Recycling of the catalyst

In comparison with classical processes involving thermal separation, biphasic techniques offer simplified process schemes and no thermal stress for the organometal-lic catalyst. The concept requires that the catalyst and the product phases separate rapidly, to achieve a practical approach to the recovery and recycling of the catalyst. Thanks to their tunable solubility characteristics, ionic liquids have proven to be good candidates for multiphasic techniques. They extend the applications of aqueous biphasic systems to a broader range of organic hydrophobic substrates and water-sensitive catalysts [48-50]. 

A related study used the air- and moisture-stable ionic liquids [RMIM][PFg] (R = butyl-decyl) as solvents for the oligomerization of ethylene to higher a-olefins [49]. The reaction used the cationic nickel complex 2 (Figure 7.4-1) under biphasic conditions to give oligomers of up to nine repeat units, with better selectivity and reactivity than obtained in conventional solvents. Recycling of the catalyst/ionic liquid solution was possible with little change in selectivity, and only a small drop in activity was observed. 

Subsequently, the enantioselective variant of this reaction202 was carried out in a biphasic medium (water + diethylene glycol) by using a mixture of amino acids and copper complexes (Eq. 3.54). When the reaction was carried out under an argon atmosphere, the recycling of the catalyst was also possible. The enantiomeric excess decreased slightly... 

Figure 1.6 Recyclability of the catalyst in (BCN)Py.NTf2 (black) and (PCN)Py.NTf2 (hatched) in the Stille reaction between phenyltributylstannane and iodobenzene...

Particular attention is now given to the characterization of the supported species, and to the control of stability and recycling of the catalyst. The behavior of the supported catalyst under the reaction conditions (temperature, pressure, and nature of reactants and products) is certainly a less well-developed area, even though these data are valuable for the conception and development of a fully recyclable catalyst. Very few chemical engineering studies have been so far reported they will become crucial if the objective is the synthesis of an industrial catalyst. 

Reports on homogeneous catalysis did not underline the recyclability of the catalysts. The supported catalysts were recycled without any significant loss of activity and selectivity Table 4 presents the recycling results for the first 3 runs using the substrate la in the presence of both catalysts. The supported catalytic system can easily be separated by centrifugation under ambient conditions without any additional treatment. 

The use of the sulfoxantphos ligand (compound (b) in Figure 7.8) in the biphasic hydroformylation of 1-octene with [BMIM][PF6] has been studied by Dupont and coworkers [58], The ligand allowed recycling of the catalyst solution up to four times with no loss in activity or selectivity. Flighly regioselective hydroformylation (n/iso = 13) was reported for a Rh/phosphine-ratio of 4 (100°C, 15 bar syngas pressure). 

In another example, a polymer-supported chromium porphyrin complex was supported on ArgoGel Cl and then employed for the ring-opening polymerization of 1,2-cyclohexene oxide and C02 [95], This complex showed higher activity than a C02-soluble equivalent, and the solid nature of the catalyst meant that recycling of the catalyst was much easier. 

An anionic rhodium iodide carbonyl complex was supported on polyvinylpyrrolidone for the carbonylation of methanol in the presence of scC02 [98], Depending on the reaction conditions and method of extraction, less than 0.08% rhodium leaching was observed. Saturation of the support with methyl iodide was found to be vital to enhance the longevity and recyclability of the catalyst. 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

The results are comparable to those of homogeneous reaction conditions (Table 41.10), and recycling of the catalyst was successful with constant ee-values over five cycles, even though conversion decreased. Amazingly, the catalyst was still active, despite being stored under atmospheric conditions for 24 h (Table 41.10, entry 7). 

Screening of the reaction media showed that, with the use of an ionic liquid/ water mixture (a so-called wet ionic liquid ), recycling of the catalyst could be improved compared to the reaction in ionic liquid without co-solvent. The com-... 

Two types of continuous membrane reactors have been applied for oligomer- or polymer-bound homogeneous catalytic conversions and recycling of the catalysts. In the so-called dead-end-filtration reactor the catalyst is compartmentalized in the reactor and is retained by the horizontally situated nanofiltration membrane. Reactants are continuously pumped into the reactor, whereas products and unreacted materials cross the membrane for further processing [57]. 

When reactions are carried out in a fluorous phase or for that matter in any biphasic system, the products can often be recovered by simple phase separation. If required, the fluorous phase can be washed with further organic solvent to recover any residual product that remains in the fluorous phase. However, in catalysed reactions, efficient recycling of the catalyst is critical to the success of the reaction. The cost of derivatizing the modifying ligands means that any unrecovered catalyst has serious implications for the economics of the process. 

Palladium-catalysed Heck reactions have been carried out in a mixture of acetonitrile and D-100 as shown in Scheme 10.3. Using a fluorinated phosphine allowed simple recycling of the catalyst by decantation [4],... 

A prime advantage of such biphasic systems is that the catalyst resides in one phase and the starting materials and products are in the second phase, thus providing for easy recovery and recycling of the catalyst by simple phase separation. A pertinent example is the aerobic oxidation of alcohols catalyzed by a water-soluble Pd-bathophenanthroline complex (Figure 9.5). The only solvent used is water, the oxidant is air, and the catalyst is recycled by phase separation. 

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