摘 ?????要: 甲烷氧化偶聯(lián)反應(yīng)可由原料甲烷直接一步制取乙烯,具備替代傳統(tǒng)石腦油制乙烯工藝的潛力,還可降低中國石油資源的對外依存度,對能源安全的保障具有重要意義。其中氧化鑭催化劑在此反應(yīng)中體現(xiàn)出較好的催化活性和低溫起活特性,備受研究者們的關(guān)注。針對甲烷氧化偶聯(lián)反應(yīng),綜述對氧化鑭催化劑活性的影響因素以及提高氧化鑭催化性能的關(guān)鍵點和改進手段,有助于推進基于氧化鑭催化劑的甲烷氧化偶聯(lián)工藝的進一步發(fā)展。
關(guān) ?鍵 ?詞:甲烷氧化偶聯(lián);La2O3;催化劑;乙烯
中圖分類號:TQ032.4??????文獻標(biāo)識碼: A ?????文章編號: 1004-0935(2024)05-0730-04
乙烯是工業(yè)生產(chǎn)的重要基礎(chǔ)原料,主要來源是石腦油的裂解。隨著石油資源的日益匱乏,尋找新的乙烯生產(chǎn)工業(yè)路線刻不容緩[1-2]。自1982年甲烷一步轉(zhuǎn)化為乙烯概念的提出,甲烷氧化偶聯(lián)(OCM)反應(yīng)引起了眾多研究者的關(guān)注[3-5]。OCM路線有望代替?zhèn)鹘y(tǒng)的石腦油裂解路線,在緩解石油短缺壓力的同時,又能夠?qū)⒓淄檫M行高效的轉(zhuǎn)化。
OCM反應(yīng)是一個高溫放熱的均相/多相催化反應(yīng)過程。目前被廣泛接受的反應(yīng)機理[6]是CH4在催化劑表面被活化失去1個H原子生成甲基自由基CH3·,CH3·在氣相中偶聯(lián)成乙烷,乙烷在催化劑表面脫氫產(chǎn)生乙烯。其中,催化劑的作用就是通過吸附、活化O2分子生成活性氧物種,再通過表面活性氧物種奪去CH4中的H原子,促使CH4活化為CH3·。在這過程中,甲烷及產(chǎn)物更傾向于深度氧化為CO和CO2,致使C2+產(chǎn)率降低,不能達到工業(yè)化的要求。因此,OCM技術(shù)的核心問題是,促進O2活化成活性氧物種、在低溫下提高甲烷的選擇性活化、抑制甲烷和產(chǎn)物的深度氧化。催化劑可以很好地解決這些問題。經(jīng)過40多年的嘗試,現(xiàn)如今有稀土金屬氧化物、堿/堿土金屬氧化物、鈣鈦礦型金屬氧化物和過渡金屬氧化物等催化劑有較好的甲烷氧化偶聯(lián)反應(yīng)性能[7-13]。La2O3催化劑具有良好的低溫反應(yīng)特性和熱穩(wěn)定性,常在500?℃以下就能激活甲烷,是當(dāng)前OCM催化劑的研究重點。本文從La2O3催化劑的OCM反影響因素和改進方法兩方面進行概述,闡明了提高OCM性能的關(guān)鍵點。
1 ?影響La2O3催化OCM性能的主要因素
1.1 ?活性氧物種
甲烷氧化偶聯(lián)反應(yīng)的速控步驟是甲烷分子的C—H鍵活化,參與其活化的活性位點是催化劑表面存在的活性氧物種,了解具體的活性氧物種類型和作用有助于深入理解OCM反應(yīng)機制,提高OCM性能[14]。WANG[15]等認(rèn)為晶格氧物種(O2-)會導(dǎo)致深度氧化,化學(xué)吸附氧物種能提高La2O3/CaO催化劑的C2+選擇性。王麗華[16]、WEN[17]等利用原位FTIR光譜表征了LaOF和BaF2/LaOF催化劑上的活性氧物種活化CH4的過程,發(fā)現(xiàn)O2分子可在催化劑表面被活化為O2-,O2-能與CH4反應(yīng)生成C2H4和CO2,并且O2-的消失速率與氣相C2H4的形成速率之間存在良好的相關(guān)性,推測O2-是La基催化劑的活性氧物種。PALMER[18]等利用密度泛函理論對La2O3催化劑上的活性位點進行研究,發(fā)現(xiàn)O2分子在La2O3催化劑表面的氧空位上解離吸附成活性氧物種,其中O22-更易剝奪CH4中的H原子,認(rèn)為O22-是活化甲烷的活性氧物種。AU[19]等通過原位拉曼表征了LaOBr和BaCO3/LaOBr催化劑上的活性氧物種,認(rèn)為O22-和O2n-(1<n<2)等活性氧物種是La基催化劑的活性氧物種。可見,過氧物種(O22-)、超氧物種(O2-)等化學(xué)吸附氧物種是La2O3催化劑的活性氧物種,有助于活化甲烷產(chǎn)生C2+產(chǎn)物,晶格氧物種(O2-)則會導(dǎo)致甲烷深度氧化為CO、CO2。
1.2 ?表面堿性位點
在OCM反應(yīng)中,催化劑的酸堿性會影響反應(yīng)性能[20],普遍認(rèn)為表面堿性位點有助于C2+選擇性的提高[21]。CHOUDHARY[22]等對堿金屬浸漬的La2O3/SA-5205催化劑進行CO2-TPD探究,發(fā)現(xiàn)OCM活性與弱堿性位點和極強堿性位點無關(guān),與中等堿性位點和強堿性位點有良好的對應(yīng)關(guān)系。武國號[23]等認(rèn)為表面強堿位點的增加有助于提高La2Ce2O7催化劑的OCM反應(yīng)活性。ZHAO[24]等發(fā)現(xiàn)在La2O3催化劑中摻雜Sr,會增加催化劑的強堿性位點,跟C2+選擇性增強有很好的相關(guān)性,認(rèn)為強堿性位點會提高C2+選擇性。因此,表面具有一定強度堿性的催化劑有助于C2+產(chǎn)物的脫附,會促進C2+選擇性的提高。
1.3 ?形貌和晶體結(jié)構(gòu)
La2O3催化甲烷氧化偶聯(lián)反應(yīng)具有形貌和結(jié)構(gòu)敏感性,不同的La2O3催化劑形貌結(jié)構(gòu)會影響OCM性能。HUANG[25]等發(fā)現(xiàn)與La2O3納米顆粒相比,La2O3納米棒在低溫下有更高的活性和選擇性,在450?℃具有9.3%的C2+收率。LE[26]等發(fā)現(xiàn)薄板狀的La2O3-LT催化劑與顆粒狀的La2O3-HT催化劑相比有更好的甲烷氧化性能,在750?℃、烷氧比為2時,CH4轉(zhuǎn)化率和C2+選擇性分別為12.2%和64.0%。?ZDEMIR[27]等合成了納米顆粒、納米棒、納米花狀、納米纖維和納米球形貌的La2O3和La2O2CO3基催化劑,發(fā)現(xiàn)La基催化劑的形態(tài)對反應(yīng)性能有很大影響,納米棒和納米纖維形貌的La2O3催化劑有更好的OCM性能,在450?℃收獲11.7%的C2+收率。除了催化劑形貌,不同的晶體結(jié)構(gòu)也會影響OCM性能。張琪[28]等發(fā)現(xiàn)晶粒尺寸會影響La2O3對O2的動態(tài)儲存。MIHAI[29]等發(fā)現(xiàn)LaFeO3催化劑上Fe—O鍵的強度受晶粒尺寸的影響,晶粒尺寸越大,F(xiàn)e—O鍵越弱,越利于分子氧和晶格氧的相互轉(zhuǎn)化,形成更多的活性氧物種。除了晶粒尺寸,暴露晶面也會影響OCM性能。XIONG[30]等發(fā)現(xiàn)X-La2O3(X=Li、Mg、Zn和Ce)催化劑優(yōu)先暴露(110)晶面,面上原子排列松散,有利于CH4和O2的活化。
2 ?改性方法
2.1 ?制備方法
制備方法可以調(diào)控La2O3催化劑的形貌結(jié)構(gòu)和晶體結(jié)構(gòu),影響活性氧物種含量和種類,提高OCM反應(yīng)性能[31]。ZHANG[32]等利用共沉淀法(CP)、水熱法(HT)和甘氨酸硝酸鹽燃燒法(GNC)制備了La2Ce2O7催化劑,發(fā)現(xiàn)制備方法可以改變催化劑的晶體結(jié)構(gòu)和表面活性氧物種,其中,La2Ce2O7-GNC催化劑晶粒尺寸和晶面長度較大,催化劑表面較松弛,能存在更多的表面化學(xué)吸附氧物種(O22-、O2-)和更多的中等堿性位點,在750?℃、烷氧比為4、18?000?mL·g-1·h-1時,達到16.6%的C2+收率。ALAHMADI[33]等比較了MOF介導(dǎo)法、溶膠凝膠法和商業(yè)購買的La2O3催化劑的OCM反應(yīng)性能,發(fā)現(xiàn)3種催化劑都為六方La2O3晶體結(jié)構(gòu),其中MOF介導(dǎo)法制備的La2O3催化劑有更小的粒徑,更大的比表面積,更多的表面堿性位點,在800?℃、 ?????85?000?mL·g-1·h-1、3.5烷氧比的條件下,可達31%的CH4轉(zhuǎn)化率和51%的C2+選擇。SOLLIER[34]等利用靜電紡絲法制備了Sr-La-Ce納米纖維催化劑,發(fā)現(xiàn)與粉末催化劑相比,納米纖維形態(tài)的Sr-La-Ce催化劑具有高傳熱傳質(zhì)性能,能在低溫下(350?℃)發(fā)生OCM反應(yīng),且相對較高的幾何表面積和納米纖維的高床孔隙率相結(jié)合,提高了超氧物種的含量,促進了OCM催化性能,SrLaCe3納米纖維催化劑在600?℃可達21.7%的C2+收率。因此,調(diào)控制備方法制備出晶粒尺寸更大,粒徑更小、結(jié)晶度更好的納米氧化鑭催化劑有助于保留更多的活性氧物種和更多的堿性位點,促進C2+產(chǎn)率提升。
CH4和O2在La2O3催化劑表面活化的本質(zhì)源自電子自表面流向CH4和O2分子的反鍵軌道,不同的晶面具有不同的原子松散度和電子結(jié)構(gòu),會造成不同強度的電子流動驅(qū)動,導(dǎo)致晶面對CH4和O2的吸附/脫附能力不一致,相對松散的原子構(gòu)型有利于形成化學(xué)吸附氧物種和適度堿性位點,從而提高OCM性能。程文敏[35]等發(fā)現(xiàn)La2O3的(110)晶面費米能級對應(yīng)的軌道形狀與π鍵型O2的LUMO相匹配,使得La2O3(110)晶面對O2分子的吸附能力最強,能在較低能壘解離O2,形成活性氧物種。JIANG[36]等用水熱法制備了0D納米顆粒狀、1D納米棒狀、2D納米片狀和3D納米花狀的La2O3催化劑應(yīng)用于OCM反應(yīng),發(fā)現(xiàn)不同維度的La2O3催化劑暴露不同的晶面,2D-La2O3暴露了La2O3(120)曲面,表面上有較多的化學(xué)吸附氧物種(O22-、O2-)和更多的中等堿性位點,可以在550?℃、72?000?h-1和CH4/O2=3條件下獲得32%CH4轉(zhuǎn)化率和15%的C2+收率。HOU[37]等制備了暴露不同晶面的La2O2CO3納米棒催化劑,發(fā)現(xiàn)不同晶面有不同的催化性,暴露(110)、(120)、(210)晶面的納米棒比暴露(001)、(100)晶面的納米棒更利于低溫下C2+產(chǎn)物的形成,歸因于(110)、(120)、(210)晶面表面原子的排布比較松散,有更大的比表面積和堿性位點來吸附氧氣,形成更多的活性氧物種(O-和O22-),增加了OCM反應(yīng)的甲烷轉(zhuǎn)化率。因此調(diào)控氧化鑭晶面使其暴露更多的La2O3(110)、(120)、(210)晶面有利于形成更多的活性位點和堿性點位,在低溫下提高催化反應(yīng)性能。
2.2 ?金屬摻雜
金屬摻雜可以促進電子-空穴對的分離,產(chǎn)生更多的晶格缺陷,增加催化劑的表面氧空位,使催化劑的活性發(fā)生改變。在La2O3催化劑中常用低價態(tài)陽離子和堿性金屬來進行異價摻雜。低價態(tài)金屬的摻雜可以形成更多的氧缺陷和堿性位點,促進氧氣的吸附活化,獲取更多的活性氧物種,提高C2+選擇性[38-40]。黎營濤[41]等將Zn-Al共摻雜進La2O3催化劑,發(fā)現(xiàn)Zn摻雜后能提高低溫反應(yīng)特性,促進更多氧氣轉(zhuǎn)化為活性氧物種,Al的摻雜能抑制Zn團聚,進一步增加活性氧物種數(shù)量,提高C2+選擇性,在800?℃收獲25.3%的甲烷轉(zhuǎn)化率和55%的C2+選擇性。堿/堿土金屬氧化物的摻雜可增加催化劑堿性位點,促進C2+生成[42-43]。KIATSAENGTHONG[44]等探究了Mg、Ca、Sr、Ba等堿金屬摻雜對甲烷氧化偶聯(lián)La2O3催化劑性能的影響,發(fā)現(xiàn)Sr的摻雜效果比其他堿金屬好,歸因于Sr能增強中等堿性位點的含量,促進O2-活性氧物種的生成,在700?℃,LaSr50催化劑能得到56.0%的C2+選擇性和17.2%的C2+產(chǎn)率。
3 ?結(jié)束語
氧化鑭催化劑良好的低溫反應(yīng)特性,可大幅度降低甲烷氧化偶聯(lián)反應(yīng)的能耗,具備良好的工業(yè)化應(yīng)用前景。但目前研究的氧化鑭催化甲烷氧化偶聯(lián)的C2+收率仍較低,未達到30%的工業(yè)化要求,需進一步提高其催化性能。
目前催化劑的表面活性氧物種、表面堿性位點和形貌晶體結(jié)構(gòu)決定著氧化鑭催化劑的反應(yīng)性能。其中化學(xué)吸附氧物種有助于激活甲烷,提高甲烷轉(zhuǎn)化率;中等堿性位點有助于促進C2+產(chǎn)物的脫附,提高C2+選擇性;暴露更多(110)晶面的納米氧化鑭催化劑有助于在低溫條件下激活OCM反應(yīng)。
因此,可通過改善制備方法、調(diào)控暴露晶面、摻雜低價態(tài)的堿金屬等手段對氧化鑭催化劑進行改良,降低OCM反應(yīng)溫度,提高化學(xué)吸附氧物種含量,增強催化劑中等堿性點位數(shù)量,從而提升OCM性能。
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Research Progress?of La2O3?Catalysts for Oxidative Coupling?of Methane
XIAO Qingyue
(Shenyang University of Chemical Technology, Shenyang?Liaoning?110142,?China)
Abstract:??The oxidation coupling of methane (OCM) reaction can directly produce ethylene, which has significant potential to replace traditional production mode of ethylene from petroleum cracking. It also can reduce the dependency of petroleum resources, which is extremely important for ensuring our energy security. Among OCM reaction, the La2O3?catalysts show excellent catalytic performance and low temperature activation characteristics, which attracts the attention of many researchers. In this paper,?aiming at the oxidative coupling reaction of methane, the influencing factors on the activity of lanthanum oxide catalyst, the key points and improvement measures to improve the catalytic performance of lanthanum oxide were reviewed, being helpful to promote the further development of methane oxidative coupling process based on lanthanum oxide catalyst.
Key words:??Oxidative coupling of methane; La2O3; Catalyst; Ethylen
收稿日期: 2023-03-16
作者簡介: 肖晴月(1997-),女,湖南省婁底市人,碩士研究生,研究方向:甲烷氧化偶聯(lián)催化反應(yīng)。