Evolution Of Oxygen-containing Functional Groups During Lignin Pyrolysis And Its Effect On Lignin Pyrolysis

Lignin is the component with the highest calorific value of lignocellulosic materials,and it is the only renewable aromatic compound.Meanwhile,the lignin is a kind of biomass renewable resource which can partially replace fossil fuel and have a high potential of development and utilization.The technology of high value utilization of lignin is not mature now,poor selectivity and low yield of the target products are still problems in the lignin pyrolysis process.In order to improve the utilization value of lignin,a series of systematic and comprehensive researches have been carried out on the oxygen-containing functional groups?methoxy,phenolic,alcohol hydroxyl,ether bond?with the highest content in lignin.Their evolution rule and the influence mechanism on the pyrolysis products have been clarified.At first,the pyrolysis gas chromatography-mass spectrometry?Py-GCMS?was modified,and the pyrolysis products of lignin at different temperature stages in the pyrolysis process were explored.At the same time,the evolution of gas and solid products in the pyrolysis process was detected by TG-IR,IR and NMR.It is found that the reactions which occurred in the pyrolysis process contain ether bond breaking,carbon bond breaking,side chain unsaturated bond removing,and methoxyl splitting,rearrangement.These lead to different components and yields of volatile products in different temperature stages.The water from the primary pyrolysis of lignin would participate in the secondary pyrolysis,and more methane would be produced in the secondary pyrolysis.With the increase of pyrolysis temperature,lignin pyrolysis residue would be dehydrogenated and deoxidized gradually,resulting in the decrease of functional groups and the increase of carbon unsaturation.At the same time,the activation energy was low and stable in the early stage of lignin pyrolysis,and it increases rapidly with the increase of lignin conversion in the late stage of pyrolysis.The evolution of lignin functional groups catalyzed by HZSM-5 zeolite and activated carbon was studied.Py-GCMS was used to study the two catalysts effects on the pyrolysis products of lignin.The evolution of lignin functional groups in rapid pyrolysis with different catalytic modes had been analyzed.Results indicate that HZSM-5 zeolite could promote the conversion of methoxy to methyl or completely remove,and it has strong ability of dehydrogenation;active carbon could promote the conversion of methoxy to phenolic hydroxyl.Under the condition of catalyst:lignin=1:1 in situ,HZSM-5 zeolite removed 25%methoxyl and 30%phenol hydroxyl,while the methoxy conversion into phenolic hydroxyl was increased6%and the amount of phenolic hydroxyl removal was increased 7%by activated carbon catalysis.Catalytic effect on the pyrolysis of lignin was affected by the catalyst amount,the catalysis/pyrolysis temperature and the catalytic mode.The coke yield and the activation energy in the later stage of lignin pyrolysis could be reduced by HZSM-5 zeolite catalysis.But activated carbon catalysis could increase the coke yield and the activation energy in the later stage of lignin pyrolysis.The lignin was modified by dimethyl sulfate and iodohexane,respectively.Modified lignin with different phenolic hydroxyl and methoxyl content was prepared.The effects of phenolic hydroxyl and methoxyl groups on the pyrolysis of lignin were revealed by the change of pyrolysis product distribution,composition and pyrolysis characteristics.Lignin model compounds was used to research the action mechanism of the two functional groups in the lignin pyrolysis.The experimental results show that 95%phenol hydroxyl in lignin could be methylated when the dosage of dimethyl sulfate was 5 ml/g lignin,and the yield of pyrolytic coke could be reduced 21.4%.When the content of iodohexane was 0.4 g/g lignin,15%of methoxyl could be changed into phenolic hydroxyl group,and the coke yield of lignin pyrolysis was reduced by 5.6%.Phenol hydroxyl group and methoxy group are the key factors to cause the pyrolytic coking of lignin,but their action mechanisms are significantly different.The major reason for the pyrolytic coking of lignin is quinone structure formation due to the dehydrogenation of phenol hydroxyl group,and methoxy group is not the necessary factor for the pyrolytic coking of lignin,but the methoxy group could promote the coking reaction which caused by phenol hydroxyl group.At the same time,there are obvious influence on the structure of aromatic compounds in lignin pyrolysis by phenolic hydroxyl and methoxy groups as electron donor groups,increasing the density of benzene ring electron cloud inhibit the coking reaction in the lignin pyrolysis process.Four lignin dimer model compounds with different side chain structure were used to explore the effects of ketone and alcohol hydroxyl groups in lignin side chain on the lignin model compounds pyrolysis.The results show that the functional groups of lignin side chain interact with each other during pyrolysis.The addition of CH₂OH group in the C_?position in the model increased the yield of pyrolytic coke from C_?=O and C_?-OH model compounds,and it increased the complexity of the pyrolysis reaction.The difference is that the interaction of C_?-keto group and CH₂OH group changed the reaction of alcohol hydroxyl group to form water and methane at low temperature,and then to form formaldehyde and methane.Furthermore,the pyrolysis activation energy of C_?=O model compound was increased by the addition of CH₂OH group,and there no benzaldehyde was produced during pyrolysis.However,the interaction between C_?-alkoxy group and CH₂OH group promoted the removal of alkoxy group and the breaking of side chain C-C bond,which made the model compound produce water and methane at low temperature.Moreover,the activation energy of C_?-OH model compounds in pyrolysis process decreased with the increase of CH₂OH group in C_?position.