Team at Univ. of North Dakota proposes alternative mechanism for the formation of cyclic hydrocarbons by thermal cracking of crop oils
Thermal cracking is a well-known pathway for transforming crop oils (i.e., triacylglyceride oils, TGs) into chemicals that can be used as replacements for petroleum transportation fuels. A unique feature of cracking compared to alternative pathways such as hydrotreating or transesterification is the formation of cyclic hydrocarbons, including aromatics; the complex mixtures from the thermal cracking of TG oils contains nearly 20% cyclic hydrocarbons. Cyclic hydrocarbons are important for fuel components, chemicals, and polymer precursors.
A team at the University of North Dakota has investigated the occurrence patterns of cyclic products obtained via the thermal cracking of several TG feedstocks, such as canola and soybean oils, to probe possible formation mechanisms.
In a study published in the ACS journal Energy & Fuels, they report that the data from their work was inconsistent with previously proposed mechanisms involving the intermolecular Diels–Alder reaction as a single pathway—the common paradigm for formation.
They proposed and supported with experimental evidence an alternate mechanism based on the intramolecular cyclization of alkenyl and alkadienyl radicals formed as a result of TG cracking, which they formulated in final form as follows:
Alkenyl radicals and alkenes are formed first, as a result of FA [fatty acid] decarboxylation and cracking. Then, as long as the products do not undergo significant aromatization and hydrogenation (which are coupled via hydrogen formation), high-temperature cyclization processes progress in time to yield, first, monocyclic and, then, polycyclic hydrocarbons (along with polymerization, leading to coke and tar). Cycloalkenes are key intermediates of this process; the ensuing aromatization of six-membered cyclic hydrocarbons and hydrogenation of alkenes (including both five- and six- membered cycloalkenes) stabilizes the final product mixture.
...In general, the presence of 5−20 wt % cyclic hydrocarbons, including aromatics, in transportation fuels improves operational performance, although this varies by engine type and the specific compression ratio and ignition characteristics of the engine. When cyclic compound concentrations become too high, fuel burning may not be complete. As a result, PAH generation may be observed. In this respect, the findings of this study, that five-membered cyclic hydrocarbons, which are abundant in the OLP [organic liquid product] of crop oil thermal cracking, are not as prone to forming PAHs as their six-membered analogues, may￼be of interest. However, only future engine testing can lead to verification that cycle size significantly affects the operational and environmental performance of a fuel.—Kubátová et al.
Alena Kubátová, Jana Št’ávová, Wayne S. Seames, Yan Luo, S. Mojtaba Sadrameli, Michael J. Linnen, Ganna V. Baglayeva, Irina P. Smoliakova, and Evguenii I. Kozliak (2011) Triacylglyceride Thermal Cracking: Pathways to Cyclic Hydrocarbons. Energy & Fuels doi: 10.1021/ef200953d