In Situ Spectroscopic Insights on the Molecular Structure of the MgO/SiO2 Catalytic Active Sites during Ethanol Conversion to 1,3-Butadiene

Ethanol is an important renewable chemical that allows for sustainable high-value product, such as 1,3-butadiene, catalytic synthesis. The MgO/SiO2 catalyst is typically utilized in a single-step ethanol-to-1,3-butadiene catalytic conversion, and the (by)product yields were shown to depend on the type, structure, and strength of the catalytic active sites. The fundamental factors describing the molecular structure and binding properties of these sites are thus of critical importance but not yet fully understood. We utilized a multimodal approach, including temperature-programmed surface-sensitive infrared mass spectrometry using probe molecules, such as CO2, NH3, and pyridine and propionic acids, to unravel the structure and persistence of these catalytic sites in situ. In particular, Mg-O-Mg, Mg-O(H)-Mg, Mg-O-Si, and Mg-O(H)-Si surface site binding configurations were proposed and scrutinized using spectroscopic methods in combination with density functional theory (DFT) calculations. A combination of NH3-temperature-programmed desorption and DFT calculations allowed to better describe the molecular structure of said catalytic sites as the presence of open and closed Lewis acid sites (LASs) was suggested. The catalyst was shown to have both open LASs with both Mg-3C and Mg-4C as LASs and also very isolated closed LASs (Mg-3C and Mg-4C). Reactive molecule surface site poisoning experiments suggested that weak basic sites were responsible for ethanol dehydrogenation and strong basic sites were responsible for aldol condensation and Meerwein-Ponndorf-Verley reduction, whereas stronger acid sites catalyze acetaldol and crotyl alcohol dehydration reactions and weak acid sites catalyzed the undesired ethanol dehydration. In situ diffuse reflectance infrared spectroscopy and fixed-bed measurements indicated the consumption of the weak basic sites during the catalytic reaction. LASs were also consumed during the adsorption and the reaction and no generation of new basic sites was observed. The fundamental surface site structure proposed here can further serve as a starting point for theoretical calculations necessary to fully model the reactive pathway during ethanol catalytic transformation to 1,3-butadiene.