Ni/Siral Catalysts for Ethylene Oligomerization: Effects of Si/Al Ratio on Ni Speciation and Catalytic Performance

Ni/Siral catalysts with different Si/Al ratios were prepared by incipient wetness impregnation (IWI) to assess the impact of support composition on Ni2+ speciation and ethylene oligomerization (EO) performance. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (TPR), X-ray diffraction (XRD), NH3 temperature-programmed desorption (TPD), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray (EDX) analysis, and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). The EO catalysts were tested in a fixed-bed reactor at 225 °C under 11 bar ethylene and at 120 °C under 26 bar ethylene. Ni/Siral-70 was the most active catalyst investigated, but Ni/Siral-30 also exhibited good performance. The active sites were inferred to be isolated Ni2+ ions on amorphous SiO2-Al2O3 containing interstitial Al3+ ions that enhance Brønsted acidity; Ni/Siral-70 displayed the highest concentration of these sites based on CO DRIFTS. Formation of NiAl2O4 surface species limited the activity of Ni/Siral-30 and especially Ni/Siral-5. The catalysts were also tested using a simulated ethane oxidative dehydrogenation (ODH) product stream containing 44% ethylene, 44% ethane, 4.5% methane, 2% H2, 4.5% CO2, 0.9% propylene, and 0.1% CO. The simulated ODH mixture gave lower EO conversion than 50/50 ethylene/N2 at 225 °C and 11 bar over Ni/Siral-30, consistent with catalyst poisoning. In contrast, EO conversion over the Ni/Siral-70 catalyst was unaffected under these conditions. Catalyst testing at 120 °C and 26 bar revealed catalyst poisoning by feed impurities for both catalysts. Low-temperature/high-pressure EO activity was not recovered by simple thermal regeneration of Ni/Siral-30 at 300 °C.