Homogeneous catalyst separation and re-use through nanofiltration of organic solvents

The separation of reaction products from catalysts is a major problem existing in many forms of homogeneous catalysis, including phase transfer catalysis and transition metal catalysed reactions. This study describes a new and generic approach to solving this separation problem using solvent resistant nanofiltration (SRNF) membranes. Data for SRNF separation of a phase-transfer catalyst (PTC), and a Heck reaction transition metal catalyst (TMC), from their respective reaction media is presented. SRNF was used to retain each catalyst from the reaction solvent and, when applied in a coupled reaction-separation system, allowed several subsequent catalyst recycles and re-uses. PTC rejection was high (99+%) whether SRNF was applied to pre- or post-reaction mixtures, and no reaction rate decline was observed for two consecutive catalyst recycles. TMC rejection, while 96% for pre-reaction mixtures, dropped to 90% for post-reaction mixtures and reaction rate decline was observed for four consecutive catalyst recycles. The clear difference between the two systems is the much higher stability of the PTC due to a less demanding catalytic cycle. The need for stable catalytic systems for effective generic use of this technology in homogeneous catalysis is highlighted.

instability of Pd in the Heck catalytic cycle. Although the exact catalytic mechanism is unclear, it is assumed that smaller Pd species are forming during the cycle, including Pd(0) entities that are both small enough to permeate the membrane and unstable enough to aggregate and precipitate as nanoclusters of Pd black [11]. As It has been demonstrated that the SRNF coupled reaction-separation technique can be generically applied to homogeneous catalysis provided that there is sufficient membrane selectivity between catalyst and product. The currently available range of SRNF membranes is small but growing, and already they are versatile enough to be used across a broad range of solvents. The key to successful application of this technology to TMC systems is catalyst stability on a par with that evident in PTC systems, such that high reaction rates can be sustained over multiple consecutive reactions. This should enable the concept to be applied to a wider range of organic syntheses.