Recycling in homogeneous catalysis

Tooze, B. Cole-Hamilton, D. J. Recovery and Recycling in Homogeneous Catalysis Kluwer Dordrecht, 2005. 

D. J. Cole-Hamilton, Recovery and Recycle in Homogeneous Catalysis, Kluwer, Dordrecht 2005. 

Catalysis in liquid-liquid biphasic systems has developed recently into a subject of great practical interest because it provides an attractive solution to the problems of separation of catalysts from products and of catalyst recycle in homogeneous transition metal complex catalysis. Two-phase systems consist of two immiscible solvents, e.g., an aqueous phase or another polar phase containing the catalyst and an organic phase containing the products. The reaction is homogeneous, and the recovery of the catalyst is facilitated by simple phase separation. 

Turnover numbers (TONs) and substrate/catalyst ratios ([S]/[C] ratios) seem the preferred quantities in homogeneous catalysis, in contrast to biocatalyst loading (units L-1) and TTNs in biocatalysis. In the case of slow homogeneous chemical catalysts, the [S]/[C] ratio can approach unity (stoichiometric conditions). In the limit of no recycle, the values for TTN and TON are identical upon re-use of catalyst, TTN increases correspondingly. Whereas recycling is very important in biocatalysis, it does not seem to be common practice in homogeneous chemical asymmetric catalysis. 

Despite the remarkable activity and selectivity attainable through sophisticated ligand design in homogeneous catalysis, broad commercial application has been hampered by the need to recover and/or recycle expensive catalysts and ligands. A number of approaches, termed biphasic catalysis , have been advanced where a soluble catalyst is immobilized in one liquid phase (often aqueous) and the substrates and products are isolated in a separate immiscible phase (Baker and Tumas, 1999 Cornils and Herrmann, 1998). Several recent reports have focused on systems using only water and C02, where water-soluble catalysts and C02-soluble substrates and products can be isolated in separate phases (Table 2.4). Hydrogenation of cinnamal-... 

Beyond these methodical developments which will prove their utility in the next decade catalyst recycling is of crucial importance in homogeneous catalysis. In contrast to heterogeneous catalysis recycling of the expensive metal is usually difficult. To facilitate catalyst/product separation often the attachment of a catalyst to an organic polymeric resin has been used. Although this concept workes nicely on a laboratory scale decreased activity and... 

Phosphine-derivatised poly(4-/i r/-butylstyrene) has been prepared for use as a soluble support in homogeneous catalysis.It was used in a monophasic medium and separation of the catalysts after reaction was effected either by cooling- or water-induced phase separation. The support was prepared by co-polymerisation of /i r/-butylstyrene with a phosphine oxide-containing styrene monomer (Scheme 19) A small quantity of a methyl red-labelled comonomer was also added to act as a colorometric tag to facilitate studies of the extent of separation and recycling of the polymeric material. The phosphine oxide was reduced to the free phosphine after the polymerisation step was complete. 

The use of liquids in homogeneous catalysis thus means not only a liquid support and from there a basic intervention in the handling and the operation of the catalyst, but also a modern separation technique for efficient work-up in organic synthesis [3], Figure 3 illustrates the enormous importance of the biphasic technique for homogeneous catalysis the catalyst solution is charged into the reactor together with the reactants A and B, which react to form the solvent-dissolved reaction products C and D. The products C and D have different polarities than the catalyst solution and are therefore simple to separate from the catalyst phase (which may be recycled in a suitable manner into the reactor) in the downstream phase separation unit. 

The basic question that has to be answered for each individual reaction examined is whether or not the products produced are of high enough value to warrant the use of immobilized catalysts. It is likely that the commercial impact of such catalyst systems will be greatest in the pharmaceutical area where the selectivities inherent in homogeneous catalysis are needed, the value of the products is high and the ability to easily separate the catalyst could pay for itself after a limited number of recycles. Impact in this area has not been obvious, probably because the polymer immobilized catalyst systems have not been in the hands of the typical synthetic organic chemist. 

Recovery and recycle of homogeneous catalysis are said to be the focal point of any new generation of, for example, hydroformylation technologies. Additionally, the costs of the new process are determined by the losses - an argument which is specially tabled at the occasion of discussions about the use of high-price rare metal catalysts such as rhodium or palladium. Obviously this is true because a new mode of recycling characterizes a new process. But on the other hand, metal losses (and supplementary ligand costs) are only a (minor) part of the overall cost as will be demonstrated in Section 12.4. 

The quest for separation and recycling has always been a key issue in homogeneous catalysis in general. This is why methods have been devised to anchor the... 

The separation of reaction products from catalysts is a recurrent problem in homogeneous catalysis. The major drawback of the common separation techniques applied in homogeneous catalysis is the extensive (and usually destmctive) postreaction workup required. OSN membranes, being selective between high MW catalysts (>450 Da) and reaction products, are able to perform this separation. Nair et al. (2002) presented a membrane-based (STARMEM 122) process for the separation of a phase transfer catalyst (PTC) and a Heck reaction transition metal catalyst from the reaction media. For the PTC catalyst the process was so efficient that rejections superior to 99% were observed for both pre- and postreaction mixtures and no reaction rate decline was observed for two consecutive catalyst recycles. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

Slightly later, and independently of Cole-Hamilton s pioneering work, the author s group demonstrated in collaboration with Leitner et al. that the combination of a suitable ionic liquid with compressed CO2 can offer much more potential for homogeneous transition metal catalysis than only being a new procedure for easy product isolation and catalyst recycling. In the Ni-catalyzed hydrovinylation of... 

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