Rhodium recycling

Recently, a new rhodium recycling system was described that takes advantage of amphiphilic ligands such as Ph2ArP (Ar = 3-hydroxyphenyl, 4-carboxyphenyl). The corresponding rhodium complexes are active in the hydroformylation of 1-octene and can be separated from the products by acidic or basic extraction into water. After neutralization of the aqueous phase, the rhodium species could be extracted into a new batch of octene, with toluene as a solvent. The recovered catalyst retained only up to 87% of its activity (72). 

Another approach to water-soluble phosphines with the emphasis on metal recycling was reported by van Leeuwen and co-workers [30], They have synthesized a number of diphosphines that, when coordinated to rhodium, form complexes having an amphiphilic character. The ligands synthesized are based on BISBI and Structures 20-22, and hydroformylation (for example) can be conducted in a homogeneous (organic) phase [30 a]. After it has been used in the hydroformylation of olefins the catalyst can be removed by acidic extraction. It was established that these novel diphosphines form active and highly selective catalysts. This amphiphilic approach, i.e., rhodium recycling abased on the extraction and re-extraction principle, will be discussed in more detail in Section 7.5. 

For the rhodium-catalyzed hydroformylation of higher alkenes, novel amphiphilic diphosphines have been reported (Structure 10-16), based on BISBI (2,2 -bis[di-phenylphosphino]methyl-l,l -biphenyl), XANTHAM, POPpy and POPam, which can be used in the rhodium recycling system [21, 22],... 

The efficiency of the rhodium recycling system using the new amphiphilic ligands was determined by re-use of the catalyst solution in a second hydroformylation run and by determination of the absolute amount of rhodium (metal) recovered using ICP-AES. 

The economics of the process depend on loss-free rhodium recycling, which is now readily achievable. A disadvantage is the corrosivity of the iodide, which requires the use of expensive stainless steels for all plant components. Up to now, alternatives such as replacement of the halogen or immobilization of the catalyst have not proved feasible. 

Buhling, A. Elgersma, J.W. Nkrumah, S. Kamer, P.C.J. Van Leeuwen, P.W.N.M. (1996) Novel amphiphilic diphosphines synthesis, rhodium complexes, use in hydroformylation and rhodium recycling, Dalton Trans., 2143-54. 

The unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

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. 

CO, and methanol react in the first step in the presence of cobalt carbonyl catalyst and pyridine [110-86-1] to produce methyl pentenoates. A similar second step, but at lower pressure and higher temperature with rhodium catalyst, produces dimethyl adipate [627-93-0]. This is then hydrolyzed to give adipic acid and methanol (135), which is recovered for recycle. Many variations to this basic process exist. Examples are ARCO s palladium/copper-catalyzed oxycarbonylation process (136—138), and Monsanto s palladium and quinone [106-51-4] process, which uses oxygen to reoxidize the by-product... 

Liquid-phase oxidation of lower hydrocarbons has for many years been an important route to acetic acid [64-19-7]. In the United States, butane has been the preferred feedstock, whereas ia Europe naphtha has been used. Formic acid is a coproduct of such processes. Between 0.05 and 0.25 tons of formic acid are produced for every ton of acetic acid. The reaction product is a highly complex mixture, and a number of distillation steps are required to isolate the products and to recycle the iatermediates. The purification of the formic acid requires the use of a2eotropiag agents (24). Siace the early 1980s hydrocarbon oxidation routes to acetic acid have decliaed somewhat ia importance owiag to the development of the rhodium-cataly2ed route from CO and methanol (see Acetic acid). 

Rhodium was about three times the price of gold through 1988—1989 until skyrocketing to 74/g ( 2300/troy oz) in early 1990. Thus precious metal catalyst costs requite an absolute minimum level of use and maximum number of catalyst recycle uses when batch processing is employed. Starting material contaminants may effect catalyst poisoning, though process routes to overcome this by feed stream pretreatment may be devised (37,60). 

Separation Processes. Separation of the catalyst from the products is a significant expense the process flow diagram and the processing cost are often dominated by the separations. Many soluble catalysts are expensive, eg, rhodium complexes, and must be recovered and recycled with high efficiency. The most common separation devices are distiUation columns extraction is also appHed. 

The processiag costs associated with separation and corrosion are stiU significant ia the low pressure process for the process to be economical, the efficiency of recovery and recycle of the rhodium must be very high. Consequently, researchers have continued to seek new ways to faciUtate the separation and confine the corrosion. Extensive research was done with rhodium phosphine complexes bonded to soHd supports, but the resulting catalysts were not sufficiently stable, as rhodium was leached iato the product solution (27,28). A mote successful solution to the engineering problem resulted from the apphcation of a two-phase Hquid-Hquid process (29). The catalyst is synthesized with polar -SO Na groups on the phenyl rings of the triphenylphosphine. 

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. 

In one patent (31), a filtered, heated mixture of air, methane, and ammonia ia a volume ratio of 5 1 1 was passed over a 90% platinum—10% rhodium gauze catalyst at 200 kPa (2 atm). The unreacted ammonia was absorbed from the off-gas ia a phosphate solution that was subsequently stripped and refined to 90% ammonia—10% water and recycled to the converter. The yield of hydrogen cyanide from ammonia was about 80%. On the basis of these data, the converter off-gas mol % composition can be estimated nitrogen, 49.9% water, 21.7% hydrogen, 13.5% hydrogen cyanide, 8.1% carbon monoxide, 3.7% carbon dioxide, 0.2% methane, 0.6% and ammonia, 2.3%. 

Hydroformylation catalyzed by rhodium triphenylphospine results in only the 9 and 10 isomers in approximately equal amounts (79). A study of recycling the rhodium catalyst and a cost estimate for a batch process have been made (81). 

Good rhodium retention results were obtained after several recycles. However, optimized ligand/metal ratios and leaching and decomposition rates, which can result in the formation of inactive catalyst, are not known for these ligands and require testing in continuous mode. As a reference, in the Ruhrchemie-Rhone-Poulenc process, the losses of rhodium are <10 g Rh per kg n-butyraldehyde. 

Moderate Reactor Productivity. The rhodium catalyst is continuously recycled, but the catalyst is inherently unstable at low CO partial pressures, for example in the post-reactor flash tank. Under these conditions the catalyst may lose CO and eventually form insoluble Rhl3 resulting in an unacceptable loss of expensive catalyst. This reaction is also more likely to occur at low water concentrations, hence in order to run the process satisfactorily catalyst concentrations are kept low and water concentrations relatively high. Hence through a combination of lower than optimum reaction rate (because of low catalyst concentrations) and water taking up valuable reactor volume the overall reactor utilization is less than optimum. 

Finally, these aqueous suspensions of rhodium(O) and iridium(O) are the most efficient systems for the hydrogenation of a large variety of mono-, di-substituted and/or functionalized arene derivatives. Moreover, in our approach, the reaction mixture forms a typical two-phase system with an aqueous phase containing the nanoparticle catalyst able to be easily reused in a recycling process. 

A method has been developed for the continuous removal and reuse of a homogeneous rhodium hydroformylation catalyst. This is done using solvent mixtures that become miscible at reaction temperature and phase separate at lower temperatures. Such behavior is referred to as thermomorphic, and it can be used separate the expensive rhodium catalysts from the aldehydes before they are distilled. In this process, the reaction mixture phase separates into an organic phase that contains the aldehyde product and an aqueous phase that contains the rhodium catalyst. The organic phase is separated and sent to purification, and the aqueous rhodium catalyst phase is simply recycled. 

Since these mixtures are immiscible at room temperature but miscible at the higher reaction temperature, there is excellent contact between the rhodium catalyst and the olefin when the reaction is carried out, increasing the reaction rate by orders of magnitude. After the reaction is complete, the reaction mixture is cooled and the phases completely separate. The product can be simply recovered by decantation and the catalyst can be recycled (Fignre 28.1). 

Precious metals reclamation Precious metals reclamation is the recycling and recovery of precious metals (i.e., gold, silver, platinum, palladium, iridium, osmium, rhodium, and ruthenium) from hazardous waste. Because U.S. EPA found that these materials will be handled protectively as valuable commodities with significant economic value, generators, transporters, and storers of such recyclable materials are subject to reduced requirements. 

Chiral thioureas have been synthesized and used as ligands for the asymmetric hydroformylation of styrene catalyzed by rhodium(I) complexes. The best results were obtained with /V-phenyl-TV -OS )-(l-phenylethyl)thiourea associated with a cationic rhodium(I) precursor, and asymmetric induction of 40% was then achieved.387,388 Chiral polyether-phosphite ligands derived from (5)-binaphthol were prepared and combined with [Rh(cod)2]BF4. These systems showed high activity, chemo- and regio-selectivity for the catalytic enantioselective hydroformylation of styrene in thermoregulated phase-transfer conditions. Ee values of up to 25% were obtained and recycling was possible without loss of enantioselectivity.389... 

Only a slight decrease in the conversion was observed during the recycling. No leaching of the immobilized rhodium complex occurred as indicated by ICP-AES analysis of the mother liquor. 

A breakthrough in hydro formylation was achieved with the introduction of a tri-arylphosphine-modified, in particular triphenylphosphine-modified, rhodium catalyst. [5] This innovation provided simultaneous improvements in catalyst stability, reaction rate and process selectivity. Additionally, products could be separated from catalyst under hydro formylation conditions. One variant is described as Gas Recycle (Figure 2.1) since the products are isolated from the catalyst by vaporization with a large recycle of the reactant gases. [6] The recycle gas is chilled to condense butanals. 

In the practice of gas recycle hydroformylation [7], rhodium complex and triphenylphosphine are dissolved in a suitable solvent. The reactor is pressurized with the... 

An alkene which will give a polar aldehyde product and syn gas are introduced into the reactor containing a non-polar ligand modified rhodium catalyst. Catalyst solution exiting the reactor enters a Flash stage where CO/H2 are purged. The catalyst solution then enters an extractor where it is contacted with a polar solvent. The product aldehyde is captured in the polar solvent in the extractor, then concentrated in the Solvent Removal Column. Polar Solvent is recycled to the Extractor. The Non-Polar catalyst solution is recycled to the reactor (see Figure 2.5). 

In hydroformylating with a polar ligand modified rhodium catalyst to give a relatively non-polar aldehyde product, after the flash column, the catalyst solution is extracted with a non-polar solvent. Polar catalyst recycles from the extractor to the reactor. The non-polar solvent is removed and recycled to the extractor (see Figure 2.6). 

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