(137f) Aqueous Surfactant Solutions As Smart Solvents for Catalytic Reactions

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.

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].

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.