(762h) Designing Catalysts for Enhanced Pairwise Selectivity of Parahydrogen Addition to Unsaturated Hydrocarbons

Parahydrogen induced polarization (PHIP) nuclear magnetic resonance (NMR) was used to investigate the effects of metal-support interactions on the pairwise addition of hydrogen to propene. In pairwise addition, two hydrogen atoms originating from the same hydrogen molecule are added across the propene double-bond. The PHIP NMR signal enhancement, which is proportional to the fraction of the total conversion occurring by pairwise addition, was measured for titania-supported iridium (Ir/TiO₂) catalysts. These enhancements were compared to those obtained for Ir/Al₂O₃ and Ir/SiO₂. While the PHIP NMR signal enhancement is important for magnetic resonance imaging, it is limited in hydrogenation reactions over heterogeneous catalysts where hydrogen atoms easily diffuse across the metal particle on the catalyst surface upon dissociative chemisorption.

To keep the particle size of Ir on the support small and to minimize hydrogen diffusion and loss of pairwise selectivity, the Ir loading was kept low (0.5 % by weight). However, this was not sufficient to yield significant pairwise addition of hydrogen to propene. Therefore, the Ir/TiO₂ catalyst was reduced at 500 °C, instead of the typical 200 °C reduction temperature, to induce strong metal support interactions. After this treatment a â?¼20-fold increase in the pairwise selectivity was observed, and this effect could be completely reversed by oxidation followed by re-reduction at 200 °C. X-ray photoelectron spectroscopy (XPS) data reveal partial reduction of the TiO₂ support, and Scanning Transmission Electron Microscopy (STEM) data reveal flattening of Ir particles after high-temperature reduction. Non-interacting supports, such as Al₂O₃ and SiO₂, did not show this behavior. It was also shown that chloride ions on the catalyst surface could increase the pair-wise selectivity for the Ir/TiO₂ catalyst, possibly by blocking hydrogen diffusion on the catalyst surface. Other methods of designing catalysts to optimize the pairwise hydrogen selectivity over supported metal catalysts are underway.