Theoretical Study of Ethylene Oligomerization by an Organometallic Nickel Catalyst

Abstract

The mechanism for ethylene oligomerization by (acac)NiH has been studied using density functional theory (DFT). The transition states for chain propagation and chain termination were optimized and the related reaction barriers calculated. Several possible mechanisms were considered for the chain termination step. Chain termination by beta-hydrogen elimination was found to be energetically unfavorable, and is not likely to be important. Instead, beta-hydrogen transfer to the incoming ethylene unit seems to be operative. The most favorable beta-hydrogen transfer pathway has two transition states. The first leads from a weak pi-complex between an incoming ethylene unit and (acac)NiCH(2)CH(2)R to an intermediate in which the two olefins C(2)H(4) and H(2)CCHR both are strongly pi-complexed to the nickel hydride (acac)NiH. The second barrier takes the intermediate to another weak pi-complex between (acac)NiCH(2)CH(3) and H(2)C=CHR from which the oligomer H(2)C=CHR can be released and the catalyst (acac)NiCH(2)CH(3) regenerated. Due to the mechanism of chain termination, the actual catalyst is proposed to be (acac)NiCH(2)CH(3) whereas (acac)NiH serves as a precursor or precatalyst.

Ethene oligomerization on Ni-SiO2-Al2O3: Experimental investigation and Single-Event MicroKinetic modeling

“…However, based on DFT calculations, Fan et al [45] concluded that Ni-H regeneration through β-hydride elimination is energetically unfavorable.…”

“…An early transition state was assumed making the transition state to resemble the corresponding reactant [45].…”

“…Upon insertion of this nickelethyl-ethene species (D), the first oligomerization, c.q., dimerization has been established. As indicated above, the formed β-agostic alkyl, c.q., butyl, complex does not undergo β-hydride elimination, which would regenerate the Ni-H site, since it is energetically unfavorable[45]. Instead, another ethene molecule has to be coordinated by the complex, leading to a similar nickel-alkylethene species as before in the catalytic cycle (E).…”

Mechanism of Ethylene Migratory Insertion and Dimerization Catalyzed by δ -Alkyl Platinum Complexes-A DFT Study

“…From IMI-2 to IMF-1, although we could not locate the transition state for this process despite detailed investigations of the potential energy surface of Pt-H β bond broken down, the barrier can be estimated to be over 75.31 kJ/mol according to the β-agostic interaction in IMI-2. 34 By comparison, the barrier of IMI-1→IMI-2 is higher in energy than that of IMH-1→IMH-2(IMF). Therefore, the pathway I is more favored.…”

Oligomerization of Ethylene Catalyzed by Iron and Cobalt in Organoaluminate Dialkylimidazolium Ionic Liquids

“…The growth in the selectivity towards hexenes can be attributed to the co-oligomerization of the primary products, butenes, with ethylene. Increasing the temperature increases both the formation of C4 and the formation of co-oligomerization products [31].…”