Aqueous Biphase Operations

The crude aldehyde is fractionally distilled into n- and isobutanal in a conventional aldehyde distillation unit. The reboiler of this n/iso column is designed as a heatabsorbing falling film evaporator incorporated in the oxo reactor, thus providing a neat, efficient method of recovering heat by transferring the heat of reaction in the reactor to cold n-butanal, which subsequently heats the n/iso column. The preferred hydroformylation temperature is 110-130 °C and is therefore used for the production of process steam. Whereas other oxo processes are steam importers, the RCH/RP process including the distillation of n-/isobutanol exports steam. No special pretreatment or even purification steps are necessary for the catalyst. This reduces the environmental burden still further. 

Flammability None High High None None High 

Thermal stability High Medium Medium High High Low 

Toxicity None Various Various S None Various Various 

Service life High High Various Unknown 

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Today, the Suzuki cross coupling of aryl halides and arylboronic acids is also carried out in aqueous-biphasic operation starting from chlorinated derivatives instead of their more costly bromo or iodo equivalents (Equation 5.6, [39]). 

A multitude of other reactions are compiled in (Table 5.4,[12m]). A proper choice of ligands and reaction conditions will make many other reactions available via aqueous-biphasic operation. 

TABLE 5.4. Recently described examples for aqueous-biphasically operated reactions... 

Fig. 4 The different methods of separation and recycling of oxo catalysts for the reaction A + B —> C + D. (4.1) Aqueous biphase operation ...

But once more, and similarly to aqueous-biphase operation, basic academic research also lagged behind the industrial research and application. 

The differences in reactivity of the hydroformylation of the alkenes C3 up to C9 are readily explained by the solubUity differences between the various olefins. This is also the case for other examples of aqueous-biphasic operation. All proposals to enhance the reactivity of higher alkenes by addition of solvents, co-ligands, cosolvents, tensides, counter ions, micelle-forming agents, surface-active ligands, etc. [11] (as a single measure or in combination the record holder used three [12]) are based on the improvement of the solubility of the feed alkene in the bulk of the aqueous catalyst solution. This is also important for the idea of reactivity, as shown in Figure 4. 

In terms of green syntheses , the community expects ongoing intensive research work with aqueous biphasic operations. One of the reasons is that water is the only green solvent being used industrially. Aspiring users of the others, such as supercritical carbon dioxide or ionic liquids, suffer from lack of experience, and with all the other biphasic variants they have to cope with the problem that the demanding catalyst systems need to be tolerated by the second phase - mostly as far as the extremely complicated and costly ligands are concerned. 

Table 12.4 Recently described examples of aqueous biphasically operated reactions"- ...

These alternative processes can be divided into two main categories, those that involve insoluble (Chapter 3) or soluble (Chapter 4) supports coupled with continuous flow operation or filtration on the macro - nano scale, and those in which the catalyst is immobilised in a separate phase from the product. These chapters are introduced by a discussion of aqueous biphasic systems (Chapter 5), which have already been commercialised. Other chapters then discuss newer approaches involving fluorous solvents (Chapter 6), ionic liquids (Chapter 7) and supercritical fluids (Chapter 8). 

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

A wide variety of new approaches to the problem of product separation in homogeneous catalysis has been discussed in the preceding chapters. Few of the new approaches has so far been commercialised, with the exceptions of a the use of aqueous biphasic systems for propene hydroformylation (Chapter 5) and the use of a phosphonium based ionic liquid for the Lewis acid catalysed isomerisation of butadiene monoxide to dihydrofuran (see Equation 9.1). This process has been operated by Eastman for the last 8 years without any loss or replenishment of ionic liquid [1], It has the advantage that the product is sufficiently volatile to be distilled from the reactor at the reaction temperature so the process can be run continuously with built in product catalyst separation. Production of lower volatility products by such a process would be more problematic. A side reaction leads to the conversion of butadiene oxide to high molecular weight oligomers. The ionic liquid has been designed to facilitate their separation from the catalyst (see Section 9.7)... 

The most common approach in organic-aqueous biphasic systems is to operate with a continuous aqueous phase, which prevents the occurrence of extreme pH spots, with a volume phase ratio up to 0.4 [35]. An adequate selection of the organic-aqueous volume phase ratio allows for high interfacial areas. The influence of the volume phase ratio in the interfacial area (a) can be predicted according to Equation 1. 

The crucial problem associated with the use of homogeneous rhodium catalysts in industrial hydroformylation is catalyst recovery. Because of the high cost of rhodium, it is necessary to recover rhodium at the ppm level to ensure economical operation. A highly successful solution to this problem was the development and application of the aqueous biphasic catalysis concept. 

The application of ILs is most surprising and intriguing in the area of biotransformations. Many enzymes are cataly tically active in ILs or in aqueous biphasic ionic liquid systems. Lipases maintain their activity in anhydrous IL media, and their enantioselectivity and operational stability are often better than in traditional media. See Table 12.5. 

The literature of biphasic hydrogenations contains plenty of substrates (al-kenes and cycloalkenes, arylaliphatic olefins, carbonyl compounds, etc.), mainly with TPPMS as water-soluble ligand (solubility approx. 200 g/1 [150] as compared with 1100 g/1 with TPPTS [37]). So far, no industrial process has been derived from these smdies. Besides the development of the basics of biphasic operation, the research concentrates on fundamental work concerning the question of where the reaction takes place phase boundary, organic phase, or aqueous phase. Wilkinson [29] concluded from his hydrogenation tests with hexenes or cyclohexenes in the presence of TPPMS that the somewhat lower rate of hydrogenation as compared with monophasic conversion should be due to the necessary diffusion of the hydrogen to the alkene/water interface. In this way the iso-... 

With optimized process operation the reaction is strongly regioselective. The process is commercialized by Rhone-Poulenc using Ruhrchemie s TPPTS and yields precursors for vitamin E cf. Section 3.1.1.1.3 [163, 164] Sc or Y triflates catalyze aqueous biphasic reactions which are alternatives to base-catalyzed processes such as aldol or Michael-type conversions [257]. 

To determine the mode of operation (see below) it is useful to group these homogeneous catalysts into aqueous biphasic systems (see Section 3.1.1.1) and nonaqueous biphasic systems (see Section 3.1.1.2). Gas-liquid-liquid reactions are also involved in organometallic phase transfer catalysis, e. g., in biphasic carbonylation of benzyl chloride to phenylacetic acid by the catalyst system NaCo(CO)4/Bu4NBr/NaOH [21]. Here, the biphasic system consists of an organic solvent and aqueous alkali. 

Table 2. Dependence of the steps in an aqueous biphasic catalytic reaction on the mode of operation.

Table 2.1 gives a comparative overview of the pro and cons of the various options in processes with multiphasic operations. On going from left to right in the table the industrial relevance of the processes decreases. The RCH/RP oxo process and the Shell Higher Olefin Process (SHOP) discussed later belong to the first class (aqueous biphase) and second class (organic biphase), respectively. 

Last but not least, the success of aqueous-phase catalysis has drawn the interest of the homogeneous-catalysis community to other biphasic possibilities such as or-ganic/organic separations, fluorous phases, nonaqueous ionic liquids, supercritical solvents, amphiphilic compounds, or water-soluble, polymer-bound catalysts. As in the field of aqueous-phase catalysis, the first textbooks on these developments have been published, not to mention Job s book on Aqueous Organometallic Catalysis which followed three years after our own publication and which put the spotlight on Job s special merits as one of the pioneers in aqueous biphasic catalysis. Up to now, most of the alternatives mentioned are only in a state of intensive development (except for one industrial realization that of Swan/Chematur for hydrogenations in scC02 [2]) but other pilot plant adaptations and even technical operations may be expected in the near future. 

The synthesis of primary amines in biphasic operation has been developed via selective reductive amination of aromatic and aliphatic carbonyl compounds using aqueous ammonia in the presence of water-soluble transition metal catalysts [17]. The use of [Rh(cod)Cl]2 with TPPTS as catalyst and ammonium acetate in water/ THF afforded benzylamine from benzaldehyde in 86% yield. This method is also feasible for the synthesis of aliphatic primary amines from aliphatic aldehydes in... 

For the different hydroformylation processes described above, the catalyst separation and recycling remain a constant preoccupation. This point is particularly crucial when a very expensive metal is used as catalyst (rhodium). The recycling is either operated by chemical transformation or by direct distillation, depending on the catalyst and its stability. In that way, the development of the aqueous biphasic process can be considered as an important breakthrough (4-6). The separation is operated by decantation, which simplifies the process scheme and limits the risks of catalyst decomposition during distillation. Even if the 0x0 Ruhrchemie/Rhone-Poulenc process presents undeniable advantages, this process remains limited to short-chain olefins (C2-C5) because of the low solubility of higher olefins in water which renders the reaction rates too low for viable processes [7]. 

The extension of classical aqueous biphasic propylene hydroformylation using additional supercritical carbon dioxide or supercritical substrate itself may provide a better catalytic performance when homogeneity of the reaction mixture at operating temperature is attained. This was investigated with [Rh(acac)(CO)2]/20 TPPTS... 

Doubtless, the hydroformylation reaction was the most investigated transition metal catalyzed carbonylation reactions in ionic liquids. This is mainly because of the industrial importance of this reaction that is in operation in various plants, which in some of them operates in aqueous biphasic regimes. The intense academic and industrial interest is mainly because of the limitations of the current aqueous-phase process to short-chain (heavier olefins in water for an effective and reasonable reaction rate to occur. 

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