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.

No abstract

No abstract

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 Ni 2+ cations are irreversibly blocked during the initial reaction stages leaving unsaturated Ni 2+ 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 Ni 2+ sites, probably through a close interaction between Al species and nearby Ni 2+ sites, as inferred from quantitative 27 Al MAS NMR and FTIR spectroscopies.

A dentigerous cyst is an epithelial-lined developmental odontogenic cavity that encloses the crown of an unerupted tooth at the cemento-enamel junction (CEJ). Dentigerous cysts are the second most common odontogenic cysts after radicular cysts, accounting for approximately 24% of all true cysts in the jaws. 1-3 They are usually solitary in occurrence. The condition is frequently seen with the permanent dentition, usually associated with impacted mandibular third molars and maxillary canines. Pain, swelling, and facial asymmetry are occasionally seen; however, they are usually asymptomatic and observed during radiographic examination. 4 This report describes a rare case of a dentigerous cyst associated with an impacted mandibular left 3rd molar with an ectopic position of the inferior alveolar canal (IAC).