Catalytic hydrogenation using supported ionic liquid membranes

Supported ionic liquid membranes (SILMs) were employed to examine the homogeneous catalytic hydrogenation of propene and ethene. In these membranes, room-temperature imidazolium perfluoroanion ionic liquids served as solvents for the homogeneous catalyst Rh(nbd)(PPh₃)₂⁺) (nbd = norbomadiene). The membranes were fabricated by supporting various ionic liquids, each containing 16-23 mM of the Rh(I) complex, in the pores of poly(vinylidene fluoride) filter membranes. During operation, the olefin was maintained at 1 atm on the feed side of the membrane, while 1 atm of hydrogen gas was swept over the permeate side and sent through a sampling loop for continuous real-time analysis. In this SILM configuration, hydrogenation of the olefin to the corresponding alkane took place as the olefin diffused through the ionic liquid region of the composite membrane structure. Rates of hydrogenation were determined as a function of the ionic liquid solvent and of total run time - the latter data providing information on the stability of the homogeneous catalyst. Maximum rates of propene hydrogenation to propane in the different ionic liquid solvents followed the order EMI(imide) > EMI(triflate) > EMIBF₄) > BMIPF₆) ≈ DMPIBF₄). Ethene hydrogenation rates followed a similar trend. Gas permeability coefficients were used to estimated olefin solubilities in each of the ionic liquids. The solubility ordering for both gases was DMPIBF₄) > BMIPF₆) > EMI(imide) > EMI(triflate) > EMIBF₄). The influence of the coordinating ability of the ionic liquid perfluoroanions on reaction rates and solubilities is discussed, and a selective distribution of the catalyst and olefin within the ionic liquid structure is postulated.