High-Resolution NMR of the Hydrogenation Reaction\nwith Parahydrogen in an Inhomogeneous Magnetic Field

Nuclear\nmagnetic resonance is extremely attractive for operando\nstudies of chemical reactors. However, the heterogeneous catalyst\nparticles placed inside an NMR probe greatly affect the uniformity\nof the magnetic field. This problem is especially acute when studying\nheterogeneous hydrogenation processes using parahydrogen. Despite\nthe increased sensitivity due to hyperpolarization, under conditions\nof a strong heterogeneity of the magnetic field, the antiphase nature\nof the NMR signals leads to a partial or even a complete loss of spectroscopic\ninformation due to significant NMR signal broadening. The use of intramolecular\nmultiple-quantum coherences in 2D NMR allows one to circumvent this\nproblem. We used the COSY pulse sequence to acquire 2D NMR spectra\nof the reaction mixture upon propene hydrogenation with parahydrogen.\nThe selection of double-quantum coherences in the resulting 2D NMR\nspectrum allowed us to obtain a highly resolved NMR spectrum under\nconditions of severe inhomogeneity of the magnetic field caused by\nthe presence of catalyst granules.