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)(2)H][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)(3)CO}(3)Al-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 < 5% C-8 (90% C-6) to 72% C-8 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.