PNNL

Abstract

Determining the kinetically dominant catalyst in a given catalytic system is a forefront topic in catalysis. The [RhCp*Cl2]2 (Cp* =[?5-C5(CH3)5]) system pioneered by Maitlis and co-workers is a classic precatalyst system from which homogeneous mononuclear Rh1, subnanometer Rh4 cluster, and heterogeneous polymetallic Rh(0)n nanoparticle have all arisen as viable candidates for the true hydrogenation catalyst, depending on the precise substrate, H2 pressure, temperature, and catalyst concentration conditions. Addressed herein is the question of whether the prior assignment of homogeneous, mononuclear Rh1Cp*-based catalysis is correct, or are trace Rh4 subnanometer clusters or possibly Rh(0)n nanoparticles the dominant, actual cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm initial H2 pressure? The observation herein of Rh4 species by in operando-X-ray absorption ?ne structure (XAFS) spectroscopy, at the only slightly more vigorous conditions of 26 °C and 8.3 atm H2 pressure, and the con?rmation of Rh4 clusters by ex situ mass spectroscopy raises the question of the dominant, room temperature, and mild pressure cyclohexene hydrogenation catalyst derived from the classic [RhCp*Cl2]2 precatalyst pioneered by Maitlis and co-workers. Ten lines of evidence are provided herein to address the nature of the true room temperature and mild pressure cyclohexene hydrogenation catalyst derived from [RhCp*Cl2]2. Especially signi?cant among those experiments are quantitative catalyst poisoning experiments, in the present case using 1,10-phenanthroline. Those poisoning studies allow one to distinguish mononuclear Rh1, subnanometer Rh4 cluster, and Rh(0)n nanoparticle catalysis hypotheses. The evidence obtained provides a compelling case for a mononuclear, Rh1Cp*-based cyclohexene hydrogenation catalyst at 22 °C and 2.7 atm H2 pressure. The resultant methodology, especially the quantitative catalyst poisoning experiments in combination with in operando spectroscopy, is expected to be more broadly applicable to the study of other systems and the “what is the true catalyst?” question. The authors would like to thank Finke Group members and Prof. Saim O¨ zkar for their valuable input as this work was proceeding. This work was supported at Colorado State University by the U.S. Department of Energy (DOE), O?ce of Science, O?ce of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences, vial DOE Grant SE-FG402-03ER15453. The work at PNNL was also supported by the U.S. Department of Energy, O?ce of Science, O?ce of Basic Energy Sciences, Division of Chemical Sciences, Geo-sciences & Biosciences. Paci?c Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for the DOE by Battelle. XSD/PNC facilities at the Advanced Photon Source and research at these facilities are supported by the U.S. Department of Energy, Basic Energy Sciences; a Major Resources Support Grant from NSERC; the University of Washington; the Canadian Light Source; and the Advanced Photon Source. Use of the Advanced Photon Source, an O?ce of Science User Facility operated for the U.S. Department of Energy O?ce of Science by Argonne National Laboratory was supported by the U.S. DOE under Contract No. DE-AC02- 06CH11357.