Cocatalyst Influence in Selective Oligomerization: Effect on Activity, Catalyst Stability, and 1-Hexene/1-Octene Selectivity in the Ethylene Trimerization and Tetramerization Reaction

The trimerization and tetramerization of ethylene to 1-hexene and 1-octene with a Cr/PNP/AlEt3 catalyst system, in combination with a variety of cocatalysts, has been investigated. The cocatalysts B(C6F5)3 (1), Al(OC6F5)3 (2), [(Et2O)2H][Al(OC6F5)4] (3), [Ph3C][Ta(OC6F5)6] (4), (Et2O)Al{OCH(C6F5)2}3 (5), (Et2O)Al{OC(CF3)3}3 (6), [Ph3C][Al{OC(CF3)3}4] (7), [Ph3C][AlF{OC(CF3)3}3] (8), [Ph3C][{(F3C)3CO}3Al−F−Al{OC(CF3)3}3] (9), and [Ph3C][CB11H6Br6] (10) have been evaluated. The relative selectivity to 1-hexene and 1-octene obtained shows a strong dependence on the nature of the cocatalyst, and a range of selectivities from 8 (90% C6) to 72% C8 have been observed. The stability of several cocatalysts toward AlEt3 has been studied, and the poor performance of 1 and 2 is linked to degradation of the cocatalyst through ethyl group exchange with AlEt3. In contrast, the [Al{OC(CF3)3}4]- anion in 7 is much more stable and gives rise to a highly active and longer lived catalyst. The overall productivity and selectivity of the catalyst is dependent upon both cocatalyst stability and the nature of the anion present, and a reason for this effect has been suggested. Selectivity control by the cocatalyst has been ascribed to interaction of the anion with the active Cr center.