Study On Green Solvents Pretreatment And Process Intensification Of Lignocellulose

Developing renewable energy is an effective measure to defuse energy crisis,decrease environmental pollution and realize sustainable development of human society.It is an important way for the development of renewable energy that converting enzymatic hydrolysis sugar form lignocellose into fuel-ethanol.However,lignocellulose has a natural barrier against degration because of its rigid and compact structure.Pretreatment is needed to break its structure and to release cellulose for enzymatic hydrolysis.In this paper,liquid hot water,supercritical CO2 and ionic liquids were used as green solvents to pretreat the straw lignocellulose.The parameters were systematically investigated and optimized in order to improve and intensify the pretreatment process.Meantime,the mechanism of the pretreatment was studied.The main research contents were as follows:(1)Liquid hot water pretreatment.Sorghum stalk and corn cob were pretreated by liquid hot water pressurized with N2 or CO2.Temperature(180-220℃),time(0-30 min),gas type(N2 or CO2)and pressure(4-9 MPa)were investigated to optimize pretreatment process in order to obtain the maximum total sugar(hemicellulose reducing sugar+enzyme hydrolysis sugar)yield.The effect of carbonic acid which was produced when CO2 dissolves in water was studied through analysis of the pH of prehydrolation and hemicellulose reducing sugar content.Furthermore,the microstructure and crystallinity of the samples before and after pretreatment was characterized by SEM and XRD.On the basis of analysis and characterization,the mechanism of liquid hot water pretreatment was also studied.Finally,the hydrolysis kinetics of sorghum stalk hemicellulose and that of corn cob was studied in liquid hot water pretreatment pressurized with CO2.The pre-exponential factors and active energies were obtained.The results from hemicellulose hydrolysis kinetics equation was compared with the experimental values.(2)Supercritical CO2(scCO2)pretreatment.scCO2 pretreatment of lignocellulose at high temperature(150-170℃)in literature required much energy and made lignocellulose hydrothermal carbonization.So,sorghum stalk,corn cob and sunflower stover were pretreated by scCO2 at low temperature in this paper.Furthermore,the mechanism of the action of water in scCO2 pretreatment was discussed and distinguished from that in steam explosion pretreatment.Temperature(40-80℃),pressure(15-25 MPa)and time(6-48 h)were investigated at the fixed water contents(75 wt%).Ultrasound was used to intensify the sole scCO2 pretreatment process and decrease pretreatment time.Water wetting,softing and swelling the lignocellulose in the pretreatment process was analyzed through SEM images and XRD results and enzymatic hydrolysis sugar yield as well as the comparision between scCO2 pretreatment at high and low temperature.CO2 infiltration and explosion in the pretreatment was presented.(3)Pretreatment of ionic liquids.Corn cob was pretreated using[EMIM]Ac.The effects of pretreatment temperature(30-90℃),time(0.5-3 h),solid-liquid ratio(1:1-1:10)and recycling times of[EMIM]Ac on the enzymatic hydrolysis sugar yield were investigated.The purpose was to reduce amount of[EMIM]Ac and the pretreatment cost,CO2,water and ethanol were used as co-solvents added to[EMIM]Ac to pretreat corn cob,respectively.Effect of temperature,time and[EMIM]Ac content were investigated in[EMIM]Ac and co-solvents pretreatment.Mechanism of ionic liquids pretreatment was studied through analysis of solid residue content after pretreatment.Finally,β value in Kamlet-Taft parameters of ionic liquids and its mixed solvents was analyzed in order to explore the ionic liquids pretreatment mechanism and give the basic priciples for selections solvent.(4)Molecular dynamics simulation.The methanism of[EMIM]Ac and[EMIM]Ac-H2O/C2H5OH was explored by molecular dynamics simulation.Long-chain to lignin model was established and optimized at the b31yp/6-311++g(d,p)group level.Molecular dynamics simulations were perforemed with Gromacs.Basically,lignin dissollution process in[EMIM]Ac-H2O/C2H5OH was studied.Both radial distribution function calcations of cations and anions and average number of hydrogen bonds with lignin were analyzed.Additionally,the interaction between anion,cation and lignin in solvent was studied,and the molecular dynamics simulations results combined with those of ionic liquids pretreatment and β values were comparied.Finally,the mechanism of ionic liquids and its mixed solvents was elaborated.