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

•The design and demonstration of a closed loop continuous flow packed bed reactor for parahydrogen enhanced nuclear magnetic resonance is presented.•The reactor system is demonstrated using two types of supported metal nanoparticle catalysts: Rh/TiO2(anatase) and Pt3Sn@mSiO2.•By recirculating the deuterated solvent and unreacted substrate, the closed loop system affords efficient and cost-effective optimization of reaction conditions (e.g., temperature and flow rate) and NMR pulse sequence variables.•Stable hyperpolarization was observed over a period of 22 min with signal enhancements of up to 626 observed at 9.4 T using 99% para-enriched H2.