Deconstruction Of Lignin And Lignin Model Under Different Thermochemical Environments

Lignin whose structure is far more complex than cellulose is second only to cellulose in biomass as natural polymers and is not fully rational use of renewable resources. It is great significance for the full utilization of renewable resources to carry out the thermal pyrolysis researches of various sources of lignin and lignin model compound in different thermal chemical environments. This paper studied the degradation, pyrolysis and cracking behaviors of non-wood fiber lignin, such as bamboo and straw, the industry lignin in pulping black liquor andβ-O-4 lignin model polymer. The possible cracking pathway of mainly chemical bonds were analyzed and calculated with simulating the structure of lignin via quantum chemical methods. The main contents in this paper were as follows:(1)The removal discipline of bamboo and straw lignin was investigated in the CO2 supercritical fluid by using dioxane/water as co-solvent. The results from orthogonal experiments indicated that the most important factor that affected the efficiency of delignification was the temperature and the followed factor is the pressure. The influences of reaction time and concentrations of co-solvent on delignification were relatively less. From the data, it could be found that the content of Klason lignin in exaction liquid would be high when the temperature rose. At 180℃and 20MPa, the delignification efficiency could reach 85% in 60 min with 95% dioxane / water as co-solvent. The GC/MS results of the extraction liquid under different temperature showed that the linear chain or branch of ethoxy and alcohol, aldehyde, ketone, acid and ester of five-member ring structure were existed. Lignin's basic structure unit just like vanillin and p-hydroxyl benzaldehydes etc were also found. The temperature had a great effect on the extraction liquid content. There would be no furfural and benzofuran generated in 160℃. But, when the temperature increased to 200℃, the benzofuran content nearly reached to 15% and furfural content run up to more than 42%. Straw extraction liquid mainly contained five-membered ring alkanes and their derivatives from carbohydrate degradation products, alkoxy alcohol, fatty acid, and degradation products from H/G/S type lignin structure.(2)The micro-distribution of lignin before and after via CO2 supercritical fluid extraction was investigated via the use of transmission electron microscopy-energy spectrum combined with technology (TEM-EDXA). The results showed that temperature were main factors which affected delignification of straw and bamboo. In the experimental set working conditions, the degree of delignification increased as the temperature was added. TEM-EDXA analysis showed that the concentration of lignin distributed between layers in straw cell wall was not obvious any more after extraction by supercritical CO2 at 160°C, 16Mpa. The lignin in cell corner and middle lamella of the bamboo fiber organization has been all dissolved at 200°C, 16MPa.(3)The oligomeric and high-poly lignin model compounds of hydroxyl-phenyl(H), guaiacyl(G) and syringyl-type(S) was synthesized , 4-hydroxybenzaldehyde, vanillin, syringyl aldehyde, 4-hydroxyacetophenone and acetovanillone as initiator. The lignin model polymers were characterized by 1H-NMR and 13C-NMR, IR spectra (IR), Matrix-Assisted Laser Desorption Ionization– Time of Flight Tandam Mass Spectrum (MALDI-TOF-MS) and Gel Permeation Chromatography (GPC). The results indicated that the lignin oligomers linked byβ-O-4 as main chemical links was successfully synthesized by LDA carbanion nucleophilic addition on polymerization methods. The molecular weight of this oligomers was about 1000 with polydispersity of 1.1 or so. The lignin high-poly model of takingβ-O-4 as the main chemical links was synthesized via CuBr₂ brominated, alkali catalyst oxygen anion nucleophilic addition reaction. The molecular weight of repeat unit of polymer is 136.16 n + 2. The structure and properties of lignin model polymer were similar with the lignin in fiber. It can be dissolved in common solvent of lignin. The weight-average molecular weight of the lignin high-poly model were 2000⁶000 and the polydispersity were 1.26¹.76.(4)Pyrolysis characteristics of technical alkali lignin and black liquor solid were studied by using TG and Py-GC-MS. The results showed that the weight-loss process of technical alkali lignin was composed of four areas. The activation energy increased slightly with the heating rate increment in the main reaction at 250⁴75℃. Below 300℃, The phenols from lignin was relatively small in pyrolysis products,while L-glucose from the carbohydrates was up to 46.7%. At 450℃, L-glucose contents from the carbohydrates was not high in pyrolysis products, while the products such as guaiacols, methoxy catechol, 2,6-dimethoxy phenol from the deconstruction of lignin was up to 62.41%. At 600℃, the yield of pyrolysis oil of inductrial alkali lignin and black liquor solids was the highest (relativily, 46% and 23%). There had more L-glucose detected in the pyrolysis oil of black liquor solids, but no L-glucose found in technical alkali lignin. The liquid products of technical alkali lignin mainly composed of low molecular weight phenols, ketones and aromatic acid compounds from pyrolysis of lignin, and the relative contents were higher than black liquor solids. (5)TG-DTG analysis indicates that pyrolysis main reaction temperature of the G-type polymer are 240℃⁴00℃and biggest weightlessness is 34.21%. Py-GC/MS(400℃- 800℃) analysis shows, from 500℃to 800℃, relative content of hydroxyl toluene and hydroxyl ethylbenzene are gradually reduce with long temperature increase in pyrolysis products of H-type polymers, while relative content of benzene, toluene, xylene are gradually increase and relative content of phenol has not changed much. It indicates the phenolic group started break away from aromatic-ring. Tubular furnace cracking (500℃⁹00℃) studies show that pyrolysis gas phase products containing mainly H2, CO, CH4, CO₂ and C₂H₄, among content of CH4 and CO₂ are less. The content of H2, CO and C₂H₄ has increase with the rise of temperature before 800℃. The rate of gasification reaches biggest at 800℃and the rate of liquefaction reaches biggest at 900℃. The liquid products mainly are low molecular weight compounds and phenols below 500℃. The product kinds significantly increased above 500℃. 800℃above, liquid products mainly constitutes by aromatic and polycyclic aromatic hydrocarbons. At 900℃, benzene content reachs 36%, the sum of contents combined benzene, toluene, naphthalene, anthraquinone reach more than 62% or more.(6)Stable conformational and cracking of mainly chemical bonds of vanillin and lignin dimer model are simulated via using quantitative chemistry calculation methods.The potential surfaces of the vanillin molecule is scanned and the stable conformation of vanillin is analyzed using HF and B3LYP method . It find out one of the most stable conformation, three of local minimum conformation and six of transient structure. all kinds bonds strength in vanillin molecules is calculated using DFT,MP2 and CBS methods. The calculation results indicate that the O-CH₃ bond strength in methoxyl is lower (only 61Kcal/mol), which maybe prior cracked in pyrolysis. The calculation about model dimer also shows that the stable order is O-4 > 1- > -β> O-4 in 1- -β-O-4 structure of model dimer. Theβ-O bond is most easy to break and O-4 bond is most stable, which is an important reasons why there are a lot of phenolic compounds in pyrolysis products of biomass or lignin.