Ionic catalytic

Most studies of catalysis in ionic liquids have focused on issues of increased selectivity and particularly the easy separation of product from the catalyst and catalyst recycling via use of a biphase. In some cases, the reaction may occur in a biphase in others, the biphase is only used for product separation. In some special cases, the second phase is exclusively product, due to insolubility of the organic products in the ionic liquid, and is easily separated by decantation, allowing the recovered ionic catalytic solution to be reused. Of course, use of an organic solvent for extraction does reduce some of the potential green benefits of the ionic liquid approach. More recently, SCCO2 has been used to extract the products. Alternatively, volatile products can be separated from the ionic liquid and catalyst by distillation. 

The transition-metal NPs dispersed in imidazolium ILs are active catalysts for the hydrogenation of alkenes, arenes and ketones (Table 6.3). Moreover, Pd(0) NPs are active catalyst precursors for C—C coupling reactions, serving as reservoirs of mononuclear catalytically active species. In most cases, the catalytic reactions are typically multiphase systems in which the NPs dispersed in the ILs form the denser phase and the substrate and product remain in the upper-phase. In these cases the ionic catalytic solution is easily recovered by simple decantation and can be reused several times without any significant loss in catalytic activity. 

Lack of organization and local nano-inhomogeneity of impure systems (IL/water or toluene) were evoked to explain the less interesting results obtained with [BMIMJIBFJ or mixtures of [BMIM][PFg] with H2O. The authors assumed that the local nano-inhomogeneity in the IL dramatically changes the partial miscibility of the alkenes, syngas and distribution products in the ionic catalytic phase . 

Multi-phase catalysis performed in ILs can lead to various phase systems where the catalyst should reside in the IL. Prior to the reaction, and in cases where there are no gaseous reactants, two systems can usually be formed a monophase, that is, the substrates are soluble in the IL and biphasic systems where one or all the substrates reside preferentially in an organic phase. If a gas reactant is involved, biphasic and triphasic systems can be formed. At the end of the reaction, three systems can be formed a monophasic system a biphasic system where the residual substrates are soluble in the ionic catalytic solution and the products reside preferentially in the organic phase and triphasic systems, formed, for example, by ionic catalytic solutions, with an organic phase containing the desired product and a third phase containing the byproducts. In most cases, catalysis performed in ILs involves two-phase systems (before and after catalysis). 

Considering that aluminum trichloride is a very important commercial catalyst wifli over 25,000 tonnes produced annually in the USA alone, such hquids containing aluminum trichloride and allowing for differing levels of acidity have been extensively studied as first generation ionic catalytic solvents in a wide variety of synthetic and catalytic processes. Ionic liquids could therefore be used as substitutes for conventional solid or suspended sources of aluminum(III) chloride. As liquid phase catalysts, they allow for tremendous control of reactor inventories and can be cleaned and recycled with ease. Therefore, ionic liquids, in ideal cases, have no waste associated with them, whereas the supported alumi-num(III) chloride catalysts will require large (and annually rising) waste disposal costs. 

The presence of several different ionic particles and therefore, centres with different reactivity, should contribute to the values of the chain growth and chain termination reaction rate constants. The presence of associated, nonassociated and isomeric forms of catalyst particles, the influence of electrolytic dissociation, intramolecular and intermolecular interaction, leading to the formation of catalytic complexes is the reason for the presence of different centres in ionic catalytic systems. 

Considering that aluminum trichloride is a very important commercial catalyst such liquids containing aluminum trichloride and allowing for differing levels of acidity have been extensively studied as first generation ionic catalytic solvents in a wide variety of synthetic and catalytic processes. Ionic liquids could therefore be used as substitutes for... 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

The rate constants for the catalysed Diels-Alder reaction of 2.4g with 2.5 (Table 2.3) demonstrate that the presence of the ionic group in the dienophile does not diminish the accelerating effect of water on the catalysed reaction. Comparison of these rate constants with those for the nonionic dienophiles even seems to indicate a modest extra aqueous rate enhancement of the reaction of 2.4g. It is important to note here that no detailed information has been obtained about the exact structure of the catalytically active species in the oiganic solvents. For example, ion pairing is likely to occur in the organic solvents. 

In determining the values of Ka use is made of the pronounced shift of the UV-vis absorption spectrum of 2.4 upon coordination to the catalytically active ions as is illustrated in Figure 2.4 ". The occurrence of an isosbestic point can be regarded as an indication that there are only two species in solution that contribute to the absorption spectrum free and coordinated dienophile. The exact method of determination of the equilibrium constants is described extensively in reference 75 and is summarised in the experimental section. Since equilibrium constants and rate constants depend on the ionic strength, from this point onward, all measurements have been performed at constant ionic strength of 2.00 M usir potassium nitrate as background electrolyte . 

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Although the alkylation of paraffins can be carried out thermally (3), catalytic alkylation is the basis of all processes in commercial use. Early studies of catalytic alkylation led to the formulation of a proposed mechanism based on a chain of ionic reactions (4—6). The reaction steps include the formation of a light tertiary cation, the addition of the cation to an olefin to form a heavier cation, and the production of a heavier paraffin (alkylate) by a hydride transfer from a light isoparaffin. This last step generates another light tertiary cation to continue the chain. 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

R. Sheldon, Catalytic reactions in ionic liquids, J Chem Soc, Chem Commun 2399-2407 2001. 

Many of the reactions of BF3 are of the Friedel-Crafts type though they are perhaps not strictly catalytic since BF3 is required in essentially equimolar quantities with the reactant. The mechanism is not always fully understood but it is generally agreed that in most cases ionic intermediates are produced by or promoted by the formation of a BX3 complex electrophilic attack of the substrate by the cation so produced completes the process. For example, in the Friedel-Crafts-type alkylation of aromatic hydrocarbons ... 

While certain TSILs have been developed to pull metals into the IL phase, others have been developed to keep metals in an IL phase. The use of metal complexes dissolved in IL for catalytic reactions has been one of the most fruitful areas of IL research to date. LLowever, these systems still have a tendency to leach dissolved catalyst into the co-solvents used to extract the product of the reaction from the ionic liquid. Consequently, Wasserscheid et al. have pioneered the use of TSILs based upon the dissolution into a conventional IL of metal complexes that incorporate charged phosphine ligands in their stmctures [16-18]. These metal complex ions become an integral part of the ionic medium, and remain there when the reaction products arising from their use are extracted into a co-solvent. Certain of the charged phosphine ions that form the basis of this chemistry (e.g., P(m-C6H4S03 Na )3) are commercially available, while others may be prepared by established phosphine synthetic procedures. 

Stoichiometric - or, more simply, non-catalytic - reactions are an important and rapidly expanding area of research in ionic liquids. This section deals with reactions that consume the ionic liquid (or molten salt) or use the ionic liquid as a solvent. 

The distribution of the products obtained from this reaction depends upon the reaction temperature (Figure 5.1-4) and differs from those of other poly(ethene) recycling reactions in that aromatics and alkenes are not formed in significant concentrations. Another significant difference is that this ionic liquid reaction occurs at temperatures as low as 90 °C, whereas conventional catalytic reactions require much higher temperatures, typically 300-1000 °C [100]. A patent filed for the Secretary of State for Defence (UK) has reported a similar cracking reaction for lower molecular weight hydrocarbons in chloroaluminate(III) ionic liquids [101]. An... 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

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