Palladium catalysts recycling

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

Snia Viscosa. Catalytic air oxidation of toluene gives benzoic acid (qv) in ca 90% yield. The benzoic acid is hydrogenated over a palladium catalyst to cyclohexanecarboxyhc acid [98-89-5]. This is converted directiy to cmde caprolactam by nitrosation with nitrosylsulfuric acid, which is produced by conventional absorption of NO in oleum. Normally, the reaction mass is neutralized with ammonia to form 4 kg ammonium sulfate per kilogram of caprolactam (16). In a no-sulfate version of the process, the reaction mass is diluted with water and is extracted with an alkylphenol solvent. The aqueous phase is decomposed by thermal means for recovery of sulfur dioxide, which is recycled (17). The basic process chemistry is as follows ... 

Apart from examples involving Suzuki reactions with standard soluble palladium catalysts, there is a growing number of publications reporting the use of immobilized, recyclable palladium catalysts for carrying out Suzuki and other cross-cou-... 

The complete transformation of a racemic mixture into a single enantiomer is one of the challenging goals in asymmetric synthesis. We have developed metal-enzyme combinations for the dynamic kinetic resolution (DKR) of racemic primary amines. This procedure employs a heterogeneous palladium catalyst, Pd/A10(0H), as the racemization catalyst, Candida antarctica lipase B immobilized on acrylic resin (CAL-B) as the resolution catalyst and ethyl acetate or methoxymethylacetate as the acyl donor. Benzylic and aliphatic primary amines and one amino acid amide have been efficiently resolved with good yields (85—99 %) and high optical purities (97—99 %). The racemization catalyst was recyclable and could be reused for the DKR without activity loss at least 10 times. 

A heterogeneous and recyclable palladium catalyst, Pd/A10(OH), is excellent for the racemization of primary amines. We have demonstrated successful DKR of various primary amines by combining the palladium catalyst and a lipase to produce the corresponding (/ )-acetamides in high yields and in high optical purities. Tables 4.3 and 4.4 show the results of the DKR of benzyhc and aliphatic primary amines. 

Mizugaki et al. 74) have recently utilized thermomorphic properties of Pd(0)-complexed phosphinated dendrimers for dendritic catalyst recycling. Using the method developed by Reetz 16), they prepared dendritic ligands containing, respectively, 2, 8, 16, and 32 chelating diphosphines. Palladium dichloride was com-plexed to the dendrimers, and a reduction in the presence of triphenylphosphine gave the Pd(0)-complexed dendrimers (80—83). The dendritic complexes were active... 

The detrimental effect of the reactive proton-donor group in the [AMIM] ion was further confirmed by the use of an ionic liquid with the proton at the C2 position replaced by a methyl group. The palladium catalyst in [BDMIM]PF6 showed high activity and selectivity in a biphasic system that allowed reaction with multiple recycles and little loss of activity. 

Palladium-catalyzed Heck reactions are important in synthetic organic chemistry (253,254). Under conventional reaction conditions, a palladium black deposit was formed from the deterioration of the homogeneous palladium complex catalyst after the reaction. Recovery and recycle of the palladium catalyst are usually not realistic. 

In contrast, ionic liquids have been reported to be suitable solvents for Heck reactions because the products can be readily separated from the ionic liquids containing the homogeneous palladium catalysts. An early test with a palladium complex in ionic liquids showed remarkably improved recyclability of the catalyst (255), but palladium black still formed after several runs with recycled catalyst. 

Palladium catalyst stability, recovery and recycle are the key to viable commercial technology. Continuous palladium recovery and recycle at 99.9% efficiency is critical to the economics of the process. Traditional catalyst recovery methods fail since the adipic acid precursor, dimethyl hex- -enedioate, is high boiling and the palladium catalytic species are thermally unstable above 125 C. Because of this problem, a non-traditional solvent extraction approach to catalyst recovery has been worked out and implemented at the pilot plant scale. Since patents have not issued, process detail on catalyst separation, secondary palladium recovery, and product recovery cannot be included in this review. 

The hydrogenation of acetic anhydride was also performed in the vapor phase over a supported palladium catalyst resulting in acetaldehyde and acetic acid in high yields (36). To avoid recycling, the reactor was designed for complete reaction of acetic anhydride. Minor selectivity loss was found in formation of ethyl acetate (0.5-1.5%) and methane (0.5%). Typical reaction conditions were 160-200 C, 30-100 psi, with a hydrogen-anhydride ratio of 3 1 to 10 1. A similar catalyst was used in the liquid phase (37). The simplicity and high selectivity of this process is certainly attractive. 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

Catalytic hydrogenation in supercritical carbou dioxide has been studied. The effects of temperature, pressure, and CO2 concentration on the rate of reaction are important. Hydrogenation rates of the two double bonds of an unsaturated ketone on a commercial alumina-supported palladium catalyst were measured in a continuous gra-dient-less internal-recycle reactor at different temperatures, pressures, and C02-to-feed ratios. The accurate control of the organic, carbon dioxide, and hydrogen feed flow rates and of the temperature and pressure inside the reactor provided reproducible values of the product stream compositions, which were measured on-line after separation of the gaseous components (Bertucco et al., 1997). 

A similar reaction was reported by Kabalka et al. where ligandless and solvent-free Suzuki couplings were performed with potassium fluoride on alumina. This reaction is very interesting as the catalyst used was palladium powder, the least expensive form of palladium available32. The authors demonstrated the simplicity of the procedure by efficient isolation of the biaryl products via a simple filtration. This could be done as the palladium catalyst remains adsorbed on the alumina surface. A small amount of water in the matrix was beneficial for the outcome of the reactions. Recycling of the catalyst was possible by adding fresh potassium fluoride to the palladium/alumina surface and the catalytic system remained effective at least through six reaction cycles (Scheme 2.6). 

Behr A, Leschinski J (2009) Application for the solvent water in two-phase telomerisation reactions and recycling of the homogeneous palladium catalysts. Green Chem 11 609-613... 

The effective removal of residual palladium has become one of the most critical issues in the implementation of homogeneously catalyzed Heck reactions in industrial syntheses, particularly in the production of pharmaceuticals, in which tight specifications (sometimes less than 0.5 ppm Pd) must be met. Only very few scalable, inexpensive removal techniques are known [47], for example, binding of Pd to N-acetylcysteine and removal of the adduct by extraction or crystallization [48], When such practical, inexpensive separation and recycling tools for palladium catalysts have been fully developed and industrially implemented, the Heck reaction will certainly find commercial use to the large extent that would be expected from its enormous synthetic utility. 

Palladium-catalyzed Suzuki cross-coupling reactions can be conducted in the ambient temperature ionic liquid, l-butyl-3-methylimidazolium tetrafluoroborate (29), in which unprecedented reactivities are witnessed, and which allows easy product isolation and catalyst recycling (Eq. (60)) [96]. 

---