Molecular Dynamics Simulation Of Methanol To Olefin(MTO) Reactions On HZSM-5 Zeolite Using ReaxFF Force Field

Methanol to Olefin (MTO) industrial process is an important process in which methanol can be effectively transformed into olefins, reactant gas methanol can be obtained from various kinds of resources such as nature gas, biomass, and even wastes, thus MTO is one of the most promising industrial processes. However, due to limited experimental technologies, the understanding of this process is still poor, the major difficulties are: how is methanol be activated by acidic zeolite, concrete mechanism for carbon chain growth, how is hydrocarbon pool formed, and how is olefin formed.In this work, we utilize molecular simulating method to investigate initial reaction mechanisms of MTO reactions. We utilize H-ZSM-5 acidic zeolite as catalyst and methanol and ethene gas as reactants. We use ab initio quantum chemistry and ReaxFF reactive molecular dynamics as computational methods. The major contents are as follows: to investigate detailed mechanism of MTO process at atomic level, ReaxFF force field has been extended and used in this work to simulate MTO reactions in H-ZSM-5 zeolite. The simulation data are consistent with previously reported experimental observations. The major innovations in this thesis are expanded understanding of MTO mechanisms, which involves: 1) By explicitly considering multi-body interactions and thermodynamic conditions explicitly, the initial reaction network of MTO in acidic zeolite has been obtained. New reaction mechanisms are proposed based on the simulations. 2) For the activation of methanol, a less possible but very important CH3 radical mechanism is identified in addition to the commonly accepted methoxyl mechanism. 3) The commonly accepted chain-growth mechanism, in which ethene interacts with methyloxide, has been observed. However, it is a small contribution to the total production. The more popular route for the chain growth is attributed to the presence of deprotonated Br?nsted sites, which are produced via the activation of methanol molecules. Therefore, the hydrocarbon pool is working with the methanol molecules involved. With the hydrocarbon pool, the chain growth is significantly accelerated. 4) Considering the collision probability, the rate-determining step for MTO is not the activation of methanol as suggested by static calculations, but the C-C chain formation.