Towards Improved Process Development for Biolubricant Production from Vegetable Oils
Biolubricant , Renewable Hydrocarbon , Vegetable Oil , Kolbe Electrolysis , Electrochemical Decarboxylation , Hydroisomerization
Biolubricants are lubricants with a base oil produced from biological feedstock. Ester-based biolubricants enjoy the most widespread commercial use, even though complex chemical modifications are required to improve their physicochemical properties. Renewable-hydrocarbon biolubricants can be more attractive because they possess desirable qualities such as superior cold-flow properties and oxidative and hydrolytic stability. In addition, hydrocarbon-based biolubricants are more similar to conventional lubricants and can be easily substituted using existing infrastructure and lubricant formulations. Current research on renewable hydrocarbons is heavily focused on hydrodeoxygenation of vegetable oils involving operating conditions in excess of 50 bar and 450 °C. Alternative routes such as those involving electrochemical decarboxylation through Kolbe electrolysis (KE) benefit from milder conditions and consume less hydrogen overall. A further benefit of Kolbe electrolysis is the potential for integration into renewable energy systems. The biolubricant production process using KE requires a hydrogenation pre-treatment stage and a hydroisomerization (HI) post-treatment stage. Theoretically, this process can be improved by combining both stages into a single pre-treatment HI. Experiments performed using HI of fatty acids have shown difficulty in converting oleic acid to isostearic acid. Key issues include the occurrence of hydrogenation without isomerization and a loss of carboxyl groups during isomerization. A factorial experimental design was used to determine how factors such as temperature and pressure might influence the isomerization process. Additionally, several experiments were performed to test the hypothesis that lactones can be converted via hydrogenation into carboxyl groups. Results indicate that temperature is a highly significant parameter in the isomerization process and that higher temperatures are beneficial for the production of branched compounds. Unfortunately, the production of branched compounds is accompanied by a loss of carboxyl groups. Lactone hydrogenation experiments were unsuccessful in restoring carboxyl functionality. It is possible that a different catalyst than platinum on alumina is required for the hydrogenation of lactones. Another possibility is that lactone formation is a minor contributor to the loss of carboxyls and the main cause for the loss is an irreversible deoxygenation reaction.