Study of the catalytic reactions of ethylene oligomerization in subcritical and supercritical media over a NiBEA catalyst

Abstract

We report a study of the catalytic reactions of ethylene oligomerization over nickel impregnated in aluminum silicate using subcritical and supercritical media. We found the BET surface area decreases with increasing nickel loading, indicating the deposition of NiO particles in the catalyst surface. We compared the performance of the NiBEA catalyst with the protonated form of the commercial support used during the preparation and showed that, although the protonated form promotes the oligomerization of ethylene, the corresponding oligomers do not desorb from the catalyst surface. Conversely, the introduction of nickel in the catalyst facilitates the desorption of the oligomers. Additionally, we used FTIR and GC-MS/FID to characterize the adsorbed and desorbed oligomers, and develop reaction pathways for the ethylene oligomerization over the NiBEA catalyst. We found that both adsorbed and desorbed oligomers are aliphatic, and the non-desorbed products constitute the coke. Additionally, we found that pressure and temperature both increase the oligomerization and desorption rates of adsorbed oligomers. Under supercritical conditions, the amount of coke formed on the catalyst and the desorbed products molecular weight both increase, relative to subcritical conditions. However, the supercritical conditions also promote the dissolution of the coke from the catalyst surface. We provided visual evidence of (1) the formation of coke in the catalyst surface, and (2) the coke dissolution phenomena under supercritical conditions. Finally, we determined that the coke molecules at supercritical conditions are aliphatic cyclic molecules with a high degree of branching.