Hydroformylation catalyst separation

The use of soluble polymers as catalysts was also explored by Bayer. His group showed that both diphenylphosphinated polystyrene and diphenylphosphinated poly(ethylene glycol) could be used as recoverable, reusable hydroformylation catalysts. Separation of the catalyst and the reaction products in these cases was achieved by taking advantage of the properties of the polymer chain. Solvent precipitation or membrane filtration both proved to be acceptable techniques to isolate products free from the polymer-bound catalyst. 

Meth5l-l,3-propanediol is produced as a by-product. The hydroformylation reaction employs a rhodium catalyst having a large excess of TPP (1) and an equimolar (to rhodium) amount of 1,4-diphenylphosphinobutane (DPPB) (4). Aqueous extraction/decantation is also used in this reaction as an alternative means of product/catalyst separation. 

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. 

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... 

The use of a water-soluble phosphine based catalyst is not a preferred choice for octene hydroformylation. Although separation of nonanal and its condensation products from an aqueous catalyst should be facile, forming nonanal at a commercially viable rate could be challenging. In order to react, octene needs to be in the same phase as the catalyst, and octane has very low solubility in water. 

A method to reduce degradation/deactivation of a phosphite modified rhodium hydroformylation catalyst in the separation system involves feeding a diene such as butadiene to the vaporizer to convert the phosphite-modified rhodium catalyst to a more stable form. [34] In the reactor, the diene is hydrogenated and catalyst activity is restored. 

C02 solubility. For example, the desired branched aldehyde formed upon hydroformylation of vinylnaphthalene is considerably more soluble in scC02 than its linear isomer. Therefore, the ratio of the branched to linear product increases from approximately 90 10 in the condensed phase to up to 98 2 in the C02 phase in a single extraction step under the low density conditions of catalyst separation. 

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)... 

I. T. Horvath, J. R abai, Facile Catalyst Separation without Water Fluorous Biphase Hydroformylation of Olefins , Science 1994, 266, 72. 

For example, conversion after two hours reaction time (T=70°C, p(H2/CO) = 50 bar) dropped from 99% in the absence of CO2 to 66.4% at a density d(C02) = 0.35 g mL to 0% at d(C02) = 0.57 g mL The observation of a yellow-orange solid precipitated during the addition of CO2 confirmed the efficient separation of catalyst and substrate. The same separation was induced in the product mixture if CO2 was introduced after the hydroformylation was completed. The mixture of regioisomeric nonanals formed during the reaction was extracted quantitatively with SCCO2 leaving an active hydroformylation catalyst behind in the reactor. 

Those solvatization effects that reflect the ongoing conversion of the hydroformylation are supposed to be similar for catalytically active complexes with molecular similarity. This knowledge opens up the perspective of a pressure-induced catalyst separation in a catalyzed reaction in a desired conversion or a desired operation range of the reactor pressure, where no dissolved metal complex remains in the CO2 phase a defined point (Sect. 5.5). 

Figure 5 Flow diagram of the Union Carbide Process for hydroformylation of higher olefins catalysed by Rhltppms in a single phase with biphasic catalyst separation.

Horvath, I.T. and Rabai, J. (1994) Facile catalyst separation without water fluorous biphase hydroformylation of olefins. Science, 266, 72. 

Horvath, I.T., Kiss, G., Cook, R.A., Bond, J.E., Stevens, P.A., Rabai, J. and Mozeleski, E.J. (1998) Molecular engineering in homogeneous catalysis one-phase catalysis coupled with biphase catalyst separation. The fluorous-soluble HRh(CO) P[CH2CH2(CF2)5CF3]3 3 hydroformylation system. J. Am. Chem. Soc., 120, 3133. 

Another method for catalyst recycling was introduced by Davies and Hanson [38]. The heterogenization of the rhodium catalyst in an aqueous phase on a silica carrier enabled simple catalyst separation via filtration. Oleyl alcohol was hydroformylated in a thin aqueous phase on the catalyst surface at 50 bar and 100°C. Under these conditions, the yield was 97%, while the catalyst could be recycled several times without any loss of activity. 

Pd-catalyzed allylic amination of crotyl acetate by piperidine, while exhibiting a reduced rate as is commonly observed for heterogenized systems (90% conversion achieved after 30 min compared with 5 min for the homogeneous system). Interestingly, the catalyst could be recycled three times via a simple filtration step. Subsequently, the catalyst was separated from the support and the support was uploaded with hydroformylation catalysts. 

The recycling potential of hydroformylation catalysts in ionic liquids is highly dependent on the way the product is isolated. In terms of catalyst lifetime, simple decantation is certainly the method of choice. However at an industrial level, distillation is the most common separation technique and evaluation of catalyst recyclability under somewhat more stressful conditions has been determined/451... 

Water soluble ligands greatly facilitate catalyst separation. Examples are (22-XLI)174 and (22-XLII),175 which are used in hydroformylations and hydrogenation catalysis, respectively. Rhodium complexes of the sulfonated phosphine (22-XLI) are used in the production of butyraldehyde, a large-scale process developed by Ruhrchemie/Rhone-Poulenc.174... 

Although the oxo synthesis has been applied industrially almost 50 years, its reaction mechanism has not been clarified in every detail. Some aspects of the proposed reaction pathway are still under investigation. Among industrial hydroformylation catalysts, major differences are observed between modified and unmodified systems and therefore they will be discussed separately. 

In commercial applications of propene hydroformylation the process underwent several modifications predominantly aimed at improvements in product/catalyst separation. The very first version of the process, which was later named the gas recycle process , effected the removal of the product aldehydes from the catalyst solution by applying a large gas recycle in order to evaporate the aldehydes [146, 196, 197]. The catalyst solution consisted of high-boiling aldehyde condensation products (dimers, trimers, and various other aldehyde consecutive products), in which an excess of TPP and the rhodium complex itself was dissolved [198, 199]. In order to keep the volume of this reaction mixture constant, the reaction conditions had to be maintained in a manner which allowed continuous evaporation of the aldehyde products generated by the hydroformylation reaction... 

It has yet to be seen whether the principle of biphasic hydroformylation can be further extended beyond C4 olefins. Bearing in mind the advantages of biphasic operation, two pathways may be considered biphasic operation in the reactor section and subsequent phase separation or a combination of homogeneous hydroformylation reaction with an auxiliary agent. This substance would require a miscibility gap with the products under conditions different from the reaction conditions. Examples of both principal methods have already been published [271, 272]. However, a general solution is not to be expected, as each feed-stock/product pair requires a specially adapted solvent. Novel developments in the field of catalyst separation and reuse of catalyst systems are noted below. 

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