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

Abstract

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.

Efficient hydrothermal deoxygenation of tall oil fatty acids into n-paraffinic hydrocarbons and alcohols in the presence of aqueous formic acid

“…k HC values estimated for the two catalysts is clear that the rate limiting deoxygenation step showed an Arrhenius type behavior similar to the reports in literature for the various carbonbased catalysts, (Fig. 4b) [18][19][20][21][22][23][24][25]. Using these plots, the apparent activation energy (E a ) of the reaction (deoxygenation step) was estimated to be 134.44 ± 31.36 kJ/mol and 148.92 ± 3.66 for 5% Pd/CSigma and 5% Ru/CSigma, respectively (Table 4), in agreement with the superior deoxygenation activity of the Pd-based catalysts [35].…”

“…Water becomes a highly reactive reaction medium thanks to its reduced dielectric constant and the increase of interphase mass transfer leading to an improved solubility with non-polar reactants [10,[18][19][20][21][22]. Pt, Pd, Ni, and Cu catalysts supported on hydrothermally stable supports like activated carbon and ZrO 2 are reported to be effective catalysts for hydrothermal decarboxylation; activated carbon has also been reported to be a effective catalyst at 380-400°C under supercritical conditions (220 bar) [23][24][25]. As a matter of fact, due to the low hydrogen requirement and favorable dehydrogenation conditions (aqueous phase reforming), hydrothermal decarboxylation has also been demonstrated to be efficient with in situ generated hydrogen (catalytic transfer hydrogenation) for production of diesel-like hydrocarbons from fats and oils [26][27][28].…”

Fatty Acid Deoxygenation in Supercritical Hexane over Catalysts Synthesized Hydrothermally for Biodiesel Production

“…Thus, no molecular hydrogen is required for the process. The following compounds are used as supercritical fluids in deoxygenation of vegetable oils, fats, and free fatty acids: carbon dioxide , water , propane , and n ‐hexane , .…”

Hydrothermal Decarboxylation of Corn Distillers Oil for Fuel‐Grade Hydrocarbons

“…There is no significant difference in the XRD patterns between fresh and spent activated carbon except in the peak intensities. The lower peak intensity of the spent activated carbon may be due to the deactivation of the catalyst during the decarboxylation reaction …”

“…[29] There is no significantd ifference in the XRD patterns between fresh and spent activated carbon except in the peak intensities.T he lower peak intensity of the spent activated carbon may be due to the deactivation of the catalyst during the decarboxylation reaction. [30] Almost all catalysts have at endency for coke deposition on their surfaces upon exposure to high temperatures.D eposited coke significantly reduces the activitya nd prevents reusabilityo ft he catalyst.X RD peaks at 29.848 and 61.928 on the spent activated carbon catalyst could be assigned to varioust ypes of coke that can be formed on the catalyst surface. [31] As these two peaks are not present in the XRD pattern of the spent activatedc arbon,n og raphitic coke deposition occurred during decarboxylation of CDO.…”