Resumen de Fischer-tropsch synthesis over carbon-supported ru-based catalysts
In order to secure the energy supplies to an increasing population and at the same time limit the damage to Earth, so that future generations may continue to live on a healthy planet, immediate actions are required. The oil is still the main source of obtaining chemicals and fuels but its use is not sustainable from an environmental point of view. The use of fossil fuels is the major contributor to CO2 emissions (this being an important greenhouse gas). Transport sector is the one most dependent on fossil energy. So, in EU-25 member states transportation stands for 30 % of the total final energy consumption and relies to 98 % on oil, mainly due to its high energy density and hitherto vast concentrated reserves. But, the high and volatile price of crude oil, the concern of governments to ensure the supply of fuels and the increasingly strict environmental laws make necessary to search for alternative energy sources. In this way, a possible alternative to the use of petroleum is the GTL process (Gas-To-Liquids). GTL process includes a set of reactions and chemical operations that transform natural gas, whose main component is methane, in different types of liquid fuels and chemicals highly versatile to the industry. A variant of this technology consists in replacing methane by coal (CTL process, Coal-To-Liquids). But, amongst the alternative energy sources, biomass plays a major role. The only natural, renewable carbon resource and with large options to substitute fossil fuels is biomass. The conversion of biomass into transportation fuels is preferentially done via its gasification into syngas followed by the liquid fuel synthesis. This is the BTL process, Biomass-To-Liquids.
In general these processes will result in an important shift from crude oil to natural gas and coal and finally to bio-wastes, as feedstock for the production of fuels and chemicals in the decades to come. Industry projections estimate that by 2020 5% of the production of chemicals could be based on Fischer-Tropsch (FT) technology with methane, coal and biomass instead of crude oil refining operations. In the FT synthesis are obtained a complex mixture of hydrocarbons of linear and branched chain, and also oxygenates (alcohols, aldehydes and esters), although the majority are linear paraffins and α-olefins. The hydrocarbons obtained, with a boiling point in the range of gasoline and diesel are high quality because they do not have heteroatoms (S, N), do not contain polyaromatic structures and the fraction of middle distillate presents a high cetane index.
Active elements in FT synthesis are the elements of groups 8-10 of the periodic table, and of them, only the metals: Fe, Co and Ru have the required FT activity for commercial application. Co and Fe are the most commonly employed however, ruthenium catalysts, despite their higher price, possess some unique features in FT synthesis. Ru catalysts possess higher intrinsic activity and can work under higher partial pressures of water or other oxygenate-containing atmospheres, being particularly important for the conversion of syngas produced from biomass. In addition, Ru catalysts are suitable for fundamental research to gain insights into the catalyst functioning and/or reaction mechanisms.
The main objective of this Doctoral Thesis is to comprehend how parameters such as nature of the support, promoters, particle size, morphology and reaction conditions affect to the performance of Ru‐based catalysts for the production of hydrocarbons from syngas.
In chapter 4 is presented a time-resolved in situ X-ray absorption spectroscopy (XANES) investigation of the local chemical environment and the electronic structure of the active metal (Ru) and the promoter (Cs) present in catalysts supported over high-surface-area graphite. XANES analysis at both Ru K and Cs L1 edges was carried out under temperature programmed reduction conditions. This allowed us to find that Ru reduction is a complex process, occurring in two steps via an intermediate oxidation state, and to show the concomitant Cs partial reduction. It is also demonstrated a close association between Cs and Ru atoms as first neighbors. Ru-Cs particle morphology is reversely changed (from flat 2D to 3D shape with low first- and second-shell coordination numbers) when the H2 atmosphere is switched to CO, staging the strong interaction of CO with the surface of the Ru-Cs nanoparticles supported on graphite, a point confirmed by microcalorimetry of CO adsorption. This in turn results in higher olefin selectivity and more long-chain hydrocarbon production in the Fischer-Tropsch reaction. To our knowledge, this is the first time that this reversible particle reconstruction upon syngas reactant switching has been demonstrated.
In chapter 5, the effect of ruthenium particle size on Fischer-Tropsch synthesis has been studied at 513 K, H2/CO = 2 and 15 bar. The use of a suitable support (graphitic materials) with large surface area and weak interactions with Ru allows to independently study these effects without the influence of parasitic metal-support interactions. Moreover, the effect of promotion with Cs was also evaluated. It is observed that the turnover frequency (TOF) for CO conversion increases significantly with Ru particle size from 1.7 to 7.1 nm or 4.2 to 7.5 nm for non-promoted and promoted catalysts, respectively, and then change slightly up to 12 nm. The selectivity to C5+ hydrocarbons increases gradually with the Ru particle size. The olefin to paraffin ratio for Cs-promoted catalysts in the C2-C4 hydrocarbons range is independent of the Ru particle size, whereas it decreases for the non-promoted catalysts. Altogether, our results demonstrate the structure sensitive nature of FT reaction over Ru catalysts for Ru particles <7 nm, and highlight the great stability of these catalysts and the potential of Cs promotion for the preferential formation of waxes, even at high conversion levels.
In chapter 6 two different ruthenium and cesium precursors were used to prepare catalysts supported on a high surface area graphite material for application in the Fischer-Tropsch process. In this work we observe significant modifications in the selectivity values for FT reaction depending on the Cs promoter precursor (CsCl vs CsNO3). Specifically the bimetallic catalyst (4Ru-4Cs), prepared from nitrogen containing metal and promoter precursors, showed a high selectivity to CO2 provably result of the water gas shift reaction (WGS).
XANES analysis at Cs L1 edge was recorded during the temperature-programmed reduction (TPR), observing a partial reduction of the CsCl species for the 4Ru-Cl-4Cs-Cl catalyst. By contrast, for the 4Ru-N-4Cs-N sample similar Cs spectrum with equal absorption edge energy values were observed along all the H2 TPR which is attributed to the formation of hardly reducible CsOH and Cs2O species over the catalyst surfaces as consequence of the CsNO3 precursor decomposition. These species are able to adsorb water molecules (co-product of the FT reaction), and then these water intermediates can interact with the CO molecules adsorbed on the Ru nanoparticles improving the WGS reaction and giving place to undesired production of CO2.
Finally, in chapter 7 molybdenum promotion effect on ruthenium based FT catalysts is investigated. The effects of Mo loading (0-5 wt%) on Ru (2 wt%) were studied at 523 K, H2/CO = 2 and 3.5 bar. Mean diameters of Ru were all close to 3 nm independently of the molybdenum loading used. The presence of molybdenum in a bimetallic ruthenium-molybdenum catalyst improved the catalytic behavior of ruthenium in terms of catalytic activity (more than four times), long-chain hydrocarbon selectivity and the olefin-to-paraffin ratio. Microcalorimetric characterization during CO adsorption at 331 K revealed a clear interaction between Ru and Mo observing an important increase of CO adsorption heats when Mo/Ru ratio was ≤ 0.26. This fact was explained because CO molecules might bind to metal oxides of Mo through the O atom while being simultaneously bound to the Ru metal through the C atom increasing significantly the adsorption heat. X-ray photoelectron spectroscopy analysis, performed to 2Ru0.5Mo/G catalyst after in-situ H2 reduction treatment, determined the presence of MoVI (MoO3) and MoIV (MoO2), MoC and Mo0 species on the catalyst surface. It is considered that Mo particles could be located around the ruthenium nanoparticles acting as a Lewis acid and therefore facilitating the CO dissociation. These conclusions represent a major contribution to the understanding of the Mo promotion effect in the FT process because realistic deductions have been obtained using a proper inert support.
