| Ethylene is a very important raw material for petrochemical products which has received more and more attention from domestic and overseas researchers. The output of ethylene can measure the development level of national petrochemical industry. And how to design more economical and environmental friendly process route for ethylene production is particularly important. In recent years, the research hot spots for ethylene production process are focus on development of catalyst. Among them, the ZSM-5 zeolite catalyst exhibits good catalytic performance in this process route. In this paper, ethylene as the target product and metal (Ag, Rh, Mo, La, Ca) modified ZSM-5 zeolite catalyst of two models as the research object which is studied under different reaction pathways. Hope to be able to choose the best reaction pathways and catalyst in ethylene process.All calculations are using the Vienna ab initio simulation package (VASP) code through the DFT-GGA calculations. Calculation process comprises:the exchange-correlation functional used to obtain the energy which is the implementation of the generalized gradient approach, and a nudged elastic band method (NEB) was used for locating the transition state (TS), the frequency analysis was performed to confirm the transition state. After calculated the reaction system of each species in activation energy to infer what kind of metal modified ZSM-5 zeolite catalyst is the best in catalytic reaction of ethylene process.The main conclusions of this work are summarized as follow:(1) This article selected two reaction pathways of ethylene process. One is oxonium ylide path and the other is dimethyl ether dehydration path. That was also choosed in the full unit cell model and 5T cluster model of ZSM-5 zeolite. Under the two reaction pathways and non-modified ZSM-5 zeolite conditions, the activation energy of full unit cell model is higher than 5T cluster model. No matter which kinds of model are used for ZSM-5 zeolite, the oxonium ylide path activation energy is higher than dimethyl ether dehydration path.(2) For oxonium ylide path and dimethyl ether dehydration path which are under the full unit cell model ZSM-5 zeolite, the activation energy from high to low in the order:ZSM-5> Ag-ZSM-5> Mo-ZSM-5>Rh-ZSM-5> La-ZSM-5> Ca-ZSM-5 and ZSM-5>Rh-ZSM-5>Ag-ZSM-5>Ca-ZSM-5>Mo-ZSM-5 > La-ZSM-5. This can be inferred that the same mtetal modified ZSM-5 zeolite catalyst for different reaction pathways activation energy is not the same. With the metal modified ZSM-5zeolite catalyst activation energy is less than unused metal modified ZSM-5 zeolite catalyst. The rules of 5T cluster model of ZSM-5 zeolite are the sames as above.(3) This paper study the three metal clusters (Ag)n, (Rh)n, (Mo)n(0<n≤5) on the full unit cell model of ZSM-5 zeolite adsorption energy and structure. As the free growth of metal clusters, three kinds of metal cluster consistent morphology and metallic bonding bond distance increases with increasing of the number of atoms in the cluster increases. When the metal clusters are absorbed on ZSM-5 zeolite, three kinds of metal clusters are consistent morphology and the metal cluster bond distance decreased obviously after the adsorption. It can be determined that the impact of ZSM-5 zeolite pores of the free growth of metal clusters. By calculating the growth energy of three kinds of metal clusters, it can be found that on the ZSM-5 zeolite (Mo)n cluster growth energy could be minimized and also on the ZSM-5 zeolite, (Mo)n is the easiest to grow in the ZSM-5 zeolite. |
PDF Full Text Request