Catalyst Hydrodeoxygenation

Hydroprocessing is a general term used for the catalytic reactions that use hydrogen to eliminate the heteroatoms such as sulfur, nitrogen, oxygen, and metals, and also to saturate the olefins and aromatics. Typical hydroprocessing reactions include hydrodesulfurization (HDS) where the sulfur is removed by breaking C-S bonds and hydrogen sulfide is formed; hydrodenitrogenation (HDN) which targets the removal of nitrogen as ammonia; hydrodeoxygenation (HDO) which removes oxygen as water; and hydrodemetalization (HDM) to remove metals such as metal sulfides.

Hydrogenation-derived renewable diesel (HDRD) production focuses on the oxygen removal from the bio-oils/fats, which correspond to a HDO reaction, to obtain hydrocarbons in diesel fuel range (Figure 10). HDO reactions of bio- oils/fats operate at moderate temperatures, between 300-600 C, and under high hydrogen pressure in the presence of a heterogeneous catalyst.

However, the reaction conditions such as temperature and pressure should be adjusted depending on the feedstock. The catalysts used for HDO are in fact the same as those that are used for HDS and HDN such as sulfided Co-Mo or Ni-Mo because the hydrogenation processes are very similar in petroleum refineries.

Figure 10. The oxygen removal from the triglycerides (HDO reaction)

It has been shown that it is possible to produce an alternative diesel fuel (green diesel) via hydrogenation of triglycerides, which can be also called HDRD. Because HDRD is characterized by a high cetane number, its preferred use is a diesel fuel additive to improve fuel ignition. Hydroprocessing of vegetable oils such as palm oil will produce hydrocarbons in the diesel boiling range (mainly C15 - C18 paraffins) which can act as a fuel ignition improvers. They carried out the process at a temperature of 350-450 C and 4.8-15.2 MPa by using a commercially available hydroprocessing catalyst such as Co-Mo and Ni-Mo

Catalyst Decarboxylation

Palladium on carbon, often referred to as Pd/C, is a form of palladium used for catalysis. It is usually used for catalytic hydrogenations in organic chemistry. When the metal is distributed over finely-divided carbon catalyst support, the surface area is larger and the catalyst is more reactive. A later study demonstrate ed that for green diesel production via decarboxylation of stearic acid over 4 wt. % Pd catalyst supported on sibunit (a new class of mesoporous carbon-carbon composite materials combining advantages of chemical stability and electric conductivity of graphite and high specific surface area and adsorpt ion capacity of active coals) is possible.39 This process was carried out in a semi-batch reactor with 300 mL volume at 17 bar helium and 300 oC, using dodecane as a solvent . Under these conditions, the catalytic decarboxylation of stearic acid resulted in n-pentadecane formation as well as n-heptadecane as the main products. Thus, it is concluded that the product distribution in catalytic decarboxylation of stearic acid changes depending on the type of the support and the nature of the surface groups in carbon material.39Some of the tested decarboxylation reactions for conversions of fatty acids are shown in TableTable 5. Literature summary of decarboxylation reactions of fatty acids.