The Synergistic Effect Of Phosphoric Acid And Hydrogen Peroxide During The PHP Pretreatment For Lignocellulosic Biomass

Bioethanol is important for solving energy shortages and environmental pollution caused by fossil fuels.Pretreatment is a key step in the bioethanol process.Phosphoric acid plus hydrogen peroxide(PHP)pretreatment is a new lignocellulose pretreatment method.The pretreatment method has strong adaptability for biomass and high ethanol yield.In this thesis,the functions of phosphoric acid and hydrogen peroxide in PHP pretreatment process were studied.In addition,the degradation pathways/mechanisms of hemicellulose and lignin during PHP pretreatment were elucidated.The main results are as follows:(1)When the study was focused on agricultural residues of wheat straw and corn stover at macro level,pretreatments with various intensity levels and different ratios of phosphoric acid to hydrogen peroxide were performed.The hemicellulose removal and delignification of these two substrates after pretreatment were determined.The effect of phosphoric acid to hydrogen peroxide ratio on hemicellulose removal and delignification were compared.Via PHP pretreatment,hemicellulose was completely removed,the highest lignin removal rates were 83.5% and 90.0% of wheat straw and corn stalk,respectively.Phosphoric acid concentrations was more influential for hemicellulose removal.As the concentration of phosphoric acid increased,the removal rate of hemicellulose increased.The delignification profiles were quite different at high and low pretreatment intensities.When the pretreatment intensity was low,as the concentration of phosphoric acid increased,the delignification showed a stronger advantage.When the pretreatment intensity was high,the ratio of phosphoric acid to hydrogen peroxide was 65%-7.1%,the delignification reached the maximum value at the same intensity level.It was found that phosphoric acid and hydrogen peroxide have synergistic effects on delignification.Higher extent of delignification of both substrates correlated well with their higher hemicellulose removal.(2)In addition,the degradation products of the PHP pretreated biomass were analyzed,and the representative degradation products were traced/analyzed at micro level in detail.It was found that the degradation products of the pretreated biomass contained a large number of small molecular carboxylic acids and aromatic compounds,indicating that biomass underwent strong chemical degradation during PHP pretreatment.It was interesting a strong oxidizing substance,peracetic acid,was found in the degradation products.Lignin was oxidatively ring-opened and degraded by peracetic acid in the PHP pretreatment system.Peracetic acid had a direct regulation function for delignification.(3)In PHP pretreatment system,the synthetic pathway of peracetic acid and the degradation mechanism of hemicellulose and lignin was proposed.The results showed that acetic acid was released from biomass during PHP pretreatment,acetic acid and hydrogen peroxide were further formed to peracetic acid.The degradation mechanism of hemicellulose and lignin components are as follows: acetyl groups on hemicellulose was degraded via acid hydrolysis of phosphoric acid,producing acetic acid.Then peracetic acid was synthesized from acetic acid and hydrogen peroxide.Peracetic acid selectively oxidized lignin through ring-opening reactions,which facilitated the subsequent lignin degradation.The degradation product of lignin contained acetic acid,which promoted the synthesis of peracetic acid and degradation reaction.In addition,peracetic acid could also attack hemicellulose and its degradation products,while further promoted the hemicellulose degradation.In PHP pretreatment system,hemicellulose is degraded by phosphoric acid,hydrogen peroxide participates in the reaction of forming peracetic acid,lignin is removed by peracetic acid.This thesis provides new information for the degradation mechanism of the pretreated lignocellulose process and the function of the chemical in the pretreatment system.