Structured fiber supports for ionic liquid-phase catalysis used in gas-phase continuous hydrogenation

Structured supported ionic liquid-phase (SSILP) catalysis is a new concept with the advantages of ionic liquids (ILs) used as solvents for homogeneous catalyst and the further benefits of structured heterogeneous catalysts. This is achieved by confining the IL with the transition metal complex to the surface of a structured support consisting of sintered metal fibers (SMFs). In an attempt to improve the homogeneity of the IL film, the SMFs were coated by a layer of carbon nanofibers (CNFs). The IL thin film immobilized on CNF/SMF supports presents a high interface area, ensuring efficient use of the transition metal catalyst. The regular structure of the support with high porosity (>0.8) allows a low pressure drop and even gas-flow distribution in a fixed-bed reactor. The high thermoconductivity of the CNF/SMF support suppresses the formation of hot spots during exothermic hydrogenation reactions. The selective gas-phase hydrogenation of 1,3-cyclohexadiene to cyclohexene over a homogeneous Rh catalyst immobilized in IL supported on CNF/SMF was used as a test reaction to demonstrate the feasibility of the SSILP concept. The catalyst [Rh(H)2Cl(PPh3)3/IL/CNF/SMF] showed a turnover frequency of 150–250 h−1 and a selectivity of >96%. High-pressure 1H NMR and 1H{31P} NMR spectroscopy was used to provide insights into the nature of the active catalytic species.