Thermal deoxygenation and pyrolysis of oleic acid

• Thermal non-catalytic pyrolysis of oleic acid was studied.
• Deoxygenation proceeded through both decarbonylation and decarboxylation.
• Deoxygenation mechanism was temperature dependent.
• Higher temperature favours decarboxylation over decarbonylation and vice versa.
• The presence of double bond in oleic acid promotes cracking at the beta position.

The primary objective of this work was to study the pyrolytic conversion of mono unsaturated fatty acids to hydrocarbon products for use as renewable chemicals and fuels. Oleic acid (cis-9-octadecenoic acid) was selected as a model mono unsaturated compound. Batch pyrolysis reactions were conducted over a combination of temperatures from 350 to 450 °C and times ranging from 0.5 to 8 h. Gas chromatography and mass spectroscopy were used to analyse and identify products in the gas and liquid product fractions. Analysis of the gas phase showed concurrent production of CO and CO2, indicating that deoxygenation reaction proceeded through both decarbonylation and decarboxylation mechanisms. The gas product contained also alkanes and alkenes with carbon numbers ranging from C1 to C5. Analysis of the liquid fraction revealed series of n-alkanes, alkenes and fatty acids, including stearic acid. The presence of the unsaturation resulted in cracking at the allylic CC and predominance of C6 to C10 hydrocarbons and C9 and C10 fatty acids. This work uncovers the dominant reaction pathways in the pyrolysis of mono unsaturated free fatty acids and demonstrates the viability of this non-catalysed conversion technology to produce renewable hydrocarbons compatible with the existing petrochemical infrastructure.