Search

X-ray absorption spectroscopy (XAS) is often employed in fields such as catalysis to determine whether transition-metal nanoparticles are oxidized. Here we show 2p3/2 XAS and 2p3d resonant X-ray emission spectroscopy (RXES) data of oleate-coated cobalt nanoparticles with average diameters of 4.0, 4.2, 5.0, 8.4, and 15.2 nm. Two particle batches were exposed to air for different periods of time, whereas the others were measured as synthesized. In the colloidal nanoparticles, the cobalt sites can have different chemical environments (metallic/oxidized/ surface-coordinated), and it is shown that most XAS data cannot distinguish whether the nanoparticles are oxidized or surface-coated. In contrast, the high-energy resolution RXES spectra reveal whether more than the first metal layer is oxidized based on the unique energetic separation of spectral features related to the formal metal (X-ray fluorescence) or to a metal oxide (d-d excitations). This is the first demonstration of metal 2p3d RXES as a novel surface science tool.

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.

Over the past decades, impending oil shortages combined with petroleum market instability have prompted a search for a new source of both transportation fuels and bulk chemicals. Renewable bio-based feedstocks such as sugars, grains, and seeds are assumed to be capable of contributing to a significant extent as fuel and bulk chemical sources.[1] However, utilization of these new types of high-oxygen-content feeds as either fuels or bulk chemical precursors is not trivial. Triglyceride or fatty acid feeds have the potential to be used as fuel because of their structural resemblance to diesel-type hydrocarbons, but further processing is required to reduce their high-oxygen content and related acidity.[2] This goal is typically achieved by hydroprocessing, thus resulting in deoxygenation (DO) of the triglyceride/fatty acid feed and yielding either saturated hydrocarbons suitable for diesel fuel, or linear olefins suitable for bulk chemical precursors. Although extensive studies have been done on deoxygenation of biomass-related feeds,[2, 3] catalyst research has so far been limited to supported noble-metal catalysts known for their hydrogenation activity and standard hydrodesulfurization (HDS) catalysts. The use of HDS catalysts can be problematic since catalyst deactivation resulting from sulfur leaching will result in contamination of the product stream.[4] The major drawback of noble-metal catalysts is their limited availability. Herein we demonstrate the use of tungsten-based catalysts for the deoxygenation of triglyceride-based feedstocks. Changes in a (pre)treatment procedure are shown to control catalyst selectivity, thus allowing high yields of unsaturated products even in the presence of hydrogen.

Several commercially available samples consisting of agglomerated small mordenite crystallites with low Si/Al ratios (5.7–10 at/at) have been treated in aqueous NaOH solution. It was found that the porosity can be enhanced when the sodium hydroxide solution is sufficiently concentrated. Treatment in 1 M NaOH for 15 min resulted in inter-crystalline porosity and the mesopore volume was increased from 0.01 to 0.21 cm3 g−1 together with an increased external surface area from 36 to 85 m2 g−1. The micropore volume and crystallinity were preserved after the treatment. Both H-MOR and Na-MOR mordenite agglomerates have been successfully treated: the Na-MOR requires a longer contact time to obtain similar porosity. By carefully choosing the alkaline concentration and contact time, intra-crystalline mesoporosity can be obtained for mordenite with Si/Al ratios as low as 10. Catalytic tests with proton mordenite showed that alkaline treatment leads to more than one order of magnitude of activity gain in the liquid-phase alkylation of benzene with propene to form cumene, while selectivity is preserved. These results demonstrate that alkaline treatment also on high-aluminum content mordenites is an effective tool to enhance accessibility and thereby its catalytic performance.

X-ray absorption spectroscopy (XAS) is often employed in fields such as catalysis to determine whether transition-metal nanoparticles are oxidized. Here we show 2p3/2 XAS and 2p3d resonant X-ray emission spectroscopy (RXES) data of oleate-coated cobalt nanoparticles with average diameters of 4.0, 4.2, 5.0, 8.4, and 15.2 nm. Two particle batches were exposed to air for different periods of time, whereas the others were measured as synthesized. In the colloidal nanoparticles, the cobalt sites can have different chemical environments (metallic/oxidized/ surface-coordinated), and it is shown that most XAS data cannot distinguish whether the nanoparticles are oxidized or surface-coated. In contrast, the high-energy resolution RXES spectra reveal whether more than the first metal layer is oxidized based on the unique energetic separation of spectral features related to the formal metal (X-ray fluorescence) or to a metal oxide (d-d excitations). This is the first demonstration of metal 2p3d RXES as a novel surface science tool.

Vegetable oil-based feeds are regarded as an alternative source for the production of fuels and chemicals. Paraffins and olefins can be produced from these feeds through catalytic deoxygenation. The fundamentals of this process are mostly studied by using model compounds such as fatty acids, fatty acid esters, and specific triglycerides because of their structural similarity to vegetable oils. In this Review we discuss the impact of feedstock, reaction conditions, and nature of the catalyst on the reaction pathways of the deoxygenation of vegetable oils and its derivatives. As such, we conclude on the suitability of model compounds for this reaction. It is shown that the type of catalyst has a significant effect on the deoxygenation pathway, that is, group 10 metal catalysts are active in decarbonylation/decarboxylation whereas metal sulfide catalysts are more selective to hydrodeoxygenation. Deoxygenation studies performed under H2 showed similar pathways for fatty acids, fatty acid esters, triglycerides, and vegetable oils, as mostly deoxygenation occurs indirectly via the formation of fatty acids. Deoxygenation in the absence of H2 results in significant differences in reaction pathways and selectivities depending on the feedstock. Additionally, using unsaturated feedstocks under inert gas results in a high selectivity to undesired reactions such as cracking and the formation of heavies. Therefore, addition of H2 is proposed to be essential for the catalytic deoxygenation of vegetable oil feeds.