The development of sustainable active and stable heterogeneous catalysts for the oligomerization of ethylene to replace the unfriendly homogenous systems based on transition metal complexes currently applied in the industry still remains a challenge. In this work we show that bifunctional catalysts comprised of Ni loaded on nanocrystalline zeolite H-Beta can efficiently catalyze the oligomerization of ethylene with high stability and selectivity to liquid oligomers under mild reaction conditions. Ni-Beta catalysts were prepared starting from a commercial nanocrystalline H-Beta sample with Si/Al ratio of 12 via both ionic exchange (1.0-2.5 wt% Ni) and incipient wetness impregnation (1.1-10.0 wt% Ni) using aqueous Ni(NO 3 ) 2 solutions, followed by air-calcination at 550ºC. The Ni-Beta catalysts exhibited no signs of deactivation under the studied conditions (T= 120ºC, P tot = 3.5 MPa, P C2H4 = 2.6 MPa, WHSV= 2.1 h Additionally, most active Ni-Beta catalysts displayed a non-Schulz-Flory product distribution with high selectivity to liquid oligomers ( 60 wt%) and high degree of branching due to the contribution of the hetero-oligomerization pathway involving zeolite Brønsted acid sites.

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. 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 2+ cations grafted on acidic silanols are the most likely active species in the working catalysts rather than the generally accepted ion-exchanged nickel cations. Based on our results, a plausible initiation mechanism involving a nickel-vinyl-hydride intermediate from which chain propagation proceeds similar to the Cossee-Arlman pathway is proposed.

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