Structural Changes of ZSM-5 Catalysts during Methanol-to-Hydrocarbons Conversion Processes

Industrial methanol-to-hydrocarbons (MTH) conversion processes comprise a series of reaction and regeneration stages performed at elevated temperatures and accompanied by steam generation. Although these conditions contribute to the progressive degradation of the catalyst structure, the restructuring processes and their implications for the MTH reaction remain elusive. This study systematically investigates the performance and structural changes of ZSM-5 catalysts during extended MTH reaction–regeneration runs by complementing the catalytic tests with in-depth characterization of aged catalysts using infrared, 27Al, 29Si, and 1H magic angle spinning nuclear magnetic resonance, as well as operando diffuse-reflectance UV–visible spectroscopic techniques. Long-term operation of these zeolites leads to partial hydrolysis of the framework aluminum, which has a marginal impact on the zeolite porosity and crystallinity and leads primarily to a decrease in Brønsted acid site concentration. Dealumination is more pronounced during the initial reaction–regeneration cycles and at higher temperatures as well as in aluminum-rich ZSM-5 zeolites. Control steaming and coke-deposition experiments indicate that the dislodgment of aluminum from framework sites mainly arises from the byproduct water evolving during the MTH reaction, although thermal stress and coke oxidation also contribute to this process. The hydrolytic potential of steam is significantly attenuated by the evolution of hydrocarbon and coke species during the MTH reaction. Catalyst aging leads to the development of a perturbed framework, framework-associated and extraframework aluminum sites, a significant fraction of which interacts with the remaining framework sites. This causes the development of Lewis acid sites in the initial phase but their concentration declines with continued operation. Dealumination processes decrease the overall catalyst activity and increase the prevalence of hydrogen-transfer and cracking reactions. Consequently, this leads to lower selectivities toward C3+ alkenes and higher propensity to coking of the aged catalysts. The findings evidence the high structural and performance dynamicity of zeolites during their prolonged operation in the MTH process, which should be considered in catalyst development and kinetic studies of this reaction.