Industrial Ziegler-Type Hydrogenation Catalysts Made from Co(neodecanoate)₂ or Ni(2-ethylhexanoate)₂ and AlEt₃: Evidence for Nanoclusters and Sub-Nanocluster or Larger Ziegler-Nanocluster Based Catalysis

Abstract

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 transition metal precatalyst plus an alkylaluminum cocatalyst (and they are not the same as Ziegler-Natta polymerization catalysts). However, for ∼50 years two unsettled issues central to Ziegler-type hydrogenation catalysis are the nature of the metal 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 as metal 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 AlEt(3). 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 the most 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 unreduced metal 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 Ir model Ziegler catalyst system...

Is It Homogeneous or Heterogeneous Catalysis Derived from [RhCp*Cl2]2?In OperandoXAFS, Kinetic, and Crucial Kinetic Poisoning Evidence for Subnanometer Rh4Cluster-Based Benzene Hydrogenation Catalysis

“…Distinguishing catalysis by a discrete metal complex “homogeneous” catalyst from a multiple metal “heterogeneous” nanoparticle catalyst remains a challenging problem in catalytic science. , It is also a forefront topic in catalysis since key catalytic propertiesincluding selectivity, activity, stability, catalytic lifetime, and poisoning as well as catalyst recovery and regenerationare inherently different for homogeneous and heterogeneous catalysts . The problem of the “identification of the true catalyst” is made more intriguing as well as compounded in complexity by the recent findings that subnanometer clusters, such as M 4 species, can be active catalysts. − …”

“…At its most basic level, the underlying conceptual basis of that 1994 approach is still the essence of the required approach today: (i) determining where the precatalyst mass resides during catalysis (i.e., in what species or forms) and then necessarily (ii) performing kinetic studiesincluding quantitative catalyst poisoning studies as the present work will make apparentsince “catalysis is a wholly kinetic phenomenon”, at least when starting from favorable reaction thermodynamics. The 1994 methodology, developed via third-row transition metals that tend to form TEM-beam stable nanoparticles (from third-row metal precatalysts that are also often TEM-beam stable), has been updated recently , by the addition of in operando spectroscopic studies that are required for a detailed, correct picture of “what is the evolved form(s) the precatalyst mass” under operating conditions, Scheme . Other notable parts of Scheme include: (i) a necessary focus throughout the research on the disproof of multiple alternative hypotheses, (ii) the idea that no single experiment can convincingly determine the true nature of the catalyst, ,, and again (iii) the required kinetic studies.…”

Synthesis and Characterization of [Ir(1,5-Cyclooctadiene)(μ-H)]4: A Tetrametallic Ir4H4-Core, Coordinatively Unsaturated Cluster

“…Such Ziegler-type industrial hydrogenation catalysts are used industrially to produce styrenic block copolymers at a level of ∼1.7 × 10 5 metric tons/year . In addition, [Ir­(1,5-COD)­(μ-H)] 4 is of considerable interest as a possible Ir–H-containing, tetrametallic Ir 4 H 4 intermediate in the nucleation and growth of Ir 0 n nanoclusters starting with (COD)­Ir + precatalysts and with stabilizers such as [P 2 W 15 Nb 3 O 62 ] 9– , HPO 4 2– , and AlEt 3 . ,,, Whether or not such polynuclear metal hydride (M–H) n species are key intermediates in M 0 n nanoparticle formationrather than the presently assumed M 0 intermediatesremains controversial. The availability of precatalysts and possible intermediates derivable from [Ir­(1,5-COD)­(μ-H)] 4 opens up the possibility of QEXAFS and other direct-method tests with such discrete, fully characterized, Ir 4 H 4 -core complexes …”

Improved Syntheses for the Compounds [(1,5-COD)M(μ-O2C8H15)]2 (M = Ir, Rh)

“…A recent paper describes the synthesis and characterization of the compounds [(1,5-COD)M(μ-O 2 C 8 H 15 )] 2 (COD = cyclooctadiene, M = Ir ( 1 ), Rh ( 2 ), O 2 C 8 H 15 = 2-ethylhexanoate) as models of industrial Co and Ni Ziegler-type hydrogenation precatalysts. − For that work, it is important to have definitively characterized, high-purity precatalyst materials readily availablecharacterization of 1 and 2 previously included single-crystal X-ray crystallographic structure determinations (Figure ). Additionally, it was noted therein that both 1 and 2 have potential in other areas, including film deposition on semiconductor devices, for which the synthesis and preliminary characterization of 1 was described previously .…”

“…In addition, the improved synthesis procedure has led to an improved yield of 1 : 76% versus the 61% yield achieved via the previous synthetic approach . The improved synthesis and simplified crystallization procedures are significant, not only because of the demonstrated usefulness of 1 as a model for study of the industrial Ziegler-type hydrogenation catalysts, but also because of the potential usefulness of both 1 and 2 in other areas and in important industrial applications. , …”