李璐,樊紅軍,胡浩權(quán)
(1中國(guó)科學(xué)院大連化學(xué)物理研究所分子反應(yīng)動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室,遼寧 大連 116023;2大連理工大學(xué)化工學(xué)院,精細(xì)化工國(guó)家重點(diǎn)實(shí)驗(yàn)室,遼寧 大連 116024)
煤及油母中常見(jiàn)C—X(X=N,S)的解離能
李璐1,樊紅軍1,胡浩權(quán)2
(1中國(guó)科學(xué)院大連化學(xué)物理研究所分子反應(yīng)動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室,遼寧 大連 116023;2大連理工大學(xué)化工學(xué)院,精細(xì)化工國(guó)家重點(diǎn)實(shí)驗(yàn)室,遼寧 大連 116024)
煤及油頁(yè)巖中除了碳、氫和氧原子是主要的組成元素外,氮和硫等雜原子大都以C—X(X=N,S)的鍵合形式存在,在其結(jié)構(gòu)及轉(zhuǎn)化利用中同樣發(fā)揮重要的作用。認(rèn)識(shí)C—X的解離能,有助于建立氮、硫熱解過(guò)程中的遷移模型,豐富對(duì)煤及油母中常見(jiàn)化學(xué)鍵性質(zhì)的認(rèn)識(shí),對(duì)發(fā)展高效清潔的能源利用技術(shù)至關(guān)重要。利用雙雜化密度泛函方法,系統(tǒng)研究了煤及油母中典型 C—X鍵的解離能(BDE)范圍。研究結(jié)果表明,煤及油母中常見(jiàn) C—N和 C—S 鍵的 BDE 值范圍分別是 154.1~55.7 kcal·mol?1和 83.0~56.6 kcal·mol?1。在熱解過(guò)程中,苯硫類自由基以及苯胺類自由基會(huì)在初期產(chǎn)生,其次才是巰基、胺類自由基等側(cè)鏈取代自由基脫落。C—S鍵的整體BDE值范圍比其他類型化學(xué)鍵更低,各類化學(xué)鍵最低BDE值的高低順序符合O—H > C—H > C—C > C—N > C—S > C—O的規(guī)律,其中,只有當(dāng)有PhO·生成時(shí),C—S > C—O,否則C—O > C—S。
煤;油母;解離能;焓;熱力學(xué);計(jì)算化學(xué)
Abstract:The carbon,hydrogen,oxygen are the majority elements in the structure of coal and oil shale,while the nitrogen and sulfur atoms,mostly in the form of C—X (X=N,S),still play a nonnegligible role in the structure and application of coal and oil shale.The homolytic bond dissociation enthalpies (BDE) of C—X for model compounds that are representative of the functionalities present in coal and kerogen were computed by using a double-hybrid method mPW2PLYP.The studies will be helpful for people to construct the transfer model of N and S in pyrolysis,and enrich the knowledge of chemical reaction of typical bonds in coal and kerogen.The BDE for C—N and C—S cover a range from 154.1—55.7 kcal·mol?1and 83.0—56.6 kcal·mol?1,respectively.The results suggest that the thiophenyl radical or phenylamino radical are the most favorable intermediates in the early pyrolytic stage.With the increase of temperature,the loss of sulfydryl and amidogen groups become feasible,and the ring cleavage reaction of five-membered ring (pyrrole and thiophene) through homolysis of C—N and C—S bond can also occur.The phenyl radical is the most difficult to form,and the directly bond dissociation of C—N bond in pyridine with six-membered ring is extremely unfavorable in the pyrolysis of coal and kerogen.Comparedto other types of bonds,C—S bond has the lowest general BDE range.The order of the lowest BDE for various bonds is O—H > C—H > C—C > C—N > C—S > C—O.The bond dissociation of C—O to generate PhO·radical is easier than to generate PhS· radical through homolysis of C—S,otherwise the BDE of C—O is higher than C—S.
Key words:coal;kerogen;bond dissociation enthalpies;enthalpy;thermodynamics;computational chemistry
能源是人類活動(dòng)的物質(zhì)基礎(chǔ),人類社會(huì)的前進(jìn)與發(fā)展離不開(kāi)優(yōu)質(zhì)能源的開(kāi)采使用和先進(jìn)能源技術(shù)的開(kāi)發(fā)利用。我國(guó)缺油、少氣和煤炭相對(duì)豐富的能源結(jié)構(gòu)決定了煤在我國(guó)能源消費(fèi)結(jié)構(gòu)中的主導(dǎo)地位。油頁(yè)巖是重要的石油補(bǔ)充能源,近年來(lái)得到廣泛關(guān)注[1-2]。低溫?zé)峤馐且环N煤及油頁(yè)巖常用的加工利用方式,但是由于煤及油頁(yè)巖本身的成分特點(diǎn)和現(xiàn)有工藝技術(shù)的缺陷,目前的熱解技術(shù)普遍存在油氣收率低、重質(zhì)組分含量高等技術(shù)難題,并且加工過(guò)程中由組分中的氮和硫轉(zhuǎn)化生成的NOx、H2S和SO2等被普遍認(rèn)為是導(dǎo)致酸雨的重要原因[3],N2O則能夠?qū)Τ粞鯇赢a(chǎn)生破壞[4],這些都會(huì)對(duì)環(huán)境造成極大的破壞。對(duì)煤及油頁(yè)巖的油母結(jié)構(gòu)中常見(jiàn)C—X(X=N,S)化學(xué)鍵在熱場(chǎng)中的解離行為的深入認(rèn)識(shí)有助于建立它們?cè)跓峤膺^(guò)程中的硫、氮遷移模型[5],對(duì)于發(fā)展高效清潔的能源利用技術(shù)具有重要的指導(dǎo)意義。
由于煤及油母初始結(jié)構(gòu)的復(fù)雜性和可變性,選擇能代表其特征官能團(tuán)性質(zhì)的模型化合物進(jìn)行研究,有助于從分子水平理解它們的化學(xué)性質(zhì)。Ling等研究了含氮原子的喹啉和異喹啉[6]以及含硫原子的噻吩[7]和苯硫酚[8]的熱解反應(yīng)機(jī)理,并指出分子內(nèi)氫轉(zhuǎn)移反應(yīng)在模型化合物的熱解過(guò)程中對(duì)產(chǎn)物的形成和分配起到了非常重要的作用,苯硫酚熱解比噻吩熱解更容易生成H2S。Dubnikova等[9]詳細(xì)研究了鄰位喹啉自由基和鄰位異喹啉自由基之間的異構(gòu)化過(guò)程以及它們各自后續(xù)的熱解反應(yīng)路徑,結(jié)果發(fā)現(xiàn)兩種自由基在熱解反應(yīng)初期會(huì)經(jīng)歷分子內(nèi)異構(gòu)化反應(yīng)迅速達(dá)到平衡,使得兩種自由基的最終熱解產(chǎn)物及分布完全一致,并且計(jì)算得出的穩(wěn)定產(chǎn)物分布與實(shí)驗(yàn)結(jié)果有良好的一致性。Liang等[10]利用ReaxFF分子動(dòng)力學(xué)模擬研究了木質(zhì)素?zé)峤夂图託錈峤庵械牧蜻w移機(jī)理,指出單純熱解中硫原子會(huì)以雜環(huán)結(jié)構(gòu)形式存在于熱解產(chǎn)物中,而在有H2條件下的加氫熱解中,H2會(huì)促使硫原子從含硫雜環(huán)形態(tài)轉(zhuǎn)變成苯硫酚/醇形態(tài),從而更易于轉(zhuǎn)變成H2S,以達(dá)到脫除硫的目的。并且,在脫硫過(guò)程中,合理控制溫度可達(dá)到最大限度脫硫的目的。
除了上述對(duì)含氮或硫模型化合物的具體產(chǎn)物生成機(jī)理研究之外,對(duì)于C—X(X=N,S)鍵解離能(bond dissociation enthalpies,BDE)的研究也引起了廣泛關(guān)注。Khrapkovskii等[11]考察了多種方法水平下硝基苯和取代硝基苯類化合物的C—NO2鍵的BDE值,提出B3LYP/6-31G(d,p)方法的計(jì)算結(jié)果能很好地吻合實(shí)驗(yàn)值,苯環(huán)上的吸電子取代基會(huì)使C—NO2鍵的 BDE值降低。Yu等[12]評(píng)價(jià)了多種密度泛函方法(density functional theory,DFT)和基組在計(jì)算有機(jī)反應(yīng)中常見(jiàn)的C—SO2R鍵解離BDE值時(shí)的表現(xiàn),指出 M06-2X/6-31G(d)的計(jì)算結(jié)果最為可靠。Yao等[13]則通過(guò)對(duì)多種化學(xué)鍵型(C—H、O—H、C—C、C—N、C—S等)BDE值的計(jì)算,得出MPW1P86方法在多種DFT方法中表現(xiàn)最為可靠。由此可見(jiàn),雖然已有文獻(xiàn)關(guān)注過(guò)C—S和C—N鍵的反應(yīng)性質(zhì)[14-22],但研究所涉及到的煤及油母結(jié)構(gòu)中的C—N和C—S鍵類型并不完善,并且未對(duì)其解離行為規(guī)律進(jìn)行系統(tǒng)研究。此外,各文獻(xiàn)選擇的最優(yōu)理論方法并不一致,有關(guān)煤及油母中化學(xué)鍵的 BDE也并未在同一計(jì)算水平下得到,以致不能直接采用已有數(shù)值進(jìn)行相互之間的比較。鑒于熱場(chǎng)中的鍵解離是反應(yīng)的第一步,因此本文選取合理的模型化合物,對(duì)煤及油母中 C—X(X=N,S)的BDE值進(jìn)行系統(tǒng)研究,其結(jié)果勢(shì)必會(huì)對(duì)理解N和S在后續(xù)過(guò)程中的遷移途徑有積極的推進(jìn)作用,并能補(bǔ)充對(duì)整個(gè)煤及油母中各個(gè)類型化學(xué)鍵熱反應(yīng)性質(zhì)的認(rèn)識(shí)。
分子A—B的鍵解離能(BDE)被定義為在標(biāo)準(zhǔn)狀況下,氣相反應(yīng)式(1)中分子A—B鍵均裂生成A·和B·兩個(gè)自由基所需的能量[23]
BDE的值只與反應(yīng)物和生成物的相對(duì)焓值有關(guān),可通過(guò)式(2)的計(jì)算獲得
其中,H(A—B)為分子A—B的標(biāo)準(zhǔn)摩爾生成焓,H(A·)和H(B·)分別為自由基 A·和 B·的標(biāo)準(zhǔn)摩爾生成焓。吡啶等雜環(huán)開(kāi)環(huán)的鍵解離能則通過(guò)鍵斷裂后形成雙自由基與母體分子的焓值差進(jìn)行計(jì)算,以吡啶為例,其開(kāi)環(huán)的C—N鍵的BDE值由反應(yīng)式(3)的能量決定
本文中所有的計(jì)算都是在Gaussian 09量化軟件包下完成,所有分子的優(yōu)化都是在沒(méi)有對(duì)稱性限制下進(jìn)行,每一個(gè)優(yōu)化結(jié)構(gòu)經(jīng)過(guò)頻率分析確認(rèn)沒(méi)有虛頻。根據(jù)作者前期對(duì)解離能計(jì)算方法評(píng)估的研究結(jié)果[24],本文中選取雙雜化密度泛函mPW2PLYP[25]方法在 cc-pVDZ[26]基組下進(jìn)行構(gòu)型優(yōu)化和頻率計(jì)算,并對(duì)優(yōu)化結(jié)構(gòu)在 cc-pVTZ[27]基組下進(jìn)行了單點(diǎn)能量計(jì)算。所有的熱力學(xué)校正數(shù)據(jù)均由頻率計(jì)算的結(jié)果獲得。為了使計(jì)算結(jié)果更準(zhǔn)確,計(jì)算中包括了基組重疊誤差(basis set superposition error,BSSE)校正[28-29]。本文中所有能量單位均為 kcal·mol?1(1 kcal·mol?1=4.184 kJ·mol?1)。
文中選擇了煤及油母中常見(jiàn)C—N和C—S鍵的模型化合物進(jìn)行研究,所選擇的化合物結(jié)構(gòu)見(jiàn)圖1。主要包括含氮或含硫側(cè)鏈以及橋鍵的芳烴化合物、含氮或含硫雜環(huán)化合物,如吡啶、吡咯和噻吩等。
圖1 所選模型化合物Fig.1 Model compounds under study in this work
表1為計(jì)算得到的不同化合物中 C—N鍵的BDE值。C—N鍵的 BDE值范圍為 154.1~55.7 kcal·mol?1。其中,吡啶環(huán)的C—N鍵斷裂的解離能最高,達(dá)到 154.1 kcal·mol?1,而 PhCH2—NHPh 鍵斷裂的BDE值最低。與先前研究報(bào)道的C—C鍵型和C—O鍵型的BDE值規(guī)律相似[30],生成Ph·自由基的 C—N鍵解離能量較高,其 BDE值范圍為82.1~99.8 kcal·mol?1。有 PhCH2·自由基和類芐基的o,m,p-C5H4NCH2·自由基生成的 C—N鍵的斷裂所需能量最低,其 BDE值范圍為 55.7~71.2 kcal·mol?1。化合物中甲基取代的胺類自由基解離出CH3·自由基的 C—N 鍵 BDE值范圍為 63.2~73.3 kcal·mol?1,其中處于橋鍵位置的 N—CH3鍵解離比苯環(huán)側(cè)鏈取代的N—CH3鍵解離所需能量更低,因此更易發(fā)生。由吡啶環(huán)替代芐胺中苯環(huán)得到的o,m,p-C5H4NCH2—NH2中的C—N鍵的BDE值分別比PhCH2—NH2高 1.8、0.6 和 1.1 kcal·mol?1。值得注意的是,不同于吡啶六元環(huán)開(kāi)環(huán)需要非常高的能量值,吡咯的 C—N鍵解離所需能量為 84.5 kcal·mol?1。各種 C—N 鍵的 BDE 值的順序見(jiàn)圖2。
表1 不同化合物中C—N鍵的解離能Table 1 BDE of C—N bonds/kcal·mol?1
表2中列出了計(jì)算得到的不同化合物中 C—S鍵 BDE值。C—S鍵的BDE值范圍為 83.0~56.6 kcal·mol?1。其中,Ph—SH 鍵斷裂產(chǎn)生的 BDE 值最高,而PhCH2—SH鍵斷裂產(chǎn)生的BDE值最低。與C—N鍵型的BDE值規(guī)律相似,生成Ph·自由基的C—S鍵解離能量較高,其BDE值范圍為80.0~83.0 kcal·mol?1。有 PhCH2·自由基生成的 PhCH2—S 鍵的斷裂所需能量最低,其 BDE值范圍為 56.6~67.4 kcal·mol?1。顯而易見(jiàn),這些 C—S 鍵的 BDE 值明顯低于相應(yīng)的C—N鍵BDE值。噻吩的C—S鍵斷裂所需能量為 80.8 kcal·mol?1,該值與吡咯的 C—N 鍵BDE值較為接近。鄰位取代的吡啶環(huán)上發(fā)生甲硫基解離所需能量明顯低于間位和對(duì)位取代,該現(xiàn)象與Hayes等[31]計(jì)算的甲基在吡啶環(huán)上不同取代位的解離能規(guī)律一致。各種C—S鍵的BDE值的順序見(jiàn)圖3。
圖 2 C—N 鍵的解離能區(qū)間(50~155 kcal·mol?1)Fig.2 C—N BDE range from 50 to 155 kcal·mol?1
表2 不同化合物中C—S鍵的解離能Table 2 BDE of C—S bonds/kcal·mol?1
圖 3 C—S 鍵的解離能區(qū)間(50~85 kcal·mol?1)Fig.3 C—S BDE range from 50 to 85 kcal·mol?1
圖4 煤及油母中常見(jiàn)鍵的解離能區(qū)間(括號(hào)中的數(shù)值為所計(jì)算鍵型的數(shù)量)Fig.4 BDE range of different bond types in coal and kerogen (data in parentheses is number of bond type calculated)
結(jié)合先前的研究結(jié)果[30],可以得到煤及油母中幾種常見(jiàn)化學(xué)鍵的BDE值范圍(圖4)。C—H、C—C、C—O、O—H、C—N和C—S鍵的BDE值范圍分別為 111.4~81.2 kcal·mol?1、102.9~62.8 kcal·mol?1、107.6~52.6 kcal·mol?1、111.2~86.6 kcal·mol?1、154.1~55.7 kcal·mol?1和 83.0~56.6 kcal·mol?1。可以看出,每一種鍵型的 BDE值都是一個(gè)很大的范圍,并且不同鍵型之間的 BDE值存在很大程度的重疊。有H·自由基生成的O—H和C—H鍵的BDE值普遍較高,C—C、C—O、C—N和 C—S鍵的最低 BDE值均來(lái)自于 PhCH2—YPh(Y=CH2、O、NH 和 S)。C—C、C—N 和 C—S鍵的高能量區(qū)域均由生成苯基和吡啶雜環(huán)的芳環(huán)自由基的鍵解離反應(yīng)組成,而在C—O鍵的BDE值區(qū)間內(nèi),芳香羧酸類及芳香脂類化合物中羧基基團(tuán)中C—O鍵斷裂所需能量最高,其次才是有芳環(huán)自由基生成相關(guān)的C—O解離。從圖4可以看出,每種鍵型的最低BDE值的高低順序?yàn)镺—H > C—H >C—C > C—N > C—S > C—O,結(jié)合先前研究成果,將同種類型化學(xué)鍵斷裂的C—O和C—S解離能進(jìn)行比較發(fā)現(xiàn),有PhS·自由基生成的C—S鍵的BDE值總是比有PhO·生成的C—O鍵的解離能高,即C—SPh > C—OPh,這得益于PhO·自由基中的O可以與苯環(huán)形成良好的共軛,使得PhO·自由基具有更高的穩(wěn)定性,從而有更低的 BDE值。而在其他同種類型的BDE值比較中,C—S鍵的BDE值均低于對(duì)應(yīng)的C—O鍵的BDE值。Shi等[32]提出脂肪醚和脂肪醇類化合物中的C—O鍵鍵長(zhǎng)遠(yuǎn)小于脂肪硫醚和硫醇類化合物中的C—S鍵鍵長(zhǎng)是導(dǎo)致這一現(xiàn)象的直接原因。
鍵解離是熱解過(guò)程的第一步,解離能的大小直接關(guān)系煤及油母中化學(xué)鍵斷裂的先后順序,從而影響后續(xù)的整個(gè)熱解過(guò)程,因此對(duì)煤及油母中常見(jiàn)C—X(X=N,S)化學(xué)鍵解離能的系統(tǒng)研究不僅有利于了解N,S原子的遷移途徑,而且對(duì)煤及油母結(jié)構(gòu)中化學(xué)鍵以及熱解反應(yīng)過(guò)程的認(rèn)識(shí)具有重要的作用。
綜合先前對(duì)煤中常見(jiàn)4種類型(C—H、C—C、C—O和O—H)化學(xué)鍵BDE值的研究結(jié)果[30]可知,煤及油母熱解過(guò)程中應(yīng)首先斷裂由苯氧類自由基、芐基類自由基、苯硫類自由基以及苯胺類自由基相互連接而形成的弱橋鍵。隨著溫度的逐漸升高,芳環(huán)的側(cè)鏈取代基開(kāi)始解離,巰基相比其他小自由基更易解離,隨后是胺類自由基、烷氧類自由基和烷基類自由基的生成,而氫自由基的直接生成較為困難。同時(shí),在此溫度區(qū)間內(nèi),五元環(huán)的吡咯和噻吩也有可能會(huì)發(fā)生開(kāi)環(huán)反應(yīng)。此外,生成苯自由基或類似的芳環(huán)自由基同樣需要較高的能量,因此這類反應(yīng)只能在高溫下發(fā)生。由于六元環(huán)的吡啶直接開(kāi)環(huán)所需的鍵解離能量過(guò)高,可以推測(cè)該六元芳香環(huán)并不是通過(guò)直接鍵斷裂的開(kāi)環(huán)反應(yīng)參與熱解反應(yīng)過(guò)程。
(1)煤及油母中常見(jiàn)C—N和C—S鍵的BDE值范圍分別是 154.1~55.7 kcal·mol?1和 83.0~56.6 kcal·mol?1,與 C—H、C—C、C—O 和 O—H 鍵的BDE值范圍間同樣存在很大程度的重疊。
(2)煤及油母中的N和S傾向于首先形成苯硫類自由基以及苯胺類自由基,巰基比胺類自由基更易形成。
(3)煤及油母中常見(jiàn)鍵型的最低 BDE值的高低順序符合O—H > C—H > C—C > C—N > C—S >C—O的規(guī)律。當(dāng)有PhO·生成時(shí),C—S > C—O,否則 C—O > C—S。
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Bond dissociation enthalpies of C—X (X=N,S) in coal and kerogen
LI Lu1,FAN Hongjun1,HU Haoquan2
(1State Key Laboratory of Molecular Reaction Dynamics,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian116023,Liaoning,China;2State Key Laboratory of Fine Chemicals,Institute of Coal Chemical Engineering,School of Chemical Engineering,Dalian University of Technology,Dalian116024,Liaoning,China)
TQ 021.2
A
0438—1157(2017)10—3900—06
10.11949/j.issn.0438-1157.20170377
2017-04-10收到初稿,2017-06-23收到修改稿。
聯(lián)系人:樊紅軍,胡浩權(quán)。
李璐(1990—),女,博士后。
國(guó)家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2016YFB0600301)。
Received date:2017-04-10.
Corresponding author:Prof.FAN Hongjun,fanhj@dicp.ac.cn;Prof.HU Haoquan,hhu@dlut.edu.cn
Foundation item:supported by the National Key R&D Program of China (2016YFB0600301).