Investigations into homogeneous transition metal catalysis by X-ray absorption spectroscopy
Investigations into homogeneous transition metal catalysis by X-ray absorption spectroscopy
In situ X-ray absorption spectroscopy (XAS) studies on a number of previously poorly characterised homogeneous metal alkyl activated organonickel alkene oligomerisation catalysts are described. Liquid-phase copper-zinc catalysts, which are reported to be active towards syngas conversion, are also investigated. Extended X-ray absorption fine structure (EXAFS) analysis, utilising curved wave theory and ab initio phaseshifts, is used to probe the structure of the solution species present during activation and catalysis. An estimation of the accuracy of the EXAFS technique for solution analysis is provided by a study of the catalyst precursors, NiX2(PR3)2 (X = Cl or Br, R = Et or Bu), M(acac)2 (M = Ni or Cu, acac = acetylacetonate), and ZnEt2; the average error in first-shell coordination radii is estimated at 0.02-0.03A*, which is consistent with previous error analyses on solids. EXAFS studies on some established organo-metallic reactions involving the formation of NiMe2(PEt3)2, Ni(acac)C2H5(PPh3), and η3-C3H5NiBr(PPh3), are used to demonstrate the applicability of the technique to working homogeneous catalyst systems. In situ XAS studies, carried out under a variety of conditions, are reported for the following nickel-based catalytic systems: Ni(acac)2/cocatalyst (cocatalyst = AlEt3, AlEt2(OEt), or ZnEt2), NiX2(PEt3)2/cocatalyst (cocatalyst = AlEt3, Al2Me3Cl3, or ZnEt2) and NiBr2(PBun3)2/AlEtCl2. Ni K-edge EXAFS data, recorded on propene dimerisation catalysts formed by the activation of solutions of NiCl2(PEt3)2 with Al2Me3Cl3 and AlEt3 at -40oC, indicates a the first coordination sphere consisting of carbon and phosphorus atoms; Ni..Alshells at Ca. 2.97A* suggest Ni-C-Al interactions which probably stabilise the nickel centre. No disruption of the Ni-C-Al interaction is observed after treatment with propene. These results offer the first direct structural parameters for such interactions in solution. No Ni..Al interaction is apparent when Ni(acac)2 is the precursor. Elevated temperature and pressure XAS studies on a NiBr2(PBun3)2/AlEtCl2 ethene-butadiene codimerisation catalyst indicate that an allylnickel phosphine species (Ni-C 2.05A*, Ni-P 2.26A*) predominates in solution under catalyst operating conditions. Allylnickel phosphine species are also directly observed by EXAFS in the NiX2(PEt3)2/ZnEt2/butadiene systems, but reduction to metallic nickel is found to occur in the absence of the diene. Metal particle formation is similarly observed in the Ni(acac)2 systems in the absence of any reacting alkene to stabilise the nickel centre. Room temperature XAS studies on solutions of Cu(acac)2 and ZnEt2, both with and without butadiene, show formation of small copper metal particles (12-25A*).
University of Southampton
1991
Corker, Judith Mary
(1991)
Investigations into homogeneous transition metal catalysis by X-ray absorption spectroscopy.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
In situ X-ray absorption spectroscopy (XAS) studies on a number of previously poorly characterised homogeneous metal alkyl activated organonickel alkene oligomerisation catalysts are described. Liquid-phase copper-zinc catalysts, which are reported to be active towards syngas conversion, are also investigated. Extended X-ray absorption fine structure (EXAFS) analysis, utilising curved wave theory and ab initio phaseshifts, is used to probe the structure of the solution species present during activation and catalysis. An estimation of the accuracy of the EXAFS technique for solution analysis is provided by a study of the catalyst precursors, NiX2(PR3)2 (X = Cl or Br, R = Et or Bu), M(acac)2 (M = Ni or Cu, acac = acetylacetonate), and ZnEt2; the average error in first-shell coordination radii is estimated at 0.02-0.03A*, which is consistent with previous error analyses on solids. EXAFS studies on some established organo-metallic reactions involving the formation of NiMe2(PEt3)2, Ni(acac)C2H5(PPh3), and η3-C3H5NiBr(PPh3), are used to demonstrate the applicability of the technique to working homogeneous catalyst systems. In situ XAS studies, carried out under a variety of conditions, are reported for the following nickel-based catalytic systems: Ni(acac)2/cocatalyst (cocatalyst = AlEt3, AlEt2(OEt), or ZnEt2), NiX2(PEt3)2/cocatalyst (cocatalyst = AlEt3, Al2Me3Cl3, or ZnEt2) and NiBr2(PBun3)2/AlEtCl2. Ni K-edge EXAFS data, recorded on propene dimerisation catalysts formed by the activation of solutions of NiCl2(PEt3)2 with Al2Me3Cl3 and AlEt3 at -40oC, indicates a the first coordination sphere consisting of carbon and phosphorus atoms; Ni..Alshells at Ca. 2.97A* suggest Ni-C-Al interactions which probably stabilise the nickel centre. No disruption of the Ni-C-Al interaction is observed after treatment with propene. These results offer the first direct structural parameters for such interactions in solution. No Ni..Al interaction is apparent when Ni(acac)2 is the precursor. Elevated temperature and pressure XAS studies on a NiBr2(PBun3)2/AlEtCl2 ethene-butadiene codimerisation catalyst indicate that an allylnickel phosphine species (Ni-C 2.05A*, Ni-P 2.26A*) predominates in solution under catalyst operating conditions. Allylnickel phosphine species are also directly observed by EXAFS in the NiX2(PEt3)2/ZnEt2/butadiene systems, but reduction to metallic nickel is found to occur in the absence of the diene. Metal particle formation is similarly observed in the Ni(acac)2 systems in the absence of any reacting alkene to stabilise the nickel centre. Room temperature XAS studies on solutions of Cu(acac)2 and ZnEt2, both with and without butadiene, show formation of small copper metal particles (12-25A*).
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Published date: 1991
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Local EPrints ID: 460560
URI: http://eprints.soton.ac.uk/id/eprint/460560
PURE UUID: 6f5faf5d-cebc-4e94-b7bb-dcd6420537a8
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Date deposited: 04 Jul 2022 18:24
Last modified: 04 Jul 2022 18:24
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Author:
Judith Mary Corker
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