Research On The Pyrolysis Mechanism Of Lignin Model Compounds

Due to the fossil fuel shortage and severe environmental pollution problems,the development and utilization of renewable biomass resources has attracted widespread attention of scholars both at home and abroad.Among various biomass utilization technologies,fast pyrolysis is an important way to achieve its efficient conversion.Lignin,one of the three major components of lignocellulosic biomass,is the most abundant resource of aromatic compounds in nature.The rapid pyrolysis of lignin will generate high value-added aromatic compounds.However,the traditional fast pyrolysis techniques have not yet achieved the selective production of specific aromatic compounds.In order to develop efficient selective pyrolysis techniques for target products,it is necessary to understand the pyrolysis mechanism of lignin and the formation pathways of pyrolytic products.Therefore,typical lignin model compounds were selected to explore the conventional and catalytic pyrolysis mechanisms of lignin by employing density functional theory(DFT)calculations combined with analytical pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS)experiments.The pyrolysis mechanisms of β-O-4 type and β-1 type lignin dimer model compounds were revealed.These lignin dimer model compounds mainly decomposed via concerted decomposition mechanism,followed by the homolytic mechanism.The β-O-4 type lignin dimer model compound was prone to undergo the Cβ-O homolytic mechanism,while the β-1 type lignin dimer model compound was prone to undergo the Cα-Cβhomolytic mechanism.Based on these studies,the effects of functional groups(hydroxyl,hydroxymethyl and methoxyl)on β-O-4 type lignin dimer model compounds were futher explored.A universal pyrolysis model for β-O-4 type lignin dimers was proposed which included 11 initial pyrolysis mechanisms(mechanism 1～11).Specific pyrolysis mechanisms would take place due to the hydroxyl,hydroxymethyl or methoxyl substituents located on the alkyl side chain and aromatic ring of lignin,and moreover,their competitiveness would be ranked with some regularity.The pyrolysis mechanism of an α,β-diether type lignin trimer model compound was theoretically investigated.The interaction of the β-O-4 and α-O-4 linkages on their homolytic cleavages was revealed.The α-O-4 and β-O-4 linkages had little interaction effects on their initial homolytic cleavage reactions,while after the initial homolytic cleavage of the α-O-4(β-O-4)linkage,the formed radical had significantly reduced bond dissociation energy for the further homolytic cleavage of the β-O-4(α-O-4)linkage.A new intermolecular interaction mechanism in the primary and secondary pyrolysis process of lignin was proposed.The intermolecular interaction mechanism was superior to the traditional unimolecular decomposition mechanism.During the lignin pyrolysis process,lignin homolysis radicals could be stabilized by lignin and its derived pyrolytic products through H-abstraction reactions.Simultaneously,these radicals could also promote the primary and secondary decomposition reactions of lignin and its derived pyrolytic products.Pyrolysis models for β-O-4 type lignin dimer model compounds catalyzed by phosphoric acid and alkali metal ions were established,and their catalytic pyrolysis mehcnaisms were also revealed.The inhibitory and promoting effects of phosphoric acid and alkali metal ions on pyrolytic products were clarified.Their participation would change the geometrical configurations of lignin model compounds and futher affect bond dissociation energies and pyrolysis pathways.During the pyrolysis process of lignin dimer model compounds,phosphoric acid could promote concerted decomposition reactions while inhibit homolytic reactions.Alkali metal ions had promoting effects on the Cβ-O homolysis and concerted decomposition but had inhibitory effects on the Cα-Cβhomolysis.