Parahydrogen-Induced Hyperpolarization inside Meso- and Micropores of Pt‑, Rh‑, Ir‑, and Pd-Containing Solid Catalysts

Homologous series of solid catalysts (silica, SBA-15, dealuminated zeolite DeA–Y, zeolite H,Na–Y) with different pore sizes and loaded with different noble metals (Pt, Rh, Ir, Pd) were applied for the gas phase hydrogenation of propene with para-enriched hydrogen (p-H2) with the aim to produce parahydrogen-induced polarization (PHIP). This hyperpolarization formed by the pairwise incorporation of p-H2 into propene was studied via in situ 1H MAS NMR spectroscopy under continuous-flow conditions. By evaluating the characteristic 1H NMR antiphase signals of the hyperpolarized reaction products, the experimental parameters and properties of noble-metal-containing porous catalysts were investigated, which affect the formation and relaxation of PHIP. For the catalysts under study, small pore diameters and interactions of the hyperpolarized product molecules with nuclei inside the pores, such as framework aluminum atoms and extra-framework sodium cations, were found to enhance the relaxation of the hyperpolarization formed by the pairwise incorporation of p-H2 into olefinic reactants. Furthermore, a very high hydrogenation activity of the noble-metal-containing catalyst (Pd/H,Na–Y) decreases the formation of PHIP, possibly caused by a too large number of reactive spillover H species without spin correlation, which hinders the pairwise incorporation of p-H2 into the olefinic reactants.