Homogeneous catalyst recovery

J. Dunnewijk, H. Bosch, A.B. de Haan, Reverse Flaw Adsorption Technology for Homogeneous Catalyst Recovery, proc. ISMR-2,2001... 

The discussion in the previous sections has evidenced that the use of biphasic systems has solved, at least in various cases, the problem of homogeneous catalyst recovery and recycle, but there still exists the problem of the cost of recycle and especially of reaction rate per volume of reactor, which derives in large part from mass- and heat-transfer limitations, but also from the low amount of catalytic centers per volume of reactor necessary to avoid side reactions and maintain a high selectivity, and/or limit catalyst deactivation or loss. These aspects often emerge only during the scaling-up and industrialization of the reaction and this is one of the reasons why many interesting reactions at the laboratory scale fail in commercialization. 

Applications for OSN have been under pilot-plant trial or demonstrated in laboratory experiments in solvent deoiling, homogeneous catalyst recovery, separation of phase-transfer agents, and solvent exchange. "... 

The separation problem of homogeneous catalysts can be addressed in different ways. In this section, we discuss the established industrial methods. One of the earliest forms of homogeneous catalyst recovery is by precipitating the metal as an insoluble salt, e.g., a hydroxide or a halide. The metal-containing precipitate is separated by filtration, converted to the active homogeneous catalyst, and then recycled. In many homogeneous catalytic processes, the ligands (see Section 2.1) present in the catalyst must be discarded or separated by some other method. 

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. 

Increasing efforts to heterogenize homogeneous catalysts for LPO are apparent (2,206—209). Significant advantages in product recovery, catalyst use, and catalyst recovery are recognized. In some instances, however, the active catalyst is reported to be material dissolved from the sotid catalyst (210). 

Cobalt in Catalysis. Over 40% of the cobalt in nonmetaUic appHcations is used in catalysis. About 80% of those catalysts are employed in three areas (/) hydrotreating/desulfurization in combination with molybdenum for the oil and gas industry (see Sulfurremoval and recovery) (2) homogeneous catalysts used in the production of terphthaUc acid or dimethylterphthalate (see Phthalic acid and otherbenzene polycarboxylic acids) and (i) the high pressure oxo process for the production of aldehydes (qv) and alcohols (see Alcohols, higher aliphatic Alcohols, polyhydric). There are also several smaller scale uses of cobalt as oxidation and polymerization catalysts (44—46). 

These advantages notwithstanding, the proportion of homogeneous catalyzed reactions in industrial chemistry is still quite low. The main reason for this is the difficulty in separating the homogeneously dissolved catalyst from the products and by-products after the reaction. Since the transition metal complexes used in homogeneous catalysis are usually quite expensive, complete catalyst recovery is crucial in a commercial situation. 

What can drive the switch from existing homogeneous processes to novel ionic liquids technology One major point is probably a higher cost-effectiveness. This can result from improved reaction rates and selectivity, associated with more efficient catalyst recovery and better environmental compatibility. 

In most cases homogeneous chiral catalysts afford higher enantioselectivities than heterogenous catalysts. Nevertheless, the development of heterogeneous chiral catalysts has attracted increasing interest because workup of the reaction, and recovery of often valuable chiral auxiliaries by simple filtration, is more convenient than in the case of homogeneous catalysts. 

The general picture of the relative merits of homogeneous and heterogeneous processes has not yet emerged clearly. The homogeneous catalyst system may offer advantages in chemical efficiency but lead to difficulties of catalyst separation and recovery, or catalysts may tend to plate out in the reactor due to slight instability. Materials of construction may have to be different for the two rival plants. All these factors will have to be considered in an economic assessment and detailed studies made of the complete process networks in both cases. 

The results obtained in reactions involving the two first examples showed a reduced catalytic activity compared to the homogeneous catalyst, a situation that may be due to diffusion problems. Enantioselectivity was similar or slightly lower than in solution, with 80% ee [21] and 58% ee [22] in the epox-idation of ds-/l-methylstyrene with NaOCl providing the best results. Only in the last example was an improvement in enantioselectivity reported from 51% to 91% ee in the epoxidation of a-methylstyrene. Recovery of the catalyst was only considered in one case [21] and a significant decrease in enantioselectivity was observed on reuse. 

Table 7. As can be seen, both Dowex and Deloxan led to poor enantioselec-tivities, which further decreased after catalyst recovery. Better results, which are comparable with those obtained in homogeneous phase, were obtained with Nation (Table 7) [53], although it was necessary to carry out the reaction at 60 °C due to the low copper content in the soHd. This low copper level is a consequence of the low surface area of this polymer (< 0.02 m g ) and, for this reason, a nafion-silica nanocomposite was used as the support [53]. With this catalyst, the reaction took place at room temperature and with similar enantioselectivity (Table 7).

An interesting and potentially u.seful variant is deployment of thermoreversibility, a situation where the reaction occurs at relatively high temperature in a homogeneous phase, which becomes a two-pha.se system at lower temperatures, facilitating catalyst recovery. Here, tailored ligands have to be u.sed. Ethoxylated phosphines have been suggested by Jin, Fell, and co-workers (1996, 1997). 

The reductive carbonylation has an advantage of low feedstock cost. A wide range of homogenous metal complexes have been tested for both reactions (1-16). The major drawback of the use of metal complex catalysts is the difficulty of catalyst recovery and purification of the reaction products (12). In addition, the gaseous reactants have to be dissolved in the alcohol/amine mixture in order to have an access to the catalyst. The reaction is limited by the solubility of the gaseous CO and 02 reactants in the liquid alcohol reactant (17). 

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 ... 

In order to overcome the catalyst recovery problem in homogeneous hydrogenation operations, the significant enhancement of catalyst activity has been pur-... 

Various types of POMs are effective catalysts for the H202- and 02-based environment-friendly oxidations. Most of these oxidations are carried out in homogeneous systems and share common drawbacks, that is, catalyst/product separation and catalyst recycling are very difficult. The heterogenization of POMs can improve the catalyst recovery and recycling. This chapter focuses on the development of (1) homogeneous catalysts with POMs and (2) the heterogenization for liquid phase-oxidations. 

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