Effect of Cocatalysts and Solvent on Selective Ethylene Oligomerization

Ethylene oligomerization activities
of chromium catalysts stabilized
by different dipyrrole-based ancillary ligands, [(Ph2C­(C4H4N)2)] (2), [Ph2C­(C4H4N)­(C5H6N)] (3), [(Et)2C­(C4H4N)2] (4), and [(C6H5)­(C5H4N)­C­(C4H4N)­(C5H6N)] (5), have been investigated using different
activation methods, and the results have been compared with the commercial
Chevron–Phillips ethylene trimerization system. Upon activation
with triethylaluminum (TEA), chromium catalysts stabilized by dipyrrole-based
ligands 2–5 showed a lower activity
and selectivity compared to the Chevron–Phillips trimerization
system based on 2,5-dimethylpyrrole (1) as the ancillary
ligand. However, unprecedented increases in both activity and selectivity
have been observed by carrying out the oligomerization in methylcyclohexane
using depleted-methylaluminoxane (DMAO) along with triisobutylaluminum
(TIBA) (1:2 ratio) as cocatalyst system under mild conditions, even
for the Chevron–Phillips system itself. Well-defined chromium
complexes, [(Ph2C­(C4H3N)2)­Cr­(Cl)­(THF)3] (6) and {[Ph2C­(C4H3N)­(C5H6N]­Cr­(THF)­(μ-Cl)}2 (7), have been synthesized and fully characterized.
Upon activating with MAO, catalyst 7 produced a statistical
distribution of oligomers, whereas under identical oligomerization
conditions catalyst 6/MAO was found to be inactive. The
use of MeAlCl2 as cocatalyst to activate 7 resulted in the switching of the catalyst’s behavior from
producing a statistical distribution of LAOs to the selective trimerization
of ethylene to 1-hexene. The addition of dialkylzinc along with MAO
resulted in an unprecedented activity increase.