Example+2

=**5. Palladium as a Possible Catalyst** = toc

Palladium is a well known green transition metal catalyst. Not only is this catalyst stable over long periods of time, but it is also recyclable in numerous cases. Recently, it has been shown that a palladium-based catalysts can be amphiphilic, soluble in both water and organic solvents - potentially opening up the process to further greening via replacement of organic solvents with water. These catalysts represent a significant improvement over the more widely-used chromium-based catalysts, which require organic solvents, and are more difficult to recover.

 5.1. Preparation of Palladium Catalysts
The Wacker process, developed over 40 years ago, is the oldest known palladium-catalyzed alcohol oxidation reaction. A co-catalyst, typically copper (II) chloride, is required for this reaction to proceed. More recently, palladium-based catalysts such as Pd(OAc) 2 /O 2 /DMSO have been used. Unfortunately, DMSO is a highly toxic solvent.  More recently, an aluminum hydroxide-supported palladium catalyst has been found to have several beneficial properties, such as solubility and activity in water, showing promise for future greening of the process. The preparation scheme for this catalyst is shown below.

 5.2. The Reaction
Several reaction schemes involving green, palladium-based catalysts have been proposed. The combination of a friendlier, reusable catalyst with a much friendlier oxidant will make the oxidation process much greener than the current processes. Two know and currently tested reactions are shown below.

5.3. Solvents
Currently, the toluene solvent is used in conjunction with palladium catalyst. The difficulties with the use of toluene include an extra distillation step along with some inefficiencies in recovering the catalyst. The next step towards further greening this process is using a water solvent instead.  Why isn't this already in practice? There are concerns that the combination of a catalyst, reactants and gaseous oxidant can lead to explosions. The solution to this problem is a inert reaction medium. Currently, critical solutions of carbon dioxide are proposed as a stable medium, or carrying this process out in a continuous flow reactor, but both are hard to put into large scale use.