| Lignin is the second largest reserves of renewable natural organic matter besides cellulose,and pulp and paper industrial production will also have a large number of lignin existing in black liquor.Lignin draws extensive attention as the only aromatic polymer that can be obtained from nature,is a green natural organic matter that can be used to produce biomass fuels and high value-added chemicals through a depolymerization process.This characteristic of lignin makes it possible to replace the increasingly depleted petrochemical resources to produce phenolic compounds.This paper studies the influence on depolymerization rate and product characteristics of alkali lignin under different conditions.The experiment uses alkali lignin as raw material,which is extracted from the black liquor of pulp and paper industry,the depolymerization experiments were carried out in micro autoclave reactor and sub-and supercritical ethanol environment.In order to lay the foundation of depolymerizing lignin to prepare bio-fuel and high value-added chemicals.The results showed that the reaction temperature and reaction time had the significant effect on the yields of depolymerization.The yield of residual lignin was the lowest at 240 oC/4 h and the highest residual char and oil yields were 26.05% and 63.92%,respectively.If the temperature was too low and the time was too short,the depolymerization would be not sufficient;If the temperature was too high and time was too long,the secondary polymerization is obvious;Both cases were not conducive to the depolymerization of the alkali lignin.The depolymerization products properties and the depolymerization mechanism of lignin structure were studied by gas chromatography/mass spectrometry(GC/MS),scanning electron microscopy(SEM)and fourier transform infrared spectroscopy(FTIR).Alkali lignin firstly formed micro-sized microspheres on the surface during the depolymerization of sub-and supercritical ethanol system.Then the increasing temperature and prolonging time would cause the diameter of microspheres to decrease even rupture.At low temperature,the ether bond broken to form esters and ketones,while other bonds such as C-C bond broken at the higher temperature to form phenolic compounds.In addition to temperature and time factors,by changing the ratio of ethanol-water and solid-liquid,it was found that the solvent had little effect on the yield of alkali lignin depolymerization.Through quantitative analysis,the results showed that the best yields of main phonelic product were as follows: phenol was 64.908 mg/g(300 oC/8 h),guaiacol was51.154 mg/g(240oC/8 h),phenol,2,6-dimethoxy was 118.922 mg/g(240oC/4 h).Acetosyringone was the main product of alkali lignin depolymerization at short time and low temperature.But the contents of guaiacol,phenol,and 2,6-dimethoxy phenol were on the rise and the amount of macromolecular products were decreasing while the reaction temperature and reaction time were increasing.The effects of nickel-based catalysts and rhodium/carbon catalysts on the yield of alkali lignin were studied.Both catalysts could increase the oil yield and reduce the residual char yield.And the best depolymerization yield also was obtained at 240 oC/4 h.The catalyst dosage was0.01 g.Nickel-based catalyst: residual char yield was 19.87%,oil yield was 67.04%;rhodium/carbon catalyst: residual char yield was 18.65% and oil yield was 75.38%.This is beneficial to further optimize the alkali lignin preparation and improve the content of phenolic compounds. |
