Unraveling the Relationship between Zeolite Structure and MTO Product Distribution by Theoretical Study of the Reaction Mechanism

Determination of the relative contributions of aromatic-based and alkene-based cycles in the MTO process is essential for understanding the reaction mechanism and estimating the product selectivity over various zeolites. However, it is a great challenge due to the high complexity of the reaction network. Thus, the olefin formation mechanisms are systematically investigated here by the density functional theory with van der Waals dispersive corrections over structurally different zeolites. It is shown that the aromatic-based cycle dominates the MTO process over H-SAPO-34 with a side-chain route as the major reaction pathway, while the alkene-based cycle plays a major role on H-BEA and H-ZSM-5. Over H-ZSM-22, the alkene-based cycle is the main route with C-5+ alkenes as primary products. The energy barriers of the aromatic-based cycle are similar over the above four types of zeolites. A linear relationship is found between the ethene/propene molar ratios and the free energy barriers of alkene-based cycles over these zeolites, indicating that the ethene and propene selectivities are mainly determined by the propagation of the alkene-based cycle. This suggests that the ultimate free energy barrier of the alkene-based cycle can be used as a measure to predict the C-2(=)/C-3(=) molar ratios obtained on topologically different zeolites.