Illustration of site-selective hydrogenation of arenes under mild conditions catalyzed by rhodium nanoparticles decorated with organic ligands.
Production of chemicals used in many industrial processes is currently carried out under the high temperatures and pressures required to break strong chemical bonds. While effective, these processes are energy intensive. Chemical catalysts that accomplish the same tasks using less energy are highly desirable.
Working toward that goal, a team of scientists in the Institute for Integrated Catalysis at Pacific Northwest National Laboratory recently achieved a key step in catalytic hydrogenation at room temperature and lower hydrogen pressure. Their discovery, detailing advances made in this crucially important reaction, is detailed in “Rh(CAAC)-Catalyzed Arene Hydrogenation: Evidence for Nanocatalysis and Sterically Controlled Site-Selective Hydrogenation”, which appeared in the journal ACS Catalysis.
Ba Tran, a PNNL postdoctoral scientist, made the crucial discovery that the molecular rhodium starting complex is not the actual catalyst. Under reaction conditions, the soluble rhodium complex is transformed into rhodium nanoparticles that carry out the catalysis. Knowing this key information will allow scientists to further hone the process and ensure only the desired reactions occur.
Why it matters: Catalysis can reduce energy costs and minimize waste, while increasing synthetic efficiency. Pursuing catalysts creatively can reshape how chemicals are transformed in reactions important to the chemical industry.
Summary: Hydrogenation of arenes (cyclic 6-membered aromatic rings) is traditionally carried out at high temperatures. The experiments recently reported yielded catalytic hydrogenation of arenes at room temperature and low pressure of dihydrogen (H2) starting from a cyclic alkyl amino carbene ligand bound to rhodium. A key accomplishment was that only the desired aromatic ring is hydrogenated, even in the presence of other groups, such as esters and amides, that could potentially also be hydrogenated. The reaction is also site-selective in that one arene is preferentially hydrogenated over other arenes in the same molecule.
The results demonstrate the importance of synergy among the scientists participating in the Institute for Integrated Catalysis (IIC). Chemists who study soluble molecular (homogeneous) catalysts generally use different techniques and approaches than heterogeneous catalysis experts. The research reported in this article relied on the combined expertise of scientists in molecular catalysis,heterogeneous catalysis, and spectroscopy. Further, the experiments relied on the specialized X-ray absorption spectroscopy technique available at the Advanced Photon Source national user facility at Argonne National Laboratory.
What’s next? Further studies center on better understanding the factors governing the rate of nanoparticle formation and how that fundamental knowledge might be used to rationally design improved catalysts.
Acknowledgments: This work addresses the Basic Research Needs for Catalysis Science as identified by U.S. Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences (BES), notably the Priority Research Direction “Understand and Control the Dynamic Evolution of Catalysts.”
Sponsors: This research was supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division. The Canadian Light Source and its funding partners also provided support. PNNL is a multi-program national laboratory operated for DOE by Battelle under contract DE-AC05-76RL01830.
Research Area: Chemical Sciences, Catalysis
User Facilities: This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory.
Research Team: Ba Tran, John Fulton, John Linehan, Johannes Lercher, and Morris Bullock (PNNL).
Reference: Tran, B. L.; Fulton, J. L.; Linehan, J. C.; Lercher, J. A.; Bullock, R. M., "Rh(CAAC)-Catalyzed Arene Hydrogenation: Evidence for Nanocatalysis and Sterically Controlled Site-Selective Hydrogenation," ACS Catal. 2018, 8, 8441-8449. DOI: 10.1021/acscatal.8b02589