Carbon NanoFiber supported Catalysts for the Deoxygenation of Fatty Acids and Derivatives

Abstract

The use of sustainable resources such as biomass is of growing importance to cope with the increasing energy demand. In this respect, vegetable oil/fat feedstocks as alternatives for fuels and chemicals are of interest, due to their structural similarities to fossil fuels, i.e. long hydrocarbon chains. However, these feeds contain too much oxygen to be directly applicable. As a result these feed are too corrosive for direct application as a fuel in the current engines. An interesting upgrading process here is deoxygenation, where the oxygen is removed from the triglyceride based feeds in the form of water (hydrodeoxygenation), CO2 (decarboxylation) or CO (decarbonylation). By selectively producing olefinic hydrocarbons from the triglyceride based feeds highly valuable precursors for bulk chemicals could be obtained, which might be more interesting from an economical point of view than making fuels. Most research has been performed on either group 10 metal catalysts or hydrodesulfurization (HDS) type catalysts. The group 10 metal catalysts showed high selectivities to the decarbonylation and decarboxylation pathways resulting in alkanes and alkenes with a carbon chain which is one carbon atoms shorter than the original chain. HDS catalysts are mainly active in the hydrodeoxygenation of vegetable oil/fat feeds resulting in alkanes/alkenes with the same chain length as in the original feed molecules. Interestingly, it was also shown that the triglycerides, fatty acid esters and fatty acids show similar reaction selectivities under typical deoxygenation reactions in the presence of H2. Under inert conditions, however, high deoxygenation selectivities were obtained for fatty acid and fatty acid esters, where triglycerides and realistic feedstocks mainly yielded cracking and coke formation. This could be conflicting in the use of fatty acids and esters as model compounds for the deoxygenation of vegetable oil/fat feeds, which mainly consists of triglycerides. It is known that the polarity of a catalyst support can significantly influence its catalytic properties. As such a new procedure was developed in order to vary the polarity of metal-loaded Carbon Nanofiber (CNF) support by introducing oxygen groups with a HNO3 gas phase oxidation. This treatment proved highly effective in increasing the polarity of the carbon support and thereby the catalytic activity of these Pd/CNF catalysts for the deoxygenation of fatty acids. Also, non-noble metal catalysts were shown to be active for the deoxygenation of fats and oils. These are tungsten based catalysts, i.e. tungsten oxide and tungsten carbide, supported on CNF. The selectivity of these tungsten based catalysts is related to the dominant tungsten phase. W2C is selective towards hydrodeoxygenation whereas WO3 shows high selectivities to decarbonylation/decarboxylation. Additionally, molybdenum carbide also obtained high activities for the hydrodeoxygenation of fatty acids. The difference in hydrogenation activity between molybdenum and tungsten carbide results in higher activities in the deoxygenation of unsaturated fatty acids for the former and higher selectivities to unsaturated products for the latter. The versatility of these group 6 metal carbides is further demonstrated by the activity of these catalysts for the hydrodeoxygenation of guaiacol, a lignin derived monomer, with high selectivities to phenolics.