Results and Discussion
The ethylene oligomerization process adopted into the calculation here
is based on a well-accepted metallacyclic mechanism, which was first
proposed by Manyik[9] at Union Carbide at 1977.
The cycle starts with the oxidative coupling of two coordinated ethylene
to form a metallacyclopentane intermediate, followed by a β -H
transfer to a third coordinated ethylene, from which 1-hexene was
released. This mechanism was soon modified by
Briggs[10] at 1989, proposing that a third
coordinated ethylene was inserted into the metallacyclopentane to give a
metallacycloheptane, from which β -H eliminate or β -agostic
3,7-H shift take place to yield 1-hexene. This metallacyclic mechanism
was experimentally verified using deuterium labeling by Agapie and
coworkers[11], and applied to ethylene
tetramerization as well, which was also tested with isotopomers by
Sasol[12]. The probing into the active center of
the catalysts, however, remained complicated because of the paramagnetic
nature of the various oxidation states of chromium formed during the
catalytic cycle[13], but there are more evidences
supporting Cr(I)/Cr(III) redox in the tetramerization catalytic
cycle[11a, 14] using electron paramagnetic
resonance and X-ray absorption spectroscopy[15],
or self-activating catalysts crystallization[16],
so Cr(I)/Cr(III) redox cycle was applied in this study.
Based on the mechanism given before, the production of α -olefin
may occur via β -H eliminate or β -agostic 3,7-H shift from
the corresponding metallacycloalkane, through which hydrogen might
modify the energy barriers of the reactions only by coordination and not
necessarily involve into the reaction, as Liu stated in the previous
studies[8]. However, we do believe it is possible
to include hydrogen into the reaction process that leads to the
production of alkene, in which hydrogen is inserted into the Cr-C bond,
offering an hydrogenolysis reaction to give an alkane chain, before aβ -H transfer to liberate α -olefin. The selectivity of the
catalyst depends on the relative equilibrium between decomposition and
ring expansion of the metallacycloalkane, so the energy required for
ring expansion is also calculated. Here we present the energy profiles
of these reactions on metallacycloheptane and metallacyclononane with or
without hydrogen, attempting to illustrate how hydrogen might affect the
reactions. All Gibbs free energies are relative to the starting material
plus the corresponding number of free ethylene molecules.