Investigation of Hydrodeoxygenation of Oils and Fats

The use of renewable biofuels in the transport sector represents an important step towards a sustainable society. Biodiesel is currently produced by the transesterification of fats and oils with methanol, but another viable method could be reaction of the feedstock with H2 to produce long-chain alkanes.

This would allow direct integration of biofuel production in existing refineries and allow use of feedstock with high amounts of free fatty acids (abattoir wastes, used fats, greases, etc.) [1], or even tall oils from the Kraft process [2]. The reaction network from oils and fats in H2 atmosphere includes the direct hydrogenation of the ester or carboxylic acid functionalities with H2 to result in H2O and alkanes (conservation of fatty acid chain length); decarbonylation or decarboxylation of the carboxylic functionality (split-off of CO or CO2, respectively, making the fatty acid chain one C-atom shorter) also yielding alkanes.

In all routes propane is formed as a by-product from the glycerol in the fats. The hydrotreatment of a model fat mixture is studied in a stainless steel autoclave between 250 and 375°C, at moderate hydrogen pressures and over catalysts of 5 wt% Pt, Pd or Ni supported on γ-Al2O3. Hydrolysis or hydrogenation of glycerides to free fatty acids is observed to be part of the alkane formation from esters.

It was seen that Pt and Pd almost exclusively facilitate the decarbonylation or decarboxylation routes. Ni also facilitated the hydrogenation route, but with a considerably lower overall activity. This may be due to differences in metal particle sizes. While decarboxylation and decarbonylation use least hydrogen during alkane formation itself, the CO and CO2 formed do react with hydrogen to form methane.

By GC-analysis of the gas phase after reaction showed that with all tested catalysts, almost all of the CO and CO2 formed yielded CH4. Further investigations of these phenomena are in progress. ------------- [1] B. Donnis, R. G. Egeberg, P. Blom, K. G. Knudsen, Topics in Catalysis, 2009, 52, 229-240. [2] G.

W. Huber, P. O’Connor, A. Corma, Applied catalysis A: General, 2007, 329, 120-129.