【作者】 郭玉华；

【导师】 陈标华； 蒲敏；

【作者基本信息】 北京化工大学 ， 化学工程， 2008， 博士

【摘要】 分子筛催化的烯烃裂解以及戊烯异构化是两个重要的石油化工过程。本文利用量子化学密度泛函理论和分层计算的方法,采用分子筛簇模型系统地研究了烯烃裂解和戊烯异构化反应机理,从微观结构和能量的角度揭示了分子筛酸催化作用本质。主要内容及结果如下:1.采用ONIOM（B3LYP/6-31G（d,p）:UFF）分层计算方法,研究了C₂-C₅直链烯烃在HY和H-ZSM-5分子筛上的吸附性质。理论计算结果表明烯烃与分子筛的酸性位相互作用形成π配位的超分子复合物。得到的乙烯在HY分子筛上的吸附能接近它的实验观测值,表明了ONIOM方法能够恰当地描述本文所研究的体系。随着烯烃碳链的增长,烯烃的吸附能增加,这个增加量近似为一个常数。与烷烃在分子筛上的吸附具有相同的规律。烯烃双键位置对烯烃的吸附能影响很大,2位烯烃的吸附能要远大于1位烯烃的。烯烃在不同类型分子筛上的吸附性能也有很大的差别,小孔径H-ZSM-5分子筛上的吸附能比大孔径的HY分子筛的大,而且碳原子数越多,这种差别越大。从微观结构上看,吸附的烯烃与H-ZSM-5分子筛酸性位的距离要远大于HY分子筛的。这些现象是由于不同类型分子筛的微孔结构产生的静电作用是不同的,孔径减小这种作用增强。2.在密度泛函B3LYP/6-31G（d,p）水平上,研究了分子筛催化的C₄-C₁₀α烯烃裂解反应机理。采用3T分子筛簇模型来模拟分子筛的酸性位。研究得出,C₄-C₁₀α烯烃的裂解具有相同的反应途径。每个反应都是酸性位质子直接进攻烯烃双键端基的碳原子,然后β位置的C-C键断裂。尽管仅有相应于C-C键断裂的过渡态被发现,但IRC计算表明分子筛催化的烯烃裂解反应实际上按照两步机理进行,存在一个吸附的、短寿命的碳正离子作为中间体。然而,在3T簇模型上并没有得到它的稳定构型。计算得到的烯烃裂解真实活化能比对应的烷烃裂解实验值低,这个结果与文献报道的烷氧基中间体裂解机理的结果相反,表明碳正离子的反应路径更有利。但由于醇盐在分子筛孔道易于形成,因而醇盐中间体的反应途径也可能发生,这样中间体就可能有两种存在形式。烯烃裂解真实的活化能几乎与碳链长度无关（约为44 kcal/mol）,这与烷烃裂解的实验结果是一致的。3.为了考察分子筛骨架结构对烯烃裂解机理的影响,采用ONIOM（B3LYP/6-31G（d,p）:UFF）方法研究了H-ZSM-5分子筛催化的1-已烯裂解反应。计算结果揭示出,H-ZSM-5上的裂解机理与在3T、5T簇模型上的相同。然而,在H-ZSM-5分子筛上优化得到了稳定的碳正离子,发现烯烃裂解反应中存在碳正离子中间体的途径,这说明分子筛的骨架结构对碳正离子的稳定性有着非常重要的影响。还发现吸附的碳正离子是一个活性的高能物种,只需要很小的活化能（7.24kcal/mol）就能发生C-C键的断裂。因此,本文推测碳正离子应该具有非常短的寿命。这样能够合理地解释为什么在分子筛孔道内通过NMR几乎很少观测到碳正离子。扩展的骨架结构对吸附能和裂解反应的表观活化能都有很大的影响,与实验上烷烃裂解情况相比考虑了分子筛环境所得到的结果是合理的。4.在密度泛函的B3LYP/6-31G（d,p）水平上,采用3T分子筛簇模型系统地研究了戊烯的双键异构、顺反异构和骨架异构反应机理。从理论上揭示了这三种反应复杂的、相互关联的微观作用机制,解释了所观测的实验现象。主要结果如下:1-戊烯的双键异构存在两个反应途径,即连续反应机理和协同反应机理。连续反应机理包括两个基元步骤,涉及烷氧基物种作为反应的中间体。协同反应仅包含一个基元步骤,质子转移和双键转移以协同方式进行。协同反应具有较低的能垒,避免了高稳定的烷氧基中间体的生成,因此在低温下这个反应途径将起主导地位。然而由于烷氧基的形成比较容易,因此高温下两个反应途径应该是相互竞争的。两个反应途径的提出很好地解释了为什么在低温和高温时所观测的实验现象不同。综合考虑两个反应途径,1-戊烯双键顺式异构的真实活化能是在19.25-19.92 kcal/mol范围内,而反式异构的真实活化能是在17.93-20.54 kcal/mol范围内,表明1-戊烯顺式异构和反式异构的能力相差不大,与实验结果相符。为了考察分子筛骨架结构对戊烯异构过程的影响,采用ONIOM（B3LYP/6-31G（d,p）:UFF）分层计算方法,研究了大孔径的HY分子筛催化1-戊烯反式双键异构的协同反应机理。结果揭示出,HY分子筛上计算的1-戊烯反式双键异构的真实活化能（20.67 kcal/mol）与3T簇上的结果几乎（20.54 kcal/mol）相等。这说明3T簇模型在一定程度上能很好地表述大孔径分子筛的酸性位。由此,后面的戊烯异构化反应均采用3T簇模型进行计算。2-戊烯的顺反异构体转化反应存在三个反应通道。在一步协同机理中,反应是通过酸性位质子转移和碳碳双键的迁移来完成的。烷氧基机理包含两个基元步骤存在两个反应通道,即2位戊氧基和3位戊氧基中间体通道。综合考虑这三个反应通道,2-戊烯顺反异构体相互转化的活化能是在22.05-23.84 kcal/mol范围内,与1-戊烯双键异构的活化能相差不大。因此,在反应过程中1-戊烯、顺式-2-戊烯和反式-2-戊烯很容易发生相互转化。戊烯的骨架异构有两种反应机理:烷氧基中间体机理和类甲基环丙烷中间体机理。而烷氧基中间体机理又包括两个反应途径,一个是甲基迁移,另一个是乙基迁移。因此,整个异构反应存在三个反应途径。（1）甲基迁移机理分三步,首先涉及3位戊氧基中间体的生成,它可以通过顺式-2-戊烯和反式-2-戊烯质子化来获得,然后是这个直链的3位戊氧基复合物通过一个环状的过渡态发生甲基迁移生成了支链的异戊氧基复合物,异戊氧基复合物再发生分解反应生成吸附的2-甲基-1-丁烯。反应的速控步骤是甲基迁移反应,其活化能是49.27kcal/mol。（2）乙基迁移机理也分三步,首先涉及2位戊烯氧基中间体的生成,它可以通过1-戊烯、顺式-2-戊烯和反式-2-戊烯质子化来获得,然后是2位戊氧基复合物发生乙基迁移生成了异戊氧基复合物,之后与甲基迁移机理相同是异戊氧基复合物发生分解。反应的速控步骤是乙基迁移反应,其活化能是49.55 kcal/mol,几乎与甲基迁移的相等,这表明两个反应途径是互相竞争的。然而由于受到分子筛骨架结构的限制,从空间位阻的角度来说,乙基发生转移的反应途径是不利的。（3）类甲基环丙烷中间体机理分两步,首先是反式-2-戊烯发生碳链扭转,生成类甲基环丙烷中间体,然后再发生甲基迁移反应生成产物。反应的中间体具有高的离子性,且是一个高能物种,这些特征与前面烯烃裂解过程中得到的碳正离子相似,所不同的是C₅H₁₁基团碳骨架发生较大的扭转,它的吸附作用是依靠碳链上的H原子与骨架氧原子之间弱的氢键。反应的速控步骤是碳链扭转反应,其活化能是35.35 kcal/mol,明显低于甲基迁移和乙基迁移两个反应途径的能垒,意味着类甲基环丙烷中间体的反应途径更容易发生。5.由戊烯的计算结果得出,双键异构反应最容易进行,而裂解反应要难于骨架异构反应,这个变化规律与实验观测的三种反应所需酸强度的变化规律是一致的。更多还原

【Abstract】 The olefin cracking and the pentene isomerization over zeolites are two very important reactions in the petrochemical industry. The reaction mechanisms of the olefin cracking and the pentene isomerization have been systematically studied by using density functional theory （DFT） and the ONIOM method with the zeolite cluster models. The microcosmic essence of the acid catalysis of the zeolite is revealed from structure and energy point of view. The main research contents and results are as following:1. The adsorption properties of linear C₂-C₅ olefins on HY and H-ZSM-5 zeolites have been studied by using the ONIOM（B3LYP/6-31G（d,p）:UFF） method. The study results indicate that the microcosmic interactions of the olefin molecules with the Br（?）nsted acid sites of the zeolites lead to the formation ofπ-complexes. The calculated adsorption energy of ethene on HY zeolite is close to its experimental data, suggesting that the ONIOM model can describe the system of our study well. The adsorption energies of olefins on zeolites increase with increasing the number of carbon atoms, and the increase amount is approximate constant, which agrees well with the adsorption properties of the alkane on zeolites. The position of the double bond has biggish effect on the adsorption energies of olefins. The adsorption energies of 2-olefins are much higher than those of 1-olefins.The adsorption energies of olefins on the different types of zeolitesalso have a significant difference. The adsorption energies of olefins onsmall pore H-ZSM-5 zeolite are much larger than those on large pore HYzeolite. Furthermore, the confinement effect in the different types ofzeolites is more obvious when the number of carbon atoms increase.From microstructure, it can be seen that the distance between theadsorbent molecule and the acidic proton in H-ZSM-5 zeolite is muchbigger than that in HY zeolite. These are mainly attributed to thedifferences in the electrostatic interactions from the different types ofzeolites, and the small pore zeolites have much stronger electric fields.2. The cracking reactions of linear C₄-C₁₀α-olefins over zeolites have been studied by using density functional theory at the B3LYP/6-31G（d,p） level. A 3T cluster model is used to simulate the Br（?）nsted acid site of the zeolite. The calculated results show that theβ-scission processes of C₄-C₁₀ olefins have the same reaction mechanism. In all cases, the attack of the zeolite acidic proton is directly on the primary carbon of the double bond of an olefin, and then the C-C bond located inβposition breaks. Although only one transition state corresponding to the rupture of the C-C bond is found, IRC calculations indicate that actually the cracking reaction follows a two-step mechanism with an adsorbed short-lifetime carbocation as intermediate species. However, the stable carbenium ion is not obtained using the bare 3T cluster model.The calculated real activation energy for this carbocation pathway is lower than the experimental value for corresponding alkane cracking contrary to the previously reported pathway via an alkoxide intermediate. Therefore, the reaction pathway via a carbocation intermediate species is energetically much more favorable. Since the alkoxide is readily formed in zeolite pores, under certain reaction conditions the pathway via an alkoxide intermediate can also occur. Thus, the intermediate species for olefin cracking likely exists in two forms. The real activation energies of olefin cracking are nearly independent of the olefin chain length （ 44 kcal/mol）, which is in agreement with the existing experimental results of alkane cracking.3. In order to understand the influence of the zeolite pore structure on the mechanism of olefin cracking, the 1-hexene cracking reaction over H-ZSM-5 zeolite has been studied by using the ONIOM（B3LYP/6-31G（d,p）:UFF） method. The obtained results display that this cracking mechanism is the same with the reaction process on the 3T and 5T cluster. The stable carbenium ion is found in the cavity of the H-ZSM-5 zeolite, which theoretically verifies the existing of carbenium ion in the reaction of olefin cracking. It indicates that the H-ZSM-5 zeolite environment plays a significant role in stabilizing the carbenium ion. The adsorbed carbenium ion is an active high energetic species, and the rupture of the C-C bond in its beta position only requires low energy barrier （7.24 kcal/mol）. Accordingly, it is anticipated that the carbenium ion should have a very short lifetime. This phenomenon can well explain why the carbocations are seldom observed inside the zeolite cavities by NMR probes. The extended zeolite framework also has profound effects on the adsorption energy and the apparent activation energy. Compared with the experimental data for alkane cracking, the results obtained in H-ZSM-5 zeolite is better reasonable than those obtained in small 5T cluster.4. The reaction mechanism of the double-bond isomerization, the cis-trans isomerization and the skeletal isomerization of pentene catalyzed by zeolites have been systematically investigated by using the B3LYP/6-31G（d,p） method with 3T cluster model. The microcosmic interaction mechanisms of the correlation among three kinds of reactions are shown theoretically, which can well explain the experimental phenomena observed. The main results are summarized as follows:The double-bond isomerization may proceed via either a stepwise or a concerted reaction pathway. The stepwise reaction consists of two elementary steps involving an alkoxy species as the intermediate. The concerted reaction includes an elementary step, and the migration of the double bond and proton transfer is concerted. The concerted mechanism has lower energy barrier than the stepwise reaction, which avoids the formation of highly stable alkoxide species. Therefore, at low temperatures, the concerted reaction should dominate the overall isomerization reaction. At high temperatures the two reaction pathways compete against each other because the formation of the alkoxy intermediate will occur relatively easily. Presenting two kinds of pathways for the double bond isomerization of olefins can well explain why the different experimental phenomena are observed at low and high temperatures. The calculated real activation energy of the cis form isomerization of 1-pentene for two pathways is in the range of 19.25-19.92 kcal/mol, while the real activation energy of the trans form isomerization of 1-pentene for two pathways is in the range of 17.93-20.54 kcal/mol. It shows that two kinds of isomerization reactions compete against each other, in agreement with experimental results. In order to investigate the effect of the zeolite framework on the process of pentene isomerization, the concerted reaction pathway of the conversion of 1-pentene to trans-2-pentene over large pore HY zeolite has been studied by using the ONIOM（B3LYP/6-31G（d,p）:UFF） method. The real activation energy of the double bond isomerization of 1-pentene over HY zeolite （20.67 kcal/mol） is nearly identical with the results of 3T cluster （20.54 kcal/mol）, demonstrating that the 3T cluster model can describe the acid site of the large pore zeolites well. Hence, the 3T cluster model is used in other isomerization reactions of pentene.The cis-trans isomerization of 2-pentene has three reaction channels. In the mechanism of the one-step concerted mechanism, the reaction proceeds through proton shift and the migration of the C=C double bond. The alkoxy intermediate mechanism includes two elementary steps and has two reaction channels, i.e. 2-pentyl alkoxide pathway and 3-pentyl alkoxide pathway. Considering three reaction channels, the real activation energies for the cis-trans isomerization of 2-pentene are in the range of 22.05-23.84 kcal/mol, which is slight higher than the data of the double bond isomerization of 1-pentene. Consequently, 1-pentene, cis-2-pentene and trans-2-pentene can easily convert each other.The skeletal isomerization can proceed by two kind of mechanism:the alkoxide intermediate mechanism and methylcyclopropane-likeintermediate mechanism. The alkoxide intermediate mechanism involvestwo reaction pathways: methyl shift and ethyl shift. Accordingly, theoverall skeletal isomerization of pentene has three reaction pathways. （1）The methyl shift mechanism consists of three elementary steps: the firststep is the formation of the line 3-pentoxide intermediate which isobtained through the protonation of adsorbed cis and trans 2-pentene; secondly, a methyl group of this intermediate transfers to another site in the residual hydrocarbon chain through a cyclic transition state, and then the branched one is formed; thirdly, the decomposition of the branched species gives adsorbed iso-pentene. The speed control elementary step is the shift of the methyl group, and its activation barrier is 49.27 kcal/mol; （2） The ethyl shift mechanism also consists of three elementary steps: firstly, the line 3-pentoxide intermediate is formed through the protonation of adsorbed 1-pentene, cis and trans 2-pentene; secondly, this intermediate converts into the branched one through a ethyl group transfer; the third step is the same with the ethyl shift mechanism. The speed control elementary step is the shift of the ethyl group, and its activation barrier is 49.55 kcal/mol. This value is nearly equivalent to that of the methyl shift process, indicating that two reaction pathways compete between each other. However, due to confinement of the pore dimension of the zeolite, the ethyl shift process is not favorable from a steric hindrance point of view; （3） The methylcyclopropane-like intermediate mechanism includes two elementary steps: the torsion of adsorbed trans-2-pentene to give the methylcyclopropane-like intermediate, and then transferring methyl group of the methylcyclopropane-like intermediate to give adsorbed iso-pentene. This intermediate has highly ionic character, and is a high energy species. These characters are similar to the carbocations obtained in the olefin cracking process. The difference is that the torsion of the carbon skeleton is significantly large. But, it can be stabilized on the small 3T cluster. The adsorption interaction between the methylcyclopropane-like intermediate and 3T cluster depends on the weak hydrogen bond. The speed control elementary step is the torsion of the carbon chain, and its activation barrier is 35.35 kcal/mol. This value obviously is lower than those of the methyl and ethyl shift process, implying that the methylcyclopropane-like intermediate pathway occurs more easily.5. Through the study results of pentene, it can be seen that the double bond isomerization reaction is the easiest; the cracking reaction is more difficult than the skeletal isomerization reaction. This change rule is consistent with the acidity change rule of three kinds of reactions observed experimentally.更多还原