Catalysts leaching

Little to no catalyst leaching and high catalyst recyclability. 

Catalyst leaching - The mixture may segregate leading to possible leaching of snlfate gronps. The leaching of catalyst was stndied in organic and in aqneons phases. 

Figure 57.6. The light microscope images of the catalysts leached from the alloys of Figure 57.5 and Table 57.2 at 110°C.

Vorlop et al. described a novel cross-linked and subsequently poly(vinyl alcohol-entrapped PaHNL for synthesis of (//(-cyanohydrins. These immobilized lens-shaped biocatalysts have a well-defined macroscopic size in the millimeter range, show no catalyst leaching, and can be recycled efficiently. Furthermore, this immobilization method is cheap and the entrapped (/ )-oxynitrilases gave similar good results compared with those of free enzymes. The (//(-cyanohydrin was obtained in good yields and with high enantioselectivity of up to >99% ee [55],... 

Poly(7V-vinyl-2-pyrrolidone)-Rh complex was used to catalyze the carbonylation of MeOH to MeOAc and AcOH in supercritical C02 at rates approximately 50% of those in liquid solution, but with minimal catalyst leaching.72... 

Supported aqueous phase (Chapter 3, Section 3.6, Chapter 5, Section 5.2.5) and supported ionic liquid phase catalysts, Chapter 7, Section 7.3) are probably not suitable for use with higher alkenes because the liquid feed slowly dissolves some of the water or ionic liquid changing the nature of the catalyst and leading to catalyst leaching. 

There are several polymer supported transition metal hydrofomylation catalysts (42 ). Most are attached by phosphine ligation and suffer fron catalyst leaching. There are no n5-cyclo-pentadienyl half sandwich systems despite the potentially, clearly advantageous presence of the relatively strong Cp-metal bond (43,MO. Resin 5 was used in the following brief study in which the potential of polystyrene-supported CpCo(C0)2 to function as a hydrofomylation catalyst was tested. 

In both the Ni(acac)2-[bmim][PF6] and fhtorous biphasic systems, catalyst leaching is very low and several further batch oxidation reactions may be carried out with similar results to those obtained in the first run. In the fluorous biphasic system, the yield of 4-chlorobenzoic acid dropped from 87 % to 70 % by the sixth reaction cycle using [bmim][PF6] as a solvent, the yield was essentially the same after four uses, and no catalyst was found to leach into the organic phase. 

Catalysis experiments were performed to investigate the telomerization of butadiene with ethylene glycol in selected TMS systems (e.g. si toluene DMF 1 5 4 or sl 2-octanol DMSO 1.35 3 5.2). With Pd/TPPTS as the catalyst a maximum yield of only 10% of the desired products could be achieved. With Pd/TPPMS the yield increased up to 43% in the TMS system si toluene isopropyl alcohol, but additional water had to be added to obtain a phase split after the reaction. The catalyst leaching is very high and 29% of the palladium used is lost to the product phase. 

Both the cyclic carbonates and NOP are able to dissolve the catalyst. It was observed that at separation temperature NOP can be found in the product phase as well as in the catalyst phase. This causes again imdesired catalyst leaching (Table 8) albeit at levels that could be reduced already down to 1% with BC as the polar phase. Thus, it was necessary to identify a mediator with a similar property like NOP that exclusively can be found in the catalyst phase. 

Table 8 Catalyst leaching dependent of cyclic carbonate ...

In general, more NMP than NOP is needed to close the miscibility gap between the cyclic carbonate and the extraction agent. In addition, a higher temperature dependency of the mixing behaviour can be recognized upon use of NMP at room temperature the NMP-TMS systems pass from closed to open systems. This effect results in an almost complete immiscibihty of the mediator and the cycHc carbonate with the extraction agent at separation temperature. In this way the catalyst leaching into the product phase can be suppressed. 

The variation of the cyclic carbonate revealed that with increasing polarity (decrease in carbon chain length) the conversion regarding frans-4-octene decreases from 100 to 94% and the selectivity to n-nonanal increases from 76 to 86%. This is accompanied with rising catalyst leaching which reflects again the increasing difference in HSP between the cyclic carbonate and the mediator (Table 10). 

Comparable experiments with N-methyl-2-pyrrolidone as the mediator produced similar results, but now with an even lower catalyst leaching between 0.06 and 0.26% for rhodium and 0.56 and 1.40% for phosphorous... 

The rhodium loss to the product phase was investigated by ICP-OES spectrometry. In all cases very low catalyst leaching can be observed and less than 1.5% of the rhodiiun is extracted (Table 12). The catalyst loss can be correlated to the polarity and the solubiUty of the mediator s3 in the product phase s2. The less polar s3, the more s3 is dissolved in s2 and the more catalyst is lost. 

Leaching can be analyzed with respect to both the catalyst (rest state) and catalyst degradation products. The above reactions involve the separation of an n-octane product solution from a 5a catalyst residue at - 30 °C, and a subsequent n-octane extraction at - 30 °C. The data in Fig. 2, together with the solvent quantities employed, predict catalyst leaching of < 0.33% per cycle (calculated from the solubility at - 20 °C). This rises to 1.0 and 3.6% if phase separations are conducted at 0 and 20 °C, respectively. 

---