肖 颯，童東紳，趙立知，任倩倩，劉豐國，俞衛(wèi)華，周春暉

（浙江工業(yè)大學(xué)化學(xué)工程學(xué)院，浙江 杭州 310014）

甘油主要來自動(dòng)植物和化工產(chǎn)業(yè)（如肥皂、生物柴油產(chǎn)業(yè)），具有可降解、可再生等優(yōu)點(diǎn)。因其獨(dú)特的性質(zhì)，甘油成為近年來的研究熱點(diǎn)。目前，甘油可轉(zhuǎn)化為各種高附加值的精細(xì)化學(xué)品[1-2]。其中，從甘油催化氫解制1,2-丙二醇（1,2-PDO）的技術(shù)最受關(guān)注。1,2-PDO 的傳統(tǒng)生產(chǎn)工藝包括環(huán)氧丙烷水解法、丙烯氧化法和酯交換法。上述工藝原料來自不可再生的石油資源，流程復(fù)雜，環(huán)境污染嚴(yán)重，經(jīng)濟(jì)效益低。而甘油氫解法使用綠色清潔的甘油作原料，產(chǎn)物種類少、收率高、易分離提純，具有替代傳統(tǒng)生產(chǎn)工藝的潛力。

甘油催化氫解反應(yīng)成功的關(guān)鍵是選擇合適的催化劑。目前研究較多的催化劑有兩大類：貴金屬催化劑（如Ru、Rh、Pt、Pd）和過渡金屬催化劑（如Cu、Ni、Co），載體研究較多的是碳材料、金屬氧化物和層狀雙氫氧化物。這些催化劑所采用的制備方法不同，工藝參數(shù)不同，催化性能也有差異。

1 催化劑研究開發(fā)

1.1 貴金屬催化劑

單一貴金屬催化劑的選擇性一般較差，添加其他組分可提高選擇性。例如Ru/C 催化劑中加入12-磷鎢酸作共催化劑后，大幅度提高了1,2-PDO 的收率[3]。

此外，載體表面的酸堿度也對(duì)催化劑的性能有一定影響[4]。酸性位有利于C-O 鍵斷裂，但酸性位過多會(huì)促進(jìn)C-C 鍵斷裂。對(duì)Ru 催化劑，TiO2載體的酸性位數(shù)量較合適[5]，HZSM5 載體的酸性導(dǎo)致副產(chǎn)物CH4選擇性提高，1,2-PDO 選擇性降低[6]。堿性氧化物負(fù)載的Ru 催化劑中，因CeO2表面呈弱堿性，甘油轉(zhuǎn)化率和1,2-PDO 選擇性較高[7]。堿性的層狀水滑石作載體對(duì)Pt 催化劑性能有促進(jìn)作用[8]。

甘油氫解反應(yīng)通常需在H2或合成氣等還原性氣氛下進(jìn)行，存在一定的安全隱患。D’Hondt 等首次發(fā)現(xiàn)在不通氫氣的條件下就能實(shí)現(xiàn)甘油氫解；采用Pt/NaY 催化劑，以20wt% 甘油水溶液為原料，在503 K 反應(yīng)15 h 后，甘油轉(zhuǎn)化率和1,2-PDO 選擇性分別為85.4% 和64.0%[9]。同樣在不加氫氣條件下，Ru/Al2O3和Pt/Al2O3混合物[10]或Pd/Fe2O3催化劑[11]也能催化甘油氫解。

過去，Ag 基催化劑催化甘油轉(zhuǎn)化制備1,2-PDO 的研究較少。最近研究表明，Ag 摻雜的分子篩催化劑[12]和Ag/γ-Al2O3催化劑[13]對(duì)于甘油催化氫解反應(yīng)也具有活性和選擇性。

1.2 Cu 催化劑

相對(duì)于貴金屬催化劑，Cu 催化劑價(jià)格低廉，對(duì)C-O 鍵加氫活性較高，對(duì)C-C 鍵斷裂活性較低，因此1,2-PDO 選擇性較高。最近，開發(fā)高效Cu 催化劑成為研究熱點(diǎn)。例如，在相對(duì)較低的壓力下（1.4 MPa）使用Raney Cu 催化劑，甘油轉(zhuǎn)化率可達(dá)100%，1,2-PDO 收率也高達(dá)94%[14]。

載體是影響催化性能的因素之一。對(duì)于氧化物改性的Raney Cu 催化劑，催化載體活性順序是MgO>ZnO>SiO2>TiO2>ZrO2>Al2O3[15]。層狀雙氫氧化物作載體能提供堿性位[16]，例如層狀雙氫氧化物Cu0.4Mg5.6Al2(OH)16CO3熱解產(chǎn)物作催化劑，在453 K，30 bar H2壓力下，反應(yīng)20 h 后，1,2-PDO選擇性98.2%，甘油轉(zhuǎn)化率80%[17]。介孔分子篩SBA-15 作載體，1,2-PDO 選擇性和甘油轉(zhuǎn)化率最高分別為92.4%和96.0%[18]。1173 K 預(yù)處理SBA-15，可提高催化劑結(jié)構(gòu)穩(wěn)定性[19]。在463 K 和0.64 MPa H2壓力下，使用Cu/ZnO/Al2O3雙載體催化劑，1,2-PDO 選擇性為92%[20]。

Cu 還能與載體MxOy結(jié)合形成CuMxOy晶相。例如Cu 和CuCr2O4[21-22]之間有明顯的相互作用，Cu/CuCr2O4活性比Cu/Cr2O3高。Cu-Fe 催化劑含有CuFe2O4晶相，在463 K，4.1 MPa H2壓力下反應(yīng)10 h，甘油轉(zhuǎn)化率和1,2-PDO 選擇性分別為47% 和92%[23]。納米CuAl2O4催化劑只有CuAl2O4一個(gè)晶相，還原性高，對(duì)氫氣吸附-脫附能力強(qiáng)，甘油轉(zhuǎn)化率和1,2-PDO 選擇性都大于90%[24]。

此外，制備方法不同，催化劑的催化性能也有差異。浸漬法制備的Cu/SiO2催化劑比離子交換法活性好，在1.5 MPa，528 K，300 mL/min H2反應(yīng)條件下，甘油可完全轉(zhuǎn)化，1,2-PDO 的選擇性為87%[25]。共沉淀法制備的Cu/Al2O3催化劑，甘油轉(zhuǎn)化率和1,2-PDO 選擇性最高分別為63%和88%，而使用固態(tài)熔融法制備的Cu/Al2O3催化劑，甘油轉(zhuǎn)化率最高僅39%[26]。溶膠凝膠法制備的Cu/ZnO催化劑表面積約為共沉淀法的兩倍，因此活性更高[29]。

影響催化性能的因素還有負(fù)載量、甘油濃度、溶劑類型、氫氣壓力、pH 和反應(yīng)溫度等。Cu 負(fù)載量較低時(shí)，Cu/MgO 催化劑活性較高[27]。添加少量NaOH 進(jìn)一步提高了Cu/MgO 的活性，可代替Ru/C (或Rh/SiO2)+Amberlyst、Pt/C (或Ru/C) +NaOH 催化劑體系[28]。反應(yīng)中生成的水會(huì)造成Cu/ZnO 催化劑失活，改用1,2-丁二醇作溶劑，甘油轉(zhuǎn)化率從5%增至55%[29]。Cu 催化劑添加助劑（如Ba[30]、Ga2O3[31]）也可使催化劑抗失活。

1.3 Ni、Co 催化劑

除了Cu 催化劑，Raney Ni[32-33]，Ni/AC[34]，Ni/NaX[35]，Ni/SiO2-Al2O3[36]，Ni/Mg/Al 層狀雙氫氧化物[37]同樣對(duì)甘油催化氫解具有很好的活性。Raney Ni 作催化劑時(shí)，可使用粗甘油作原料[38]。KBH4處理Ni/AC 催化劑，使得AC 表面的羰基還原為酚基，大大提高了催化劑酸性[34]。此外，Ni 還可以作為摻雜劑或者載體。例如，將金屬Ni 摻在Cu-Cr催化劑中有助于提高1,2-PDO 的選擇性[39]。若增加原料中水的含量，甘油的轉(zhuǎn)化率和1,2-PDO 的選擇性都會(huì)增加，這是因?yàn)榈蜐舛雀视涂蓽p少脫水反應(yīng)和加氫裂化[40]。

Co 催化劑的研究相對(duì)較少。在較高溫度下處理Co/MgO 催化劑時(shí)，Co3O4和MgO 之間的相互作用得到了加強(qiáng)，形成了MgCo2O4晶體和Mg-Co-O 固溶體[41]，減少了鈷氧化物的還原性，同時(shí)阻止了Co 粒子的聚集，所得的Co/MgO 催化劑表現(xiàn)出良好的穩(wěn)定性。Co/Zn/Al 催化劑在反應(yīng)前后物理性質(zhì)能保持幾乎不變，可重復(fù)使用[42]。

1.4 雙金屬催化劑

使用雙金屬催化劑如Pt-Ru、Au-Ru 可提高1,2-PDO 的收率[43]。Re 的加入對(duì)抑制C-C 鍵斷裂發(fā)揮了作用，Ru-Re[44-46]、Pd-Re[47]、Pt-Re[48]催化劑均比單一Ru、Pd、Pt 或Re 催化劑活性好。

此外，銅和貴金屬結(jié)合的雙金屬催化劑的研究也有較多報(bào)道。例如，在453 K、2.0 MPa 氫氣下，使用Pd0.04Cu0.4/Mg5.5Al2O8.56催化劑，反應(yīng)10 h后，甘油的轉(zhuǎn)化率為88.0%，1,2-PDO 選擇性為99.6%[49]。使用Rh0.02Cu0.4/Mg5.6Al1.98O8.57催化劑，甘油轉(zhuǎn)化率和1,2-PDO 的選擇性最高分別為91.0%和98.7%[50]。此外，碳納米管[51]、膨潤(rùn)土[52]、HMS[53]也可作為Cu-Ru 催化劑的載體。對(duì)于Cu-Ag/γ-Al2O3催化劑，Ag 的加入使CuO 原位還原為Cu，這有助于提高銅在載體表面上分散性[54]，還能抑制甘油C-C 鍵斷裂生成乙二醇，1,2-PDO 的選擇性最高98.3%，此時(shí)甘油轉(zhuǎn)化率為100%[55]。Ni-Cu/Al2O3催化劑能將溶劑中的氫轉(zhuǎn)移到甘油之上[56-58]。當(dāng)甲酸作氫源，45 bar N2，493 K，Ni-Cu/Al2O3催化劑，反應(yīng)24 h，甘油轉(zhuǎn)化率和1,2-PDO選擇性分別為90% 和82%[59]。

2 反應(yīng)機(jī)理

要想設(shè)計(jì)出高效的催化劑，認(rèn)識(shí)甘油氫解反應(yīng)的機(jī)理至關(guān)重要，根據(jù)文獻(xiàn)報(bào)道，可能的反應(yīng)機(jī)理有5 種。其中脫氫-脫水-加氫機(jī)理[60]、脫水-加氫機(jī)理[61]和螯合氫解機(jī)理[62]已有較多綜述提到，本文不予贅述，只對(duì)直接氫解機(jī)理和原位氫解機(jī)理進(jìn)行解釋說明。

圖1 甘油氫解過渡態(tài)模型[63]

直接氫解機(jī)理由Shinmi 等提出，后來被該研究小組進(jìn)一步完善[63-64]。首先，甘油的-CH2OH 基團(tuán)吸附在ReOx表面上形成醇鹽，然后活化氫攻擊醇鹽的3 號(hào)位，使C-O 鍵斷裂，最后醇鹽水解得到產(chǎn)物1,2-PDO（見圖1）。

Chia 等提出了一種不同的直接氫解路徑[65]。第一步甘油通過質(zhì)子化-脫水反應(yīng)形成碳正離子，-CH2OH 基團(tuán)發(fā)生氫轉(zhuǎn)移，碳正離子形成更穩(wěn)定的含氧碳正離子。最后氫轉(zhuǎn)移生成1,2-PDO 或1,3-PDO[65]。這與Qin 等的研究結(jié)果一致[66]。反應(yīng)過程如下所示：

原位氫解機(jī)理是基于脫水-加氫機(jī)理發(fā)展而來的，理論上甘油自身可以通過水相重整（APR）產(chǎn)生氫氣[67]，將甘油制氫和甘油氫解結(jié)合起來，使生成的氫氣被消耗,即APR 原位氫解機(jī)理（見圖2）。除甘油本身作氫源，乙醇[11]、甲酸[56]、異丙醇[56]、甲醇[58]等也可作氫源，通過催化轉(zhuǎn)移加氫（CTH）實(shí)現(xiàn)甘油催化氫解，即CTH 原位氫解機(jī)理。首先載體酸中心吸附甘油形成醇鹽，金屬活性位活化氫源分解產(chǎn)生的氫，然后醇鹽和吸附的活性氫原子相互作用生成1,2-PDO。

圖2 甘油水相重整與氫解制備1,2-PDO[68]

3 結(jié)論

甘油催化氫解制1,2-PDO 已成為甘油轉(zhuǎn)化的研究熱點(diǎn)，近五年來有了新的進(jìn)展。研究表明，金屬催化劑性能順序?yàn)镽u≈Cu≈Ni>Pt>Pd，其中Cu 催化劑價(jià)格相對(duì)低廉，1,2-PDO 選擇性較高，具有工業(yè)化前景；雙金屬催化劑可提高金屬分散性、減小金屬粒徑，不同金屬針對(duì)不同反應(yīng)步驟發(fā)揮作用，因此催化性能比單一金屬好；載體、制備方法等也是影響催化劑性能的關(guān)鍵因素，特別是采用溶膠凝膠法制備的催化劑，甘油轉(zhuǎn)化率和1,2-PDO 選擇性較高；酸堿添加劑能提高催化劑的熱穩(wěn)定性，加快反應(yīng)速率或減少副產(chǎn)物生成；金屬氧化物類助劑可提高催化劑表面酸性，或使活性組分不易被氧化；此外，認(rèn)識(shí)甘油氫解反應(yīng)的各種機(jī)理，對(duì)設(shè)計(jì)高效催化劑具有重要意義。目前，甘油氫解制備1,2-PDO 的技術(shù)已趨于成熟，如何實(shí)現(xiàn)工業(yè)化應(yīng)是未來探索的主要方向。

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