歐陽四余,徐瓊,伏再輝,張超,劉賢響,尹篤林
(湖南師范大學(xué)化學(xué)化工學(xué)院石化新材料與資源精細利用國家地方聯(lián)合工程實驗室,化學(xué)生物學(xué)及中藥分析教育部重點實驗室,湖南 長沙 410081)
石油等遠古生物質(zhì)轉(zhuǎn)化的化石類資源為近代人類社會的快速發(fā)展提供了充足的能源和豐富的石化產(chǎn)品。然而由于其不可再生性,以化石資源為原料生產(chǎn)的基礎(chǔ)化工產(chǎn)品面臨資源枯竭的威脅逐步增強?;诘厍虮砻婀夂献饔醚h(huán)產(chǎn)生的生物質(zhì)資源來源廣泛、廉價易得,高效轉(zhuǎn)化和綜合利用生物質(zhì)資源有望成為代替化石能源而解決相關(guān)問題最重要的途徑之一[1]。5-羥甲基糠醛(5-HMF)是實現(xiàn)生物質(zhì)資源綜合利用的一種極為重要的平臺化合物[1-4],它可由葡萄糖和果糖等直接轉(zhuǎn)化生成(圖1),通過加氫、氧化脫氫、酯化、鹵化、聚合等化學(xué)反應(yīng)既可生產(chǎn)液體燃料,又可轉(zhuǎn)化為一系列重要的有機化工原料[5-8]。如何實現(xiàn)生物質(zhì)向5-HMF的高效轉(zhuǎn)化、開發(fā)綠色催化體系、實現(xiàn)5-HMF的工業(yè)化生產(chǎn)是近年來本領(lǐng)域研究的重要熱點之一。王軍等[8]從反應(yīng)溶劑的角度綜述了 2008年前相關(guān)制備5-HMF的研究成果;姜楠等[4]從制備方法及溶劑體系等角度重點綜述了 2006—2010年由單糖制備5-HMF的研究成果;Robert-Jan等[1]從反應(yīng)機理、制備工藝等方面全面地整理了近年來制備 5-HMF的研究成果。本文首次從原料的角度分別綜述了近5年來由纖維素、淀粉、菊糖、蔗糖制備5-HMF的研究成果及近 3年來由葡萄糖、果糖轉(zhuǎn)化生成5-HMF的最新研究進展,總結(jié)歸納了不同原料制備5-HMF的酸催化劑類型,分析了不同酸催化體系的優(yōu)缺點。
纖維素是地球上最豐富的可再生生物質(zhì)資源,是構(gòu)成植物骨架和細胞的主要成分。從化學(xué)結(jié)構(gòu)上說,纖維素是由吡喃型葡萄糖通過1,4-β-糖苷鍵形成的有機高分子聚合物。綜合利用生物質(zhì)資源的技術(shù)瓶頸就在于把纖維素解聚成小分子單糖,這也是從纖維素或木質(zhì)纖維素類生物質(zhì)直接轉(zhuǎn)化成5-HMF的技術(shù)難點之一,因此大多數(shù)研究者認(rèn)為由纖維素水解而得的葡萄糖需經(jīng)過異構(gòu)成果糖再脫水生成5-HMF,如圖1所示。Fu研究組[9]的報道說明在酸催化條件下纖維素水解為單糖的過程中有少量5-HMF生成,但高選擇性地將纖維素轉(zhuǎn)化為5-HMF仍需要進一步優(yōu)化現(xiàn)有催化體系和開發(fā)高效的催化體系。
Kim等[10]的研究工作表明,對葡萄糖和果糖為原料制備 5-HMF有較好催化效果的金屬氯化物催化劑對纖維素的催化轉(zhuǎn)化效果也不錯。而其他的研究組[11-12]也發(fā)現(xiàn),在低熔點離子液體中纖維素結(jié)晶度降低、溶解性更好,有利于其后的催化轉(zhuǎn)化。早在2007年,Li等[13]就報道了在無機酸和1-丁基-3-甲基咪唑氯離子液體體系中水解纖維素得到了較高的葡萄糖收率,此后在纖維素直接轉(zhuǎn)化為 5-HMF的研究中主要采用離子液體溶劑。近5年來研究者發(fā)展了以金屬氯化物為催化劑、離子液體為溶劑的催化體系,提高了纖維素轉(zhuǎn)化率和5-HMF產(chǎn)率,表1列出了近5年研究纖維素直接轉(zhuǎn)化為5-HMF的典型結(jié)果。
以纖維素為原料制備 5-HMF 的方法分為一鍋法和兩步法。Yu等[14]在離子液體中,溫度120℃,反應(yīng)8h,以CuCl2-CrCl2為催化劑一鍋法制備 5-HMF,產(chǎn)率高達 55%。Qi等[15]在離子液體中,150℃下微波照射反應(yīng) 10m in,產(chǎn)率即有54%。后又采用兩步法[16],在離子液體體系中,首先以強酸性陽離子交換樹脂做催化劑將纖維素水解為葡萄糖,分離出樹脂后添加 CrCl3為催化劑,使葡萄糖異構(gòu)脫水生成5-HMF,產(chǎn)率提高到70%。Zhang的研究小組[17]先將纖維素水解為水溶性短鏈碳水化合物,再用 CuCl2催化將產(chǎn)物轉(zhuǎn)化為5-HMF,水解產(chǎn)物總收率高達97%,5-HMF產(chǎn)率高達89%。
圖1 生物質(zhì)原料制備5-HMF的轉(zhuǎn)化路徑
表1 近5年來纖維素轉(zhuǎn)化制備5-HMF的研究結(jié)果
研究者們還嘗試使用不同的氯化物催化劑[21-22]、使用雙功能離子液體[24]、金屬負載型離子液體[25]為催化劑,使用離子液體為助劑[17]。與金屬氧化物作催化劑的水或兩相溶劑體系[26-27]相比,金屬氯化物-離子液體體系的轉(zhuǎn)化效果更好,但仍存在催化劑用量大、反應(yīng)時間較長等缺點。值得一提的是,Shi等[23]發(fā)現(xiàn),采用MnCl2/[BM IM]Cl雙催化劑加適量的水作溶劑,在較短的反應(yīng)時間內(nèi)也能得到不錯的轉(zhuǎn)化率和5-HMF產(chǎn)率。然而如果使用重金屬有害成分作為催化劑而又不能全循環(huán)有效利用,在實際應(yīng)用技術(shù)發(fā)展時將會受到限制,加強5-HMF制備中的催化材料滿足綠色化學(xué)規(guī)則的探索應(yīng)該受到進一步的重視。
淀粉是葡萄糖的高聚體,在生物質(zhì)資源中,淀粉類農(nóng)產(chǎn)品是最容易被工業(yè)化利用的。淀粉于180~240℃、酸性條件、酶催化條件下水解為葡萄糖,同時也產(chǎn)生 5-HMF、麥芽糖和果糖,但是關(guān)于淀粉直接轉(zhuǎn)化制備5-HMF的研究報道比較少,可能是以淀粉制備 5-HMF存在與糧食爭原料之嫌。Chheda等[28]在水/甲基異丁基酮-仲丁醇兩相體系中用鹽酸(pH=1.0)催化轉(zhuǎn)化淀粉為 5-HMF,其選擇性為 43%,收率為26%。Hu等[29]采用SnCl4作為催化劑,在離子液體[EM IM]BF4溶劑中,100℃反應(yīng) 24h,得到5-HMF產(chǎn)率約為50%。Yu等[30]使用雙功能催化劑SO/ZrO2-A l2O3,在DMSO溶劑中,150℃反應(yīng)6h,得到5-HMF產(chǎn)率為55%。曲永水[31]發(fā)現(xiàn)[C2OHM IM]BF4在碳水化合物水解過程中表現(xiàn)出了優(yōu)異的催化性能,在優(yōu)化條件下催化淀粉轉(zhuǎn)化為5-HMF的收率接近43%。考慮到一些變質(zhì)的或污染的淀粉難以作為糧食或作為生物化工原料,開展將其轉(zhuǎn)化為5-HMF的高效催化體系的研究對于這類淀粉的資源化利用仍具有一定價值。
由于菊糖較果糖價格便宜且可直接降解成果糖,近5年來從菊糖類生物質(zhì)資源直接制備5-HMF的研究也取得了較快的發(fā)展。表2列舉了菊糖轉(zhuǎn)化為5-HMF的一些研究結(jié)果。從催化劑類型來看,離子液體催化劑和易于分離的固體酸催化劑是近幾年催化菊糖轉(zhuǎn)化為5-HMF的主要催化劑。反應(yīng)溶劑也仍以離子液體溶劑為主,以DMSO[32-33]、DMF[34]作溶劑的反應(yīng)也獲得了較高的 5-HM F產(chǎn)率。如Zhang等[32]發(fā)現(xiàn),菊糖在DMSO溶劑中以酸性離子液體[CM IM]Cl為催化劑,120℃反應(yīng) 3h,可獲得5-HMF的產(chǎn)率為88.4%。Hu等[35]制備了一種新型高酸性固體酸催化劑,在水/2-丁醇中160℃反應(yīng)2h,可獲得5-HMF的產(chǎn)率為61.5%。Qi等[36]通過兩步反應(yīng)得到了 82%的 5-HM F產(chǎn)率,他們首先以[BM IM][HSO4]為溶劑和催化劑,5min內(nèi)將菊糖催化水解成果糖,產(chǎn)率達84%;再以[BM IM]Cl為溶劑,樹脂為催化劑即可將果糖進一步轉(zhuǎn)化為5-HMF。此外,Tong等[33]發(fā)現(xiàn)SnCl4/四丁基溴化銨在DMSO中可以高效地催化葡萄糖、果糖、菊糖轉(zhuǎn)化為5-HMF,其中菊糖轉(zhuǎn)化為5-HMF的產(chǎn)率可達62.1%。Wu等[41]還開發(fā)了CO2/水雙相體系,通過控制CO2的壓強可以調(diào)節(jié)5-HMF的產(chǎn)率,在CO2壓強6MPa,160℃下反應(yīng)45m in,可得到5-HMF產(chǎn)率為53%。
表2 近5年來菊糖轉(zhuǎn)化制備5-HMF的研究進展
蔗糖是自然界分布最廣的寡糖,是一分子果糖和一分子葡萄糖的縮合產(chǎn)物,在酸或酶的催化作用下,可水解成葡萄糖和果糖的等量混合物。相對于果糖和葡萄糖,蔗糖用于5-HMF 合成的報道較少,但因其價格低廉、來源廣泛,近年來關(guān)注度也逐步提高,相應(yīng)的催化體系也不斷被研發(fā)。表3總結(jié)了一些研究小組近年來在不同酸催化體系中以蔗糖制備5-HMF的研究結(jié)果。
雖然研究者對離子液體催化體系的研究越來越深入,但是在蔗糖轉(zhuǎn)化為5-HMF的反應(yīng)中,離子液體催化劑并沒有表現(xiàn)出更好的催化性能。Jadhav等[42]以蔗糖為原料,在雙陽離子室溫離子液體[TetraEG(M IM)2][OM s]2中 120℃反應(yīng) 150m in,5-HMF產(chǎn)率為 67.2%。Shi等[34]開發(fā)了[AM IM]Cl/ DMF催化體系,不再需要添加任何催化劑,該體系在100℃下反應(yīng)1h,可獲得產(chǎn)率為40.3%的5-HMF。Qu等[43]還開發(fā)了堿性離子液體[BM IM]OH/DMSO體系,在160℃反應(yīng)3h,可得蔗糖轉(zhuǎn)化率90%和產(chǎn)率55%的5-HMF。而王娜妮[44]以蔗糖作原料,磷酸為催化劑,在 140℃條件下合成 5-HMF產(chǎn)率達到69%。Wang等[45]考察了金屬氯化物催化劑-助劑體系對蔗糖脫水合成5-HMF的反應(yīng)。在各種助劑中,NH4Br的效果非常突出,在 CrCl3-NH4Br、SnCl4-NH4Br 體系中,5-HMF 的產(chǎn)率分別達到87.0%和 66.3%。Takagaki等[46]采用固體酸催化劑樹脂-15和固體堿催化劑 Mg-Al水滑石(HT)在N,N-二甲基甲酰胺中聯(lián)合催化蔗糖轉(zhuǎn)化反應(yīng),5-HMF的產(chǎn)率為54%。
表3 近5年來蔗糖轉(zhuǎn)化制備5-HMF的研究進展
果糖是一種含6個碳原子的單糖,它的呋喃型結(jié)構(gòu)使其能直接脫水生成5-HMF,與其他生物質(zhì)原料相比,以果糖為原料生成5-HMF的轉(zhuǎn)化率和產(chǎn)率都更高。2012年,Li等[49]報道了在離子液體[BM IM]Br中,果糖在100℃反應(yīng)1h即可完全轉(zhuǎn)化,5-HMF的產(chǎn)率高達95%。也正因為如此,以果糖為原料制備 5-HMF的研究并未因其相對昂貴的價格而減少。在進一步提高果糖轉(zhuǎn)化率和5-HMF產(chǎn)率的同時,研究者致力于開發(fā)綠色環(huán)保的催化體系。從表4中可以看出,對環(huán)境污染小、不腐蝕設(shè)備、易分離、可重復(fù)使用的固體酸催化劑的研究明顯增加。而種類繁多的固體酸催化劑中以磺酸型催化劑的研究較多,效果較好。
炭基固體磺酸是近年來出現(xiàn)的一類新型高效的磺酸催化劑,在酯化、縮合、水解、脫水等典型酸催化反應(yīng)中都有優(yōu)異的催化活性[50]。近3年中研究者以纖維素、木質(zhì)纖維素等生物質(zhì)原料制備了不同炭基固體磺酸,用于催化果糖脫水生成5-HMF取得了良好的效果。Guo等[51]以葡萄糖、果糖、纖維素、木質(zhì)素、竹粉和小桐子果皮等作為催化劑載體原料,通過炭化和磺化兩步法合成炭基固體酸。在離子液體-二甲基亞砜([BM IM][Cl]-DMSO)協(xié)同作用下,在110℃微波加熱反應(yīng)10min,果糖轉(zhuǎn)化率和5-HMF得率分別達98%和84%。
磺酸樹脂是已工業(yè)化生產(chǎn)的磺酸型催化劑,將其用于本反應(yīng)有助于加快 5-HMF生產(chǎn)的工業(yè)化。Li等[52]發(fā)現(xiàn)在離子液體中采用D001-大孔樹脂,在75℃反應(yīng) 20m in,即可以獲得 93.0%的產(chǎn)率,同時發(fā)現(xiàn)樹脂的比表面積越大、酸性越強,則越利于5-HMF的生成。Kannan等[53]發(fā)現(xiàn),磺酸樹脂在DMF和DMSO兩種溶劑中催化劑的性能差別顯著,原因是磺酸樹脂在 DMF中易被氨基中和,因此使用DMF等含氨基的物質(zhì)作溶劑時需要把失活的酸中心用酸沖洗以恢復(fù)活性。
沸石等擇型催化劑的使用能更好地提高5-HMF的選擇性,Ordomsky等[54]就證明在沸石催化體系中添加M IBK能填充分子篩中有副反應(yīng)活性位點的孔道,從而提高5-HMF的選擇性。
無機酸[55-57]、金屬氯化物[57-58]和離子液體催化劑[49,59-60]對果糖制備5-HMF也有較好的催化效果,但相比于固體酸催化劑,上述催化劑存在環(huán)境污染大、分離困難等問題,環(huán)境友好的高效固體酸催化劑應(yīng)是該體系今后研究的重點。
表4 近3年果糖轉(zhuǎn)化生成5-HMF的研究進展
研究者也在反應(yīng)裝置和分離技術(shù)上做出努力,促進 5-HM F產(chǎn)率的提高。Herm ans等[64]用 Amberlyst-15為催化劑,1,4-二氧雜環(huán)己烷/DMSO作溶劑,采用固定床連續(xù)反應(yīng),通過調(diào)節(jié)樹脂的孔徑、進料的流速等優(yōu)化反應(yīng)條件,當(dāng)反應(yīng)溫度為110℃,進料流速為 0.09m L/m in時,反應(yīng) 3min,5-HMF的產(chǎn)量即可達 92%,時空收率為反應(yīng)釜的75倍。Takashi等[60]發(fā)現(xiàn),在室溫下適當(dāng)比例的水/乙腈/[MBCIM]SO3Cl可將反應(yīng)體系分層,從而開發(fā)了一種新的兩相催化分離體系,促進5-HMF的分離。
近年來,一些新的制備方法也被開發(fā)出來。Tsutsumi等[65]使用半導(dǎo)體材料Si負載—OH基團,80℃,可見光照射下反應(yīng)5h,能獲得97%的5-HMF產(chǎn)率,表明光催化果糖制備 5-HMF也是非常有效的。Liu等[68]發(fā)現(xiàn),廉價可持續(xù)反應(yīng)的CO2/氯化膽堿系統(tǒng)可以將高濃度果糖有效地轉(zhuǎn)化為5-HMF。
葡萄糖與果糖互為同分異構(gòu)體,是自然界分布最廣且最為重要的一種單糖,可以大量由淀粉和纖維素直接水解得到。以葡萄糖為原料制備 5-HMF比果糖的轉(zhuǎn)化難度要大,但葡萄糖是一種更為經(jīng)濟、更有潛力的生物質(zhì)原料,人們對葡萄糖轉(zhuǎn)化成5-HMF的關(guān)注方興未艾。表5歸納了近3年來葡萄糖在不同酸催化體系中制備5-HMF的研究結(jié)果。
目前的研究表明,對葡萄糖轉(zhuǎn)化為5-HMF有較好催化活性的催化劑是鉻的氯化物,但鉻的化合物屬于重毒性物質(zhì),不適于大規(guī)模工業(yè)生產(chǎn)。研究者們在近3年對其他金屬的氯化物[69-70]、無機酸[69-72]、固體酸[51,73-74]、酸堿雙功能催化劑[75-76]及離子液體催化劑[26,77]做了廣泛的嘗試。各種酸催化劑中金屬的氯化物[69]、固體酸[51]、磷酸緩沖鹽體系[56]和離子液體[77]表現(xiàn)出較好的催化活性,如表5所示。Moller等[78]在 220℃微波條件下亞臨界水中無催化轉(zhuǎn)化葡萄糖,獲得了高達50%的5-HMF產(chǎn)率,乙酰丙酸等其他副產(chǎn)物總量也不到16%。雖然堿中心是催化異構(gòu)反應(yīng)的活性中心[1],但研究者嘗試采用酸堿雙功能催化劑來達到葡萄糖的異構(gòu)和脫水時沒有收到預(yù)期效果。Peng等[75]在納米二氧化硅上負載—SO3H、—NH2基團制備了雙功能催化劑,在120℃的水溶液中反應(yīng) 3h,得到 5-HMF的產(chǎn)率為13%。Lee等[76]也按照酸堿共催化的機理使用SO/ZrO2雙催化劑在120℃反應(yīng)6h,5-HMF產(chǎn)率僅4.6%。
Grande等[79]開發(fā)了兩步法制備 5-HMF,即第一步采用海水為溶劑在異構(gòu)酶作用下將葡萄糖轉(zhuǎn)化為果糖,第二步把反應(yīng)液通入另一個反應(yīng)釜采用草酸為催化劑進一步生成5-HMF,140℃反應(yīng)1h能得到 57%的 5-HMF。值得一提的是,Chidambaram等[80]在以葡萄糖為原料探索兩步法制備 2,5-二甲基呋喃時,第一步采用 12-磷鉬酸為催化劑,在[EM IM]Cl中添加乙腈作為溶劑,在 120℃下反應(yīng)3h,葡萄糖轉(zhuǎn)化率達到99%,5-HMF產(chǎn)率高達98%,這是令人興奮的超常結(jié)果,催化反應(yīng)的效率幾乎與點擊反應(yīng)相當(dāng)。作者認(rèn)為在該催化體系中,乙腈除了降低黏度之外還能調(diào)節(jié)酸質(zhì)子的活性,葡萄糖沒有經(jīng)過異構(gòu)成果糖這一步,而是通過 1,2-烯二醇直接生成5-HMF。葡萄糖直接轉(zhuǎn)化為5-HMF的新型高效催化體系的研究是值得期待的。
表5 近3年葡萄糖在不同體系中制備5-HMF的研究結(jié)果
Fu課題組[83-84]研究在微波輻射下竹炭基磺酸催化竹材加工中的固體廢物竹粉水解和CuCl2-離子液體官能化炭基磺酸催化竹粉水解的過程中,發(fā)現(xiàn)在該“固-固-液”催化過程中得到還原糖的同時,產(chǎn)物中有少量5-HMF(圖2)。Li課題組[85]用磺酸功能化離子液體多相催化甘蔗皮液化的產(chǎn)物中既有纖維素水解和脫水產(chǎn)物,也有木質(zhì)素的降解產(chǎn)物。Zhao等[86]以玉米秸稈、稻殼等生物質(zhì)原料,以離子液體為溶劑,三氯化鉻為催化劑,采用微波加熱的方式,得到5-HMF收率分別為45%及47%。
圖2 竹炭基磺酸催化竹粉成還原糖和5-HMF示意圖
這些工作成功地為人們揭示了將木質(zhì)素和纖維素共存植物組織的農(nóng)林副產(chǎn)物及固體廢物通過復(fù)合酸催化直接轉(zhuǎn)化成 5-HMF等一系列功能分子的可能性。雖然產(chǎn)物組成的復(fù)雜性增加,但相比由這些原料先通過繁瑣的預(yù)處理變成簡單單一的葡萄糖、果糖,再進一步地制備成5-HMF等功能分子,該工藝顯得更簡便、節(jié)能,加強多功能催化體系的研發(fā)成為指導(dǎo)該工藝發(fā)展的重要思想。
目前生物質(zhì)降解制備 5-HMF已經(jīng)備受廣大研究者的關(guān)注,對纖維素、淀粉、菊糖等多糖原料轉(zhuǎn)化的研究正廣泛而深入的開展,也獲得了一些喜人的成果。在今后的研究中,對纖維素轉(zhuǎn)化有較好催化作用的金屬氯化物-離子液體體系應(yīng)進一步優(yōu)化,如通過負載等方法使金屬氯化物催化劑固相化,以利于后續(xù)分離和重復(fù)使用,減少重金屬對環(huán)境的污染。對于經(jīng)濟易得的葡萄糖、蔗糖等原料,應(yīng)著力開發(fā)對水解、異構(gòu)反應(yīng)具有高效催化作用的新型催化劑,以提高原料轉(zhuǎn)化率和5-HMF的選擇性。對成本相對較高但容易轉(zhuǎn)化完全的果糖原料,應(yīng)盡量采用廉價綠色的催化體系,發(fā)展高效低能耗的分離技術(shù)以降低生產(chǎn)成本,為實現(xiàn)5-HMF的工業(yè)化開發(fā)提供基礎(chǔ)。雖然目前對木質(zhì)纖維素類植物原料轉(zhuǎn)化制備5-HMF的研究較少,但因其價廉、易得、量大且有利于農(nóng)林固廢的高附加值利用,是工業(yè)化生產(chǎn)最適宜的原料,因而在今后的研究中也應(yīng)重視農(nóng)林副產(chǎn)物催化液化工藝的探索,促進其直接、高選擇性地向5-HMF轉(zhuǎn)化。此外,生物質(zhì)原料制備5-羥甲基糠醛是一個十分復(fù)雜的多步反應(yīng)過程,研究者也應(yīng)著力于探索反應(yīng)機理,為優(yōu)化制備工藝提供理論指導(dǎo)??傊訌姶呋牧系木G色設(shè)計、加強催化劑循環(huán)利用的工程研究及高效低耗的分離工藝技術(shù)的研發(fā)是加快 5-HMF生產(chǎn)技術(shù)工業(yè)化的重心和關(guān)鍵。
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