Using Subcritical Water for Decarboxylation of Oleic Acid into Fuel-Range Hydrocarbons

Zakir Hossain, Md.; Jhawar, Anil Kumar; Chowdhury, Muhammad B. I.; Xu, William Z.; Wu, Wei; Hiscott, David V.; Charpentier, Paul A.

Using Subcritical Water for Decarboxylation of Oleic Acid into Fuel-Range Hydrocarbons

2017

Current interest in renewable fuel production is focused on high-performance fuels such as jet fuel because of their premium value in the marketplace. Currently, lower-value fuels such as biodiesel can be obtained using a variety of feedstocks, but contain significant amounts of oxygen, hence lowering their fuel value. In this work, we examined a one-pot catalytic hydrothermal process for the decarboxylation with an activated carbon catalyst of oleic acid as a model compound for free fatty acids. Temperature (350–400 °C), water-to-oleic acid ratio (2:1–4:1, v/v), catalyst, catalyst-to-total feed ratio (0.15–0.75), and residence time (1–2 h) were found to be key factors for removing oxygen from oleic acid. The complete removal of the carboxylic group from the upgraded liquid phase was achieved at 400 °C with a water-to-oleic acid ratio of 4:1 (v/v) and a residence time of 2 h as confirmed by FTIR and ¹³C NMR results. The pseudo-first-order reaction rate constant was found to follow Arrhenius behavior with the activation energy determined to be 90.6 ± 3 kJ/mol. GC-FID results showed a high selectivity to heptadecane conversion, whereas the GC-TCD results indicated that decarboxylation was the dominating chemical reaction. High heating values and fuel densities in the range of commercial jet fuels were obtained using this approach without the addition of high-pressure hydrogen or a hydrogen-donor solvent.

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