Figure 6. Relative Gibbs free energy profiles (kcal/mol) for
the formation of active center on Cr/PNP catalyst with hydrogen, with
hydrogen coordinated onto the metal center opposite the ethylene
molecule
When hydrogen is introduced into the system, the reduction elimination
could originate from the β -H on the alkyl group, or from the
hydrogen molecule, thus enable other possible scenarios to occur. Figure
6 presents the Gibbs free energy profiles with hydrogen adsorbed axially
onto the metal center. In this case direct reduction from hydrogen
adsorbed onto the metal center to liberate methane off could occur via
A-TS0 -1-H2 with an energy barrier of 3.6 kcal/mol. The
other methyl group could be eliminated in the same way via
A-TS1-2-H2 with an energy barrier of 13.3 kcal/mol.
Similar as before, ethylene could insert into the M-C bond via
A-TS0-3-H2 with an energy barrier almost the same as
that without hydrogen (5.8 kcal/mol), from which the β -H
eliminate could occur via A-TS3’-4-H2 after endergonic
conformation change from A3-H2 to A3’-H2to bring the propyl group close to the methyl group, this time overall
energy barrier reduced to 12.2 kcal/mol. This pathway is exactly the
same as when hydrogen is absent, in which hydrogen function as a ligand
and does not participate into the reaction. The energy barrier is
reduced from 14.4 kcal/mol to 12.2 kcal/mol upon introduction of
hydrogen. A third possible route would be a combination of the two
developments, in which elimination from hydrogen occur after ethylene
insertion to release first methane and then propane. However, this
reduction via A-TS3-8-H2 has an exceptionally high
energy barrier of 33.4 kcal/mol, hence unlikely to happen. It is also
possible that after ethylene insertion, another ethylene is adsorbed
onto the available site of the metal center to give
A5-H2, from which another insertion occurs via
A-TS5-6-H2 to form two alkyl chain. A β -H
eliminate via A-TS6’-7-H2 would produce the active
center, but a conformation change bringing two propyl group close to
each other is required, which is sterically unfavorable, as is showed in
the high energy barrier (20.7 kcal/mol required to change from
A6-H2 to A6’-H2).
Judging from energy profile present in Figure 6, two pathways are
energetically more favorable among these possible developments:
elimination from the hydrogen molecule adsorbed onto the metal center,
and ethylene insertion followed by conformation change and β -H
elimination, namely the same pathway without hydrogen. Either way, the
imposition of hydrogen reduced the energy barrier to form the active
center (3.6 kcal/mol, 13.3 kcal/mol; or 5.8 kcal/mol, 12.2 kcal/mol;
compared to 5.9 kcal/mol, 14.4 kcal/mol without hydrogen), the only
difference would be whether hydrogen participate into the reaction, or
function as a second ligand. From the calculations we tested, this
reduction of energy barrier is the only compartment to demonstrate an
unquestionable easing effect, hence we propose that the formation of
active center is a key factor regarding catalyst activity, thus coincide
with the assumption raised by Jiang et al. in
2016[6] that hydrogen might play an important role
in the generation of active centers. In this case, hydrogen reduces the
energy barrier to generate active centers on the catalyst, hence more
active centers are available for reaction, leading to the significant
increase of catalyst activity.