Ethylene oligomerization in zeolite-grafted Cr(III)-diphosphinoamine catalysts using triisobutylaluminium as cocatalyst: Change from dimerization to trimerization due to confinement effect

Zeolite-supported Cr(Ill)-diphosphinoamine (Cr(III)-PNP) catalysts were prepared through grafting PNP on HY and NaY zeolites followed by complexing with CrCl3(THF)(3) for ethylene oligomerization. The structure of supported Cr(III)-PNP catalysts was characterized by scanning electron microscopy, X-ray diffraction, nitrogen adsorption and desorption, thermogravimetric analyses and Fourier transform infrared, and the influence of the supported pattern on reactivity for ethylene oligomerization were investigated. The results revealed that the complex of Cr(III)-PNP was grafted on silicon hydroxyls in the pore channel of HY zeolite to decrease pore size but to maintain pore structure. Comparing with homogeneous Cr(III)-PNP producing 1-butene as main product, HY-supported catalyst had higher activity and selectivity toward 1-hexene increased from 4.07% to 73.24% using triisobutylaluminium as cocatalyst. The increase is attributed to confinement effect of the pore channel, which increases the stability of the chromacycloheptane intermediate to 1-hexene. The confinement effect for ethylene oligomerization was revealed in experiment.