Quantum Chemistry Study On The Reactions Of Light Hydrocarbons Conversion Over Cation-Exchanged Zeolites

C4 hydrocarbons catalytic cracking over ZSM-5 zeolites is an effective way enchancing propene and ethene production, which is important to utilize C4-riched hydrocarbon and increase the profits of petrochemical industry. In addition, catalytic conversion of light hydrocaron in zeolites, especially methane into valuable raw chemicals, is an important approach to optimize the utilization of hydrocarbon. Serveral reactions were investigated by using density functional theory (DFT) with cluster model in this paper, including the reaction of C4 hydrocarbon catalytic cracking over acidic zolites, the simulation of zeolitic acidity, light olefine dimerizaiton over acidic zeolites, methane conversion over Ag-ZSM-5 and In-ZSM-5 zeolites, and structure, hydrothermal stability and adsorption performance of Re-ZSM-5. The main and important conclusion are summarized as follows.1. The reaction of n-butane monomolecular cracing over acidic zeolites was investigated. The mechanism of n-butane dehydrogenating over acidic zeolites was revealed by comparing the dehydrogenation at different order C atoms in n-butane. The calculated results show that the dedydrogenation is expected to be perferred atβ-C atoms in n-butane. The reactivity sequence found for n-butane cracking over acidic zeolite is: Secondary cracking > Primary cracking > dehydrogenation atα-C atom > dehydrogenation atβ-C atom.2. The relation between the acidity and structure of the cluster shows that the acidity effect of the zeolite can be simulated by modifying the peripheral bonds of the cluster for the 5T cluster. With the increase of the length of terminal Si-H bond, the acidity of zeolites will increase and the deprotonation energy will decrease. The reaction barriers of n-butane monomolecular cracking decrease with the increase of zeolitic acidity. In addition, the reaction of primary C-C bond cracking is most sensible to zeolitic acidity. The relationship between reaction barriers and deprotonation energies was deduced by applying Bransted-Polanyi principle. Therefore, from the correlations, the activation barriers could be predicted for the reactions on other zeolites from the calculated deprotonation energy.3. The reaction of 1-butene monomolecular catalytic cracking over acidic zeolites was investigated in this paper. The results show that the reaction of C-C cracking does not proceed with the protonated 1-butene but with butoxide. Compared with the reaction barriers of n-butane, 1-butene is cracked over acidic zeolites more easily. The reactivity sequence found for light alkene dimerization over acidic zeolite is: 1-butene > propene > ethane.4. The reactions of light alkene dimerization were investigated in this paper. The results show that the reactions proceed with the dimerization between the second alkene molecule and the first alkene molecule chemsorbed product alkoxide. The reactions of alkene dimerization are exothermic, and the controlled step of reaction is the dimeraztion step. The calculated results of reactions barriers show that the activation barriers decrease with the increase of molecular chain. The reactivity sequence found for light alkene dimerization over acidic zeolite is: 1-butene > propene > ethane. It is found that the reaction of 1-butene bimolecular cracking is easier to occur over acidic zeolites.5. Two different pathways of methane catalytic activation over Ag-ZSM-5 and In-ZSM-5 were taken into account in this paper: the "carbenium" and the "alkyl" pathways. It is found that the "alkyl" is the preferential reaction pathway for methane catalytic activation over Ag-ZSM-5 and In-ZSM-5. Consequently, the mechanism of methane catalytic conversion in the presence of ethane was proposed in this paper. The mechanism of methane activation over Ag-ZSM-5 and In-ZSM-5 are different because of different Lewis acid-base pairs existing in Ag⁺ and InO⁺ ion-exchanged ZSM-5. In addition, it is found that the Ag⁺ and InO⁺ cations play an important role in the methane activation by comparing with the reaction of methane activation over H-ZSM-5.6. The structure, hydrothermal stability and alkanol adsorption performances of Re-ZSM-5 are investigated in this paper. It is found that the Re-ZSM-5 remain stable and the ReO₃⁺ cations do not lost from the framework of zeolites even in the presence of steam in 823 K. As a consequence, the Re-ZSM-5 still remain its catalytic activity at high temperatures and water-rich conditions. The results of alkanol adsorption on Re-ZSM-5 show that the adsorption energies will be decrease with the increase of molecular chain.