Supercritical systems hydroformylation

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). 

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. 

As a unique medium for asymmetric hydroformylation, supercritical carbon dioxide has recently been examined, which can be carried out in an extremely low catalyst concentration. The reactions of styrene (16a) and pentafluorostyrene (16e) catalyzed by Rh-BINAPHOS appear to give mixed results that are highly dependent on the reaction conditions [77,78], Enantioselectivity up to 92-95% ee for 16a or 85 % ee for 16e has been observed [78]. A biphasic reaction system has also been examined for the reaction using Rh(acac)(CO)2 with a sulfonated diphosphine ligand BINAS [79], The reaction proceeds smoothly at 40°C and 100 atm in high conversion with excellent branched aldehyde selectivity (95%), but enantioselectivity is very low (18% ee). The use of these newer reaction conditions is still in the very early stage and further development is expected in the next decade. 

As a further development in the exploration of the role of solvent in hydroformylation systems, the use of supercritical carbon dioxide (SCCO2), ionic liquids and fluorous systems has been reported. A review by Jessop, Ikariya, and Noyori presents information about the utility of SCCO2 in a variety of homogeneous reactions. ... 

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... 

Because of their high solubility in supercritical carbon dioxide (scCOj), similar fluorous catalyst systems have also been successfully used for enantioselective hydroformylation of olefins in this environmentally benign reaction medium [19] (Scheme 3.4). 

A highly fluorous C02-philic rhodium catalyst was effectively immobilized in an inverted H20/scC02 system for the prototypical hydroformylation reaction shown in Eq. (5) [57]. Emulsion-type mixtures are formed under the reaction conditions upon stirring, which separate rapidly when stirring is stopped. After removal of the aqueous phase from the bottom of the reactor, a clear supercritical catalyst phase remains in the reactor that can be re-used for subsequent reactions. Recycling is very efficient at moderate catalyst loadings, but noticeable deactivation occurs at very low rhodium concentrations, probably caused by the low pH of the aqueous solution in the presence of C02. 

A novel catalyst, RhH(CO)(P(p-CFjPh)j)3, was synthesized for the homogeneous catalytic hydroformylation of olefins in supercritical carbon dioxide. The incorporation of p-(trifluoromethyl) groups in the conventional hydroformylation catalyst, HRhCO(PPh3)3, provided enhanced solubility in supercritical carbon dioxide while maintaining catalyst activity and selectivity in the hydroformylation of 1-octene. The reaction rate showed a first-order dependence on the catalyst concentration. The total system pressure had no effect on either the reaction rate or selectivity. However, selectivity was found to depend on the concentration of the catalyst [63]. 

In the research on a supercritical hydroformylation process, all the important dynamic and equilibrium steps in the catalytic system had to be measured for comparisons with conventional media. The hydrogenation of dicobaltocta-carbonyl... 

HCo(CO)4 is held below its equilibrium value for the reaction in eq (3.2-1), which is only achieved after the alkene is fully consumed. The results for propylene hydroformylation in supercritical CO2 have been compared [53] with those of Mirbach [55] for the reaction of 1-octene in methylcyclohexane solution. Under comparable conditions, the steady-state concentrations of the intermediates do not differ greatly (i.e. by little more than a factor of three), and the overall hydroformylation rates are quite similar, d[aldehyde]/dt = 1.2 x 10" M s and 0.77 x 10 M s , for the methylcyclohexane and CO2 systems, respectively. Although different alkenes were used in the two studies, the comparisons are believed to be meaningful, as Wender et al. [56] have shown that hydroformylation rates for a wide range of straight-chain terminal alkenes vary only slightly with chain length for cobalt catalysts. 

The most severe dra wback in homogeneous catalysis is the separation of the catalyst from the reaction mixture. The industrial success of the aqueous two-phase hydroformylation ofpropene to n-butanal [1] in Ruhrchemie AG in 1984 represents the considerable progress in this field. However, aqueous/organic biphasic catalysis has its limitations when the water solubility of the starting materials proves too low, as in hydroformylation of higher olefins (see Chapter 1). To solve this issue, a variety of approaches have been attempted. Additions of co-solvents [2] or surfactants [3, 4] to the system or application of tenside ligands [5, 6] and amphiphilic phosphines [7, 8] are ways to increase the reaction rates. Other approaches such as fluorous biphase system (FBS see Chapter 4) [9], supported aqueous phase catalysis (SAPC see Section 2.6) [10], supercritical CO2 (cf. Chapter 6) [11] and ionic liquids (cf Chapter 5) [12] have also been introduced to deal with this problem. 

Of all the catalyst systems studied, Rh-TPPTS is the most suitable and commercially proven catalyst system for biphasic hydroformylation. Several modifications of the water-soluble catalysts using co-solvents [15], surfactants and micelle-forming reagents [16], a supercritical C02-water biphasic system [17], supported aqueous-phase catalysis [18], and catalyst-binding ligands (interfadal catalysis) [19] have been proposed to overcome the lower rates observed in biphasic catalysis due to poor solubilities of reactants in water (see Sections 2.2.3.2 and 2.3.3.3). So far, endeavors have been centered on innovating novel catalyst systems from the viewpoint of efficient catalyst recycle and rate enhancement, but limited information is available on the kinetics of biphasic hydroformylation. 

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