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.