Formaldehyde-Mediated Initial Carbon-Carbon Bond Formation in Zeolite-Catalyzed Methanol-to-Hydrocarbon Conversion.

Zeolite-catalyzed methanol-to-hydrocarbon conversion is a promising technology for the sustainable production of valuable hydrocarbon products. However, the mechanism behind the formation of the first carbon-carbon bond has been a subject of controversy for several decades. By comprehensive consideration of previous experimental phenomena and theoretical studies, a formaldehyde (HCHO)-based first carbon-carbon formation mechanism is proposed. Within the new mechanism, hydrated or methylated products of HCHO (methanediol, methyloxymethanol, and dimethyloxymethane) with much weaker C-H bond strengths replace methane in the traditional methane-HCHO mechanism, allowing energetically and kinetically favorable pathways to form the first C-C bond. The formed C-C bond products are further converted to ketene and olefins via the methylation-decarbonylation route. The plausibility of the newly proposed mechanism is confirmed by both theoretical calculations and experiments in various MTH zeolite catalysts. A key intermediate in this mechanism is glycolaldehyde, which was captured in situ by both mass spectrometry and Fourier transform infrared spectroscopy. The viability of the mechanism in different zeolites, as predicted theoretically, was also confirmed by gas chromatography. Not only does this new mechanism introduce an innovative pathway for the first C-C bond formation, but it also provides a comprehensive explanation of the specific role of HCHO in the early stage of the MTH process and associated reactions.