Recently,
the decarboxylation of oleic acid (9(
Z
)-octadecenoic
acid) catalyzed by triruthenium dodecacarbonyl, Ru
3
(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 Ru
3
(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.