Coating of Pd/C catalysts with Lewis-acidic ionic liquids and liquid coordination complexes – SCILL induced activity enhancement in arene hydrogenation

Abstract

New solid catalysts with ionic liquid layer (SCILLs) were prepared by the coating of heterogeneous Pd/C catalysts with a thin film of acidic liquid: a known chloroaluminate ionic liquid (IL), [BMIM]Cl/AlCl₃, and a recently developed liquid coordination complex (LCC), urea/AlCl₃. Both coatings improved the activity of Pd/C catalyst against toluene hydrogenation; at lower temperatures (40 °C) the IL-SCILL performed better, whereas at 80 °C the LCC-SCILL gave higher conversion. A thin film of ionic liquid proved to be sufficient to increase activity, while larger volumes of ionic liquid resulted in a decrease in activity due to pore blocking and hence mass transport limitations. Kinetic investigations in batch mode revealed a zero order with respect to toluene concentration, a first order in hydrogen pressure, and activation energy of 64.7 kJ mol⁻¹ for the Pd/C catalyst. The SCILL catalysts had slightly lower values of 53.1 kJ mol⁻¹ for [BMIM]Cl/AlCl₃ and 61.4 kJ mol⁻¹ for urea/AlCl₃, respectively.