Heterogeneous oligomerization of ethylene to liquids on bifunctional Ni-based catalysts: The influence of support properties on nickel speciation and catalytic performance

In this work we present a comparative study of the catalytic performance for the heterogeneous oligomerization of ethylene to liquids of bifunctional catalysts comprising Ni (ca. 5 wt%) impregnated on three aluminosilicate supports exhibiting distinct acidic, textural, and structural properties. Specifically, a nanocrystalline Beta zeolite, a mesostructured Al-MCM-41 sample, and a commercial silica-doped alumina (Siralox-30) were employed as supports. The materials were characterized by ICP-OES, XRD, N-2-physisorption, FTIR-pyridine, Al-27 MAS NMR spectroscopy, electron microscopy (HAADF-STEM, TEM), and low-temperature FTIR-CO. The characterization results revealed significant differences in the nickel speciation depending on the support identity. Thus, while isolated Ni2+ cations in ion exchange positions were the prevailing species on the zeolitic Ni/Beta catalyst, Ni2+ interacting with weakly acidic surface hydroxyl groups (i.e., silanols and aluminols) and under-coordinated Ni2+ on the surface of sub-5 nm NiO nanoparticles predominated on Ni/Al-MCM-41 and Ni/Siralox-30 catalysts. In the absence of appreciable deactivation the latter two catalysts displayed significantly higher catalytic activity (related to the Ni sites), thus questioning the general believe that ion exchanged Ni2+ cations are the unique active Ni species in Ni-based catalysts. On the other hand, the higher density and strength of the Bronsted acid sites in Ni/Beta determined a higher selectivity to the targeted liquid (C5+) oligomers and a higher degree of branching (i.e., higher octane-number) of the gasoline-range product in comparison to the amorphous catalysts. The C5+ productivity, however, was maximum for the most active Ni/Al-MCM-41 catalyst, reaching values as high as 12 mmol/(Kg(cat) s). (C) 2015 Elsevier B.V. All rights reserved.