The Nature of Active Ni Sites and the Role of Al Species in the Oligomerization of Ethylene on Mesoporous Ni-Al-MCM-41 Catalysts

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.