Catalysts triphase

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. 

The catalysts mentioned above are soluble. Certain cross-linked polystyrene resins, as well as alumina and silica gel, have been used as insoluble phase-transfer catalysts. These, called triphase catalysts, have the advantage of simplified product work up and easy and quantitative catalyst recovery, since the catalyst can easily be separated from the product by filtration. 

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... 

Although phase transfer agents have been attached to clays, silica and alumina, the vast majority of studies have used organic polymers, especially polystyrene, as the support. The earliest of these triphase catalysts was prepared from 12% chloromethylated polystyrene crosslinked with 2% divinylbenzene by reaction with a tertiary amine. A wide range of triphase catalysts has since been reported, some examples of which are shown in Figure 5.16. 

Note 1 Polymer phase-transfer catalysts in the form of beads are often referred to as triphase catalysts because such catalysts form the third phase of the reaction system. 

Polymer phase-transfer catalysts (also referred to as triphase catalysts) are useful in bringing about reaction between a water-soluble reactant and a water-insoluble reactant [Akelah and Sherrington, 1983 Ford and Tomoi, 1984 Regen, 1979 Tomoi and Ford, 1988], Polymer phase transfer catalysts (usually insoluble) act as the meeting place for two immiscible reactants. For example, the reaction between sodium cyanide (aqueous phase) and 1-bromooctane (organic phase) proceeds at an accelerated rate in the presence of polymeric quaternary ammonium salts such as XXXIX [Regen, 1975, 1976]. Besides the ammonium salts, polymeric phosphonium salts, crown ethers and cryptates, polyethylene oxide), and quaternized polyethylenimine have been studied as phase-transfer catalysts [Hirao et al., 1978 Ishiwatari et al., 1980 Molinari et al., 1977 Tundo, 1978]. 

Triphase catalyst. Commercial neutral alumina can function as a triphase catalyst in solid-liquid-solid systems. It can catalyze displacement reactions as well as the permanganate oxidation of alcohols. 

Phase Transfer Catalysis. Initial evaluations of the ability of polymer 2 to function as an insoluble or triphase catalyst involved examination of relative rates compared to 18-crown-6. 

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... 

Arrad, O., and Y. Sasson, Silica Impregnated with Tetrametl -ammonium Salts as Solid-Solid-Liquid Triphase Catalysts, /. Org. Chem., 55, 2952 (1990). 

Kondo, S., H. Yasui, and K. Tsuda, Insoluble Polymeric Sulfoxides as Liquid-Solid-Liquid and Solid-Solid-Liquid Triphase Catalysts, Makromol. Ghent, 190, 2079 (1989). 

A fascinating triphase catalyst has been developed this past year for catalytic epoxidation of allylic alcohols. The combination of phosphotungstic acid with an amphiphilic poly(fV-isopropylacrylamidc)-dcrived polymer provides a macroporous complex 5, which functions as a recoverable catalyst in aqueous conditions. Thus, treatment of allylic alcohol 6 with 0.003 mol% catalyst 5 and 2 equiv of hydrogen peroxide in aqueous medium resulted in the formation of the corresponding epoxide 7 in 96% yield. The catalyst exhibits... 

Triphase catalysts (in which the catalyst is anchored to a polymer for ease of removal)... 

Table 19.5 Derivation of a mechanistic model for nucleophilic substitution reactions in the presence of a triphase catalyst (Satrio et al., 2000) ... 

The reversible ion-exchange reaction is expressed by using the traditional notation of heterogeneous catalysis and assuming the formation of a transitional site S Y between the forward and reverse reaction steps, where S " is the site equivalent of the triphase catalyst s cation Q. ... 

It is desired to conduct the transesterification of benzyl chloride with sodium acetate using polymer-supported tributylmethylammonium chloride as a triphase catalyst. The reactor is maintained at isothermal conditions, but temperature gradients can exist within the pellets. Using the following data, generate plots of reactant concentration as a function of time for both isothermal and nonisothermal pellets. 

---