Homogeneous catalyst separation methods

A number of potential methods for homogeneous catalyst separation and recovery have been discussed in the preceding chapters. This chapter addresses the separation of homogeneous catalysts by means of advanced filtration techniques. Separation of homogeneous catalysts by size exclusion (ultra- or nanofiltration, defined in detail in Section 4.3.1) offers several advantages ... 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

TABLE 9.1. Rhodium catalysed hydroformylation of 1-octene using a variety of separation methods and catalyst systems compared with results from homogeneous... 

Liquid multiphasic systems, where one of the phases is catalyst-philic, are attractive for organic transformation, as they provide built-in methods of catalyst separation and product recovery, as well as advantages of catalytic efficiency. The present chapter focuses on recent developments of catalyst-philic phases used in conjunction with heterogeneous catalysts. Interest in this field is fueled by the desire to combine the high catalytic efficiency typical of homogeneous catalysis with the easy product-catalyst separation features provided by heterogeneous catalysis and in situ phase separations. 

Undoubtedly, the most notable feature of these new dendrimeric organometallic molecules is their ability to act successfully as effective homogeneous catalysts for the Kharasch addition reaction of polyhalogenoalkanes to olefinic C=C double bonds. Indeed, they show catalytic activity and clean regiospecific formation of 1 1 addition products in a similar way to that observed in the mononuclear compounds. Likewise, the nanoscopic size of these first examples of soluble dendritic catalysts allows the separation of such macromolecules from the solution of the products by ultrafiltration methods. 

Another approach is to separate the products from the homogeneous catalyst using a two phase liquid system. For example, this method is used in the oligomerization step of the Shell Higher Olefins Process for the manufacture of linear a-olefins.5,9-11,330 A polar nickel catalyst containing a P- chelate ligand is dissolved in a polar solvent e.g. 1,4-butanediol, which is immiscible with higher oc-olefins, and recovery of the catalyst is easily achieved by simple phase separation. 

Homogeneous catalysis by transition metal complexes almost always involves processes in which product-catalyst separation and catalyst recycling are important issues. For years, researchers have worked to find effective ways to isolate metal-complex catalysts in phases separate from those containing the catalyst, usually by anchoring the metal complex to a solid surface. As summarized by Driessen-Holscher, it is now evident that the method that has met with most practical success in this direction involves the use of multiple liquid phases. For example, rhodium complexes with water-soluble sulfonated ligands are used to catalyze alkene hydroformyla-tion, and the aqueous-phase catalyst and the organic products are easily separated as insoluble liquid phases. 

Although not all of the factors that influence homogeneous hydrogenation and hydroboration in sc C02 are fully understood, it is clear that the use of sc C02 can lead to an increase in selectivity for some reactions. Additional work is needed to understand the opportunities for further selectivity enhancements and catalyst separation/recycle strategies. Even sc C02 systems that exhibit similar selectivities to those obtained in organic solvents could offer a practical, environmentally responsible method for the production of many important chiral building blocks. 

The synthesis of Prosulfuron , reaction 7.32 involves hydrogenation of the intermediate with Pd/C catalyst followed by other reactions. Suggest a method that would address both the problems of separation of the homogeneous catalyst and the subsequent hydrogenation step. 

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