Industrial ziegler-type hydrogenation catalysts made from Co(neodecanoate)₂ or Ni(2-ethylhexanoate)₂ and AlEt ₃: Evidence for nanoclusters and sub-nanocluster or larger ziegler-nanocluster based catalysis

Ziegler-type hydrogenation catalysts are important for industrial processes, namely, the large-scale selective hydrogenation of styrenic block copolymers. Ziegler-type hydrogenation catalysts are composed of a group 8-10 transitionmetal precatalyst plus an alkylaluminumcocatalyst (and they are not the same asZiegler-Natta polymerization catalysts). However, for̃50 years two unsettled issues central to Ziegler-type hydrogenation catalysis are the nature of themetal species present after catalyst synthesis, and whether the species primarily responsible for catalytic hydrogenation activity are homogeneous (e.g., monometallic complexes) or heterogeneous (e.g., Ziegler nanoclusters defined asmetal nanoclusters made from combination of Ziegler-type hydrogenation catalyst precursors). A critical review of the existing literature (Alley et al. J. Mol. Catal. A: Chem. 2010, 315, 1-27) and a recently published study using an Ir model system (Alley et al. Inorg. Chem. 2010, 49, 8131-8147) help to guide the present investigation of Ziegler-type hydrogenation catalysts made from the industrially favored precursors Co(neodecanoate)₂ or Ni(2-ethylhexanoate)₂, plus AlEt₃. The approach and methods used herein parallel those used in the study of the Ir model system. Specifically, a combination of Z-contrast scanning transmission electron microscopy (STEM), matrix assisted laser desorption ionization mass spectrometry (MALDI MS), and X-ray absorption fine structure (XAFS) spectroscopy are used to characterize the transition metal species both before and after hydrogenation. Kinetic studies including Hg(0) poisoning experiments are utilized to test which species are themost active catalysts. The main findings are that, both before and after catalytic cyclohexene hydrogenation, the species present comprise a broad distribution of metal cluster sizes from subnanometer to nanometer scale particles, with estimated mean cluster diameters of about 1 nm for both Co and Ni. The XAFS results also imply that the catalyst solutions are a mixture of the metal clusters described above, plus unreducedmetal ions. The kinetics-based Hg(0) poisoning evidence suggests that Co and Ni Ziegler nanoclusters (i.e., M_≥4) are the most active Ziegler-type hydrogenation catalysts in these industrial systems.Overall, the novelty and primary conclusions of this study are as follows: (i) this study examines Co-and Ni-based catalysts made from the actual industrial precursor materials, catalysts that are notoriously problematic regarding their characterization; (ii) the Z-contrast STEM results reported herein represent, to our knowledge, the best microscopic analysis of the industrial Co and Ni Ziegler-type hydrogenation catalysts; (iii) this study is the first explicit application of an established method, using multiple analytical methods and kinetics-based studies, for distinguishing homogeneous from heterogeneous catalysis in these Ziegler-type systems; and (iv) this study parallels the successful study of an IrmodelZiegler catalyst system, thereby benefiting from a comparison to those previously unavailable findings, although the greaterM-Mbondenergy, and tendency toagglomerate, of Ir versusNi or Co are important differences to be noted.Overall, the main result of this work is that it provides the leading hypothesis going forward to try to refute in future work, namely, that sub,M_≥4 to larger, M_n Ziegler nanoclusters are the dominant, industrial, Co-and Ni-plus AlR₃ catalysts in Ziegler-type hydrogenation systems.