Two-phase catalyst

One important variant of Rh/PPhs catalysis is the two-phase catalyst system developed by Kuntz at Rhone-Poulenc in 1981, using a sulfonated tri-phenylphosphine ligand, P(C6H4-m-S03Na)3 (TPPTS) to generate the water soluble catalyst RhH(C0) P(C6H4-m-S03Na)3]3. 

In the application of membrane technology (cf. Section 3.2.3) for the separation of the Rh complexes and the re-immobilized ligands after the reaction, a further remarkable enlargement of the ligands was desirable. Unfortunately, the combination of diamines with TPPTS yields highly crosslinked polymeric materials, which cannot be handled. A reduction of the degree of crosslinking is possible by use of the disulfonated TPPDS (cf. Section 3.1.1.1). So, in a combination with TCD-diamine (tricyclodecane diamine) a salt was formed that was partly soluble in toluene and soluble in THF. The same was the case with the use of MA -dimethyl-TCD-diamine. These salts may be useful in water-free two-phase catalyst systems. 

Not surprisingly the ideal form of the process is aqueous biphase catalysis, in which the organometallic two-phase catalyst resides in a stationary aqueous solution in the reaction system. This is not only the most convenient arrangement on both the laboratory and industrial scale, but also the optimal modification wich respect to cost and environmental considerations. Use of water as the second phase has its limitations however, especially when the water solubility of the starting materials proves too low, preventing adequate transfer of organic substrate into the aqueous phase or at the phase boundary, and consequently reducing the reaction rate to such an extent that it becomes unacceptable. Cases... 

In this concept, catalysis is carried out in a homogeneous, one-phase reaction system, while a two-phase catalyst extraction is done afterwards. A one-phase reaction seems to be preferred, when the type III two-phase... 

The alternative is to fabricate CCLs as ultrathin two-phase composites 100 mn -200 mn), in which electroactive Pt could form the electronically conducting phase, or Pt nanoparticles could be supported on a conductive substrate. The remaining volume should be filled with liquid water, as the sole medium for proton and reactant transport. The ultra-thin two-phase catalyst layer was explored by using the Poisson-Nemst-Planck (PNP) equations as employed for water-filled spherical agglomerates [69, 118]. The equations in Section 8.5.2 can be rewritten for the ID planar situation... 

Figure 8.17. Effectiveness factor of ultrathin two-phase catalyst layers as a function of thickness, Lcl. at different current densities. (Reproduced by permission of ECS — The Electrochemical Society, from Wang Q, Eikerling M, Song D, Liu S. Modeling of ultrathin two-phase catalyst layers in PEFCs.)...

Wang Q, EikerUng M, Song D, Lin S. Modeling of ultrathin two-phase catalyst layers in PEFCs. J Electrochem Soc 2007 154(6) F95-101. 

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