木質(zhì)素的催化加氫轉(zhuǎn)化

張穎，翟勇祥

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木質(zhì)素的催化加氫轉(zhuǎn)化

張穎，翟勇祥

（能源材料化學(xué)協(xié)同創(chuàng)新中心，中國(guó)科學(xué)技術(shù)大學(xué)化學(xué)系，中科院城市污染物轉(zhuǎn)化重點(diǎn)實(shí)驗(yàn)室，安徽合肥 230006）

木質(zhì)素是來源于木質(zhì)纖維素的一種重要的可再生生物質(zhì)資源，可用于制備化學(xué)品和燃料。由于木質(zhì)素本身結(jié)構(gòu)的復(fù)雜性和穩(wěn)定性使其難以有效利用。目前大量的制漿和造紙工業(yè)的木質(zhì)素沒有得到有效利用，大部分用于燃燒供能，并且造成了一定程度的環(huán)境污染。為了保護(hù)環(huán)境、實(shí)現(xiàn)可持續(xù)發(fā)展，催化轉(zhuǎn)化木質(zhì)素制備高附加值化學(xué)品成為了研究的熱點(diǎn)。木質(zhì)素轉(zhuǎn)化的研究眾多，但是進(jìn)展依然相對(duì)緩慢。目前主要的轉(zhuǎn)化方法包括堿催化解聚、酸催化解聚、熱化學(xué)轉(zhuǎn)化、加氫處理解聚、氧化解聚等。由于加氫處理解聚木質(zhì)素可以獲得低聚木質(zhì)素、酚類等有價(jià)值的化學(xué)品和制備烴類燃料，是目前研究的熱點(diǎn)和最有效的方法之一。但是，催化劑失活和解聚產(chǎn)物產(chǎn)率不高等依然是需要進(jìn)一步解決的問題。基于此，梳理了近年來木質(zhì)素加氫處理主要的催化體系和相關(guān)結(jié)果，提出了尚待解決的問題，以期為今后建立有效的木質(zhì)素解聚體系并實(shí)現(xiàn)其高值化利用的相關(guān)研究提供參考。

生物質(zhì)；木質(zhì)素；催化劑；加氫；降解

引 言

木質(zhì)素是植物的重要組成結(jié)構(gòu)之一，在自然界中的儲(chǔ)量豐富[1]。木質(zhì)素是復(fù)雜的三維無定形聚合物，主要由芥子醇、松柏醇、香豆醇3種結(jié)構(gòu)單元（表1）[2]通過碳氧鍵（包括-O-4, 4-O-5,-O-4等）和碳碳鍵(包括-,-5,-4, 5-5等)（圖1）[1,3]無規(guī)聚合形成，是自然界中非石油資源的可再生芳香化合物的主要來源[4]。

表1 木質(zhì)素結(jié)構(gòu)單元在各種植物中的含量[2]

木質(zhì)素轉(zhuǎn)化研究的難點(diǎn)在于其結(jié)構(gòu)和組成的不確定性。而木質(zhì)素結(jié)構(gòu)的差別，是由生物質(zhì)自身的種類不同，生長(zhǎng)環(huán)境和生長(zhǎng)季節(jié)甚至來源部位不同造成的[5]。對(duì)木質(zhì)素進(jìn)行結(jié)構(gòu)分析需要將木質(zhì)素先分離出來，這就導(dǎo)致了分析出來的結(jié)構(gòu)與分離過程相關(guān)，木質(zhì)素的天然結(jié)構(gòu)依然難以確定[6]。木質(zhì)素結(jié)構(gòu)和預(yù)處理方法相關(guān)，不同的處理方法得到的木質(zhì)素結(jié)構(gòu)有時(shí)差異巨大[7]。Dale等[8]將不同的預(yù)處理方法分為4類：物理處理法（例如球磨）、溶劑分離法、化學(xué)方法和生物處理方法。每種處理方法得到的木質(zhì)素都有其自身的優(yōu)點(diǎn)和缺點(diǎn)，對(duì)于進(jìn)一步降解木質(zhì)素得到的產(chǎn)物和降解過程也都有一定的影響[7]。目前，木質(zhì)素的主要來源是制漿和造紙工業(yè)（如堿木素、木質(zhì)素磺酸等）以及專門用于生物質(zhì)生產(chǎn)的能源作物（如有機(jī)溶劑木質(zhì)素）。根據(jù)工藝的不同，主要獲得的是磺酸木質(zhì)素、堿木質(zhì)素、酶解木質(zhì)素和有機(jī)溶劑木質(zhì)素等。堿木質(zhì)素和磺酸木質(zhì)素是堿法和亞硫酸法造紙的副產(chǎn)物。

木質(zhì)素的復(fù)雜酚類聚合結(jié)構(gòu)具有化學(xué)穩(wěn)定性，其轉(zhuǎn)化所需條件非?？量?。目前應(yīng)用于木質(zhì)素解聚的方法眾多，主要有堿催化的解聚、酸催化的解聚、熱化學(xué)催化的木質(zhì)素解聚、氧化解聚、加氫處理的解聚等[9]。其中木質(zhì)素的加氫處理是木質(zhì)素轉(zhuǎn)化最常見和最有效的方法之一，報(bào)道的文獻(xiàn)眾多，但是所用的催化劑體系復(fù)雜，且仍有許多問題未解決。因此，本文主要介紹加氫處理木質(zhì)素的解聚方法，歸納總結(jié)目前報(bào)道的催化劑及其產(chǎn)率，并展望未來木質(zhì)素加氫處理的催化劑發(fā)展方向，以期對(duì)木質(zhì)素加氫解聚工作提供理論指導(dǎo)。

1 木質(zhì)素加氫解聚處理過程

木質(zhì)素的加氫處理過程就是利用氫（氫氣或其他氫源）對(duì)木質(zhì)素進(jìn)行熱還原的過程。通過加氫處理，可以獲得解聚的木質(zhì)素、酚類和其他具有高附加值的化學(xué)品，以及制備小分子量的碳?xì)淙剂?。木質(zhì)素的加氫處理過程涉及的主要反應(yīng)類型包括氫解（hydrogenolysis）、加氫烷基化（hydroalkylation）、加氫脫氧（hydrodeoxygenation）、加氫（hydrogenation）以及綜合的加氫過程（integrated hydrogen-processing）[9]。

氫解是氫斷裂碳碳鍵或碳雜原子（O、N、S等）鍵的化學(xué)反應(yīng)[10]。氫解是木質(zhì)素中加氫處理斷裂碳氧鍵的主要方式。加氫脫氧（HDO）可以去除酚類分子中的氧，用于制備碳?xì)浠衔铩＜託涿撗跏巧镉娃D(zhuǎn)化的重要方法，屬于氫解的一種反應(yīng)。加氫反應(yīng)是利用一對(duì)氫原子飽和或者還原有機(jī)化合物的化學(xué)反應(yīng)。碳碳雙鍵、碳碳叁鍵、碳氧雙鍵在加氫過程中飽和，增加了產(chǎn)物中氫原子的含量。通過選擇合適的催化劑[11-12]和反應(yīng)條件可以選擇性控制芳香基團(tuán)中的碳碳雙鍵和非芳香環(huán)中的碳碳雙鍵、碳碳叁鍵、碳氧雙鍵的飽和。通常加氫反應(yīng)和氫解反應(yīng)是同時(shí)發(fā)生的。由于木質(zhì)素的復(fù)雜結(jié)構(gòu)，難以獲得單一的產(chǎn)物。其加氫過程也不是單一的一種反應(yīng)，在氫解或加氫過程中，實(shí)際上還包含其他的反應(yīng)過程。綜合的加氫過程既包括木質(zhì)素的解聚也包括解聚產(chǎn)物的轉(zhuǎn)化。

加氫處理過程對(duì)反應(yīng)條件的要求較高，因此催化劑對(duì)于加氫處理就顯得非常重要。其中用于木質(zhì)素加氫處理研究較多且效果較好的金屬主要是鎳、鉬、鈀、銠、釕和鉑等。單金屬催化劑、雙金屬催化劑和雙功能催化劑用于加氫處理木質(zhì)素及其模型物的工作也有報(bào)道。

2 鎳催化木質(zhì)素加氫處理

早在20世紀(jì)40年代，鎳基催化劑就已經(jīng)應(yīng)用于木質(zhì)素的加氫、氫解反應(yīng)[13]。Wenkert等[14]報(bào)道了鎳化合物和格氏試劑高效催化氫解芳基碳氧鍵的研究結(jié)果。

Sergeev等[15]報(bào)道了可溶性鎳化合物用于催化氫解二芳醚的研究。該催化劑對(duì)于底物有較寬的適用范圍，不論是富電子基團(tuán)取代還是缺電子基團(tuán)取代的二芳基醚都能有效氫解。其對(duì)于碳氧鍵的斷裂活性為Ar—O—Ar>>Ar—OMe>ArCH2—OMe。隨后，非均相的鎳基催化劑[16]也相繼出現(xiàn)，并且在負(fù)載量低至0.25%（mol）時(shí)，催化效果依然明顯。在這些鎳催化的反應(yīng)中，添加堿很重要，但是具體的堿在反應(yīng)中的作用依然不是很清楚。同時(shí)，均相和非均相的鎳催化劑對(duì)于產(chǎn)物的選擇性有很大的影響，這些結(jié)果都列在表2中。

Zhao等[24]報(bào)道的Ni/SiO2催化劑，能在水相中催化斷裂芳香基的碳氧鍵。與之前報(bào)道的均相催化體系不同，該催化劑可以在水相中使用。通過催化氫解可以解開-O-4和-O-4鍵中碳氧鍵連接。由于水的存在，4-O-5鍵的斷裂是氫解和水解共同作用的結(jié)果。但是，在斷裂碳氧鍵的同時(shí)，不可避免地發(fā)生了苯環(huán)的加氫，在產(chǎn)物中能檢測(cè)到環(huán)己醇的存在。

鎳負(fù)載在碳和氧化鎂上制成的催化劑不但能催化氫解模型物中的碳氧鍵，也能斷裂磺酸木質(zhì)素中的-O-4鍵，并使苯環(huán)不加氫[19,25]。Wang等[20]利用Ni/AC催化劑在醇溶劑中直接氫解真實(shí)木質(zhì)素。在該反應(yīng)中，醇溶劑亦可以作為氫源。Rinaldi等[26]也發(fā)現(xiàn)了相似的氫轉(zhuǎn)移解聚木質(zhì)素的反應(yīng)，并用鎳基催化劑直接解聚白楊木質(zhì)素。

對(duì)于許多反應(yīng)，在催化劑中引入第2種金屬可以有效地提升催化效果[27]。Zhang等發(fā)現(xiàn)Ni-W2C負(fù)載在碳上制備的催化劑，不但可以催化纖維素轉(zhuǎn)化到乙二醇[28-30]，還可以催化木質(zhì)素得到單酚類，產(chǎn)率可以達(dá)到46.5%[21]。有趣的是，不論單獨(dú)使用Ni/AC還是單獨(dú)使用W2C/AC進(jìn)行反應(yīng)，單酚的產(chǎn)率都不超過20%，Ni和W2C的協(xié)同作用影響了木質(zhì)素的轉(zhuǎn)化反應(yīng)。相似的Ni-TiN[18]、NiAu[23]和NiM（M=Rh，Ru，Pd）[22]也都被制備并用于催化有機(jī)溶劑木質(zhì)素氫解。協(xié)同作用的機(jī)理研究[17, 22]顯示主要有3個(gè)方面的影響因素：①增加了表面金屬活性位點(diǎn)；②提高了H2和底物的反應(yīng)活性；③阻礙了苯環(huán)的進(jìn)一步加氫。

表2 鎳催化的木質(zhì)素加氫處理

表3 鉬催化的木質(zhì)素加氫處理

3 鉬催化木質(zhì)素加氫處理

20世紀(jì)80年代鉬氧化物、鉬硫化物、鉬氮化物和鉬碳化物已用于催化木質(zhì)素及其模型物的加氫脫氧反應(yīng)[31]。在氧化鉬催化的木質(zhì)素模型物加氫脫氧的反應(yīng)中，氧化鉬優(yōu)先斷裂Ph—O—Me鍵中酚氧鍵[32]。在碳化鉬催化的苯甲醚加氫脫氧[33]的反應(yīng)中也有相同的結(jié)果。在載體和氮化方法的研究中發(fā)現(xiàn)，載體通過對(duì)活性位點(diǎn)的修飾可以改變產(chǎn)物的選擇性，同時(shí)，分散度和氮化程度對(duì)于鉬的催化活性也有很大的影響[10, 34]。碳負(fù)載的硫化鉬作為催化劑用于木質(zhì)素加氫脫氧反應(yīng)，硫化鉬的活性與所用的碳的結(jié)構(gòu)和化學(xué)性質(zhì)并沒有太大關(guān)系[35]。Smith等[36]比較了沒有負(fù)載的低表面積的鉬基催化劑（MoS2、MoO2、MoO3和MoP）在4-甲苯酚加氫脫氧反應(yīng)中的活性?；贑O脫附的催化TOF值為MoP>MoS2>MoO2>MoO3，活化能遞增順序?yàn)镸oP

4 鉑族金屬催化木質(zhì)素加氫處理

在涉及氫的反應(yīng)中，鉑族金屬（鉑、鈀、釕、銠、銥）擁有優(yōu)異的催化性能[9]。與鎳基催化劑不同，鉑族金屬擁有更高的加氫活性，常常用于原始木質(zhì)素或預(yù)處理的木質(zhì)素直接加氫。不過，鉑族催化劑更傾向于使用溫和的催化條件，鉑族催化劑催化氫解得到的氫解產(chǎn)物在較高的條件下不穩(wěn)定。在溫和條件下，Al-SBA-15負(fù)載的Ni、Pd、Pt、Ru[39]和Pd/C[40]催化降解木質(zhì)素得到單酚，二聚體和低聚物，選擇性地?cái)嗔袮r—O—R和Ar—O—Ar鏈接。單體單元（芥子醇、松柏醇、香豆醇）組成比例不同的稻殼木質(zhì)素可以通過釕碳[41]選擇性地催化轉(zhuǎn)化得到4-乙基苯酚。來自不同原料的木質(zhì)素和不同方法提取的木質(zhì)素有明顯的區(qū)別，Bouxin等[42]研究了Pt/Al2O3催化的不同木質(zhì)素氫解產(chǎn)物的區(qū)別。他們發(fā)現(xiàn)木質(zhì)素中-O-4鍵的含量越高，單體的產(chǎn)率也就越高。高度縮合的木質(zhì)素產(chǎn)生的主要是無烷基的酚類產(chǎn)物，而沒有縮合的木質(zhì)素主要產(chǎn)生的是保留了側(cè)鏈碳的酚類。

木質(zhì)素二聚體模型物氫解[19]的反應(yīng)中，鈀碳催化解聚的產(chǎn)物主要是二聚體、環(huán)己烷和Ni/C的產(chǎn)物。鈀碳不僅催化-O-4鍵的斷裂還會(huì)使得芳環(huán)加氫。Abu-Omar等[43]證明在鈀催化劑中加入鋅可以有效增加催化劑的活性，相較于鈀碳催化劑更加有效地?cái)嗔蚜?O-4鍵。氫解之后芳香醇的加氫脫氧反應(yīng)沒有對(duì)芳環(huán)進(jìn)行加氫，保留了產(chǎn)物的芳香性減少了氫氣的消耗。當(dāng)對(duì)真實(shí)的木質(zhì)素原料進(jìn)行氫解時(shí)，加入一定量的無機(jī)酸[44]或者固體酸[45]可以有效降低氫解的條件并提升解聚效率。通常，鉑族催化[46]的氫解反應(yīng)都伴隨有加氫反應(yīng)。根據(jù)反應(yīng)條件的不同，兩步反應(yīng)相繼或同時(shí)發(fā)生。

除了芳香醚結(jié)構(gòu)，木質(zhì)素中還存在許多脂肪族醚和呋喃結(jié)構(gòu)，然而這些碳氧鍵因?yàn)槿鄙傧┍推S基連接，反應(yīng)活性較低。Marks等[47]利用均相三氟磺酸過渡金屬鹽和負(fù)載型鈀納米粒子催化劑在離子液中催化醚鍵氫解。形成飽和醇并且不會(huì)有芳香基團(tuán)的損失。如果反應(yīng)是在Hf(OTtf)4和Pd/AC催化下的無溶劑體系中反應(yīng)，底物范圍可以拓寬到脂肪醚和呋喃。

均相釕催化劑在催化-O-4鍵斷裂中也表現(xiàn)出很高的催化活性，并且能保留芳香環(huán)[46, 48-49]。均相釕催化劑，如Ru(Cl)2(PPh3)3、RuH2(CO)(PPh3)3、Ru-xantphos，通過氧化還原過程同時(shí)發(fā)生脫氫和C—O鍵斷裂過程。

5 金屬磷化物催化的木質(zhì)素加氫處理

Ni2P、Fe2P、Co2P和WP都被應(yīng)用于木質(zhì)素衍生產(chǎn)物的加氫脫氧反應(yīng)（表4）。與貴金屬催化劑相比，這些金屬磷化物催化劑在轉(zhuǎn)化率沒有降低的情況下可以提高產(chǎn)物的選擇性；與商業(yè)的CoMoS催化劑相比，在氣相加氫脫氧反應(yīng)中有更好的穩(wěn)定性[52-55]。

表4 金屬磷化物催化的木質(zhì)素加氫處理

6 雙金屬催化劑催化木質(zhì)素加氫處理

與單金屬催化劑相比較，雙金屬催化劑能調(diào)節(jié)催化性能和產(chǎn)物的選擇性。Co、Ni、Mo和W的混合硫化物催化劑以及PtSn[56]、PtRh[57]、NiRe[58]、PtRe[59]和ZnPd[49]等雙金屬催化劑常用于木質(zhì)素的加氫脫氧反應(yīng)。這些雙金屬催化劑相較于單金屬催化劑在加氫脫氧反應(yīng)中表現(xiàn)出了更好的選擇性。早在約130年前就有工作報(bào)道了CoMo催化劑在苯酚加氫脫氧反應(yīng)中表現(xiàn)出了較高的反應(yīng)活性[60]。與單獨(dú)的MoS2催化劑相比，Co或Ni提高了Mo催化芳香化合加氫脫氧反應(yīng)的速率[61-63]。

木質(zhì)素的加氫脫氧主要包括兩條路徑：加氫然后脫氧或者直接的脫氧[62, 64]。一些報(bào)道指出，Co或者Ni添加到Mo催化劑中可以顯著增強(qiáng)直接脫氧的能力[62]，但是也有報(bào)道認(rèn)為是脫甲氧基能力增強(qiáng)的結(jié)果[61, 63]。硫化CoMo催化劑在愈創(chuàng)木酚加氫脫氧反應(yīng)中受載體效應(yīng)影響。與γ-氧化鋁和二氧化鈦相比，二氧化鈦載體的催化劑在HDO反應(yīng)中表現(xiàn)出更好的催化活性[61]。

在木質(zhì)素加氫脫氧反應(yīng)中，CoMo催化劑要優(yōu)于Ni化合物，因?yàn)镃oMo較低的加氫活性可以很好地保留原料中的芳環(huán)[63, 65-66]。在不同底物的反應(yīng)中，Weckhuysen等[60]發(fā)現(xiàn)CoMo硫化催化劑加氫脫氧反應(yīng)中-O-4和-5比5-5連接更易解開，并且主要的產(chǎn)物是不完全脫氧的酚或者兒茶酚。

引入第2種金屬的優(yōu)點(diǎn)主要可以歸結(jié)為：① 增加催化活性；② 增加催化劑穩(wěn)定性；③ 改變選擇性。而這又主要由4種效應(yīng)控制，分別是：幾何效應(yīng)、電子效應(yīng)、協(xié)同效應(yīng)和雙功能效應(yīng)[67]。一般來說，催化活性和選擇性主要受幾何效應(yīng)和電子效應(yīng)影響，而反應(yīng)速率受到協(xié)同效應(yīng)和雙功能效應(yīng)的影響。值得注意的是，后兩種效應(yīng)的影響往往會(huì)產(chǎn)生新的反應(yīng)路徑。

雙金屬催化的優(yōu)點(diǎn)顯而易見，但是仍然有許多問題需要克服。首先，碳在（Ni,Co）和（Mo,W）催化劑[68]表面的沉積就是一個(gè)大問題，結(jié)焦隨催化劑酸性的增加而增加，但是加氫脫氧反應(yīng)又需要酸性位點(diǎn)[69]。其次，催化劑容易發(fā)生氧化失活現(xiàn)象，但是生物油中氧和硫的含量是比較高的[70]。氧化物（底物）的性質(zhì)和載體的表面性質(zhì)對(duì)催化劑中毒都有重要的影響[71]。為了解決這些問題，Yang等[70, 72]發(fā)展了一系列無定形的(Co,Ni)-(Mo,W)基催化劑。其中，Mo和W氧化物主要作為布朗斯特酸位點(diǎn)起到脫水的作用，而Ni和Co起到催化加氫的作用。再之，催化劑和雜質(zhì)間的相互作用也尚未明確。這個(gè)問題在真實(shí)木質(zhì)素作為原料時(shí)尤為突出。對(duì)于結(jié)構(gòu)和催化效果之間的關(guān)系仍需要大量的研究。有關(guān)雙金屬催化劑的反應(yīng)結(jié)果列于表5。

表5 雙金屬催化劑催化的木質(zhì)素加氫處理

7 雙功能催化體系催化木質(zhì)素加氫處理

為了克服傳統(tǒng)含硫催化劑的失活問題，科學(xué)家構(gòu)建了包括金屬和酸性化合物的雙功能催化劑。Kou等[74]報(bào)道了Pd/C、Pt/C、Ru/C或Rh/C和磷酸組成的催化體系，可以選擇性地催化酚類化合物加氫脫氧得到環(huán)烷烴和甲醇。與含硫催化劑不同[75-76]，在該反應(yīng)中，金屬催化加氫，酸催化水解或脫水，兩者耦合在一起。系統(tǒng)的動(dòng)力學(xué)研究[77]表明兩種催化能力是相互獨(dú)立的，但是酸催化的步驟決定了加氫脫氧反應(yīng)速率。因此，高效的加氫脫氧催化劑中需要高濃度的酸性位點(diǎn)。此外，Kou等[78]將Pd/C替換為金屬納米粒子和布朗斯特酸離子液，可以更加有效地催化反應(yīng)進(jìn)行。

固體布朗斯特酸組成的雙功能催化劑在加氫脫氧反應(yīng)中同樣有效[79-83]。與其他固體酸（硫酸氧化鋯、大孔樹脂15、全氟磺酸/SiO2、Cs2.5H0.5OW12O40）相比，HZSM-5表現(xiàn)出較高的反應(yīng)速率和較低的表面活化能，因?yàn)榉惺椎垒^高的酸密度[79]。此外，Pd/C和HZSM-5組成的催化劑體系不僅能催化酚類單體加氫還可以催化酚類二聚體加氫[79]。鎳基的加氫催化劑，如Raney Ni與全氟磺酸（或二氧化硅）[80]，Ni/HZSM-5[84-85]，Ni/Al2O3-HZSM-5[82]也能有效催化加氫脫氧反應(yīng)。Ni和酸性沸石分子篩雙組分催化劑[86]用于纖維素水解酶木質(zhì)素解聚。當(dāng)使用Raney Ni作為催化劑時(shí)單酚產(chǎn)率只有12.9%（質(zhì)量），只有分子篩時(shí)產(chǎn)率不到5%，但是Ni和分子篩的雙功能催化劑可以使單酚的產(chǎn)率增加到21%～27.9%（質(zhì)量）。有關(guān)雙功能催化體系的反應(yīng)結(jié)果列于表6。

表6 雙功能催化劑催化的木質(zhì)素加氫處理

8 總結(jié)展望

木質(zhì)素作為生物質(zhì)的重要組成部分，其催化轉(zhuǎn)化制備高附加值的化學(xué)品的研究一直是學(xué)術(shù)界和工業(yè)界的關(guān)注重點(diǎn)。木質(zhì)素轉(zhuǎn)化是現(xiàn)代生物精煉中的重要部分，并且木質(zhì)素的結(jié)構(gòu)和組成決定木質(zhì)素制備精細(xì)化學(xué)品的路徑是獨(dú)一無二的。加氫處理作為木質(zhì)素解聚的一種手段已經(jīng)取得了一定的成果，但是仍然無法滿足木質(zhì)素的工業(yè)轉(zhuǎn)化的要求。

目前，含有-O-4、-O-4，4-O-5連接鍵的木質(zhì)素模型物的加氫解聚催化體系有大量報(bào)道并獲得了較好的結(jié)果。但是，針對(duì)模型物的催化體系在真實(shí)木質(zhì)素的解聚過程中并不是都能起到很好的作用。這主要是因?yàn)檎鎸?shí)木質(zhì)素結(jié)構(gòu)的穩(wěn)定性和復(fù)雜性以及解聚過程中的高活性中間產(chǎn)物易重新聚合形成更加穩(wěn)定的聚合物。雖然Ni、Pd、Ru和Pt的單金屬催化劑在真實(shí)木質(zhì)素解聚中的應(yīng)用較多，但是雙金屬和雙功能催化劑在真實(shí)木質(zhì)素解聚中表現(xiàn)出更加優(yōu)異的效果。與單金屬催化劑相比較，雙金屬催化劑可以通過調(diào)節(jié)金屬間的幾何效應(yīng)和電子效應(yīng)從而實(shí)現(xiàn)協(xié)同調(diào)節(jié)催化劑的催化性能和產(chǎn)物的選擇性并且具有更高的反應(yīng)活性。Ni、Ru、Rh、Pd的雙金屬催化劑可以催化有機(jī)溶劑木質(zhì)素解聚得到單酚。由金屬和酸性化合物組成的雙功能催化劑，可以解決傳統(tǒng)催化劑失活的問題，同時(shí)也提高了木質(zhì)素的解聚效率。從現(xiàn)有報(bào)道的結(jié)果來看，雙金屬和雙功能催化劑更具潛力。

總之，真實(shí)木質(zhì)素催化加氫處理難點(diǎn)主要是兩方面：其一，木質(zhì)素的三維結(jié)構(gòu)的復(fù)雜性，使得其在溶劑中的溶解性以及與金屬活性中心的接觸都受到阻礙，因而難以有效解聚；另外，真實(shí)木質(zhì)素加氫解聚需要較高的反應(yīng)溫度、催化劑酸性中心易結(jié)焦、解聚產(chǎn)物易重聚合、含有大量雜質(zhì)等[92]都是需要進(jìn)一步解決的問題。針對(duì)上述問題，要實(shí)現(xiàn)木質(zhì)素的有效加氫解聚，需要進(jìn)一步設(shè)計(jì)和篩選合適的溶劑體系和具有較高活性和穩(wěn)定性的催化劑。

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Catalytic hydroprocessing of lignin

ZHANG Ying, ZHAI Yongxiang

(Collaborative Innovation Center of Chemistry for Energy Material, Department of Chemistry, CAS Key Laboratory of Urban Pollutant Conversion, University of Science and Technology of China, Hefei 230006, Anhui, China)

Lignin derived from lignocellulose is a renewable resource for the production of chemicals and fuels. However, due to its highly irregular polymeric structure and the carbon based inactive property, lignin valorization is very difficult. Lignin is usually viewed as a waste by-product in the current biorefinery processes and most of the lignin is burned to produce heat and power for the biorefinery processes. There were a series of studies on the lignin conversion such as depolymerization over acid or base, pyrolysis, hydroprocessing and oxygenation. The degradation of lignin over hydroprocessing was the most efficient method to produce alkane fuels and high value-added chemicals such as phenols. However, there were some problems remained to be solved such as catalyst deactivation and low yield. This review focuses on the catalytic systems for lignin hydroprocessing and current challenges in order to provide a reference for efficient and large-scale application of lignin.

biomass; lignin; catalyst; hydrogenation; degradation

10.11949/j.issn.0438-1157.20161250

O 643.3

A

0438—1157（2017）03—0821—10

國(guó)家自然科學(xué)基金項(xiàng)目（21572213）；國(guó)家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃項(xiàng)目（2012CB215306）。

2016-09-06收到初稿，2016-11-03收到修改稿。

聯(lián)系人及第一作者：張穎（1977—），女，副教授。

2016-09-06.

ZHANG Ying, zhzhying@ustc.edu.cn

supported by the National Natural Science Foundation of China (21572213) and the National Basic Research Program of China (2012CB215306).