Catalysis, triphase

In most of the examples described in this section, the catalyst could be reused with little loss of activity. Triphase catalysis has the advantages of selectivity, recoverability, and reusability, and clearly has applications in clean technology. 

Solid supported reagents offer advantages over liquid reagents because they may be recovered and recycled simply by filtration [58]. This approach has been 

These polystyrene-based catalysts are effective for the cyanide displacements of 1-bromooctane and 1-chlorooctane, and also for the generation of dichlorocarbene from chloroform and aqueous sodium hydroxide, giving quantitative yields of (2,2-dichlorocyclopropyl)benzene from styrene. The catalysts may be recovered simply by filtering the reaction mixture. Unfunctionalized polystyrene does not catalyse these reactions. As well as improving product purification and catalyst recovery, this approach also avoids 

Another proposal is that catalysis occurs primarily at the surface. This is not becanse the rate of reaction is significantly faster than the rate of diffusion, but because the pores of the catalyst become filled with one phase only, which prevents the influx of the other reactant [62]. 

Water is a unique solvent because of its high polarity and ability to form a network of H-bonds. It is immiscible with many organic solvents and is therefore a suitable solvent for use in biphasic reactions in which catalysts are made preferentially soluble in the aqueous phase. Phase transfer catalysis allows the use of aqueous reagents with substrates that have low solubility in water. That water is abundant and totally non-toxic make it the perfect clean solvent, provided that solubility issues can be overcome, and it is in use as a solvent on an industrial scale for polymerization, hydroformylation, and a range of organic chemistry involving PTC. These applications are discussed further in Chapters 7-11. 

Matlack A. S. Introduction to Green Chemistry, Marcel Dekker, New York, 2001. 

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Phases. Often there are two Hquid phases (Hquid—Hquid PTC) or one soHd, one Hquid (soHd—Hquid PTC). If the catalyst is bound to a polymeric matrix this may comprise a third phase (triphase catalysis). Examples of gas—Hquid, gas—soHd, and soHd—soHd PTC are stiH relatively rare. In the latter two cases, a small amount of Hquid, eg, water, is probably present as an uimoticed third phase. 

In practice, 1—10 mol % of catalyst are used most of the time. Regeneration of the catalyst is often possible if deemed necessary. Some authors have advocated systems in which the catalyst is bound to a polymer matrix (triphase-catalysis). Here separation and generation of the catalyst is easy, but swelling, mixing, and diffusion problems are not always easy to solve. Furthermore, triphase-catalyst decomposition is a serious problem unless the active groups are crowns or poly(ethylene glycol)s. Commercial anion exchange resins are not useful as PT catalysts in many cases. 

Crown ethers attached to insoluble polymeric substrates (see the following discussion for examples) have been used as phase transfer catalysts for liquid/liquid systems. In using such systems, the catalyst forms a third insoluble phase the procedure being referred to as triphase catalysis (Regen, 1979). This arrangement has the advantage that, on completion of the reaction, the catalyst may be readily separated from the reaction solution and recycled (Montanari, Landini Rolla, 1982). As... 

A particularly ingenious approach is that of triphase catalysis which was developed by Regen (1979). The catalyst is a quaternary ammonium residue which is covalently bound to an insoluble polystyrene resin, and reactions of anionic reagents are carried out in a two-phase water-organic solvent mixture. 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

Triphase catalysis is applicable not only to liquid-liquid-solid (L, L, S) systems, but also to liquid-solid-solid (L, S, S) systems. For instance, McKenzie and Sherrington (1978) studied the substitution of phenoxide for bromide in 1-bromobutane using immobilized polyethyleneglycol monoethers such as [131] as triphase catalyst they found (Table 38) that under L, L, S... 

The simplest C-C bond formation reaction is the nucleophilic displacement of a halide ion from a haloalkane by the cyanide ion. This was one of the first reactions for which the kinetics under phase-transfer catalysed conditions was investigated and patented [l-3] and is widely used [e.g. 4-12], The reaction has been the subject of a large number of patents and it is frequently used as a standard reaction for the assessment of the effectiveness of the catalyst. Although the majority of reactions are conducted under liquiddiquid two-phase conditions, it has also been conducted under solidrliquid two-phase conditions [13] but, as with many other reactions carried out under such conditions, a trace of water is necessary for optimum success. Triphase catalysis [14] and use of the preformed quaternary ammonium cyanide [e.g. 15] have also been applied to the conversion of haloalkanes into the corresponding nitriles. Polymer-bound chloroalkanes react with sodium cyanide and cyanoalkanes under phase-transfer catalytic conditions [16],... 

Catalysts have been bonded to insoluble polymers to allow, in principle, an appreciable simplification of PTC the catalyst represents a third insoluble phase which can be easily recovered at the end of the reaction by filtration, thus avoiding tedious processes of distillation, chromatographic separation and so on. This is of potential interest mainly from the industrial point of view, due to the possibility of carrying on both discontinuous processes with a dispersed catalyst and continuous processes with the catalyst on a fixed bed. This technique was named "triphase catalysis" by Regen (13,33,34). 

Triphase Catalysis by Quaternary Ammonium and Phosphonium Ions. 57... 

The method and speed of mixing affect observed rates in triphase catalysis when the chemical reactions are fast. For the reaction of 1-bromooctane in toluene with aqueous sodium cyanide (Eq. (3))... 

Intraparticle diffusion limits rates in triphase catalysis whenever the reaction is fast enough to prevent attaiment of an equilibrium distribution of reactant throughout the gel catalyst. Numerous experimental parameters affect intraparticle diffusion. If mass transfer is not rate-limiting, particle size effects on observed rates can be attributed entirely to intraparticle diffusion. Polymer % cross-linking (% CL), % ring substitution (% RS), swelling solvent, and the size of reactant molecule all can affect both intrinsic reactivity and intraparticle diffusion. Typical particle size effects on the... 

The most important point about the alkyl halide reactivities in triphase catalysis is that the reactions which have the highest intrinsic rates are the most likely to be limited by intraparticle diffusion. The cyanide ion reactions which showed the greatest particle size and cross-linking dependence with 1-bromooctane had half-lives of 0.5 to 2 h and with benzyl bromide had half-lives of 0.13 to 0.75 h. The reactions of 1-bromooctane and of benzyl chloride which were insensitive to particle size and cross-linking had half-lives of 14 h and 3 h respectively. Practical triphase liquid/ liquid/solid catalysis with polystyrene-bound onium ions has intraparticle diffusional limitations. 

The % ring substitution of the polymer is a critical factor in catalytic activity. Its importance was demonstrated clearly in Regen s first full paper on triphase catalysis 89). Catalysts 2 and 13 (2% CL) were active for cyanide displacement on 1-bromooctane (Eq. (3)) only at 21 % or lower RS (Table 3). Commerical anion exchange resins, polystyrenes highly substituted as benzyltrimethylammonium ions 2 or benzyldimethyl-(2-hydroxyethyl)ammonium ions 14, were inactive. 

Substantial variations of the organic solvent used in triphase catalysis with polystyrene-bound onium ions have been reported only for the reactions of 1-bromo-octane with iodide ion (Eq. (4))74) and with cyanide ion (Eq. (3)) 73). In both cases observed rate constants increased with increasing solvent polarity from decane to toluene to o-dichlorobenzene or chlorobenzene. Since the swelling of the catalysts increased in the same order, and the experiments were performed under conditions of partial intraparticle diffusional control, it is not possible to determine how the solvents affected intrinsic reactivity. 

Polymer supports different from polystyrene show different response to % RS in triphase catalysis of nucleophilic displacements. Poly(4-vinylpyridines) 66-71 % alkylated with C8, C12, and C16 chains (6) give high yields of cyanide displacement on 1-bromooctane and on 1-iodooctane and of bromide/iodide exchange of the 1-halooctanes81,87). An 81.6% RS n-butyl-quaternized poly(4-vinylpyridine) (6)... 

Polystyrene has been used most often as the support for phase transfer catalysts mainly because of the availability of Merrifield resins and quaternary ammonium ion exchange resins. Although other polymers have attrative features, most future applications of polymer-supported phase transfer catalysts will use polystyrene for several reasons It is readily available, inexpensive, easy to functionalize, chemically inert in all but strongly acidic media, and physically stable enough for most uses. Silica gel and alumina offer most of these same advantages. We expect that large scale applications of triphase catalysis will use polystyrene, silica gel, or alumina. 

The hydrophobicity of TS-1 could also explain why the oxidation of hydrocarbons in aqueous H2C>2 is faster without added organic solvent (triphase catalysis) than in organic solution (biphase catalysis) e.g. benzene hydroxylation under triphase conditions was up to 20 times faster than in acetonitrile or acetone (biphase conditions).1741 Indeed, benzene competes more favourably with water than with organic solvents for adsorption within the micropores of hydrophobic TS-1, as furthermore confirmed through adsorption experiments.1471... 

Bhaumik A., Mukherjee P. and Kumar R., Triphase catalysis over titanium-silicate molecular sieves under solvent-free conditions -1. Direct hydroxylation of benzene, J. Catal. (1998) 178 pp. 101-107. 

Phase transfer catalysis and the use of crown ethers are also of particular advantage in alkanenitrile synthesis (Table 1). Usually quaternary ammonium and phosphonium salts serve quite well as catalysts. Another modification is represented by the use of a solid catalyst, which is insoluble in the two-phase system, for instance alumina or anion-exchange resins (triphase catalysis). Crown ethers again capture the cations and generate naked cyanide ions in fairly nonpolar solvents, leading to exceptionally mild reaction conditions. 

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