Recovery, catalyst

In many homogeneous catalyst-based industrial processes efficient recovery of the metal is essential for the commercial viability of the technology (see Section 1.4). This is especially true for noble metal-based homogeneous catalytic reactions. Apart from economic reasons spent catalyst recovery is also essential to prevent downstream problems, such as poisoning of other catalysts, deposition on process equipment, waste disposal, etc. Several different techniques are being followed industrially for the recovery of the catalyst from the reaction medium after the end of the reaction  

Precipitation of the catalyst from the reaction medium, followed by filtration, as in the cobalt-based hydroformylation process (see Section 5.4). Here cobalt is removed from the reaction products in the form of one of its salts or as the sodium salt of the active carbonyl catalyst. The aqueous salts can be recycled directly, but sometimes they are first converted into an oil-soluble long-chain carboxylic acid salt, such as the corresponding naphthenate, oleate, or 2-ethylhexanoate. 

Selective crystallization of the product. This leaves the catalyst and the residual substrate in the liquid phase. The liquid phase with the catalyst is normally recycled. 

Flash distillation of the product where very high vacuums are applied at moderate temperatures so the solvents and products vaporize, which are collected and condensed in a condenser, leaving the catalyst behind in the vessel. In the Monsanto acetic acid process, the catalyst rhodium iodide is left behind in the reboiler once the products are flashed off (see Section 4.9). 

Liquid/liquid extraction of the catalyst, as in the DuPont adiponitrile process, where the nickel complex is extracted out of the product mixture after the reaction, with a solvent (see Section 7.7). In Shell s SHOP process the soluble nickel catalyst is also extracted from the reaction medium with a highly polar solvent, and reused (see Section 7.4.1). 

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. 

The most widely used industrial unit operation for the separation of a soluble catalyst from solution is distillation. Distillation could be of two types—flash distillation and distillation external to the reaction. In flash distillation, the reaction is carried out at elevated temperatures to continuously evaporate the products, while the catalyst remains in the solution. Thus the soluble catalyst always remains in the reactor and does not have to be recycled. The reactants that evaporate with the products can be reused by recycling them back to the reactor. 

In distillation external to the reactor, a part of the solvent is evaporated together with the reactants and products, while the high boiling homogeneous catalyst solution is recycled via the bottom section of the distillation column. An obvious drawback of distillation is the decomposition of the homogeneous catalyst at elevated temperatures. The maximum temperatures of both flash and external distillations are limited by the temperature at which the homogeneous catalyst decomposes. 

Another well-documented separation method is phase separation and/or extraction. In extraction, the differences in the solubilities of various compounds, and/or miscibilities of two liquids present in the reaction mixture, are exploited. Many organic liquids and water do not mix. This effect can be exploited if the products and reactants have very different solubilities in aqueous and organic phases. Recovery of homogeneous catalysts from a mixture of two immiscible liquids by phase separation is a relatively recent, successful industrial method. We discuss these and related methods in more detail in Chapter 5. 

Finally, at a research level, heterogenizations of homogeneous catalysts have been extensively studied. The motivation behind this method is to combine the advantages of heterogeneous and homogeneous catalysts, i.e., easy separation with high selectivity. Basically, in this method 

---

Carbon monoxide High-pressure Off-gas Light ends Catalyst recovery Dehydration... 

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). 

Water formed in the reaction as well as some undesirable by-products must be removed from the acetic acid solvent. Therefore, mother Hquor from the filter is purified in a residue still to remove heavies, and in a dehydration tower to remove water. The purified acetic acid from the bottom of the dehydration tower is recycled to the reactor. The water overhead is sent to waste treatment, and the residue still bottoms can be processed for catalyst recovery. Alternatively, some mother Hquor from the filter can be recycled directiy to the reactor. 

Heterogeneous hydrogenation catalysts can be used in either a supported or an unsupported form. The most common supports are based on alurnina, carbon, and siUca. Supports are usually used with the more expensive metals and serve several purposes. Most importandy, they increase the efficiency of the catalyst based on the weight of metal used and they aid in the recovery of the catalyst, both of which help to keep costs low. When supported catalysts are employed, they can be used as a fixed bed or as a slurry (Uquid phase) or a fluidized bed (vapor phase). In a fixed-bed process, the amine or amine solution flows over the immobile catalyst. This eliminates the need for an elaborate catalyst recovery system and minimizes catalyst loss. When a slurry or fluidized bed is used, the catalyst must be separated from the amine by gravity (settling), filtration, or other means. 

Eor trace removals, eg, catalyst recovery, usually to <100 ppm often difficult to regenerate solvent to such low levels. 

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

Ex situ or off-site, regeneration of base metal catalysts is a service offered by several vendors worldwide, including Catalyst Recovery, Inc., of Lafayette, Louisiana, and Medicine Hat, Alberta, Canada Catalyst Recovery, Europe of Rodange, Luxembourg Nippon CRI of Miyako, Japan Englehard (formerly Edtrol) of Salt Lake City, Utah Eurecat, U.S., of Pasadena, Texas and Eurecat, SA of La Voulte, Erance (22—28). 

Marston Sala high-gradient magnetic separator Electromagnet, SI 1 percondi icting 20,000 50,000 Steel wool, expanded metal, steel halls 25x Itf Strongly to very weakly 0.0001-2 Iron ores, industrial minerals, coal, liquefied coal, wastewaters, purifiers, catalyst recovery, chemical industry... 

The problem of efficient catalyst recovery from the flue gas has since been overcome by Shell, which has an external separator design... 

Fluid cat cracking required identifying stable operating regimes for beds of fine catalyst at high gas flow rates. Highly efficient cyclone and electrostatic systems had to be developed for catalyst recovery. Finally, the principles of pressure... 

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. 

A most important issue in the present method was the ability to reuse the catalyst, without losing its activity. This proved to be one of the most salient advantages offered by 4. Catalyst recovery was straightforward, since it involved a simple filtration step. In our hands, it was possible to perform several cycles of catalyst reuse by simple filtration and washing steps (see Table 7). 

The catalysts mentioned above are soluble. Certain cross-linked polystyrene resins, as well as alumina and silica gel, have been used as insoluble phase-transfer catalysts. These, called triphase catalysts, have the advantage of simplified product work up and easy and quantitative catalyst recovery, since the catalyst can easily be separated from the product by filtration. 

The activity and enantioselectivity of the complex 6a-Cu were shghtly lower than those observed in dichloromethane, but the catalysts were stable after two reuses. After each reaction, the products were extracted with hexane and the catalyst remained in the ionic hquid phase, ready for reuse. In the case of the complex 6b-Cu, the enantioselectivities obtained in the former reactions (entry 3 in Table 5) were significantly lower than those observed in dichloromethane ( 90% ee) under the same conditions. When the complex was prepared in situ in the ionic hquid, as opposed to dissolving the preformed complex, the enantioselectivity results improved considerably (entry 5) but decreased again after catalyst recovery. 

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).

Cobalt catalysts such as HCo(CO)4 are widely used for hydroformyla-tion of higher alkenes, despite the higher temperatures and pressures required. The main reason for this is that these catalysts are also efficient alkene isomerization catalysts, allowing a mix of internal and terminal alkenes to be used in the process. Catalyst recovery is more of a problem here, involving production of some waste and adding significantly to the complexity of the process. A common recovery method involves treating the catalyst with aqueous base to make it water soluble, followed by separation and subsequent treatment with acid to recover active catalyst (4.3). 

The immobilization of metal catalysts onto sohd supports has become an important research area, as catalyst recovery, recycling as well as product separation is easier under heterogeneous conditions. In this respect, the iron complex of the Schiff base HPPn 15 (HPPn = iVA -bis(o-hydroxyacetophenone) propylene diamine) was supported onto cross-linked chloromethylated polystyrene beads. Interestingly, the supported catalyst showed higher catalytic activity than the free metal complex (Scheme 8) [50, 51]. In terms of chemical stability, particularly with... 

Scheme 3.7 REMP Intramolecular metathesis of pre-catalyst 75 to form catalyst 76 incorporation of monomers, release of a cyclic polymer and catalyst recovery...

Hence, P-C bond-cleavage followed by isomerization is responsible for the formation of side products. Furthermore, due to destabilization of the catalyst complex, deactivation occurs and palladium black is formed, which is a notorious disadvantage of Pd-phosphine catalysts in general. Catalyst decomposition and the formation of side products causes additional separation and catalyst recovery problems. These problems have been solved by the discovery of novel catalyst complexes, which are active and stable at temperatures of over 250 °C (Cornils and Herrmann, 1996). 

Complete catalyst recovery is important for several reasons ... 

Thermal operations such as distillation, decomposition, transformation, and rectification often cause thermal degradation. Furthermore, with these processes quantitative catalyst recovery is generally not possible, which results in loss of productivity. 

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). 

Betzemeier et al. (1998) have used f-BuOOH, in the presence of a Pd(II) catalyst bearing perfluorinated ligands using a biphasic system of benzene and bromo perfluoro octane to convert a variety of olefins, such as styrene, p-substituted styrenes, vinyl naphthalene, 1-decene etc. to the corresponding ketone via a Wacker type process. Xia and Fell (1997) have used the Li salt of triphenylphosphine monosulphonic acid, which can be solubilized with methanol. A hydroformylation reaction is conducted and catalyst recovery is facilitated by removal of methanol when filtration or extraction with water can be practised. The aqueous solution can be evaporated and the solid salt can be dissolved in methanol and recycled. 

Pd/Al203-FeCl3, and Ce-Pd/Al203-FeCl3 catalysts exhibit activity for the synthesis of ethylphenylcarbamate from the reductive carbonylation of nitrobenzene with ethanol at 453 K and 2.07 - 2.93 MPa. The advantage of the use of Al203-supported Pd catalyst is the easy of catalyst recovery form the reactants/product mixture. 

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). 

---