PNNL

The Science

Transformations of carbon-based, or organic, molecules often involve hydrogenation, or the addition of hydrogen to the molecule. In liquid-phase reactions, the solvent can play a key, but challenging to understand, role. Researchers used the hydrogenation of benzaldehyde on palladium (Pd) as a model reaction to study the influence of solvent on reactivity. They found that only the strength of hydrogen adsorbed on Pd notably affects the overall speed of the reaction. Varying the solvent enables changes in the resulting reaction rate by up to one order of magnitude.

The Impact

Metals are critical for catalyzing, or speeding up, the hydrogenation of organic compounds. Hydrogenation is a necessary reaction for producing fuels and chemical compounds from a variety of carbon sources. Solvents can play a significant role in solution-phase reactions, but understanding what parameters influence the performance of a metal catalyst under solution conditions is a challenge. This work decouples these different solvent effects and contributes to efforts to define and control the interactions that can strongly affect catalysis, therefore enhancing reactivity.

Summary

Metals play a key role in the catalytic hydrogenation of organic compounds. In liquid-phase catalysis, solvents can impose significant effects on the ability of metals to perform the reaction. Identifying which parameters directly cause these effects remains challenging. To understand how to control the efficacy of metals for liquid-phase catalysis, a team of scientists investigated the impact of four solvents—methanol, water, dioxane, and tetrahydrofuran—on the reduction of benzaldehyde on activated-carbon-supported palladium. These four solvents encompass organic/aqueous, polar/non-polar, and protic/aprotic solvents, covering a range of properties exhibited by frequently used solvents. The researchers used kinetic methods and performed detailed kinetic modeling to quantify the reversibility as well as the kinetic relevance of the reaction steps. Their results underscore the importance of how a solvent can help stabilize hydrogen adsorbed onto the metal surface but indicate that the solvent interactions with the substrate, in this case benzaldehyde, has less of an influence on reaction rates. The next step in this research is broadening its scope and performing a wider set of experiments to assess the generality of these correlations of solvent effects to hydrogenation rates.

PNNL Contact

Johannes Lercher, Pacific Northwest National Laboratory, Johannes.Lercher@pnnl.gov

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences Program, Division of Chemical Sciences, Geosciences and Biosciences.