PNNL

The Science

Biomass-derived carboxylic acids can be upgraded through ketonization, which couples two acids to form a larger, less oxygenated ketone. Researchers studied acetic acid ketonization on faceted TiO₂ nanoparticles to determine whether ketene is a side product or can participate in the catalytic pathway. Experiments and calculations showed directly that ketene can act as a mobile gas-phase intermediate.

The Impact

Ketonization is a well-established route for upgrading biomass-derived carboxylic acids into larger, less oxygenated ketones that can serve as chemicals and fuel precursors. This research shows that the selectivity of heterogeneous catalysts can depend not only on the chemistry of local active sites but also on transport of reactive intermediates through the catalyst bed. This introduces catalyst morphology, porosity, and intermediate residence time as potential design variables for controlling reaction pathways and selectivity.

Summary

Reactive gas-phase intermediates are often overlooked in heterogeneous catalysis, despite their potential for controlling key catalytic steps. A team of researchers used faceted TiO₂(101) nanoparticles assembled into layers of controlled thickness to test whether ketene is a side product or reactive intermediate during acetic acid ketonization. The nanoparticle layers preserve local TiO₂(101)-like surface chemistry while creating interparticle voids that enable gas-phase transport and repeated desorption-readsorption events before gas-phase products escape, creating a complex yet tractable model system.

Temperature-programmed reaction spectroscopy and batch-reactor measurements showed that ketene and acetone production are correlated with how deeply the catalyst layers are saturated with acetic acid. At low acid coverage, the nanoparticles behave like planar TiO₂(101), producing mostly ketene and recombined acetic acid. At higher coverage in thicker layers, acetone production increases superlinearly, consistent with ketene traveling through the particle bed and reacting on neighboring surfaces. Density functional theory calculations further showed that ketene can couple with surface enolates or monodentate acetic acid through low-barrier pathways that converge on beta-ketoacid intermediates. Together, the results show that ketene can act as a mobile gas-phase intermediate and reveal how spatially separated elementary steps can influence catalytic reactivity and selectivity.

Contact

Zdenek Dohnálek, Pacific Northwest National Laboratory, Zdenek.Dohnalek@pnnl.gov 

Funding

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences (CSGB) Division, Catalysis Science Program, FWP 47319. A.S. acknowledges support by DOE BES, CSGB Division under Award DE-SC0007347. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830.