Mechanism Study On The Oxidative Depolymerization Of Lignin For Aromatic Acids By PMS

Lignin was the most abundant source of renewable aromatics in nature.Compared with cellulose and hemicellulose,lignin was more promising to replace fossil energy and benefit mankind as a renewable energy.However,due to the complex three-dimensional network structure of lignin,it has a high degree of complexity and strong resistance to degradation.Moreover,the components obtained from lignin degradation have high reactivity and were prone to condensation reactions to form more complex polymers,resulting in poor selectivity and low yield of the products obtained from lignin degradation and conversion.In addition,existing lignin degradation methods require very strict depolymerization conditions such as high temperature and high pressure.Advanced oxidation system had extremely high oxidation activity at room temperature and could be used to depolymerize lignin,but its mechanism of action was not clear.Therefore,the mechanism of preparing aromatic acid by potassium bisulfate dissolved polylignin was studied.The influence of different catalysts on activation of potassium persulfate was studied.The scientific correlation between catalyst structure and activation of potassium persulfate was established.The mechanism of oxygen dissolution of polybenzyl phenyl ether,4-benzoxy-phenol,3-benzoxy-4-methoxybenzaldehyde in acidic and alkaline systems of potassium persulfate was studied.The mechanism of catalyst/potassium persulfate system on the structure of dealkali lignin was studied.Four carbon-based catalysts including g-C₃N₄,NC-800,B_1:5 and Co&C were prepared.The catalysts were characterized by XRD,XPS,SEM and BET,and the yield of the catalysts in potassium bisulfate system was studied.The results showed that NC-800 catalyst could activate the model compound of potassium persulfate depolymerization in a short time.Combined with the characterization of materials,it was found that the main role in the activation process of potassium persulfate was the content of graphite N structure,and the N element could promote the electron transfer between potassium persulfate and catalyst.The depolymerization ofα-O-4 bond lignin dimer model compounds in NC-900/potassium persulfate and Na OH/potassium persulfate systems was studied.The types of free radicals in the system were verified by free radical quenching experiment,and the effects of reaction temperature,reaction time and p H value on the depolymerization products of the model compound were investigated.Results show that the heat could be activated potassium hydrogen sulfate to produce free radicals,potassium hydrogen sulfate was mainly produced in acidic system OH^·,and of SO₄^·-,potassium hydrogen sulfate can be activated in alkaline solution generated ¹O₂and O₂^·-,Carbon-based catalysts increase the rate and quantity of free radical production.The mechanism of depolymerization of lignin by potassium bisulfate was proposed.In potassium persulfate oxidation systems,reactive oxygen species were first produced by potassium persulfate at low temperatures.The lignin model compound was converted to the free radical of the model compound by the active oxygen species,and the free radical rapidly condensed into oligomers.The oxygen depolymerization results of dealkalized lignin in catalyst/potassium persulfate and Na OH/potassium persulfate systems were studied.2D HSQC NMR and gel permeation chromatography were used to analyze the reaction results.It was found that the reactive oxygen radicals could oxidize the ether bond and the hydroxyl structure of the side chain in lignin.In addition,the ¹O₂and O₂^·-produced by the activation of alkaline conditions had stronger oxidation activity,which could depolymerize the dealkalized lignin into small molecular weight compounds.