Oligomerization of supercritical ethylene over nickel-based silica-alumina catalysts

Abstract We used supercritical ethylene as both a solvent and reactant for ethylene oligomerization over silica-alumina catalysts with different pore structures and decorated with Ni nanoparticles namely Ni-Hβ and Ni-Al-SBA-15. The catalysts were characterized by electron microscopy, X-ray diffraction, nitrogen physisorption, and Fourier transformed infrared spectroscopy. The effect of pressure and temperature on the overall conversion and product selectivity over Ni-Hβ were evaluated in the range from 0.0 to 52.5 bar (partial pressure of ethylene) and 30 to 120 °C. At subcritical conditions over Ni-Hβ (120°C and 11.2/22.4 bar), the ethylene conversion reached a plateau of around 50 % after six hours. By increasing the pressure past the critical point of ethylene, the steady-state conversion significantly increased to 71 %. The drastic increase in conversion can be attributed to the high butene and hexene solubilities in supercritical ethylene, promoting the desorption of di- and trimerization products from the catalytic active sites and making those sites available for incoming ethylene molecules. At supercritical conditions, high ethylene conversion (85 %) and C4 selectivity (74 %) was achieved using Ni-Al-SBA-15 (impregnated mesoporous catalyst), while Ni-Hβ (the microporous catalyst) yielded less C4 (32 wt.%) and more C6 (22 wt.%) possibly because of confinement effects of the micropore channels. We also examined the influence of the reaction temperature over Ni-Hβ. The catalytic activity increased with temperature from 30 to 120 °C. Even though there was little activity at 30 °C for ethylene oligomerization, the solvation abilities at this condition provide opportunities for in-situ removal of heavy molecular weight species from the catalyst for regeneration purposes, provided that the catalyst does not strongly adsorb ethylene molecules at low temperature. This work shows for the first time that supercritical ethylene conditions have great potential for practical catalytic applications.