In the pioneering work describing this method, variety of catalysts was tested for deoxygenation of stearic acid at 300OC and 0.6 MPa of helium [9]. Catalyst screening was performed for Pd, Pt, Mo, Ni, Ru, Rh, Ir, and Os metals, bimetallic Pd-Pt catalyst as well as Raney nickel and oxides Ni-Mo/Al2O3. It was shown that Pd and Pt active metals on carbon as a support have the highest activity and selectivity to hydrocarbons (Table 6.2).

Reaction conditions: mstearic acid = 4.5 g, mdodecane = 86 g, mcatalyst = 1 g, T = 300o C, p 0.6 MPa, Vcarrier gas = 25 mL/min (He). Adopted from Ref. [14]. The metal loading is in wt%. a Conversion of stearic acid and selectivities towards products after 6 h of reaction b Selectivity to C35 symmetrical ketones

c Crack denotes cracking products consisted of shorter fatty acids, C10-C17 acids and shorter hydrocarbons, C13-C16 hydrocarbons

d Heavy denotes dimeric products formed via unsaturated acids and olefins e Other denotes unidentified products

Furthermore, different catalyst supports were used such as Al2O3, SiO2, Cr2O3, active carbon, and zeolites, from which active carbon was the most suitable for decarboxylation/decarbonylation of fatty acids. The zeolite-supported metal cata­lysts show high activity, but low selectivity to long-chain hydrocarbons, which was caused by cracking of the feedstock. Therefore, high acidity of the support is not suitable for obtaining fuel with high cetane number. On the other hand, basicity of the supports is not desired as well due to very low selectivity to hydrocarbons. The Ru/MgO catalyst with a basic support used for deoxygenation of stearic acid, converted 99% of fatty acids into ketones containing 35 carbons (Table 6.2).

The bimetallic metal oxides Ni-Mo/Al2O3 show low activity and selectivity towards hydrocarbons at the temperature of 300°C and 0.6 MPa helium pressure. This result is in agreement with the other works (Table 6.1) which show low activity of Ni-Mo oxide catalyst at temperatures around 300°C.

Since fatty acids, their esters, and triglycerides are relatively large molecules it could be beneficial to use catalysts support with its mesoporous structure. Sibunit carbon was used as a support for deoxygenation of dodecanoic acid [8]. The advantage of Sibunit over activated carbon is mesoporous structure (pores larger than 2 nm) and higher thermal and mechanical strength which are more suitable for industrial application of the catalyst. Despite change of the properties of Sibunit compared to active carbon the activity and selectivity of palladium sup­ported on Sibunit are still the same.

The influence of mesoporous support structure was shown in deoxygenation of stearic acid and ethyl stearate over palladium on mesocellular silica foam support [26]. The catalysts proposed for deoxygenation of fatty acids have amorphous cell and window of 37 and 17 nm, respectively. High porosity of the material ensured good accessibility of substrates to the palladium nanoparticles, thus decreasing internal diffusion limitations.

The effect of metal dispersion was studied over 1% Pd/C at temperature of 300°C with pressure 1.75 MPa of hydrogen in argon [27]. The catalysts with metal dispersion between 18 and 72% were used for deoxygenation of stearic and pal­mitic acid. The optimum results for deoxygenation of fatty acids were achieved with the catalysts with palladium dispersion between 47 and 65%. In the case of the catalyst with the lowest palladium dispersion (18%), the metallic surface area is too low to provide sufficient deoxygenation activity. Moreover, extensive cat­alyst deactivation occurs. For the catalyst with the most dispersed palladium on the surface (72%), activity was lower than for catalysts with dispersion of 65 and 47%. This result could be explained by deposition of the smallest palladium particles in the micropores which could be easily blocked by coke deposits.