the maximum content of 23.19%at 600?.2.Structures and pyrolytic characteristics of organosolv lignins from herbaceous biomassFour herbaceous lignins from corn stalk,bamboo,triarrhena lutarioriparia,sorghum bicolor stalk were extracted using organosolv 1,4-dioxane,and NMR,GPC,TG-FTIR and Py-GC/MS methods were adopted to research their chemical structures and pyrolytic characteristics thoroughly.The results showed that four herbaceous lignins enriched pCA units and revealed similar molecular fragment distribution.The main differences were that corn stalk lignin contained the most S units(80.7%)and bamboo lignin contained the highest oxidized syringyl units bearing a carboxyl group at C?(S')units(17.2%)among four lignins.Compared with other lignins,bamboo lignin had a minimum charcoal residue yield during the pyrolytic process.4-VP was the most abundant product in the pyrolytic products of four herbaceous lignins,and its content from corn stalk lignin was the highest,while that from bamboo lignin was the lowest.In addition,the contents of 4-VP in the pyrolytic products from four lignins evolved differently as temperatures rising:the content of 4-VP in bamboo lignin reached the maximum of 26.37%at 300?,while that in corn stalk,triarrhena lutarioriparia and sorghum bicolor stalk lignins reached the maximum of 43.90%,40.91%and 35.97%at 500?,respectively.