Immobilized PTC

The separation of soluble PTC is a matter of concern in the industry not only due to environmental considerations, but also due to contamination of the product with the catalyst. Further research should be oriented towards development of novel catalyst separation techniques and of novel reactor-separator combo units. As outlined before, the development of a membrane reactor with PT catalyst immobilized on the membrane surface seems to be a novel and viable candidate for accomplishing PTC reactions on an industrial scale. Another aspect of PTC which needs urgent consideration is the development of engineering technology for immobilized PTC. This would require the development of supports with low diffusional limitations and with the right hydrophilic-lipohilic balance to ensure adequate contact of the aqueous and organic phases with the supported catalyst. 

We shall use immobilized PTC and triphase catalysis interchangeably in this chapter. 

Onium salts, such as tetraethylammonium bromide (TEAB) and tetra-n-butylammonium bromide (TBAB), were also tested as PTCs immobilized on clay. In particular, Montmorillonite KIO modified with TBAB efficiently catalyzed the substitution reaction of a-tosyloxyketones with azide to a-azidoketones, in a biphasic CHCI3/water system (Figure 6.13). ° The transformation is a PTC reaction, where the reagents get transferred from the hquid to the solid phase. The authors dubbed the PTC-modified catalyst system surfactant pillared clay that formed a thin membrane-hke film at the interface of the chloroform in water emulsion, that is, a third liquid phase with a high affinity for the clay. The advantages over traditional nucleophilic substitution conditions were that the product obtained was very pure under these conditions and could be easily recovered without the need for dangerous distillation steps. 

Kenawy 64) immobilized ammonium and phosphonium peripheral functionalized dendritic branches on a montmorillonite supported chloromethylstyrene/methyl methacrylate copolymer (74-75). These polymer/montmorillonite-supported dendrimers were used as phase transfer catalysts (PTC) for the nucleophilic substitution reaction between -butyl bromide and thiocyanate, cyanide, and nitrite anions in a toluene or a benzene/water system. These PT catalysts could be recycled by filtration of the functionalized montmorillonite from the reaction mixture. Generally,... 

Chiral phase-transfer catalysis (PTC) is a very interesting methodology that typically requires simple experimental operations, a mild reaction conditions and inexpensive and/or environmentally benign reagents, and which is amenable to large-scale preparations [15]. The possibihty of developing recoverable and recyclable chiral catalysts has attracted the interest of many groups. Indeed, the immobilization of chiral phase-transfer catalysts has provided the first demonstrations of the feasibility of this approach. 

Other chiral fragments such as N-methylephedrine, N,N-dimethyl a-methylben-zylamine or strychnine have been immobilized, as quaternary ammonium salts, on PS-DVB and tested for different reactions under PTC conditions, but only low levels of enantioselectivity were obtained [262-264]. 

In a true biphasic system (i.e. with no PTC agent) with toluene as the organic phase, nitrobenzene and nitrotoluene were reduced with high efficiency with CO + H20 catalyzed by RhCl3.3H20, [Rh CO ] or [ RhCl(l,5-hexadiene) 2] (12 mol aniline/mol Rh.h, at room temperature and 1 bar CO) [364], Other nitroarenes reacted similarly, however, no or only negligible reaction took place with nitroaliphatics. Nitrobenzene is the standard substrate in such reactions, its reduction to aniline was also catalyzed by c/s-[Rh(CO)2(2-picoline)2]PF6 [365] and analogous cis-[Rh(CO)2(amine)2]PF6 complexes immobilized on poly(4-vinylpyridine) [366],... 

However, the main disadvantages of PTC, espeeially in commercial applications, is the need to separate the catalyst from the product organic phase. Some general separation techniques are discussed in the Conclusion Section. Another method to overcome the problems associated with catalyst recovery is to immobilize the PTC on a solid support. This is discussed in the subseetion on ultrasound in PTC systems. 

PTC reactions can be broadly classified into two main classes soluble PTC and insoluble PTC (Figure 1). Within each class, depending on the actual phases involved, reactions are further classified as liquid-liquid PTC (LLPTC), gas-liquid PTC(GLPTC), and solid-liquid PTC(SLPTC). In some cases, the PT catalyst forms a separate liquid phase, and this variant of PTC can be grouped along with traditional insoluble PTC, where the PT catalyst is immobilized on a solid support. Other nontypical variants of PTC include inverse PTC (IPTC) and reverse PTC via a reverse transfer mechanism (Halpem et al., 1985). 

Although capsule membrane PTC is not suitable for direct scale-up to industrial level due to the inconveniences of working with capsules, the principles can be exploited in membrane reactors, with the PT catalyst immobilized on the membrane surface. This would not only enable easy recovery of both aqueous and organic phases after reaction without any problems of emulsification, but also ensure that the PT catalyst does not contaminate the product in the organic phase. Using a membrane reactor will also ensure high mass-transfer rates due to high interfacial areas per unit volume of reactor. More importantly, it will open up possibilities for continuous operation. 

So far, much research has gone into finding new synthetic routes, new products and novel selective syntheses, and in the analysis of important factors affecting yield and in some cases selectivity. However, other practical constraints relevant to process development for industrial-scale synthesis have to be tackled. For example, new insights are needed to develop cost-effective, stable, and selective PT catalysts (especially effective immobilized triphase catalysts). Other relevant factors include the recovery and recycle of the PT catalyst, catalyst decomposition, environmental issues such as catalyst toxicity, and ease of product recovery. Catalyst costs are not very high when quats are used, as against the more expensive crown ethers or cryptands. In most cases, the overall process is more than cost-effective since PTC allows the use of cheap alternative raw materials, prevents the use of costly dipolar solvents, is less energy intensive (due to lower temperatures) than alternative methods, alleviates the need... 

Catalyst separation from reaction mixtures has been efficiently carried out by using solvent-resistant nanofiltration membranes [75]. Following an alternative approach to solving this problem a quaternary ammonium salt has been immobilized on a soluble poly(ethylene glycol) polymer support. The supported catalyst thus obtained, soluble in solvents commonly used in PTC such as dichloromethane and acetonitrile, was used in a series of standard reactions under PTC conditions with comparable results to those obtained with traditional PTC catalysts [76]. Moreover, it compares favorably to other quaternary salts immobilized on insoluble polystyrene supports [77]. The catalyst can be easily recovered by precipitation with ethereal solvent and filtration and shows no appreciable loss of activity when recycled three times. 

The use of PTC in electroorganic oxidation and reduction reactions is widespread because it involves in situ generation and regeneration of oxidizing and reducing agents. Most applications are in liquid-liquid systems, but it can also be used in solid-liquid PTC systems (Chou et al., 1992). Recently, Do and collaborators extensively studied the electrochemical oxidation of benzyl chloride in the presence of a PT catalyst (soluble and immobilized) in both batch (Do and Chou, 1989, 1990, 1992) and in continuous (Do and Do, 1994a,b,c) electrochemical reactors. Mathematical models for both types of reactors were also proposed. 

It should be noted that this reaction is a special case of solid-liquid catalysis, because the anionic species, solubilized by the catalyst, is both reactant and substrate. Also of importance is the fact that because the tetrabutylammonium salt is used in catalytic quantities, the concentration of active species (the tetrabutylammonium carboxylate) remains very low throughout the reaction, which is therefore conducted in high dilution conditions. As expected, the yields are excellent. Another type of PTC is the triphase catalysis, having as a peculiar feature, the catalyst immobilized on a polymer (solid phase), which is in contact with the aqueous and organic phases containing the reactant and the substrate, respectively. This method presents the major advantage of avoiding the problem of catalyst and product separation. Various macrolides have been synthesized by this procedure. 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

The importance of structural factors in determining the activity of PTC catalysts bonded to a polymer appears, however, to have been largely underestimated It is indeed clear that the same factors governing the lipophilicity, and from a certain point of view the anion activation of soluble quaternary salts, must play a fundamental role in the case of immobilized catalysts as well Short alkyl chains,... 

---