Study On The Mechanism Of Hemicellulose Pyrolysis Based On Density Functional Theory

Hemicellulose as a major component of biomass,the pyrolysis behavior largely reflects the whole biomass pyrolytic rule,the study of the pyrolysis mechanism helps to understand the characteristics of biomass pyrolysis.In order to understand the mechanism of thermal decomposition of hemicellulose and to identify the chemical pathways for the formation of key products during pyrolysis,the pyrolysis processes of three typical monomers in the xylan structure of hemicellulose investigated by using density functional theory(DFT)methods at M062X/6-311++G(d,p)level in this thesis.The main studies and finding are summarized as follows:Firstly,the main chain of hemicellulose xylopyranose pyrolysis reaction mechanism are analyzed in the theoretical calculation.Nine possible reaction paths were designed,and all the reactants,transition states,intermediates and products were optmized,and the standard thermodynamic and kinetic parameters in each reaction pathway were calculated.In the pyrolysis,two categories of pyrolytic reaction pathways were proposed.In the first category,xylopyranose is firstly converted to acyclic carbonyl isomer via transition state TS1 with energy barrier of 188.7 kJ /mol,and then the isomer further decomposes to form all sorts of small molecules through five possible pyrolytic reaction pathways.In the second category,xylopyranose first undergoes dehydration,and then four possible pyrolytic reaction pathways were proposed through simultaneous cracking of C-O bond and C-C bond.The calculation results show that the major products are low molecular products such as ethanol,acetaldehyde,furfural,acetone,acids,CO2 and CO.Secondly,the branched-chain of hemicellulose 4-O-methyl-glucuronic acid pyrolysis reaction mechanism are analyzed in the theoretical calculation.4-O-methylglucuronic acid was first activated by hydrogen atom intramolecular transfer occurs ring-opening reaction to form chain intermediates,for the intermediate design of a total of four possible pyrolysis reaction pathway.The calculation results show that the pathway(1-1)and(1-2)were the main reaction channels.The majorpyrolysis products are methanol,glycolaldehyde,2-hydroxy-3-methoxy-butyl aldehyde acid,glyoxal,2-hydroxy-butyl aldehyde acid and so on;In competitive reaction pathways(2)and(5),the competitive pyrolysis products are formic acid,CO2,CO,4-hydroxy-3-vinylmethylketone,methoxyethene and so on.In hemicellulose pyrolysis,CO2 is more likely to be formed through decarboxylate reaction of unsaturated reactants or intermediates,and acetic acid is more likely to be formed through elimination reaction of O-acetyl.Finally,in the pyrolysis,arabinofuranose is firstly converted to intermediate with a low energy barrier of 181.5 kJ/mol by intramolecular hydrogen atom transfer reaction,the isomerization reaction is endothermic and the absorption heat is 14.1kJ/mol.And then compound decomposes to all sorts of small molecules through 3pathways.Six possible pathways were proposed,the equilibrium geometries of the reactants,transition states,intermediate and products were fully optimized,and the standard thermodynamic and kinetic parameters of every reaction pathway were calculated.The calculation results show that reaction(2),(3)and(5)are major reaction channels,in pyrolysis of arabinofuranose and speed control step reaction energy barrier of 321.5 kJ/mol,300.2kJ/mol and 339.9 kJ/mol.The main products are low molecular products such as glycolaldehyde,acetaldehyde,CO,furfuraldehyde and acetone,which is according with related analysis of experimental results.