Wilkinson-type catalysts in ionic liquids for hydrogenation of small alkenes: understanding and improving catalyst stability

In this work, we apply supported ionic liquid phase (SILP) catalysts to hydrogenate 1-alkenes in a continuous fixed-bed reactor. SILP catalyts were prepared using the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(4)C(1)Im][NTf2] and tris(triphenylphosphine)rhodium(i) chloride [RhCl(PPh3)(3)] as a dissolved catalyst. Under isothermal conditions, we observe an increase in activity related to restructuring of the catalyst via formation of supported metal nanoparticles (NPs). Combined in situ infrared spectroscopic and theoretical studies show that NP formation is a step-wise process including the sequential loss of ligands and formation of isolated, supported Rh+-ions as intermediates. To stabilize the highly selective homogeneous catalyst, we systematically vary the experimental conditions (co-feeding of CO) and catalyst design (application of strongly binding CO or chelating xantphos ligand). This knowledge-driven approach results in a stabilization of the molecular Rh complex under hydrogenation conditions.