Hydroformylation catalyst immobilization

An interesting new concept of catalyst immobilization is the use of supported aqueous phase catalysts. Here, the catalyst is immobilized in a thin water layer adhered within the pores of a high-surface-area porous support. A new Rh catalyst of this class with ligand 11 is stable, recyclable, and highly selective in the hydroformylation of higher alkenes to linear aldehydes.236... 

Supercritical hydrogenation is just one example of continuous reactions which can be carried out in SCCO2 solution. Other reactions which have been carried out successfully include Friedel-Crafts alkylation of aromatics by alcohols [64], the dehydration of alcohols to form ethers [65] (using acid catalysts), and the hydroformylation of alkenes [52] (using rhodium catalysts immobilized on Si02). In each of these reactions, it is possible to obtain a selectivity which is at least as good, and often better, than with conventional solvents. However, the precise role of the scCC>2 in these reactions is not as obvious as in supercritical hydrogenation. 

Membrane technology is a recent development to separate (or concentrate) water-soluble catalysts (mainly hydroformylation catalysts) [147, 149], although a prior art is known [194, 195]. There are proposals for the use of immobilized or re-immobilized aqueous phases for large-scale processes (cf. Ref. [222] and Section 3.1.1.6). Carbon dioxide as a solvent for biphasic hydroformylations has been described by Rathke and Klinger [184], although the use of CO2 for hydroformylation purposes was described earlier [185]. For the use of supercritical CO2 cf. Section 3.1.13 with non-aqueous ionic liquids cf. Section 3.1.1.2.2. Investigations with supercritical water are in an early state (e. g., Ref. [223]). 

Table 2. Principles of catalyst immobilization exemplified for hydroformylation.

In a first approximation, the new methods correspond to the conventional solvent techniques of supported catalysts (cf Section 3.1.1.3), liquid biphasic catalysis (cf Section 3.1.1.1), and thermomorphic ( smart ) catalysts. One major difference relates to the number of reaction phases and the mass transfer between them. Owing to their miscibility with reaction gases, the use of an SCF will reduce the number of phases and potential mass transfer barriers in processes such as hydrogenation, carbonylations, oxidation, etc. For example, hydroformylation in a conventional liquid biphasic system is in fact a three-phase reaction (g/1/1), whereas it is a two-phase process (sc/1) if an SCF is used. The resulting elimination of mass transfer limitations can lead to increased reaction rates and selectiv-ities and can also facilitate continuous flow processes. Most importantly, however, the techniques summarized in Table 2 can provide entirely new solutions to catalyst immobilization which are not available with the established set of liquid solvents. 

Supported Liquid-phase Hydroformylation. - A potentially attractive alternative to chemically anchored hydroformylation catalysts is the use of supported liquid-phase catalyst (SLPC) systems for gas-phase hydroformyl-ations. The homogeneous catalyst is dissolved in a non-volatile solvent and then condensed in the pores of a support, where the strong negative capillary forces effectively immobilize the catalyst, thereby preventing metal loss. In addition one might expect that the environment of the homogeneous system... 

Other Systems. - The commercial application of Rh hydroformylation catalysts is currently limited to alkenes of relatively low molecular weights where the product can be removed by distillation. Similar constraints apply to SLPC systems and for chemically immobilized systems low activites can be expected for high molecular weight alkenes because of problems connected with reactant diffusion. A number of publications have appeared over the past five years which address this problem. 

Table I illustrates some of the catalysts we have prepared using polyethylene oligomers. Basically, we have carried out this chemistry with a wide variety of homogeneous catalysts, including various isomerization catalysts, hydrogenation catalysts, hydroformylation catalysts, etc. This approach to catalyst immobilization and recovery seems to be very general. In all cases, we recover 99.9 percent of the catalysts after the first or second cycle whenever we measure it by looking for metal in the filtrate.

Advanced Catalyst Immobilization Technologies for Hydroformylation Catalysis... 

Immobilization of Homogeneous Hydroformylation Catalysts on Solid Surfaces by Covalent Anchoring... 

Figure 6.14.8 Example of a molecularly enlarged triphenylphosphine-type ligand obtained by core functionalization of a carbosilane dendrimer with triphenylphosphine ligands of this type have been used for the immobilization of a Rh-hydroformylation catalyst in membrane reactors. Adapted from Oosterom et al. (2002).

New concepts are under intense academic and industrial development that allow facile product/catalyst separation (i) immobilization of homogeneous hydroformylation catalysts on solid surfaces by covalent anchoring ... 

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