Catalytic HDO over sulfided Ni-Mo/y-Al2O3 and Co-Mo/y-Al2O3 catalysts was proven to be a complex reaction [14], in which not only active metal is responsible for conversion of fatty acids and their derivatives, but also alumina support and sulfur compound exhibit activity as well.

The reaction pathway and mechanism for transformation of fatty acids esters over sulfided catalysts was proposed by [14]. Figure 6.2a shows a complete

pathway of reactions that can take place over sulfided catalyst. The fatty acid esters can be transformed to fatty acids by hydrolysis and thereafter converted directly to hydrocarbons containing one less carbon number by decarboxylation/decarbony — lation or transform indirectly via aldehyde as an intermediate. Hydrocarbons with the same carbon number can be produced by reduction of fatty acids via aldehyde and alcohol. Separately, esterification of the fatty acids and their alcohols can occur, forming esters with the same number of carbons in the chain on both sides of the ester group.

Apart from elucidating deoxygenation pathways of fatty acid esters over Ni-Mo and Co-Mo catalysts, influence of sulfur on the reaction mechanism was also shown [14]. Dehydration reaction can be catalyzed by acid. Therefore, it was proposed that, in conversion of alcohols to alkenes, elimination reactions (E1, E2) can occur (Fig. 6.2b). Although elimination reaction E1 is unlikely to occur, elimination reaction E2 is the plausible mechanism which produces alkenes. The supports-catalyzed acid reactions were, however, excluded because in the experi­ment using only y-Al2O3 no hydrocarbons were formed using fatty acids esters as a feedstock at 300°C and under 1.5 MPa hydrogen pressure. Fatty acids and alcohols originating from the transformation of esters were the only products found [11].

The elimination reaction does not explain the presence of sulfur compounds in the reaction mixture. Hence mechanism of substitution was proposed (Fig. 6.2c). Although substitution of SN1 is unlikely to occur due to the unstable carbenium ion as an intermediate, SN2 substitution occurs which is confirmed by the formation of thiol compounds [14].

The role of sulfur in the deoxygenation mechanism over sulfided Ni-Mo and Co-Mo catalysts is still unclear. Although elimination and substitution reactions can explain the formation of sulfur compounds and n-carbon alkenes, by dehy­dration of alcohols, the role of reactions on overall conversion has not yet been clarified. However, effect of the addition of the sulfur can be stated [12]. With increasing content of H2S in the reaction, the following was observed:

• increase of the conversion of aliphatic esters to hydrocarbons

• increase of selectivity toward decarboxylation/decarbonylation products

• increase of unsaturation of hydrocarbon products

To avoid sulfidation of the catalyst and addition of the sulfur to the feedstock, metal nitrides catalysts supported on alumina were tested for the deoxygenation of oleic acid and canola oil [15]. Although oxygen removal from canola oil in a continuous reactor, at 400°C and 8.35 MPa hydrogen pressure, reached 90% level, the yield of hydrocarbons which can be used as diesel fuel did not exceed 50/100 g of oleic acid.

When investigated separately Ni, Mo and Ni-Mo/y-Al2O3 catalysts, it was found that decarboxylation/decarbonylation reaction occur at the nickel surface, where reduction of the carboxylic group occurs on the Mo sites [16]. Therefore, Ni-Mo alloy is selective toward decarboxylation/decarbonylation depending on the proportions between the active metals. When the proportion of Ni to total amount of Ni-Mo on y-Al2O3 was varied between 0.2 and 0.4, the selectivity to

hydrogenation of carboxylic group was in the range of 60-80% (at 100% con­version, temperature between 260 and 2800C and under 3.5 MPa of hydrogen pressure). Although Ni and Mo on c-Al2O3 show lower conversion of triglycerides than Ni-Mo catalysts, their selectivity within the hydrocarbon products was close to 100% for Ni to decarboxylation/decarbonylation reaction and for Mo to hydrogenation of carboxylic group.

HDO of rapeseed oil has been recently studied over sulfided Ni-Mo catalyst using mesoporous alumina as a support. When compared to y-Al2O3, the sulfided Ni-Mo catalyst with mesoporous alumina structure showed higher activity in temperature range of 260-280°C. The catalyst with mesoporous support outper­formed the catalyst with micropores alumina by 50% yield at 260°C [17]. These results can be explained by good accessibility of long-chain fatty acids to the active sites of Ni-Mo catalyst.

Influence of hydrogen pressure and volume ratio between hydrogen and sun­flower oil was investigated over a non-sulfifed Co-Mo/Al2O3 catalyst in the temperature range between 320 and 380°C [18]. The results from the experiments indicate that with an increase of temperature, an increase in conversion and increase of ratio, between decarboxylation/decarbonylation and hydrogenation of carboxylic group, occur. On the other hand, an increase of hydrogen pressure favors hydrogenation reactions of triglycerides as well as slightly increases the conversion level. The results of experiments with different volume ratio between hydrogen and sunflower oil indicate that the highest conversion of sunflower oil was obtained at the ratio between 400 and 600 Nm3/m3.