(111h) Understanding Solvent and Pretreatment Effects on the Surface Chemistry of Small Oxygenates on Molybdenum Trioxide

Sugars derived from biomass possess a high degree of functionality and offer promise as a potential feedstock for production of commodity chemicals. Understanding the pathway sugars take towards their desired end point is key because it allows for a more controlled chemical route. Heterogeneous catalysts are able to promote different routes based on their acidity and other physical parameters. Identifying descriptors for sugar interactions with a catalyst is key, but the role of solvents in these systems must not be neglected. Pretreatments can also affect the catalyst surface and must be considered as well. This work focuses on retro aldol condensation (RAC) and similar reactions by considering the reaction with simple oxygenates over different states of molybdenum trioxide (MoO₃).

This study highlights the role of ethanol on the surface of fully oxidized and partially reduced MoO₃ containing defect sites created through partial reduction. By combining diffuse reflectance infrared spectroscopy (DRIFTS) with temperature programmed desorption (TPD), surface species were monitored along with changes to the catalyst. X-ray absorption spectroscopy (XAS) measurements determined how the oxidation state was affected by the temperature and chemical environment during pretreatments. Ethanol, acetaldehyde and crotonaldehdye were introduced over the three different pretreated catalysts, and DRIFTS showed how these molecules bind to the surface. TPD analysis showed that reducing pretreatments significantly increased the yields of dehydrogenation and coupling products. Theoretical calculations gave vibrational frequencies and showed the catalyst with a vacancy defect adsorbed oxygenates more easily. These results collectively show that alcoholic species can interact with the fully oxidized surface, whereas aldehydes prefer surfaces containing defect sites. The oxidation state was also shown to be heavily dependent on which solvents were used and what pretreatment was done prior to reaction.