DFT Study On The Key Steps Of The Dimethyl Ether Prepared From Syngas On ZSM-5 Zeolite Catalyst

At present research territory, studies on dimethyl ether (DME) synthesis from syngas in a single reactor on ZSM-5 zeolite are forced on the mechanism of methanol synthesis from syngas.While fewer investigations has been carried out to the mechanism of subsequent key steps. In the present work we employ periodic DFT-GGA calculation method and five models(5T cluster model,10T cluster model, full cell unit moel, oxygen and silicon vacancy model respectively) to explore the possible six pathways of DME synthesis from syngas,and analyze the effect of framework confinement and vacancy site. CH3, CH3OH, CH3O(H)CH3, CH3OCH3 and H are recognized by literature adsorbed on the above models in the process of catalytic reaction, the reaction mechanism of dimethyl ether synthesis from methanol have been calculated and discussed systematically. The main conclusions are summarized as follows:(1) To compare the reaction mechanism of DME synthesis on 5T cluster model and 10T cluster model we can draw some significant conclusions: Adsorption strength of adsorbates are more stable on 5T cluster model than on 10T cluster model due to the effect of framework confinement. The optimal pathway for DME synthesis involving the form of intermediate DMO (dimethyl ether oxonim ion) and then the products:CH3+CH3OH→CH3O(H)CH3→CH3OCH3+H.The first step is the rate-limiting step and the reaction barriers are 118.67kJ/mol,89.73kJ/mol respectively.While the second step can occur easily with relatively lower barriers of 86.83kJ/mol and 19.30kJ/mol.(2) In this study, we calculate and discuss the adsorption energy, reaction mechanism and rate constant on full cell unit model of ZSM-5 zeolite of DME synthesis. There exists a linear relationship between the adsorption energy and the corresponding bond length in this model, the shorter bond length can leads to the more stable adsorption. The calculate results showed that the mechanism for DME synthesis on full cell unit model of ZSM-5 is the same as 10T cluster model.(3) In this study, we also discuss the adsorption energy, reaction mechanism and rate constant on oxygen and silicon vacancy model of ZSM-5 zeolite of DME synthesis. The adsorption of the silicon vacancy model is more stable than oxygen vacancy model. For silicon vacancy, there are two pathways for DME synthesis, which is adsorbed CH3 group reaction with adsorbed CH3OH molecule or CH30 group. Refer to the oxygen vacancy model, except the two pathways similar to silicon vacancy model, there is another pathway: firstly two adsorbed CH30H molecules form a intermediate oxonium ion and then the intermediate oxonium ion dehydrogenation to form DME.(4) Comparing the reaction barriers in this paper, we find that there exists a relationship of barriers:Ea(5T cluster model)> Ea(10T cluster model)> Ea(full cell unit model)> Ea(silicon vacancy model)> Ea(oxygen vacancy model). The effect of framework confinement of ZSM-5 only influences the reaction barriers while doesn’t change the mechanism of reaction. The influence of oxygen vacancy for DME synthesis is larger than that of silicon vacancy.