C-C Coupling Catalyzed by Zeolites: Is Enolization the only Possible Pathway for Aldol Condensation?

MBEA zeolites are known to catalyze carbon-carbon coupling reactions such as acetaldehyde condensation, which is an important step in a range of industrially relevant processes, e.g., a sustainable butadiene synthesis. The widely accepted mechanism of the reaction includes a separate enolization step via an α-proton transfer to zeolite. By combining the H-D exchange activity studies, FTIR spectroscopy, and DFT calculations, we show that such a mechanism is indeed feasible for the SnBEA zeolite. For the ZrBEA and TiBEA zeolites, on the other hand, experimental evidence suggests that a separate enolization step is unlikely. We propose the possibility of an alternative concerted single-step mechanism that involves coadsorption of two aldehyde molecules at the open M(IV) Lewis acid site and a subsequent proton transfer between the adsorbates in a collective transition state stabilized by the M-OH group of the open site. The study suggests that the nature of a zeolite dopant can thus be used to control the activity of the catalyst by modifying the reaction mechanism.