La chimie de surface de petites nanoparticules métalliques ( ~ 1 nm), principalement de ruthénium ou d'alliages de ruthénium, a été étudiée par une approche théorique au niveau DFT. Cela est appuyé par le développement d'outils d'analyse de propriétés structurales, électroniques et thermodynamiques de ces nanoparticules. Une première partie est consacrée à l'étude des propriétés structurales de nanoparticules métalliques. La variété de morphologie des nanoparticules ainsi que la nécessité de pouvoir générer des modèles appropriés sont mises en évidence. En particulier, l'affinement de la génération de modèles structuraux théoriques est rendu possible via l'implémentation de méthodes de modélisation de nanoparticules génériques couplées à l'utilisation de la méthode de Monte Carlo inversé permettant se rapprocher au plus près de la réalité expérimentale. L'application à ces nanoparticules de descripteurs électroniques ou morphologiques, tels que le d-band center ou le nombre de coordination généralisé, est par la suite proposée en relation avec leur capacités d'adsorption, et plus généralement dans le cadre du principe de Sabatier. Un descripteur électronique de la liaison chimique (COHP) est appliqué aux différentes nanoparticules, pour mettre en évidence les différences entre structures aussi bien que la nature des interactions au sein du cœur métallique, ainsi qu'entre ce cœur et les espèces de surface. Enfin, l'adsorption d'espèces à la surface de ces modèles est étudiée. L'adsorption d'un seul ligand à la surface d'une nanoparticule modèle est utilisée comme sonde de détermination de sites d'adsorption préférentiels, puis des taux d'adsorption plus élevés sont considérés dans le but d'étudier l'influence de celui-ci sur l'adsorption de ligands surnuméraires, ainsi que pour rendre compte de l'influence des ligands de surface sur la morphologie du cœur métallique. Pour cela, les propriétés thermodynamiques des systèmes adsorbés ont été modélisées par prise en compte de l'influence de la pression et de la température sur la stabilité relative des diverses structures via une modélisation de thermodynamique ab initio. Enfin, cette même approche à été utilisée pour étudier la co-adsorption de ligands H2 et CO à la surface de nanoparticules de ruthénium et de rhénium dans le cas particulier de la synthèse de Fischer-Tropsch, permettant notamment de proposer un intermédiaire thermodynamiquement favorable pour cette réaction. Une étude préliminaire de cette réaction, d'un fort intérêt chimique et sociétal, conclut ce manuscrit. L'utilisation combinée des approches structurale, électronique et thermodynamique permet alors d'avoir un point de vue élargi sur certains aspects de la chimie de ces nanoparticules de ruthénium. / Surface chemistry of small metallic nanoparticles ( ~ 1 nm), mainly ruthenium or ruthenium alloys, has been studied at the DFT level via a theoretical approach. This study is supported by the development of analytical tools, that allow to investigate structural, electronic and thermodynamical properties of those nanoparticles. A first part is dedicated to the structural properties of metallic nanoparticles. Morphological diversity is highlighted as well as the necessity of being able to desing reliable models. The refinement of structural models is made possible via the combined use of generic nanoparticles structure design and of the reverse Monte Carlo method in order to fit experiments. Electronic or morphologic descriptors such as d-band center or generalized coordination number are applied to those nanoparticles, in relationship with their adsorption possibilities and, to a larger extent, with the Sabatier principle. An electronic descriptor of the chemical bond (COHP) is applied to the considered nanoparticles in order to show differences between structures, as well as the interactions within the metallic core and between the core and surface species. Finally, adsorption of surface species is studied. A single ligand probe is used to spot favorable adsorption sites, then higher coverages are considered so as to test its influence on the adsorption of extra ligands, and to investigate the effect of surface ligands on the metallic core morphology. To do this, thermodynamical properties of adsorbed systems have been modeled by taking into account the effect of pressure and temperature on the nanoparticles relative stabilities via ab initio thermodynamics. The same approache was eventually applied to H2/CO coadsorbed at ruthenium and rhenium nanoparticles surface, in the context of the Fischer-Tropsch synthesis, allowing to propose a thermodynamically favorable intermediate for this reaction. Preliminary study of this reaction, of high chemical and societal interest, conclude this manuscript. The combined use of structural, electronic and thermodynamical approaches widens the overview on some aspects of ruthenium nanoparticles chemistry

L'intérêt pour la synthèse de Fischer-Tropsch (FTS) est d'actualité. Elle permet la conversion de matière première (biomasse) en combustible liquide. Comparés aux catalyseurs à base de cobalt, ceux à base de fer présentent une désactivation rapide, une activité et une sélectivité faibles en produisant une quantité non désirable de CO2. Après plusieurs décennies d'études, l'origine de ces défauts reste méconnue. Les catalyseurs classiques sont généralement fortement chargés en fer (>70 wt.%) et composés de nombreuses phases empêchant l'établissement d'une relation structure-activité. Il est nécessaire de développer des catalyseurs contenant du fer plus actifs, plus sélectifs et plus stables par une approche rationnelle. La synthèse de nanoparticules de taille contrôlée (3.5 nm) encapsulées dans les murs d'une silicalite-1 creuse (Fe@hollow-silicalite-1) est présentée. L'encapsulation empêche le frittage pendant la synthèse de Fischer-Tropsch, permettant de garder une bonne dispersion du fer. Contrairement aux autres catalyseurs, le catalyseur Fe@hollow-silicalite-1actif ne produit pas de CO2. L'hydrophobicité de la silicalite-1 est très certainement à l'origine de la non-production de CO2 par inhibition de la réaction directe du gaz à l'eau. On démontre que le catalyseur Fe@hollow-silicalite-1convertit le CO2 en CO par réaction du gaz à l'eau inversée (R-WGS). Afin d'établir une relation structure-activité, des catalyseurs à base de fer de taille bien contrôlée sont synthétisés et caractérisés (MET, in-situ XANES, in-situ Mössbauer). Deux catégories de TOF suivant la taille des particules, ~10-2 s-1 pour les plus larges (>20 nm) et ~10-3 s-1 pour les plus petites, sont observées / Fischer-Tropsch synthesis (FTS) is gaining renewed interests as it allows converting alternative feedstocks (biomass) into liquid fuels. Compared to Co-based catalysts, state of the art Fe catalysts show lower activity, faster deactivation and lower selectivity as it produces an undesirable amount of CO2. Despite decades of studies, the origins of low activity and selectivity and fast deactivation are still unclear. Typical Fe based catalysts are highly metal loaded (>70 wt.%) and composed of many different phases, which strongly impedes the establishment of structure-activity relationships. There is a need to develop more active, more selective and more stable iron FTS catalysts by rational approaches.The synthesis of well-controlled 3.5 nm iron nanoparticles encapsulated in the walls of a hollow-silicalite-1 zeolite (Fe@hollow-silicalite-1) is presented. The encapsulation prevents particle sintering under FTS conditions leading to a high and stable Fe dispersion. The catalyst Fe@hollow-silicalite-1 is active and highly selective in FTS. Most importantly, Fe@hollow-silicalite-1 does not produce CO2 in contrast to all other Fe-based catalysts. The strong hydrophobicity of the silicalite-1 is likely the origin of the lack of CO2 production by inhibition of the forward WGS reaction. We demonstrated that Fe@hollow-silicalite-1converts CO2 into CO by the reverse WGS reaction. In order to establish a structure-activity relationship, a series of Fe-based catalysts with well-controlled particle sizes were synthesized and characterized (TEM, in-situ XANES, in-situ Mössbauer, XRD). We observed two distinct categories of TOFs depending on the particle size, ~10-2 s-1 for larger (>20 nm) and ~10-3 s-1 for smaller ones

La synthèse Fischer-Tropsch (FT) permet de convertir un mélange d'hydrogène et de monoxyde de carbone (gaz de synthèse) sélectivement en hydrocarbures avec une distribution large de longueur de chaine. Le gaz de synthèse peut être produit à partir de différentes ressources comme le gaz naturel, le charbon et la biomasse. A la lumière de la volonté de diversifier les sources d'énergies, la synthèse FT peut apporter une contribution cruciale pour la production de carburants liquides. Les catalyseurs à base de Cobalt supportés sur alumine sont utilisés pour produire des cires lourdes. L'activité et la sélectivité dépendent des propriétés structurales et texturales du catalyseur. Cette étude a pour but d'établir un lien entre les propriétés structurales des catalyseurs à base de Cobalt supportés sur alumine et des paramètres cinétiques spécifiques. Pour atteindre cet objectif, il a été mis en oeuvre une étude cinétique en régime permanent couplé à la technique « SSITKA » sur différents échantillons de Cobalt. En utilisant cette méthodologie, il a été trouvé que la conversion en CO sur 5 catalyseurs à base de Cobalt dépend uniquement du nombre de site initial sur la surface atomique de Cobalt réduit. Aucune influence de la taille de particule, de l'orientation de la phase cristalline ou du promoteur n'a pu être mis en évid ence. Les expériences SSTIKA réalisées sur une longue durée ont permis d'estimer le nombre de sites actifs dans les conditions de travail. Enfin, la modélisation cinétique a démontré que l'espèce la plus abondante sur la surface est le monoxyde de carbone adsorbé et que deux intermédiaires distincts de surface conduisent à la production de méthane et des hydrocarbures plus lourds / The Fischer-Tropsch synthesis (FTS) converts a mixture of hydrogen and carbon monoxide (syngas) selectively into hydrocarbons with a large chain length distribution. Syngas can be produce from different resources such as natural gas, coal and biomass. In the light of energy resource diversi fication, FTS can make a crucial contribution to the production of liquid fuels. Alumina supported cobalt catalysts are used to produce heavy waxes. The activity and selectivity depend on the structural and textural properties of the catalyst. This study aims at establishing a link between the structural properties of alumina supported cobalt catalysts and specific kinetic parameters. To this purpose, the steady-state and SSITKA kinetics over different cobalt samples have been carried out. By using this met hodology, it was found that the CO conversion over 5 cobalt catalysts only depends on the initial number of reduced cobalt surface atoms. No influence of the cobalt particle size, phase orientation or promotor could be identified. SSITKA experiments during long-term catalyst testing allowed estimating the number of active sites under working conditions. Further modelling showed that the most abundant surface species is adsorbed carbon monoxide and that two distinct surface intermediates lead to the production of methane and higher hydrocarbons

Lower C2–C4 olefins are important commodity chemicals usually produced by steam cracking of naphtha or fluid catalytic cracking of vacuum gas oil. The Fischer–Tropsch synthesis of lower olefins (FTO) with iron-based catalysts uses synthesis gas as an alternative feedstock. Nanostructured carbon materials are widely applied as supports for the iron nanoparticles due to their weak interaction with the metal species, facilitating the formation of catalytically active iron carbide. Numerous synthetic approaches towards carbon-supported FTO catalysts with various structures and properties have been published in recent years but structure-performance relationships remain poorly understood. We apply ordered mesoporous carbon (CMK-3) as a support material with well-defined pore structure to investigate the relationships between calcination/activation conditions and catalytic properties. After loading of iron and sodium/sulfur as the promoters, the structures and properties of the FTO catalysts are varied by using different calcination (300–1000 °C) and activation (350 or 450 °C) temperatures followed by FTO testing at 1 bar, 350 °C, H2/CO = 1. Carbothermal reduction of iron oxides by the support material occurs at calcination temperatures of 800 or 1000 °C, leading to a higher ratio of catalytically active iron(carbide) species but the catalytic activity remains low due to particle growth and blocking of the catalytically active sites with dense graphite layers. For the samples calcined at 300 and 500 °C, the formation of non-blocked iron carbide can be enhanced by activation at higher temperatures, leading to higher catalytic activity. Olefin selectivities of ∼60%C in the formed hydrocarbons with methane of ∼10%C are achieved for all catalysts under FTO conditions at low CO conversion. The influence of the calcination temperature is further investigated under industrially relevant FTO conditions. Promoted CMK-3-supported catalysts obtained at low calcination temperatures of 300–500 °C show stable operation for 140 h of time on stream at 10 bar, 340 °C, H2/CO = 2.

[pt] A reação de síntese de Fischer-Tropsch tem despertado interesse pelo seu caráter tecnológico e científico uma vez que este processo é capaz de converter o gás natural em produtos de elevado valor agregado. No presente trabalho, catalisadores de ferro suportados em sílica alumina ou em SBA-15 foram sintetizados, caracterizados, ativados e empregados na síntese de Fischer Tropsch. Os catalisadores foram preparados via impregnação incipiente do ponto úmido utilizando cloreto de ferro III, obtendo-se catalisadores de porcentagem mássica próximos a 5%, 10% e, no caso da sílica alumina, 2% também. As amostras foram caracterizadas por análise termogravimétrica (ATG), por medidas de adsorção física de N2, difração de raios-X (DRX) e redução à temperatura programada (RTP). O teor de ferro foi determinado pela espectroscopia de raios X por dispersão de energia (EDX). A ativação dos catalisadores e, em seguida, os testes catalíticos foram conduzidos em um reator tubular de bancada e os produtos gasosos foram analisados através de cromatografia gasosa com detector por ionização de chama (CG-DIC), cuja análise mostrou a formação de hidrocarbonetos C1-C5. A adsorção física de N2 revelou que, a introdução de ferro nos suportes, diminuiu a área superficial específica e o volume de poro dos materiais. A difração de raios X foi realizada para os catalisadores após a calcinação, mostrando a presença das fases α-Fe2O3 e Fe3O4; após a ativação e após a reação, onde se encontrou Fe0 e carbetos de ferro Fe5C2 e Fe2C. Os testes catalíticos mostraram maior seletividade aos hidrocarbonetos C2-3 para os catalisadores suportados em SBA-15. / [en] The Fischer-Tropsch synthesis reaction has showed interest due to its technological and scientific character since the process is able to convert natural gas into high added value products. In the present work, iron catalysts supported on SBA-15 or silica doped alumina were synthesized, characterized, activated and employed in the Fischer Tropsch synthesis. The catalysts were prepared via incipient wetness impregnation method using iron III chloride, obtaining catalysts with a weight percentage of about 5%, 10% and, in the case of silica doped alumina, 20% as well. The samples were characterized by thermogravimetric analysis (ATG), with measurements of N2 physisorption, X-ray diffraction (DRX) and temperature programmed reduction (RTP). Iron content was determined by energy dispersive X-ray spectroscopy (EDX). The catalysts activation and, subsequently, their catalytic tests were conducted in a bench tubular reactor and the products were analyzed by gas chromatography with flame ionization detector (CG-DIC), where it was detected the formation of C1-C4 hydrocarbons. The N2 physisorption showed that the introduction iron in the supports decreased the specific surface area and the pore volume of the materials. X-ray diffraction was performed for catalysts after calcination, showing the presence of α-Fe2O3 and Fe3O4 phases, after activation and after reaction, where it was found crystals of Fe0 and iron carbides Fe5C2 and Fe2C. The catalytic tests showed greater C2-3 selectivity for SBA-15 supported catalysts.

I den här rapporten utvärderas det nya konceptet Power & Biomass to Liquid (PBtL). PBtL är ett alternativ till den tidigare och mer etablerade Biomass to Liquid (BtL) processen. Med PBtL förbättras utbytet av kol jämfört med BtL genom att elektricitet läggs till i processen. Elektriciteten används för att producera H2, som används för att höja H2/CO förhållandet istället för att använda WGS som i vanlig BtL process. Rapporten är en del i ett större PBtL projekt som bedrivits vid Institutt for kjemisk prosessteknologi vid NTNU och på SINTEF. Utvärderingen utfördes genom flera simuleringar av lågtemperaturs Fischer-Tropsch reaktorer i simuleringsprogrammet Aspen Plus. Omvandling och katalytiska reaktorer utvecklades och togs fram i programmet. Produktfördelningen i omvandlingsreaktorn modellerades med ASF distribution theory tillsammans med en metod för sammanslagning av högre kolväten. Fördelningen av paraffiner, olefiner och oxygenater baserades på experimentella resultat från Shafer et al. som studerade en slurryreaktor under liknande förhållanden. Den kinetiska reaktorn modellerades med en variant av ASF fördelningsteori kallad ”consorted vinylene mechanism” från Rytter och Holmen. Reaktorerna adderades till förgasningsprocess, som utvecklats tidigare av PBtL gruppen, I förgasningsprocessen förgasas biomassa till syntesgas, dvs H2 och CO. För att möjliggöra en utvärdering av det efterföljande steget med separering av vax, mellandestillat och lättare kolväten så antogs en väl fungerande separation av Fischer-Tropsch produkterna. En enklare separation av med flash förångning gjordes också, dels för fortsättningen av PBtL processen och för att kunna studera tailgasrecirkulering. Ett mindre bidrag var studier av en torkningsprocess för biomassa innan inloppet till förgasningsprocessen. PBtL konceptet diskuterades även ur ett praktiskt perspektiv.  Resultaten visar att vid driftbetingelser på 210 °C, 25 bar och H2/CO = 1,95 så gav omvandlingsreaktorn en kolselektivitet för CH4 respektive C5+ på 14,77 respektive 75,40 mol% C. Högre temperatur, tryck och H2/CO förhållande i reaktorn resulterar i en högre kolselektivitet mot lägre kolväten. Vid samma driftbetingelser gav den katalysreaktorn en kolselektivitet för CH4 respektive C5+ på 7,612 respektive 86,00 mol% C. Resultaten visar att C8-C16 produktionen var högre än C17+ med avseende på molflöde men lägre beträffande massflöde för katalysreaktorn. Generellt så ökar kolselektiviteten med ökande kolnummer till ett maximum runt 13 för att sedan minska. / In this report, the new Power & Biomass to Liquid (PBtL) concept was evaluated. The PBtL concept is a new alternative to the more well-established Biomass to Liquid (BtL) concept where electricity is added to the process. The main purpose for developing the PBtL is that the BtL process exhibits poor carbon efficiency compared to the PBtL process. The electricity here is used to produce H2 in electrolysis. The report is part of a larger PBtL project pursued for several years at the Department of Chemical Engineering at NTNU and SINTEF. The evaluation was done by simulating different types of low temperature Fischer-Tropsch reactors in simulation software Aspen Plus. A conversion reactor and a kinetic reactor was developed. A conversion reactor based on the result from the kinetic reactor was also developed.  The conversion-based reactor was modeled with the ASF distribution theory which describes the distribution of products formed in Fischer-Tropsch synthesis along with a method of lumping higher hydrocarbons. The distribution between paraffins, olefins and oxygenates was based on experimental data from Shafer et al. with similar operating condition with a Slurry reactor. The kinetic-based reactor was modeled with ASF distribution theory with a consorted vinylene mechanism previously described in Rytter and Holmen. The reactors were added to a process for which the biomass gasification section had previously been developed by the PBtL group. The Fischer-Tropsch products were as well separated in order to evaluate the subsequent step of separation of waxes, middle distillate and lighter hydrocarbons. This enabled the option of recycling of tail gas to the Fischer-Tropsch reactor to be evaluated. A smaller contribution included addition of a biomass dryer prior the biomass gasification section. The PBtL concept is also shortly discussed from a practical point-of-view.  It was found that for the operating condition of 210 °C, 25 bar and H2/CO = 1.95 for the conversion-based reactor yielded a carbon selectivity towards CH4 and C5+ of 14.77 and 75.40 mol C% respectively. For the same operating condition, the kinetic-based reactor yield a carbon selectivity towards CH4 and C5+ of 7.612 and 86.00 mol C% respectively. It could be seen from the conversion-based reactor that elevating temperature, pressure and H2/CO (to a certain extent) results in higher carbon selectivity towards lower hydrocarbons. From the product separation with the kinetic reactor, it was observed that C8-C16 production was higher than the C17+ production in terms of mole flow but lower in terms of mass flow. For both models, carbon selectivity increases with carbon number and peaks around carbon number 13 and then starts to decrease.

The technical feasibility of gasoline and diesel range hydrocarbons production through oligomerisation of olefins, starting from biomass with the intermediary steps of gasfication, water-gas shift reaction and syngas-to-olefins synthesis was investigated, through mathematical modelling and simulation on Matlab. The model for the gasifier was based on minimisation of Gibbs free energy and its results showed that higher carbon efficiencies could be achieved at lower pressures and steam inlet, and more inlet energy, by pre-heating the gasifying agents. The water-gas shift reactor was used to increase the ratio of hydrogen to carbon monoxide from the gasifier, before entering the syngas-to-olefins process. A 1-D model was employed to determine the concentration, temperature, and pressure profiles in the reactor. High inlet pressure and temperature were shown to be beneficial, by requiring smaller reactors for the desired ratios to be reached. Experimental data from scientific literature was used for empirical modelling of the Fischer-Tropsch reactor. Partial pressure of CO and H2 amounting to 1 bar, high temperature and H2/CO showed better production of the low olefins. A reaction mechanism and accordingly, rate equations were developed and employed in a plug-flow type reactor model, calculating the concentration profile of the olefins up to C20. High pressures were favourable for the production of heavier fractions, while elevated temperatures showed to cause more cracking of heavy hydrocarbons and consequently, less conversion. Based on the results of individual reactors, an integrated process flow diagram was suggested and optimised for maximum production of low olefins to the oligomerisation reactor (C2-4). The optimisation showed overall carbon efficiency of the process to be around 20%. The reason for this was associated with the choice of catalyst in the FTO process, due to its high selectivity to carbon dioxide. / Den tekniska genomförbarheten av bensin- och dieselproduktion genom oligomerisering av olefiner, från biomassa via förgasning, vatten-gasförskiftsreaktion och syngas-till-olefiner syntes undersöktes genom matematisk modellering och simulering på Matlab. Förgasningsmodellen baserades sig på Gibbs energi minimisering. Ju mindre förgasningstryck desto högre uppnås koleffektivitet. Vatten-gasskift reaktorn användas för att anpassa väte/kolmonoxid förhållande från förgasningsreaktorn till syngas till olefiner rektorn. En 1-D modell utvecklades och beräknade reaktorns koncentrationer, temperatur och tryckprofiler. Högre inlopps tryck och temperaturer leder till mindre reaktorer.   Experimentella data från vetenskaplig litteratur användes för att modellara Fischer-Tropsch reaktorn (syngas till olefiner). Partialt tryck av CO och H2 lika med 1 bar, hög temperatur och H2 / CO visade högre produktion av lätta olefiner. En reaktionsmekanism och följaktligen hastighetsekvationer utvecklades för oligomerisering och användades i en pluggflödesreaktor. Modellen beräknade koncentrationer profiler av olefiiner upp till C20.Högre tryck producerar tyngre fraktioner (diesel) medan högra temperaturer främjar krakning. Baserat på resultaten från enskilda reaktorer föreslogs ett integrerat processflödesdiagram och optimerades för maximal produktion av låga olefiner till oligomeriseringsreaktorn (C2-4). Optimeringen visade att den totala koleffektiviteten i processen var cirka 20%. Anledningen till detta var förknippat med valet av katalysator i FTO-processen på grund av dess höga selektivitet för koldioxid.

Climate change is one of the biggest global threats of the 21st century. Fossil fuels constitute by far the most important energy source for transportation and the different governments are starting to take action to promote the use of cleaner fuels. Biomass-derived fuels are a promising alternative for diversifying fuel sources, reducing fossil fuel dependency and abating greenhouse gas emissions. The research interest has quickly shifted from first-generation biofuels, obtained from food commodities, to second-generation biofuels, produced from non-food resources. The subject of this PhD thesis is the production of second-generation biofuels via thermochemical conversion: biomass is first gasified to synthesis gas, a mixture of mainly H2 and CO; synthesis gas can then be catalytically converted to different fuels. This work summarizes six publications, which are focused on the synthesis gas conversion step. Two processes are principally examined in this summary. The first part of the PhD thesis is devoted to the synthesis of ethanol and higher alcohols, which can be used as fuel or fuel additives. The microemulsion technique is applied in the synthesis of molybdenum-based catalysts, achieving a yield enhancement. Methanol cofeeding is also studied as a way of boosting the production of longer alcohols, but a negative effect is obtained: the main outcome of methanol addition is an increase in methane production. The second part of the PhD thesis addresses wax hydroconversion, an essential upgrading step in the production of middle-distillate fuels via Fischer-Tropsch. Bifunctional catalysts consisting of noble metals supported on silica-alumina are considered. The deactivation of a platinum-based catalyst is investigated, sintering and coking being the main causes of decay. A comparison of platinum and palladium as catalyst metal function is also carried out, obtaining a fairly different catalytic performance of the materials in terms of conversion and selectivity, very likely due to dissimilar hydrogenation power of the metals. Finally, a kinetic model based on the Langmuir-Hinshelwood-Hougen-Watson formalism is proposed to describe the hydroconversion reactions, attaining a good fitting of the experimental data. / Klimatförändringarna är ett av de största globala hoten under det tjugoförsta århundradet. Fossila bränslen utgör den helt dominerande energikällan för transporter och många länder börjar stödja användning av renare bränslen. Bränslen baserade på biomassa är ett lovande alternativ för att diversifiera råvarorna, reducera beroendet av fossila råvaror och undvika växthusgaser. Forskningsintresset har snabbt skiftat från första generationens biobränslen som erhölls från mat-råvaror till andra generationens biobränslen producerade från icke ätbara-råvaror. Ämnet för denna doktorsavhandling är produktion av andra generationens biobränslen via termokemisk omvandling. Biomassa förgasas först till syntesgas, en blandning av i huvudsak vätgas och kolmoxid; syntesgasen kan sedan katalytiskt omvandlas till olika bränslen. Detta arbete sammanfattar sex publikationer som fokuserar på steget för syntesgasomvandling. Två processer är i huvudsak undersökta i denna sammanfattning. Den första delen av doktorsavhandlingen ägnas åt syntes av etanol och högre alkoholer som kan användas som bränsle eller bränsletillsatser. Mikroemulsionstekniken har använts vid framställningen av molybden-baserade katalysatorer, vilket gav en höjning av utbytet. Tillsatsen av metanol har också studerats som ett sätt att försöka få en högre koncentration av högre alkoholer, men en negativ effekt erhölls: huvudeffekten av metanoltillsatsen är en ökad metanproduktion. Den andra delen av doktorsavhandlingen handlar om vätebehandling av vaxer som ett viktigt upparbetningssteg vid framställning av mellandestillat från Fischer-Tropsch processen. Bifunktionella katalysatorer som består av ädelmetaller deponerade på silica-alumina valdes. Deaktiveringen av en platinabaserad katalysator undersöktes. Sintring och koksning var huvudorsakerna till deaktiveringen. En jämförelse mellan platina och palladium som funktionella metaller genomfördes också med resultatet att det var en ganska stor skillnad mellan materialens katalytiska egenskaper vilket gav olika omsättning och selektivitet, mycket sannolikt beroende på olika reaktionsmönster hos metallerna vid vätebehandling. Slutligen föreslås en kinetisk modell baserad på en Langmuir-Hinshelwood-Hougen-Watson modell för att beskriva reaktionerna vid vätebehandling. Denna modell ger en god anpassning till experimentella data. / El cambio climático es una de las mayores amenazas del siglo XXI. Los combustibles fósiles constituyen actualmente la fuente de energía más importante para el transporte, por lo que los diferentes gobiernos están empezando a tomar medidas para promover el uso de combustibles más limpios. Los combustibles derivados de biomasa son una alternativa prometedora para diversificar las fuentes de energía, reducir la dependencia de los combustibles fósiles y disminuir las emisiones de efecto invernadero. Los esfuerzos de los investigadores se han dirigido en los últimos años a los biocombustibles de segunda generación, producidos a partir de recursos no alimenticios. El tema de esta tesis de doctorado es la producción de biocombustibles de segunda generación mediante conversión termoquímica: en primer lugar, la biomasa se gasifica y convierte en gas de síntesis, una mezcla formada mayoritariamente por hidrógeno y monóxido de carbono; a continuación, el gas de síntesis puede transformarse en diversos biocombustibles. Este trabajo resume seis publicaciones, centradas en la etapa de conversión del gas de síntesis. Dos procesos se estudian con mayor detalle. En la primera parte de la tesis se investiga la producción de etanol y alcoholes largos, que pueden ser usados como combustible o como aditivos para combustible. La técnica de microemulsión se aplica en la síntesis de catalizadores basados en molibdeno, consiguiendo un incremento del rendimiento. Además, se introduce metanol en el sistema de reacción para intentar aumentar la producción de alcoholes más largos, pero los efectos obtenidos son negativos: la principal consecuencia es el incremento de la producción de metano. La segunda parte de la tesis estudia la hidroconversión de cera, una etapa esencial en la producción de destilados medios mediante Fischer-Tropsch. Los catalizadores estudiados son bifuncionales y consisten en metales nobles soportados en sílice-alúmina. La desactivación de un catalizador de platino se investiga, siendo la sinterización y la coquización las principales causas del problema. El uso de platino y paladio como componente metálico se compara, obteniendo resultados catalíticos bastante diferentes, tanto en conversión como en selectividad, probablemente debido a su diferente capacidad de hidrogenación. Finalmente, se propone un modelo cinético, basado en el formalismo de Langmuir-Hinshelwood-Hougen-Watson, que consigue un ajuste satisfactorio de los datos experimentales. / <p>QC 20160308</p>

In this study nanometric iron carbide particles were produced by using an induction thermal plasma reactor. There are several applications for iron carbide particles in research and industry, such as in ferrofluids, magnetic recording and biosensors. We are focused in this project on its application as catalyst for Fischer-Tropsch reaction. Two different injection methods were used in this study. Suspension injection was used because of its capability to inject heterogeneous precursors, and solid injection was used to inject reactants with any desired molar ratio. The effect of several process parameters was investigated (plate power, injection rate, probe position, particle size and reactant ratio) and composition and morphology of produced powder were characterized using several characterization techniques including X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Thermogravimetric Analysis (TGA), and specific surface area measurement using BET method. XRD results showed that the produced powder has about 50% of iron carbide alongside other phases such as pure iron, austenite and graphite. SEM and TEM images revealed that nanometric particles with a diameter between 10-50 nm were produced alongside larger particles with diameter between 1 to 3 [micrometer]. High resolution TEM images showed that the produced nanometric particles have a core-shell structure and that they are embedded in an amorphous carbon. A new method has also been developed to collect the produced nanopowder in a liquid in order to minimize nanoparticle dispersion into the air, and protect pyrophoric nanoparticles from air exposure.

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2005. / In a Fischer-Tropsch refinery environment carboxylate complexes are of interest since the carboxylic acids present in product streams lead to formation of carboxylate salts through leaching of process equipment and catalysts. It is widely accepted that decomposition of organic (carboxylic) acids catalysed by metals is controlled by the decomposition of metal salts or complexes previously formed with such an acid. The determination of physical and structural properties of caboxylate complexes could contribute to the explanation of the mechanism involved in the decarboxylation of carboxylic acids. We have successfully determined the molecular structures of copper(II) allyl acetate, zinc(II) formiate, zinc(II) isovaleroate, yttrium(III) acetate and lanthanum(III) propionate. It has been established that zinc has a preferred tetrahedral coordination in carboxylate complexes compared to the octahedral coordination of copper, lanthanum and yttrium complexes considered. The carboxylate O-C-O angle in these complexes range between 119° and 125° and the conformation of the carbon chains is anti in all cases except for copper(II) allyl acetate, where a gauche conformation is adopted. Using structural methods such as TGA, infrared spectroscopy and X-Ray powder diffraction and combining it with existing knowledge of yttrium carboxylates and the effective use of computational chemistry – to calculate favourable internal parameters, using DFT calculations and B-LYP level theory - a likely structure for yttrium(III) propionate is proposed. The use of infrared measurements were especially valuable towards predictions of possible structures and the postulations of Nakamoto, on the relation between carboxylate carbonyl stretching frequencies and the nature of the carboxylate bond, could be used to accurately identify – except for the formiate salts of zinc(II) and yttrium(III) – the bonding mode present in the relevant compounds. We systematically tuned the non-cyclic organic part of the mono carboxylate ligand by lengthening and branching of the alkyl chain and determined that thermal decomposition and heat capacity of zinc complexes are a strong function of the ligand, while the behaviour of analogous yttrium complexes is hardly affected. The thermal investigation of lanthanum(III) propionate yielded a result that is in contrast with a previous study - where only CO2 was reported as byproduct - and we report an alternative result which indicates formation of symmetric ketones when the compound is heated to a high enough temperature. Earlier general assumptions about the layer-like crystal structure of lanthanum complexes coordinated by alkyl chain carboxylate are contradicted by the crystallographic data we collected for this compound. The crystal packing of lanthanum(III) propionate clearly shows a layered structure which is unexpected for a carboxylate with such a short alkyl.