Operando Spectroscopy on the Reaction Mechanism and Deactivation of Methanol-to-Olefins Catalysts

Abstract

Lower olefins, such as ethylene and propylene, are important chemical building blocks, and many products, such as plastics, are made from olefins. Traditionally, these lower olefins are produced from crude oil. Because of changing demands and changes in feedstock composition, “on-purpose” methods to produce specific olefins have to be applied in order to meet future olefin demands. Among the processes that can be used to produce on-purpose olefins from renewable sources are Methanol-to-Olefins (MTO) processes. MTO processes are catalytic processes, and zeolite materials are used as catalysts. Olefins are formed from methanol via the hydrocarbon pool mechanism. Catalyst deactivation during this process is caused by the buildup of inactive hydrocarbon species inside the zeolite catalyst material. However, the border between active hydrocarbon pool species that produce olefins and inactive hydrocarbons that deactivate the catalyst is not very well-defined. Studying the hydrocarbon pool during the reaction is a good way to understand deactivation during MTO processes. In Part I of this PhD thesis, three small-pore zeolite catalyst materials were compared. During the reaction, the formation and evolution of the hydrocarbon pool were followed using operando UV–vis spectroscopy and related to the activity and selectivity of the catalyst. In this way, different hydrocarbon pool species were identified for the three different zeolite catalysts, and related to the different stages of the MTO process. In addition, the expansion of the zeolite lattices caused by the accumulation of hydrocarbons was studied during the reaction using operando X-ray diffraction (XRD). In Part II, two medium-pore zeolite materials were studied during the MTO process. Zeolite H-ZSM-5 was compared with magnesium-modified ZSM-5. Using three high-temperature UV–vis probes along the reactor bed, the formation of retained hydrocarbons at different locations along the reactor bed was studied during the reaction. Mg-ZSM-5 showed differences in catalytic behavior and formation of retained hydrocarbons compared to H-ZSM-5. The differences were related to the differences in acidic properties between the two materials.