Decarboxylation of Fatty Acids with Triruthenium Dodecacarbonyl: Influence of the Compound Structure and Analysis of the Product Mixtures.

Abstract

Recently, the decarboxylation of oleic acid (9(Z)-octadecenoic acid) catalyzed by triruthenium dodecacarbonyl, Ru3(CO)12, to give a mixture of heptadecenes with concomitant formation of other hydrocarbons, heptadecane and C17 alkylbenzenes, was reported. The product mixture, consisting of about 77% heptadecene isomers, 18% heptadecane, and slightly >4% C17 alkylbenzenes, possesses acceptable diesel fuel properties. This reaction is now applied to other fatty acids of varying chain length and degree of saturation as well as double-bond configuration and position. Acids beyond oleic acid included in the present study are lauric (dodecanoic), myristic (tetradecanoic), palmitic (hexadecanoic), stearic (octadecanoic), petroselinic (6(Z)-octadecenoic), elaidic (9(E)-octadecenoic), asclepic (11(Z)-octadecenoic), and linoleic (9(Z),12(Z)-octadecadienoic) acids. Regardless of the chain length and degree of unsaturation, a similar product mixture was obtained in all cases with a mixture of alkenes predominating. Monounsaturated fatty acids, however, afforded the alkane with one carbon less than the parent fatty acid as the most prominent component in the mixture. Alkylbenzenes with one carbon atom less than the parent fatty acid were also present in all product mixtures. The number of isomeric alkenes and alkylbenzenes depends on the number of carbons in the chain of the parent fatty acid. With linoleic acid as the starting material, the amount of alkane was reduced significantly with alkenes and alkylaromatics enhanced compared to the monounsaturated fatty acids. Two alkenes, 9(E)-tetradecene and 1-hexadecene, were also studied as starting materials. A similar product mixture was observed but with comparatively minor amount of alkane formed and alkene isomers dominating at almost 90%. The double-bond position and configuration in the starting material do not influence the pattern of alkene isomers in the product mixture. The results underscore the multifunctionality of the Ru3(CO)12 catalyst, which promotes a reaction sequence including decarboxylation, isomerization, desaturation, hydrogenation, and cyclization (aromatization) to give a mixture of hydrocarbons simulating petrodiesel fuels. A reaction pathway is proposed to explain the existence of these products, in which alkenes are dehydrogenated to alkadienes and then, under cyclization, to the observed alkylaromatics. The liberated hydrogen can then saturate alkenes to the corresponding alkane.