Insight into Three-Coordinate Aluminum Species on Ethanol-to-Olefin Conversion over ZSM-5 Zeolites

Abstract

Commercial bioethanol can be readily converted into ethylene by a dehydration process using solid acids, such as Brønsted acidic H-ZSM-5 zeolites, and thus, it is an ideal candidate to replace petroleum and coal for the sustainable production of ethylene. Now, strong Lewis acidic extra-framework three-coordinate Al³⁺ species were introduced into H-ZSM-5 zeolites to improve their catalytic activity. Remarkably, Al³⁺ species working with Brønsted acid sites can accelerate ethanol dehydration at a much lower reaction temperature and shorten the unsteady-state period within 1-2 h, compared to >9 h for those without Al³⁺ species, which can significantly enhance the ethanol dehydration efficiency and reduce the cost. The reaction mechanism, studied by solid-state NMR, shows that strong Lewis acidic EFAl-Al³⁺ species can collaborate with Brønsted acid sites and promote ethanol dehydration either directly or indirectly via an aromatics-based cycle to produce ethylene.

Keywords: MFI zeolite; acidity; aluminum; ethanol to olefin; solid-state NMR spectroscopy.