Aqueous biphasic hydroformylation

In the aqueous biphasic hydroformylation reaction, the site of the reaction has been much discussed (and contested) and is dependent on reaction conditions (temperature, partial pressure of gas, stirring, use of additives) and reaction partners (type of alkene) [35, 36]. It has been suggested that the positive effects of cosolvents indicate that the bulk of the aqueous liquid phase is the reaction site. By contrast, the addition of surfactants or other surface- or micelle-active compounds accelerates the reaction, which apparently indicates that the reaction occurs at the interfacial layer. 

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

Figure 5.8. Membrane steps as a constituent part of aqueous-biphasic hydroformylation A+B->C+D...

The aqueous biphasic hydroformylation concept is ineffective with higher olefins owing to mass transfer limitations posed by their low solubility in water. Several strategies have been employed to circumvent this problem [22], e.g. by conducting the reaction in a monophasic system using a tetraalkylammonium salt of tppts as the ligand, followed by separation of the catalyst by extraction into water. Alternatively, one can employ a different biphasic system such as a fluorous biphasic system or an ionic liquid/scC02 mixture (see later). 

The success of aqueous biphasic hydroformylation stimulated a flurry of activity in the application of the concept to other reactions involving organometallic catalysis [14—27]. 

The POP-Xantphos ligand, which at the present affords the best turnover frequencies and l b ratios is considerably more expensive than the rather simple TPPTS ligand employed in aqueous biphasic hydroformylation. Compared to the classical rhodium-phosphine process, manufacturing costs in the aqueous biphasic process are about 10% lower. Accordingly, requirements for a fairly expensive catalyst will be exceptional if it should be considered in an industrial process, especially if one bears the additional costs for the required ionic liquid in mind. Nevertheless, the prospect of biphasic ionic liquid hydroformylation looks very promising. 

There is still a dispute among experts as to the place in which the biphasic aqueous reaction actually takes place, although it is very probably not the bulk of the liquid but the interfacial layer between the aqueous and organic phases. In the case of aqueous biphasic hydroformylation, this question has been decided by methods of reaction modeling and comparison with experimentally proven facts, thus leading to scale-up rules and appropriate kinetic models as a basis for optimal reactor design [34]. 

The great number of very different proposals which address widely differing points in the 0x0 process does prove that there is currently no single ideal way of reacting higher olefins in aqueous biphasic hydroformylations. In addition, the 0x0 products, which are under a great deal of cost pressure, tolerate no cost-... 

Various papers describe the aqueous biphasic hydroformylation for simple olefins as well as for functionalized olefins or dienes [154-174] (cf. the Section 6.1). In recent work [175], the synthesis of n-nonanal by consecutive isomerization and hydroformylation reactions of trans-4-octene has been described. The catalyst used was the in situ combination of Rh(acac)(CO)2 and the chelate phosphite BIPHE-PHOS. Performing the reaction in propylene carbonate the selectivity to n-nonanal could be raised up to 95%. If after the reaction the product is extracted with dodec-... 

The aqueous biphasic hydroformylation of propene, namely the Ruhrchemie/ Rhone Poulenc (RCH/RP) process, has been widely used to produce n-butanal and many attempts have been proposed to improve this catalytic system, such as a thermoregulated phase transfer (TRPT) Rh(I) complex catalyst [74]. Moreover, Bonnemann et al. [75] have proved the in situ formation of Rh colloids when such a catalyst was applied to the aqueous biphasic hydroformylation of 1-octene. 

The kinetics of low-carbon olefins, ethylene [21] and propylene [22], in aqueous systems was reported and different rate models proposed. Wachsen et al. [13] proved that aqueous biphasic hydroformylation of propylene took place at the interfacial region, in contrast to two preliminary kinetic models that incorporate mass transport. 

Thermoregulated phase-transfer catalysis has been used successfully for the aqueous biphasic hydroformylation of higher olefins [13, 18]. A reasonable explanation for the satisfactory catalytic reactivity is that it results from the thermoregulated properties of Rh/TRL complexes. As shown in Table 1, average turnover frequencies (TOFs) of250 h for 1-dodecene and 470 h for styrene have been achieved. Even the hydroformylation of oleyl alcohol, an extremely hydrophobic internal olefin, exhibits a yield of 72% [24]. 

Although the scope of the aqueous biphasic hydroformylation of functionalized olefins stiU needs to be deeply investigated, these few studies demonstrate clearly that fimctionalized olefins can be hydroformylated efficiently in an aqueous biphasic medium. However, it should be kept in mind that water is not only an inert mobile phase. Water can also act as a reactant or a coordinating solvent that modifies catalytic species. So, in some cases, imexpected increases or decreases in the activity or selectivity can be observed. 

Accompanying Ruhrchemie/Rhone-Poulenc s industrial realization of the aqueous biphasic hydroformylation reaction (cf Sections 2.4.1.1.3 and 2.5.1) and Kuraray s hydrodimerization process (see Section 2.4.4.2), there are some other minor processes for the production of fine chemicals. 

In spite of the importance of the field, the last reviews covering a broad area in hydroformylation are outdated (Falbe 1980, Pruett 1977) and it was felt timely to bring together the recent developments. Only in the area of aqueous biphasic hydroformylation there are several exhausting reviews available. This is the first monograph on hydroformylation of this type and for other processes there not many examples. 

When a TPPTS [P(w-CgH S03Na)3]-modified cationic complex [HRu(CO) (CH3CN)(TPPTS)3]BF was screened in the aqueous-biphasic hydroformylation in a methoxyethanol—water mixture, the following order of activity was noted, which correlates with the results obtained with the Rh-based congener [30] ... 

Hexene was transformed into -heptanal with a relatively low Hb ratio of 2 1. Noteworthy, up to a thiophene concentration of 500 ppm no intoxication effect was observed in the catalytic reaction. Alternatively, for the aqueous biphasic hydroformylation of 1-hexene, the complex RuCl2(DMS0)2(PyS03Na)2 was utilized. Remarkably, it converted cleanly also technical naphtha containing thiophene impurities of up to 50 ppm [31]. 

A similar approach was followed by Monflier and coworkers [94], who sulfonated biphenyl phosphines and screened the resulting BiphTS ligands in the cobalt-catalyzed aqueous biphasic hydroformylation. These phosphines have a solubihty in water of 1.0kgl . Compared to TPPTS, BiphTS ligands are more basic. Remarkably, PfBiPhljTS possesses a basicity similar to that of PPhj, which means that the deactivating effect of the sulfonate groups is fully suspended. 

OctMIM]Br = [OMIM]Br) to the aqueous biphasic hydroformylation, where a significant acceleration of the reaction rates with different olefins were noted (Figure 7.6) [24]. 

The effect of methylated cydodextrins on the RhH(CO)(TPPTS)3 complex in hydroformylation conditions (50 bar, CO H2 = 1 1, and80°C) were investigated by high-pressure P NMR spectroscopy. It was found that the formation of the stable inclusion complex between methylated P-cyclodextrin and TPPTS influences the TPPTS dissociation equilibrium. The methylated a-cydodextrin does not interact with the TP PTS and the methylated y-cydodextrin can only weakly bind to the TP PTS. These results explain why a decrease in the normal-to-branched aldehydes ratio is always observed when cydodextrins are used as mass-transfer agents in aqueous biphasic hydroformylation processes [104]. 

Amino acids and oligopeptides were used as ligands for Rh(CO)2(acac) in the aqueous biphasic hydroformylation of styrene. The water-soluble catalytic system maintained its activity practically unchanged during three recyded experiments [113]. 

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