Nature of Active Nickel Sites and Initiation Mechanism for Ethylene Oligomerization on Heterogeneous Ni-beta Catalysts

Abstract

Higher olefins produced via ethylene oligomerization are versatile commodity chemicals serving a vast range of industries with large global economic impact. Nickel aluminosilicates are promising candidates to replace the homogeneous catalysts employed in industrial ethylene oligomerization processes. The current poor understanding of the true nature of the active nickel centers and the nickel-mediated oligomerization mechanism in these materials, however, hampers the rational design of improved catalysts. Here we applied in situ time- and temperature-resolved FTIR spectroscopy with simultaneous MS analysis of products to disentangle these fundamental issues using nanocrystalline Ni-beta zeolite as catalyst. We elucidate that isolated Ni cations grafted on acidic silanols are the most likely active species in the working catalysts rather than the generally accepted ion-exchanged nickel cations. On the basis of our results, a plausible initiation mechanism involving a nickel vinyl hydride intermediate from which chain propagation proceeds similarly to the Cossee-Arlman pathway is proposed.