Hydrogenation of aromatics followed by ring opening is a process to produce liquid transportation fuels (i.e., diesel, jet fuels). Processes such as these are attractive for feedstocks with large quantities of mono- and polyaromatic species, such as light cycle oil (LCO) or renewable oil. This upgrading approach produces fuels with improved combustion characteristics, particularly cetane number. Previous research has demonstrated that aromatic molecules could be hydrogenated over high-surface-area nickel catalysts, but the cetane number was still below specifications. In this research, we aim to determine the reaction mechanism for the ring opening of alkylnaphthenes. Because there is no commercial process for ring opening (hydrodecyclization) reactions, there has been little catalyst development, especially in the production of paraffins or isoparaffins from cycloparaffins. In our down-flow trickle-bed reactor, we study the ring opening of six alkylnaphthene model compounds over three different catalysts (Ir/SiO2, Ir/Csβ, and Pt/SiO2). Our research has shown that when studying mono-branched alkylnaphthenes, the trends are dependent on the catalytic metal. Iridium catalysts tend to ring open away from the substituent, and that trend is enhanced as the bulkiness of the R group increases. However, the trend is the opposite for platinum, as larger R groups promote endocyclic C-C bond scission adjacent to the substituent. We also calculated the corresponding reaction rates (and turnover frequencies), which showed a decrease for each catalyst as the R group gets bulkier. Additionally, we analyzed the ring opening products of two trimethylcyclohexanes. The trimethylcyclohexane rings tended to open via the metallocyclobutane mechanism for all catalysts, whereas the mono-branched alkylnaphthenes preferentially opened via the dicarbene mechanism (Ir) or the multiplet mechanism (Pt). We propose hypotheses for why these mechanisms occur and why the trimethylcyclohexane product distribution differs from theoretical distribution.

Ring opening of alkylnaphthenes over supported Ir and Pt catalysts

ACS Spring 2022

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