| Fuel ethanol is a kind of clean energy, which could be direct co-fired with gasoline without vehicles transformation. Cellulosic ethanol produced from lignocellulose is a new type of fuel ethanol, this kind of fuel is burned with nearly zero-emission greenhouse gas.Pretreatment technology is the first and the most important step in cellulosic ethanolproduction, it will determine sugar recovery, crystallinity and accessibility from cellulase to substrate. This paper mainly focus on phosphoric acid-acetone pretreatment method, poplar (Populus hopeiensis Hu et Chow) is regarded as raw material to carry out a systematic research. Poplar main components were measured by high performance liquid chromatography, results showed that: cellulose40.77%, xylan18.62%, lignin25.04%, following study is based on this composition result.Effect of pre-immersion temperature to component separation is the largest, followed by reaction time, liquid/solid ratio has limited effect. Although increased temperature and prolonged time could enhance dissolution and hydrolysis of cellulose, lignin removal is also raised, however, if pre-immersion conditions are too severe, it will lead to material loss and sugar over-hydrolyzed. XRD characterization showed that color rendering index CrI fall from39.9%to27.7%after pretreatment, CrI decrease and crystal structure transition further improve enzymatic hydrolysis. After24h enzymatic test, enzymatic hydrolysis efficiency reached93.36%.Using Saccharomyces cerevisiae to ferment the hydrolysate, total conversion is regarded as the index to optimize the pretreatment conditions through response surface method. Analysis of variance analysis showed that, phosphoric acid concentration, pre-immersion temperature and reaction time all had significant effect (p<0.001) on total conversion. Given response values weight, optimum pretreatment process is determined, it is pretreated at53.9℃with84.7wt%phosphoric acid concentration and liquid/solid ratio8.51:1for43.6min, the highest total conversion reached18.92%.Simulation results of pre-immersion process showed that, phosphoric acid molecules was adsorbed into cellulose fragments and formed four-hydrogen bonds structure, the binding energy is-1.61eV, far greater than binding energy of cellulose fragments in acetone or water. Energy gap of phosphoric acid-cellulose system mounted up to4.46eV, significantly less than energy gap of cellulose-acetone system and cellulose-water system. Molecular dynamics results in phosphoric acid-cellulose system revealed cellulose clusters spread to entire reaction system in3ns, cellulose molecules dissolved in phosphoric acid. However, bond cleavage energy barrier of cellulose clusters in phosphoric acid, acetone and water were3.301eV,3.201eV and3.132eV, which are much higher than0.75eV, they all have high chemical stability.Guaiacyl glyceryl-β-O-coniferyl alcohol is set as lignin model, dissolution energy of lignin in phosphoric acid is1.58times than that in acetone, and in acetone activation energy, reaction energy of β-O-4bond cleavage and hydrogen bond cleavage energy are both lower than energy in phosphoric acid, In acetone, the internal force of lignin fragmentsis quite loose, piles of lignin formmany holes, surface area of hole is634.21cm2/mol. So lignin fragments dissolved easily in acetone. However, β-O-4bond cleavage and hydrogen bond cleavage of lignin in phosphoric acid and acetone were both endothermic. Reaction activation energy, reaction energy and hydrogen bond cleavage all have relatively high values, β-O-4bond is not easy to fracture, lignin molecules could maintain high chemical stability.After1h pretreatment under50℃, acetone extracted lignin yield could reach21.57%. In infrared absorption spectroscopy, the absorptionsin typical lignin monomer absorption peak are obvious, so lignin modification degree is relatively small. The1H-NMR spectrum showed that it has absorption in absorption peaks oftypical chemical shift, close to the absorption peak position and shapeof standard lignin spectrum. P-hydroxyphenyl lignin monomer becomes main part of extracted lignin, accountes for almost half of total lignin. Extracted lignin only underwent slight group changes, degradation degree is small, so this kind of lignin has a high practical value, it could be used as a by-product to reduce ethanol production cost. |
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