Pt+促進(jìn)丙烷1,2-脫氫氣相反應(yīng)路徑中結(jié)構(gòu)和能量學(xué)的理論分析

趙改云，郭玥，張干兵，鐘欣欣，胡瑋，夏清華

(有機(jī)化工新材料湖北省協(xié)同創(chuàng)新中心，有機(jī)功能分子合成與應(yīng)用教育部重點(diǎn)實(shí)驗(yàn)室，

湖北大學(xué)化學(xué)化工學(xué)院，湖北 武漢 430062)

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Pt+促進(jìn)丙烷1,2-脫氫氣相反應(yīng)路徑中結(jié)構(gòu)和能量學(xué)的理論分析

趙改云，郭玥，張干兵，鐘欣欣，胡瑋，夏清華

(有機(jī)化工新材料湖北省協(xié)同創(chuàng)新中心，有機(jī)功能分子合成與應(yīng)用教育部重點(diǎn)實(shí)驗(yàn)室，

湖北大學(xué)化學(xué)化工學(xué)院，湖北 武漢 430062)

摘要：采用密度泛函理論方法UB3LYP，結(jié)合相對(duì)論贗勢(shì)，優(yōu)化Pt+活化丙烷1,2-脫氫反應(yīng)路徑中各基元步驟涉及的反應(yīng)物、中間體、過(guò)渡態(tài)和產(chǎn)物的幾何構(gòu)型，并在同一水平上計(jì)算其振動(dòng)頻率和能量.結(jié)果表明，整個(gè)反應(yīng)路徑放熱195.7 kJ/mol，決速步驟是第二個(gè)C—H鍵的活化，能壘是48.7 kJ/mol.說(shuō)明1,2脫氫反應(yīng)在常溫常壓下特別容易進(jìn)行，計(jì)算結(jié)果與實(shí)驗(yàn)觀察的結(jié)果一致.

關(guān)鍵詞：丙烷活化；1,2-脫氫路徑；氣相鉑離子催化；反應(yīng)機(jī)理；密度泛函理論

0引言

1計(jì)算方法

用密度泛函理論方法UB3LYP[13-15]對(duì)鉑離子與丙烷1,2-脫氫反應(yīng)路徑所有駐點(diǎn)(反應(yīng)物、中間體、過(guò)渡態(tài)和產(chǎn)物)的幾何構(gòu)型進(jìn)行了全優(yōu)化，并得到其能量.通過(guò)振動(dòng)分析確證所優(yōu)化的駐點(diǎn)對(duì)應(yīng)局域極小或過(guò)渡態(tài)，并得到零點(diǎn)振動(dòng)能(ZPVE)和熱化學(xué)量.計(jì)算中用相對(duì)論贗勢(shì)[16-17]來(lái)描述Pt的含60個(gè)內(nèi)層電子的惰性原子實(shí)([Kr]4d104f14)，而其5s/5p電子與5d/6s/6p價(jià)電子一起做明確處理.Pt采用經(jīng)Couty和Hall改進(jìn)的LANL2DZ相對(duì)論贗勢(shì)基組[18]，外加f極化函數(shù)[19]，C和H采用標(biāo)準(zhǔn)的3-ξ基6-311G**[20].通過(guò)內(nèi)稟反應(yīng)坐標(biāo)(IRC)[21-23]計(jì)算證實(shí)過(guò)渡態(tài)確實(shí)沿反應(yīng)坐標(biāo)分別連接反應(yīng)物和產(chǎn)物.所有計(jì)算用Gaussian 09程序[24]完成.

為標(biāo)定所選理論方法，先用所選方法計(jì)算了體系的有關(guān)性質(zhì)，與相應(yīng)實(shí)驗(yàn)值比較.計(jì)算得到鉑的電離能為8.74 eV，與實(shí)驗(yàn)值9.0 eV[25]接近；Pt+-CH2解離能的計(jì)算值為493.2 kJ/mol，和實(shí)驗(yàn)值479.0±4.2 kJ/mol一致[10].因此，可以認(rèn)為所選方法和基組對(duì)研究該反應(yīng)來(lái)說(shuō)是合適的.

2結(jié)果討論

按脫氫方式的不同，Pt+使丙烷脫氫的反應(yīng)可能存在多種不同路徑.因Uggerud等人的同位素實(shí)驗(yàn)

確認(rèn)按1,2-脫氫方式進(jìn)行，本文中只簡(jiǎn)報(bào)對(duì)1,2-脫氫路徑的理論研究結(jié)果.鉑離子的基態(tài)組態(tài)為5d9，計(jì)算所得的二重態(tài)的能量最低，與洪特規(guī)則一致.計(jì)算結(jié)果顯示Pt+活化丙烷脫氫反應(yīng)中沒(méi)有涉及自旋交叉，在下文的討論中只涉及能量最低的二重態(tài)勢(shì)能面.計(jì)算得到各個(gè)駐點(diǎn)(反應(yīng)物、中間體、過(guò)渡態(tài)以及產(chǎn)物)的優(yōu)化的幾何結(jié)構(gòu)展示在圖1中；各駐點(diǎn)的相對(duì)能量列于表1，相應(yīng)的勢(shì)能曲線見(jiàn)圖2.為方便敘述，下文中用粗體的數(shù)字表示各物種.

在丙烷1，2-脫氫路徑中，Pt+分別活化一個(gè)C(α)—H和一個(gè)C(β)—H鍵，消除一分子氫，產(chǎn)物為Pt[CH2CHCH3]+.由表1可知，沿該路徑整個(gè)反應(yīng)放熱195.7 kJ/mol(ΔG298=-196.5 kJ/mol)，說(shuō)明經(jīng)該路徑反應(yīng)是熱力學(xué)可能的.這與

實(shí)驗(yàn)發(fā)現(xiàn)該反應(yīng)在常溫常壓下發(fā)生的事實(shí)一致.

如圖1、2和表1所示，Pt+和丙烷形成穩(wěn)定的反應(yīng)絡(luò)合物[PtC3H8]+(1)，其中一個(gè)C(β)—H(1)鍵從0.107 nm被拉長(zhǎng)至0.133 nm，而Pt-H(1)和Pt-C(β)鍵長(zhǎng)值分別為0.163和0.236 nm，這表明Pt+和丙烷間形成了一個(gè)典型的agostic鍵.這種相互作用更加穩(wěn)定了中間體1，使其能量比分離反應(yīng)物的低180.1 kJ/mol.接著1經(jīng)過(guò)過(guò)渡態(tài)TS1/2形成了插入中間體H-Pt+-C3H7(2).從鍵長(zhǎng)值可以看到，中間體2中形成了正常的Pt—H鍵和Pt—C鍵.虛頻(309.7i cm-1)表明TS1/2確實(shí)為一級(jí)鞍點(diǎn).TS1/2中，C—H(1)、Pt—H(1) 和Pt—C(β)鍵長(zhǎng)分別為0.141、0.159和0.231 nm，分別和1中的對(duì)應(yīng)鍵長(zhǎng)相近，表明TS1/2是一個(gè)典型的“早”過(guò)渡結(jié)構(gòu)，以致于該活化步幾乎無(wú)能壘(在不考慮零點(diǎn)校正的情況下，TS1/2的能量略高于中間體1).這一步放出77.6 kJ/mol的熱量.

圖1　Pt+活化丙烷1,2-脫氫反應(yīng)路徑中各駐點(diǎn)的優(yōu)化幾何(鍵長(zhǎng)的單位是nm，鍵角的單位是

圖2　用UB3LYP計(jì)算Pt+活化丙烷1，2-脫氫反應(yīng)路徑的二重態(tài)勢(shì)能曲

表1　Pt+活化丙烷1,2-脫氫反應(yīng)中各駐點(diǎn)的相對(duì)能量ΔErel (考慮零點(diǎn)能)，ΔEelec (不考慮零點(diǎn)能)，298 K下的焓ΔH298以及吉布斯自由能ΔG298(單位：kJ/mol)

然后，2中的第二個(gè)C—H鍵C(α)—H(2)被活化斷開(kāi)，同時(shí)Pt—H(2)鍵長(zhǎng)由0.159 nm縮短為0.154 nm，從而形成金屬二氫化物中間體3.此步經(jīng)過(guò)過(guò)渡態(tài)TS2/3，翻越48.7 kJ/mol的能壘，放出12.1 kJ/mol 的熱量.TS2/3的虛頻939.0i cm-1確證為一個(gè)過(guò)渡態(tài).從幾何上來(lái)看，TS2/3明顯地接近3的特征，表明TS2/3是典型的“遲”過(guò)渡結(jié)構(gòu).此步是整個(gè)催化過(guò)程中位壘最高的，因此成為整個(gè)過(guò)程的決速步.中間體3中兩個(gè)氫配體還原消去，經(jīng)過(guò)過(guò)渡態(tài)TS3/4，翻越25.8 kJ/mol的能壘，形成金屬分子氫絡(luò)合物4.這一步放熱12.0 kJ/mol.過(guò)渡態(tài)TS3/4(266.8i cm-1)與4的結(jié)構(gòu)相近，其中H—Pt—H平面和丙烯基平面是近垂直的，因此也具有“遲”過(guò)渡結(jié)構(gòu)特征.從3到4，H(1)—H(2)距離縮短到0.083 nm，此距離已很接近于游離H2中的H—H鍵長(zhǎng)(0.075 nm)，而同時(shí)Pt—(H2)鍵長(zhǎng)由0.154變長(zhǎng)到0.179 nm，Pt—C(α)鍵長(zhǎng)由0.222 nm縮短為0.219 nm.這說(shuō)明4中Pt與丙烯配體作用增強(qiáng)，而與氫分子配體間僅有較弱的agostic相互作用.這有利于下一步H2的消去，事實(shí)上H2比較容易消去，僅需82.0 kJ/mol的能量，且生成的脫氫產(chǎn)物(H3CC(H)Pt CH2)+(5)與4中的相應(yīng)部分結(jié)構(gòu)幾乎一樣.中間體4在整個(gè)體系的勢(shì)能面中具有最小值.

3結(jié)論

結(jié)合相對(duì)論贗勢(shì)，用密度泛函理論方法UB3LYP計(jì)算了Pt+活化丙烷1,2-單分子脫氫反應(yīng)路徑的結(jié)構(gòu)和勢(shì)能面.結(jié)果顯示，該反應(yīng)按1,2-脫氫方式的⊿G?0；而1,2-消除反應(yīng)的決速步(第二個(gè)C—H的活化步)的能壘很小，僅為48.7 kJ/mol.表明反應(yīng)沿1,2-脫氫路徑，在熱力學(xué)和動(dòng)力學(xué)上都是有利的，與實(shí)驗(yàn)發(fā)現(xiàn)該反應(yīng)可以在常溫常壓下按1,2-脫氫方式發(fā)生是一致的.

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(責(zé)任編輯胡小洋)

Theoretical analysis for the structures and energetics on the reactionpathway of 1,2-dehydrogenation of propane by gas-phase Pt+

ZHAO Gaiyun,GUO Yue,ZHANG Ganbing,ZHONG Xinxin,HU Wei,XIA Qinghua

(Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory

for the Synthesis and Application of Organic Functional Molecules, College of Chemistry and Chemical Engineering,

Hubei University,Wuhan 430062,China)

Abstract:The geometries,vibrational frequencies,and energetics of all stationary points including reactants,intermediates,transition states,and products on the 1,2-dehydrogenation pathway of the activation of propane by Pt+in gas phase were calculated by DFT method with UB3LYP functional combining with relativistic effective core potential for understanding its mechanism.Calculated results show that the whole reaction on 1,2-dehydrogenation pathway is exothermic by 195.7 kJ/mol.The rate-determining step is the step of the activation of the second C—H bond,which has a lower barrier of 48.7 kJ/mol.These mean that the 1,2-dehydrogenation of propane by gas-phase Pt+can take place readily at room temperature.These results agree well with the experimental observation.

Key words:activation of Propane；1,2-dehydrogenation pathway；gas-phase platinum ion catalysis；the reaction mechanism；density functional theory

中圖分類號(hào):O643.38

文獻(xiàn)標(biāo)志碼:ADOI:10.3969/j.issn.1000-2375.2016.01.013

文章編號(hào):1000-2375(2016)01-0069-04

通信作者

作者簡(jiǎn)介:趙改云(1989-)，女，碩士生；張干兵，，副教授，E-mail: gbzhang@hubu.edu.cn

收稿日期:2015-05-29