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.@en

Fourier transform infrared spectroscopy and density functional theory calculations have been used to elucidate the nature of active sites of ZrBEA zeolite responsible for the catalytic synthesis of butadiene. We show that the content of open Zr(IV) Lewis acid sites, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure connected to three -O-Si linkages and one OH group, correlates with the catalytic activity in the process of conversion of ethanol into butadiene. The higher catalytic activity of the open sites is attributed to their higher acid strength and steric accessibility. The study suggests that the control of such open sites plays a crucial role for the further design of the optimal multifunctional zeolite-based catalysts. (Figure Presented).

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