(MECP), as shown in Scheme 17, reaction 1. The following reaction may occur
from the chromacyclopentane species h5 on the triplet surface to generate a
methyl-chromacyclobutane species h6 through an intramolecular 2,1-hydrogen
shift, which would lead to a metathesis reaction to produce propylene and butene
molecules, as observed experimentally [79]. However, the hydrogen transfer was
prohibited by a much higher energy barrier of 57.2 kcal mol
À1 without formaldehyde adsorption, as shown in Scheme 17, reaction 2. Alternatively, the reaction
from h5 to a dimerization product 1-butene was found to be finished in a two-step
manner via a Cr-hydride intermediate h7 with an energy barrier of 39.4 kcal mol
À1 .
The reaction crossover to the quintet surface before the second triplet barrier
through another MECP and 1-butene was finally released on the quintet surface,
as shown in Scheme 17, reaction 3. For the ring expansion step, ethylene molecule
may be directly inserted into the Cr–C bond of h5, generating a chromacycloheptane structure h10 on the triplet surface with an insertion barrier of
26.9 kcal mol
À1 , as shown in Scheme 17, reaction 4. In contrast to h5, the ring
opening of h10 took place in a one-step manner assisted by a direct β-H agnostic
interaction. The trimerization product 1-hexene was also released on the quintet
surface with a barrier of 28.0 kcal mol
À1 and the corresponding spin crossing took
place in the product channel, as shown in Scheme 17, reaction 5. The further ring
expansion from chromacycloheptane h10 was prohibited because ethylene coordination complex could not be located for h10 due to steric hindrance.
The calculated activation barriers over models 4g, 4g-1, and 4g-2 for all the
typical reactions (similar to reactions 1–5 in Scheme 17) during the induction
period are summarized in Table 4. After a complete desorption of the formaldehyde
molecules, the first initiation reaction occurred on 4g via a MECP to generate a
chromacyclopentane species on the triplet surface. The following ring expansion
gave a chromacycloheptane species and a subsequent one-step reductive elimination yielded 1-hexene on the quintet surface through another MECP. On the site
4g-1, the ring expansion step was forbidden because a third ethylene molecule
could not be adsorbed on the chromacyclopentane species with one formaldehyde
coordinated on the Cr center. Therefore, the reaction of ethylene trimerization on
model 4g-1 was absent. Although the dimerization on model 4g-1 was likely to take
place with an energy barrier of 35.5 kcal mol
À1 , a metathesis reaction was still
possible on site 4g-1 to produce short olefins. There was no reaction initiated by
4g-2 because the chromium site was completely shielded by the two coordinated
formaldehyde molecules.
Table 4 also summarizes the calculated activation barriers of all the typical
reactions (in Scheme 17) during the induction period over catalyst models similar to
4g, 4g-1, and 4g-2 except that both Si atoms within each model were fully
fluorinated. Fluorination of the silica support for the F-modified Phillips catalyst
showed negligible influence on ethylene dimerization to 1-butene and metathesis to
propylene [160]. However, the energy barrier was increased significantly in
reaction 5 of Scheme 17, in which 1-hexene was formed from the chromacycloheptane species through a one-step intramolecular hydrogen shift. Fluorination showed a positive effect on ring expansion in reaction 4 of Scheme 17.
Phillips Cr/Silica Catalyst for Ethylene Polymerization
183
from the chromacyclopentane species h5 on the triplet surface to generate a
methyl-chromacyclobutane species h6 through an intramolecular 2,1-hydrogen
shift, which would lead to a metathesis reaction to produce propylene and butene
molecules, as observed experimentally [79]. However, the hydrogen transfer was
prohibited by a much higher energy barrier of 57.2 kcal mol
À1 without formaldehyde adsorption, as shown in Scheme 17, reaction 2. Alternatively, the reaction
from h5 to a dimerization product 1-butene was found to be finished in a two-step
manner via a Cr-hydride intermediate h7 with an energy barrier of 39.4 kcal mol
À1 .
The reaction crossover to the quintet surface before the second triplet barrier
through another MECP and 1-butene was finally released on the quintet surface,
as shown in Scheme 17, reaction 3. For the ring expansion step, ethylene molecule
may be directly inserted into the Cr–C bond of h5, generating a chromacycloheptane structure h10 on the triplet surface with an insertion barrier of
26.9 kcal mol
À1 , as shown in Scheme 17, reaction 4. In contrast to h5, the ring
opening of h10 took place in a one-step manner assisted by a direct β-H agnostic
interaction. The trimerization product 1-hexene was also released on the quintet
surface with a barrier of 28.0 kcal mol
À1 and the corresponding spin crossing took
place in the product channel, as shown in Scheme 17, reaction 5. The further ring
expansion from chromacycloheptane h10 was prohibited because ethylene coordination complex could not be located for h10 due to steric hindrance.
The calculated activation barriers over models 4g, 4g-1, and 4g-2 for all the
typical reactions (similar to reactions 1–5 in Scheme 17) during the induction
period are summarized in Table 4. After a complete desorption of the formaldehyde
molecules, the first initiation reaction occurred on 4g via a MECP to generate a
chromacyclopentane species on the triplet surface. The following ring expansion
gave a chromacycloheptane species and a subsequent one-step reductive elimination yielded 1-hexene on the quintet surface through another MECP. On the site
4g-1, the ring expansion step was forbidden because a third ethylene molecule
could not be adsorbed on the chromacyclopentane species with one formaldehyde
coordinated on the Cr center. Therefore, the reaction of ethylene trimerization on
model 4g-1 was absent. Although the dimerization on model 4g-1 was likely to take
place with an energy barrier of 35.5 kcal mol
À1 , a metathesis reaction was still
possible on site 4g-1 to produce short olefins. There was no reaction initiated by
4g-2 because the chromium site was completely shielded by the two coordinated
formaldehyde molecules.
Table 4 also summarizes the calculated activation barriers of all the typical
reactions (in Scheme 17) during the induction period over catalyst models similar to
4g, 4g-1, and 4g-2 except that both Si atoms within each model were fully
fluorinated. Fluorination of the silica support for the F-modified Phillips catalyst
showed negligible influence on ethylene dimerization to 1-butene and metathesis to
propylene [160]. However, the energy barrier was increased significantly in
reaction 5 of Scheme 17, in which 1-hexene was formed from the chromacycloheptane species through a one-step intramolecular hydrogen shift. Fluorination showed a positive effect on ring expansion in reaction 4 of Scheme 17.
Phillips Cr/Silica Catalyst for Ethylene Polymerization
183