Study On Thermochemical Conversion Of Technical Lignin To Aromatic Chemicals

The alternative green and renewable resources are imminently needed with the consumption of fossil energy and serious environmental pollution.Biomass has attracted attention because of its abundant reserves and renewability.Lignin,one of the three major components of lignocellulosic biomass,is a renewable resource with rich aromatic units.It can be transformed into high value-added chemicals and fuels to replace the nonrenewable fossil resources.In this dissertation,lignin was degraded by fast pyrolysis and microwave-assisted depolymerization,considering the effects of lignin type,reaction process,reaction conditions and catalyst on the distribution and yield of degraded products.It can provide theoretical basis and technical support for efficient and directional conversion of lignin.The main conclusions of this dissertation are as follows:The pyrolysis behavior of kraft lignin fractions with different molecular weights fractionated by ultrafiltration membrane technology was investigated by pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS).Results indicated that the predominant products derived from pyrolysis of lignin fractions with different molecular weights changed in the relative content,but not in the compound species.At 500?,high molecular weight lignin favored the generation of guaiacol-type(G-type)phenolic compounds(57.33%),and low molecular weight lignin produced more syringol-type(S-type)phenolic compounds(34.33%).As temperature increased,methoxy groups in S and G units were easier to cleave from the benzene ring of the high molecular weight lignin,leading to an increase in the relative content of phenol-type(H-type)phenolic compounds.At 800?,demethoxylation and dehydroxylation were enhanced.High molecular weight lignin was found to produce the highest amounts of H-type phenolic compounds(46.22%)and aromatic hydrocarbons(17.25%).The pyrolysis behavior of kraft lignin fractions fractionated by gradient acid precipitation was investigated.At 500?,LpH6 fraction favored the generation of guaiacol and syringol.At 650?,compared to other fractions,the methoxy groups in the syringyl building block of LpH4 fraction were easier to be cracked from the benzene ring,which was benefical to generate more H-type phenolic compounds.At 800?,relative content of H-type phenolic compounds from pyrolysis of KL fraction was the highest(43.24%).The relative contents of H-type,G-type and S-type phenolic compounds from pyrolysis of LpH6 and LpH2 fractions were higher,while H-type and G-type phenolic compounds were the dominant products from pyrolysis of LpH4 fraction.The effect of stepwise pyrolysis process on the distribution and yield of enzymatic hydrolysis lignin(EHL)pyrolysis products with and without a catalyst was investigated.In the absence of catalyst,2,3-dihydrobenzofuran(DHBF)and G-type phenolic compounds were the dominant products at low temperature during the direct pyrolysis process.As temperature increased,demethoxylation reaction was enhanced.The relative contents of DHBF and G-type phenolic compounds decreased,while relative contents of H-type phenolic compounds increased.Stepwise pyrolysis of lignin could concentrate the phenolic compounds in bio-oil.When the first stage pyrolysis temperature was 260?,the relative content of total phenolic compounds was 86.2%.After adding HZSM-5 catalyst,demethoxylation,dehydroxylation,and aromatization reactions were enhanced as temperature increased during the direct pyrolysis process.The number of aromatic hydrocarbons in products increased,together with the relative content.The relative contents of DHBF and total phenolic compounds decreased,while relative content of aromatic hydrocarbons increased during the stepwise pyrolysis process.The highest relative content of aromatic hydrocarbons(30.4%)was achieved when the first stage pyrolysis temperature was 260?.Microwave-assisted degradation of EHL in methanol/formic acid media was investigated,concerning the effect of formic acid dosage,reaction temperature and reaction time on EHL depolymerization.It was found that the highest bio-oil yield of 72.0%including 6.7%monomers was achieved at 160? and a FA-to-lignin mass ratio of 4 after a reaction time of 30 min.Among the monomers,the yield of 2,3-dihydrobenzofuran was the highest,followed by p-coumaric acid.Formic acid acted mainly through acid-catalyzed cleavage of the linkages in lignin.Oligomers(molecular weight:253-510 g/mol)in bio-oil were mainly composed of dimers and trimers according to the Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry(MALDI-TOF MS)analysis.Microwave-assisted degradation of EHL in methanol/formic acid media was carried out using Ru/C cooperated with Lewis acids.The synergistic catalytic effects of Ru/C and Lewis acids and the effect of reaction parameters on EHL depolymerization were investigated.Among the Lewis acids,ZnCl2 showed the optimal depolymerization.A bio-oil yield of 93.4%including 13.4%aromaric monomers was achieved under the optimum condition(2 mmol ZnCl2,0.2 g Ru/C,160?,30 min).Higher amounts of ZnCl2 and Ru/C,higher reaction temperature and longer reaction time could increase the yield of H-type phenolic compounds in products,but reduce the yields of bio-oil and total aromatic monomers.Formic acid could not only act through acid-catalyzed cleavage of the linkages,but also act as hydrogen donor to hydrodeoxygenate the benzene ring side chain in the presence of catalyst.This study can provide experimental and theoretical guidance for efficient and directional conversion of lignin.