(607g) Design Strategies for Improved Pairwise Addition of Parahydrogen to Unsaturated Compounds over Heterogeneous Catalysts

Designing oxide-supported metal catalysts for parahydrogen-induced hyperpolarization (PHIP) nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) is challenging. Production of hyperpolarized molecules using parahydrogen requires pairwise addition to an unsaturated molecule. However, oxide-supported metal catalysts mediate facile and reversible dissociation of dihydrogen, rapid diffusion of hydrogen atoms across the metal surface, and step-wise addition to the unsaturated molecule, leading to the loss of singlet spin-correlation. Limiting the metal particle size can restrict hydrogen adatom diffusion but is often not sufficient to prevent randomization of the singlet spin order for highly active hydrogenation catalysts. An alternative strategy is to limit the diffusion of hydrogen by blocking sites on the metal nanoparticles with an oxide layer. We have successfully demonstrated that strong metal-support interactions can improve the pairwise selectivity over titania-supported metal catalysts. A high-temperature reduction at 773 K results in migration of a TiO_x layer (where 1.5 < x < 2.0) over the metal nanoparticles, and this has been shown to increase the pairwise selectivity in the propene hydrogenation reaction over Rh/TiO₂ and Ir/TiO₂ catalysts (compared with catalysts reduced at only 473 K). The activity (propene conversion) is higher at lower temperatures for the Rh/TiO₂ catalysts, but the pairwise selectivity, and thus the NMR signal enhancement, is significantly higher over the Ir/TiO₂ catalysts (Figures 1a and b). To gain more control over the oxide overlayer, we employed atomic layer deposition to deposit Al₂O₃ and ZrO₂ over Rh/TiO₂ and Pt/Al₂O₃ catalysts. As expected from the general inverse relationship between catalytic activity and selectivity, the overlayers reduce activity (conversion of propene, Figure 1c) while increasing pairwise selectivity. For in-vivo MRI, achieving pairwise selective addition by heterogeneous catalysis would offer important advantages over homogeneous hydrogenation where rapid separation from hyperpolarized adducts is problematic and catalyst stability is lacking.