Confinement of Rh nanoparticles in triphenylphosphine oxide-functionalized core-crosslinked micelles for aqueous biphasic hydrogenation catalysis

Abstract

The introduction of phosphine oxide as anchoring groups in the hydrophobic core of amphiphilic star-block copolymers leads to greatly improved confinement of rhodium nanoparticles (RhNPs) inside the nanoreactors with a benefit in aqueous biphasic catalysis. The copolymers are specially designed core-crosslinked micelles (CCMs) forming a stable latex by reversible addition-fragmentation chain-transfer (RAFT) polymerization. They possess a hydrophilic shell made of polycationic 4-vinyl-N-methylpyridinium iodide P(4VPMe+I−) chains, a triphenylphosphine oxide (TPPO)-functionalized polystyrene core and are crosslinked at the inner end of the polystyrene chains by diethylene glycol dimethacrylate (DEGDMA) (TPPO@CCM-C). Ex-situ synthesized RhNPs readily cross the hydrophilic shell and remain anchored within the CCM nanoreactor cores. The RhNP-loaded TPPO@CCM-C latex was applied as catalyst in the hydrogenation of styrene under mild conditions with complete selectivity towards ethylbenzene and average turnover frequency (TOF) up to 12000 h−1, corresponding to a corrected TOF (cTOF) up to 16800 h−1 based on only surface atoms of the RhNPs. Moreover, the catalytic phase proved recyclable after product extraction with diethyl ether, demonstrating efficient retention of the RhNPs by the core TPPO ligands. Although the activity decreased after the first catalytic run, it converged to a stable average TOF of ca. ∼1025 h−1 (cTOF of ca. ∼1440 h−1), which was similar to that of an extensively pre-washed RhNP-TPPO@CCM-C latex. This phenomenon is attributed to a promoter effect of adsorbed ligands, which were used as stabilizer for the RhNPs synthesis and were gradually removed during the work-up washings between recycles.