PNNL

The Science                                

Biomass conversion requires the formation of new carbon–carbon bonds and the removal of oxygens from different molecules. Ketonization reactions of carboxylic acids are important to both those processes. New work examined the ketonization of acetic and propanoic acids on titanium dioxide, or TiO₂. Researchers explored the effects of both water (H₂O) and hydroxyl groups on the TiO₂ surface on the overall reaction. They found that H₂O changes the identity of the intermediates on the TiO₂ surface and leads to lower ketonization activity, while the hydroxyl groups react immediately with the acids and play no major role in the ketonization.

The Impact

Biomass represents a potential source of fuel and valuable chemicals. Carboxylic acid ketonization is an important part of the carbon–carbon bond formation and deoxygenation reactions in converting biomass to useful forms. Better understanding how water, ubiquitous in these reactions, affects ketonization can help researchers develop more efficient catalysts to produce renewable fuels by reducing the overall oxygen content and building platform molecules with target carbon chain lengths.

Summary

Carboxylic acids are important intermediates for converting biomass to fuels and useful chemicals. The major routes of biomass conversion all produce carboxylic acids. Ketonization of carboxylic acids forms ketones, building blocks that can undergo further reactions to increase the fraction of molecules in the gasoline/diesel molecular size range. Metal oxides with Lewis acid–base site pairs have been widely reported as efficient catalysts for carboxylic acid ketonization, with most studies based on clean metal oxide surfaces. However, the surfaces of metal oxides are often populated with hydroxyl groups generated from H₂O. Water is always present in catalytic biomass conversion, but few studies address the role of surface hydroxyl groups and the catalytic consequences of water on ketonization reactions.

Researchers used acetic acid and propanoic acid as model compounds to understand the effects of H₂O on gas-phase ketonization activity over TiO₂ catalysts. Kinetic measurements showed that adding H₂O vapor decreases the rate and increases the activation barrier of the carboxylic acid ketonization reaction. By combining spectroscopic approaches, researchers identified multiple species present on the TiO₂ surface upon carboxylic acid adsorption, including molecular carboxylic acid, monodentate carboxylate, and chelating/bridging bidentate carboxylates. The presence of H₂O vapor increases the coverage of the less reactive bridging bidentate carboxylate, leading to the lower ketonization activity of hydrated TiO₂. Surface hydroxyl groups are consumed by interaction with carboxylic acid upon the formation of surface acetate species and do not affect the ketonization reaction.

PNNL Contact

Yong Wang, Pacific Northwest National Laboratory and Washington State University, yong.wang@pnnl.gov

Funding

This work was primarily supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences within the Catalysis Science program and carried out at the Pacific Northwest National Laboratory, a multiprogram national laboratory operated for the DOE by Battelle. F.L. and H.W. were also partly supported by the DOE Office of Energy Efficiency and Renewable Energy and Bioenergy Technologies Office for this work. W.H. is also partly supported by Chambroad Scholarship for pursuing his PhD. Part of the work was conducted at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility sponsored by the Biological and Environmental Research program. Computing time was granted by a user proposal at the EMSL and the National Energy Research Scientific Computing Center.