Perpetual hyperpolarization of allyl acetate from parahydrogen and continuous flow heterogeneous hydrogenation with recycling of unreacted propargyl acetate

A novel closed loop, continuous flow (CF) reactor system for parahydrogen enhanced nuclear magnetic resonance (NMR) of liquids via heterogeneous catalysis is introduced which enables recycling of unreacted liquid substrate reactant. This system consists of an HPLC pump, a liquid substrate reservoir incorporating a gas diffuser, an all-metal packed bed catalytic reactor, and an AF-2400 tube-in-tube gas permeable membrane for removal of normal H₂. Two types of supported metal nanoparticle catalysts were tested: mesoporous silica encapsulated Pt₃Sn intermetallic nanoparticles and a Rh on anatase TiO₂ support. In the CF hydrogenation of propargyl acetate to allyl acetate, the hyperpolarized signals exhibited stability over 20 min of recirculation, with signal enhancements of up to 626 using 99% p-H₂ and negligible leaching of the catalyst into the flowing solutions. These results demonstrate the practicality of performing systematic optimization of conditions for continuous flow catalysis and polarization transfer to heteronuclei with important implications for biomedical magnetic resonance imaging.