Study On The Effect Of Modification Of Functional Group Of Lignin Model Compound On Its Pyrolysis Process

The consumption of non-renewable fossil energy sources such as oil,coal,and natural gas has led to the deterioration of the ecological environment.Therefore,replacing non-renewable fossil energy with renewable resources is of great significance to alleviate energy shortages and environmental degradation.At present,as a green renewable resource,wood fiber biomass resources are playing an increasingly important role in replacing traditional fossil fuels to produce sustainable fuels and chemicals.Lignin is an important component of lignocellulosic biomass resources.It is the first rich natural aromatic polymer in nature and the most abundant renewable oxygen-containing aromatic hydrocarbon resource.It has the potential to replace fossil energy to produce phenolic chemicals.In the past ten years,the rapid pyrolysis technology of lignin has aroused widespread concern among scholars,but the traditional rapid pyrolysis technology of lignin has not yet achieved the directional depolymerization of lignin.Therefore,if lignin is to be subjected to directional cracking during the thermochemical conversion process to improve the yield of specific products,it is necessary to clarify the thermochemical conversion pathway of lignin and the formation mechanism of the main products.Based on this,this thesis adopts the research method of the combination of density functional theory calculation and pyrolysis experiment,focusing on exploring the influence mechanism of the C_α-OH、Ph-OH functional groups modulation ofβ-O-4 type lignin model compounds on the pyrolysis process.The research results provide theoretical support for clarifying the influence mechanism of lignin active group modulation on the pyrolysis process,and are of great significance for the further development of lignin high-value utilization technology.Eight kinds of lignin dimer model compounds and three kinds of GG type lignin hyperpolymer model compounds were synthesized and characterized by ¹H-NMR and ¹³C-NMR.The characterization data showed that they were of high purity and could be used as experimental materials for subsequent research.Density functional theory method was used to calculate the bond dissociation energies of the C_aromatic-C_α,C_α-C_β,C_β-O bond,and C_aromatic-O bonds in four lignin dimer model compounds,the homolytic cracking reaction during pyrolysis of these dimers was then invetigated and the formation pathways of pyrolysis products of different dimers were analyzed.The results show that the homogenization of C_β-O bond is the main reaction in the initial pyrolysis of dimer,whereas the homolysis of C_α-C_βbond is a competitive reaction.After the oxidation and acetylation of C_α-OH,the bond dissociation energy of C_β-O bond decreases,whereas the dissociation energy of C_α-C_βbond increases,ccompanied with an increase in the probability of the C_β-O bond dissociation and a decrease in the competitive ability of C_α-C_βbond homolysis.For the pyrolysis of four model compounds,the main aromatic products include benzyl alcohol,toluene,benzaldehyde,guaiacol,etc.The selective modification of the C_α-OH functional groups can regulate the types of pyrolysis products.In particular,the product types for the pyrolysis of model compounds modified by oxidation become less,accompanied with an increase in the selectivity to ceratin products.Two dimer model compounds were used as experimental raw materials,pyrolysis experimental analysis and density functional theory calculation were combined to study the influence mechanism of oxidation modulation of C_α-OH group on the pyrolysis process.The results showed that:When C_α-OH is changed into C_α=O,the generation of coke during pyrolysis can be inhibited,the conversion efficiency of raw materials is improved and the selectivity of pyrolysis products is enhanced.During the pyrolysis process,after the dimer is homogenized by C_α-C_βbond,the free radicals generated by B unit will undergo rearrangement reaction and remove the hydrogen radicals to provide hydrogen donor for the whole pyrolysis system.At the same time,it was found that only the homogenization reaction mechanism existed in the pyrolysis process after the model compound was oxidized and modified.The types of monomer phenolic compounds in pyrolysis products decreased and the selectivity was enhanced.Taking two kinds of hyperpolymer model compounds as research objects,the influence of acetylation modification of C_α-OH、Ph-OH functional groups of model compounds on pyrolysis process was explored by combining TGA、Py-GC/MS、in-FTIR experiment and gauss 09theoretical calculation,The results show that:After acetylation modification of functional groups of polymer model compounds,its thermal stability is obviously reduced,and carboxylic acid is decomposed at lower temperature.At the same time,the difference in the dissociation energy of C_α-C_βbond and C_β-O bond became larger,the homogenization reaction competitiveness of C_α-C_βbond was weakened,and the yield of monomer phenolic compounds formed based on the homogenization of C_β-O bond in the pyrolysis products increased.Meanwhile,acetylation modulation can inhibit the formation of oligomers and coke and improve the pyrolysis efficiency of raw materials.The high polymer formed by acetylation and modification forms the intermediate product containing C_α=C_βstructure through the transition state of five-element ring at low temperature.In the subsequent pyrolysis process,the homogenization reaction of C_β-O bond is the main reaction,which will greatly improve the selectivity of pyrolysis products.Three dimer model compounds were used as research objects to explore the influence of the modification of phenolic hydroxyl on the pyrolysis process of the model compound,TGA experiments show that:when the model compound contains phenolic hydroxyl groups,the yield of pyrolytic coke is increased.Closed system u-tube thermal cracking system showed that the yield of liquid products of the three model compounds decreased with the increase of pyrolysis temperature.When Ph-OH is modified into Ph-H,the yield of benzaldehyde in the pyrolysis product will increase.The results of density functional theory show that the dissociation energy difference between C_α-C_βbond and C_β-O bond is the highest when Ph-OH is modified into Ph-OMe,the competitiveness of C_α-C_βbond homolysis reaction is weakened,and the content of C_α-C_βbond homolysis products in the pyrolysis products is reduced.