ACHEMA 2009 location:Frankfurt am Main (Germany) date:11-15 May 2009
The palladium-catalyzed hydrogenolysis of aromatic ketones to alkylbenzenes was studied in mixtures of ionic liquids to explore the promotional effect of these innovative reaction media. First, the miscibility of a wide range of ionic liquids with aromatic ketones and alkylbenzenes was evaluated. From these miscibility tests, it emerged that choline-based ionic liquids display the desired mixing behaviour, i.e., complete miscibility with the aromatic ketone substrate at reaction temperature and a clear phase separation of the derived alkylbenzene product at room temperature. Next, the selected ionic liquids were assessed as reaction media in the hydrogenolysis of aromatic ketones over palladium catalysts. A binary mixture of choline and betainium bis(trifluoromethylsulfonyl)imide ionic liquids resulted in the highest conversion and selectivity values in the acetophenone hydrogenolysis. Presumably, the carboxylic acid group in the betainium-based ionic liquid acts as a co-catalyst, which makes these task-specific ionic liquids an excellent alternative for the conventionally applied acidic additives. At the end of the reaction, the immiscible alkylbenzene separates from the ionic liquid mixture and the pure product phase can
be isolated by simple decantation. Furthermore, the ionic liquid mixture acts as an immobilization medium for the catalyst which allows efficient catalyst recycling. This
catalyst system was then applied in the hydrogenolysis of diverse aromatic ketones.
After optimization of the reaction conditions, high alkylbenzene yields (> 90 %) were obtained in all cases. The catalyst and the ionic liquid could be used at least 3 times without any loss of activity or selectivity.
This work shows that one should take advantage of the unique properties of ionic liquids as designer solvents to tune their physical properties and the chemical
reactivities of the reagents dissolved in them. In many cases, not a single component ionic liquid, but rather an ionic liquid mixture will give the best performance.