(721d) Study of the Catalytic Reactions of Ethylene Oligomerization in Subcritical and Supercritical Media over a Nibea Catalyst

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We report a fundamental study for the production of alpha-olefins via the heterogeneous ethylene oligomerization reaction over NiBEA catalyst. This study proposes the use of ethylene to produce alpha-olefins, which are feedstock in oxidation, halogenation, alkylation, hydration, and oligomerization, for the production of a vast range of chemicals. Additionally, alpha-olefins are potential additives for transportation fuels, and a single hydrogenation step can convert alpha-olefins to high octane number fuels. We propose the conversion of ethylene to alpha-olefins using a heterogeneous catalyst as opposed to the conventional method commercially available, which uses a homogeneous catalyst. Heterogeneous catalysts are more advantageous from an environmental perspective, relative to homogeneous catalysts, because they do not require disposal of non-environmentally friendly solvents. Furthermore, it is easier to separate the catalyst from the effluent in a heterogeneous medium as opposed to homogenous systems. We chose the NiBEA catalyst for this study because this catalyst has been proved to oligomerize ethylene under mild conditions, therefore, reducing the oligomerization energy requirement. In addition to the study of ethylene oligomerization over heterogeneous catalysts, we propose the use of ethylene under supercritical fluid conditions. Catalyst deactivation is one of the main drawbacks associated with heterogeneous catalytic systems. Therefore, we suggest that ethylene, which is the reactant in the heterogeneous oligomerization process, can act as a solvent, therefore, solubilizing the coke from the catalyst surface under supercritical conditions.

In this work, 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 on the catalyst surface. We compared the performance of the NiBEA catalyst with the protonated form of the commercial support and showed that, although the protonated form promotes the chain growth, 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 developed 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 chain growth 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 on 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.