陳 倩 陳京環(huán) 王 堃 蔣建新 孫潤(rùn)倉(cāng)
(1.北京林業(yè)大學(xué) 北京 100083; 2.中國(guó)制漿造紙研究院 北京 100102)
熱水預(yù)處理生物質(zhì)原料及其生物轉(zhuǎn)化研究進(jìn)展*
陳 倩1陳京環(huán)2王 堃1蔣建新1孫潤(rùn)倉(cāng)1
(1.北京林業(yè)大學(xué) 北京 100083; 2.中國(guó)制漿造紙研究院 北京 100102)
在能源問(wèn)題日益緊張的時(shí)局下,尋求可再生清潔能源是亟待解決的關(guān)鍵問(wèn)題。由農(nóng)林廢棄物轉(zhuǎn)化獲得新能源、新材料已經(jīng)成為重要的發(fā)展趨勢(shì),其中生物乙醇作為環(huán)保、可持續(xù)的新型能源得到了廣泛關(guān)注。預(yù)處理作為生物乙醇制備的第一個(gè)重要環(huán)節(jié)備受重視,傳統(tǒng)化學(xué)預(yù)處理技術(shù)能量消耗大、對(duì)設(shè)備要求高、半纖維素降解嚴(yán)重且對(duì)環(huán)境造成污染,沒(méi)有充分考慮半纖維素和木質(zhì)素的高值化回收利用,單一化降解纖維素使得經(jīng)濟(jì)利用價(jià)值很低;生物預(yù)處理作為一種環(huán)境友好和低成本的預(yù)處理技術(shù),也存在著轉(zhuǎn)化效率低、作用周期長(zhǎng)和碳水化合物損失嚴(yán)重的缺點(diǎn)。熱水預(yù)處理通過(guò)條件參數(shù)優(yōu)化可在盡量保留天然大分子原位結(jié)構(gòu)的基礎(chǔ)上,一定程度地破壞植物細(xì)胞壁的致密結(jié)構(gòu),且僅利用水作為反應(yīng)試劑,具有無(wú)化學(xué)藥劑使用、對(duì)環(huán)境友好、操作成本低等優(yōu)點(diǎn),其對(duì)生物質(zhì)細(xì)胞壁的主要作用為使木素-碳水化合物復(fù)合體(LCC)連接鍵斷裂并除去部分半纖維素,使木質(zhì)素性質(zhì)發(fā)生改變并進(jìn)行再分配,且在一定程度上降低了纖維素的聚合度。熱水預(yù)處理過(guò)程中生成的糠醛、5-羥甲基糠醛、甲酸、乙酸等產(chǎn)物,會(huì)對(duì)生物降解產(chǎn)生抑制作用,可以通過(guò)優(yōu)化條件來(lái)控制其含量。酶解過(guò)程是指利用纖維素酶及其輔助酶將預(yù)處理后的纖維素降解為可發(fā)酵單糖,若直接將預(yù)處理后的產(chǎn)物進(jìn)行發(fā)酵則需要較長(zhǎng)時(shí)間且僅能獲得極低濃度的乙醇。酶水解過(guò)程中由于半纖維素和木質(zhì)素的保護(hù)作用,阻礙了纖維素酶與纖維素底物的接觸,而預(yù)處理過(guò)程則會(huì)削弱或完全破壞這種阻礙作用,增大酶與纖維素的接觸面積使酶解效率提高。提高預(yù)處理溫度會(huì)使乙醇發(fā)酵得率提高,但是預(yù)處理溫度過(guò)高會(huì)導(dǎo)致纖維素降解從而使乙醇得率降低。本文對(duì)熱水預(yù)處理過(guò)程中纖維素、半纖維素、木質(zhì)素物理化學(xué)性質(zhì)的改變和處理過(guò)程中抑制物的轉(zhuǎn)化生成進(jìn)行總結(jié),分析比較在不同預(yù)處理?xiàng)l件下生物質(zhì)中各主要組分和降解產(chǎn)物不同程度的變化及其對(duì)后續(xù)酶水解、酵母發(fā)酵的影響。
熱水預(yù)處理; 木質(zhì)生物質(zhì); 抑制物; 酶水解; 生物乙醇
化石能源使人們生活發(fā)生了翻天覆地的變化,人們對(duì)能源的消耗也越來(lái)越多,交通運(yùn)輸、制造業(yè)、電力等部門(mén)都在消耗大量能源。對(duì)化石能源的高度依賴(lài),不僅使能源逐漸短缺,而且對(duì)環(huán)境造成了負(fù)面影響。隨著化石能源的枯竭以及面臨的全球變暖問(wèn)題,人們開(kāi)始尋求新型環(huán)保能源,其中使用生物質(zhì)轉(zhuǎn)化為乙醇是目前最受人們關(guān)注的生產(chǎn)液體燃料的方法。基于木質(zhì)纖維生物質(zhì)來(lái)源廣、成本低等特點(diǎn),使用這類(lèi)原料制備生物乙醇的研究已經(jīng)取得了很大進(jìn)展(李文等, 2009); 然而,目前乙醇的生產(chǎn)技術(shù)還不能滿(mǎn)足市場(chǎng)要求,大規(guī)模地生產(chǎn)乙醇燃料需要新的思路。
木質(zhì)生物質(zhì)轉(zhuǎn)化為乙醇主要包括4個(gè)步驟: 1) 木質(zhì)生物質(zhì)的預(yù)處理; 2) 纖維素經(jīng)酶水解轉(zhuǎn)化為葡萄糖; 3) 將葡萄糖發(fā)酵轉(zhuǎn)化為乙醇; 4) 乙醇的分離和脫水(Mosieretal., 2005a)。其中,木質(zhì)生物質(zhì)的預(yù)處理是十分關(guān)鍵的一步,其原因在于木質(zhì)生物質(zhì)結(jié)構(gòu)的復(fù)雜性。木質(zhì)生物質(zhì)主要由纖維素(30%~45%)、半纖維素(20%~35%)和木質(zhì)素(10%~30%)3種高分子聚合物組成,在分子水平上纖維素、半纖維素和木質(zhì)素分子間存在不同的結(jié)合力。木質(zhì)素為不定形、非均一、非線(xiàn)性的三維立體高聚物,除分子間的化學(xué)連接外,還同時(shí)以共價(jià)鍵和半纖維素結(jié)合,而纖維素線(xiàn)性分子間和分子內(nèi)氫鍵形成的結(jié)晶結(jié)構(gòu),使其難溶于水,導(dǎo)致纖維素在自然條件下水解速率極低。典型的物理化學(xué)預(yù)處理方式,如蒸汽爆破、有機(jī)溶劑、硫酸和熱堿等,對(duì)反應(yīng)設(shè)備要求嚴(yán)格,耗費(fèi)大量的能源(Sunetal., 2002; Silversteinetal., 2007; Dataretal., 2007),并且產(chǎn)生對(duì)后續(xù)酶水解有抑制作用的物質(zhì),生產(chǎn)成本高昂且污染環(huán)境;生物預(yù)處理作為一種環(huán)境友好和低成本的預(yù)處理技術(shù),也存在著效率低、作用周期長(zhǎng)和碳水化合物嚴(yán)重?fù)p失的缺點(diǎn)(Kelleretal., 2003; Hwangetal., 2008; Zhangetal., 2007; Shietal., 2008; 2009)。熱水預(yù)處理通過(guò)高溫高壓來(lái)處理木質(zhì)纖維素,能部分水解半纖維素并破壞木質(zhì)素及纖維素的結(jié)構(gòu),增加生物轉(zhuǎn)化底物的酶可及面積(Mosieretal., 2005a),被認(rèn)為是成本效益最優(yōu)的木質(zhì)生物質(zhì)預(yù)處理方式(Yuetal., 2010a)。在120~230 ℃的溫度條件下熱水預(yù)處理生物質(zhì)原料245 min,會(huì)有40%~60%的物料損失,其中包括4%~22%的纖維素、35%~60%的木質(zhì)素和幾乎全部的半纖維素組分(Mosieretal., 2005a),該預(yù)處理方法具有腐蝕性小、無(wú)沉淀產(chǎn)生且操作成本低等特點(diǎn)(Liuetal., 2010)。
木質(zhì)生物質(zhì)具有復(fù)雜、精細(xì)的細(xì)胞壁結(jié)構(gòu),了解預(yù)處理過(guò)程中纖維素、半纖維素、木質(zhì)素的化學(xué)轉(zhuǎn)化和預(yù)處理后的結(jié)構(gòu)特點(diǎn)對(duì)于消除生物質(zhì)抗降解屏障十分有幫助(Caoetal., 2012)。熱水預(yù)處理過(guò)程對(duì)細(xì)胞壁的作用可以歸結(jié)為以下3方面: 1) 木素-碳水化合物復(fù)合體(LCC)連接鍵斷裂并除去部分半纖維素; 2) 木質(zhì)素性質(zhì)改變和再分配; 3) 纖維素的消晶化(Chundawatetal., 2011)。
1.1熱水預(yù)處理過(guò)程木質(zhì)素理化性質(zhì)變化
木質(zhì)素主要存在于細(xì)胞壁的次生壁S2層中,其主要結(jié)構(gòu)單元為對(duì)羥苯基丙烷(H)、愈創(chuàng)木基丙烷(G)和紫丁香基丙烷(S),主要通過(guò)C—C鍵和醚鍵進(jìn)行連接,其中以β—O—4芳基醚鍵連接鍵最多,主要功能基團(tuán)為甲氧基、羥基(酚羥基和醇羥基)和羰基。諸多因素(物種、科屬、生長(zhǎng)環(huán)境等)會(huì)造成植物間木質(zhì)素含量和組成的不同,闊葉木中木質(zhì)素含有大量紫丁香基結(jié)構(gòu),其次為愈創(chuàng)木基結(jié)構(gòu),針葉木中以愈創(chuàng)木基為主,而禾本科(Gramineae)中3種結(jié)構(gòu)單元均有一定分布(鄭大峰等, 2005)。圖1為不同反應(yīng)條件下樣品預(yù)處理前后的掃描電鏡(SEM)圖像,試驗(yàn)將高壓反應(yīng)釜內(nèi)進(jìn)行熱水預(yù)處理后的樣品與較為溫和的處理方法——索氏提取和回流(以水為溶劑)處理后的樣品進(jìn)行對(duì)比,可以觀察到后者處理的樣品其表面形態(tài)與原料相似,而在高溫水熱處理時(shí),原料破損嚴(yán)重,顏色變深,并且表面有液滴狀顆粒(Nitsosetal., 2013)。熱水預(yù)處理可以除去一小部分木質(zhì)素(溶于酸的部分),但在處理過(guò)程中木質(zhì)素熔融、固化并在纖維表面再聚合,同時(shí)也使部分木質(zhì)素與碳水化合物之間的連接斷裂,改變了木質(zhì)素的化學(xué)結(jié)構(gòu)和分布狀況(Aitaetal., 2010)。一些研究表明,酸不溶木素經(jīng)過(guò)酸性條件處理后會(huì)比處理前的含量更高,這種現(xiàn)象被認(rèn)為是多糖降解產(chǎn)物(如糠醛)重新聚合或者多糖與木素聚合形成的木素類(lèi)化合物,被稱(chēng)為“假木素”(Jungetal., 2010; Maoetal., 2010; Lietal., 2005)?!凹倌舅亍敝泻恤然Ⅳ驶?、脂肪族和芳香族結(jié)構(gòu),并且在不同預(yù)處理?xiàng)l件下分子質(zhì)量變化不大(Mn~1 000 g·mol-1; Mw~5 000 g·mol-1)(Huetal., 2012)。與此同時(shí),木質(zhì)素在高溫水解離的H離子的作用下會(huì)形成芐醇結(jié)構(gòu)中間體,醚鍵不穩(wěn)定容易發(fā)生斷裂,斷裂后的木素碎片與相鄰芳香環(huán)中的正碳離子形成C—C連接。在較為劇烈的條件下,木質(zhì)素再縮合阻礙了熱水預(yù)處理過(guò)程中的解聚作用和木質(zhì)素的脫除,雖然醚鍵斷開(kāi),但是β—O—4′鍵脫除的速率接近于0(Lietal., 2007)。
圖1 原料和不同溫度熱水預(yù)處理后樣品的掃描電鏡(SEM)圖像(Nitsos et al., 2013)Fig.1 SEM images of the parent and representative hydrothermally treated biomass samples at increasing temperature and constant reaction time (15 min) (Nitsos et al., 2013)
1.2熱水預(yù)處理過(guò)程纖維素理化性質(zhì)變化
經(jīng)熱水預(yù)處理后,固體殘?jiān)欣w維素含量隨預(yù)處理強(qiáng)度的增加而減少,同時(shí),水解液中葡萄糖、纖維二糖、纖維素低聚物等纖維素降解產(chǎn)物的含量相應(yīng)增加(Petersenetal., 2009; Liuetal., 2003)。在不同熱水預(yù)處理溫度下,進(jìn)一步分析處理液中的產(chǎn)物發(fā)現(xiàn),纖維素?zé)o定形結(jié)構(gòu)中的一些短鏈仍與結(jié)晶纖維素通過(guò)氫鍵等連接在一起(Yuetal., 2010b)。在較低的預(yù)處理溫度(100 ℃)下,無(wú)定形區(qū)內(nèi)的短分子鏈會(huì)生成C4-C13的低聚物,而完全斷裂糖苷鍵形成葡萄糖單體的最低溫度為150 ℃左右,纖維素結(jié)晶區(qū)開(kāi)始水解為葡萄糖單體的溫度為180 ℃左右,這是由于纖維素結(jié)晶區(qū)分子內(nèi)和分子間的氫鍵作用,極大削弱了H離子對(duì)糖苷鍵的作用效果。纖維素?zé)o定形區(qū)與結(jié)晶區(qū)之間的氫鍵和鏈長(zhǎng)也存在差異,這些差異會(huì)在很大程度上影響熱水預(yù)處理水解液中葡萄糖低聚物的分布。通常,在相同水解溫度下,無(wú)定形纖維素相對(duì)于結(jié)晶部分在主要降解物中會(huì)生成更多葡萄糖及其低聚物,但生成低聚物的選擇性并不隨纖維素聚合度的降低而增加(Yuetal., 2010b)。徐紹華等(2013)利用X射線(xiàn)衍射法研究經(jīng)高溫?zé)崴A(yù)處理后的桉木(Eucalyptus)發(fā)現(xiàn),纖維素的結(jié)晶度在高溫?zé)崴A(yù)處理前后變化很小,但在微觀形態(tài)(圖2)上,處理后的桉木比原料纖維長(zhǎng)度明顯變短,碎片化現(xiàn)象顯著,纖維之間的結(jié)合變得松弛,可以清晰觀察到纖維的斷裂。Saha等(2013)以麥草桿為原料在200 ℃下進(jìn)行熱水預(yù)處理,處理時(shí)間小于25 min時(shí),隨著時(shí)間增加,越來(lái)越多的纖維素被水解,但是處理時(shí)間超過(guò)25 min直至60 min,纖維素的水解量緩慢降低。
圖2 桉木高溫?zé)崴A(yù)處理前后及酶解殘?jiān)膾呙桦婄R(SEM)圖像(徐紹華等, 2013)Fig.2 SEM images of Eucalyptus before and after hot water pretreatment and enzymolysis (Xu et al., 2013)a.桉木原料Eucalyptus materials; b.預(yù)處理后 Materials after pretreatment.
1.3熱水預(yù)處理過(guò)程半纖維素理化性質(zhì)變化
與纖維素不同,半纖維素的組成與細(xì)胞組織、植物種類(lèi)、糖苷鍵類(lèi)型、側(cè)鏈組成和聚合度有關(guān)(Fengeletal., 1984; Jeffries, 1994)。聚木糖的水解機(jī)制通常采用擬均相反應(yīng)動(dòng)力學(xué)模型和預(yù)處理強(qiáng)度參數(shù)來(lái)表達(dá),但是聚木糖的動(dòng)力學(xué)研究非常困難,不僅反應(yīng)步驟復(fù)雜,而且還受很多因素影響(馬靜, 2015)。Conner等(1986)提出,木聚糖水解為低分子質(zhì)量的木寡糖為第1階段,第2階段的主要產(chǎn)物為木糖。在熱水預(yù)處理過(guò)程中,木質(zhì)生物質(zhì)中的碳水化合物會(huì)發(fā)生降解反應(yīng)或者溶解在水溶液中,尤其是半纖維素主要分解為可溶性低聚物和單糖。隨著預(yù)處理溫度和時(shí)間增加,更多的木聚糖被降解,在糖苷鍵斷裂的同時(shí),從半纖維素上脫離下來(lái)的乙?;谒芤褐行纬梢宜幔禾墙到猱a(chǎn)生5-羥甲基糠醛而戊糖降解為糠醛,并可進(jìn)一步降解生成甲酸(Klinkeetal., 2003; Larssonetal., 1999; Luetal., 2009)。反應(yīng)過(guò)程中生成的乙酸可以促進(jìn)水解,乙酸中的水合氫離子比水中產(chǎn)生的水合氫離子對(duì)水解過(guò)程的作用更大(Garroteetal., 1999)。Yu等(2013)在熱水預(yù)處理過(guò)程中根據(jù)木糖和降解產(chǎn)物的變化,得出在較高的預(yù)處理溫度和壓強(qiáng)下,體系中的木糖分解得更多更快。李海龍等(2012)用熱水對(duì)相思木(Acacia)進(jìn)行預(yù)處理發(fā)現(xiàn),處理液pH隨著預(yù)處理時(shí)間增加逐漸下降,水解結(jié)束時(shí)pH最低可達(dá)到3.0,期間處理液中的糖含量呈現(xiàn)先增加后降低的趨勢(shì)。當(dāng)處理液pH達(dá)到3.2左右時(shí),糖的含量達(dá)到最大值,然后隨著pH下降,糖含量以較快的速度下降。
熱水預(yù)處理過(guò)程中會(huì)生成許多化學(xué)中間體和副產(chǎn)物,對(duì)水解酶(包括纖維素酶、木聚糖酶、β-葡萄糖苷酶等)和發(fā)酵微生物產(chǎn)生抑制作用,從而使水解和發(fā)酵速率減慢。Kont等(2013)通過(guò)高效液相色譜和質(zhì)譜分析證實(shí)熱水預(yù)處理過(guò)程中會(huì)生成抑制纖維素酶活性的低聚糖(聚合度7~10),主要由半纖維素?cái)嗔研纬刹⑷芙庠谔幚硪褐?Puetal., 2013; Holopainen-Mantilaetal., 2013)。在乙醇發(fā)酵過(guò)程中,抑制物主要包括3類(lèi): 弱酸(甲酸、乙酸、葡萄糖醛酸、乙酰丙酸、阿魏酸和對(duì)香豆酸)、呋喃醛類(lèi)(糠醛、羥甲基糠醛)和酚類(lèi)(對(duì)苯二酚),其中弱酸會(huì)使細(xì)胞內(nèi)環(huán)境酸化,是抑制細(xì)胞生長(zhǎng)的主要原因,呋喃醛類(lèi)化合物對(duì)釀酒酵母的影響主要是抑制酵母生長(zhǎng),而酚類(lèi)化合物對(duì)發(fā)酵具有最強(qiáng)的抑制作用,并且低分子質(zhì)量酚類(lèi)化合物毒性更強(qiáng)(李洪興等, 2009; 李志強(qiáng)等, 2015; 肖領(lǐng)平, 2014)。在熱水預(yù)處理的初始階段,基本沒(méi)有糠醛生成,當(dāng)預(yù)處理溫度從25 ℃升至170 ℃、升溫速率為1.2 ℃·min-1、保溫時(shí)間為120 min的條件下,反應(yīng)時(shí)間和H-因子(不同溫度下相對(duì)反應(yīng)速率常數(shù)與其時(shí)間的函數(shù)的積分面積)分別達(dá)到150 min和450時(shí),生成的糠醛含量隨著反應(yīng)時(shí)間和H-因子增加呈直線(xiàn)增加;在熱水預(yù)處理溫度140~160 ℃時(shí)得到的糠醛[(0.31±0.00) mg·g-1)]和5-羥甲基糠醛[(0.11±0.01) mg·g-1]含量較低,在180 ℃糠醛含量增加到(0.63±0.02) mg·g-1,5-羥甲基糠醛含量依然為(0.11±0.01) mg·g-1,在200 ℃時(shí)二者生成含量分別為(12.00±2.00) mg·g-1和(1.00±0.10) mg·g-1(李海龍等, 2012)。乙酸在熱水解過(guò)程中的含量與糠醛和5-羥甲基糠醛類(lèi)似,200 ℃時(shí)每克原料可以生成(32±0.0) mg乙酸(Sahaetal., 2013)。Min等(2015)將熱水預(yù)處理分2步進(jìn)行,證明當(dāng)?shù)?步熱水預(yù)處理低溫、較長(zhǎng)時(shí)間進(jìn)行,第2步熱水預(yù)處理高溫、較短時(shí)間進(jìn)行,相比單一的在較高或較低熱水預(yù)處理強(qiáng)度下,可以更有效地減少抑制物含量從而提高糖的生物轉(zhuǎn)化效率。
通過(guò)酶水解和微生物降解從植物細(xì)胞中獲得較高得率的葡萄糖,預(yù)處理是十分關(guān)鍵的一步。植物細(xì)胞中的木質(zhì)素和半纖維素對(duì)纖維素具有保護(hù)作用,預(yù)處理過(guò)程可以破壞或除去木質(zhì)素和半纖維素,使酶更容易接觸到纖維素從而促進(jìn)酶解效率(Kumaretal., 2009; Mosieretal., 2005a)。在熱水預(yù)處理過(guò)程中,木質(zhì)素從木材細(xì)胞中轉(zhuǎn)移到纖維素表面,可降低酶的可及度,減慢酶水解速率,但是這種抑制作用隨著水解時(shí)間增加會(huì)有所減弱,在纖維素轉(zhuǎn)化率較高的情況下幾乎消除。研究表明,生物酶制劑與木質(zhì)素液滴之間的結(jié)合并不是抑制作用的主要原因,纖維素表面的木質(zhì)素液滴形成的阻礙作用才是抑制酶水解的重要因素,其作用大小主要取決于木質(zhì)素聚合物分子的化學(xué)性質(zhì)和粒徑大小(Lietal., 2014)。Jeoh等(2007)認(rèn)為,預(yù)處理強(qiáng)度會(huì)影響酶解過(guò)程中酶與底物的反應(yīng)程度。熱水預(yù)處理后的酶水解效率主要取決于預(yù)處理溫度和時(shí)間,較長(zhǎng)的預(yù)處理時(shí)間不利于半纖維素的回收,但是隨著預(yù)處理時(shí)間增加纖維素的水解是先增加后降低的。熱水預(yù)處理溫度對(duì)于酶水解過(guò)程十分重要,在預(yù)處理溫度達(dá)到160 ℃之前,隨著預(yù)處理溫度升高,酶解糖含量緩慢增加,但是在溫度達(dá)到180 ℃時(shí)得到的酶解糖比160 ℃時(shí)增加了55%,預(yù)處理溫度200 ℃又比180 ℃時(shí)增加了43.5%(Sahaetal., 2013)。Dien等(2006)通過(guò)對(duì)比經(jīng)熱水預(yù)處理和未經(jīng)預(yù)處理的無(wú)淀粉玉米纖維,發(fā)現(xiàn)在經(jīng)熱水預(yù)處理原料上培育的真菌產(chǎn)生的酶活性更強(qiáng),并且當(dāng)預(yù)處理時(shí)間從20 min增加到30 min時(shí),木糖的得率增加了14%,延長(zhǎng)預(yù)處理時(shí)間可以除去更多非碳水化合物的側(cè)鏈基團(tuán)或者更進(jìn)一步使細(xì)胞結(jié)構(gòu)打開(kāi)從而增加木糖的可消化性。Mosier等(2005b)對(duì)比研究了190 ℃、15 min和200 ℃、5 min 2種熱水預(yù)處理強(qiáng)度后發(fā)現(xiàn),二者的固體損失分別為35%和30%,而前者最終經(jīng)酶水解后的總糖得率較高,說(shuō)明對(duì)于總碳水化合物利用的生物轉(zhuǎn)化效率與熱水預(yù)處理過(guò)程中的物料損失有關(guān),其中主要是半纖維素過(guò)度降解。Park等(2010)通過(guò)X射線(xiàn)衍射和核磁共振等方法測(cè)定纖維素結(jié)晶度對(duì)酶解效率的影響,結(jié)果發(fā)現(xiàn)結(jié)晶度對(duì)酶解效率的影響并不清晰,除了結(jié)晶度外,木質(zhì)素/半纖維素含量和分布、孔隙率和顆粒大小也會(huì)產(chǎn)生影響。一些研究甚至認(rèn)為結(jié)晶度指數(shù)在酶水解過(guò)程中是增加的,但是增加量很少(Caoetal., 2005; Chenetal., 2007; Wangetal., 2006)。
Negro等(2003)對(duì)經(jīng)熱水處理后的黑楊(Populusnigra)進(jìn)行同步糖化發(fā)酵發(fā)現(xiàn),隨著預(yù)處理溫度升高,糖化發(fā)酵得率升高,最大得率達(dá)到理論值的60%(理論值為原料中含有的葡萄糖成分全部進(jìn)行糖化發(fā)酵,每克葡萄糖可以生成0.51 g乙醇)。Díaz等(2010)研究表明,油菜(Brassicacampestris)籽經(jīng)217.7 ℃、42.2 min熱水預(yù)處理和酶水解處理后,得到了原料中葡萄糖含量的70%,相應(yīng)地,同步糖化發(fā)酵也會(huì)得到了較高的得率;但是如果預(yù)處理強(qiáng)度過(guò)于劇烈,會(huì)造成部分纖維素降解,使生物乙醇得率降低。Romaní等(2010)用桉木在195~250 ℃下進(jìn)行預(yù)處理,經(jīng)酶解糖化發(fā)酵后發(fā)現(xiàn),當(dāng)預(yù)處理溫度達(dá)到210 ℃時(shí),全部纖維素已為葡萄糖,當(dāng)預(yù)處理溫度超過(guò)230 ℃時(shí),部分纖維素降解損失,導(dǎo)致乙醇得率降低。Da Cruz等(2012)以甘蔗(Saccharumofficinarum)作為原料優(yōu)化熱水預(yù)處理?xiàng)l件,當(dāng)處理?xiàng)l件為190 ℃、17.2 min時(shí),用重組啤酒酵母進(jìn)行發(fā)酵乙醇得率最高(理論最高產(chǎn)量的50.1%),并且熱水預(yù)處理后的酶水解產(chǎn)物無(wú)需進(jìn)行脫毒操作即可直接進(jìn)行發(fā)酵操作(Sahaetal., 2013)。
在能源問(wèn)題日益緊張的時(shí)局下,尋求可再生清潔能源是亟待解決的關(guān)鍵問(wèn)題,而乙醇燃料無(wú)疑是化石能源的最佳替代者。基于木質(zhì)纖維生物質(zhì)來(lái)源廣、成本低等特點(diǎn),使用這類(lèi)原料制備生物乙醇的研究取得了很大進(jìn)展。作為地球上儲(chǔ)量最豐富的生物有機(jī)質(zhì),木質(zhì)纖維素每年通過(guò)光合作用固定的太陽(yáng)能達(dá)4×1021J,但是其利用率僅為0.27%(Márquezetal., 2007),如何合理利用這一巨大的生物質(zhì)資源已成為該領(lǐng)域研究的重點(diǎn)和熱點(diǎn)。木質(zhì)生物質(zhì)細(xì)胞壁結(jié)構(gòu)復(fù)雜,導(dǎo)致纖維素在自然條件下水解速率極低,預(yù)處理是生產(chǎn)乙醇燃料十分關(guān)鍵的一步。熱水預(yù)處理是成本效益最優(yōu)的木質(zhì)生物質(zhì)預(yù)處理方式之一,且對(duì)環(huán)境友好,雖然通常要在高溫下進(jìn)行,需要配備耐高壓的反應(yīng)容器,但是反應(yīng)過(guò)程中不需要添加其他化學(xué)試劑,反應(yīng)時(shí)間較短,在盡量保留天然大分子原位結(jié)構(gòu)的基礎(chǔ)上一定程度地破壞了植物細(xì)胞壁的致密結(jié)構(gòu)。木質(zhì)生物質(zhì)復(fù)雜的理化組成和多級(jí)結(jié)構(gòu)的不均一性,是阻礙其經(jīng)濟(jì)高效水解產(chǎn)糖的另一關(guān)鍵問(wèn)題,盡管輔助一些經(jīng)濟(jì)有效的預(yù)處理手段,但添加單一纖維素酶實(shí)現(xiàn)木質(zhì)纖維素-糖轉(zhuǎn)化依然難以跨越酶解成本過(guò)高的障礙,常常需要由纖維素酶和輔助酶(其他水解酶)構(gòu)成的組合酶(催化水解同一底物但不同來(lái)源和特性的2種或2種以上酶配合而成的酶制劑,這些酶之間具有互補(bǔ)性)、復(fù)合酶(催化水解不同底物的多種酶混合而成的酶制劑,這些酶的來(lái)源相同或不同)或組合型復(fù)合酶等混合酶共同作用,才能實(shí)現(xiàn)預(yù)期目標(biāo)(馮定遠(yuǎn)等, 2008)。熱水預(yù)處理過(guò)程中生成的副產(chǎn)物可以通過(guò)進(jìn)一步處理生成更有價(jià)值的產(chǎn)品,其中糠醛用途廣泛,可以應(yīng)用在醫(yī)藥、塑料、橡膠等領(lǐng)域,5-羥甲基糠醛的化學(xué)性質(zhì)也較為活潑,可以應(yīng)用在化工領(lǐng)域,甲酸和乙酸在諸多領(lǐng)域中均有良好應(yīng)用,乙酸還可以作為催化劑促進(jìn)生物質(zhì)原料中化學(xué)鍵的斷裂。熱水預(yù)處理作為一種預(yù)處理方式具有很多優(yōu)勢(shì),相信經(jīng)過(guò)不斷努力和發(fā)展,可以得到更廣泛的應(yīng)用,為實(shí)現(xiàn)生物質(zhì)能源工業(yè)化生產(chǎn)帶來(lái)更多契機(jī)。
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(責(zé)任編輯 石紅青)
ResearchProgressontheHydrothermalPretreatmentofLignocellulosicBiomassandItsBioconversion
Chen Qian1Chen Jinghuan2Wang Kun1Jiang Jianxin1Sun Runcang1
(1.BeijingForestryUniversityBeijing100083; 2.ChinaNationalPulpandPaperResearchInstituteBeijing100102)
With the energy problem increasingly serious, to seek renewable clean energy has become the key problem to be solved. Transformation of agricultural and forestry wastes into new materials, calorific value of energy, chemical materials are becoming an important new tendency. Bioethanol is attacking more and more attentions since it is a sustained development model of environmental protection type. Pretreatment is an initial step in ethanol preparation process and traditional chemical-pretreatment methods consume a lot of energy, require highly on equipment, seriously degrade hemicellulose and pollute environment seriously. Besides, the traditional pretreatment methods were not focused on hemicellulose and lignin for high value applications, facing the low-coefficient utilization of whole biomass and the financial upside of single product model. As one of the pretreatment methods, biological process is an eco-friendly and low-cost process, but it also has disadvantages of hydrothermal pretreatment tries to keep the structure of natural macromolecules, and break plant cells at a certain degree, in which water is the only reagent. It has such advantages as lower chemical dosage, environmental friendly, low operation costs and the major effects of hydrothermal pretreatment on biomass cell wall are breaking the bonds of lignin-carbohydrate connection (LCC), removing parts of hemicellulose, modifying lignin and reducing cellulose crystallinity. Inhibitors (furfural, 5-hydroxymethyl furfural, formic acid, acetic acid) are generated in the hydrothermal pretreatment process and restrain the biological degradation process. The quantity of inhibitors varies indifferent pretreatment conditions and can be controlled by changing the conditions. Enzyme hydrolysis is a bioprocess transforming cellulose into monosaccharide, more time and lower ethanol content could be obtained by direct fermentation of pretreated substrate. Hemicellulose and lignin hinder enzyme contacting with cellulose. When hemicellulose and lignin are broken by pretreatment, the contacting efficiency of enzyme and cellulose could be increased. The yield of ethanol increased with the increasing temperature, however, the yield decreased when the temperature was too high. The chemical and structural changes of cellulose, lignin and hemicelluloses are summarized, in this paper, as well as the generated inhibition in the hydrothermal process. The effects of various pretreatment conditions on enzyme hydrolysis and ethanol fermentation are also discussed in detail.
hydrothermal pretreatment; lignocellulosic biomass; inhibitors; enzyme hydrolysis; bioethanol
TQ35
A
1001-7488(2017)09-0097-08
10.11707/j.1001-7488.20170912
2016-04-05;
2016-06-20。
北京市自然科學(xué)基金項(xiàng)目(6174046); 北京林業(yè)大學(xué)材料科學(xué)與技術(shù)學(xué)院創(chuàng)新計(jì)劃項(xiàng)目。
*王堃為通訊作者。