Biphasic homogeneous catalysis

An important development in the past 15 years in hydroformylation technology was the introduction of biphasic homogeneous catalysis. Kuntz (62) expressed the basic idea of a new generation of water-soluble oxo catalysts with triphenylphosphane trisulfonate (tppts as the sodium salt) as a ligand for a rhodium-complex-catalyzed hydroformylation process. Ruhrchemie AG adapted the idea on the basis of research done at Rhone-Poulenc and developed it into an industrially viable process, which was... 

Aqueous homogeneous catalysts depend on the development of polar, and thus water-soluble, ligands and their incorporation into organometallic complexes. Therefore, the history of biphasic homogeneous catalysis begins with preparatory work on various water-soluble ligands (cf. Table 1). 

Keywords Hydroformylation, Oxo process. Aqueous biphasic homogeneous catalysis. 

All these observations were empirical, individual results of unsystematic experiments. Since water had been judged, as mentioned, to be incompatible with the metal carbonyl catalysts of the oxo process, this solvent was not a seriously considered alternative. This paper points the way to the introduction of water as a future-oriented solvent for industrial homogeneous catalysis. Applications of phase transfer catalysis will not be considered here (since they require additional, cost-increasing phase transfer agents), but the emphasis will be placed on aqueous biphasic homogeneous catalysis and its status and possibilities. 

The most important and oldest application of aqueous biphasic, homogeneous catalysis is hydroformylation (oxo process, Roelen reaction). This process is used to produce n-butyraldehyde, the desired main product of the reaction of propylene, which is converted by aldolization into 2-ethyUiexenal and this is finally hydrogenated to give 2-ethylhexanol (2-EH), the most economically important plasticizer alcohol (Scheme 1) ... 

The history of biphasic homogeneous catalysis starts with Manasserfs statement... 

The field is in flux. As a consequence of the increased scientific study of aqueous biphasic homogeneous catalysis an increasing number of commercial applications may be expected in the future [14, 17]. Several processes are the subject of detailed surveys and reviews [17]. 

The third possibility comprises a two-phase system made up from a liquid phase and a supercritical fluid. The liquid phase may be considerably swollen by dissolved gas, as is for example observed with mixtures of organic liquids and compressed CO2 [13,14]. This can alter some physical properties, such as the solubility of gases in the liquid phase, to some extent. Type III can be further differentiated into two extremes, depending on the location of the catalyst. In Type Ilia, the catalyst is dissolved in the liquid phase only, whereas the substrates and/or products are mainly in the SCF. Type mb refers to a system with the catalyst separating exclusively in the supercritical phase. Both systems are very similar to those used in conventional biphasic homogeneous catalysis [8]. 

The same is true for combinations of two or more measures to ensure biphasic homogeneous catalysis, such as aqueous-biphase + supercritical solvents [23a-d], aqueous-biphase + iluorous solvents [23e-h], supercritical solvents + ionic liquids [23i], Iluorous -i- supercritical solvents [23j], or other combinations. By the way, some papers argue a new concept for the separation of heterogeneous catalysts by usage of two different and immiscible solvents. Against gravity, surface polarity modified catalysts are located only in one of the two phases. The catalyst phase can be separated by simple decantation [22j,k]. 

To demonstrate the advantages of surfactant-stabilized emulsions for biphasic homogeneous catalysis, the hydrogenation of styrene using the water-soluble catalyst RhCl(tppds)3 (tppds = tris(3,5-disulfonatophenyl)phosphine) was investigated. 

Innovative Concepts fir Catalyst Separation in Biphasic Homogeneous Catalysis 131... 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

The use of thermomorphic systems has recently been studied as a way of achieving catalyst separation in homogeneous catalysis. For example, a biphasic hydroformylation catalyst system was developed to take advantage of the unusual solvent characteristics of perfluorocarbons combined with typical organic solvents (4). Fluorous/organic mixtures such as perfiuoromethylcyclohexane... 

Biphasic techniques for recovery and recycle are among the recent improvements of homogeneous catalysis - and they are the only developments which have been recently and successfully applied in the chemical industry. They are specially introduced into the hydroformylation (or "oxo") reaction, where they form a fourth generation of oxo processes (Figure 5.1 [1]). They are established as the "Ruhrchemie/Rhone-Poulenc process" (RCH/RP) [2] cf. also Section 5.2.4.1), with annual production rates of approximately 800,000 tonnes y"1 (tpy). 

Aqueous biphasic catalysis is a special case of the two-phase processes of homogeneous catalysis. Despite the academic literature s provocative question "Why water " [18a, 18b], the advantages of water as the second phase and the "liquid support" are numerous. On the one hand, the search for the necessary solubility gap is much easier with water than with various organic-phase liquids (Figure 5.2). Additionally, water has many properties which predestine it as a ideal liquid support in homogeneous catalysis (see T able 5.1)[18c,18d]. 

Figure 5.20. The various possibilities for biphasic operation of homogeneous catalysis...

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)... 

Another solution to the problem of catalyst/product separation is the biphasic catalysis. The liquid biphasic catalysis became an attractive technology for potential commercial application of enantioselective homogeneous catalysis. The most important features of such systems are related to the fact that both reaction rate and e.s. may be influenced by the number of ionic groups in water-soluble ligand or by addition of surfactants. Descriptions of water-soluble ligands and the recent results in the rapidly progressing area of biphasic enantioselective catalysis are available in recent reviews [255,256],... 

There are numerous types of multiphasic chemical processes. The most common are biphasic although triphasic, tetraphasic and even higher number of phases can also be used to conduct chemical synthesis. All the multiphasic methods aim to overcome the major problem of homogeneous catalysis, which is catalyst recovery and product separation. The simplest systems are biphasic ones that involve immobilizing a catalyst in one solvent, which is immiscible with a second solvent in which the substrates/products are dissolved. If a gas is required as a substrate then the system could be regarded as triphasic (i.e. liquid-liquid-gas), although for the purposes of this book (and as is most commonly defined elsewhere) such as system will be referred to as biphasic. In other words, only the number of different liquid solvent phases will be used to define the phasicity of a system. 

CO oxidation, 38 236 differential heat of adsorption, 38 217 Biphasic systems, catalysis see Multiphase homogeneous catalysis BiPMo catalysts, 34 39 in formamide to nitrile reaction, 34 39 Bi-postdosing thermal desorption spectroscopy cyclohexene, 42 240... 

For instance, catalysis in liquid/liquid two phases is generally referred to as biphasic catalysis and has widened the practical scope of homogeneous catalysis the catalyst is present in one liquid phase, while reactants and products are present in the other liquid phase. Thus, the catalyst can be separated by simple phase separation. Celanese is operating a 300 000 t/a plant for propylene hydroformylation using a water-soluble rhodium phosphine complex in a biphasic mode of operation at the Ruhrchemie site in Oberhausen [142],... 

Recovery ofthe soluble cattalysts presents the greatest difficulty in large scale appfications of homogeneous catalysis. In a way, aqueous biphasic catalysis itself provides a solution of this problem. It is not the aim of this book to discuss the various other methods of heterogenization of homogeneous catalysts. The only exception is the use of water-soluble... 

In general, it can be concluded, that although a large scale biphasic solution process for hydrodesulfurization and hydrodenitrogenation is not likely to come soon, there are promising results in homogeneous catalysis which can lead to constmction of such processes in the future. 

The concept of this biphasic catalysis implies that the organometallic catalyst is soluble in only one phase whereas the reactants and products are confined almost entirely to the other phase. In most cases, the catalyst phase can be reused, and the products and reactants are simply removed from the reaction mixture by decantation. In successful processes involving biphasic catalysis, the advantages of homogeneous catalysis listed above may be realized without the disadvantages of expensive separation of catalysts from products. 

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