Catalyst continued rhodium

The rhodium catalyst (46 mg) is dissolved in acetone (10 ml) in a microhydrogenation apparatus which is then flushed three times with deuterium gas. After stirring the solution in an atmosphere of deuterium for about 1 hr the deuterium uptake ceases and constant pressure is attained. 5a-Cholest-2-ene (136, 19.5 mg) is added and the stirring continued until deuterium uptake ceases (about 3/4 hr). The solvent is evaporated to dryness and the residue is extracted with hexane and the resulting solution filtered through a small alumina column (3 g, activity 111). Evaporation of the hexane gives 2, 3 -d2-5oc-cholestane (137) 18 mg, 92% mp 78-79° isotope composition 94%d2,5%d, andl%do. ... 

Moderate Reactor Productivity. The rhodium catalyst is continuously recycled, but the catalyst is inherently unstable at low CO partial pressures, for example in the post-reactor flash tank. Under these conditions the catalyst may lose CO and eventually form insoluble Rhl3 resulting in an unacceptable loss of expensive catalyst. This reaction is also more likely to occur at low water concentrations, hence in order to run the process satisfactorily catalyst concentrations are kept low and water concentrations relatively high. Hence through a combination of lower than optimum reaction rate (because of low catalyst concentrations) and water taking up valuable reactor volume the overall reactor utilization is less than optimum. 

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. 

Going around the reaction system in Fig. 16, the first problem are poisons for rhodium such as traces of sulfur compounds in the raw materials. 3 valent P-compounds as ligands are highly prone to oxidation according to PR3 + [O] -> 0=PR3. In a continuous process, even traces of peroxides in the starting olefin and traces of oxygen in the synthesis gas accumulate over the time, so meticulous purification steps are a must if ligand-modified rhodium catalysts are used. 

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. 

Introduction. Homogeneous catalytic hydrogenation with cationic rhodium catalysts has been extensively explored by Schrock and Osborn. Use of these complexes in stereoselective organic synthesis has been a topic of more recent interest, and has been recently reviewed. The reagent of choice for many of these directed hydrogenations has continued to be [Rh(nbd)(dppb)]BF4 (1). 

Figure 13. Continuous formation of (S )-4-phenyl-2-butanol from 4-phenyl-2-butanone using the electrochemical enzyme membrane reactor under indirect electrochemical NADH regeneration with a high-molecular-weight rhodium catalyst [26,29,30,65].

C) on a continuous basis, and product solution flows from the reactor into a flash-tank where the initial separation of product from catalyst is achieved. Reduction of pressure in the flash-tank causes vaporization of most of the volatile components while the catalyst remains dissolved in the liquid phase and is recycled back to the reactor. The product stream is directed into a distillation train to remove methyl iodide, water, and heavier by-products from the acetic acid product. The "heavies" include propionic acid and higher-molecular-weight organics arising from condensation reactions of acetaldehyde. Higher alkyl iodides can also form, especially if iodide salts are added to the rhodium catalyst. 

---