Tuning the Acid Nature of the ZSM-5 Surface for Selective Production of Ethylene from Ethanol at Low Temperatures

Abstract

Solid-acid ZSM-5 catalysts stand out as highly reactive for ethanol dehydration, but the selective production of ethylene at low temperatures, however, is still a challenge. Herein, two ZSM-5 zeolites with a distinct Si/Al ratio have been modified with Ce, La, or P species or treated with H2O or NH3 to get a better understanding on the contribution of acid sites to the ethanol-to-ethylene catalysis. The doping of ZSM-5 affects both the number and strength of acid sites, of which the Ce content inversely increases the population of weak acid sites (WAS). Atomically dispersed oxygen vacancy-rich CeOx synergistically contribute to the dissociation of H2O during the synthesis of Al-rich ZSM-5 to modify the local Al environment by forming new Al–OH bonds, acting as WAS. This significantly enhances the conversion of ethanol into intermediates and ultimately into ethylene (selectivity up to 92%). Further poisoning of strong acid sites (SAS) by in situ NH3-titration confirms that ethanol-to-ethylene catalysis at low temperatures occurs mostly over WAS, while the contribution of SAS is minor. The underpinning insights can serve as a basis for further developments in the combination of other multivalence species with ex situ water treatments or in situ cofeeding to design robust catalysts that are able to efficiently dehydrate bioethanol into ethylene at low temperatures.