Hydrogenation of olefins and polymerization of ethene over chromium oxide/silica catalysts V. In situ Infrared measurements and investigation of the polymer
“…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.…”
The surface organometallic fragment (tSiO) 2 CrdCHCMe 3 initiates the polymerization of ethylene and 1-hexene at room temperature in the absence of organoaluminum or other activators. The kinetics at the gas-solid interface were studied by in situ IR spectroscopy in a constant volume, variable pressure reactor. At low pressures, the reaction is first order in P(C 2 H 4 ), first order in the number of moles of chromium, and linearly dependent on the Cr loading on silica. The second-order rate constant for polymerization is (177 ( 3) s -1 (mol Cr) -1 at 21 °C, and is virtually unchanged upon subsequent additions of ethylene to the catalyst. The kinetic isotope effect, k(C 2 H 4 )/k(C 2 D 4 ), is a meagre 1.29. The temperature dependence of the second-order rate constants yields ∆H ‡ obs ) (30.2 ( 0.9) kJ/mol and ∆S ‡ obs ) (-99 ( 3) J/(K‚mol). These values are consistent with a propagation mechanism of agostically assisted migratory insertion at an alkylchromium(IV) active site. They are inconsistent with an alternating alkylidene/metallacycle mechanism requiring ratedetermining C-H cleavage. The rate of polymerization is accelerated by a factor of 3.2 in the presence of 18 mol % H 2 . Homopolymerization of 1-hexene is only 4.7 times slower than that of ethylene, an effect attributed to weaker preequilibrium binding of the substituted olefin which is partially compensated by faster migratory insertion. Copolymerization of ethylene and 1-hexene is random. It results in slower incorporation of ethylene but faster incorporation of 1-hexene relative to their respective homopolymerization rates.
“…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.…”
The surface organometallic fragment (tSiO) 2 CrdCHCMe 3 initiates the polymerization of ethylene and 1-hexene at room temperature in the absence of organoaluminum or other activators. The kinetics at the gas-solid interface were studied by in situ IR spectroscopy in a constant volume, variable pressure reactor. At low pressures, the reaction is first order in P(C 2 H 4 ), first order in the number of moles of chromium, and linearly dependent on the Cr loading on silica. The second-order rate constant for polymerization is (177 ( 3) s -1 (mol Cr) -1 at 21 °C, and is virtually unchanged upon subsequent additions of ethylene to the catalyst. The kinetic isotope effect, k(C 2 H 4 )/k(C 2 D 4 ), is a meagre 1.29. The temperature dependence of the second-order rate constants yields ∆H ‡ obs ) (30.2 ( 0.9) kJ/mol and ∆S ‡ obs ) (-99 ( 3) J/(K‚mol). These values are consistent with a propagation mechanism of agostically assisted migratory insertion at an alkylchromium(IV) active site. They are inconsistent with an alternating alkylidene/metallacycle mechanism requiring ratedetermining C-H cleavage. The rate of polymerization is accelerated by a factor of 3.2 in the presence of 18 mol % H 2 . Homopolymerization of 1-hexene is only 4.7 times slower than that of ethylene, an effect attributed to weaker preequilibrium binding of the substituted olefin which is partially compensated by faster migratory insertion. Copolymerization of ethylene and 1-hexene is random. It results in slower incorporation of ethylene but faster incorporation of 1-hexene relative to their respective homopolymerization rates.
“…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
“…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) …”
The reaction of ethylene with CO prereduced
CrO
x
/silica polymerization catalyst has been
monitored by
quadrupole mass spectrometry. Evolution of 1-hexene is observed
shortly after ethylene adsorption. This
corresponds to the formation of a chromacyclopentane intermediate
species which can also participate in
ethylene polymerization.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
