| Lanthanum-modified ZSM-5 zeolite is a very important shape selective catalyst for the technical route to convert olefins in FCC gasoline into propylene. Also, it is very important to increase the profits of petrochemical industry. For the design and development of new and more effective FCC catalysts, it is inevitable to understand both the modification and catalysis mechanisms of La-modified ZSM-5 zeolite. To this end, in this dissertation, the La-modified mechanism of ZSM-5 zeolite, the mechanism of steam dealumination of ZSM-5 zeolite, the mechanism of improvement in hydrothermal stability of La-modified ZSM-5 zeolite, and the mechanisms of activation and double-bond isomerization of 1-hexene over ZSM-5 zeolite before and after modification were systematically studied by applying density functional theory (DFT) and the ONIOM method with cluster models.Firstly, the two-layer ONIOM method and DFT were employed for investigating the accommodation and function of La species in ZSM-5 zeolite. The results show that the introduced lanthanum cations lie in the symmetrical six-membered oxygen rings with Al situated at T11 sites in the straight channels. The channel wall of ZSM-5 zeolite has profound effects on the structure of La(OH)2+ species. There are weak intramolecular hydrogen bonds between the hydroxyls in La(OH)2+ species and the lattice oxygens in La-modified ZSM-5 zeolite. The stable species for the lanthanum cations are La(OH)2+ up to a typical hydrocarbon cracking temperature of 950 K. The lanthanum cations interact with the acidic protons of strong Br(?)nsted acid sites in ZSM-5 zeolite and lead to the formation of weak acidic sites. Also, the weak acidic sites exhibit O-H stretching frequencies of 3742 and 3762 cm-1 and are in the form of Si-OLa(OH)2-Al. The presence of lanthanum cations improves the hydrothermal stability of ZSM-5 zeolite, and reduces both the steam and catalytic deactivation rates of ZSM-5 zeolite.Secondly, the mechanism of steam dealumination of ZSM-5 zeolite, and the mechanism of improvement in hydrothermal stability of La-modified ZSM-5 zeolite were investigated using DFT with 12T cluster models simulating the local structures of the zeolite. The results show that because of the hydrogen bond interaction between the first adsorbed water molecule and the ZSM-5 zeolite framework, the Al-O bond is elongated and weakened. As the second water molecule is adsorbed, the Al-O bond near to the second water molecule is further weakened and eventually broken because of the hydrogen bond interaction between the second adsorbed water molecule and the ZSM-5 zeolite framework. As more water molecules are adsorbed, the other Al-O bonds are broken sequentially, resulting in the dealumination of ZSM-5 zeolite. The adsorbed water molecule is far away from the Al atom in La-modified ZSM-5 zeolite. The lanthanum species coordinate with four zeolite framework oxygen atoms, lay over the framework Al atom and partially prevent polar H2O molecules from attacking the Al-0 bond, thereby retarding the weakening of Al-O bond and improving the hydrothermal stability of ZSM-5 zeolite.Thirdly, the mechanism of double-bond isomerization of 1-hexene to trans-2-hexene was investigated by using DFT with a 3T cluster model simulating the Br(?)nsted active site of the zeolite. Furthermore, the influence of the ZSM-5 zeolite pore structure on the double-bond isomerization of 1-hexene to cis, trans-2-hexene were studied by using the ONIOM2 method. The results show that the double-bond isomerization of 1-hexene to trans-2-hexene has three reaction pathways, i. e., stepwise reaction pathway by means of the bifunctional (acid-base) nature of the zeolite active sites, stepwise reaction pathway by means of the Bronsted acid sites solely and concerted reaction pathway. The real activation energy for the concerted reaction pathway is lower than those for the two stepwise reaction pathways. Furthermore, the concerted reaction pathway avoids the formation of highly stable alkoxide species. From an energetic perspective, the concerted reaction pathway dominates the overall reaction process at low temperatures. However, since the formation of the alkoxy intermediate occur relatively easily at high temperatures, the two stepwise reaction pathways can proceed and are competitive with the concerted reaction pathway. The real activation energy for the stepwise reaction pathway by means of the Br(?)nsted acid sites solely is highest, but the transition state is least sensitive to the zeolite pore structure. Owing to the restricted transition state-type selectivity, the double-bond isomerization of 1-hexene to cis, trans-2-hexene can only occur via the stepwise reaction pathway which involves the Br(?)nsted acid part of the zeolite solely in the straight channels of the ZSM-5 zeolite. In the straight channels of the ZSM-5 zeolite, the apparent activation energy for the isomerization of 1-hexene to cis-2-hexene is 14.34 kcal/mol, while the apparent activation energy for the isomerization of 1-hexene to trans-2-hexene is 12.65 kcal/mol. The results show that cis form isomerization of 1-hexene is competitive with the trans form isomerization of 1-hexene, which is in agreement with reported experimental observations.Lastly, the two-layer ONIOM method was carried out to study the mechanism of the activation and double-bond isomerization of 1-hexene over La-modified ZSM-5 zeolite. The results show that there is no formation ofπ-complex but weaker interaction between the C=C and the acidic center. The activation of 1-hexene proceeds via the heterolytic O-H bond cleavage directly in La species, in which case, the acidic proton of the La species transfers to one carbon atom of the double bond of 1-hexene, while the other carbon atom of the double bond of the 1-hexene bonds with the oxygen of the La species, yielding a stable alkoxy activation product. Thereafter, the oxygen of the La species abstracts a hydrogen atom from the C6H13 fragment and the C-O bond of the alkoxy intermediate is broken, restoring the zeolite acidic site and yielding trans-2-hexene. The activation barrier for the activation of 1-hexene is 38.39 kcal/mol, and the real activation energy for the trans form isomerization of 1-hexene is 45.98 kcal/mol. The results show that the addition of La leads to a decrease in adsorption capacity and catalytic activity of ZSM-5 zeolite, which is in agreement with reported experimental results. |
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