Hydroformylation in supercritical fluids

The cobalt-catalyzed hydroformylation of 1-octene was investigated using bis tri(3-fluorophenyl)phosphane hexacarbonyldicobalt and Go2(GO)g as precatalysts in 

The rhodium-catalyzed hydroformylation of 1 -octene with the new P-donor ligands PPh3 (OC9Hig) (n = 3, 2,1) containing branched alkyl chains was investigated in supercritical carbon dioxide and toluene as solvents. The selectivities for aldehydes were higher in the supercritical medium than in toluene [119]. 

The combination of supercritical CO2, Rh(CO)2(acac), and a fluorous polymeric phosphane 38 was found to be a highly effective catalytic system for the chemose-lective hydroformylation of usually unreactive alkyl acrylates (Equation 7.11) [120]. 

A new inverted biphasic catalysis system using supercritical CO2 as the stationary catalyst phase and water as the continuous phase was described for rhodiumotalyzed hydroformylation of polar substrates. Product separation and catalyst recycling was possible without depressurizing the autoclave. Turnover numbers of up to 3560 were obtained in three consecutive runs and rhodium leaching into the aqueous phase was below 0.3 ppm [125]. Hydroformylation of propene was carried out in supercritical carbon dioxide + water and in supercritical propene + water mixtures using Rh(acac) (CO)2 and P(m-C6H4S03Na)3 as catalysts. Compared to traditional hydroformylation technology, the supercritical reactions showed better activity and selectivity [126]. 

Hydroformylation of 1-hexene in supercritical carbon dioxide was investigated using a rhodium-phosphane catalyst tethered to a silica support. The performance of the tethered catalyst was compared with a homogeneous rhodium-phosphane 

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Webb, P.B.and Sellin, M.F. and Kimene, T.E. and Williamson, S. and Slawin, A.M.Z. and Cole-Hamilton, D.J. (2003). Continuous Flow Hydroformylation of Alkenes in Supercritical Fluid-Ionic Liquid Biphasic System. J. Am. Chem. Soc., 125, 15577-15588. 

There are an increasing number of applications of high pressure NMR in supercritical fluids to homogeneous catalysis [266]. Using their toroidal pressure probe, Rathke and coworkers [249, 267-269] have extensively studied the Co2(CO)g-cata-lyzed hydroformylation of olefins in scCOj (Eq. (14)). The hydrogenation of Co2(CO)g (Eq. (15)) is a key step in this reaction. 

Sellin MF, Webb PB, Cole-Hamilton DJ (2001) Continuous flow homogeneous catalysis hydroformylation of alkenes in supercritical fluid-ionic liquid biphasic mixtures. Chem Commun 8 781-782... 

When a fluid is compressed and heated above the critical conditions (or to supercritical conditions, sc), the differences between gas and liquid disappear. For carbon dioxide, this occurs for temperatures above 31 °C and pressures above 7.3 MPa. For reactions (such as alkylations, aminations, hydroformylations, hydrogenations and Fischer Tropsch synthesis) occurring in supercritical fluids, the reaction rate is often increased dramatically because of improved desorption of heavy molecules minimizing the oxygen and hydrogen solubility limitations, improved heat transfer, and improved selectivity by a catalyst by minimizing pore diffusion limitations. 

The same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

This chapter has focused on heterogeneous catalysis in supercritical media, but the relationship between supercritical fluids and catalysis is much broader. There have been numerous studies of homogeneous catalysis in SCFs. Examples include hydroformylation via cobalt carbonyl complexes in supercritical CO2, oxidation via metal salts dissolved in supercritical water, and acid-catalyzed dehydration of alcohols in supercritical water. 

It must be also noted that supported ionic liquid phase (SILP) catalysis can also be successfully combined with supercritical fluids. Cole-Hamilton et al. [127] have reported recently high activity (rates up to 800 h ), stable performances (>40 h) and minimum rhodium leaching (0.5 ppm) in the hydroformylation of 1-octene using a system that involves flowing the substrate, reacting gases and products dissolved in... 

As mentioned earlier, supercritical fluids have a broad potential for application in new processes. In recent years a number of publications reviewed the role of supercritical fluids in technical applications [9-16], and more industrial applications of sc-fluids were established in the synthesis of fine chemicals. In a variety of reactions such as hydrogenations, hydroformylations and Friedel-Crafts reactions, the advantageous use of sc-fluids as solvents has been demonstrated with respect to yield, selectivities, and no work-up procedure [17]. 

In order to overcome this limitation, the elegant concept of biphasic catalysis for hydroformylation reaction was further extended to media other than water [8]. The identity of this ideal second phase is far from obvious as very few solvents present chemical and physical properties in accordance with the hydroformylation requirements. Without giving a complete list, one can cite perfluorinated solvents [9] (see Chapter 4), supercritical fluids [10] (see Chapter 6), and nonaqueous ILs [11]. At... 

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