A study by X-ray absorption spectroscopy of novel supported molybdenum olefin metathesis catalysts
A study by X-ray absorption spectroscopy of novel supported molybdenum olefin metathesis catalysts
An in situ X-ray absorption spectroscopy (XAS) study of novel supported molybdenum olefin metathesis catalysts is described. Several aspects of molybdenum XAS are also examined. An extended X-ray absorption fine structure (EXAFS) study of molybdenum compounds of both known and unknown structure is used to prove the validity of the analysis techniques using curved wave theory with theoretical phaseshifts. For nearly all the compounds studied coordination radii which agree to within 1% with data from crystallographic studies are obtained. The theoretical X-ray absorption near-edge structure (XANES) of several molybdenum compounds calculated using the program DLXANES are presented. These are compared with both the experimental XANES and the calculated XANES of distortions of the real structures. These studies show that, while the broad features of XANES spectra can be reproduced by such modelling, more accurate phaseshifts are required to produce theoretical spectra which closely mirror the experimental data. The position of the K-edge of molybdenum in a range of compounds is compared with some measures of the nominal coordination charge on the molybdenum atom and no simple relationship is found between the two. The LII,III-edge spectra of several molybdenum compounds are examined in two ways, first a study shows that the compounds have branching ratios lower than the statistical value, secondly the second derivatives of the L edges show splittings that may be related to the energy levels in the molybdenum 5d orbitals. The preparation of novel room temperature propene metathesis catalysts by supporting Mo2X4L4 [L = P(n-Bu)3j, X = Cl or Br, L = py, X = Cl], β-Mo_2Cl_4(dpe)_2, Mo_2(BzO)_4 and Li_4Mo_2Me_8.4Et_2O on alumina is described. In situ XAS studies on three catalytic systems, those formed from the precursors Mo_2(AcO)_4, Li_4Mo_2Me_8.4Et_2O and Mo_2Cl_4[P(n-Bu)_3]_4, using a purpose-built cell are reported. Though these studies do not provide conclusive evidence of the structure of the catalysts, they indicate a molybdenum-carbon interaction of 1.80-1.90AA in the active catalysts. This is strongly suggestive of a carbene species and is the first such structural evidence of a molybdenum-alkylidene. This supports the Chauvin-Herrison mechanism for metathesis in the heterogeneous reaction.
University of Southampton
Mosselmans, Julian Frederick Willem
1990
Mosselmans, Julian Frederick Willem
Mosselmans, Julian Frederick Willem
(1990)
A study by X-ray absorption spectroscopy of novel supported molybdenum olefin metathesis catalysts.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
An in situ X-ray absorption spectroscopy (XAS) study of novel supported molybdenum olefin metathesis catalysts is described. Several aspects of molybdenum XAS are also examined. An extended X-ray absorption fine structure (EXAFS) study of molybdenum compounds of both known and unknown structure is used to prove the validity of the analysis techniques using curved wave theory with theoretical phaseshifts. For nearly all the compounds studied coordination radii which agree to within 1% with data from crystallographic studies are obtained. The theoretical X-ray absorption near-edge structure (XANES) of several molybdenum compounds calculated using the program DLXANES are presented. These are compared with both the experimental XANES and the calculated XANES of distortions of the real structures. These studies show that, while the broad features of XANES spectra can be reproduced by such modelling, more accurate phaseshifts are required to produce theoretical spectra which closely mirror the experimental data. The position of the K-edge of molybdenum in a range of compounds is compared with some measures of the nominal coordination charge on the molybdenum atom and no simple relationship is found between the two. The LII,III-edge spectra of several molybdenum compounds are examined in two ways, first a study shows that the compounds have branching ratios lower than the statistical value, secondly the second derivatives of the L edges show splittings that may be related to the energy levels in the molybdenum 5d orbitals. The preparation of novel room temperature propene metathesis catalysts by supporting Mo2X4L4 [L = P(n-Bu)3j, X = Cl or Br, L = py, X = Cl], β-Mo_2Cl_4(dpe)_2, Mo_2(BzO)_4 and Li_4Mo_2Me_8.4Et_2O on alumina is described. In situ XAS studies on three catalytic systems, those formed from the precursors Mo_2(AcO)_4, Li_4Mo_2Me_8.4Et_2O and Mo_2Cl_4[P(n-Bu)_3]_4, using a purpose-built cell are reported. Though these studies do not provide conclusive evidence of the structure of the catalysts, they indicate a molybdenum-carbon interaction of 1.80-1.90AA in the active catalysts. This is strongly suggestive of a carbene species and is the first such structural evidence of a molybdenum-alkylidene. This supports the Chauvin-Herrison mechanism for metathesis in the heterogeneous reaction.
This record has no associated files available for download.
More information
Published date: 1990
Identifiers
Local EPrints ID: 461909
URI: http://eprints.soton.ac.uk/id/eprint/461909
PURE UUID: e02d22d9-9a49-4716-baf9-bf202182ad25
Catalogue record
Date deposited: 04 Jul 2022 18:58
Last modified: 04 Jul 2022 18:58
Export record
Contributors
Author:
Julian Frederick Willem Mosselmans
Download statistics
Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.
View more statistics