1983
DOI: 10.1016/0021-9517(83)90060-x
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Hydrogenation of olefins and polymerization of ethene over chromium oxide/silica catalysts V. In situ Infrared measurements and investigation of the polymer

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Cited by 53 publications
(5 citation statements)
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“…The polymerization reactions are also first-order with respect to total Cr, requiring that the fraction of active sites be reproducible, constant, and independent of catalyst loading. , In contrast, heterogeneous Cr/silica catalysts show increasing activity with decreasing Cr loading, and the active site concentration appears to increase as polymerization proceeds. , Our observed first-order behavior in n Cr rules out potential mechanisms that require two surface Cr sites for the activation of ethylene. ,,, In addition, the magnetic properties of 2 indicate that the major, even exclusive, surface species is mononuclear. , We consider monomer-induced migration of active sites to be unlikely since the magnetic behavior of the still-active catalyst remains unchanged after polymerization. Our results further imply that (IV) is the resting oxidation state of Cr in the active catalyst.…”
Section: Discussionmentioning
confidence: 86%
“…The polymerization reactions are also first-order with respect to total Cr, requiring that the fraction of active sites be reproducible, constant, and independent of catalyst loading. , In contrast, heterogeneous Cr/silica catalysts show increasing activity with decreasing Cr loading, and the active site concentration appears to increase as polymerization proceeds. , Our observed first-order behavior in n Cr rules out potential mechanisms that require two surface Cr sites for the activation of ethylene. ,,, In addition, the magnetic properties of 2 indicate that the major, even exclusive, surface species is mononuclear. , We consider monomer-induced migration of active sites to be unlikely since the magnetic behavior of the still-active catalyst remains unchanged after polymerization. Our results further imply that (IV) is the resting oxidation state of Cr in the active catalyst.…”
Section: Discussionmentioning
confidence: 86%
“…After the creation of Y 2 groups (initiation), the mechanism proceeds via insertion of ethylene with formation of the precursor represented in square brackets in Scheme . This model explains the first-order behavior with respect to the monomer pressure. ,,
17 General Scheme of the Initiation Step in Ethylene Polymerization on the Phillips Catalyst a a Y 1 can be a (C 2 H 4 ) n complex containing 1, 2, or 3 coordinated molecules, and the Y 2 species can be one of those reported in Scheme .
…”
Section: 2 Ethylene Polymerization Mechanism On Cr/sio2 Catalyst521 I...mentioning
confidence: 98%
“…A prerequisite for this scheme is that there must be a Cr−H bond present prior to the onset of polymerization. Some groups have suggested that surface silanol groups provide a source of additional hydrogen atoms. , Hydride transfer may occur between a silanol group and a supported Cr(II) ion to yield an O 2- species and a Cr(IV)−H bond, into which the first ethylene can insert (see Scheme a) . Alternatively, it has been proposed that ethylene adsorption directly onto a surface silanol group is followed by its coordination to an adjacent chromium ion along with the migration of a proton from the silanol group onto the metal center (see Scheme b) …”
Section: 22 Cossee Model For Initiation and Propagationmentioning
confidence: 99%
“…A prerequisite for this scheme is that there must be a M−H bond present prior to the onset of polymerization. Some groups have suggested that surface silanol groups provide a source of additional hydrogen atoms. , Hydride transfer may occur between a silanol group and a supported Cr 2+ ion to yield an O 2- species and a Cr 3+ −H bond (into which the first ethylene can insert) …”
Section: Introductionmentioning
confidence: 99%