Ionic liquids in the production of new materials. ionic liquid
PETROCHEMISTRY, 2007, Volume 47, No. 5, p. 339-348
UDC 541.48-143:542.97
F. A. Nasirov, F. M. Novruzova, A. M. Aslanbeyli, and A. G. Azizov
Institute of Petrochemical Processes, National Academy of Sciences of Azerbaijan, Baku E-mail: [email protected] Received February 6, 2007
Data on the processes of catalytic conversion of olefins and dienes using ionic liquids (ILs) as solvents are summarized. The role of these compounds in solving environmental problems from the point of view of green chemistry is discussed. Some industrial processes involving ionic liquids are considered.
The general definition of "green chemistry" is the design and development of chemical products and processes that reduce or eliminate the use and production of hazardous substances. Any substance and the method of obtaining it through chemical transformations can be considered in connection with their possible impact on the environment. The task of "green chemistry" is reduced to the development of chemical processes, on the one hand, economically acceptable, on the other - minimally polluting nature. When developing such "clean" industrial processes, one should be guided by the 12 principles of "green chemistry" given in the works.
The use of environmentally friendly solvents or the conduct of processes without solvents at all is one of the most important areas of "green chemistry". Typical organic solvents are often sufficiently volatile compounds that, in addition to being hazardous air pollution, they tend to be highly flammable, toxic, or carcinogenic. The use of ILs instead of them is of great scientific and practical interest in the development of new "green chemistry" processes.
Advances in the application of ILs in catalysis are described in detail in numerous books and review articles, including .
Significant progress has been made using ILs in such processes of the catalytic conversion of olefins and dienes as dimerization, oligomerization, alkylation, and metathesis. The potential of ILs as new media for the mentioned reactions of homogeneous catalysis was fully appreciated thanks to the pioneering work and in-depth studies of a whole group of chemists.
CONCEPT OF IONIC LIQUIDS
Ionic liquids, as a new class of alternative solvents, are attracting much attention due to their low vapor pressure, lack of toxicity, and the possibility of interaction with organometallic compounds, which opens up wide prospects for their use in catalysis. In principle, a huge variety of ILs is achieved by varying the combination of cation and anion, which, in turn, can be chosen for each specific reaction. At the same time, the toxicity and cost issues of this new class of solvents must be evaluated on a case-by-case basis.
ILs, consisting of a large nitrogen-containing organic cation and a much smaller inorganic anion, are compounds with Gm usually below 100-150°C.
Numerous cation–anion associations capable of forming room temperature ILs (RBIs) have been mentioned in the literature. This circumstance distinguishes them from classical molten salts (e.g., NaCl with Mm = 801°C, Na3AlF3 with Mm = 1010°C, tetrabutylphosphonium chloride with Mm = 80°C, LiCl:KCl = 6:4 mixtures with Gm = 352°C, etc.). IZHKT - liquids Ch. arr. with large asymmetric cations in the molecule preventing close packing of anions. ILs contain ammonium, sulfonium, phosphonium, lithium, imidazolium, pyridinium, pi-colinium, pyrrolidinium, thiazolium, triazolium, oxazolium and pyrazolium cations with various substituents.
Of particular interest are liquid salts based on ^^ dialkylimidazolium cation, from -
characterized by a wide range of physicochemical properties, which are usually obtained by anion exchange from imidazole halides.
IL anions are divided into two types. The first is composed of polynuclear anions (for example,
A12 C1-, A13 C1 10, Au2C17, Fe2C17 and 8b2B-!), formed by the interaction of the corresponding Lewis acid with a mononuclear anion (for example,
A1C1-) and are especially sensitive to air and water. The second type is mononuclear anions that are part of neutral stoichiometric ILs,
e.g., VB4, RB6, 2pS133, SiS12, 8pS1-,
N#802)-, N(#802)-, C(SBz802)3, SBzS02,
SB3803, CH380- etc.
By changing the alkyl groups of the starting compound (imidazole, pyridinium, phosphonium, etc.), as well as the type of associated anions, the synthesis of a huge variety of ILs with different physicochemical properties is theoretically possible. The authors of the work suggest the existence of up to one trillion (1018) possible cation/anion combinations in ILs.
The most commonly used are chloraluminate, tetrafluoroborate, or hexafluorophosphate ILs based on N-alkylpyridinum or 1,3-dialkylimidazolium. Organochloraluminate ILs obtained from N-alkylpyridinium or 1,3-dialkylimidazolium chlorides and aluminum trichloride have a wide liquid phase limit up to 88°C.
The physical and chemical properties of ILs (density, electrical conductivity, viscosity, Lewis acidity, hydrophobicity, ability to form hydrogen bonds) can be controlled by changing the type and ratio of cationic and anionic components. In this case, it becomes possible to create ILs with desired properties suitable for use in catalysis.
ILs are called "green solvents" - due to their low vapor pressure, they are not volatile and therefore do not ignite; moreover, they are immiscible with a number of common organic solvents, which provides a real alternative for creating two-phase systems. This property makes it easier to separate the products from the reaction mixture, as well as to regenerate the catalyst and return it to the system together with the IL. Two-phase liquid-liquid catalysis promotes the "heterogenization" of a homogeneous catalyst in one phase (usually polar, in this case in an IL), and organic products in another. The product is separated from the catalyst solution by simple decantation, and the catalyst is used repeatedly without reducing the efficiency.
efficiency, selectivity and activity of the process. An ionic type catalyst can be easily retained in the IL phase without the need for the synthesis of special purpose ligands. In the case when the catalyst is not charged, the transition (washout) of an expensive transition metal into the organic phase can be limited by using functional ligands specially introduced into the structure of the IL. The thermodynamic and kinetic characteristics of chemical reactions carried out in IL differ from those in traditional volatile organic solvents, which is also of great interest.
The literature reports on many chemical reactions in which ILs are used as a medium. Such reactions include cracking, hydrogenation, isomerization, dimerization, oligomerization, etc. It is known that ILs used in a number of catalytic systems exhibit greater activity, selectivity, and stability than in the case of traditional solvents. They often provide better yields, highly selective distribution of reaction products, and in some cases faster process kinetics. Reactions in IL also proceed at lower pressures and temperatures than conventional reactions, thus leading to a significant reduction in energy and capital costs.
IONIC LIQUIDS IN CATALYTIC PROCESSES OF OLEFIN AND DIEN CONVERSION
The catalytic processes of dimerization, oligomerization, alkylation, and metathesis of olefins and dienes in IL open up new opportunities for their conversion into more valuable olefins and other products. The role of the solvent in these homogeneous catalytic processes is to dissolve and stabilize the molecules of monomers, ligands, and catalysts without interaction with them and without competition with monomers for a vacant coordination center.
As solvents, ILs are unique in their weak coordination ability, which, with respect to the catalytic complex, depends on the nature of the anion. ILs with low nucleophilicity do not compete with the organic molecule for coordination in the electrophilic center of the metal. In some cases, their role is simply to provide a polar, weakly coordinating environment for the organometallic complex catalyst (as a "harmless" solvent) or as a cocatalyst (for example, in the case of chloroaluminate or chlorostannate ILs), so they can note-
act as a direct solvent, co-solvent and catalyst.
It is known that most ILs form two-phase mixtures with many olefins, and these systems have all the advantages of both homogeneous and heterogeneous catalysis (e.g., mild process conditions, high efficiency/selectivity ratio characteristic of homogeneous catalysts, easy separation of reaction products, optimal consumption of heterogeneous catalysts).
At present, the most studied reaction in IL is the dimerization of lower olefins catalyzed by nickel compounds using a chloraluminate type of solvent.
The French Petroleum Institute (FIN) has developed a catalytic process of propylene dimerization in a chloraluminate IL based on 1-bu-
tyl-3-methylimidazolium chloride (bmimCl) - so-called. nickel process. The catalyst consists of L2NiCl2 (L = Ph3P or pyridine) in combination with EtAlCl2 (bmimCI/AlQ3/EtAlQ2 = 1/1.2/0.25) and active catalytic
ionic complex of nickel(II) +AlCl– formed in situ upon alkylation of L2NiCl2 with EtAlCl2 in acidic alkyl chloroaluminate ILs. Since the latter promote the dissociation of ionic metal complexes, it was assumed that they have a beneficial effect on this reaction. At 5°C and atmospheric pressure, the productivity of the process reaches ~250 kg dimer/g Ni, which is much higher than
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A. G. Azizov, R. V. Alieva, F. M. Velieva, B. V. Guliyev, M. D. Ibragimov - 2008
Ionic liquids belong to the so-called "green solvents", which correspond to the principles of green chemistry. Some ionic liquids, such as 1-butyl-3-methylimidazolium chloride, are relatively effective solvents for cellulose. In classical solvents, this process occurs only under very harsh conditions.
History
The first publication appeared in 1888. Gabriel reported in it about ethanol ammonium nitrate, which has a melting point of 52-55 ° C. In 1914, Paul Walden obtained the first ionic liquid with a melting point below room temperature: ethylammonium nitrate + − , which has a melting point of 12 °C. After that, ionic liquids were forgotten for a while, and were considered only a laboratory curiosity. In 1951, Harley obtained ionic liquids from chloroaluminates, which he used for aluminum electrodeposition. In 1963 Yoke reported that mixtures of copper(I) chloride with alkylammonium chlorides were often liquid. In 1967, Swain used tetra-n-hexylammonium benzoate to study the kinetics of electrochemical reactions. In the period from the 1970s to the 1980s, chloroaluminates were used for spectro- and electrochemical studies of transition metal complexes. In 1981, for the first time, they were used as a solvent and a catalyst simultaneously to carry out the Friedel-Crafts reaction. In 1990, Nobel laureate Yves Chauvin applied ionic liquids to two-phase catalysis. In the same year, Österjong used ionic liquids to polymerize ethylene with a Ziegler-Natta catalyst. A breakthrough in the research came in 1992, when Wilkes and Zavorotko, working on the search for new electrolytes for batteries, reported the production of the first ionic liquids resistant to air and moisture - imidazolium salts with anions − and MeCO 2 − . After that, an active study of ionic liquids began. The number of published articles and books is constantly growing. In 2002 there were more than 500 publications, in 2006 almost 2000. Chemical dealers now offer a large selection of commercially available ionic liquids. In 2009, the US Department of Energy (DOE) awarded a $5.13 million grant to Arizona startup Fluidic Energy to build prototypes of durable metal-air batteries with an order of magnitude greater specific capacity than lithium-ion batteries. The role of the electrolyte should be played not by an aqueous solution, but by an ionic liquid. Accordingly, the new type of battery was named Metal-Air Ionic Liquid Battery.
Properties
Physical properties
Ionic liquids in the solid state are powders or waxy substances of white or yellowish color. In the liquid state, they are colorless, or with a yellowish tint, which is due to a small amount of impurities. One of the characteristic properties of ionic liquids is their high viscosity, which makes them difficult to work with. The main characteristic of ionic liquids is their low melting point, due to the steric hindrance of the structure, which will complicate crystallization. For example, 1-ethyl-3-methylimidazolium dicyanamide, , melts at Tm = −21°C, pyridinium chloride, Cl, melts at Tm = 144.5°C but 1-butyl-3,5-dimethylpyridinium bromide, [ N-butyl-3,5-dimethyl-Py]Br, vitrifies only below Tg = −24 °C.
Classification
Receiving and cleaning
The synthesis of ionic liquids can be reduced to two steps: cation formation, and anion exchange (when required). Often, the cation is commercially available as a halide salt, and it only remains to replace the anion to obtain the desired ionic liquid.
Quaternization reactions
The formation of the cation can be carried out either by reaction with an acid or by quaternization of an amine, phosphine, or sulfide. To perform the latter, haloalkanes or dialkyl sulfates are often used. The quaternization reaction is very simple - the original amine (or phosphine) is mixed with the desired alkylating agent, heated with stirring, in most cases without solvent. The reaction time and heating temperature depend on the haloalkane. Reactivity increases from chlorine to iodine. Fluorine derivatives cannot be obtained in this way.
Anion exchange reactions
Can be divided into two categories: the direct reaction of halide salts with Lewis acids and the metathesis (exchange) of anions. Preparation of ionic liquids by the reaction of a Lewis acid (most often AlCl 3 ) with a halide salt was the dominant method in the early stages of research.
For example, the reaction of obtaining an ionic liquid by the reaction of ethylmethylimidazolium chloride with aluminum chloride (Lewis acid):
+ Cl − + AlCl 3 → + AlCl 4 −
The meaning of the salt metathesis reaction is to form a new pair of salts that could be easily separated based on their different physical properties. For example, obtaining silver halides (which precipitate), or acids, which can be easily separated by washing the ionic liquid with water (only for water-immiscible ionic liquids). For example, the reaction of ethylmethylimidazolium chloride with hexafluorophosphoric acid
+ Cl − + HPF 6 → + PF 6 − + HCl
As a result of the reaction, a water-immiscible ionic liquid is formed, and the by-product, hydrochloric acid, remains dissolved in water.
Receipt in industry
Despite the ease of obtaining ionic liquids in the laboratory, not all methods are applicable on an industrial scale due to their high cost. Ionic liquids are marketed as "green solvents", but they often use large amounts of organic solvents in their manufacture, often to remove halogens from ionic liquids. All these shortcomings must be eliminated in the transition to large-scale syntheses. For example, Solvent Innovation has proposed, patented and produces tons of ionic liquid, which received the trade name ECOENG 212. It meets all the requirements of green chemistry: it is non-toxic, it can decompose when released into the environment, it does not contain halogen impurities, it does not solvents are used, and ethyl alcohol is the only by-product.
cleaning
Since ionic liquids cannot be purified by distillation (their saturated vapor pressure is practically zero), in practice, the starting compounds are purified, from which the ionic liquid is going to be obtained. Theoretically, it is possible to drive off any organic impurities from the ionic liquid, since many of the latter are resistant to heating to very high temperatures: they do not decompose up to 400 °C. It is also possible to purify ionic liquids with activated carbon, followed by filtration through a short neutral alumina column. The water is distilled off by heating for several hours to 60 °C under reduced pressure. In industry, the ability of ionic liquids to be cleaned for reuse is of paramount importance due to the high cost of the latter. Efficiency varies from poor to very good. Various innovative methods are proposed. For example, extraction of products with supercritical CO 2 or membrane techniques. In addition, the direction of leasing ionic liquids to enterprises for single use seems promising. Thus, one firm will supply and clean the solvent for another, which will save money by reusing the solvent.
see also
Sources
- Remember LISA (indefinite) . geektimes.ru. Retrieved February 15, 2016.
- Ignatyev, Igor; Charlie Van Doorslaer, Pascal G.N. Mertens, Koen Binnemans, Dirk. E. de Vos. Synthesis of glucose esters from cellulose in ionic liquids (English) // Holzforschung: journal. - 2011. - Vol. 66, no. 4 . - P. 417-425. - DOI:10.1515/hf.2011.161 .
- S. Gabriel, J. Weiner. Ueber einige Abkömmlinge des Propylamins (German) // Chemische Berichte (English) Russian: shop. - 1888. - Bd. 21, no. 2. - S. 2669-2679. - DOI:10.1002/cber.18880210288 .
- P. Walden,. Molecular weights and electrical conductivity of several fused salts. (English) // Bull. Acad. sci. : journal. - 1914. - P. 405-422.
- Frank. H. Hurley, Thomas P. Wier Jr. Electrodeposition of metals from fused quaternary ammonium salts. (English) // Journal of the Electrochemical Society (English) Russian: journal. - 1951. - Vol. 98 . - P. 203-206.
- Yoke, John T., Weiss, Joseph F.; Tollin, Gordon. Reactions of triethylamine with copper(I) and copper(II) halides. (English) // Inorganic Chemistry: journal. - 1963. - Vol. 2(6) . - P. 1209-1216.
- Chauvin, Yves; Gilbert, Bernard; Guibard, Isabelle. Catalytic dimerization of alkenes by nickel complexes in organochloroaluminate molten salts. (English) // Chemical Communications (English) Russian: journal. - 1990. - Vol. 23. - P. 1715-1716.
D. G. Loginov, V. V. Nikeshin
APPLICATIONS OF IONIC LIQUIDS IN THE CHEMICAL INDUSTRY
Key words: ionic liquids, solvent, catalyst.
The types of ionic liquids, basic properties, methods of preparation and main areas of application in chemical technologies are considered.
Key words: ionic liquid solvent, catalyst.
The types of ionic liquids, the main properties, methods of preparation and the main applications in chemical technologies.
Despite the existence of a wide range of known catalysts, chemical engineering and organic synthesis are constantly in need of new, more efficient and environmentally friendly catalysts,
reaction media and solvents. At
The development and improvement of industrial processes of basic and fine organic synthesis, as well as in petrochemistry, require new approaches to solving existing economic and environmental problems associated with high energy costs and environmental pollution. Modern
an approach to solving the problem of replacing volatile organic compounds used as solvents in organic synthesis involves the use of ionic liquids. The use of ionic liquids as new reaction media can solve the emission problem
solvents and reuse of expensive catalysts.
The term "ionic liquids" means
substances that are liquids at temperatures below 100°C and consist of organic cations, for example, 1,3-dialkylimidazolium, N-
alkylpyridinium, tetrakylammonium,
tetraalkylphosphonium, trialkylsulfonium and various anions: 01", [BP4]", [PP6]", [$LP6]", CF3SO3", [(CF3SO2)2N]", POB3", RSO3", Ar0O3", CP3CO2", CH3CO2 ", NOO3", [A12C17]".
The nature of the anion has a great influence on the properties of ionic liquids - melting point, thermal and electrochemical stability and viscosity. The polarity as well as the hydrophilicity or hydrophobicity of ionic liquids can be optimized by appropriate selection of the cation/anion pair, and each new anion and cation provides further possibilities for varying the properties of ionic liquids.
Increased attention to ionic liquids is due to the presence of the following specific properties:
1. Wide range of liquid state (> 300 °C) and low melting points (Tmelt< 100 °С).
2. High electrical conductivity.
3. Good dissolving power
in relation to various inorganic, organometallic and organic
compounds and polymers of natural and synthetic origin.
4. Catalytic activity, causing an increase in the selectivity of organic reactions and the yield of the target product.
5. Non-volatile, reusable.
6. Non-combustibility, non-explosion hazard, non-toxicity and the resulting absence of harmful effects on the environment.
7. Limitless possibilities in the directed synthesis of ionic liquids with desired properties.
Qualities 3 and 4 make ionic solvents particularly attractive in polymer synthesis.
Ionic liquids are unique objects for chemical research, their use in catalysis, organic synthesis, and other areas, including biochemical processes. The number of ionic liquids described in the literature is currently very large (about 300). Potentially, the number of ionic liquids is practically unlimited and is limited only by the availability of suitable organic molecules (cationic particles) and inorganic, organic and
metal complex anions. According to various estimates, the number of possible combinations of cations and anions in such ionic liquids can reach 1018. Figure 1 shows some of the most studied ionic liquids described in the literature.
The cooking methods are quite simple and can be easily scaled up. There are three main synthesis methods that are most commonly used:
An exchange reaction between a silver salt containing
the necessary anion B", and a halogen derivative with the necessary cation A +: Ad + B "+ A + Na1" ^
A+B" + AdHa1
Quaternization reaction N
alkyl halide derivative with metal halides: \u003d N + - A1kNa1 "+ MNa1p ^ N+ - A1kMNa1" n + 1
Ion exchange reactions on ion exchange resins or clays.
Rice. 1 - Ionic liquids
^ = H, alkyl, aryl, hetaryl, allyl, etc., including functional groups, x = 1-4, m=2, 3. X- = ^4]", ^6]", ^6]" . [AI4]-, [AGS^]-, [A12C17]-, [A13C1yu]-, (CF3SO2)2N-, [B^]-, -, [Me(C0)n]-, etc.)
Another practically important direction in the synthesis of ionic liquids is their preparation directly in the reactor. In this case, the corresponding M-alkyl halide and the metal halide are mixed in the reactor and an ionic liquid is formed just before starting the chemical process or catalytic reaction. Most often, ionic liquids are prepared on the basis of a mixture of aluminum chloride with organic chlorides. When two solids are mixed, an exothermic
reaction, and eutectic mixtures are formed with melting points down to -90 °C. It is, as a rule, a transparent colorless or yellow-brown liquid (the color is due to the presence of impurities and local overheating of the reaction mass during the preparation of the ionic liquid).
Ionic liquids, due to the diversity and peculiarities of their properties, have proved to be very attractive for catalysis and organic synthesis. As regards the "environmental friendliness" of ionic liquids, much should and will be reassessed in subsequent studies, although, in general, the fact that they are recyclable, non-flammable and have a low saturated vapor pressure makes them full participants in "green" chemistry, even without taking into account those gains in productivity and selectivity, examples of which were given in the review. Obviously, due to their high cost, ionic liquids are unlikely to find widespread use in large-scale processes unless additional advantages are found.
heterogeneous systems. At the same time, low-tonnage chemistry, primarily metal complex catalysis, may turn out to be a fertile area for their use, as well as electrochemistry in general and electrocatalysis in particular.
Literature
1. A.F. Yagfarova, A.R. Gabdrakhmanova, L.R. Minibaeva, I.N. Musin, Vestnik Kazan. technol. un-ta, 15, 13, 192-196(2012)
2. A.R. Gabdrakhmanov, A.F. Yagfarova, L.R. Minibaev,
A.V. Klinov, Vestnik Kazan. technol. Univ., 15, 13, 6366 (2012).
© D. G. Loginov - Master of the Dept. processes and devices of chemical technology KNRTU, [email protected];
V. V. Nikeshin - Ph.D. tech. sciences, ved. programm. cafe processes and devices of chemical technology KNRTU, [email protected]
Introduction
I. Literature review 8
1.1, Heterogeneous acid-base catalysts (oxide catalysts, zeolites, solid superacids, heteropol and acids, metal chlorides) 8
1.2. Homogeneous acid-base catalysts. Protic acids and liquid superacids 15
1.3. Inorganic ionic liquids (melts of metal salts) 17
1.5. Ionic liquids 20
II. Experimental 49
1. Starting materials and catalysts. Hardware 49
2. Preparation of ionic liquids 53
3. Methods and conditions of the experiment 57
III- Results and discussion 62
1. Study of the isomerization of C-Ca n-alkanes in the presence of ionic liquids 62
2. Transformations of cycloalkanes: isomerization of methylcyclopentane and cyclohexane in the presence of ionic liquids 85
3. Catalytic transformations of xylenes in the presence of ionic liquids: the effect of temperature and catalyst composition on the activity and selectivity of their isomerization 97
IIIA Catalytic properties of systems based on supported ionic liquids 102
Findings 112
References 114
Appendix 130
Introduction to work
Despite the huge number of known catalysts, catalysis and organic synthesis are constantly in need of new, more efficient and more environmentally friendly catalysts, catalytic media and solvents. In most industrial processes of basic and fine organic synthesis, as well as in petrochemistry, new approaches are needed to solution of existing economic and environmental problems associated with high energy costs and environmental pollution. Recent advances in the chemistry of melts of mixtures of organic and inorganic salts, which are commonly called "ionic liquids" or "low-temperature salt melts," can partially solve the problems mentioned above.
Most known liquids are molecular. This means that, whether they are polar or non-polar, they are made up of molecules. In the early 1980s, a new class of liquids was discovered called ionic liquids. Unlike molecular liquids, regardless of the degree of dissociation, these systems consist mainly of ions. The properties and behavior of such liquids when used as solvents or catalysts (catalyst media) are very different from the properties of molecular liquids,
In recent years, there has been an intensive growth in the number of publications and patents, as well as reviews on various aspects of the preparation, study of the properties and use of ionic liquids, including in catalysis. The first ionic liquid was obtained by the Russian scientist Paul Walden in 1914 and had the following composition : *". In the period from 1940 to 1980, ionic liquids of various classes were synthesized, however, until the 90s, systematic studies of ionic liquids were not carried out. Also, the possibility of their use as catalysts was not investigated. However, since 1990
interest in ionic liquids began to grow at an accelerating pace. The number of publications in central journals by 2001 increased to 600, and the number of patents reached about 60. Several reviews have appeared on ionic liquids +H2SO4 This stage is analogous to the well-known reaction of carbenium ions with isoalkanes in the gas phase. Under these conditions, the result is a proton transfer during the dissociation of a nonclassical carbonium ion: R, + + HR - H2S04 + HSCV - + 2H2S04 + RiR This reaction is the reverse of the well-known isoalkane cracking reaction proven for liquid superacids and zeolites.
Thus, the key point of the new mechanism of two-step alkylation of isoalkanes with olefins is the assumption that this reaction involves the direct alkylation of isoalkanes with protonated esters through the intermediate formation of nonclassical carbonium ions. Using this mechanism, it is possible to describe the course of the process of alkylation of isoalkanes with olefins through the following elementary stages: olefin - a mixture of alkyl sulfates - protonated esters - non-classical carbonium ions - alkylation products.
Ionic liquids can also include aprotic organic superacids containing an acyl halide and a double molar excess of a Lewis acid. The RCOX-2A1X3 complexes are active catalysts for the transformation of normal alkanes at low temperatures (20C). As a rule, they are superior in reactivity to active protic superacids and differ significantly from the equimolar RCOX-AIX3 complexes, which are inert with respect to alkanes under such mild conditions.
It is known that RCOX-2A1X3 complexes in CH2X2 solution are an equilibrium mixture of acylium salts RCO AI2X7" and donor-acceptor complexes RC(X)=0 AbX6.
The RCOXAIX3 complexes exist in solutions exclusively as coordination complexes RC(X)=0 A1X3. Thus, only RCOX 2AlXj complexes active in reactions with alkanes are able to efficiently generate acylium cations, which qualitatively distinguishes them from RCOX-AIX3 complexes. However, it remains unclear what the activity of these complexes is associated with: whether it is with the ability to generate an acylium cation or with the presence of the dimeric A12X7 anion in their composition. The authors of the work attempted to answer this question. The study of mesitoyl bromide complexes with AlBrj of the composition MstCOBr-AIBr3 and MstCOBr-2AlBr3 (Mst=2,4,6-Me3C6H4) made it possible to conclude that the reactivity of acylium salts depends on the structure of the anion. It is known that substituents in positions 2 and 6 of the aromatic ring sterically prevent the formation of ArC(X)=O MXn coordination complexes with the sp2 hybridized carbonyl carbon atom and practically have no effect on the formation of ArCO+ cations. Thus, when aluminum bromide was used as the Lewis acid, complexes were obtained that in CH2X2 solution are exclusively ionic salts of MstCO+AlBr4 and MstCO+Al2Br7\, which was unambiguously confirmed by NMR spectra on the 3C and 27A1 nuclei. Also, it was found that when a saturated hydrocarbon is added, the homogeneity of the solutions is not disturbed in both cases. According to NMR data, the ionic structure of the salts is also preserved.
The MstCO+AIBr4" salt turned out to be inert in the reactions of octane and dodecane destruction. On the contrary, the MstCO+AbBr7 salt initiates this reaction and the quantitative splitting of the above hydrocarbons is observed within 30 minutes. A similar qualitative difference between the two mesitoylium salts is also observed in the reaction involving trimethylenenorbornane: in the presence of MstCO+AlBr4" reaction is not observed, while the MstCO+AbBr7 salt initiates the formation of adamantane. These data indicate that only complex salts with a dimeric anion are active in reactions with saturated hydrocarbons, while salts with a monomeric anion at 20°C are practically inactive in these processes. However, the reason for this, in the opinion of the authors, cannot be the different degree of screening of the positive charge by the counterion. The 13С NMR spectra for the salts of the composition M3YuEVgAshz and MstCOBr-2AIBr3 are practically identical, which makes the assumption of different electrophilicity of acylium cations with monomeric and dimeric anions doubtful.
Aprotic organic superacids
Similar in their properties to aprotic organic superacids are ionic liquids, discovered back in 1914 by the Russian scientist Paul Walden, but have been widely developed and used in catalysis only in the last decade. In this part of the literature review, literature and patent data on ionic liquids will be considered, including their preparation, physicochemical properties and use in hydrocarbon processing catalysis, basic organic synthesis and, to a lesser extent, fine organic synthesis. Ionic liquids are unique objects for chemical research, both in the direction of synthesis and for their use in catalysis, organic synthesis and other areas, including biochemical processes. The number of ionic liquids described in the literature is currently very large, and includes both ionic liquids already well known before the 90s, in particular: pyridinium, imidazolium and polyalkylammonium, and a large number of ionic liquids synthesized relatively recently: Guanidine, Pipyridinium , Polycyclic, Bridged ionic liquids, Binuclear or polynuclear ionic liquids, Hydrophobic ionic liquids (fluorinated) Interest in fluorinated ionic liquids is currently constantly growing, since fluorinated systems are insensitive to the presence of water and other protic substances, have low melting points, low viscosity and have a number of other advantages of ionic liquids. Recently, a number of articles have been published on the synthesis and properties of fluorinated ionic liquids - Composites obtained from ionic liquids and polymer gels, as well as complexes prepared using using anions HftFn+i" .
At present, there are a fairly large number of publications devoted to combinatorial synthesis and screening of ionic liquids in catalytic reactions and organic synthesis. Of particular note is the Symyx patent, which describes a large number of ionic liquids and discusses a range of catalytic reactions in which they can be used. Potentially, the number of ionic liquids is practically unlimited and is limited only by the availability of suitable organic molecules (cationic particles) and inorganic anions.
Ionic liquids containing aluminum chloride are the most commonly used and have been studied in detail. Typical examples are salt melts obtained from anhydrous aluminum chloride and a quaternized ammonium salt, for example, 1-ethyl-3-methylimidazolium chloride (EtMelmCl), alkyl pyridinium, etc. The ionic liquid AIOS - EtMelmCl contains a whole set of ionic liquids, the physical properties of which and Lewis acidity are determined by the molar ratio of its constituent salts.
Ionic liquids with Lewis acidity "in addition to the organic cation, contain mainly A12C17" and AICI "anions, while the main ionic liquids contain an organic cation and advantages such as low melting point (up to -90C at certain ratios of organic and inorganic salts), chemical and thermal stability, high intrinsic electrical conductivity and a wide potential window. A series of imidazolium ionic liquids have been investigated by DTA/GGA and DSC, and some conclusions have been drawn about their thermal stability. For example, compared to pyridinium ionic liquids, imidazolium ionic liquids are less stable, provided that they contain the same anions.
To study chloroaluminate ionic liquids, NMR, IR, PMR, UV and Raman spectroscopy, various electrochemical methods are used. In addition, synthesized ionic liquids are often characterized by X-ray diffraction analysis.
The melting temperature. The ability of low-temperature salt melts to remain liquid over a wide temperature range is an important characteristic of ionic liquids, especially if they are used as solvents. Currently, there is no theory about how the melting point of ionic liquids depends on their composition and the nature of the cation and anion in them. composition, However, empirical studies have shown that the phase diagram of the ionic liquid 1-ethyl-3-metschmidazolium chloride / AICIz has two clear minima at a molar content of aluminum chloride of about 0.4 and 0.65, corresponding to acidic ionic liquids.
The dependence of the melting temperature on the length of the radical of typical ionic liquids containing imidazolium or pyridinium derivatives as cations has a clear minimum for the Cj - C5 radicals. A decrease in the length of the radical leads to an increase in the ionicity of the structure, while its increase leads to an increase in the molecular weight and, thus, to an increase in the melting temperature 10C. The following features of organic cations positively influence the decrease in the melting point of ionic liquids: low symmetry, weak intermolecular interactions, the absence of hydrogen bonds, and a uniform charge distribution in the cation. It is also commonly believed that an increase in the anion size leads to a decrease in the melting point. In addition, the introduction of fluorine into the structure of ionic liquids, as a rule, lowers the melting point, and systems with melting points from -40C to -90C are known.
Preparation of ionic liquids
Ionic liquids were synthesized using the appropriate procedures described in . Below are the methods for preparing ionic liquids used in this work to study their catalytic properties, in particular, in the isomerization reactions of alkanes, cycloalkanes, and aromatic hydrocarbons. All operations were carried out in an inert atmosphere. An ionic liquid N-n-butylpyridinium chloride - aluminum chloride was obtained from an organic amine salt - N-w-butylpyridinium chloride, preliminarily dried over P2O5, and freshly distilled aluminum chloride (Fluka) in an inert medium (Ag). The first step in the synthesis of an ionic liquid is the preparation of N-n-butylpyridinium chloride.
Method for the synthesis of N-n-buthyigtridinium chloride. Into a 100 ml two-necked flask equipped with a reflux condenser, an inlet and outlet of an inert gas (Ar or N2), was placed 0 L M (7–9 g) of pyridine (Aldrich, 98%), previously dried over alkali. OL M (9.2 g) of n-butyl chloride (Aldrich, 98%) was added with stirring with a magnetic stirrer. The resulting mixture was refluxed for 5 hours under an inert gas atmosphere (no solvent). After 7 hours, unreacted starting materials were decanted from the white crystals formed. The crystals were washed with acetonitrile and dried in vacuum at room temperature for 1 hour. The yield was 30% (5.6 g). ]H NMR spectrum (CDC13) was taken for the obtained substance.
The following compounds were synthesized using a similar procedure: N-/mropylpyridinium chloride, N-k-penthishiridinium chloride, N-n-hexylpyridinium chloride, Nf-octylpyridinium chloride, N-H-hexadecylpyridinium chloride.
Method for preparing the ionic liquid N-n-butgtpyridinium chloride - aluminum chloride. 0.03 M (5.6 g) of Nf-butschiridinium chloride was placed into a 100-mL round-bottom flask, and 0.06 M (8.0 g) of anhydrous aluminum chloride was gradually added under stirring in a stream of inert gas. In this case, the mixture spontaneously heated up, so it was cooled so that the temperature did not exceed 30C. The reaction mixture was kept for 2 h (until the formation of a homogeneous system) with constant stirring in a stream of inert gas. The density of the ionic liquid was М.3-1.4 g/cm3.
It has been established that the formation of a complex of N-w-butchshiridinium chloride with aluminum chloride leads to a change in the chemical shifts in the PMR spectra. First of all, this is expressed in an abrupt increase in chemical shifts for all protons of the complex by 1 - 1.7 mD. In addition, the proton signals are broadened, which indicates an intense interaction involving protons in the complex.
Aluminum chloride (P.5 g, 0.086 M) was gradually added to crystalline 1-n-pentyl-3-methylimidazolium chloride (8.2 g, 0.043 M) at room temperature and vigorous stirring. The reaction temperature did not exceed 30C. The mixture was stirred for 2 h in a stream of nitrogen at room temperature until a homogeneous system was formed. As a result, a viscous light brown ionic liquid with a density of -1.3 -1.4 g/cm3 was obtained.
The ionic liquid 1-n-butyl-3-mstylimidazolium chloride - aluminum chloride (1:2 mol) was prepared by a similar method. Ionic liquid trimethylammonium hydrochloride - aluminum chloride (1:2 mol.) was obtained from trimethylammonium hydrochloride (Aldrich, 99%) and fresh aluminum chloride in an inert atmosphere. To this end, 13.4 g (0.05 M) of aluminum chloride was slowly added to 4.8 g (0.05 M) of trimethylammonium hydrochloride dried over P2055 with vigorous stirring. Since the reaction proceeds with the release of heat, the reaction mass, if necessary, was cooled so that its temperature did not exceed 50C. The resulting mixture of salts was stirred for 2 hours at room temperature. As a result, a transparent light brown ionic liquid was formed, which is very mobile at room temperature, with a specific gravity of -1.4 g/cm3. In the case of preparing ionic liquids with a molar ratio of 1:1.5 or 1:1–25, 10.0 g (0.075 M) or 8.4 g (0–0625 M) of aluminum chloride were taken, respectively.
Preparation of catalysts based on supported ionic liquids. To prepare the deposited ionic liquids, the carriers shown in Table 1 were used. Before use, the carriers were preliminarily calcined in a stream of dry air at 450–520C for 3–4 h and additionally evacuated immediately before the deposition of ionic liquids at 250C for 1.5 h.
Dry carriers in an inert atmosphere were placed in a three-necked flask with a magnetic stirrer. Then, with vigorous stirring, an ionic liquid was added dropwise (from 20 to 100 wt.%, i.e., the mass ratio of ionic liquid: carrier varied from 0.2: 1 to 1: 1 ), after which the catalyst was stirred in an argon flow at 30C for 2 h, then the substrate was added.
Cycloalkane transformations: isomerization of methylcyclopentane and cyclohexane in the presence of ionic liquids
It is known that the reactions of mutual transformations of methylcyclopentane (MCP) and cyclohexane (CT) proceed on heterogeneous acid-type catalysts, such as oxide systems modified with Group VIII metals, zeolites, heteropolyacids, sulfated transition metal oxides, for example, SO/ /ZrOa, etc. d. . However, the use of these catalysts requires the use of high temperatures, up to 400C and higher for alumina-platinum catalysts and about 250C for sulfated zirconia.
The use of ionic liquids as catalysts for the isomerization of cycloalkanes was first reported in our work, where it was shown that ionic liquids are active in this reaction. , compared with heterogeneous catalysts, is the high selectivity of the reaction, which is close to 100%, while the maximum selectivity achieved, for example, on S0427Zr02 is 90%. The remaining heterogeneous catalysts have even lower selectivity due to dehydrogenation, cracking, and ring opening reactions followed by the formation of benzene, isohexanes, and lighter paraffins. The purpose of this part of the work was to compare the activity of ionic liquids trimethylammonium hydrochloride - A1CH (1: 2 mol.), triethylammonium hydrochloride - AICIs (1: 2 mol.), N-n-butylpyridinium chloride - AICIs (1: 2 mol.) and N-w -pentylpyridinium chloride - АІСІз (1: 2 mol.) in the reaction of mutual isomerization of methylcyclopentane and cyclohexane. The structures of the studied ionic liquids are given below: (direct and reverse reaction). It is known that at 60C the equilibrium mixture of methylcyclopentanocyclohexane consists of 23% MCP and -77% CT; the equilibrium constant Kp is 3.35. To find out whether it is possible to achieve thermodynamic equilibrium in these reversible reactions under mild reaction conditions using ionic liquids as catalysts, we carried out experiments in which the substrates were artificial mixtures of MCP and CT with a component content of 15 and 85 wt. %; as well as 30 and 70 wt. %, respectively. Thus, an attempt was made to achieve thermodynamic equilibrium on both sides. An ionic liquid trimethylammonium hydrochloride - AlCl (1: 2 mol.) Was used as a catalyst.
Taking into account the obtained data on the course of the reaction of mutual transformations of methylcyclopentane and cyclohexane to thermodynamic equilibrium, when describing the kinetics of the isomerization of cyclohexane to methylcyclopentane, it is necessary to take into account the reversibility of the reaction.
In this regard, we used the following method. Near equilibrium, the rate of reversible catalytic reactions involving one substrate and product is described by the following equation: The form of these functions depends on which kinetic scheme the process obeys. At certain ratios of rate constants and concentrations of reactants and effectors, the parameters b\ and b2 will only be functions of the constants of the elementary stages, and they are directly proportional to the concentration of the catalyst. For example, at low concentrations of the substrate and product, the parameters b] and bz are equal to the ratios of the corresponding maximum rates to the Michaelis constants, if the process obeys the reversible Michaelis scheme. These parameters characterize the rates of forward and reverse reactions, respectively.
Since the kinetic parameters b[ and br are not the rate constants of the elementary stages, but characterize the rates of the forward and reverse reactions, they are often called "effective" or "apparent" rate constants. Therefore, in this chapter, the parameters bi and b2 will be called the effective rate constants of the forward and reverse catalytic reactions and will be denoted by k] and k_i, respectively.
Knowing the initial and equilibrium concentrations of methylcyclopentane or cyclohexane and approximating the experimental curve by equation (2), we obtain the sum of the constants of the forward and reverse reactions (ki + k_i). To calculate the constant k.] we use the expression:
Equation (2) was chosen to process the kinetic curves presented in Fig. 11. As can be seen from the figure, this equation satisfactorily describes the experimental data. To calculate the rate constants of the forward and reverse reactions, we used the experimental data obtained when the reaction was carried out near thermodynamic equilibrium in the presence of an additive and without it. Also, the effect of an activating additive on the reaction rate of mutual transformations of cyclic hydrocarbons was studied. The content of the additive varied from 2 to 8% by weight. The resulting curves were also processed using equation (2).
The miscibility of ionic liquids with various solvents is presented in Table 1.4.
Table 1.4. Miscibility of IL with various solvents. No. Solvent e I
А1С13 - base - AICI3 - acid 1 Water 80.1 Immiscible Reacting Reacting 2 Propylene carbonate 64.4 Miscible Miscible Miscible 3 Methanol 33.0 Miscible Reacting Reacting 4 Acetonitrile 26.6 Miscible Miscible Miscible 5 Acetone 20.7 Miscible Miscible Reacting 6 Methylene chloride 8.93 Miscible Miscible Miscible 7 THF 7.58 Miscible Miscible Reacting 8 Trichlorethylene 3.39 Immiscible Not
miscible Not
miscible 9 Carbon disulfide 2.64 Not miscible Not
miscible Not
miscible 10 Toluene 2.38 Immiscible Miscible Reacts 11 Hexane 1.90 Immiscible Not
miscible Not
mixed up
Ionic liquid (+PF) Typically, processes in ionic liquids are compared with those in typical organic solvents. From this point of view, with respect to compounds that exhibit weak basic properties, the main IL will behave like DMF. On the other hand, acid-type ILs lead At room temperature, ionic liquids are excellent solvents and, at the same time, are able to play the role of catalysts for a number of reactions, such as Friedel-Crafts, Diels-Alder, isomerization and reduction reactions.
[EM1sh]C1-A1C13 and other haloaluminate ionic liquids have Lewis acidity, which can be controlled by changing the molar ratio of the two components A1C13A1C13. All this makes ionic liquids interesting as non-aqueous reaction media. The Lewis acidity of these systems is determined by the activity of chloride. Equilibrium in a chloraluminate liquid at room temperature can be described by two equations:
AICI4" + AICI3 AI2C17*
The first describes the process in basic melts, when the molar ratio A1C13AmC1 is less than one, and the second - in acidic ones, where the ratio is greater than one. This means that more anions C G, AICI4", AI2CI7" are formed, and their relative amounts are determined by the equilibrium: 2A1SC" *
ACL" + SG Heptachloroaluminate ion is a strong Lewis acid, due to the chlorine ion in the conjugate Lewis base. A neutral ionic liquid is a liquid where the molar ratio of A1C13AltC1 is equal to one and only the AICI4* ion is present. At present, it has become possible to neutralize buffer acid ILs solid metal alkyl chlorides.
The complete solubility of ionic liquids in solvents makes them convenient for spectrophotometric measurements, especially in the visible and UV regions. They can be used together with organic solvents; in this case, as a result of solvation, IL ions are dispersed and, as a result, some physicochemical properties change: a decrease in viscosity and an increase in the conductivity of the solution. When comparing the IR spectra of acidic and basic ionic liquids, a slight distortion of the aromatic ring is found, which is less stressed in contrast to the salt, which has a smaller cation. This means that the hydrogen bond between the hydrogen atom on the second carbon atom of the ring and the chloride ion is either very weak or non-existent. In basic type ILs, the hydrogen bond tension is still significant. One of the advantages of ILs is their thermal stability over a wide temperature range, which makes it possible to successfully control the reactions occurring in these liquids. Thus, +PF6" begins to decompose at a temperature of ~620 K, and with a noticeable rate at 670 K. Decomposition of IL proceeds according to the same mechanism both in air and in a nitrogen atmosphere. It was found that when heated in air, IL oxidation does not occur.
Ionic liquids are convenient to use and inexpensive to produce. They are good solvents, and the possibility of creating catalytic systems on their basis makes them preferable for carrying out catalytic reactions. By selecting ionic liquids, it is possible to achieve the separation of reaction products into another phase.
The behavior of ILs under the action of ionizing radiation has practically not been studied. A preliminary assessment of the radiation stability of one of the most well-known IL based on 1,3 dialkylimidazole cation (+PF6") shows that it is relatively resistant to ionizing radiation (similar to benzene) and more stable than the system based on a mixture of tributyl phosphate and kerosene. It has been shown that that under the conditions studied, ionic liquids do not decompose into their constituent organic components under the action of ionizing radiation in detectable amounts.
More on the topic 1.5.2. Properties of ionic liquids:
- 3.5. Study of the radiation-chemical process of polymerization of elemental phosphorus in organic solvents in the presence of ionic liquids 3.5.1. Dielectric Properties of Initial Solutions
