UNIVERSITE LIBRE DE BRUXELLES November 2010 Faculté des Sciences Appliquées / Ecole Polytechnique
Faculty of Applied Sciences / Polytechnic School
Kinetic and Mechanistic Studies of CO Hydrogenation over Cobalt-based Catalysts
Julien SCHWEICHER
Dissertation presented to obtain a PhD degree in Engineering Sciences (Specialization in Heterogeneous Catalysis)
Supervisor: Prof. Norbert KRUSE
Laboratory of Chemical Physics of Materials (Catalysis-Tribology)
UNIVERSITE LIBRE DE BRUXELLES November 2010 Faculté des Sciences Appliquées / Ecole Polytechnique
Faculty of Applied Sciences / Polytechnic School
Kinetic and Mechanistic Studies of CO Hydrogenation over Cobalt-based Catalysts
Julien SCHWEICHER
Dissertation presented to obtain a PhD degree in Engineering Sciences (Specialization in Heterogeneous Catalysis)
Supervisor: Prof. Norbert KRUSE
Laboratory of Chemical Physics of Materials (Catalysis-Tribology)
The goal is to be as simple as possible, but not simpler
Albert Einstein
Acknowledgements/Remerciements
Pour commencer, je voudrais tout naturellement remercier mon promoteur, le Prof.
Norbert Kruse, pour m’avoir accueilli si chaleureusement dans son laboratoire et pour m’avoir permis de réaliser cette thèse de doctorat sous sa direction. Je lui suis vraiment reconnaissant de la confiance et du soutien qu’il m’a accordés.
Je veux, pour suivre, remercier le Prof. Alfred (Freddy) Frennet. Bien que retraité depuis de nombreuses années déjà, il a toujours été un conseiller omniprésent pour le bon déroulement de ce travail. Ses idées, son enthousiasme et ses qualités pédagogiques m’ont particulièrement marqué.
Un tout grand merci aussi à mes collègues Adam Bundhoo et Matthieu Moors. Nous avons traversé ensemble tous les bons - mais aussi les moins bons - moments d’une thèse de doctorat et d’une tranche de vie. Si l’on m’avait dit il y a quelques années que deux de mes meilleurs amis seraient un Français écoutant du « métal » et écrivant des nouvelles fantastiques (qu’en plus j’allais lire !) et un Bruxellois du CCM qui a pour passion la photographie, j’aurais probablement bien rigolé (encore plus si l’on m’avait dit qu’ils auraient tous les deux des chats comme animaux de compagnie !). Pourtant, nous avons ensemble refait maintes fois le monde aux quatre coins du globe, mais souvent à côté d’une bonne bière !
Un autre compagnon de route que je veux remercier est Gérôme Melaet. Arrivé au labo un an après moi, il a toujours été là durant les bons (Salamanca par exemple) et les mauvais moments (course poursuite pour une cartouche par exemple). Sans lui, l’ambiance au labo ne serait pas celle que j’ai connue. Je lui souhaite tout le meilleur (et surtout du courage) pour la dernière ligne droite !
Je voudrais également remercier le Dr. Thierry Visart de Bocarmé. Il a toujours prêté une oreille attentive à mes questions, que ce soit au niveau scientifique ou autre.
Merci également à Jean-Marie Bastin. Toujours de bon conseil, ses connaissances techniques, commerciales et administratives m’ont fait gagner énormément de temps durant ces années de thèse.
Il me reste à remercier chaleureusement tous les autres membres du laboratoire de Chimie Physique des Matériaux (Catalyse-Tribologie) (CPMCT) de l’ULB sans qui cette thèse n’aurait pas été pareille. Merci en particulier au Dr. Rafal Szukiewicz (we shared some great time at the synchrotron in Berkeley and without him, I would probably have gone crazy during the night shifts and I would for sure have missed one plane!), à Viacheslav (Slavik) Iablokov (I will remember for a long time the Spanish fiestas!), à Olivier Croquet, à Yannick Herremans, à Aline Desantoine et au Dr. Sergey Chenakin. Je voudrais aussi souhaiter bon vent à la relève : Cédric, Ervin,… et j’ai également une petite pensée pour les anciens (Aless, Dai, Fred, Vincent, Valérie,…).
En plus de mes collègues du CPMCT, je voudrais aussi remercier d’autres scientifiques qui ont, de près ou de loin, contribué à la réalisation de cette thèse de doctorat : le Prof. Marie-Paule Delplancke ainsi que le Dr. Jean Dille, Tiriana Segato et Patrizio Madau (Matières et Matériaux, ULB), le Prof. Véronique Halloin (Génie chimique, ULB et maintenant Secrétaire générale du F.R.S.-FNRS), le Dr. Frédéric Meunier (Queen’s University of Belfast, United Kingdom, et maintenant à l’Université de Caen, France), le Dr. Helen Daly (Queen’s University of Belfast, United Kingdom), le Dr. Heiko Oosterbeek (Shell Global Solutions, Pays-Bas), le Prof. John Geus (Universiteit Utrecht, Pays-Bas), le Prof. Miquel Salmeron
(Lawrence Berkeley National Laboratory, USA) et le Dr. Ferenc Borondics (Lawrence Berkeley National Laboratory, USA, et maintenant Beamline Scientist au Canadian Light Source).
Je profite aussi de cet espace pour remercier mes nombreux « sponsors » : l’ULB et Shell Global Solutions (1 an de bourse de doctorat), le Fonds pour la formation à la Recherche dans l’Industrie et dans l’Agriculture (F.R.I.A.) (3 ans de bourse de doctorat), le Fonds David et Alice Van Buuren (prix pour fin de doctorat), l’EU Transnational Access Programme (séjour à la Queen’s University of Belfast, United Kingdom), le Bureau des Relations Internationales et de la Coopération (BRIC) de l’ULB (2 séjours au Lawrence Berkeley National Laboratory, USA), le F.R.S.-FNRS (congrès en Finlande, Corée du Sud et Espagne), la bourse de voyage de Brouckère-Solvay de l’ULB (congrès en Finlande), le Fonds Agathon De Potter de l’Académie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique (congrès en Corée du Sud) et la Fondation L. et H. Fredericq de l’Académie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique (congrès en Espagne).
Pour terminer, je tiens à remercier infiniment mes parents sans qui je ne serais certainement pas en train de défendre une thèse de doctorat. Leur soutien en toutes circonstances m’a toujours beaucoup aidé.
Je ne peux pas finir sans aussi remercier du fond du cœur mon frère Guillaume. Nous avons tellement partagé durant toutes ces années ! Je lui souhaite tout le meilleur pour sa thèse, mais aussi pour tout le reste !
Bien évidemment, mon tout dernier remerciement va à Sabine qui illumine ma vie (et aussi qui me supporte) depuis plus de 2 ans. J’espère que cela durera encore très longtemps !
Abstract
During this PhD thesis, cobalt (Co) catalysts have been prepared, characterized and studied in the carbon monoxide hydrogenation (CO+H2) reaction (also known as “Fischer- Tropsch” (FT) reaction). In industry, the FT synthesis aims at producing long chain hydrocarbons such as gasoline or diesel fuels. The interest is that the reactants (CO and H2) are obtained from other carbonaceous sources than crude oil: natural gas, coal, biomass or even petroleum residues. As it is well known that the worldwide crude oil reserves will be depleted in a few decades, the FT reaction represents an attractive alternative for the production of various fuels. Moreover, this reaction can also be used to produce high value specialty chemicals (long chain alcohols, light olefins…).
Two different types of catalysts have been investigated during this thesis: cobalt with magnesia used as support or dispersant (Co/MgO) and cobalt with silica used as support (Co/SiO2). Each catalyst from the first class is prepared by precipitation of a mixed Co/Mg oxalate in acetone. This coprecipitation is followed by a thermal decomposition under reductive atmosphere leading to a mixed Co/MgO catalyst. On the other hand, Co/SiO2
catalysts are prepared by impregnation of a commercial silica support with a chloroform solution containing Co nanoparticles. This impregnation is then followed by a thermal activation under reductive atmosphere.
The mixed Co/Mg oxalates and the resulting Co/MgO catalysts have been extensively characterized in order to gain a better understanding of the composition, the structure and the morphology of these materials: thermal treatments under reductive and inert atmospheres (followed by MS, DRIFTS, TGA and DTA), BET surface area measurements, XRD and electron microscopy studies have been performed. Moreover, an original in situ technique for measuring the H2 chemisorption surface area of catalysts has been developed and used over our catalysts.
The performances of the Co/MgO and Co/SiO2 catalysts have then been evaluated in the CO+H2 reaction at atmospheric pressure. Chemical Transient Kinetics (CTK) experiments have been carried out in order to obtain information about the reaction kinetics and mechanism and the nature of the catalyst active surface under reaction conditions. The influence of several experimental parameters (temperature, H2 and CO partial pressures, total volumetric flow rate) and the effect of passivation are also discussed with regard to the catalyst behavior.
Our results indicate that the FT active surface of Co/MgO 10/1 (molar ratio) is entirely covered by carbon, oxygen and hydrogen atoms, most probably associated as surface complexes (possibly formate species). Thus, this active surface does not present the properties of a metallic Co surface (this has been proved by performing original experiments consisting in switching from the CO+H2 reaction to the propane hydrogenolysis reaction (C3H8+H2) which is sensitive to the metallic nature of the catalyst). CTK experiments have also shown that gaseous CO is the monomer responsible for chain lengthening in the FT reaction (and not any CHx surface intermediates as commonly believed). Moreover, CO chemisorption has been found to be irreversible under reaction conditions.
The CTK results obtained over Co/SiO2 are quite different and do not permit to draw sharp conclusions concerning the FT reaction mechanism. More detailed studies would have to be carried out over these samples.
Finally, Co/MgO catalysts have also been studied on a combined DRIFTS/MS experimental set-up in Belfast. CTK and Steady-State Isotopic Transient Kinetic Analysis (SSITKA) experiments have been carried out. While formate and methylene (CH2) groups have been detected by DRIFTS during the FT reaction, the results indicate that these species play no role as active intermediates. These formates are most probably located on MgO or at the Co/MgO interface, while methylene groups stand for skeleton CH2 in either hydrocarbon or carboxylate. Unfortunately, formate/methylene species have not been detected by DRIFTS over pure Co catalyst without MgO, because of the full signal absorption.
Résumé
Au cours de cette thèse de doctorat, nous avons préparé, caractérisé et testé des catalyseurs de cobalt (Co) pour la réaction d’hydrogénation du monoxyde de carbone (CO+H2), également appelée réaction de Fischer-Tropsch (FT). Les industriels utilisent la réaction FT pour produire des hydrocarbures à longues chaînes comme l’essence ou le diesel. L’intérêt réside dans le fait que les réactifs (CO et H2) sont obtenus à partir d’autres sources de carbone que le pétrole : le gaz naturel, le charbon, la biomasse ou même des résidus pétroliers. Puisqu’il est de notoriété publique que les réserves de pétrole seront épuisées d’ici quelques dizaines d’années, la réaction FT représente une alternative attrayante pour la production de carburants. Cette réaction peut également être utilisée pour synthétiser des composés de chimie fine à haute valeur ajoutée (alcools à longues chaînes, alcènes légers,…).
Deux classes de catalyseurs ont été étudiées durant cette thèse : du cobalt avec de la magnésie utilisée comme support ou dispersant (Co/MgO) et du cobalt avec de la silice utilisée comme support (Co/SiO2). Chaque catalyseur de la première catégorie est préparé par précipitation d’un oxalate mixte de Co/Mg dans l’acétone. Cette coprécipitation est suivie d’une décomposition thermique sous atmosphère réductrice conduisant à un catalyseur mixte Co/MgO. Quant aux catalyseurs Co/SiO2, ils sont préparés par imprégnation d’une silice commerciale avec une solution de chloroforme contenant des nanoparticules de cobalt. Cette imprégnation est suivie d’une activation thermique sous atmosphère réductrice.
Les oxalates mixtes de Co/Mg ainsi que les catalyseurs Co/MgO ont été caractérisés en détails afin de mieux connaître leur composition, leur structure et leur morphologie. Nous avons pour ce faire réalisé des traitements thermiques sous atmosphères réductrice et inerte (suivis par MS, DRIFTS, TGA et DTA), des mesures de superficie BET, des expériences XRD et de microscopie électronique. De plus, nous avons développé une méthode originale et opérant in situ pour mesurer la surface de chimisorption de H2 de nos catalyseurs.
Les catalyseurs Co/MgO et Co/SiO2 ont alors été testés à pression atmosphérique pour la réaction CO+H2. Nous avons réalisé des expériences de Cinétique Transitoire Chimique (Chemical Transient Kinetics, CTK) pour obtenir des informations sur la cinétique et le mécanisme de la réaction ainsi que sur la nature de la surface active du catalyseur en conditions réactionnelles. L’influence de plusieurs paramètres expérimentaux (température, pressions partielles de H2 et de CO, débit volumétrique total) et l’effet de la passivation sur le déroulement de la réaction sont aussi discutés.
Nos résultats indiquent que durant la réaction FT, la surface active de Co/MgO 10/1 (rapport molaire) est entièrement recouverte d’atomes de carbone, d’oxygène et d’hydrogène, très probablement associés en tant que complexes de surface (peut-être des formiates). La surface active ne possède donc pas les propriétés d’une surface métallique de Co (cela a été prouvé par des expériences originales consistant à passer rapidement de la réaction CO+H2 à la réaction d’hydrogénolyse du propane (C3H8+H2) qui est sensible à la nature métallique du catalyseur). Les expériences CTK ont également montré que CO en phase gazeuse joue le rôle du monomère responsable de l’allongement de chaîne pour la réaction FT (et non pas un intermédiaire de surface CHx comme souvent postulé dans la littérature). De plus, nous avons vu que la chimisorption de CO est irréversible sous nos conditions réactionnelles.
Les résultats CTK obtenus sur les catalyseurs Co/SiO2 sont par contre assez différents mais ne permettent pas d’aboutir à des conclusions quant au mécanisme réactionnel. Des études plus approfondies devraient être réalisées sur ces échantillons.
Finalement, nous avons aussi étudié les catalyseurs Co/MgO sur un appareillage combinant les analyses par DRIFTS et MS, à Belfast. Nous y avons réalisé des expériences CTK et de transitoires isotopiques (Steady-State Isotopic Transient Kinetic Analysis, SSITKA). Alors que nous avons détecté par DRIFTS des formiates et des groupements méthylène durant la réaction FT, les résultats indiquent que ces espèces ne jouent aucun rôle en tant qu’intermédiaires réactionnels actifs.
Ces formiates sont probablement situés sur MgO ou à l’interface Co/MgO, et les groupements méthylène font partie du squelette d’un hydrocarbure ou d’un carboxylate. Malheureusement, aucun groupement formiate ou méthylène n’a pu être détecté par DRIFTS sur le catalyseur de Co pur sans MgO, cela étant dû à l’absorption totale du signal par ce matériau.
Contents
1. Introduction...1
1.1. Present energy situation...1
1.2. Synthesis gas production ...6
1.2.1. Synthesis gas production via reforming...6
1.2.1.1. Steam reforming ...6
1.2.1.2. Catalytic Partial Oxidation (CPO) ...7
1.2.1.3. AutoThermal Reforming (ATR)...7
1.2.2. Synthesis gas production via gasification...8
1.3. Reactions involving synthesis gas...9
1.4. Fischer-Tropsch (FT) synthesis...10
1.4.1. A bit of history and present situation ...10
1.4.2. Chemistry of the FT reaction and industrial applications...13
1.4.2.1. Basic chemistry and thermodynamics of the FT reaction...13
1.4.2.2. FT industrial conditions...15
1.4.2.3. FT reactors ...16
1.4.3. Catalysts of the FT reaction...19
1.4.4. Product distribution of the FT reaction...23
1.5. Kinetics and mechanisms for the CO+H2 reaction ...26
1.5.1. “Alkyl” mechanism ...26
1.5.2. “Alkenyl” mechanism ...28
1.5.3. “Enol” (or “Hydroxycarbene”) Mechanism ...29
1.5.4. “CO insertion” mechanism (on the metal)...29
1.5.5. “Formate” mechanism ...30
1.5.6. FT mechanism concluding remark ...32
2. Aim of the thesis...33
3. Experimental section...34
3.1. Catalysts preparation ...34
3.1.1. Co/MgO catalysts ...34
3.1.1.1. Oxalates (co)precipitation...34
3.1.1.2. Thermal decomposition of the oxalates...36
3.1.2. Co/SiO2 catalysts...37
3.1.2.1. Co nanoparticles synthesis...37
3.1.2.2. Incipient Wetness Impregnation over SiO2...37
3.1.2.3. Activation leading to Co/SiO2 catalysts...37
3.2. Methods and experiments at the CPMCT (ULB) ...38
3.2.1. Experimental set-up for characterization and catalytic tests ...38
3.2.1.1. Set-up overview...38
3.2.1.2. Reactors ...41
3.2.1.3. Control of the volumetric flow rates ...47
3.2.1.4. Temperature control ...48
3.2.1.5. Mass spectrometry analysis ...48
3.2.2. Temperature-programmed decompositions ...54
3.2.3. In situ measurement of the total surface area (BET)...54
3.2.4. In situ measurement of the H2 chemisorption surface area...59
3.2.5. Catalytic tests ...63
3.2.5.1. Steady-state experiments...63
3.2.5.2. Chemical Transient Kinetics experiments ...63
3.3. Other methods and experiments at the ULB and at the Utrecht University...70
3.3.1. Thermal analyses ...70
3.3.2. X-Ray Diffraction ...70
3.3.3. Electron Microscopy ...72
3.4. Methods and experiments at the Queen’s University of Belfast ...75
3.4.1. DRIFTS/MS experimental set-up...75
3.4.2. Sample conditioning ...76
3.4.3. Transient experiments (CTK and SSITKA)...77
4. Characterization experiments...79
4.1. Temperature-programmed decompositions ...79
4.1.1. TPR of the Co oxalate ...79
4.1.2. TPR of the Co/Mg 10/1 oxalate ...81
4.1.3. TPR of the Co/Mg 1/1 oxalate ...83
4.1.4. Comparison of the TPR of the different oxalates and discussion...84
4.2. BET surface area measurements...88
4.3. Thermal analyses ...89
4.4. X-Ray Diffraction experiments...94
4.5. Thermal decompositions followed by DRIFTS/MS (Belfast)...101
4.6. Electron Microscopy ...112
4.7. Characterization conclusions ...119
5. Development of a new method to measure in situ the H2 chemisorption surface area of catalysts...121
5.1. Method development...121
5.2. Method validation ...123
5.3. Method applications ...124
6. CO+H2 Catalytic Tests at the CPMCT (ULB) - Chemical Transient Kinetics (CTK)..127
6.1. Reactor characterization ...127
6.2. Preliminary remarks ...130
6.2.1. Reaction total pressure...130
6.2.2. Transport limitations ...130
6.3. CO+H2 over Co/MgO catalysts...131
6.3.1. Reference experiment ...131
6.3.1.1. Results...131
6.3.1.2. Discussion ...141
6.3.2. Influence of the experimental parameters ...144
6.3.2.1. Influence of temperature...144
6.3.2.2. Influence of H2 and CO partial pressures ...147
6.3.2.3. Influence of total volumetric flow rate ...148
6.3.2.4. Determination of FT termination step ...149
6.3.2.5. Effect of catalyst passivation ...149
6.4. Hydrogenolysis reactions to test FT active surface of Co/MgO catalysts...151
6.4.1. Reaction Transient Kinetics (RTK) with ethane hydrogenolysis...151
6.4.2. Reaction Transient Kinetics (RTK) with propane hydrogenolysis...153
6.5. CO+H2 over Co/SiO2 catalysts ...156
7. CO+H2 Catalytic Tests in Belfast - DRIFTS/MS Studies during CTK and SSITKA...160
7.1. Results ...160
7.1.1. DRIFTS/MS during steady-state and CTK ...160
7.1.2. DRIFTS/MS during SSITKA ...166
7.2. Discussion ...168
7.3. Conclusions...170
8. General conclusions and Outlooks...171
9. References...177
1. Introduction
1.1. Present energy situation
Mankind has evolved for centuries using various types of energy. Worldwide energy consumption is continuously increasing and this tendency is not forecast to change, as can be seen in Figure 1.1 (from the BP Statistical Review of World Energy, June 2009)1.
Fig. 1.1: BP estimation of the world primary energy consumption over the last 25 years Figure 1.1 also shows that our main energy resource is still crude oil, closely followed by coal and natural gas. Hydroelectricity and nuclear energy are clearly visible in this representation but their contribution is much less important. However, this graph from BP does not show the contribution of biomass (and waste) and of other renewables (wind, solar, geothermal, tidal, and wave energies). In its World Energy Outlook 20082, the International Energy Agency (IEA) estimates that in 2006, biomass (and waste) had a contribution of 1186 Million tonnes oil equivalent (Mtoe), and that the contribution of other renewables was 66 Mtoe (1 toe is equal to ~42 Giga Joule)3.
In addition to providing the same data as those from BP, the IEA has established a reference scenario for the 20 coming years2. Figure 1.2 shows this forecasting along with the IEA estimated values for energy consumption for the years 2000 and 2006. The first observation that can be made is that the values from BP and IEA for 2000 and 2006 are in very good agreement. The IEA forecasting data indicate that over the period 2006-2030, coal and gas will undergo an (average) annual rate of growth of approximately 2% whereas the oil rate of growth will only be 1%. Despite a very small overall contribution, the category “other renewables” will exhibit the highest rate of growth (approximately 7%) over this period.
Nevertheless, crude oil will remain the dominant energy source in 2030.
Fig. 1.2: IEA estimation of the world primary energy consumption for the years 2000 and 2006 and forecasting for the years 2015 and 2030
The largest crude oil consumption occurs in the transportation sector where there are almost no competitive alternatives to petroleum3. The share of transport in the primary oil demand was 52% in 2006 and is expected to increase to 57% in 20302.
Despite the convenience of crude oil utilization, people are aware that the world reserves of black gold will be depleted in a few decades. It is thus very important to have other energy options when the crude oil depletion will occur.
Moreover, crude oil prices are varying a lot, which is not convenient for a system/organization relying on this only resource. Figure 1.3 illustrates these huge variations.
Fig. 1.3: Recent evolution of petroleum products prices in Rotterdam (in US $ per barrel)1
Crude oil prices depend of course on the available stocks and on demand and supply, but they are also strongly influenced by geopolitical events. In order to face these variations in the price of petroleum products, it would be desirable to be able to produce them by alternative petroleum-free routes.
For the time being, two realistic candidates that could easily replace crude oil are natural gas (which is mainly composed of methane) and coal. Indeed, the reserves for such resources are still important, especially for coal. Figure 1.4 shows the world reserves-to- production (R/P) ratios at end 2008 for crude oil (42 years), natural gas (60.4 years) and coal (122 years)1. There seems to be a common agreement about the values for crude oil and natural gas but another source estimates the coal reserves to last for 250 years3.
Fig. 1.4: World fossil fuel reserves-to-production (R/P) ratios at end 20081
These numbers are of course an estimation of the available reserves and they will slightly change from one year to another, because the known reserves will evolve, so will the production rate. Figures 1.5 and 1.6 provide examples of this evolution of the proved reserves for crude oil and natural gas. Although we produce and use more and more oil and gas, the proven reserves for both resources have increased over the past 20 years. On one hand, this is due to exploration and on the other hand, to the improvement of recovery techniques (that permit to increase the productivity of a field).
Fig. 1.5: Oil proved reserves1
Fig. 1.6: Natural gas proved reserves1
Optimistic people will of course say that it would be much better to rely on “green energies” (environmentally friendly energies) for the future. Fossil fuels (oil, natural gas and coal) present two major drawbacks: they are non-renewable and they seem to contribute to the global warming of our planet3. Residual products of fossil fuels such as carbon dioxide (CO2, product of combustion), methane (CH4) and nitrous oxide (N2O) belong to the group of
“greenhouse gases”3. Normally, when the sun rays strike the Earth’s surface, some part of them is reflected back towards space as infrared radiation (or in other terms, as heat). The greenhouse gases absorb this reflected infrared radiation and trap it into the atmosphere.
This provokes an increase of the Earth’s surface temperature. That phenomenon is known as the “greenhouse effect”. Over time, it could lead to an excessive warming of the planet and to serious damage (hurricanes, tsunamis, floods and droughts, seas level rise…)3-5. Despite more and more global warming alarmists, the correlation between the presence of greenhouse gases in the atmosphere and the increase of the Earth’s surface temperature is still not clear5. A complete analysis of the problem shows that it is far more complex than anyone imagined5. The roles of urban heating and changes in ocean circulation may have been underestimated by modelers5. The only fact that is 100% established is that fossil fuels are non-renewable resources and other energetic options will have to be developed to meet our growing demand for energy. Nevertheless, natural gas and coal appear to be very realistic and viable candidates to replace crude oil in the short and mid-terms, before renewable energies advent in the long-term. Indeed, renewable and/or clean energies could currently not meet the world energetic needs6-9. Moreover, these resources (e.g. biomass, wind, solar,…) vary over time and between locations10.
Biomass for example is a very attractive energy resource but it is not ready to play a major role in the world energy supply11. Crops especially dedicated to produce biomass are not very popular right now, because there is a competition between two utilizations for them:
food and production of biofuels3,11. This competition makes the price of cereals increase11, which is not convenient at all for poor people. At present, the only acceptable feedstock for biofuels production is plant residues and waste3,11. However, the production of these “second generation” biofuels is currently not cost effective due to remaining technical barriers11.
Solar energy is obviously a very elegant resource too. However, its current potential is very small by comparison with our energetic needs. The photovoltaic cells using semiconductor materials to generate electricity from the sun rays12 are still very expensive and their efficiency is quite low3. More research and development (R&D) will have to be carried out if we want that photovoltaic cells play a role in the future. Moreover, other energy options have to be used during the night and when the weather is cloudy9 (except if an efficient way of electricity storage is developed). Beside photovoltaic cells we can also think about solar thermal electricity generation (by focusing the Sun’s energy to produce steam to drive generators)12. Despite its high price, solar thermal energy could make a considerable
contribution in summer and in hot regions9. However, this could be more challenging in winter and at less ideal sites9.
Although being a little player in the global energy game, wind is a commercially viable and economically competitive renewable source13. Of the renewable energies, the technology for transforming wind energy into electricity is one of the most mature3. Despite the variability of wind force, it seems that this technology will continue to grow in the years to come. The current trends in the wind industry are the shift towards offshore installations and the use of larger machines13.
Hydroelectricity is another clean energy option (it consists in generating electricity from the energy of falling water). Its share in the global picture is not large for the moment. It is considered that only a third of the economic potential of this source has been operated at present3. Several developing countries have plans to increase their hydroelectric capacity, especially in Asia (China, India and Laos) and Central and South America3.
Another energy alternative is the use of hydrogen and fuel cells. However, this is not going to happen before a few decades3. People always see H2 as the perfect fuel for the future: its combustion produces only H2O and absolutely no CO2. This common thought is quite funny because H2O vapor is by far the most important greenhouse gas (it absorbs more irradiance from the Earth than CO2)5. Moreover, the H2 production is currently performed by steam reforming or partial oxidation of natural gas or gasification of coal (see also section 1.2.)3,4. This means that for the moment, H2 is not at all a clean source of energy: its methods of production require the use of fossil fuels and also generate some CO24. The development of CO2 capture and sequestration techniques will at least have to be carried out rapidly if one wants to give a chance at H2 produced from fossil fuels3,4. Other approaches for producing cleaner H2 are biomass gasification and H2O electrolysis using power generated by renewable energy sources such as solar cells4. Unfortunately, both of these methods are not cost competitive for the moment4. Beside the development of cleaner H2 production techniques, much effort is done in the improvement of fuel cells4 (a fuel cell is an electrochemical device that converts the chemical energy of a fuel and an oxidant (e.g. H2 and O2 from the air) directly into electricity, H2O and heat). The major advantage of fuel cells is their high electrical conversion efficiency4. However, they cannot financially compete with gasoline/diesel-powered engines at the moment4,14. Moreover, several technical issues remain to be overcome for fuel cells to be used practically: the lifetime of the membrane is not so long at present and the deactivation of the electrocatalyst happens too fast3,4.
Despite the fact that it is neither renewable nor clean, nuclear fission could be a medium player in the energy game in the future. The biggest issue of nuclear energy is its waste that lives long and causes a lot of worries in the world societies3,15. Despite the fact that there are safe ways to store nuclear waste in underground special containers3,15, nuclear fission still causes lots of discussions at the political level in a lot of countries12,15.
In the future, nuclear fusion will perhaps be “the” clean solution to supply energy to the world3. The only problem is that this technology is not yet controlled3,12. Despite the increasing efforts to foster nuclear fusion research (e.g. the International Thermonuclear Experimental Reactor (ITER) project in Cadarache (France))3, a practical application cannot be envisaged before several decades12.
After this concise review of energy options, we clearly see the great interest in using natural gas and coal as short and mid-term alternatives to crude oil. Precisely from natural gas and coal (or from biomass or petroleum residues), it is possible to form a gas mixture of carbon monoxide (CO) and hydrogen (H2)16-20. This mixture is called “synthesis gas” or
“syngas” because it allows one to produce numerous chemical compounds such as hydrocarbons or alcohols16,17,21,22. The “Fischer-Tropsch” (FT) reaction is this reaction of carbon monoxide hydrogenation aiming at producing various fuels for engines. These synthetic gasoline or diesel fuels exhibit the same characteristics than the petroleum-derived products, except that they are less harmful to the environment. In particular, their sulphur content is extremely low23,24. The CO hydrogenation reaction can also be used to produce high value specialty chemicals (long-chain alcohols, light olefins,…)16,17,25. The general name
of “Gas-To-Liquids” (GTL) describes this whole range of reactions involving CO and H2 as reactants.
Nowadays, the FT synthesis enables us to produce mainly linear products. If the desired compounds are branched hydrocarbons, a subsequent step of isomerization is required after the synthesis. However, some researchers are currently investigating the possibility to directly produce iso-paraffins from CO and H2 in a single step by using catalysts with zeolites26-29.
1.2. Synthesis gas production
Two main categories of techniques permit to produce the synthesis gas from carbonaceous resources: reforming and gasification. The term reforming is used for conversion of gaseous or light liquid feedstock to synthesis gas and the term gasification for conversion of solid or heavy liquid feedstock to synthesis gas16. The two most commons feeds for syngas generation are natural gas (rich in methane) and coal (rich in carbon). Other possible feedstocks include biomass, wastes (e.g. municipal solid wastes, paper pulp), refinery by-products (e.g. petroleum coke, heavy oils, bitumen) and coal bed methane. The main difference among the various feeds that could be used for syngas generation is the composition of this syngas: natural gas produces a syngas with a ratio H2/CO=2 or greater, coal produces a syngas with a ratio H2/CO=0.5 to 1 and biomass produces a syngas with a ratio H2/CO=1 or less20.
From a financial point of view, it has to be known that the most expensive part of a Fischer-Tropsch plant is the synthesis gas generation facility16. It typically accounts for 60- 70% of the capital and running costs of the total plant18,23,30,31. The FT synthesis part of the global plant usually accounts for 25% of the total capital cost and the hydrocracking part accounts for the remaining 15%23.
The processing of coal is more expensive than natural gas conversion16,30,32. Not only is the capital cost for natural gas reforming 30% lower than for coal gasification but the process is also more efficient. In methane reforming, only 20-25% of the carbon is converted to CO2 whereas this number is about 50% with coal, due to the lower hydrogen content of the latter16,30,32.
However, coal gasification could still be worthwhile if the coal price is low and if both electricity and higher value hydrocarbon products are co-produced with liquid fuels on a large scale16.
1.2.1. Synthesis gas production via reforming
There are three major techniques for producing the syngas via natural gas reforming16,18,33-35: steam reforming, catalytic partial oxidation and autothermal reforming.
1.2.1.1. Steam reforming
Key reactions in steam reforming:
1) Natural gas steam reforming: CH4+H O2 RCO+3H2 ΔH298K0 = 206 kJ/mol 2) Natural gas CO2 reforming: CH4 +CO2 R2CO+2H2 ΔH298K0 = 247 kJ/mol 3) « Water-Gas Shift » reaction (WGS): CO H O+ 2 RCO2+H2 ΔH298K0 = - 41 kJ/mol Steam reforming is an endothermic process (the standard enthalpy of reaction
0
H298K
Δ is positive for reaction #1). A continuous supply of energy is required for this process.
High temperatures are thus needed to give high equilibrium conversions. As reaction #1 results in gas expansion (2 gaseous molecules on the left and 4 gaseous molecules on the right), its conversion is favored by working at low pressures.
Because of its high energy consumption, steam reforming is not the preferred technology for production of synthesis gas for the large-scale GTL applications. Processes based on oxidative reforming are generally preferred for that purpose.
Steam reforming is always accompanied by the “Water-Gas Shift” reaction (WGS, reaction #3). This reaction is generally fast and we may consider that it is in equilibrium at most conditions.
Steam can be partially substituted by carbon dioxide to perform CO2 reforming (reaction #2). Adding CO2 to the steam could be more economical, especially if there is a source of cheap CO2 near the syngas generation facility. However, the use of CO2 reduces the H2/CO ratio in the produced syngas.
Steam reforming catalysts are usually based on nickel (Ni) as the active metal. Cobalt (Co) and noble metals (ruthenium (Ru), rhodium (Rh)) also catalyze this reaction but they are too expensive to find widespread use. A number of different supports including alumina (Al2O3), magnesium-aluminum spinel (MgAl2O4), and zirconia (ZrO2) are employed.
In industrial practice, steam reforming may be performed in two types of reactors:
steam reformers (fired heaters with catalyst-filled tubes placed in the radiant part of the heater) and heat exchange reformers (the heat required for the reaction is supplied from a flue gas or a process gas).
1.2.1.2. Catalytic Partial Oxidation (CPO)
Key reactions in Catalytic Partial Oxidation (CPO)
1) Natural gas partial oxidation: 4 1 2 2 2 2
CH + O →CO+ H ΔH298K0 = - 38 kJ/mol 2) Natural gas steam reforming: CH4+H O2 RCO+3H2 ΔH298K0 = 206 kJ/mol 3) Natural gas CO2 reforming: CH4 +CO2 R2CO+2H2 ΔH298K0 = 247 kJ/mol 4) « Water-Gas Shift » reaction (WGS): CO H O+ 2 RCO2+H2 ΔH298K0 = - 41 kJ/mol CPO is an exothermic process (the standard enthalpy of reaction ΔH298K0 is negative for reaction #1) where the main reaction is the partial oxidation of CH4 into CO. Both air and oxygen may be used as oxidant in a CPO reactor where no burner is used. The gas mixture entering the reactor reacts by heterogeneous catalyzed reactions (noble metals catalysts).
This entering gas mixture consists in the hydrocarbon feed, the oxidant, H2O and CO2. Steam and carbon dioxide are added to the stream in order to obtain the desired H2/CO ratio in the produced syngas.
The methane steam reforming (reaction #2) and shift reactions (reactions #3 and #4) are typically at or close to equilibrium at the reactor exit.
In practice, the products of the partial oxidation of CH4 can be further oxidized in CO2 and H2O. The process has thus to be very well controlled in order to stop the oxidation at the CO stage.
1.2.1.3. AutoThermal Reforming (ATR)
Key reactions in AutoThermal Reforming (ATR)
1) Natural gas combustion: 4 3 2 2 2 2
CH + O →CO+ H O ΔH298K0 = - 519 kJ/mol 2) Natural gas steam reforming: CH4+H O2 RCO+3H2 ΔH298K0 = 206 kJ/mol 3) « Water-Gas Shift » reaction (WGS): CO H O+ 2 RCO2+H2 ΔH298K0 = - 41 kJ/mol
ATR is a combined combustion and catalytic process carried out in an adiabatic reactor. The reactants are natural gas, oxygen and steam. The idea is to use the heat produced by reaction #1 (exothermic process) to provide the required energy for reaction #2 (endothermic process). ATR is thus the combination of CPO and Steam reforming.
An ATR reactor design consists of a burner, a combustion chamber and a catalyst bed section. At first, the gas mixture is partially converted in the pressurized combustion chamber. Then, heterogeneous steam reforming reactions occur over the catalyst bed to produce the equilibriated syngas. In addition to equilibriating the syngas, the catalysts also destroy the soot precursors that may have been formed before entering the catalyst bed section.
The most used ATR catalysts are based on nickel, with carriers of alumina (Al2O3) or magnesium-aluminum spinel (MgAl2O4).
For very large GTL capacities, ATR has become the preferred technique for generating the syngas. Indeed, it is the best economic option for large plants.
1.2.2. Synthesis gas production via gasification
Gasification is the conversion of coal or other solid or liquid carbonaceous resource to synthesis gas. This conversion is usually performed by partial combustion of coal in an air/steam or oxygen/steam mixture16.
The processes occurring during coal gasification are more complicated than the ones involved for gas reforming. Indeed, coal is a more complicated substance to work with. It consists of hydrocarbon chains, mineral matter and moisture, while other components (e.g.
halogens) are present in smaller concentrations16. Each rank of coal presents specific applications due to its respective composition and structure. These two parameters are essential to determine if a particular coal is suitable for gasification. The coal caking properties, the water content and the ash properties are important parameters in this decision too.
A lot of technologies exist for coal gasification. They mainly differ on the method of contact between the solid and the gas phases during gasification. Coal can flow counter- current with O2/H2O (fixed bed or moving bed), co-current (entrained flow gasifier) or the coal and the gases can be well-mixed (fluidized bed gasifier).
In all gasifiers, coal undergoes several transitions. First of all, it dries, having for result that the moisture is evaporated. Then, the pyrolysis occurs, producing gas, vaporized tars and oils and solid char residue. Pyrolysis is a complex process resulting in the decomposition of organic substances by heat. It involves cracking, hydrogenation and free radical reaction mechanisms. After drying and pyrolysis, gasification (reaction #1) and combustion (reaction
#2) occur. The partial oxidation exothermicity supplies the energy required by the carbon/steam reaction (which is endothermic).
Key reactions in O2/H2O gasification
1) Carbon/Steam reaction: C H O+ 2 RCO H+ 2 ΔH298K0 = 119 kJ/mol 2) Partial oxidation: 1 2
C+2O →CO ΔH298K0 = -123 kJ/mol 3) « Water-Gas Shift » reaction (WGS): CO H O+ 2 RCO2+H2 ΔH298K0 = - 41 kJ/mol 4) Methanation: CO+3H2RCH4+H O2 ΔH298K0 = -206 kJ/mol
Here again, the WGS reaction (reaction #3) is present and it can be used to manipulate the H2/CO ratio of the final syngas to a certain extent. Methanation (reaction #4) can also occur in a gasifier. These two reactions are usually near their chemical equilibrium in most of the gasifiers.
As for CPO, there is a possibility of further oxidation of CO in CO2. The temperature control has to be very accurate for that matter.
1.3. Reactions involving synthesis gas
The carbon monoxide hydrogenation reaction is very interesting because it allows one to produce a wide range of organic molecules from the two simple “inorganic” molecules constituting the syngas: CO and H236.
Although this thesis will only deal with the Fischer-Tropsch reaction (FT) (i.e. the production of hydrocarbons from CO and H2), the syngas also presents other applications and uses. Figure 1.7 shows an overview of a lot of possible applications for syngas, along with the catalysts used for each process17,37-47.
Fig. 1.7: Main applications of the syngas mixture (CO/H2)
As will be explained in details later, the Fischer-Tropsch reaction producing paraffins, olefins and alcohols is generally operated with cobalt (Co) or iron (Fe) catalysts. A nickel (Ni) catalyst can also be used in that purpose but will produce almost exclusively methane (CH4).
Ruthenium (Ru) could be utilized to produce very long-chain hydrocarbons (chemically identical with polyethylene)17 but is not much used in practice (we will explain that later).
Syngas can also be used to produce ethylene glycol over a homogeneous rhodium (Rh) catalyst39. Another very important use of the syngas is the production of aldehydes and higher alcohols via the hydroformylation process (also known as the OXO synthesis)41,42. In this process, olefins react with syngas over cobalt complexes catalysts (such as Co2(CO)8 and HCo(CO)4). Another large utilization of the syngas is the production of methanol (CH3OH), which is one of the major building blocks of the chemical industry in the world.
Typically, the conversion of syngas into methanol is realized over a catalyst made of copper supported on zinc oxide (Cu/ZnO). The methanol that is produced can then be used to synthesize a lot of chemical compounds: olefins43,44, formaldehyde45, acetic acid40,… The last major use of syngas is the production of pure H2 that will be used in fuel cells48. For that purpose, CO has to be removed from the syngas as completely as possible, in order to prevent any poisoning of the cells.
1.4. Fischer-Tropsch (FT) synthesis
1.4.1. A bit of history and present situation
The synthesis of hydrocarbons from CO and H2 was discovered by Sabatier and Senderens in 190249. They performed the CO+H2 reaction over cobalt and nickel-based catalysts and they highlighted the production of methane (CH4).
In 1913, Badische Anilin und Soda Fabrik (BASF) patented a process for the catalytic hydrogenation of CO to give hydrocarbons other than methane, alcohols, ketones and acids50. The catalysts were at this time the metals cerium, cobalt or molybdenum, or their alkali-containing (sodium hydroxide) metallic oxides. The operating conditions were quite harsh: 300-400°C and 120 atm (=12159000 Pa). Because of World War I and priority given to the ammonia and methanol syntheses, BASF never continued the hydrocarbon synthesis50.
Despite the industrial interest of BASF, it is the work of two German chemists, Franz Fischer (1877-1947) and Hans Tropsch (1889-1935) (see their pictures in Figure 1.8), that has lead a few years later to the biggest scientific and commercial milestones for the CO+H2
reaction16,25,50-52. They both worked at the Kaiser Wilhelm (presently Max Planck) Institute for Coal Research (KWI) in Mülheim (Germany). They made coal react with steam to produce the gaseous mixture of CO and H2 (syngas) that they subsequently converted into petroleum- like liquids. Around 1925-1926, they were able to produce by this method hydrocarbon gases (ethane, propane, butane) and liquids (octane, nonane) over cobalt-iron catalysts at 250- 300°C and 1 atm (101325 Pa) with a ratio H2/CO=250,53.
Fig. 1.8: Pictures of Franz Fischer (left) and Hans Tropsch (right)50
In 1934, Ruhrchemie AG acquired the patent rights from Fischer and Tropsch and constructed a CO+H2 pilot plant near Essen in Germany50. It had a daily capacity of 20 barrels of motor gasoline, diesel oil and lubricating oil (as comparison, the present total production of crude oil in 2008 was 81820000 barrels per day!1). In 1935, the Germany’s Nazi government initiated the push for petroleum independence and four commercial-size Ruhrchemie licensed FT plants were under construction. Their total daily capacity was around 2200 barrels of gasoline, diesel and lubricating oil. These plants were working at 1 atm (101325 Pa) or medium pressure (5-15 atm = 506625-1519875 Pa) and at temperatures between 180°C and 200°C. They used a new catalyst developed by Otto Roelen (1897-1993) between 1933 and 1938: 100 Co-5 ThO2-8 MgO-200 kieselguhr (SiO2)32. This catalyst became the standard FT catalyst because of its greater activity and lower reaction temperature. Five additional plants were constructed in Germany and the total capacity was approximately 15000 barrels per day (bpd) at the outbreak of World War II in
September 193950. The production peaked at 11200 bpd in 194450. The price of the synthetic gasoline was more than double the price of imported gasoline but Nazi Germany had no other option during World War II, since all the crude oil importations were blocked. Germany thus utilized its naturally abundant supplies of bituminous and brown coal50. The production of the 9 FT plants fell significantly because of Allied bombing and in March 1945, the production was only 28000 barrels (equivalent to a daily production of 930 barrels)50. After the end of World War II, Germany did not continue the production of synthetic fuel because the Potsdam Conference (16 July 1945) prohibited it. This ban on coal hydrogenation was completely removed in 1951 in an effort to provide employment for several thousand workers. Despite some production after this ban removal, none of the German synthetic fuel plants produces synthetic fuels today50.
During the 1950s there were some unsuccessful attempts in the USA to convert syngas (generated from natural gas) into gasoline17. For example, a FT plant was built in Brownsville (Texas) in 1950 but it operated only briefly as the increase in the price of natural gas made it uneconomic32. In the 1940s, people were already worried about a future depletion of the crude oil reserves and a related increase in the price of fuels. That explained the interest in the FT process. Nevertheless, the price of fuels did not increase as expected because of the discovery of the huge oil deposits in the Middle East32.
The discovery of “cheap” crude oil hindered the development of any FT process in the world, except in South Africa17. The South African large reserves of coal which could be mined at low price led to the construction of their first FT commercial plant in Sasolburg in 1955: SASOL I (South African Synthetic Oil Ltd.)16,17,32. As a result of sharply rising crude oil prices in the 1970s (in particular due to the Yom Kippur war in 1973), SASOL II and III plants came on stream in Secunda in 1980 and 198316,17,32. They produce mainly waxes, gasoline and diesel fuel. The combined production of these two plants is around 52000 bpd. A picture of the Sasol Secunda facility is presented in Figure 1.9, and a Sasol FT reactor is depicted in Figure 1.10 (courtesy of Anne Buchanan and Marsja Hall-Green from Sasol). Up until 2004, the raw syngas for all 3 Sasol plants was produced from coal16. From 2004, the Sasolburg plant (SASOL I) uses methane piped in from Mozambique16.
Fig. 1.9: Picture of the Sasol facility in Secunda (South Africa)
