Effect Of ScCO₂ Pretreatment On Isolation Of The Lignin From Eucalyptus

Lignocellulosic biomass is the most abundant and renewable resource.As a natural composite material,the efficient and environmental utilization of plant fiber has been impeded by its innate close structure.Supercritical carbon dioxide(scCO2)pretreatment is an effective physical-chemical combined technology which is efficient and environmentally friendly.As a pretreatment method for biomass resources,it has attracted more and more attention.At present,the scCO2 pretreatment is mainly applied in the fields of extraction and bioethanol production.However,different applications of plant fiber require inconsistent pretreatments,and scCO2 pretreatment also has different effects on different raw materials.Therefore,it is of great significance to study the principle and effect of scCO2 pretreatment on the composition and structure of plant fibers.These findings on the scCO2 pretreatment could potentially expand the application of scCO2 pretreatment in the preparation of nanomaterials or components separation.Firstly,eucalyptus fiber was pretreated by steam explosion,scCO2 explosion and scCO2-steam explosion(SCS).The structural and compositional changes of eucalyptus fibers was studied to explore the mechanism of different explosion.From the result,we find that the effect of scCO2 explosion is not as destructive as steam explosion.But in SCS explosion,scCO2 and steam form carbonic acid to promote the degradation of hemicellulose and lignin.The degraded productions of hemicellulose and lignin migrated and eluted under the flow of scCO2 fluid and steam.When pressure was relief instantaneously,the pressure differ from internal and external cell.The fluids that carried degradation productions leaked out through pit membrane.The pit membrane was also broke by the explosive force.SCS can destroyed the structure of eucalyptus fibers greatly.Surface pore size and crystallinity of eucalyptus fiber after scCO2-steam(SCSE)is the largest.Hemicellulose and lignin degraded and eluted so that the thermal stability of SCSE increased significantly.Secondly,the orthogonal experiments was designed using four factors(temperature,pressure,time and moisture content)to analyze their effects on specific surface area(SSA),crystallinity and thermo stability of eucalyptus.With increasing temperature,the carbonic acid formed by steam and scCO2 promoted the degradation of hemicellulose and lignin as well as the softening of lignin.As a result,the damage to eucalyptus structure intensified and the holes on the surface increase.And the amorphous material is removed so that the crystallinity increased.With the increase of time,it is beneficial to further degradation and removal of hemicellulose and lignin.The crystallinity and SSA of eucalyptus fibers increased.As the pressure increased,the explosive force on the pit film increased when the pressure is released instantaneously,resulting in the increases of the hole on the surface.At the temperature of 200?,pressure of 12 MPa,reaction time of 30 min and liquid to solid ratio of 10:1,the surface pore size,crystallinity,SSA and thermal stability were the highest.The crystallinity increased from 58.4%of the raw material to 69.4%and SSA increased from 0.74 m2/g to 1.25 m2/g.The formation of carbonic acid accelerated the degradation of amorphous components such as hemicellulose and lignin,which resulted in the loosening of the eucalyptus structure.The addition of ethanol(67%)formed a co-solvent system that can further extracted lignin of eucalyptus(SCEL).Orthogonal test was used to study the effects of temperature,pressure,reaction time and liquid-solid ratio on the yield of lignin and the structure and properties of the residue.The results indicated that the most important factor that affected lignin yield was liquid-solid ratio.The increase of liquid-solid ratio is conducive to the complete dissolution and separation of lignin,so that the yield of lignin is significantly increased.The increasing temperature was beneficial to degrade hemicellulose and break its bonds with lignin so that and promote the dissolution of lignin.As a result,the yield of lignin as well as the crystallinity,SSA and pores on surface increased.With increasing reaction time,hemicellulose can be further degraded and removed,so that SSA,crystallinity of eucalyptus residues increase as well as the yield of lignin increased.At 180? and 12 MPa,the yield of SCEL reached 33.1%after extracted for 20 min in 45 ml 67%ethanol/water as co-solvent.The yield of SCEL is higher than lignin extracted by phosphoric/ethanol method(PL,25.5%)and ethanol method(EL,24.0%).The structures of EL,SCEL and PL lignin was compared.The strong acid environment in the phosphor/ethanol extracted method broke the ?-O-4 aryl ether bond and resulted in the condensation of lignin,which lead to an increases of molecular weight and thermal stability.And the condensation of PL against the later use of lignin.In ethanol extracted method,it is difficult to extract those lignin with high molecular weight so the yield of lignin and molecular weight are low.There was hemicellulose in EL and the thermal stability decreased.The yield of SCEL is high and acidic environment is conducive to the degradation of the hemicellulose and can increase the purity of the lignin.scCO2-ethanol extracted method do less destruction of the aromatic ether bonds.In conclusion,carbonic acid was formed in scCO2 explosion and resulted in the degradation of hemicellulose and lignin.The structure of eucalyptus was destroyed and chemical components migrated.Therefore the accessibility of chemical reagents is increased.At the same time,the scCO2-ethanol method is an efficient lignin extracted method and provided high purity lignin.CO2 escape when pressure is released without residue in eucalyptus and no additional separation step is required.