磷酸—丙酮预处理法制取纤维乙醇的工艺优化与机理研究
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摘要
燃料乙醇是一种可以直接与汽油混烧、无需车辆改造直接应用的清洁能源。产自木质纤维素的纤维乙醇则是一种近零温室气体排放的新型燃料乙醇。
预处理技术是纤维乙醇制备中的第一步也是最为关键的步骤,决定着物料的糖回收率、结晶度和酶可及度等决定后续酶解率高低的因素。本文聚焦磷酸-丙酮预处理法,以杨木(Populus hopeiensis Hu et Chow)为原料开展系统全面的组分变化研究。杨木原料主要成分为纤维素40.77%,木聚糖18.62%,木质素25.04%。
系统研究了预浸温度、预浸时间、磷酸/杨木液固比和磷酸浓度对后续酶解发酵的影响规律。预浸温度的提高和处理时间的延长有利于综纤维素的溶解和水解,也利于木质素的脱除,然而过于激烈的预浸条件会导致的物料损失和糖类水解。预处理后的物料结晶度指数CrI值会降至27.7%,酶解24h后酶解率最高可到93.36%。选用酿酒酵母(Saccharomyce scerevisiae)对多组水解液进行发酵,以总转换率为考察指标,发现磷酸浓度、预浸温度和反应时间均对总转化率均有着显著的影响(P<0.001),而液固比的影响较小,最优预处理工艺为温度53.9℃、时间43.6min84.7wt.%磷酸浓度、液固比8.51：1,物料到乙醇的总转化率高达18.92%。
纤维素分子在预处理中的模拟计算结果表明,纤维素与磷酸、丙酮和水的结合能分别为-1.61eV、-0.01eV、-0.31eV。纤维素-磷酸系统、纤维素-丙酮系统和纤维素-水系统的能隙值分别为4.46eV、5.48eV、5.38eV。纤维素-溶剂系统中的自发性动力学研究显示,3ns内纤维素团簇会快速分散并溶解于磷酸；而纤维素团簇加入丙酮中,1ns后便迅速聚集而发生沉降。纤维素团簇在磷酸、丙酮和水溶剂中的键断裂活化能分别为3.301eV,3.201eV和3.132eV,均远高于0.75eV,具有化学稳定性。因此,磷酸分子会被吸附于纤维素片段中间而形成四氢键结构,纤维素分子会部分溶解于磷酸中,添加丙酮后又会发生沉降的现象。
以愈创木基甘油-p-O-松柏醇醚为木质素模型,模拟计算结果表明,木质素在磷酸中的溶解能是在丙酮中的1.58倍,而且木质素在丙酮中p-0-4键断裂的反应活化能、反应能和氢键断裂能均低于在磷酸中的能量。在丙酮中,木质素片段的微观结构较为松散,堆积木质素形成空穴的表面积可达634.21cm2/mol,因此木质素片段在丙酮中容易发生溶解。木质素在磷酸和丙酮中的β-0-4键断裂和氢键断裂均为吸热过程,反应活化能、反应能和氢键断裂能均较高,不易发生键断裂,可以保持化学稳定性。
木质素是分散剂、粘结剂的重要原料。杨木经磷酸预浸和丙酮提取后,木质素产率最高可达21.57%。针对预处理过程中的提取木质素进行化学结构研究。热解-气相色谱-质谱结果表明,对羟苯基成为最主要的木质素单体,占到木质素单体总量的50%,提取木质素仅发生了轻微程度的基团变化。红外吸收图谱和核磁共振氢谱结果表明,提取木质素的图谱与标准木质素图谱的差异较小。本预处理法的提取木质素单体降解程度较小,没有引入新的官能团,具有较好的实用价值。
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.
预处理技术是纤维乙醇制备中的第一步也是最为关键的步骤,决定着物料的糖回收率、结晶度和酶可及度等决定后续酶解率高低的因素。本文聚焦磷酸-丙酮预处理法,以杨木(Populus hopeiensis Hu et Chow)为原料开展系统全面的组分变化研究。杨木原料主要成分为纤维素40.77%,木聚糖18.62%,木质素25.04%。
系统研究了预浸温度、预浸时间、磷酸/杨木液固比和磷酸浓度对后续酶解发酵的影响规律。预浸温度的提高和处理时间的延长有利于综纤维素的溶解和水解,也利于木质素的脱除,然而过于激烈的预浸条件会导致的物料损失和糖类水解。预处理后的物料结晶度指数CrI值会降至27.7%,酶解24h后酶解率最高可到93.36%。选用酿酒酵母(Saccharomyce scerevisiae)对多组水解液进行发酵,以总转换率为考察指标,发现磷酸浓度、预浸温度和反应时间均对总转化率均有着显著的影响(P<0.001),而液固比的影响较小,最优预处理工艺为温度53.9℃、时间43.6min84.7wt.%磷酸浓度、液固比8.51：1,物料到乙醇的总转化率高达18.92%。
纤维素分子在预处理中的模拟计算结果表明,纤维素与磷酸、丙酮和水的结合能分别为-1.61eV、-0.01eV、-0.31eV。纤维素-磷酸系统、纤维素-丙酮系统和纤维素-水系统的能隙值分别为4.46eV、5.48eV、5.38eV。纤维素-溶剂系统中的自发性动力学研究显示,3ns内纤维素团簇会快速分散并溶解于磷酸；而纤维素团簇加入丙酮中,1ns后便迅速聚集而发生沉降。纤维素团簇在磷酸、丙酮和水溶剂中的键断裂活化能分别为3.301eV,3.201eV和3.132eV,均远高于0.75eV,具有化学稳定性。因此,磷酸分子会被吸附于纤维素片段中间而形成四氢键结构,纤维素分子会部分溶解于磷酸中,添加丙酮后又会发生沉降的现象。
以愈创木基甘油-p-O-松柏醇醚为木质素模型,模拟计算结果表明,木质素在磷酸中的溶解能是在丙酮中的1.58倍,而且木质素在丙酮中p-0-4键断裂的反应活化能、反应能和氢键断裂能均低于在磷酸中的能量。在丙酮中,木质素片段的微观结构较为松散,堆积木质素形成空穴的表面积可达634.21cm2/mol,因此木质素片段在丙酮中容易发生溶解。木质素在磷酸和丙酮中的β-0-4键断裂和氢键断裂均为吸热过程,反应活化能、反应能和氢键断裂能均较高,不易发生键断裂,可以保持化学稳定性。
木质素是分散剂、粘结剂的重要原料。杨木经磷酸预浸和丙酮提取后,木质素产率最高可达21.57%。针对预处理过程中的提取木质素进行化学结构研究。热解-气相色谱-质谱结果表明,对羟苯基成为最主要的木质素单体,占到木质素单体总量的50%,提取木质素仅发生了轻微程度的基团变化。红外吸收图谱和核磁共振氢谱结果表明,提取木质素的图谱与标准木质素图谱的差异较小。本预处理法的提取木质素单体降解程度较小,没有引入新的官能团,具有较好的实用价值。
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.
引文
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