PNNL

The Science

Converting carbon dioxide (CO₂) and hydrogen into methanol (MeOH) represents an avenue for turning an abundant feedstock into a key commodity chemical and a potentially valuable fuel. Researchers studied how foundational periodic trends, such as electronegativity, can be used to design catalysts by controlling local electric fields within their molecular structures. They specifically focused on pendant phosphorous- and nitrogen-containing groups that can be incorporated to introduce charge into ligand backbones. The team prepared a series of structurally similar metal complexes to directly compare pendant phosphonium and ammonium groups with a neutral carbon reference complex. They demonstrated that the more electropositive phosphonium group generates a more localized positive charge, which increases the rate of catalysis by stabilizing charge buildup during a critical step in the reaction.

The Impact

Hydrogen is an attractive fuel given its high energy density by mass, but storing and transporting it remains challenging. Hydrogenating CO₂ to methanol stores hydrogen in stable chemical bonds, creating a liquid hydrogen carrier that can leverage existing infrastructure for transport and storage. By answering fundamental questions in molecular catalyst design and how ligand effects tune metal centers, researchers can optimize hydrogen utilization toward desired products, such as methanol, while minimizing unwanted side products. This work also addresses a broader problem of better understanding and controlling the factors that determine molecular catalyst performance. While changing the molecular structures surrounding catalytic metal centers can have a large effect on catalysis, connecting specific structural properties to changes in activity and selectivity remains difficult. This work explores how local charge and electric fields can be used as a design principle for controlling catalyst behavior.

Summary

Pendant ammonium ions are frequently used to introduce localized electric fields to stabilize key intermediates for a wide range of catalytic reactions. Because of periodic trends in electronegativity, phosphonium groups can be expected to display greater charge localization and generate stronger local electric fields near the catalyst active site compared to ammonium groups. However, directly comparing pendant ammonium groups with their heavier counterparts has been largely unexplored in homogenous catalysts. 

Researchers investigated the impact of charge localization by incorporating a quaternary phosphonium group into the ligand backbone of a Ru(triphos) hydrogenation catalyst. Operando ¹H nuclear magnetic resonance spectroscopy was used to measure the kinetics for hydrogenation of CO₂ to MeOH via a series of isostructural complexes containing carbon, ammonium, and phosphonium bridgehead groups. The phosphonium-based ligand led to a higher turnover frequency and increased selectivity for production of MeOH than the lighter ammonium analog. The phosphonium-containing catalyst exhibits an overall tenfold increase in the rate of MeOH production relative to the parent catalyst that does not have a charged ligand. The researchers elucidated the details of the catalytic mechanism from a combination of speciation studies, substrate spiking experiments, and electronic structure calculations. The results indicate that the phosphonium bridgehead (a) generates a larger electrostatic potential within the internal ligand cavity and (b) provides the greatest stabilization of charge buildup in the rate-limiting intramolecular hydride transfer to the bound formate intermediate.

Contact

Eric Wiedner, Pacific Northwest National Laboratory, Eric.Wiedner@pnnl.gov 

Funding

This work was primarily supported by the U.S. Department of Energy, Office of Science, through the Basic Energy Sciences program, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science program, FWP 47319. Synthesis and characterization of the phosphonium-based catalyst was supported by the Discovery Research Grant Program of the Natural Sciences and Engineering Research Council of Canada. Computational resources were provided by a user proposal at the National Energy Research Scientific Computing Center located at Lawrence Berkeley National Laboratory.