Liquid hydrocarbon production via ethylene oligomerization over Ni-Hβ

Abstract We carried out oligomerization of ethylene using nickel supported on an acidic zeolite β (Ni-Hβ) catalyst in a laboratory-scale packed bed reactor for the synthesis of liquid hydrocarbons in milliliter quantities. We evaluated the effect of several process variables (temperature, pressure, weight hourly space velocity (WHSV), and nickel loading) on the ethylene conversion, liquid hydrocarbon/coke yield, and oligomeric product selectivity. Increases in pressure resulted in higher ethylene conversion, leading to a liquid yield of 13.0 wt%, with 10.0 wt% coke at 65 bar. As the pressure increased to 65 bar, the selectivity towards octenes reached 10%, along with a decrease in C4 to 34%, which suggested that higher pressures promoted butene dimerization. Under the conditions studied, a minimum temperature of 120 °C was required to produce liquid hydrocarbons. The liquid yield increased with temperature, with a maximum of 17.0 wt% at 190 °C. Higher temperatures led to the formation of odd-numbered oligomers primarily due to acid-catalyzed cracking reactions. In the range of WHSV tested, a moderate 2.0 h −1 resulted in a local maximum of 10.6 wt% of liquid hydrocarbon yield. A moderate nickel loading of 3.4 wt% also resulted in the highest liquid yield (10.6 wt%) out of the three loadings tested. The results strongly suggest that the ionic form of nickel is much more active catalytically than the NiO form. The variation in nickel loading revealed the importance of having a synergistic balance of nickel and acid sites on the catalyst to maximize ethylene conversion and maintain high liquid hydrocarbon yield. The results from this work formed the basis for pilot scale runs carried out at the Southwest Research Institute (SWRI) for the Advanced Hardwood Biofuels (AHB) project. The pilot runs highlighted the need for efficient removal of ethanol from the ethylene feed after the dehydration process.