Study On The Effect Mechanism Of Nitrogen-Containing Components On Biomass Pyrolysis For High-Value Chemicals

Biomass pyrolysis for high value-added products is an important way for biomass utilization.The composition of bio-oil from biomass pyrolysis is complex,and contains the potential high value-added components.Due to its low content,the extraction process of these high value-added components is complex and the cost is high,which limits the high value utilization of bio-oil.Based on this,it is necessary to effectively improve the content of high-value components and control the pyrolysis process of biomass to achieve the selective preparation for specific target products.The purpose of this paper is to study the mechanism of nitrogen-containing components on the formation of high-value components form biomass pyrolysis.The migration and transformation of nitrogen-containing amino acids in the pyrolysis process and its influence on the cellulose pyrolysis and the main products were explored by pyrolysis experiments and density functional theory（DFT）calculation.In the pyrolysis process of cellulose and lignin,exogenous nitrogen ammonia was introduced to prepare high value-added components（pyrrole,levoglucosan,4-vinylphenol）,and the effect of ammonia on the target products formation was explored in detail.The nitrogen-containing components（phenylalanine and glutamic acid）,cellulose unit glucose and the mixtures were pyrolyzed by pyrolysis-gas chromatrography/mass（Py-GC/MS）,pyrolysis-photoionization time-of-flight mass spectrometer（Py-PI-TOF-MS）and closed U-tube fixed-bed reator.The interaction path between amino acids and glucose was studied by DFT calculation,and the interaction mechanism between endogenous nitrogen（amino acids）and glucose was revealed.The interaction of glucose with phenylalanine and glutamic acid was obviously different.The glucose and phenylalanine mainly polymerized,and it supplied hydrogen to promot the phenylethylamine deamination for styrene.The glucose and glutamic acid mainly polymerized to promote the glutamic acid decarboxylation for 2-pyrrolidone.The calculation results showed that the hydroxyl group at C1 of glucose provided hydrogen for the amino group connected with phenylethylamine C2,which promoted the phenylethylamine deamination to form styrene.The reaction energy barrier of the reaction path was 200.58 k J/mol.The aldehyde group of chain glucose binded to the amino group of glutamic acid,which can reduce the energy barrier of glutamic acid decarboxylation,and promote the formation of 2-pyrrolidone with the energy barrier of285.01 k J/mol.Cellulose pyrolysis behavior was investigated at 400-800°C in a fixed-bed reactor,simultaneously,the formation mechanisms of levoglucosan（LG）and pyrrole were explored with DFT calculations.Results showed that the yield of CO in pyrolysis gas was the highest,reaching 147.66 m L/g at 800℃.The yields of H₂ and CH₄ increased significantly with CO₂decrease.NH₃ increased the content of oxygen-containing substances and inhibited the formation of aromatic hydrocarbons and aliphatic hydrocarbons.In addition,some nitrogen-containing products were produced,and the relative content increased with the increase of temperature.And the main nitrogen-containing product was pyrrole.The relative content of LG increased about 2 times,and the absolute yield increased about 4.6 times,and reached10.96 wt.%at 600℃.The calculation results showed that NH₃can promote the cellulose pyrolysis for LG,and the reaction energy barrier of of the optimal reaction pathway was194.05 k J/mol.The C2 hydrogen of cellobiose was transferred to O position of glycosidic bond,which breaked the glycosidic bond to form intermediate a4-i1 and glucose.The intermediate a4-i1 combined with NH₃ to form LG by hydrogen transfer,while the glucose produced combinesd with NH₃ to form LG by dehydration.The results showed that NH₃promoted the dealdylation of FF to form furan,and the combination of NH₃ and furan promoted the ring opening reaction of furan more than that of H₂O.After ring opening,the amino group at C2 position was connected to the C5 position to form a ring,and the O atom is dehydrated to form pyrrole.The energy barrier of pyrrole formation from cellulose pyrolysis was 280.08 k J/mol in the presence of NH₃.A simple method to produce 4-vinylphenol through lignin pyrolysis under NH₃atmosphere was proposed.Lignin pyrolysis behavior was investigated in a fixed-bed reactor,simultaneously,the formation mechanism of 4-vinylphenol was explored with DFT calculations.Results showed that H₂ yield increased significantly and became the main component at higher temperature（reaching 190 m L/g at 800°C）,while that of CH₄ and CO₂decreased largely.The introduction of NH₃ also promoted the formation of phenols without methoxy through removing-O-CH₃ branches of phenols with methoxy,while inhibited the generation of O/S-species and aromatics.The 4-vinylphenol yield increased about five times,and reached 4.89 wt.%（based on lignin）with the relative content of 23.43%at 600°C.The DFT calculation results showed the reaction path of NH₃ and lignin pyrolysis for 4-vinylphenol had a low reaction energy barrier（260.63 k J/mol）.For the formation mechanism of 4-vinylphenol,in the initial pyrolysis stage,hydrogen abstraction of amino can promote the C_β-O bond cleavage.In the branched chain crack stage,NH₃ provides more H free radicals and synergetic effect can promote dehydration and decarbonylation reaction to form2-methoxy-4-vinylphenol,and reduce the high energy barrier of demethoxylation significantly in the final stage.The multiple action mechanism of NH₃ significantly reduced the energy barrier of lignin pyrolysis for 4-vinylphenol.