An alternative to transesterification of TGs contained in vegetable oils to obtain biofuels is to transform these renewable sources via different chemical processes in conventional petroleum refineries.

The production of high-quality diesel fuel from vegetable oils can be obtained by hydrocracking of TGs treated with high-molecular weight hydrocarbons in conventional oil refineries, as described by Huber et al.64 In this way, renewable liquid alkanes can be produced by treatment of mixtures of vegetable oils and fractions of heavy oil vacuum (HVO), under hydrogen flows and conventional catalysts (sulphured NiMo/Al2O3) at standard temperature conditions (300- 450°C). The reaction involves the hydrogenolysis of C=C bonds in vegetable oils, which leads to a mixture of lower molecular weight alkanes by three different

routes: decarbonylation, decarboxylation and hydrodeoxygenation (Fig. 7.5). Waxes can be formed. Straight-chain alkanes can be isomerized and cracked. The organic acids formed by hydrotreating could catalyze the isomerization and cracking reactions.

The yield of straight-chain alkanes C15-C18 obtained by hydrotreating of pure vegetable oil is about 71% (for sunflower oil), with a theoretical maximum yield of 75%. These yields can be increased by diluting pure vegetable oils with petroleum feedstocks such as HVO. The straight-chain C15-C18 yield of a 5% sunflower oil-95% HVO mixture has been reported to be 87%, higher than that obtained using pure sunflower oil (75%).64

In conclusion, the hydrotreating of vegetable oils also seems to be a promising alternative to produce biofuels from renewable sources, especially because it has the advantage of using existing petroleum refineries without the need to purchase additional capital equipment.