Product Separation and Catalyst Recycling

In an ideal case, an ionic liquid dissolves the catalyst and displays a partial miscibility with the reactants under reaction conditions (giving a relatively high reaction rate) and negligible miscibility with the product (giving enhanced selectivity and yield). At the termination of the reaction, the product can be removed by simple decantation without the need to extract the catalyst. This mode of operation eliminates heating and therefore results in reduced loss of catalyst by thermal decomposition (/). 

When the products are partially or totally miscible in the ionic liquid, the separation of the products can be more complicated. It is however possible to reduce the solubility of typical organic products in the ionic liquid by introducing a more polar solvent that can be separated by distillation afterward at a lower temperature (27). Because of the low vapor pressure of the ionic liquid, direct distillation can be applied without azeotrope formation (28). However, such operation is often limited to highly volatile or thermally labile products because of the general thermal instability of organometallic catalysts. 

As an alternative to distillation, extraetion with a eo-solvent that is poorly mis-eible with the ionie liquid has often been used. There are many solvents that can be used to extract product from the ionic liquid phase, whether from a monophase reaction or from a partially miscible system. Typical solvents are alkanes and ethers (15). Supercritical CO2 (SCCO2) was recently shown to be a potential alternative solvent for extraction of organics from ionic liquids (22). CO2 has a remarkably high solubility in ionic liquids. The SCCO2 dissolves quite well in ionic liquids to facilitate extraction, but there is no appreciable ionic liquid solubilization in the CO2 phase in the supercritical state. As a result, pure products can be recovered. For example, about 0.5 mol fraction of CO2 was dissolved at 40°C and 50 bar pressure in [BMIMJPFe, but the total volume was only swelled by 10%. Therefore, supercritical CO2 may be applied to extract a wide variety of solutes from ionic liquids, without product contamination by the ionic liquid (29). 

However, because water has a high polarity, the solubility of CO2 in ionic liquids, exemplified by [BMIMJPFg is markedly reduced by the presence of water (30). 

It has recently been demonstrated that solutes can be extracted from ionic liquids by perevaporation. This technique is based on the preferential partitioning of the solute from a liquid feed into a dense, non-porous membrane. The ionic liquids do not permeate the membrane. This technique can be applied to the recovery of volatile solutes from temperature-sensitive reactions such as bioconversions carried out in ionic liquids (34). 

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Easy product separation and catalyst recycling Lower cost of chemical processes... 

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

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. 

Turnover frequencies could be further increased (reaction rates as high as 7,500 mol mor lf1) if LiCl was added instead of an organic base, however at a pronounced cost to the selectivity. While a temperature of 50°C is required in toluene to activate the catalyst, complex 40 exhibits activity already at -10°C in the ionic liquid. This indicates that the in situ generation of the catalyst, which is believed to require the formation of a Ni-hydride complex, proceeds more efficiently in the ionic liquid. On the other hand, the use of aluminiumalkyles as the proton scavenger led to poor results and the catalyst decomposed rapidly at ambient temperature. The catalyst stability was sufficient at low temperature, -10°C, but the linear product was formed with only 12% selectivity under these conditions. The biphasic nature of the system allows for easy product separation and catalyst recycling. Accordingly, the performance was also tested in a continuous mode and catalytic activity was maintained for at least three hours.1 71 After that time,... 

Interest in coal-derived syntheses of base chemicals has led to a fast growing number of publications in open and patent literature concerning homologation reaction. -Most of that work is devoted to the hydrocarbonylation of methanol and aims at the optimi7ation of catalysis, product separation, and catalyst recycling. 

An inverted scC02-aqueous biphasic system has been used as reaction medium for Rh-catalyzed hydrogenation of polar substrates (Equation 4.36). Chiral and achiral C02-philic catalysts were efficiently dissolved and immobilized in scC02 as the (upper) stationary phase, while water, as the mobile phase, contained the polar substrates and products. Notably, product separation and catalyst recycling were conducted by maintaining the pressure in the reaction vessel. The catalyst phase was reused several times with high conversion and product yields of more than 85% [70]. 

Sulfinate anions have been used as nucleophiles in palladium-catalyzed allylic alkylation [143]. More recently, both Cu- and Pd-catalyzed couplings of sulfinate anions with aryl halides have also been reported as a means to generate unsymmetrical diaryl sulfones, which are common motifs in bioactive molecules [38, 93, 144—148]. Similarly, Cu-catalyzed coupling of arylboronic acids with sulfinate anions has been reported [95,149,150]. Notably, Kantam and co-workers found that the use of ionic liquids permits Cu(OAc)2-catalyzed sulfone synthesis at ambient temperature and with convenient product separation and catalyst recyclability (17) [150]. 

Water (-fco-solvent) Organic liquid Easy product separation and catalyst recycling Lower cost of chemical processes Lack of toxicity of water Low reaction rate for water poorly miscible substrates Mass transfer limits rate of reaction Treatment of spent water... 

Heck reaction has some important applications in industry because it is one of the effective tools for the formation of new C-C bonds [49]. The traditional Heck reaction is performed with a Pd catalyst with phosphine ligands in the presence of a base under an inert atmosphere. However, the expensive Pd complex is often lost at the end of the reaction, which limits the large-scale application of Heck reactions. On the other hand, phosphine ligands, especially the electron-rich ones, are often toxic, and water and air sensitive. For industrial application, it is important to have good strategies for catalyst-product separation and catalyst recycling. SIL catalyst is one of the promising alternatives for the development of eco-friendly processes. 

Solid catalyst, Amberlyst-15 sulfonic ion-exchange resin, proved to be the most effective solid catalyst that gave 82% yield of 5-HMF after 1 minute at 120°C [37]. Immobilization of homogeneous catalyst on solid support also demonstrated good activity. It was foimd that the immobilized ILs and acid modified silica gel were effective for the dehydration of fructose to 5-HMF. The advantages of using solid catalyst are in the area of product separation and catalyst recycling. 

The troublesome process of product separation and catalyst recycling in carbonylation reactions using ionic liquids can be considerably simplified by using a solid ionic phase [68,69] or by introducing of an inert solid support [70]. The continuous liquid-phase carbonylation of methanol has been performed using the rhodium carbonyl iodide complex [Rh(CO)2l2] immobilized on a methylpyridinium cation resin [68,69]. The catalytic activity remains constant for the 2000-h operation with virtually no Rh leaching. IL-impregnated silica was used as a solid support for the Monsanto-type catalyst system [Rh(CO)2l2]-BMM [70]. 

A new inverted biphasic catalysis system using supercritical CO2 as the stationary catalyst phase and water as the continuous phase was described for rhodiumotalyzed hydroformylation of polar substrates. Product separation and catalyst recycling was possible without depressurizing the autoclave. Turnover numbers of up to 3560 were obtained in three consecutive runs and rhodium leaching into the aqueous phase was below 0.3 ppm [125]. Hydroformylation of propene was carried out in supercritical carbon dioxide + water and in supercritical propene + water mixtures using Rh(acac) (CO)2 and P(m-C6H4S03Na)3 as catalysts. Compared to traditional hydroformylation technology, the supercritical reactions showed better activity and selectivity [126]. 

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