Hydroformylation ultrafiltration

After a hydroformylation run, the reaction solution was subjected to ultrafiltration using an asymmetric polyethersulfone membrane (MWCO 50 kDa) supplied by Sartorius. A retention of 99.8% was found. When the catalyst solution was recycled, virtually the same catalytic activity was observed again (165 TO h 1). Repetitive recycling experiments resulted in 2-7% loss of rhodium, which was subscribed to partial oxidation of the phosphine ligand. 

Since none of the commercially available nano- or ultrafiltration membranes so far shows real long-term resistance against organic solvents under the reaction conditions needed for a commercially interesting hydroformylation process and since no prices are available for bulk quantities of membranes for larger scale applications, considerations about the feasibility of such processes are difficult and would be highly speculative. 

The small droplets act as microreactors when they contain the water-soluble catalysts. For hydroformylation reactions with water-soluble Rh/TPPTS in the droplets, the alkene, carbon monoxide and hydrogen approach the micelle surface where the reaction occurs, as is illustrated in Fig. 5.13. After the reaction is completed, phase separation can be achieved by changing the temperature of the reaction mixture. When the mixture is cooled down an aqueous bottom phase, containing most of the surfactant and the water-soluble catalyst separates from the organic upper phase, which contains the hydrophobic products and unconverted reactants. In case of incomplete catalyst recovery the micelle remaining in the product phase can be separated by means of ultrafiltration. 

Although organic solvents were used in the workup, it might be possible to use filtration plus ultrafiltration, followed by distillation instead. Hydroformylation of 1-hexene has been achieved in polyethylene glycol by using a cobalt catalyst containing polyethyleneoxy substitu-tents.254 The catalyst phase was separated for reuse. 

Just as with hydrogenations, hydroformylations, etc., a major reason for performing catalytic oxidations in water is to provide for facile recovery of the catalysts, by simple phase separation, from the product which is in an organic phase. However, many examples of catalytic oxidations in water involve water-soluble substrates and/or products. In this case catalyst recovery can be facilitated by using polymeric water-soluble ligands (see below) in conjunction with separation with an ultrafiltration membrane [3] or by other measures. 

Industrial interest in soluble polymer-bound catalysts has been closely linked to the development of ultrafiltration membranes with sufficient long-term stability in organic solvents. Membranes fulfilling these requirements were prepared first in the late 1980s. Today, solvent-stable flat sheet membranes and membrane modules are available from several suppliers. As for the viability of ultrafiltration in organic solvents, rhodium-catalyzed hydroformylation of dicydopentadiene with continuous catalyst recovery and recycling has been demonstrated successfully on a pilot plant scale over an extended period of time [5]. The synthesis of other fine chemicals by asymmetric reduction and other reactions has also been carried out in continuously operated membrane reactors (also cf Section 7.5) [6-9]. The extent of commercial interest in catalysts bound to soluble polymers appears to fluctuate at intervals. Amongst other factors, the price of precious metals can be a driver. 

For ultrafiltration as a unit operation for the separation of polymer-bound soluble catalysts in particular, the recovery process for a rhodium catalyst from the hydroformylation of dicyclopentadiene is an illustrative example (for another detailed example, see Section 7.5) [26, 27]. Toluene can be used as a solvent with the polyaramide membrane employed. TPPTS or also a sulfonated bidentate phosphine with large ammonium counterions, are used as ligands. For efficient recovery, molecular weights of the catalyst of more than 3000 g mofi were required on the membrane used. Separation is performed in two steps [28]. A pilot plant was run successfully over an extended period of time of three months. 

One interesting design of macromolecular soluble metal complexes is binding of a rhodium complex with sulfonated triphenylphosphine to a soluble polyelectrolyte, polydiallyldimethylammonium tetrakis(3,5-bis(trifluorome-thyl)phenyl borate. In this ease, the content of phosphine groups enables us to control the catalyst solubility in methanol at high content of the phosphine counter ion, the polymer is insoluble and if the ratio of tetraalkylammonium groups to phosphine groups is 4 1 and 10 1, the polymer is soluble in methanol and is an active catalyst of 1-hexene hydroformylation. The catalyst was separated by ultrafiltration [177]. 

Schweb and Mecking have reported one of the first examples of noncovalent anchoring of catalysts to soluble polymeric supports in 2001. This noncovalently anchored catalyst featured phosphine ligands that were bound by multiple sulfonate groups to soluble polyelectrolytes using electrostatic interactions. The catalyst system was employed in the hydroformylation of 1-hexene and exhibited typical selectivity for a bis-triphenylphosphine-bound rhodium catalyst. The complex was readily recovered and recycled by ultrafiltration. Independently, Reek et al. described similar systems, but these systems made use of a soluble... 

By comparison with the non-immobilized catalyst, similar reaction rates in the hydroformylation of 1-hexene were noted. By ultrafiltration on an asymmetric polyethersulfone membrane, the catalyst could be repeatedly recycled with 2-7% loss of rhodium. 

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