Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes

UNIVERSITY OF SANTIAGO DE COMPOSTELA SCHOOL OF ENGINEERING (E.T.S.E.) Department of Chemical Engineering DESULFURIZATION OF FUELS WITH IONIC LIQUIDS BY EXTRACTION AND OXIDATIVE EXTRACTION PROCESSES A thesis submitted by BORJA RODRÍGUEZ CABO for the degree of Doctor in Chemical and Environmental Engineering to the University of Santiago de Compostela Santiago de Compostela, November 2013 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes iii Authorization for submission by the thesis directors (in Spanish) Alberto Arce Arce, Catedrático de Ingeniería Química, Ana M. Soto Campos, Catedrática de Ingeniería Química, y Eva Rodíl Rodríguez, Profesora titular de Ingeniería Química INFORMAN: Que la presente memoria titulada “Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes”, que para optar al grado de Doctor en Ingeniería Química, Programa de Doctorado de Ingeniería Química y Ambiental, presenta Borja Rodríguez Cabo, fue realizada bajo nuestra inmediata dirección en el Departamento de Ingeniería Química de la Universidad de Santiago de Compostela. Considerando que constituye trabajo de Tesis, autorizan su presentación al Centro de Postgrado de la Universidad de Santiago de Compostela. Y para que así conste, firman el presente informe en Santiago de Compostela a 4 de Noviembre de 2013 Alberto Arce Arce Ana Soto Campos Eva Rodíl Rodríguez Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes v Acknowledgements Five years. Probably the best five years in my life. And it is mainly due to this thesis work. In the end of 2008 I had the honor of joining a leading research group in its field not only in Spain but also in the world. The Group of Phase Equilibria and Separation Processes from the University of Santiago de Compostela has taken me in, becoming my second family. Thus, I want to express my acknowledgements to all the people that somehow were part of this amazing period. In first place I would like to thank Professors Alberto Arce and Ana Soto. They gave me the great opportunity of being part of this ship they always crew with good judgment and professionalism. When someone teaches you principles like rigor, sacrifice and honesty, you are not only being formed as a good worker, but also as a good person. Moreover, they trusted me to let me help in the organization of the 24th European Symposium on Applied Thermodynamics, what was assumed by me as a challenge and a sign of confidence. Thank you. Of course this acknowledgement is also extended to Eva Rodíl. Thanks to her I had the opportunity to (slightly) go into the nanotechnology world (completely unknown for me until that moment), which is one of the things I appreciate the most of this period. Coffees, advices and confidences don’t need to be mentioned. I especially thank Héctor Rodríguez, who always was there to offer his valuable help and to try to make things work. Thanks to all of them I discovered the ionic liquids, an amazing and interesting field that I already really love. When I arrived to this group I had the pleasure to coincide with María Francisco for more than one year. She took my hand for my first steps in the research world, showing me how to get along in the lab. But during these years I also was lucky to know other many people that have passed through the group: Alfonsina, Yamil, Magda, Kasia, Cristina, Jara, María, Oscar and Alberto. Alicia and Iago, who besides being my thesis-period lab mates, are very good friends. They always cheer the lab up and make the work easier. Elio, Adrián and Iria arrived to the group not too long ago, but we quickly had a very nice harmony that led to a good friendship. And of course, the person who I must strongly thank is Sara. She accompanied me since the beginning, being the best friend, the best workmate. A person who always is happy and makes people around her feel the same. She never let me give up. All the help and good moments shared with the people in the Department are also very valuable for me. I want to thank Susana for her inestimable help and patience with the patent application. Thanks to the RIAIDT staff who perfectly knew the urgency of many analysis and always treated me with generosity. In the last period I also had the opportunity to spend three amazing months in the Delft University of Technology (TU Delft), Netherlands. Professor Andreas Schmidt-Ott allowed me to form part Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes of his excellent group for this period, from which I learned many things. Jicheng and Marco were my best friends and advisors in the group, but I can’t forget Tobias, George, Julien, Luyi, Isaac, Aaike, Anne and Kostis. But the support received during these years does not only come from the work. Thus I want to acknowledge Toño for his understanding and patience, as well as all my friends from Lugo, Santiago, Sanxenxo and Delft. They were able to forgive my absences in many evenings, weekends or holidays. Finally, but very, very important I want to thank my family. An especial mention is referred to my parents. This thesis work is now finished thanks to and for them. It wouldn’t have made sense without them by my side. Closing these lines, I want to express my acknowledge to the University of Santiago de Compostela, and to the Ministry of Economy and Competitiveness of the Spanish Government for financial support through project CQT2012-33359, and for having awarded me with a FPI grant (ref. BES-2010-033448). Thanks to Repsol and CYTEC for kindly supplying the fuel samples and some ionic liquids, respectively. B. Rodríguez Cabo Santiago de Compostela, November 2013. ABSTRACT Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes xi CONTENTS 1. OBJECTIVES.................................................................................................... 1 2. INTRODUCTION ............................................................................................. 5 2.1. IONIC LIQUIDS ........................................................................................ 7 2.2. IONANOFLUIDS..................................................................................... 12 2.3. DESULFURIZATION OF FUEL OILS ......................................................... 14 2.4. IONIC LIQUIDS IN DESULFURIZATION OF FUEL OILS ............................ 19 2.4.1. EXTRACTIVE DESULFURIZATION .................................................... 19 2.4.2. OXIDATIVE DESULFURIZATION ...................................................... 21 3. EXPERIMENTAL ............................................................................................ 25 3.1.1. IONIC LIQUIDS SYNTHESIS ............................................................. 29 3.1.2. IONANOFLUIDS SYNTHESIS ........................................................... 30 3.1.1.1. METHODS ............................................................................... 31 3.1.1.2. NANOPARTICLES CHARACTERIZATION ................................... 31 3.2. EQUIPMENT ......................................................................................... 33 3.3. EXPERIMENTAL PROTOCOL .................................................................. 38 3.3.1. LIQUID-LIQUID EQUILIBRIA ........................................................... 38 3.3.2. DESULFURIZATION OF SYNTHETIC FUELS ..................................... 38 3.3.2.1. EXTRACTIVE DESULFURIZATION ............................................. 40 3.3.2.2. OXIDATIVE DESULFURIZATION ............................................... 41 3.3.3. DESULFURIZATION OF REAL FUELS ............................................... 41 4. RESULTS AND DISCUSSION .......................................................................... 43 4.1. LIQUID-LIQUID EQUILIBRIA .................................................................. 45 4.1.1. CORRELATION ............................................................................... 61 4.2. DESULFURIZATION OF SYNTHETIC FUELS ............................................ 79 4.2.1. EXTRACTIVE DESULFURIZATION .................................................... 79 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 4.2.1.1. SYNTHETIC GASOLINE ............................................................ 79 4.2.1.2. SYNTHETIC DIESEL OIL ........................................................... 81 4.2.2. OXIDATIVE DESULFURIZATION ..................................................... 84 4.2.2.1. SYNTHETIC GASOLINE ............................................................ 84 4.2.2.2. SYNTHETIC DIESEL OIL ........................................................... 84 4.3. DESULFURIZATION OF REAL FUELS ..................................................... 93 4.3.1. EXTRACTIVE DESULFURIZATION ................................................... 93 4.3.1.1. GASOLINE .............................................................................. 93 4.3.1.2. DIESEL OIL .............................................................................. 94 4.3.2. OXIDATIVE DESULFURIZATION ..................................................... 94 4.3.2.1. GASOLINE .............................................................................. 94 4.3.2.2. DIESEL OIL .............................................................................. 94 5. CONCLUSIONS ............................................................................................ 97 LIST OF SYMBOLS .......................................................................................... 103 Symbols ............................................................................................ 105 Greek letters ..................................................................................... 106 Subscripts ......................................................................................... 107 Superscripts ...................................................................................... 107 Symbols on top ................................................................................. 107 REFERENCES ................................................................................................. 109 APPENDIX A (List of publications)................................................................. 127 APPENDIX B (Summary in Spanish, Resumen) ............................................. 131 1. OBJECTIVES 1. Objectives 3 As it is well-known, the growing use of sulfur-containing fuels in the industry and in the transport sector is leading to serious pollution problems. In an effort to invert this tendency, authorities are approving each time more severe directives aimed at obtaining ultralow sulfur fuels. The adaptation of refineries to those requirements implies high operation costs and less safe processes. The main objective of this thesis is to define a system for the desulfurization of fuels by using Ionic Liquids (ILs). In particular, two different operations are going to be studied, the extractive desulfurization and the oxidative desulfurization, so as to look for a suitable solvent and find the optimal operational conditions for the process. The first part of this study is the analysis of the viability of using several ILs for the desulfurization of fuels by liquid-liquid extraction. The objective is to complete a previous work, carried out in our research group, where the IL 1-hexyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide, [C63,5mmpy][NTf2], was proposed for that aim, but there were some outstanding issues. Now, the experiments intend to analyze the effect of the position of the cation substituents on the desulfurization process. Taking this into account, the IL 1-hexyl-2,4-dimethylpyridinium bis(trifluoromethylsulfonyl)imide, [C62,4mmpy][NTf2], having asymmetric substituents, will be tested. Moreover, the IL 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2mim][NTf2], was not previously studied for this purpose despite the fact that it showed preliminary good results. For this reason, this IL will also be evaluated. On the other hand, as no solubility of the IL was observed in the hydrocarbon phase in the extraction process, the presence of sulfur in the anion should not be a problem. Nonetheless, since undetectable traces of the IL could increase the sulfur content of the raffinate, we decided to continue with the study of ILs based on this last cation (cheaper than the pyridinium one) and to avoid the presence of sulfur in the anion. Accordingly, the ILs 1-ethyl-3- methylimidazolium acetate, [C2mim][OAc], and 1-ethyl-3- methylimidazolium diethylphosphate, [C2mim][DEP], will also be assessed. For all chosen solvents, the liquid-liquid equilibria of the systems {IL + hexane + thiophene}, {IL + toluene + thiophene}, and {IL + hexane + pyridine} will be determined. Thermodynamic parameters will be used for the evaluation of the efficacy of the solvents in the desulfurization process, paying attention to parallel and relevant processes that may also take place, such as the denitrogenation or dearomatization of the fuel. The obtained data will be correlated in order to facilitate their use in processes simulation and design. As a following step, the extractive desulfurization (EDS) of synthetic fuels (gasoline and diesel oil) will be carried out in a three step extraction process. The fuels are simulated as a mixture of aliphatics, aromatics, and nitrogen and sulfur containing compounds. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes In the case of the diesel oil model, heavier alkanes and a more refractive sulfur component will be used. Composition of feed and raffinate after each one of the extraction steps will be analyzed with the aim of following the evolution of fuel constituents along the process. In spite of being a forgotten matter in most bibliographic studies, the effect of the desulfurization in all the fuel compounds is a key factor to be considered during the implementation of the process in real refineries. As the next goal, and in order to enhance the desulfurization yield, the possibility of combining the extraction with an oxidation step (oxidative desulfurization process, ODS) will be evaluated. The oxidation of sulfur compounds makes them easier to be extracted. The highly effective AcOH + H2O2, will be tested as an oxocatalytic system. However, another objective of this thesis is to find a method to synthetize stable ionanofluids of metal oxides (nanoparticles of Al2O3, TiO2 and Cr2O3 dispersed in an IL). These oxides can be used as catalytic agents with H2O2, since the specific area of the solid is increased due to the nanometer size, and, thus, its catalytic activity is improved. The best oxocatalytic system will be selected, and an optimization of the oxidative conditions will be carried out. Several parameters, such as the ratios of catalyst and oxidizing agent, the temperature, the mixing technique, or the stirring and settling down times will be studied. Afterwards, the three steps extraction process will be carried out with the optimized oxidesulfurization system and the results will be compared to those obtained for the EDS. Finally, samples of real diesel oil and gasoline will be subjected to the three stages of desulfurization and oxidesulfurization. The evolution of the total sulfur content in the fuel will be determined, to verify the conclusions obtained in the previous stages of development of this thesis. 2. INTRODUCTION Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes - Their vapor pressure is negligible, what confers them a great potential as “green solvents” [15]. Since they have not vapor pressure they are not volatile, and then they do not generate air pollution events. This is, probably, their most important and appreciated characteristic. - ILs also have a wide electrochemical window [16]. - Brønsted, Lewis and Franklin acidity, as well as superacidity, can be found in them [17]. - They are easy to synthesize and most of them easy to handle. - ILs can, depending on their substitutions and functional groups, act not only as solvent media but also as catalytic agents in many processes [18,19], developing a double role. - There is, as previously explained, a huge number of possible ILs in comparison with traditional organic solvents. Moreover, and taking into account the possibility of designing their properties by means of tuning their ions, an appropriate IL can be searched for an specific task. - Miscibility with water depends on their constituent ions. Hydrophobic ILs can be used in biphasic processes as waterimmiscible phase. In general, the hydrophobicity of ILs increases when decreasing the coordination ability of the anion. The toxicity of ILs is also an important factor to consider. Despite the existence of several works warning about their toxicity for aquatic environments and animals [20,21], it is possible to design less toxic and more biodegradable ILs [22,23]. ILs have been even proposed as active pharmaceutical ingredients [24]. The whole of these properties and characteristics make ILs very interesting solvents for the industry. Their absence of atmospheric emissions in comparison to the common volatile organic solvents, and their amazing properties as solvent media caused a great interest in the researching activities world-wide. With this aim, the use of ILs for different processes has been studied during last decades. For example, they have been widely used in electrochemistry: an IL electrolyte was useful to create a lithium ion cell with a good performance and at a level of practical utility [25]. RTILs have also been thought as solvents in lithium batteries [26], showing promising results. The metal and alloy depositions (sometimes in the nanoscale) in ILs [27] or the films deposits of ILs on electrode surfaces [28] have been also reported. Other related usages are as electrolytes in electrochemical synthesis of conducting polymers [29] and electromechanical actuators [30], or as electrochemical sensors to detect fire or ethanol [31,32]. Due to the capacity of ILs to solve polar and non-polar compounds, they were also studied as alternative phase for gas chromatography columns [33,34] and as suitable candidates for replacement of volatile organic solvents in extraction processes. In this sense, ILs can be used for separation of chiral substances, 2. Introduction 11 biological agents or even as able entrainers to break a variety of azeotropic systems [35-37]. The study of ILs for liquid-liquid extraction became necessary and there are, thus, a big number of works dealing with this issue [38-41]. The extraction of metal ions from aqueous solutions was achieved with ILs, showing partitioning values in agreement with traditional liquid-liquid separations [42-44]. Other interesting possible applications studied are: solvents for cellulose [45,46], fluids for the storage of solar thermal energy [47], reprocessing agents for nuclear fuels [48,49], lubricants [50,51], fluids for the synthesis of enhanced thermometers [52], etc. The capture of pollutant gases in ILs has been extensively studied during last years. CO2, SO2 and SH2 are probably de most studied gases [53-56]. The enhanced oil recovery (EOR) by using ILs is also gaining great attention by the researchers [57,58]. Moreover, there is also a huge amount of samples of researching activity with ILs focused on their use as synthesis media [59-61], acting in some cases as catalyst too [62,63]. Nonetheless, and besides the endless researching work, the use of ILs in the industry is still quite limited nowadays. The BASIL [64] and DIFASOL [65] are probably, the best known examples of use of ILs for industrial processes. The former, Biphasic Acid Scavenging utilizing Ionic Liquids of BASF, is used for the production of the generic photoinitiator precursor alkoxyphenylphosphine with an increased yield in comparison with the traditional process. Moreover, the volume of the reactor needed is reduced. The latter, from the Institut Français du Pétrole, uses chloroaluminate (III) ILs for biphasic dimerization reactions, and they achieved with this new procedure a smaller reactor, and a higher yield and dimer selectivity. Moreover, Eastman Chemical Company was using an IL-based process since 1996 for the isomerization of 3,4-epoxybut-1-ene to 2,5-dihydrofuran [66]. Other companies, such as IoLiTec, Air Products or Central Glass Company, also use ILs technology. Recently, Petronas (with the support of QUILL Research Center) has developed an interesting method (HycaPure Hg) for scrubbing toxic and corrosive mercury from natural gas streams by using ILs [67]. Taking into account the unstoppable research in this field it would be logic to think that the list of real industrial applications will carry on growing. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 2.2. IONANOFLUIDS Nanotechnology is a growing and very important researching field. Since the first time this word was used by Taniguchi [68], a remarkable tenacity was put by the scientific community on the development of this area. Nevertheless, the unlimited possibilities of nanotechnology could already be imagined some years before, when Feynman enlightened the world with his famous talk entitled "There's plenty of room at the bottom" [69]. Nanotechnology can be defined, so, as the science to manipulate matter on the molecular or atomic scale [70]. The great importance of working at the nanoscale is that at this level matter can interact in a different way with what surrounds it. This means that the physical and/or chemical properties of materials in the nanoscale are different from those shown in the bulk solids [71]. There is no need to delve into the numerous applications of nanotechnology in the present industry, as varied as: catalysis, electronics, medicine, food, batteries and cells, sports, environmental treatment, cosmetics, textiles or military uses [72-81]. The methods for synthesizing nanomaterials (either nanofilms or nanowires or nanoparticles) are also plentiful and very different. They can be divided in top-down and bottom-up processes. The former (litographic and milling methods) consist in the reduction of the solid to the nanoscale. The "bottom-up" techniques (colloids, sol-gel, epitaxy...) are based on the chemical synthesis and/or the controlled deposition and growing of the material. Among them, the chemical synthesis in liquid phase is probably the most used method and the easiest to upscale. It normally consists in the reaction of two precursors or the reduction of a precursor within a liquid medium, and many kinds of nanomaterials have been synthesized by this method [82-86]. But the preparation of isolated solid materials is not the only objective of nanotechnology. Nanofluids are dispersions of nanomaterials in a conventional base fluid (water, oil, glycol, etc). This term was used for the first time by Choi in 1995 [87], and due to the enhanced properties of this new kind of material, its popularity and interest grew very fast. They are mainly fluids in which certain properties are enhanced because of the nanosolid they contain. The advantages of working with a solid material in the nanosize instead of a bulky solid lie in a better flow and better interaction of the particle with the fluid: the specific area is increased and a physical property can more easily be transmitted if needed [88]. Moreover the size of the particles avoids corrosion phenomena. Nanofluids can be obtained by two different methods. When the nanoparticles are directly synthesized within the fluid which will form the nanofluid we will talk about one-pot synthesis. This technique allows much more stable nanofluids. The other option is to synthesize the nanomaterial and then transfer it to the fluid base. This is called a "two-pot synthesis" method. These special materials are mostly used 2. Introduction 13 as thermal fluids [89-92], but other applications are also found for nanofluids. For example, Zhang [93] dispersed a Mo-S nanocluster in mineral oils for automotive lubrication applications, reducing wear and friction. Huang et al. [94] prepared biocompatible and bioconjugatable magnetite nanofluids which could be applied to several bioapplications including magnetic resonance imaging, highly efficient magnetic bioseparation, or targeted drug delivery. But, could ILs be useful for the synthesis of nanofluids? Actually, they are. These solvents form extent bonding systems in the liquid state and they are highly structured [95]. This allows the obtaining of well-defined and ordered nanostructures. The dispersion of nanostructured materials within an IL as base fluid is an ionanoluid [96]. As it was explained before, the formed nanoparticles can be dispersed in an IL (what would be a two-pot synthesis) or they can be directly formed inside the IL, where they will be stabilized (one-pot method). The main advantages of ionanofluids in comparison with other nanofluids are those related to the important properties of ILs. Thus, dispersions of nanoparticles can be achieved with enhanced applications, due to higher thermal stabilities as well as wider ranges of temperatures in which the nanofluid will be liquid. There is a thriving growth of publications of ionanofluids [97-101], but most of them are related to the dispersion of other-way formed nanoparticles in an IL [97-99]. Just some of them are prepared in the IL itself [100,101], involving a reaction step between precursors. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 2.3. DESULFURIZATION OF FUEL OILS With the industrialization and development of many countries, serious atmospheric pollution episodes started to appear worldwide. It is usual to make the assumption that the only cause of air pollution is big factories, but the truth is that automobile emissions are a big contributor to this phenomenon facing the entire world. With the number of vehicles on the road and the number of vehicle miles traveled, it is needed to control this problem. These "mobile sources" include cars, motorcycles, trucks, buses, non-road recreational vehicles, farm and construction machines, lawn and garden equipment, marine engines, aircraft, locomotives... One of the sadly most known air pollution problems is the acid rain [102]. It is produced when the emissions of SO2 and NOx react with the water molecules in the atmosphere, leading to acidic species. Then, they are spread by the effect of winds for hundreds of miles, and when they reach the land they sink into the soil and go to water sources. This way lakes, streams and aquatic systems become acidic and, thus, toxic for many aquatic animals. But the acid rain also affects forests, where trees are less able to take up water and nutrients, and their leaves and needles are harmed by the acid. Materials and structures are affected by these depositions, which obviously can damage humans. Moreover, sulfur inhibits the use of advanced technology to control total pollutant emissions, including NOx, CO, and particulate matter. All these reasons make evident the necessity of diminishing the sulfur content in fuels, and many countries established directives for this effect. The European Union fixed the maximum content of sulfur in gasoline and gasoil to 10 ppm, starting from 2009 [103], and for non-road mobile machinery, agricultural and forestry tractors and recreational craft from 2011 [104]. As for the Unites States of America, the regulations established a maximum of 15 ppm of sulfur in diesel from 2006 and of 30 ppm for gasoline from 2005 [105]. The current industrial process for the desulfurization of fuels is the hydrodesulfurization (HDS). It consists in the reaction of the hydrocarbons present in the untreated fuel with hydrogen, leading to the cleavage of the C-S bonds. Hydrogen sulfide is formed as byproduct, and normally it is treated to obtain sulfur-free gas (richhydrogen gas) which is used again in the hydrogenolysis. The reaction takes place in a catalytic fixed-bed reactor working at high pressures (20 – 70 atm) and temperatures (between 543.15 and 673.15 K). The catalyst is, typically, a sulfide of molybdenum supported on alumina and promoted by either cobalt or nickel [106]. With this process it is expected to remove the organic sulfur compounds, such as mercaptans, sulfides and disulfides, thiophenes, dibenzothiophenes and their alkyl-substituted derivatives, present in the fuels due to the natural formation of the crude oil. The presence of each compound varies in each type of fuel (the main three transportation fuels are gasoline, diesel and jet fuels), and their reactivity in the HDS process 2. Introduction 15 diminishes when increasing the size of the molecule (Figure 2.3). Thus, the most difficult components to remove by the HDS process are the refractory dibenzothiophenes, especially those with substitutions in the positions 4 and 6. Figure 2.3. Reactivity of the sulfur containing compounds of transportation fuel versus their size. The thiophenic compounds are reactive to the HDS process depending on both the electron structure and the steric hindrance of alkyl groups. The HDS reactivity is higher for thiophene and benzothiophene than for dibenzothiophene due to a higher π-electron density [107,108]. Moreover, the more substituted the rings are, the more difficult to extract, since the steric hindrance is stronger. The sulfur-containing compounds that dominantly remain in the diesel fuels after the hydrodesulfurization are dibenzothiophenes with alkyl substituents at the 4- and/or 6- positions [109-112]. The HDS process implies the denitrogenation of the fuels in a certain extension, too. The hydrogenolysis reaction also transforms N-containing compounds, as pyridine, to alkanes and ammonia. A certain rate of dearomatization also takes place, but this elimination has to be controlled, since the total elimination of aromatic compounds can be bad for the fuel. The adaptation of refineries to the each time more strict legislative requisites is being carried out through a change in their processes. Several desulfurization steps, recycling, more severe pressure and temperature conditions, high hydrogen consumptions... are the pathways to achieve the demanded desulfurization levels. Nonetheless, these proposals imply high operation costs and less safe processes. With the aim of solving these limitations and enhancing the extraction of sulfur from fuels several alternative techniques were Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes studied during last years to replace or to be coupled to the HDS process. Most studied methods are: - Reactive alkylation: if the boiling point of the sulfur compounds is increased, it will be easier to separate these components by distillation. This can be achieved by inserting alkylic groups in the molecule, what is known as reactive alkylation. British Petroleum patented this technology under the olefinic alkylation thiophenic sulfur process (OATS) [113]. This technique allowed obtaining high desulfurization yields with a minimal octane loss [114]. - Precipitation: this technique is based on the formation and later precipitation of insoluble-in-fuel sulfur compounds. The original sulfur derivatives are mixed with a π-acceptor, forming insoluble charge-transfer complexes, which precipitate and are withdrawn [115,116]. Other option is to form insoluble crystalline powders of the sulfur compound by, for example, methylation with iodomethane [117-119]. - Biodesulfurization: it consists in the use of biological agents to metabolize the sulfur compounds and transform them into sulfur-free molecules or easier extractable ones. This technique uses mild conditions and it can be used accompanying other desulfurization methods. Different kinds of microorganisms and/or enzymes can be used for the biodesulfurization of fuels, such as Rhodococcus rhodochrous, Sulfolobus acidocaldarius, Pseudomonas or the catalytic system Dsz protein-flavin reductase [120-123]. - Adsorption: adsorptive techniques are based on the ability of certain solids to adsorb the sulfur containing compounds [124- 126]. This mechanism can be carried out by physical interactions, physisorption, or by means of chemical interactions between sulfur compounds and the adsorbent, chemisorption. In the first case the selectivity of the adsorbent is lower, but they can be more easily recovered. Nevertheless, when chemical interactions take place the behavior is the opposite, and the selectivity and efficiency of the adsorbent is higher bur they are much more difficult to regenerate. - Extractive desulfurization (EDS): the desulfurization of fuels via extraction implies the use of a solvent in which the sulfur compounds are more soluble than in the fuel. Then, they are transferred into the solvent phase, which is immiscible with the fuel. Distillation is used to separate the organosulfur compounds and to recycle the solvent. A general process flow is shown in Figure 2.4 [112]. 2. Introduction 17 Figure 2.4. General process for the EDS of fuels. Many solvents were tested to carry out a high-performance extraction. The solubility of sulfur compounds is not the only important parameter to select the appropriate solvent: its capability to extract the desired compounds and not the rest is also decisive. A lot of organic solvents such as polyalkyleneglycol, acetone or ethers were tested but the performances of the extraction were not satisfactory. Best results when using common organic solvents were achieved by means of mixtures of solvents: acetone–ethanol, tetraethyleneglycol–methoxytriglycol, etc [127,128]. These “solvent cocktails” enhanced the solubility of the sulfur compounds, making easier their extraction. - Oxidative desulfurization (ODS): one way to improve the extraction of the sulfur containing compounds is to oxidize these extracted components and then, the solvent can carry on extracting more sulfur components. This is the basis of the ODS. It is, thus, a complementary technique to adsorption, extraction, thermal decomposition… The selection of a suitable oxidizing agent to convert the sulfur compounds into sulfoxides and then to sulfones (Figure 2.5) is a critical factor. To get the process more efficient, a catalyst for the oxidation reaction will be needed. Figure 2.5. Oxidation of sulfur containing compounds. An important feature for application of the ODS is that its relative efficiency is different from that of HDS. The oxidative fuel S-bearing solvent recovered solvent desulfurized fuel oil oil compounds mixing tank separator distillator thiophene sulfoxide sulfone Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes reactivity seems to increase when the electron density of the sulfur specie is higher [130]: dibenzothiophene > 4,6- dimethyldibenzothiophene > benzothiophene >> thiophene [129-131]. As consequence, refractory sulfur compounds in HDS are easily removed by ODS [132]. The most used oxidant is hydrogen peroxide, H2O2, and the usual catalyst is a carboxylic acid, like acetic or formic acid. These two components form a peroxycarboxylic acid, which is the real responsible for the oxidation [133-136]. But other oxidants and catalysts were also studied, thus leading to different yields of oxidation and to different performances of desulfurization [137-141]. Among all these techniques, EDS and ODS are probably the most studied methods due to, in part, the huge amount of possible solvents for an extraction process, as well as to the effectiveness of combining both methods. Nonetheless, the lack of a solvent with the suitable requirements means that none of these techniques could be an industrial reality nowadays. 2. Introduction 19 2.4. IONIC LIQUIDS IN DESULFURIZATION OF FUEL OILS The research about the use of ILs to desulfurize fuels is quite widespread because of their excellent properties as solvents for extraction purposes, being an alternative to traditional organic solvents. As it will be explained below, they have been widely studied for EDS processes as extracting agents. But moreover, due to the endless possibilities and complexity of ILs, they were also studied for oxidation of sulfur containing compounds, acting in some cases as a solvent that can contain oxidizing and/or catalytic systems within itself, showing a great value for ODS processes. 2.4.1. EXTRACTIVE DESULFURIZATION The use of ILs for the selective extraction of sulfur from fuels was reported for first time by the group of Wassercheid [142] in 2001. Lewis (1-butyl-3-methylimidazolium chloride with AlCl3) and Brønsted (a mixture of diethyl(hexyl)ammonium methanesulfonate and tributylammonium methanesulfonate) acidic ILs were studied and compared to a neutral one (1-butyl-3-methylimidazolium octylsulfate). The Lewis-acidic IL showed a much higher efficiency in sulfur extraction from real diesel than the other two. The effect of the extraction of dibenzothiophene was analyzed using the same acidic IL but with FeCl3 instead of AlCl3 [143]. The amount of dibenzothiophene extracted increased with an increasing molar ratio of FeCl3/IL. The high extraction ability of these ILs was suggested to be due to the πcomplexation bonding of these species with the aromatic sulfur compounds. Despite the good desulfurization performances, these ILs are sensitive to water [144] and they can generate HCl, being unstable in air and not applicable to real processes. Zhang and coworkers [145] used imidazolium ILs with hexafluorophosphate and tetrafluroborate anions. They found that a butyl chain in the cation enhanced the desulfurization in comparison to an ethyl one, obtaining best results when using the hexafluorophosphate anion. They also observed that the sulfur extraction is higher when the density of aromatic π electrons is increased. Other authors worked with this kind of ILs [146,147] obtaining promising results. Once more it was checked that the desulfurization rate increased with the length of the alkyl chain of the cation, but this time for pyridinium-based ILs [148]. This work also revealed the negligible solubility of these ILs in the fuel, and that the extraction is favored for sulfur compounds with higher density of aromatic π-electrons. The extraction mechanism was postulated to be due to the π-π interactions between the aromatic sulfur compounds and the pyridinium ring of the IL. Nevertheless, these anions imply the possible formation of their hydrolysis products such as HF [149], which is observed in presence of water at moderate temperatures. 3. EXPERIMENTAL 3. Experimental 27 3.1. MATERIALS For the fuel models, hexane (>99 wt%), heptane (>99.5 wt%), isooctane (≥99.5 wt%), dodecane (>99 wt%), hexadecane (99 wt%), toluene (≥99.5 wt%), thiophene (>99 wt%) and dibenzothiophene (98 wt%) were purchased from Sigma-Aldrich and used without purification. Pyridine was obtained from Riedel-de Haën (≥99.5 wt%). Samples of real gasoline and diesel oil were kindly supplied by Repsol. For the oxidation of the sulfur compounds, hydrogen peroxide (30 wt%), acetic acid (≥99 wt%), anatase TiO2 (99.8 wt%), Cr2O3 (≥98 wt%) and Al2O3 (99.7 wt%) were purchased from Sigma-Aldrich and used without purification. The structure of the four ILs used in this work is showed in Figure 3.1. The IL [C2mim][OAc] was purchased from Iolitec (>95 wt%) and purified under vacuum (5·10-3 mbar) for 48 h at 343.15 K. The ILs [C62,4mmpy][NTf2], [C2mim][NTf2] and [C2mim][DEP] were synthesized in our laboratory. Figure 3.1. Studied ILs. Upper row: [C62,4mmpy][NTf2] (left) and [C2mim][NTf2] (right). Lower row: [C2mim][OAc] (left) and [C2mim][DEP] (right). The materials needed for the synthesis of the ILs were purchased from Sigma-Aldrich: 1-bromohexane (98 wt%), 1-bromoethane (≥99 wt%), acetonitrile (99.8 wt%), ethyl acetate (99.7 wt%), 2,4-lutidine (99 wt%), triethylphosphate (≥99.8 wt%) and 1-methylimidazole (≥99 wt%). Diethylether (99.5 wt%) was acquired from Panreac. The salt lithium bis(trifluoromethylsulfonyl)imide (99.9 wt%) was acquired from Solvionic. The synthesis of each IL is detailed below. The purity of all ILs, commercial and synthesized, was verified by 1H NMR and 13C NMR spectroscopy. The water content of the pure components, along with their experimental densities and refractive indices are showed in Table 3.1 and compared to bibliographic data [202-212]. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 28 Table 3.1 CAS-register numbers, water content (ω H2O ), densities (ρ) and refractive indices (n D ) of the pure components at 298.15 K and atmospheric pressure Component CAS ω H2O (ppm) ρ (g cm -3 )n D Exp. Lit. Exp. Lit. Hexane 110-54-3 40 0.65511 0.65484 [202] 1.37243 1.37226 [202] Heptane 142-82-5 5 0.67947 0.67946 [202] 1.38517 1.38511 [202] iso -Octane 540-84-1 5 0.68784 0.68778 [203] 1.38921 1.3895 [203] Dodecane 112-40-3 31 0.74532 0.74518 [202] 1.42011 1.41952 [202] Hexadecane 544-76-3 7 0.77115 0.77003 [203] 1.43232 1.4325 [203] Toluene 108-88-3 18 0.86215 0.86219 [203] 1.49352 1.49413 [203] Thiophene 110-02-1 257 1.05858 1.05887 [202] 1.52519 1.52572 [202] Pyridine 110-86-1 358 0.97811 0.97824 [202] 1.50663 1.50745 [202] Acetic acid 64-19-7 440 1.04397 1.0446 [206] 1.37072 1.3720 [206] [C 62,4 mmpy][NTf 2 ]Not found 285 1.33939 Not found 1.45336 Not found [C 2 mim][NTf 2 ]174899-82-2 115 1.51842 1.518 [207] 1.42306 1.42251 [208] [C 2 mim][OAc] 143314-17-4 1300 1.09902 1.0993 [209] 1.50069 1.4992 [210]* [C 2 mim][DEP] 848641-69-0 215 1.14840 1.1464 [211] 1.47304 1.4789 [212] *Interpolated value 3. Experimental 29 3.1.1. IONIC LIQUIDS SYNTHESIS − 1-hexyl-2,4-dimethylpyridinium bis(trifluoromethylsulfonyl)imide, [C62,4mmpy][NTf2]. Following the procedure previously published [156], 1-bromohexane was reacted with 2,4-lutidine (1.05:1 in mass) for 48 hours at 353.15 K, as explained in Figure 3.2. Some ml of acetonitrile were added to the resulting brown solid, and it was washed several times with ethyl acetate. Then the mixture was left to separate in a decanting funnel and the excess of solvents were removed by a rotary evaporator firstly, and by high vacuum later. Figure 3.2. Alkylation of the pyridinium ring. In a second step (see Figure 3.3), the formed IL, 1-hexyl-2,4- dimethylpyridinium bromide, [C62,4mmpy]Br, was reacted with lithium bis(trifluoromethylsulfonyl)imide (1:1.02 in mass). For that purpose, both solids were solved in distilled water and then the two aqueous solutions were mixed for 12 hours at room temperature. Figure 3.3. Metathesis with Li[NTf2] salt. The obtained IL was washed several times with fresh distilled water and separated by decanting. The aqueous phase contained the salt excess and the LiBr by-product. The water content in the IL was decreased under 300 ppm by using a rotary evaporator and afterwards high vacuum (5·10-3 mbar) for 48 hours at 343.15 K. − Synthesis of 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2mim][NTf2]. The synthesis of this IL (Figure 3.4) follows a classical metathesis reaction [9] similar to the above-explained one. 1-Bromoethane was reacted for 48 hours with 1-methylimidazole (mass ratio of 1.05:1) at 313.15 K. The obtained IL was then purified in the rotary evaporator and solved in water to be reacted with an aqueous solution of lithium Li + +N - SS O O OO FF F F FFN - SS O O OO FF F F FF +Li + Br - Br - N + CH 3 CH 3 CH 3 N + CH 3 CH 3 CH 3 R. T., 12 h + Br - CH 3 Br N CH 3 CH 3 N + CH 3 CH 3 CH 3 353.15 K, 48 h Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 30 bis(trifluoromethylsulfonyl)imide (1:1.02 in mass) for 12 hours at room temperature. Figure 3.4. Steps for the synthesis of the IL [C2mim][NTf2]. Afterwards the IL was washed with water several times and separated in a decanting funnel. As in the previous case, the LiBr byproduct and the excess of lithium bis(trifluoromethylsulfonyl)imide were eliminated with the aqueous phase. A rotary evaporator and high vacuum (5·10-3 mbar) for 48 hours at 343.15 K were used to purify the obtained IL. − Synthesis of 1-ethyl-3-methylimidazolium diethylphosphate, [C2mim][DEP]. In this case, the 1-methylimidazole was reacted for 3 days at 393.15 K with triethylphosphate (mass ratio of 1:1), as described by Kuhlmann et al. [213]. The reaction is showed in Figure 3.5. Figure 3.5. Synthesis of the IL [C2mim][DEP]. The resulting IL was washed with diethylether for three times to remove the possible excess of reactants. After extraction, water content of the IL was decreased by using a rotary evaporator and a further purification under high vacuum (5·10-3 mbar) for 48 h at 343.15 K. 3.1.2. IONANOFLUIDS SYNTHESIS Ionanofluids of titanium dioxide (TiO2), aluminum oxide (Al2O3) and chromium oxide (Cr2O3) in the IL ([C62,4mmpy][NTf2]) were prepared. 313.15 K, 48 h R. T., 12 h N N CH 3 + CH 3 Br N N CH 3 CH 3 + Br - N + N CH 3 CH 3 + Br - Li + +N - SS O O OO FF F F FF N - SS O O OO FF F F FF N N CH 3 CH 3 + +Li + Br - 393.15 K, 72 h N N CH 3 + N N CH 3 CH 3 + P O O O O CH 3 CH 3 CH 3 P O O - O O CH 3 CH 3 3. Experimental 31 3.1.1.1. METHODS a) Disolution-reprecipitation method This method, developed in this thesis and recently patented [214], does not need any volatile solvent or solid precursor, avoiding air pollution and the formation of by-products. Thus, the technique permits a “greener fabrication” of nanoparticles/nanofluids. Moreover, high temperatures or pressures are not needed, and the method is quite simple. The bulk powder was added to the IL (1 wt%) in a roundbottomed flask at room temperature, and the reaction mixture was heated at 393.15 K and vigorously stirred for 4 h in an argon atmosphere (Figure 3.6). Figure 3.6. Procedure for the formation of the ionanofluids (dissolutionreprecipitation method). The resulting colored mixture was allowed to cool down to room temperature, and then it was centrifuged at 4000 rpm for 15 min, to separate the excess bulk solid from the nanodispersion. b) Spark discharge method The other method used in the preperation of the ionanofluid was the spark discharge, introduced in 1988 [215] and widely studied by Schmidt-Ott group [216,217]. A spark was generated between two electrodes of the target material within an inert gas flow. This spark breakdowns the gas (nitrogen) and generates a plasma channel. So, the electrode material evaporates in the vicinity of the spark, obtaining primary particles in the vapor phase. This gas flow was then mixed with an air current to obtain the oxide of the metal. Then, the gas containing the formed nanoparticles was bubbled into the IL so that the solid was retained transferred to it. 3.1.1.2. NANOPARTICLES CHARACTERIZATION The optical properties of the synthesized nanoparticles were measured by UV–Vis absorption spectroscopy. An aliquot taken from the nanodispersion was dissolved in acetone. The blank consisted in the same volume of pure IL also dispersed in acetone. Each sample was measured after having done a wavelength screening between 200 and 1000 nm, to obtain the maximum absorption peak. The distributions of sizes of the nanoparticles were determined by the Dynamic Light Scattering (DLS) technique. The suitable parameters (viscosity, absorption, dielectric constant, and refractive index) were needed for the nanomaterial and dispersant. Histograms with the distributions of sizes were recorded. Moreover, in order to know the approximate Bulk solid Ionic liquid Stirring(4h) Heating Dispersion mixture Centrifugation + Nanodispersion Excess solid + (393,15 K) Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 38 3.3. EXPERIMENTAL PROTOCOL 3.3.1. LIQUID-LIQUID EQUILIBRIA To determine the experimental tie-lines for each ternary system, mixtures of their components in the immiscible region were mixed in 30 mL glass jacketed vessels (Figure 3.17). The temperature was kept constant at 298.15 K by means of a Selecta Ultraterm 200 thermostatic bath. The mixtures were magnetically stirred for 2 h, and then settled down for a minimum of 4 h. These times were enough to ensure a good mixture degree and the posterior achievement of equilibrium. Figure 3.17. View of a jacketed cell (left), and different cells connected to the thermostatic bath (right). Then, samples from both layers were withdrawn with syringes. Compositions of hydrocarbons and nitrogen or sulfur compounds were determined by gas chromatography. The IL composition was calculated by difference. The operational conditions for the analysis are shown in Table 3.2. 3.3.2. DESULFURIZATION OF SYNTHETIC FUELS In order to simulate the behavior of real fuels, gasoline and diesel oil models were prepared and used to study the extraction of sulfur compounds with ILs. The synthetic gasoline is composed by hexane (26 wt%), heptane (26 wt%), isooctane (26 wt%), toluene (10% wt%), pyridine (6 wt%) and thiophene (6 wt%). In the case of the diesel oil model, the alkanes become heavier and the sulfur proportion in the fuel is shared with a more refractive compound, resulting the following mixture: heptane (26 wt%), dodecane (26 wt%), hexadecane (26 wt%), toluene (10 wt%), pyridine (6 wt%), thiophene (3 wt%) and dibenzothiophene (3 wt%). 3. Experimental 39 Table 3.2 Gas chromatograph operational conditions. Column SPB TM -1 SULFUR (30m x0.32mm x4µm) Injection volume 4 µL Carrier gas Helium Liquid -Liquid Equilibria Flow rate 1.5 ml·min -1 for 14 min Oven temperature 373 K for 10 min 473.15 K (50 K·min -1 ) for 2 min Injector temperature 473.15 K Split ratio 75:1 Detector temperature 548.15.15 K Detector FID Synthetic gasoline Flow rate 4 ml·min -1 for 8 min 6 ml·min -1 (1 ml·min -1 ) for 4 min Oven temperature 343.15 K for 9 min 553.15 K (100 K·min -1 ) for 2 min Injector temperature 548.15 K Split ratio 25:1 Detector temperature 523.15 K Detector FID Synthetic diesel oil Flow rate 4 ml·min -1 for 14 min Oven temperature 413.15 K for 4 min 553.15 K (100 K·min -1 ) for 8 min Injector temperature 548.15 K Split ratio 25:1 Detector temperature 523.15 K Detector FID and FPD Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 40 3.3.2.1. EXTRACTIVE DESULFURIZATION With the aim of studying the EDS of the fuel models with ILs, three extraction steps were carried out. The synthetic fuel was added to a thermostated cell (at 298.15 K) with the corresponding IL. The proportion was 1:1 in weight. The mixture was stirred for 12 hours (Figure 3.18) to ensure equilibrium conditions between both phases. Then, the stirring was stopped and the phases were left to settle down. After 12 hours of separation, a sample from the model (upper phase) was withdrawn and analyzed by GC. The IL was removed, and the rest of the organic phase was stirred with fresh IL for the second extraction step, and later the process was repeated for the third one (see Figure 3.19). This protocol was followed for both synthetic gasoline and diesel oil. Figure 3.18. Jacketed cell with IL and synthetic fuel. Figure 3.19. Three steps EDS process. Solvent: IONIC LIQUID Extract 1 Raffinate 1 Extract 2 Raffinate 3 Raffinate2 Diesel: 26% heptane 26% dodecane 26% hexadecane 10% toluene 3% thiophene 3% dibenzothiophene 6% pyridine Gasoline: 26% hexane 26% heptane 26% isooctane 10% toluene 6% thiophene 6% pyridine Synthetic Fuels Extract 3 RECOVERY 3. Experimental 41 3.3.2.2. OXIDATIVE DESULFURIZATION Regarding the ODS experiments, firstly an optimization of the parameters in the oxidation process was carried out. For this optimization, the IL [C62,4mmpy][NTf2] was used, due to the high solubility of sulfur compounds in it. Hydrogen peroxide was selected as oxidizing agent, and the use of two possible catalysts was analyzed: acetic acid (AcOH) and metal oxide nanoparticles (TiO2, Al2O3 or Cr2O3). With the former, different molar ratios of acetic acid and of H2O2 to the sulfur content in the synthetic oil were tested. In the case of the metal oxide nanoparticles, their concentration in the IL was the maximum allowed to have a stable ionanofluid. Once the best oxidizing-catalytic system was selected, an optimization of the most suitable temperature for the ODS process was carried out. The influence of a one-pot operation (IL, model oil, oxidant, and catalyst added all together) versus a two-step operation (first mixing the IL and the model oil to carry out the extraction, and then adding the oxidant and the catalyst) was also studied. Finally, the stirring and settling down times for equilibration and phases separation were optimized by analyzing (by gas chromatography) the evolution of the sulfur components in the raffinate at different times. With the optimized oxocatalytic system, operation mode, temperature, and stirring and settling down times for the ODS procedure, a protocol similar to that described previously for EDS was carried out. 3.3.3. DESULFURIZATION OF REAL FUELS EDS and ODS processes carried out with synthetic fuels were subsequently repeated with real gasoline and diesel oil. In this case, due to the large number of components in the real fuels, the oil-rich phases were studied using a total sulfur analyzer (as it is usually done in refineries), following the decrease of sulfur in the fuel. 4. RESULTS AND DISCUSSION 4.Results and discussion 45 4.1. LIQUID-LIQUID EQUILIBRIA LLE data for the ternary systems consisting in {IL + thiophene + hexane}, {IL + thiophene + toluene}, and {IL + thiophene + pyridine} were obtained at 298.15 K and atmospheric pressure. Tables 4.1 to 4.12 show the compositions of the tie-lines ends for each IL and each system. The corresponding triangular diagrams are shown in Figures 4.1 to 4.12. From the experimental LLE data, the solute distribution ratio, β, and selectivity, S, were calculated for each tie-line as follows: 𝛽=𝑥2 𝐼𝐼 𝑥2 𝐼 (1) 𝑆= 𝑥2 𝐼𝐼 𝑥2 𝐼·𝑥1 𝐼 𝑥1 𝐼𝐼 (2) where x is the mole fraction. Subscripts 1 and 2 indicate hydrocarbon (inert) and thiophene or pyridine (solute). Superscripts I and II refer to the hydrocarbon-rich phase and the IL-rich phase, respectively. These parameters are related to the amount of solvent needed for the separation (solute distribution ratio) and to the number of needed stages to carry it out (selectivity). Thus, they are decisive parameters to determine the suitability of a solvent as extracting agent. Tables 4.1 to 4.12 also show the β and S values obtained for the LLE data of studied ternary systems. These values are plotted versus the solute molar fraction in the hydrocarbon-rich phase in Figures 4.1 to 4.12. As it can be seen (Figures 4.1, 4.4, 4.7 and 4.10) the systems containing thiophene and hexane show a Type II behavior, with two pairs of partially immiscible compounds and one biphasic region. The slope of the tie-lines in the lowest part of the diagram (interesting region for practical applications) is positive, but this tendency changes in the upper part of the ternary diagram, showing a solutropic behavior. In the case of the systems {IL+ thiophene + toluene} the diagrams are also Type II, as it is showed in Figures 4.2, 4.5, 4.8 and 4.11. The slope of tie-lines are negative through all the diagram. Nevertheless, when studying the IL [C2mim][DEP] (Figure 4.11) it can be seen that the slope of the tie-lines is higher than the unity for the lower part of the diagram, and it becomes negative when increasing the fraction of thiophene (solutropic behavior). For the systems with pyridine, the obtained ternary diagrams show a Type I behavior, with only one pair of components partially immiscible and one immiscible region (see Figures 4.3, 4.6, 4.9 and 4.12). All the systems have positive slopes in all the range of compositions, except in the case of the [C2mim][OAc] IL, whose diagram presents a solutropic behavior and the slope of the tie-lines becomes negative in the upper region. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 46 Table 4.1 Experimental LLE data for the ternary system {[C62,4mmpy][NTf2] (1) + thiophene (2) + hexane (3)} at 298.15 K and atmospheric pressure. Hydrocarbon-rich phase IL-rich phase β S x1I x2I x3I x1II x2II x3II 0.000 0.000 1.000 0.833 0.000 0.167 0.000 0.039 0.961 0.704 0.140 0.156 3.59 22.1 0.000 0.072 0.928 0.627 0.230 0.143 3.19 20.7 0.000 0.167 0.833 0.464 0.402 0.134 2.41 15.0 0.000 0.273 0.727 0.366 0.515 0.119 1.89 11.5 0.000 0.414 0.586 0.290 0.608 0.102 1.47 8.44 0.000 0.481 0.519 0.261 0.636 0.103 1.32 6.66 0.000 0.568 0.432 0.225 0.670 0.105 1.18 4.85 0.000 0.655 0.345 0.202 0.702 0.096 1.07 3.85 0.000 0.745 0.255 0.172 0.743 0.085 1.00 2.99 0.000 0.892 0.108 0.135 0.815 0.050 0.91 1.97 0.000 1.000 0.000 0.108 0.892 0.000 0.89 Standard uncertainties: υ(p) = 5 kPa; υ(T) = 0.05 K; υ(xI) = 0.003; υ(xII) = 0.005. Figure 4.1. Down: Experimental (●, solid line) and correlated with UNIQUAC model (○, dotted line) tie-lines for the LLE of the ternary system {[C62,4mmpy][NTf2] + thiophene + hexane} at 298.15 K and atmospheric pressure. Up: Experimental (●) and correlated (dotted line) solute distribution ratios (β ) and selectivities (S). [C62,4mmpy][NTf2] 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Thiophene 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Hexane 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 x2I 0,0 0,2 0,4 0,6 0,8 1,0 S 0 10 20 30 x2I 0,0 0,2 0,4 0,6 0,8 1,0 β 0 1 2 3 4 4.Results and discussion 47 Table 4.2 Experimental LLE data for the ternary system {[C62,4mmpy][NTf2] (1) + thiophene (2) + toluene (3)} at 298.15 K and atmospheric pressure. Hydrocarbon-rich phase IL-rich phase β S x1I x2I x3I x1II x2II x3II 0.000 0.000 1.000 0.167 0.000 0.833 0.000 0.031 0.969 0.169 0.030 0.801 0.97 1.17 0.000 0.093 0.907 0.181 0.088 0.731 0.95 1.18 0.000 0.180 0.820 0.166 0.172 0.662 0.96 1.18 0.000 0.282 0.718 0.161 0.265 0.574 0.94 1.18 0.000 0.411 0.589 0.152 0.380 0.468 0.92 1.16 0.000 0.523 0.477 0.137 0.484 0.379 0.92 1.16 0.000 0.570 0.430 0.138 0.522 0.340 0.92 1.16 0.000 0.669 0.331 0.131 0.610 0.259 0.91 1.17 0.000 0.766 0.234 0.124 0.692 0.184 0.90 1.15 0.000 0.847 0.153 0.116 0.762 0.122 0.90 1.13 0.000 0.913 0.087 0.114 0.818 0.068 0.90 1.15 0.000 0.956 0.044 0.115 0.850 0.035 0.89 1.12 0.000 1.000 0.000 0.108 0.892 0.000 0.89 Standard uncertainties: υ(p) = 5 kPa; υ(T) = 0.05 K; υ(xI) = 0.002; υ(xII) = 0.003. Figure 4.2. Down: Experimental (●, solid line) and correlated with UNIQUAC model (○, dotted line) tie-lines for the LLE of the ternary system {[C62,4mmpy][NTf2] + thiophene + toluene} at 298.15 K and atmospheric pressure. Up: Experimental (●) and correlated (dotted line) solute distribution ratios (β ) and selectivities (S). 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Thiophene 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Toluene 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 [C62,4mmpy][NTf2] x2I 0,0 0,2 0,4 0,6 0,8 1,0 β 0,0 0,5 1,0 1,5 2,0 x2I 0,0 0,2 0,4 0,6 0,8 1,0 S 0,0 0,5 1,0 1,5 2,0 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 54 Table 4.9 Experimental LLE data for the ternary system {[C2mim][OAc] (1) + pyridine (2) + hexane (3)} at 298.15 K and atmospheric pressure. Hydrocarbon-rich phase IL-rich phase β S x1 I x2 I x3 I x1 II x2 II x3 II 0.000 0.000 1.000 0.998 0.000 0.002 0.000 0.051 0.949 0.889 0.108 0.003 2.12 670 0.000 0.129 0.871 0.737 0.252 0.011 1.95 155 0.000 0.204 0.796 0.658 0.332 0.010 1.63 130 0.000 0.325 0.675 0.503 0.482 0.015 1.48 66.7 0.000 0.454 0.546 0.426 0.551 0.023 1.21 28.8 0.000 0.618 0.382 0.342 0.626 0.032 1.01 12.1 0.000 0.726 0.274 0.312 0.654 0.034 0.90 7.26 0.000 0.798 0.202 0.253 0.712 0.035 0.89 5.16 Standard uncertainties: υ(p) = 5 kPa; υ(T) = 0.05 K; υ(xI) = 0.006; υ(xII) = 0.007. Figure 4.9. Down: Experimental (●, solid line) and correlated with UNIQUAC model (○, dotted line) tie-lines for the LLE of the ternary system {[C2mim][OAc] + pyridine + hexane} at 298.15 K and atmospheric pressure. Up: Experimental (●) and correlated (dotted line) solute distribution ratios (β ) and selectivities (S). [C2mim][OAc] 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Pyridine 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Hexane 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 S 0 200 400 600 800 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 β 0,0 0,5 1,0 1,5 2,0 2,5 4.Results and discussion 55 Table 4.10 Experimental LLE data for the ternary system {[C2mim][DEP] (1) + thiophene (2) + hexane (3)} at 298.15 K and atmospheric pressure. Hydrocarbon-rich phase IL-rich phase β S x1 I x2 I x3 I x1 II x2 II x3 II 0.000 0.000 1.000 0.942 0.000 0.058 0.000 0.016 0.984 0.896 0.046 0.058 2.88 48.8 0.000 0.090 0.910 0.735 0.210 0.055 2.33 38.6 0.000 0.168 0.832 0.611 0.340 0.049 2.02 34.4 0.000 0.304 0.696 0.506 0.450 0.044 1.48 23.4 0.000 0.431 0.569 0.405 0.540 0.055 1.25 13.0 0.000 0.608 0.392 0.332 0.617 0.051 1.01 7.80 0.000 0.710 0.290 0.269 0.678 0.053 0.95 5.23 0.000 0.804 0.196 0.239 0.712 0.049 0.89 3.54 0.000 0.917 0.083 0.166 0.802 0.032 0.88 2.27 0.000 1.000 0.000 0.116 0.884 0.000 0.87 Standard uncertainties: υ(p) = 5 kPa; υ(T) = 0.05 K; υ(xI) = 0.004; υ(xII) = 0.007. Figure 4.10. Down: Experimental (●, solid line) and correlated with UNIQUAC model (○, dotted line) tie-lines for the LLE of the ternary system {[C2mim][DEP] + thiophene + hexane} at 298.15 K and atmospheric pressure. Up: Experimental (●) and correlated (dotted line) solute distribution ratios (β ) and selectivities (S). [C2mim][DEP] 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Thiophene 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Hexane 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 S 0 10 20 30 40 50 60 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 β 0 1 2 3 4 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 56 Table 4.11 Experimental LLE data for the ternary system {[C2mim][DEP] (1) + thiophene (2) + toluene (3)} at 298.15 K and atmospheric pressure. Hydrocarbon-rich phase IL-rich phase β S x1 I x2 I x3 I x1 II x2 II x3 II 0.000 0.000 1.000 0.471 0.000 0.529 0.000 0.112 0.888 0.420 0.123 0.457 1.10 2.14 0.000 0.226 0.774 0.385 0.217 0.398 0.96 1.87 0.000 0.311 0.689 0.360 0.291 0.349 0.94 1.85 0.000 0.452 0.548 0.309 0.409 0.282 0.91 1.76 0.000 0.597 0.403 0.270 0.511 0.219 0.86 1.58 0.000 0.712 0.288 0.236 0.603 0.161 0.85 1.51 0.000 0.870 0.130 0.191 0.724 0.085 0.83 1.27 0.000 1.000 0.000 0.134 0.866 0.000 0.87 Standard uncertainties: υ(p) = 5 kPa; υ(T) = 0.05 K; υ(xI) = 0.004; υ(xII) = 0.007. Figure 4.11. Down: Experimental (●, solid line) and correlated with UNIQUAC model (○, dotted line) tie-lines for the LLE of the ternary system {[C2mim][DEP] + thiophene + toluene} at 298.15 K and atmospheric pressure. Up: Experimental (●) and correlated (dotted line) solute distribution ratios (β ) and selectivities (S). [C2mim][DEP] 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Thiophene 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Toluene 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 S 0,5 1,0 1,5 2,0 2,5 3,0 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 β 0,0 0,5 1,0 1,5 2,0 4.Results and discussion 57 Table 4.12 Experimental LLE data for the ternary system {[C2mim][DEP] (1) + pyridine (2) + hexane (3)} at 298.15 K and atmospheric pressure. Hydrocarbon-rich phase IL-rich phase β S x1 I x2 I x3 I x1 II x2 II x3 II 0.000 0.000 1.000 0.940 0.000 0.060 0.000 0.020 0.980 0.851 0.100 0.049 5.13 103 0.000 0.049 0.951 0.758 0.196 0.046 4.00 82.7 0.000 0.084 0.916 0.671 0.274 0.055 3.26 54.3 0.000 0.143 0.857 0.552 0.397 0.051 2.78 46.7 0.000 0.274 0.726 0.420 0.529 0.051 1.93 27.5 0.000 0.322 0.678 0.377 0.572 0.051 1.78 23.6 0.003 0.427 0.570 0.299 0.650 0.051 1.52 17.0 0.000 0.555 0.445 0.209 0.706 0.085 1.27 6.66 Standard uncertainties: υ(p) = 5 kPa; υ(T) = 0.05 K; υ(xI) = 0.006; υ(xII) = 0.007. Figure 4.12. Down: Experimental (●, solid line) and correlated with NRTL model and α=0.1 (○, dotted line) tie-lines for the LLE of the ternary system {[C2mim][DEP] + pyridine + hexane} at 298.15 K and atmospheric pressure. Up: Experimental (●) and correlated (dotted line) solute distribution ratios (β ) and selectivities (S). [C2mim][DEP] 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Pyridine 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Hexane 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 β 1 2 3 4 5 6 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 S 0 20 40 60 80 100 120 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 58 Practical applications of desulfurization process correspond to lowest sulfur concentrations. Taking into account the slopes of the tielines in the lowest part of the diagrams, thermodynamic parameters for all the tested ILs indicate a favorable separation of thiophene and pyridine from hexane. Nonetheless, separation of the sulfur compound from toluene becomes more difficult. A better comparison among solvents can be carried out through solute distribution ratios, β , and selectivities, S. Figure 4.13. β and S values, as a function of the solute concentration in the hydrocarbon-rich phase, for the ternary systems {[C6mmpy][NTf2] + hexane + thiophene} (up), {[C6mmpy][NTf2] + toluene + thiophene} (middle) and {[C6mmpy][NTf2] + hexane + pyridine} (down). The compared ILs are [C62,4mmpy][NTf2] (●) and [C63,5mmpy][NTf2] (). IL + hexane + thiophene IL + toluene + thiopheneIL + hexane + pyridine 4.Results and discussion 59 As the first goal of this work is to analyze the effect of the position of the cation substituents on the desulfurization process, a comparison between the results obtained with the previously studied 1-hexyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide IL [156], [C63,5mmpy][NTf2], and with the 1-hexyl-2,4-dimethylpyridinium bis(trifluoromethylsulfonyl)imide, [C62,4mmpy][NTf2], is going to be carried out. Figure 4.13 shows β and S, as a function of the solute concentration in the hydrocarbon-rich phase, for the {[C6mmpy][NTf2] + thiophene + hexane}, {[C6mmpy][NTf2] + thiophene + toluene}, and {[C6mmpy][NTf2] + thiophene + pyridine} ternary systems. It can be seen that values and trends of β and S are really similar for both ILs. This figure demonstrates that the separation of thiophene from hexane with these ILs is favorable in terms of solubilities and selectivities, especially at low mole fractions of thiophene. The values of this last parameter are slightly higher in the case of [C62,4mmpy][NTf2]. In the separation of thiophene from toluene, also slightly higher values of β and S were found in the case of IL with methyls asymmetrically placed in the pyridinium cation ring. Nonetheless, the low values of these parameters indicate that desulfurization would be accompanied with a certain degree of aromatic compound removal. Figure 4.13 also shows that with desulfurization a good denitrogenation would be achieved, with no appreciable differences between both ILs. In Figures 4.14 to 4.16, β and S values can be observed as a function of the solute concentration in the hydrocarbon-rich phase for the different ternary systems and the four ILs studied in this work. When studying the solubilities and selectivities of the systems with hexane and thiophene (Figure 4.14) it can be noticed that they are higher than the unity for the four ILs, what means they are good candidates for desulfurization. The ranking of solubilities is [C62,4mmpy][NTf2] > [C2mim][DEP] > [C2mim][NTf2] > [C2mim][OAc]. This results agree with the establishments done by Holbrey et al. [161], which state the dialkylpyridinium as the best solvents to extract sulfur compounds. Taking into account that the IL could be recovered, the selectivity becomes a decisive parameter to the evaluation of the ILs as solvents for the extraction. This parameter is related to the ability of extracting the target (sulfur) compound while keeping the other fuel components. For the extraction of thiophene from hexane the ranking of selectivities is [C2mim][OAc] > [C2mim][NTf2] ≈ [C2mim][DEP] > [C62,4mmpy][NTf2]. The best selectivity values are obtained for the IL [C2mim][OAc] while the pyridinium IL achieves the worst results. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 60 Figure 4.14. β (left) and S (right) values, as a function of the solute concentration in the hydrocarbon-rich phase, for the ternary system {IL + thiophene + hexane} at 298.15 K and atmospheric pressure. The compared ILs are: [C62,4mmpy][NTf2] (●, solid line), [C2mim][NTf2] (○, dotted line), [C2mim][OAc] (▼, dashed line), and [C2mim][DEP] ( , dash-dotted line). For the separation of thiophene from toluene, the solubilities ranking is [C2mim][DEP] ≈ [C62,4mmpy][NTf2] > [C2mim][NTf2] ≈ [C2mim][OAc], being all values too close (see Figure 4.15). β values close to the unity point out that a high amount of solvent is needed, but selectivity values, which are higher than the unity (especially in the case of the IL with acetate anion), ensure the viability of the separation. In the case of the selectivities, the ILs are ranked as follows: [C2mim][OAc] > [C2mim][DEP] > [C2mim][NTf2] > [C62,4mmpy][NTf2]. They are higher than the unity, but in the two latter they are too close to this value. Once more, the highest selectivities values are found for the [C2mim][OAc] IL. Figure 4.15. β (left) and S (right) values, as a function of the solute concentration in the hydrocarbon-rich phase, for the ternary system {IL + thiophene + toluene} at 298.15 K and atmospheric pressure. The compared ILs are: [C62,4mmpy][NTf2] (●, solid line), [C2mim][NTf2] (○, dotted line), [C2mim][OAc] (▼, dashed line), and [C2mim][DEP] ( , dash-dotted line). x2I 0,0 0,2 0,4 0,6 0,8 1,0 β 0 1 2 3 4 x2I 0,0 0,2 0,4 0,6 0,8 1,0 S 0 20 40 60 80 100 120 140 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 β 0,6 0,8 1,0 1,2 1,4 x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 S 0 1 2 3 4 5 4.Results and discussion 61 Finally, the values of solubilities and selectivities are higher than one for all the ILs when studying the separation of pyridine from hexane. This would imply a denitrogenation process simultaneous to the desulfurization. As it can be seen in Figure 4.16, the ranking for the solubilities is [C62,4mmpy][NTf2] > [C2mim][NTf2] ≈ [C2mim][DEP] >[C2mim][OAc]. The ability of the ILs to preferentially extract pyridine ranks [C2mim][OAc] > [C2mim][NTf2] ∼ [C2mim][DEP] > [C62,4mmpy][NTf2]. Figure 4.16. β (left) and S (right) values, as a function of the solute concentration in the hydrocarbon-rich phase, for the ternary system {IL + pyridine + hexane} at 298.15 K and atmospheric pressure. The compared ILs are: [C62,4mmpy][NTf2] (●, solid line), [C2mim][NTf2] (○, dotted line), [C2mim][OAc] (▼, dashed line), and [C2mim][DEP] ( , dash-dotted line). 4.1.1. CORRELATION Equilibrium conditions are deduced from the First and Second Laws of Thermodynamics. A closed system, heterogeneous and multicomponent, is at equilibrium when at constant temperature (T) and pressure (P), the Gibbs free energy (G) is a minimum. Mathematically: 𝑑(𝐺)𝑇,𝑃= 0 (3) and this expression can be considered as a criteria or definition of equilibrium. From this equation, equilibrium conditions can be established in terms of activities. For a multi-phasic and multi-component system, the equilibrium conditions are: 𝑇(1)= 𝑇(2)=⋯= 𝑇(𝑚) (4) 𝑃(1)= 𝑃(2)=⋯= 𝑃(𝑚) (5) x 2 I 0,0 0,2 0,4 0,6 0,8 1,0 β 0 2 4 6 8 10 x2I 0,0 0,2 0,4 0,6 0,8 1,0 S 0 50 100 150 200 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 62 𝑎𝑖(1)= 𝑎𝑖(2)=⋯= 𝑎𝑖(𝑚) (6) where i refers to all the components of the system and the superscripts make reference to all the distinct phases. The activity is related to composition through the activity coefficient, which is defined as the ratio of the two, hence the expression turns into: (𝑥𝑖.𝛾𝑖) 1= (𝑥𝑖.𝛾𝑖)(2)=⋯= (𝑥𝑖.𝛾𝑖)(𝑚) (7) which is probably the most practical expression of the equilibrium criterion. To correlate the experimental data, a model of the activity coefficient as function of composition and temperature is needed. The experimental obtained data for the compositions of the equilibria systems were correlated using two well-known models, NRTL (Non Random, Two-Liquid) [218] and UNIQUAC (UNIversal QUAsi Chemical) [219]. The NRTL equation was developed by Renon and Prausnitz (1968) [218]: ln 𝛾𝑖= ∑𝑥𝑖𝜏𝑗𝑖𝐺𝑗𝑖 ∑𝑥𝑘𝐺𝑘𝑖 + ∑𝑥𝑗𝐺𝑖𝑗 ∑𝑥𝑘𝐺𝑘𝑗 �𝜏𝑖𝑗 − ∑𝑥𝑚𝐺𝑚𝑗𝜏𝑚𝑗 ∑𝑥𝑘𝐺𝑘𝑗 � (8) 𝜏𝑖𝑗 = �𝑔𝑖𝑗 − 𝑔𝑗𝑗�𝑅𝑇 ⁄= ∆𝑔𝑖𝑗 𝑅𝑇 ⁄ (9) 𝐺𝑖𝑗 =𝑒𝑥𝑝�−𝛼𝑖𝑗𝜏𝑖𝑗� (10) where gij is the residual Gibbs free energy and ij = ji is the parameter that takes into account the non-randomness aleatory disposition of the molecules. In principle, the non-randomness parameter can be specified (from 0.2 to 0.47) according to a set of rules devised by the authors [218] that depend on the chemical nature of the components in the mixture, but it is usually given fixed value in an empirical way. The NRTL equation is applicable to multicomponent systems with only binary parameters (Δgij). Abrams and Prausnitz [219] combined the theoretical concepts of the NRTL equation background with the quasi-chemical lattice model. The resulting UNIQUAC equation for the excess Gibbs energy consists of two parts, the first is a combinatorial part that attempt to describe the dominant entropic contribution, and the second is a residual part that is primarily due to intermolecular forces that are responsible for the enthalpy of mixing. For any component i, the expression of the activity coefficient provided by the UNIQUAC model is: 4.Results and discussion 63 ln 𝛾𝑖=ln ∅𝑖 𝑥𝑖+ 𝑧 2𝑞𝑖𝑙𝑛𝜃𝑖 ∅𝑖 + 𝑙𝑖− ∅𝑖 𝑥𝑖∑𝑥𝑗𝑙𝑗− 𝑞𝑖ln�∑𝜃𝑗𝜏𝑗𝑖 𝑚 𝑗=1 �+ 𝑞𝑖− 𝑞𝑖∑�𝜃𝑗𝜏𝑖𝑗 ∑𝜃𝑘𝜏𝑘𝑗 𝑚 𝑘=1 � 𝑚 𝑗=1 (11) 𝑙𝑗= 𝑧 2�𝑟𝑗− 𝑞𝑗�− �𝑟𝑗−1� (12) where z is the lattice coordination number, arbitrarily given a value of 10; and the segment fraction , the area fraction θ and the parameter τ are given by: ∅𝑖= 𝑟𝑖𝑥𝑖 ∑𝑟𝑗𝑥𝑗 𝑚 𝑗=1 (13) 𝜃𝑖= 𝑞𝑖𝑥𝑖 ∑𝑞𝑗𝑥𝑗 𝑚 𝑗=1 (14) 𝜏𝑖𝑗 =𝑒𝑥𝑝�−𝑢𝑖𝑗− 𝑢𝑗𝑗 𝑅𝑇 � (15) where (uij–ujj) = Δuij are the adjusting parameters of the model and represent the description of the energy interaction between a molecule of i and a molecule of j; and the parameters r and q are purecomponent molecular-structure constants depending on molecular size and external surface areas. These structural parameters can be obtained from crystallographic data, although it is also possible to estimate them through a group contribution method, using the Van der Waals volumes and areas of the functional groups and making them dimensionless by reference to the volume or area of a methylene element in an infinite hydrocarbon chain. In the NRTL correlation, the non-randomness parameter, , was pre-fixed at the values of 0.1, 0.2 and 0.3. For the UNIQUAC model, the structural parameters, r and q, for the hydrocarbons are obtained using the molecular volume and area calculated by Bondi [220], and for the ILs from group contribution data [221, 222]. They are showed in Table 4.13. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 70 Table 4.18 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][NTf2] (1) + thiophene (2) + toluene (3)}. NRTL  = 0.3; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.9483 14.5 1-2 892.31 285.52 1-3 14193 -1927.4 2-3 15006 -3738.0  = 0.3; optimized β∞ = 1.08 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 1.3782 3.1 1-2 -2.5411 90.062 1-3 14179 -1922.0 2-3 15019 -3572.8 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.1183 0.8 1-2 -2696.5 7009.3 1-3 -2198.9 5694.4 2-3 -344.38 429.23 Optimized β∞ = 0.89 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.1198 0.6 1-2 -2697.0 7013.9 1-3 -2209.8 5726.6 2-3 -158.86 258.72 4.Results and discussion 71 Table 4.19 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][NTf2] (1) + pyridine (2) + hexane (3)}. NRTL  = 0.3; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.7358 27.9 1-2 -8698.6 7459.1 1-3 10468 18369 2-3 6740.5 1377.3  = 0.3; optimized β∞ = 7.14 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.9929 7.8 1-2 -5275.0 5244.8 1-3 10808 21705 2-3 7531.3 2150.2 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.5614 10.7 1-2 -7839.4 2812.4 1-3 2848.5 1170.8 2-3 355.46 -2786.1 Optimized β∞ = 4.55 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.7246 7.2 1-2 8160.8 -4493.6 1-3 3408.6 1094.3 2-3 -323.42 2606.7 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 72 Table 4.20 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][OAc] (1) + thiophene (2) + hexane (3)}. NRTL  = 0.2; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.7914 5.0 1-2 -4751.2 15410 1-3 9693.8 16369 2-3 2603.8 -1278.0  = 0.2; optimized β∞ = 1.61 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.7913 4.9 1-2 -4749.6 15406 1-3 9730.4 16377 2-3 2591.4 -1264.0 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.3500 2.8 1-2 -1420.1 4007.2 1-3 970.13 7250.4 2-3 -1058.0 2620.6 Optimized β∞ = 1.75 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.3564 2.1 1-2 -1387.2 3903.5 1-3 913.26 7244.0 2-3 -971.13 2436.3 4.Results and discussion 73 Table 4.21 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][OAc] (1) + thiophene (2) + toluene (3)}. NRTL  = 0.3; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.5681 7.5 1-2 -2799.3 13506 1-3 2756.7 12700 2-3 348.58 -1681.1  = 0.3; optimized β∞ = 0.91 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.6945 4.6 1-2 -2630.3 12741 1-3 2911.2 13464 2-3 1241.9 -1467.3 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.3665 8.3 1-2 -1575.2 4464.6 1-3 -655.72 3993.3 2-3 -335.54 480.44 Optimized β∞ = 0.95 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.4942 1.5 1-2 8160.8 -4493.6 1-3 3408.6 1094.3 2-3 -323.42 2606.7 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 74 Table 4.22 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][OAc] (1) + pyridine (2) + hexane (3)}. NRTL  = 0.2; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 1.4427 6.5 1-2 -51868 1765.7 1-3 11477 15498 2-3 15053 -53116  = 0.2; optimized β∞ = 2.05 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 1.4603 6.3 1-2 -51000 1624.5 1-3 11582 15270 2-3 15223 -52256 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.4886 3.1 1-2 -1863.6 4125.2 1-3 1359.9 3288.5 2-3 -321.46 1596.4 Optimized β∞ = 2.27 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.4994 3.0 1-2 -1897.4 4292.3 1-3 1377.5 3056.7 2-3 -196.16 1409.6 4.Results and discussion 75 Table 4.23 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][DEP] (1) + thiophene (2) + hexane (3)}. NRTL  = 0.2; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 3.0229 11.4 1-2 -46468 405.13 1-3 7269.8 34435 2-3 47521 -45696  = 0.2; optimized β ∞ = 3.23 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 3.0351 11.0 1-2 -43573 724.61 1-3 7171.4 27720 2-3 46342 -42913 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.4314 2.0 1-2 -2632.4 6277.9 1-3 -478.10 10940 2-3 -1383.7 3628.6 Optimized β∞ = 3.03 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.4288 1.9 1-2 -2745.3 6677.6 1-3 -348.20 9555.8 2-3 -1321.3 3474.6 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 76 Table 4.24 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][DEP] (1) + thiophene (2) + toluene (3)}. NRTL  = 0.3; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.6112 3.7 1-2 -4423.5 13726 1-3 -445.04 16617 2-3 -1394.2 459.90  = 0.3; optimized β∞ = 1.49 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.6502 2.1 1-2 -4451.5 13704 1-3 5743.9 25720 2-3 -3185.7 277.40 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.4321 4.5 1-2 -1628.6 4031.0 1-3 -1814.5 9259.0 2-3 -251.97 451.97 Optimized β∞ = 1.18 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.6192 2.1 1-2 -1611.9 4031.8 1-3 -1798.8 9498.4 2-3 -746.74 1233.9 4.Results and discussion 77 Table 4.25 Residual function (F), mean error of the solute distribution ratio (Δβ) and binary interaction parameters for the correlations of the ternary system {[C2mim][DEP] (1) + pyridine (2) + hexane (3)}. NRTL  = 0.1; not optimized F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.3748 3.0 1-2 -11213 16755 1-3 -1588.8 19828 2-3 5122.9 -2043.4  = 0.1; optimized β∞ = 6.25 F Δβ Components Parameters i-j Δgij (J/mol) Δgji (J/mol) 0.3761 2.5 1-2 -10893 16658 1-3 -1622.6 20149 2-3 5027.3 -1759.8 UNIQUAC Not optimized F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.5556 9.6 1-2 -1537.8 2507.5 1-3 -320.21 8658.6 2-3 -995.65 2463.0 Optimized β∞ = 5.88 F Δβ Components Parameters i-j Δuij (J/mol) Δuji (J/mol) 0.4066 3.6 1-2 -6277.2 -161.72 1-3 577.16 2292.1 2-3 2813.2 -6538.5 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 78 To establish a comparison, the correlations that best fitted the experimental data for each ternary system are presented in Table 4.26. Table 4.26 Best-fitting correlations for the studied ternary systems. IL {IL+thiophene+ hexane} {IL+thiophene+ toluene} {IL+pyridine +hexane} [C62,4mmpyr][NTf2] UNIQUAC UNIQUAC NRTL  = 0.1 [C2mim][NTf2] UNIQUAC UNIQUAC NRTL  = 0.3 [C2mim][OAc] UNIQUAC UNIQUAC UNIQUAC [C2mim][DEP] UNIQUAC UNIQUAC NRTL  = 0.1 4.Results and discussion 79 4.2. DESULFURIZATION OF SYNTHETIC FUELS 4.2.1. EXTRACTIVE DESULFURIZATION 4.2.1.1. SYNTHETIC GASOLINE Figures 4.17 to 4.20 show the composition (%wt) of each component in the simulated gasoline and raffinate product after one, two and three extraction stages with each of the four ILs studied. It can be seen that the reduction of thiophene is stronger in the case of the pyridinium IL, achieving 84.9 wt% of extraction of this compound after three steps. In the case of the other ILs, the thiophene composition is reduced around 80.6 wt%, 79.3 wt%, and 61.1 wt% using [C2mim][DEP], [C2mim][OAc], and [C2mim][NTf2], respectively. The best performance of the pyridinium IL is probably due to the electronic π-π interactions of this ring with the aromatic sulfur specie, as it was previously explained in other works [148]. As expected from equilibria previously determined (section 4.1), an important dearomatization in the extraction process is also observed. The reduction of toluene content is around 67.0 wt%, 23.7 wt%, 14.2 wt%, and 32.8 wt% using [C62,4mmpy][NTf2], [C2mim][DEP], [C2mim][OAc], and [C2mim][NTf2], respectively. After three steps of extraction, the denitrogenation of the gasoline is total with the four ILs. In all the cases, no IL was found in the raffinate phases. Figure 4.17. Three stages EDS of synthetic gasoline at 298.15 K and atmospheric pressure with the IL [C62,4mmpy][NTf2]. Model oil components n-HEXANE HEPTANE i-OCTANE TOLUENE PYRIDINE THIOPHENE Weight percentage 0 10 20 30 40 FRESH RAFF. 1 RAFF. 2 RAFF. 3 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 86 Figure 4.28 shows the removal (%wt) of thiophene and dibenzothiophene when the synthetic diesel oil was mixed with the same quantity of ionanofluid (oxide nanoparticles dispersed within the IL [C62,4mmpy][NTf2]) at a fixed content of oxidant agent (nH2O2/nS=9), after one oxidative extraction stage. The ionanofluids were prepared by the dissolution-reprecipitation method, as it was explained above. The best desulfurization rates were found when using the Al2O3 and Cr2O3 nanoparticles, being the alumina option the most favorable due to its lower toxicity. Figure 4.28. Removal of thiophene (black) and dibenzothiophene (grey) from synthetic diesel oil using the IL [C62,4mmpy][NTf2] (nH2O2 / nS = 9) with different nanocatalyst prepared by dissolution/reprecipitation. Nevertheless, the desulfurization rate is not enhanced in comparison to the use of a classical oxidative-catalytic system (H2O2 + AcOH), and thus the use of ionanofluids to improve the ODS technique was dismissed. Following with the IL [C62,4mmpy][NTf2] and the H2O2-AcOH as oxidizing-catalytic system, the effect of the temperature in the ODS was tested. Figure 4.29 shows the removal (%wt) of thiophene and dibenzothiophene when the stirring and settle down processes are carried out at 298.15 K and 323.15K (no higher temperatures were tested due to the possible evaporation of the more volatile compounds of diesel oil). As it can be seen, an enhancement was found when working at 323.15 K (from 92.1 wt% to 99.0 wt% of dibenzothiophene reduction). Nanoparticles Sulfur compound removal (%) 0 20 40 60 80 Al2O3TiO2Cr2O3 4.Results and discussion 87 Figure 4.29. Removal of thiophene (black) and dibenzothiophene (grey) from synthetic diesel oil using the IL [C62,4mmpy][NTf2] (nH2O2 / nS = 9, nAcOH / nS = 9) at different temperatures. Figure 4.30 shows the removal of the diesel oil sulfur compounds when one-pot or two-step operation is used. When the model, the IL, and the oxidizing system is mixed at once the extraction of sulfur compounds is higher than when carrying out a normal EDS and oxidizing the mixture afterwards. Figure 4.30. Removal of thiophene (black) and dibenzothiophene (grey) from synthetic diesel oil using the IL [C62,4mmpy][NTf2] (nH2O2 / nS = 9, nAcOH / nS = 9, 323.15 K) at different number of steps for the process. Working in EDS, previous tests [156] showed that 12h stirring and 12h settle down ensured the equilibrium achievement. For this reason, all the previous tests were carried out with these times. Nonetheless, as oxidation is a kinetic process, we finally followed the T (K) 298.15 323.15 Sulfur compound removal (%) 0 20 40 60 80 100 Mixing technique One-pot Two-step Sulfur compound removal (%) 0 20 40 60 80 100 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 88 evolution in time of the desulfurization in the optimized system (onepot mixing, 323.15 K, mIL / mmodel = 1, nH2O2 / nS = 9, nAcOH / nS = 9) at different stirring and settle down times (Figures 4.31 and 4.32). Figure 4.31 shows the evolution of thiophene removal and the behavior corresponds to that found in the usual phase equilibria. Stirring 12h the extraction is higher than when less time is used (higher stirring times do not show any change in the curve), and settle down times of 12h are needed to do not appreciate variation in thiophene extraction. Figure 4.31. Removal of thiophene from synthetic diesel oil using the IL [C62,4mmpy][NTf2] at 323.15 K (nH2O2 / nS = 9, nAcOH / nS = 9) and different mixing times. A very different behavior is found for dibenzothiophene (Figure 4.32) where 12h of good mixing are needed to achieve the highest extraction, having the posterior settle down time little effect. To ensure the best effectiveness in the extraction of light and heavy sulfur compounds, 12h stirring and 12h settle down were fixed to all the desulfurization experiments. Settling time (min) 0200 400 600 800 Thiophene removal (%) 0 20 40 60 80 100 5 min 30 min 12 hours 4.Results and discussion 89 Figure 4.32. Removal of dibenzothiophene from synthetic diesel oil using the IL [C62,4mmpy][NTf2] at 323.15 K (nH2O2 / nS = 9, nAcOH / nS = 9) and different mixing times. b) Results Figures 4.33 to 4.36 show the composition (%wt) of each component in the simulated diesel oil and raffinate products after one, two and three ODS stages, carriedo out with the previously optimized conditions, and with each of the four studied ILs. The best oxidesulfurization was achieved using the IL [C62,4mmpy][NTf2], extracting more than the 94 wt% of thiophene and all the dibenzothiophene. This IL also reduces around 66.1 wt% the toluene content. [C2mim][NTf2] extracts 87.1 wt% of thiophene, all the dibenzothiophene and reduces around 42.2 wt% the toluene composition. [C2mim][DEP] extracts 59.8 wt% of thiophene, 61.6 wt% of dibenzothiophene and reduces around 48.4 wt% the toluene composition. Finally [C2mim][OAc], that achieved the worst results, extracts 58.6 wt% of thiophene, 59.5 wt% of dibenzothiophene and reduces around 36.7 wt% the toluene composition. One more time, denitrogenation is total for all the ILs after the three stages of extraction and no IL was found in the raffinate phases. Settling time (min) 0200 400 600 800 Dibenzothiophene removal (%) 0 20 40 60 80 100 5 min 30 min 12 hours Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 90 Figure 4.33. Three stages ODS of synthetic diesel oil at 323.15 K and atmospheric pressure with the IL [C62,4mmpy][NTf2]. Figure 4.34. Three stages ODS of synthetic diesel oil at 323.15 K and atmospheric pressure with the IL [C2mim][NTf2]. Model oil components HEPTANE DODECANE HEXADECANE TOLUENE PYRIDINE THIOPHENE DIBENZOTHIOPHENE Weight percentage 0 10 20 30 40 FRESH RAFF. 1 RAFF. 2 RAFF. 3 Model oil components HEPTANE DODECANE HEXADECANE TOLUENE PYRIDINE THIOPHENE DIBENZOTHIOPHENE Weight percentage 0 10 20 30 40 FRESH RAFF. 1 RAFF. 2 RAFF. 3 4.Results and discussion 91 Figure 4.35. Three stages ODS of synthetic diesel oil at 323.15 K and atmospheric pressure with the IL [C2mim][OAc]. Figure 4.36. Three stages ODS of synthetic diesel oil at 323.15 K and atmospheric pressure with the IL [C2mim][DEP]. Model oil components HEPTANE DODECANE HEXADECANE TOLUENE PYRIDINE THIOPHENE DIBENZOTHIOPHENE Weight percentage 0 10 20 30 40 FRESH RAFF. 1 RAFF. 2 RAFF. 3 Model oil components HEPTANE DODECANE HEXADECANE TOLUENE PYRIDINE THIOPHENE DIBENZOTHIOPHENE Weight percentage 0 10 20 30 40 FRESH RAFF. 1 RAFF. 2 RAFF. 3 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 92 In the case of [C62,4mmpy][NTf2] and [C2mim][NTf2], best results were found than when using simple extraction. This is in agreement with usual behavior found in bibliography [224,225]. Once the sulfur compounds are extracted to the IL-phase they are oxidized, and then the IL is able to keep on extracting more thiophene and dibenzothiophene. Nonetheless, it seems that desulfurization ability of the two water-miscible ILs ([C2mim][DEP] and [C2mim][OAc]) is diminished when acetic acid and hydrogen peroxide are solved within them. This behavior had been previously found for water-miscible ILs, such us [C4mim][BF4] [226] or the [C2mim][DEP] itself [173]. The worsening in the extraction ability of ILs can be strongly affected by their water content. Since these two hydrophilic ILs are mixed with water, their efficiency in the ODS process is lower than when working with dry ILs in a simpler EDS. A case in point is [C2mim][NTf2], when the cell was left to go down to room temperature after having taken the sample, a white crystalline precipitate started to appear. It was filtered out and washed several times with distilled water. Then, it was dried under soft vacuum for one day and the collected crystals (Figure 4.37) were analyzed by X- ray powder diffraction. The pattern of the sample matches with the diffractogram of dibenzothiophene dioxide, what means that the oxidation product could be separated from the IL-phase without the need of other extracting agents Figure 4.37. Precipitate after the ODS with [C2mim][NTf2]: a) Image of the crystals in the cell; b) X-ray powder diffraction of the collected crystals (red line) and of dibenzothiophene dioxide (blue line). COUNTS 10 1000 0 2000 3000 20 30 Position (2θ) ab 4.Results and discussion 93 4.3. DESULFURIZATION OF REAL FUELS 4.3.1. EXTRACTIVE DESULFURIZATION 4.3.1.1. GASOLINE Figure 4.38 shows how total sulfur concentration diminishes when a real gasoline is submitted to the same multistage extraction performed for the synthetic fuel. According to the results obtained for synthetic fuel, the expected ranking in the extraction capacity would be [C62,4mmpy][NTf2] > [C2mim][DEP] ≈ [C2mim][OAc] > [C2mim][NTf2]. Nonetheless, in the case of the real fuel, desulfurization achieved with the IL [C2mim][OAc] is much more important (desulfurization yield of 38.7 wt% from the first stage) than the obtained for the other ILs. The ranking for the other ILs is maintained according to the synthetic gasoline. Figure 4.38. Three stages EDS of real gasoline at 298.15 K and atmospheric pressure: [C62,4mmpy][NTf2] (●, solid line), [C2mim][NTf2] (○, dotted line), [C2mim][OAc] (▼, dashed line), and [C2mim][DEP] ( , dash-dotted line). The difference between the studied gasoline model (based in thiophene) and the real gasoline (which contains derivatives of thiophene and small mercaptans) lies in the sulfur components. This fact leads to think that the acetate IL has better ability to extract compounds as mercaptans than the other ILs, and the desulfurization of the gasoline is increased for this reason. In order to corroborate this behavior, a mixture of a mercaptan (1-butanethiol) and [C2mim][OAc] was prepared and stirred. Then it was left to settle down and a sample of the upper phase (mercaptan) was analyzed by GC-MS. Results pointed out that the 1-butanethiol reacted in the presence of the IL to form dibutylsulfide and/or dibutyldisulfide, which would be easier to Stage FRESH RAFF.1 RAFF. 2 RAFF. 3 Sulfur content (ppm) 120 140 160 180 200 220 240 Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 94 extract from the gasoline. This interaction could explain the high desulfurization ability of this IL in comparison with the other ones. 4.3.1.2. DIESEL OIL The desulfurization ability of the ILs by EDS of the real diesel oil is ranked as for the synthetic fuel: [C62,4mmpy][NTf2] > [C2mim][DEP] > [C2mim][OAc] > [C2mim][NTf2]. The pyridinium IL achieved a good desulfurization (37.8 wt% after three steps) in comparison with the imidazolium ones. The obtained desulfurization evolution is showed in Figure 4.39. Figure 4.39. Three stages EDS of real diesel oil at 298.15 K and atmospheric pressure: [C62,4mmpy][NTf2] (●, solid line), [C2mim][NTf2] (○, dotted line), [C2mim][OAc] (▼, dashed line), and [C2mim][DEP] ( , dash-dotted line). 4.3.2. OXIDATIVE DESULFURIZATION 4.3.2.1. GASOLINE Since no improvement was observed with respect to the EDS technique with the synthetic gasoline, no ODS experiments were carried out with real gasoline. 4.3.2.2. DIESEL OIL In Figure 4.40 it can be seen the ability of the four studied ILs to extract sulfur by ODS from real diesel oil, which can be ranked as follows: [C62,4mmpy][NTf2] > [C2mim][NTf2] >> [C2mim][DEP] > [C2mim][OAc]. The behavior is similar to that found with the synthetic diesel oil. The desulfurization ability is clearly higher in the case of the [NTf2]- based ILs and lower for the other two. The pyridinium and Stage FRESH RAFF.1 RAFF. 2 RAFF. 3 Sulfur content (ppm) 5000 6000 7000 8000 9000 10000 4.Results and discussion 95 imidazolium [NTf2]- based ILs are able to extract the 96.5 wt% and the 82.0 wt%, respectively, of sulfur from the fuel. Figure 4.40. Three stages ODS of real diesel oil at 323.15 K and atmospheric pressure: [C62,4mmpy][NTf2] (●, solid line), [C2mim][NTf2] (○, dotted line), [C2mim][OAc] (▼, dashed line), and [C2mim][DEP] ( , dash-dotted line). Stage FRESH RAFF.1 RAFF. 2 RAFF. 3 Sulfur content (ppm) 0 2000 4000 6000 8000 10000 LIST OF SYMBOLS List of symbols 105 Symbols a activity APILs aprotic ionic liquids CAS chemical abstracts service EDS extractive desulfurization F residual function Fa objective function (activities) Fb objective function (compositions) FRESH fresh model, gasoline or gasoil G Gibbs energy HDS hydrodesulfurization IL ionic liquid l parameter in the UNIQUAC model LLE liquid-liquid equilibrium M number of tie-lines n number of moles nD refractive index NMR nuclear magnetic resonance NRTL Non-Random Two Liquid ODS oxidative desulfurization P pressure Pn binary interaction parameter PILs protic ionic liquids POM polyoxometalate q area molecular-structure constant in the UNIQUAC model Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 106 Q empirical constant, 10-6 in Eq. (16) and 10-10 in Eq. (17) r volume molecular-structure constant in the UNIQUAC model R universal constant of gases RAFF. Raffinate RTILs room-temperature ionic liquids S selectivity S sulfur T temperature TSILs task-specific ionic liquids UNIQUAC UNIversal QUAsiChemicaL wt weight x mole fraction z lattice coordination number Greek letters α non-randomness parameter in the NRTL model β solute distribution ratio; partition coefficient γ activity coefficient ∆β mean error of the solute distribution ratio Δg binary energy interaction parameter in the NRTL model Δu binary energy interaction parameter in UNIQUAC model θ area fraction in the UNIQUAC model ρ density τ parameter in the NRTL or UNIQUAC models υ standard uncertainty List of symbols 107 φ segment fraction in the UNIQUAC model ωH2O water content Subscripts a activity i=1,2,…,n component j=1,2,…,n component j counter for phases k=1,2,…,n component k counter for tie-lines m phase P constant pressure S solute T constant temperature ∞ infinite dilution Superscripts I raffinate phase II IL phase m=1,2,…,n phase Symbols on top ^ calculated value REFERENCES References 111 [1] J. D. Holbrey, R. D. Rogers, Ionic Liquids in Synthesis, Ed. P. Wasserscheid, T. Welton, Wiley-VCH, Weinheim 2002, 41-55. [2] K. R. Seddon, The International George Papatheodorou Symposium: Proceedings, Ed. S. Boghosian, V. Dracopoulos, C. G. Kontoyannis, G. A. Voyiatzis, Patras 1999, 131-135. [3] J. H. Davis Jr., Chemistry Letters 2004, 33, 1072-1077. [4] C. Baudequin, D. Brégon, J. Levillain, F. Guillen, J. C. Plaquevent, A. C. Gaumont, Tetrahedron: Asymmetry 2005, 16, 3921-3945. [5] M. Y. Machado, R. Dorta, Synthesis 2005, 15, 2473-2475. [6] M. Freemantle, An Introduction to Ionic Liquids, Ed. The Royal Society of Chemistry, Cambridge 2010, 1-11. [7] J. S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey, Inorganic Chemistry 1982, 21, 1263-1264. [8] J. S. Wilkes, M. Zaworotko, Journal of the Chemical Society, Chemical Communications 1992, 13, 965-966. [9] P. Benhôte, A. P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Inorganic Chemistry 1996, 35, 1168-1178. [10] C. Friedel, J. M. Crafts, Comptes Rendues 1877, 84, 1392 & 1450. [11] N. Nambu, N. Hiraoka, K. Shigemura, S. Hamanaka, M. Ogawa, Bulletin of the Chemical Society of Japan 1976, 49, 3637-3640. [12] P. Walden, Bulletin de l'Academie Imperiale des Sciences de Saint- Pétersbourg 1914, 8, 405–422. [13] S. Werner, M. Haumann, P. Wasserscheid, Annual Review of Chemical and Biomolecular Engineering 2010, 1, 203–230. [14] Y. Patell, K. R. Seddon, L. Dutta, A. Fleet, Green Industrial Applications of Ionic Liquids, Ed. R. D. Rogers, K. R. Seddon, S. Volkov, NATO Science Series, Dordtrecht 2002, 499-510. [15] M. J. Earle, K. R. Seddon, Pure and Applied Chemistry 2000, 72, 1391-1398. [16] S. P. Ong, O. Andreussi, Y. Wu, N. Marzari, G. Ceder, Chemistry of Materials 2011, 23, 2979-2986. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 118 [107] X. Ma, K. Sakanishi, I. Mochida, Industrial & Engineering Chemistry Research 1994, 33, 218-222. [108] Z. Zhang, X. Guo, S. Liu, X. Zhu, L. Xu, Fuel Processing Technology 2008, 89, 1135-1141. [109] D. D. Whitehurst, T. Isoda, I. Mochida, Advances in Catalysis 1998, 42, 345-471. [110] I. V. Babich, J. A. Moulijn, Fuel 2003, 82, 607-631. [111] C. Song, Catalysis Today 2003, 86, 211-263. [112] C. Song, X. Ma, Applied Catalysis B: Environmental 2003, 41, 207-238. [113] G. A. Huff, O. S. Owen, B. D. Alexander, D. N. Rundell, W. J. Reagan, J. S. Yoo, WO 9909117 1999. [114] M. Arias, D. Laurenti, C. Geantet, M. Vrinat, I. Hideyuki, Y. Yoshimura, Catalysis Today 2008, 130, 190-194. [115] V. Meille, E. Shulz, M. Vrinat, M. Lemaire, Chemical Communications 1998, 3, 305-306. [116] A. Milenkovic, E. Shulz, V. Meille, D. Lofferda, M. Forissier, M. Vrinat, P. Sautet, M. Lemaire, Energy & Fuels 1999, 13, 881-887. [117] Y. Shiraishi, Y. Taki, T. Hirai, I. Komasawa, Industrial & Engineering Chemistry Research 2001, 40, 1213-1224. [118] R. M. Acheson, D. R. Harrison, Journal of the Chemical Society D 1969, 13, 724-724. [119] R. M. Acheson, D. R. Harrison, Journal of the Chemical Society C 1970, 13, 1764-1784. [120] K. A. Gray, O. S. Pogrebinsky, G. T. Mrrachko, L. Xi, D. J. Monticello, C. H. Squires, Nature Biotechnology 1996, 14, 1705-1709. [121] F. Kargi, J. M. Robinson, Applied and Environmental Microbiology 1982, 44, 878-883. [122] M. F. Luo, J. M. Xing, Z. X. Gou, S. Li, H. Z. Liu, J. Y. Chen, Biochemical Engineering Journal 2003, 13, 1-6. References 119 [123] T. Ohshiro, Y. Izumi, Bioscience, Biotechnology and Biochemistry 1999, 63, 1-9. [124] V. Selvavathi, V. Chidambaram, A. Meenakshisundaram, B. Sairam, B. Sivasankar, Catalysis Today 2009, 141, 99-102. [125] J. H. Kim, X. Ma, A. Zhou, C. Song, Catalysis Today 2006, 111, 74-83. [126] Z. Y. Zhang, T. B. Shi, C. Z. Jia, W. J. Ji, Y. Chen, M. Y. He, Applied Catalysis B: Environmental 2008, 82, 1-10. [127] I. Funakoshi, T. Aida, US 5753102 1998. [128] P. Forte US 5582714 1996. [129] M. Te, C. Fairbridge, Z. Ring, Applied Catalysis A: General 2001, 219, 267-280. [130] S. Otsuki, T. Nonaka, N. Takashima, W. Qian, A. Ishihara, T. Imai, T. Kabe, Energy & Fuels 2000, 14, 1232-1239. [131] J. M. Campos-Martin, M. C. Capel-Sanchez, P. Perez-Presas, J. L. G. Fierro, Journal of Chemical Technology and Biotechnology 2010, 85, 879-890. [132] E. Torres-García, A. Galano, G. Rodriguez-Gattorno, Journal of Catalysis 2011, 282, 201-208. [133] M. F. Ali, A. Al-Malki, B. El-Ali, G. Martinie, M. N. Siddiqui, Fuel 2006, 85, 1354-1363. [134] L. Chen, S. Guo, D. Zhao, Chinese Journal of Chemical Engineering 2007, 15, 520-523. [135] V. Toteva, A. Georgiev, L. Topalova, Fuel Processing Technology 2009, 90, 965-970. [136] F. A. Duarte, P. A. Mello, C. A. Bizzi, M. A. G. Nunes, E. M. Moreira, M. S. Alencar, H. N. Motta, V. L. Dressler, E. M. M. Flores, Fuel 2011, 90, 2158-2164. [137] H. Lü, Y. Zhang, Z. Jiang, C. Li, Green Chemistry 2010, 12, 1954-1958. [138] M. A. Ramos-Luna, L. Cedeño-Caero, Industrial and Engineering Chemistry Research 2011, 50, 2641-2649. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 120 [139] L. Cedeño-Caero, M. Ramos-Luna, M. Méndez-Cruz, J. Ramírez- Solís, Catalysis Today 2011, 172, 189-194. [140] W. Guo, C. Wang, P. Lin, X. Lu, Applied Energy 2011, 88, 175- 179. [141] X. M. Yan, P. Mei, J. Lei, Y. Mi, L. Xiong, L. Guo, Journal of Molecular Catalysis A: Chemical 2009, 304, 52-57. [142] A. Bösmann, L. Datsevich, A. Jess, A. Lauter, C. Schmitz, P. Wasserscheid, Chemical Communications 2001, 23, 2494-2495. [143] N. H. Ko, J. S. Lee, E. S. Huh, H. Lee, K. D. Jung, H. S. Kim, M. Cheong, Energy & Fuels 2008, 22, 1687-1690. [144] R. Schmidt, Energy & Fuels 2008, 22, 1774-1778. [145] S. Zhang, Z. C. Zhang, Green Chemistry 2002, 4, 376-379. [146] S. Zhang, Q. Zhang, Z. C. Zhang, Industrial and Engineering Chemistry Research 2004, 43, 614-622. [147] C. Xuemei, H. Yufeng, L. Jiguang, L. Qianqing, L. Yansheng, Z. Xianming, P. Xiaoming, Y. Wenjia, Chinese Journal of Chemical Engineering 2008, 16, 881-884. [148] H. S. Gao, M. F. Luo, J. M: Xing, Y. Wu, Y. G: Li, W. L. Li, Q. F. Liu, H. Z. Liu, Industrial and Engineering Chemistry Research 2008, 47, 8384-8388. [149] R. P. Swatloski, J. D. Holbrey, R. D. Rogers, Green Chemistry 2003, 5, 361-363. [150] J. Eber, P. Wasserscheid, A. Jess, Green Chemistry 2004, 6, 316-322. [151] Y. Mochizuki, K. Sugawara, Energy & Fuels 2008, 22, 3303- 3307. [152] C. C. Cassol, A. P. Umpierre, G. Ebeling, B. Ferrera, S. S. X. Chiaro, J. Dupont, International Journal of Molecular Science 2007, 8, 593-605. [153] L. Alonso, A. Arce, M. Francisco, A. Soto, Journal of Chemical Engineering Data 2007, 52, 2409-2412. References 121 [154] L. Alonso, A. Arce, M. Francisco, A. Soto, Journal of Chemical Engineering Data 2008, 53, 1750-1755. [155] L. Alonso, A. Arce, M. Francisco, A. Soto, Fluid Phase Equilibria 2008, 263, 176-181. [156] A. Arce, M. Francisco, A. Soto, Journal of Chemical Thermodynamics 2010, 42, 712-718. [157] C. Asumana, G. Yu, X. Li, J. Zhao, G. Liu, X. Chen, Green Chemistry 2010, 12, 2030-2037. [158] G. Yu, X. Li, X. Liu, C. Asumana, X. Chen, Industrial & Engineering Chemistry Research 2011, 50, 2236-2244. [159] A. R. Hansmeier, G. W. Meindersma, A. B. de Haan, Green Chemistry 2011, 13, 1907-1913. [160] M. M. Taib, T. Murugesan, Asia-Pacific Journal of Chemical Engineering 2012, 7, 469-473. [161] J. D. Holbrey, I. López-Martín, G. Rothenberg, K. R. Seddon, G. Silveiro, X. Zheng, Green Chemistry 2008, 10, 87-92. [162] M. Królikowski, K. Walczak, U. Domanska, Journal of Chemical Thermodynamics 2013, 65, 168-173. [163] K. Kedra-Królik, F. Mutelet, J: N. Joubert, Industrial & Engineering Chemistry Research 2011, 50, 2296-2306. [164] A. Marciniak, M Królikowski, Journal of Chemical Thermodynamics 2012, 49, 154-158. [165] M. Francisco, A. Arce, A. Soto, Fluid Phase Equilibria 2010, 294, 39-48. [166] L. Alonso, A. Arce, M. Francisco, O. Rodríguez, A. Soto, Journal of Chemical Engineering Data 2007, 52, 1729-1732. [167] L. Alonso, A. Arce, M. Francisco, A. Soto, Journal of Chemical Thermodynamics 2008, 40, 966-972. [168] L. Alonso, A. Arce, M. Francisco, O. Rodríguez, A. Soto, AIChE Journal 2007, 53, 3108-3115. [169] L. Alonso, A. Arce, M. Francisco, A. Soto, Journal of Solution Chemistry 2008, 37, 1355-1363. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 122 [170] L. Alonso, A. Arce, M. Francisco, A. Soto, Fluid Phase Equilibria 2008, 270, 97-102. [171] H. Rodríguez, M. Francisco, A. Soto, A. Arce, Fluid Phase Equilibria 2010, 298, 240-245. [172] J. Feng, C. X. Li, H. Meng, Z. Wang, Petrochemical Technology 2006, 3, 272-276. [173] Y. Nie, C. Li, A. Sun, H. Meng, Z. Wang, Energy & Fuels 2006, 20, 2083-2087. [174] X. Jiang, Y. Nie, C. Li, Z. Wang, Fuel 2008, 87, 79-84. [175] C. Yansheng, L. Changping, J. Qingzhu, L. Qingshan, Y. Peifang, L. Xiumei, U. Welz-Biermann, Green Chemistry 2011, 13, 1224-1229. [176] C. Jiang, J. Wang, S. Wang, H. Guan, X. Wang, M. Huo, Applied Catalysis B: Environmental 2011, 106, 343-349. [177] J. Wang, D. Zhao, K. Li, Energy & Fuels 2010, 24, 2527-2529. [178] H. Li, L. He, J. Lu, W. Zhu, X. Jiang, Y. Wang, Y. Yan, Energy & Fuels 2009, 23, 1354-1357. [179] L. Chuang, J. Huang, W. Lo, G. Wei, Journal of the Chinese Chemical Society 2012, 59, 324-330. [180] W. Zhu, H. Li, Q. Gu, P. Wu, G. Zhu, Y. Yan, G. Chen, Journal of Molecular Catalysis A: Chemical 2011, 336, 16-22. [181] H. Lü, J. Gao, Z. Jiang, F. Jing, Y. Yang, G. Wang, C. Li, Journal of Catalysis 2006, 239, 369-375. [182] W. Huang, W. Zhu, H. Li, H. Shi, G. Zhu, H. Liu, G. Chen, Industrial & Engineering Chemistry Research 2010, 49, 8998-9003. [183] J. Ge, Y. Zhou, Y. Yang, M. Xue, Industrial & Engineering Chemistry Research 2011, 50, 13686-13692. [184] J. Zhang, A. Wang, X. Li, X. Ma, Journal of Catalysis 2011, 279, 269-275. [185] D. Xu, W. Zhu, H. Li, J. Zhang, F. Zou, H. Shi, Y. Yan, Energy & Fuels 2009, 23, 5929-5933. References 123 [186] D. Liu, J. Gui, J. Ding, J. Ma, J. Lee, Z. Sun, Reaction Kinetics, Mechanisms and Catalysis 2011, 104, 111-123. [187] E. Lissner, W. F. de Souza, B. Ferrera, J. Dupont, ChemSusChem 2009, 2, 962-964. [188] D. Zhao, Z. Sun, F. Li, R. Liu, H. Shan, Energy & Fuels 2008, 22, 3065-3069. [189] D. S. Zhao, H. W. Ren, J. L. Wang, Y. Yang, Y. Zhao, Energy & Fuels 2007, 21, 2543-2547. [190] W. Zhu, J. Zhang, H. Li, Y. Chao, W. Jiang, S. Yin, H. Liu, Royal Society of Chemistry Advances 2012, 2, 658-664. [191] H. Li, W. Zhu, Y. Wang, J. Zhang, J. Lu, Y. Yan, Green Chemistry 2009, 11, 810-815. [192] J. Zhang, W. Zhu, H. Li, W. Jiang, Y. Jiang, W. Huang, Y. Yan, Green Chemistry 2009, 11, 1801-1807. [193] D. Zhao, J. Wang, E. Zhou, Green Chemistry 2007, 9, 1219- 1222. [194] J. Wang, D. Zhao, K. Li, Energy & Fuels 2009, 23, 3831-3834. [195] L. Lu, S. Cheng, J. Gao, G. Gao, M. He, Energy & Fuels 2007, 21, 383-384. [196] W. Zhang, K. Xu, Q, Zhang, D. Liu, S. Wu, F. Verpoort, X.-M. Song, Industrial & Engineering Chemical Research 2010, 49, 11761- 11763. [197] G. Yu, J. Zhao, D. Song, C. Asumana, X. Zhang, X. Chen, Industrial & Engineering Chemical Research 2011, 50, 11690-11697. [198] B. Zhang, Z. Jiang, J. Li, Y. Zhang, F. Lin, Y. Liu, C. Li, Journal of Catalysis 2012, 287, 5-12. [199] H. Gao, C. Guo, J. Xing, J. Zhao, H. Liu, Green Chemistry 2010, 12, 1220-1224. [200] D. Zhao, Z. Sun, F. Li, H. Shan, Journal of Fuel Chemistry and Technology 2009, 37, 194-198. [201] W. Keim, W. Korth, P. Wasserscheid, WO Pat. WO 0016902 2000. Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes 124 [202] J. A. Riddick, W. B. Bunger, T. K. Sakano, Organic Solvents. Physical Properties and Methods of Purification, 4th ed., John Wiley & Sons, 1986. [203] M. Fermeglia, G. J. Toriano, Chemical Engineering Data 1999, 44, 965-969. [204] B. Luo, G. C. Benson, B. C.-Y. Lu, Journal of Chemical Thermodynamics 1988, 20, 267-271. [205] T. E. Daubert, R. P. Danner, Physical and thermodynamic properties of pure components: Data compilation, Hemisphere Publishing Corporation, 1989. [206] D. R. Lide, CRC Handbook of Chemistry and Physics, 87th ed.; Taylor and Francis: Boca Raton, FL, 2007. [207] E. Guillet, D. Imbert, R. Scopelliti, J. C. G. Bunzil, Chemistry of Materials 2004, 16, 4063-4070. [208] M. Tariq, P. A. S. Forte, M. F. Costa Gomes, J. N. Canongia Lopes, L. P. N. Rebelo, Journal of Chemical Thermodynamics 2009, 41, 790-798. [209] M. G. Freire, A. R. R. Teles, M. A. A. Rocha, B. Schroder, C. M. S. S. Neves, P. J. Carvalho, D. V. Evtuguin, L. M. N. B. F. Santos, J. A. P. Coutinho, Journal of Chemical Engineering Data 2011, 56, 4813- 4822. [210] A. P. Fröba, M. H. Rausch, K. Krzeminski, D. Assenbaum, P. Wasserscheid, A. Leipertz, International Journal of Thermophysics 2010, 31, 2059-2077. [211] J. Wang, C. Li, C. Shen, Z. Wang, Fluid Phase Equilibria 2009, 279, 87-91. [212] L. Hadlich de Oliveira, M. Aznar, Journal of Chemical Engineering Data 2011, 56, 2005-2012. [213] E. Kuhlmann, S. Himmler, H. Giebelhaus, P. Wasserscheid, Green Chemistry 2007, 9, 233-242. [214] A. Arce Arce, A. Soto Campos, E. Rodil Rodríguez, B. Rodríguez Cabo, WO 2012013852 A2 20120202 2012. References 125 [215] S. Schwyn, E. Garwin, A. Schmidt-Ott, Journal of Aerosol Science 1988, 19, 639-642. [216] N. S. Tabrizi, M. Ullmann, V. A. Vons, U. Lafont, A. Schmidt-Ott, Journal of Nanoparticle Research 2009, 11, 315-332. [217] N. S. Tabrizi, Q. Xu, N. M. van der Pers, A. Schmidt-Ott, Journal of Nanoparticle Research 2010, 12, 247-259. [218] H. Renon, J.M. Prausnitz, AIChE Journal 1968, 14, 135-144. [219] D. S. Abrams, J. M. Prausnitz, AIChE Journal 1975, 21, 116- 128. [220] J. M. Prausnitz, T. F. Anderson, E. A. Grens, C. A. Eckert, R. Hsieh, J. P. O´Connel, Computer Calculations for Multicomponent Vapor–Liquid and Liquid–Liquid Equilibria, Prentice Hall, Englewood Cliffs, New York, 1980. [221] Z. Lei, J. Zang, Q. Li, B. Chen, Industrial & Engineering Chemical Research 2009, 48, 2697-2704. [222] R. Kato, J. Gmehling, Journal of Chemical Thermodynamics 2005, 37, 603-619. [223] J. M. Sørensen, W. Arlt, Liquid-Liquid Equilibrium Data Collection, DECHEMA Chemistry Data Series, Frankfurt, 1980. [224] L. W. Lo, H. Y. Yang, G. T. Wei, Green Chemistry 2003, 5, 639- 642. [225] D. Zhao, Y. Wang, E. Duan, J. Zhang, Fuel Process Technology 2010, 91, 1803-1806. [226] H. Zhang, J. Gao, H. Meng, C. X. Li, Industrial & Engineering Chemical Research 2012, 51, 6658-6665. APPENDIX A (List of publications)