Theoretical Study Of The Olefin Cracking And The Pentene Isomerization Over Zeolites

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 ofÏ€-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.