Mechanism Study Of Potassium Catalytic Pyrolysis Of Lignin And Lignin Model Polymers

Biomass energy can effectively alleviate energy shortages,environmental pollution and excessive CO₂ emission caused by excessive use of fossil energy due to its advantages of renewable,large reserves,clean and neutral CO₂ emissions.Biomass consists mainly of three components,cellulose,hemicellulose and lignin.Unlike the relatively simple glycan structure of cellulose and hemicellulose,with a large number of benzene rings and oxygen-containing functional groups,lignin is the most complicated structure.At the same time,lignin is the only natural renewable aromatic polymer,and high value-added platform chemicals obtained by the directional regulation can be widely used in the chemical industry.In recent years,the pyrolysis mechanism of lignin has received continuous attention from researchers all over the world.However,previous studies lacked sufficient comparison of the catalytic effects of different kinds of potassium compounds.Secondly,model dimers have been mainly used to simulate lignin pyrolysis,but it's quite different from the real lignin due to the simple structure and poor thermal stability.Third,transition metal elements,zeolites,biological enzymes,graphite and precious metals have been widely used to study the catalytic mechanism of lignin and its monomers.The study of catalytic pyrolysis mechanism of potassium using lignin model polymer is still blank.Finally,the comprehensive analysis of the catalysis of organic bound and ionic potassium by quantum chemical calculation has not been carried out.Aiming at these problems,this paper selects alkali lignin and synthetic?-O-4 lignin model polymer to study the pyrolysis mechanism of lignin and the catalytic effects of potassium by combining the experiment and numerical calculation.Firstly,KCl,KOH and K₂CO₃ were added into the alkali lignin by leaching method,and then the pyrolysis experiments were carried out on a thermogravimetric analyzer coupled with a flourier transform infrared spectrometer?TG-FTIR?and analytical pyrolysis-gas chromatography/mass spectrometry?Py-GC/MS?.The results show that the temperature for maximum weight loss rate is greatly advanced by about 60°C under the catalytic effects of potassium.The formation of phenolic compounds and CO₂ is in the low temperature section?<300°C?,while the formation of phenolic compounds were inhibited by the potassium compounds at this temperature.In the main lignin pyrolysis stage,the decarbonylation and demethylation of potassium compounds promoted the yield of CO₂ and guaiacol,respectively,and the decarbonylation and demethylation effects of KOH were the strongest among the three potassium compounds.Secondly,two kinds of linear polymers with?-O-4 type linkages containing hydroxyl and carbonyl groups respectively on C?were synthesized as a lignin model polymer.They were used as lignin model polymers for pyrolysis analysis.It is clearly confirmed that the source of CO₂ release is the removal of carbonyl,not the hydroxyl or the ether in C_?-O bond,the decarbonylation effect enhances with increasing temperature.The carbonyl on C_?can activate the polymer,decrease the breakage temperature of the C_?-O bond,and make it dominant in the depolymerization process of the C_?=O polymer,with simultaneous breakage of the C_?-C_?bond.The hydroxyl on C_?causes the breakage of the C_?-O bond to absolutely dominate the depolymerization process of the C_?-OH polymer,and the breakage of C_?-C_?bond hardly occurs.Thirdly,the lignin model polymers were applied to study the catalytic mechanism of potassium.The specific catalytic effects of KCl,KOH and K₂CO₃ on lignin depolymerization mode and oxygen-containing functional groups were studied respectively.The potassium can promote decarbonylation and dealkylation in the pyrolysis process of lignin model polymers,which result in the yield of CO₂ and phenols increased dramatically.The presence of carbonyl can promote the catalytic effect of potassium.The presence of potassium significantly reduce the depolymeri-zation temperature of lignin model polymers,strongly promote the breakage of C_?-O bond,thereby changing the depolymerization pathway of the polymers.The catalytic effects of KCl,KOH and K₂CO₃ are obviously different,which is attributed to the interaction between different anions and polymer organic matrix.The catalytic activity of KOH and K₂CO₃ was stronger than that of KCl.Furthermore,the possible pyrolysis pathways of most structurally representative fragments in the lignin model polymer were studied by quantum chemical calculation,and the calculated results were in good agreement with the experimental results.The specific catalytic effects of organic bound and ionic potassium on linkage fracture mechanism and product formation pathway were studied.The calculation results confirmed that the main pyrolysis mechanism of C_?=O type and C_?-OH type lignin model polymer is C_?-O fracture,followed by C_?-C_?fracture.It is clarified that ketene is an important intermediate of CO₂emission.Compared with C_?-OH type dimer,the C_?=O type dimer is more easily cleaved to form ketene,which makes the C_?=O type polymer more prone to CO₂ generation than the C_?-OH type polymer during pyrolysis.The organically bound potassium promoted the synergistic reaction of two lignin model polymers,but had little effect on the homogeneous cracking reaction mechanism.K⁺promotes the synergistic reaction mechanism of C_?-O cleavage,and inhibits the homogenization reaction,especially the C_?-C_?homogeni-zation reaction.Organically bound potassium and K⁺catalyze the cracking of p-hydroxyacetone and hydroxystyrene alcohol into the precursors of phenol and CO₂,thus promoting the generation of phenol and CO₂.The conclusions of quantum chemical calculation are in good agreement with the experimental results.The mechanism of lignin pyrolysis and the catalytic effects of potassium are revealed from the aspects of free radical reaction,linkages rupture,transformation and removal of oxygen-containing functional groups.