Ruthenium-polydopamine-supported carbon catalysts for the oxidative cleavage of unsaturated fatty acids from ecuadorian palm oil

Oxidative cleavage involves the rupture of carbon-carbon double bonds in alkenes for the production of carbonyl compounds of high added value. Most of the works related to this matter are focused on small and aromatic alkenes, but only few of them address unsaturated fatty acids (UFAs). Yet UFAs can be cleaved into dicarboxylic acids, eventually with an odd number of carbon atoms rare in nature and highly demanded for polymer and pharmaceutic applications. This is typically the case of azelaic acid accessible by oxidative cleavage of oleic and linoleic acids, making these reactions particularly interesting. To date, the oxidative cleavage of UFAs is mainly performed by ozonolysis which is high energy demanding process with several risks associated to ozone handling. The other alternative is related to the use of transition metal catalysts in homogenous phase which are quite active in terms of conversion and yield, but they are pollutant and cannot be recovered. On the other hand, current heterogeneous catalysts display poor activity due to the poor contact amid the UFA and the active phase. To solve this issue, we propose new heterogeneous catalysts formed by oxidized carbon black (CBO) and calcined carbon submicrometric particles (CCN) functionalized with polydopamine (PDA). The latter is a polymer with amine and catechol groups which act as tweezers for the complexation of metal cations. These functionalized carbon supports were impregnated with Ru which is a transition metal quite active in the oxidative cleavage reaction. All catalytic tests were carried out with oleic acid in a biphasic solution formed by H2O/MeCN/AcOEt (4/2/1) at room temperature with NaIO4 as oxidizing agent. These catalysts completed the transformation of oleic acid into pelargonic and azelaic acids after 7 h of agitation. Moreover, they remained active even after 5 consecutive catalytic tests demonstrating their stability. Further characterization showed that the solid catalysts released Ru ions in the liquid phase which were oxidized by NaIO4 into RuVIIIO4. The latter reacted with oleic acid via a pericyclic [3+2] reaction yielding a perruthenate ester which in turn was hydrolyzed producing 9,10 dihydroxystearic acid (diol) and H2RuVIO4. The diol is cleaved by NaIO4 into nonanal and 9-oxononanoic acid which are over oxidized into pelargonic and azelaic acids. The addition of N-Methylmorpholine N-oxide (NMO), a greener oxidant, reduced the reaction time to 5 h without sacrificing the yields of the desired oxidative cleavage products. Finally, the catalysts were assessed in the oxidative cleavage of UFAs from a real natural blend such as palm oil. This vegetable oil was submitted to hydrolysis (to break triglycerides) and esterification (to transform all fatty acids into methyl esters) prior catalytic tests. Conversions and yields attained were similar to those obtained with pure oleic acid demonstrating that the rest of the components from palm oil do not interfere with the normal ongoing of the reaction. Therefore, these new catalysts open a novel route for the production of high value-added products with medical applications (i.e. azelaic acid) from a cheap feedstock as palm oil.