Mechanism Research On The Catalytic And Co-pyrolysis Of Seaweed Polysaccharides And Cellulose By Experimental And Subsequent Density Functional Theory Studies

Most researches on the pyrolysis of biomass were focused on the analysis based on macroscopic experiments in the past years,while less attention was paid to the mechanisms of main product formation.Co-pyrolysis and catalytic pyrolysis are the important approaches to upgrade the quality of pyrolysis bio-oil.However,the chemical reaction mechanism of the two approaches are less studied.The pyrolysis mechanisms of soluble polysaccharide and cellulose,as the main constitutions of algal and lignocellulosic biomass respectively,can help to understand the pyrolysis mechanisms of the two kinds of biomass.In this thesis,the formation mechanisms of main products during the pyrolysis of seaweed soluble polysaccharide,catalytic pyrolysis of soluble polysaccharide or cellulose with the ZSM-5 catalyst and co-pyrolysis of soluble polysaccharide and cellulose were analyzed in detail based on the Py-GC/MS analysis,molecular dynamics simulation and subsequent density functional theory.The research content and conclusions are as following:1)The pyrolysis mechanisms of seaweed soluble polysaccharide were gained insight into based on the macroscopic experiments,molecular dynamics simulations and subsequent density functional theory.The experiment results showed that the pyrolysis products of seaweed soluble polysaccharide are mainly furans,of which the relative content of 5-methyl furfural is the most.The molecular dynamics simulation results showed that the decomposition of polysaccharide mainly occurred at middle temperature range(500-800 K).Seven potential pyrolysis pathways of seaweed model compound,sulfated polysaccharides,were proposed to interpret the pyrolysis mechanism based on the subsequent density functional theory.The theoretical calculations were in good accordance with related experimental results.It is meaningful for the seaweed utilization to gain insight into the pyrolysis mechanism.2)The catalytic pyrolysis mechanisms of seaweed soluble polysaccharide or cellulose with the ZSM-5 catalyst were studied based on the combination of Py-GC/MS and subsequent density functional theory.The experimental results showed that the relative contents of furan and aromatic hydrocarbons increased during the catalytic pyrolysis of cellulose,and fufural was the main products during the catalytic pyrolysis of sulfated polysaccharides.The calculation results based on subsequent density functional theory during the catalytic pyrolysis of sulfated polysaccharides or cellulose with the ZSM-5 catalyst showed that the ZSM-5 catalyst could boost the pyrolysis and upgrade the quality of bio-oils due to the promotion of hydrodeoxygenation during the catalytic reactions.Due to the hydrogen bonding and van der Waals force interactions,the pyrolysis pathways were changed and the energy barrier decreased,which promoted the pyrolysis.This indicated that ZSM-5 had a catalytic influence on the pyrolysis of sulfated polysaccharides.3)The co-pyrolysis mechanisms of sulfated polysaccharides and cellulose at 823 K were studied based on the combination of Py-GC/MS,molecular dynamics simulations and subsequent density functional theory.Compared to the pyrolysis products of sulfated polysaccharides or cellulose individually,the relative contents of aldehydes,ketones and organic acids decreased in the co-pyrolysis bio-oils,while that of esters increased significantly.The results from molecular dynamics simulation showed that the temperature range of the individual pyrolysis did not completely overlap and co-pyrolysis could improve the conversion rate of sulfated polysaccharides and cellulose,which gave a preliminary explanation on the synergistic effects of the co-pyrolysis of seaweed and lignocellulosic biomass.Finally,the co-pyrolysis synergistic effect of sulfated polysaccharides and cellulose at 823 K was studied based on the subsequent density functional theory.The results indicated that acetic acid acted as the catalyst to promote the dehydration reaction of cellulose,which accelerated the pyrolysis.Acetic acid and cellulose underwent the esterification reaction firstly,which was in accordance with experimental results.