Studies Of Molecular Level Pyrolysis Mechanism Of Lignin Model Compounds

As the energy and environment problems in the world wide becomemore and more severe and emergency, developing renewable andsustainable new energy has become one of the most important challengesfaced by the mankind in the21stcentury. Biomass resources has receivedgrowing public concern as the important source of energy and industrialchemicals, for it has many advantages, such as, vast reserves, renewable,carbon neutral, etc.Biomass pyrolysis is one of the most important and promisingapproaches of biomass utilization. Pyrolysis products are composed ofliquid products (bio-oil), gas products (combustible gas), solid products(coke). Bio-oil has many advantages, such as, easy storage, easytransportation, high energy density, etc. In addition, bio-oil can providesome valuable industrial chemicals. Lignin is the main component ofbiomass (secondly to cellulose), its pyrolytic behavior reflects the law ofbiomass pyrolysis in a large part. As a result, the study of lignin pyrolysismechanism contributes to understanding bio-oil performance and itsformation mechanism. In order to explore lignin pyrolysis mechanism fromthe molecular level, this paper investigates the pyrolytic process of ligninand the formation mechanism of main products by quantum chemistrytheory methods. The following are the main research works and results:Phenethyl phenyl ether (PPE) was used as dimeric lignin modelcompound. Ten possible pyrolytic pathways were proposed including twoconcerted reactions and eight free-radical reactions. The standardthermodynamic and kinetic parameters in each reaction pathway werecalculated at different temperatures by density functional theory methods inGaussian03package at B3LYP/6-31G(d) level. The calculation resultsshow that all reactions are endothermic. Both thermodynamic and kineticanalyses support reaction pathway9and10in concerted mechanism asmajor reaction channels. The activation energies of rate-determining stepsin pathway9and10are60.1kcal/mol and52.8kcal/mol respectively.Weget four better pathways from all the reaction pathways of PPE pyrolysis in four different bond rupture ways. We can sequence the four reactionpathways in descending order of kinetically supporting priority as: path2>path4> path7> path6. As a result, it can be infered that major products oflignin pyrolysis are styrene, phenol and the compounds that can be formedby combinations of phenoxy, benzyl, phenyl and hydrogen radical.Guaiacol was chosen as monomeric lignin model compound. Fivepossible pyrolytic pathways with a focus on the methoxy group wereproposed. Pathway1is the pyrolysis pathway of guaiacol through O-CH3homolysis. Pathway2~pathway4are all pyrolysis pathways of guaiacolthrough demethoxylation. Pathway5is the pyrolysis pathway of guaiacolthrough OCH3rearrangement generating o-quinonemethide that isconsidered as polymerization intermediate and coke precursor. Thestandard thermodynamic and kinetic parameters in each reaction pathwaywere calculated at different temperatures by density functional theorymethods in Gaussian03package at B3LYP/6-31G++(d,p) level. Thethermodynamic analysis results totally agree with the kinetic analysisresults. Analyses results show all the reaction pathways are endothermicexcept pathway3. We can sequence the five reaction pathways indescending order of kinetically supporting priority as: Path3>Path1>Path4>Path2> Path5. Among the three demethoxylation reactionpathways(Path2,Path3, Path4), analyses results firstly support Path3,then Path4, at last Path2. This indicates that the action of adding hydrogenatom to the benzene ring effectively lowered the energy of the bondconnecting the methoxy and the benzene ring. Pathway5uncovered thepossible formation mechanism of the o-quinonemethide that is consideredas polymerization intermediate and coke precursor during pyrolysis.