Transformation of fatty acids over palladium catalysts is highly selective to pro­duce hydrocarbons with n-1-carbon number, where n is carbon number of fatty acid substrate. The formation of gaseous products, CO, and CO2, indicates that transformation of fatty acids occurs via decarboxylation and decarbonylation [9]. Hydrogen is not needed for the reaction to occur but low amounts are beneficial for catalyst stability. The reaction pathway was proposed for deoxygenation of stearic acid over Pd/C catalyst (Fig. 6.4) [9]. The pathways have been updated from the original work of [14] by adding the intermediate steps of the formation of aldehyde and its hydrogenation to alcohol based on our recent data [28], where it was shown that catalytic pathway in deoxygenation of fatty acids depends on the hydrogen content in gas atmosphere. In hydrogen-free conditions the main reaction is decarboxylation whereas in hydrogen-rich conditions decarbonylation dominates. The latter reaction proceeds through an aldehyde intermediate, which is trans­formed at high rate to (n-1) hydrocarbon. The alcohol intermediate can be formed in these conditions via hydrogenation of aldehyde; however, further dehydroxy — lation to the corresponding hydrocarbon does not proceed, while alcohol is

I_ TI I (isomerization) ,1 1————————

С17Н36 |-*—————————- *—— ► saturated Cl 7

+/-H2

(dimenzation) ^

Fig. 6.4 Reaction pathway for deoxygenation of stearic acid over a Pd/C catalyst. Adopted from Ref. [9] decomposed over palladium via an alkyl intermediate to (n-1) hydrocarbon [28]. Despite numerous of reactions that can occur on the palladium catalyst the selectivity toward by-products is very low. The 95 mol% of stearic acid was converted to n-heptadecane and 3 mol% to n-heptadecenes, over 5 wt% Pd/C (Table 6.2).

The desired products of deoxygenation are long-chain hydrocarbons. By-products of the reaction can by created by cracking (not observed over Pd/C catalyst) and due to transformation of the unsaturated products (olefins) or unsaturated fatty acids. Olefins can be transformed to cycloalkanes by cyclization reaction which goes via dehydrogenation forming aromatics (e. g. benzyl unde­cane) [29], while unsaturated fatty acids can form dimers by Diels-Alder reaction [30].

Deoxygenation of fatty acid esters over palladium catalyst occurs via formation of fatty acid followed by decarboxylation/decarbonylation reaction [31, 32]. Aromatic C17 hydrocarbons were also found as by-products in small extents.

In the case of unsaturated fatty acids the reaction pathway is extended by isomerization reaction of the feedstock (Fig. 6.5). In the studies of linoleic and oleic acid deoxygenateon over Pd/C catalyst, it was proven that cis/trans izomerization can occur [33] together with migration of the double bond through aliphatic chain giving high number of fatty acid isomers [29].

In the first work describing the deoxygenation of fatty acids over noble metal catalysts [9] it was shown that decarboxylation is the main reaction giving CO2 as a gas product. Recently it was proven that during reaction, the ratio between

Heavy byproducts (dimers, aromatics)

OMOUirCH]

Hep lade carte

Fig. 6.5 Reaction scheme of oleic acid deoxygenation over a Pd/C catalyst. Taken from Ref. [33]

decarboxylation and decarbonylation can change. Deoxygenation over a fresh Pd/C catalyst starts with high selectivity toward decarboxylation. However, in time the selectivity toward decarboxylation can decrease followed by an increase of decarbonylation reaction [34]. This phenomenon could occur due to accumulation of carbon monoxide in the reaction atmosphere (see Sect. 6.2.2.3).

Hydrogen per se is not needed for decarboxylation/decarbonylation reaction to occur. Its presence can, however, influence reaction pathway. It was shown that increase of hydrogen content in the reaction atmosphere is increasing hydroge­nation rate of carboxylate group. Thereafter, an instant decarbonylation of the created aldehyde species occurs [28].