The nature of active Ni sites and the role of Al species in the oligomerization of ethylene on mesoporous Ni-Al-MCM-41 catalysts

Abstract

[EN] Although nickel dispersed in mesoporous aluminosilicates are efficient catalysts for the oligomerization of ethylene, the nature of the active nickel sites still remains controversial. Here we applied in situ FTIR-CO spectroscopy during reaction with ethylene combined with reaction kinetics with online MS analysis of reaction products to unravel the nature of the active nickel species in working mesoporous Ni-Al-MCM-41 catalysts. The results revealed that isolated ion-exchanged Ni2+ cations are irreversibly blocked during the initial reaction stages leaving unsaturated Ni2+ cations grafted on silanols and at the surface of small (confined) NiO nanoparticles as the active species in the pseudo-steady state. The low activity of these species in pure silica materials suggested a promotional role of aluminum in the Al-MCM-41 matrix on enhancing the activity of Ni2+ sites, probably through a close interaction between Al species and nearby Ni2+ sites, as inferred from quantitative Al-27 MAS NMR and FTIR spectroscopies