Supported ionic liquid catalysts (SILCA) in the hydrogenation reactions

Ionic liquids or room temperature salt melts have been under intensive study recently. Some of the most important features of ionic liquids are neglible vapour pressure, wide liquidus range, unique solvation properties, wide electrochemical window and good ion conductivity.^1.2 They have also shown good performance in various kinds of catalytic reactions and in preparation of nano structured materials and nanoparticles for catalysis. ^3,4 However, the wide use of ionic liquids has still been prevented by high cost, limited knowledge of their physical and chemical properties and biodegradability. That's why supported ionic liquid catalysts with catalytic amount of ionic liquid immobilized on solid surface are a noteworthy alternative in industrial point of view.³ We introduce a preparation method for heterogeneous catalyst by using an immobilized ionic liquid layer on activated carbon, into which the transition metal particles have been dissolved. ⁶ Ionic liquid and palladium acetylacetonate were dissolved into dry acetone. Solution was poured over support, which was dried activated carbon cloth (ACC) Kynol®. Acetone was evaporated leaving Pd⁴⁺ ions dissolved in ionic liquid on active carbon support. Catalyst was pre-reduced in the reactor (1 h hydrogen flow in 100°C). During this pre-treatment, Pd ⁴⁺ ions were reduced to Pd²⁺, Pd⁺ ions or Pd nanoparticles, depending on the ionic liquid Pdprecursor was dissolved in. In hydrogenation reactions, Pd has proved to be selective catalyst for carbon-carbon double bond instead of carbonyl group. To demonstrate the feasibility of the concept the catalysts were used in the hydrogenation of citral and cinnamaldehyde. Citral itself and its different hydrogenation products are used e.g. by the perfumery industry. Citral and cinnamaldehyde are also interesting model compounds in hydrogenations because they contain three different double bonds: isolated and conjugated carbon-carbon double bonds and a carbonyl group. During hydrogenation of them many, competing and consecutive reactions can occur. Our experiments showed that ionic liquid can improve the hydrogenation reaction rate or affect selectively the product distribution.