280508 Aqueous Surfactant Solutions As Smart Solvents for Catalytic Reactions

Monday, October 29, 2012: 4:55 PM
330 (Convention Center )
Michael Schwarze1, Tobias Hamerla2, Anke Rost2, Katja Seifert1 and Prof. Dr. Reinhard Schomäcker2, (1)Department of Chemistry, Berlin Institute of Technology, Berlin, Germany, (2)Department of Chemistry, Berlin University of Technology, Berlin, Germany

With the growing need for sustainability, the aim of chemical processes is not only to provide a product; moreover it should follow the principles of Green Chemistry formulated by Paul Anastas and John Warner [1]. The practical use of these principles should help to prevent pollution, save energy, and reduce the potential for chemical accidents. One decisive approach is the replacement of harmful or toxic organic solvents by water. An essential problem of this approach is that many reactants are poorly soluble in water, but solubility can be significantly increased by adding surfactants. In this context, the application of surfactant-based reaction media has increased during the last years and it is shown in the literature that many noble-metal catalyzed homogeneous reactions can be successfully performed in these media, e.g. hydrogenation reactions [2], coupling reactions [3], hydroformylation reactions [4] or methathesis reactions [5]. Moreover, surfactant systems allow for catalyst recovery which is a major problem in homogenous catalysis.

For aqueous-micellar solutions catalyst recovery is possible by micellar enhanced ultrafiltration (MEUF), a well known technique from the field of waste-water purification.

Retentat & Permeat TX100-Lösung_klein.JPG

Figure 1: Aqueous-micellar reaction solution (dissolved rhodium catalyst) before (yellow) and after (colorless) ultrafiltration process (membrane MWCO=10 kDa).

In the case of microemulsion systems, a mixture of water/organic solvent/surfactant (here the organic solvent is only partially replaced by water), catalyst recovery can be performed by phase separation. In Figure 2, the different states of a microemulsion system are shown. Under optimized conditions, the catalyst can be recycled after the reaction from a 2- or 3 phase system.



Figure 2: States of a microemulsion system with dissolved rhodium catalyst.

Although there are many examples of reactions in these media, the development of a combined reaction-separation-process is no easy task. In this contribution we will show results for the rhodium catalyzed hydrogenation of itaconates in aqueous-micellar solutions and discuss the main parameters which are important with respect to catalyst recycling and product isolation using MEUF, e.g. the partition coefficient of the reactants and the influence of the surfactant and surfactant concentration.

Table 1: Partition coefficients of itaconates in aqueous-micellar TX-100 solutions [6].









log KMW






Furthermore, we will show examples for noble-metal catalyzed reactions in microemulsions, e.g. coupling reactions and hydroformylation reactions. In the case of microemulsions, a detailed analysis of the phase behavior is required to find the best conditions for reaction and catalyst recycling by phase separation. If this information is available, several combined reaction and separation cycles can be performed as recently reported by H. Nowothnick et al. [7].     

[1] P.T. Anastas, J.C. Warner, Green Chemistry: Theory and Practice., Oxford and New York: Oxford University Press, 1998; [2] M. Schwarze et al., RSC Adv. 1 (2011) 474; [3] B.H. Lipshutz et al., Org. Lett. 10 (2008) 1333; [4] M. Gottardo et al., Adv. Synth. & Cat. 352 (2010) 2251; [5] B.H. Lipshutz et al., Org. Lett. 10 (2008) 1325; [6] M. Schwarze et al., Ind. & Eng. Chem. Res. 51 (2012) 1846; [7] H. Nowothnick et al., Angew. Chem. Int. Ed. 50 (2011) 1918.

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