60th Annual Report on Research 2015

Nuclear magnetic resonance (NMR) spectroscopy is an important analytical method which utilizes radio waves to probe the magnetism of atomic nuclei in an applied magnetic field. NMR spectra contain valuable information about molecular structure and dynamics, and the technique is important to many fields of science. In the standard NMR experiment, signal is derived from the weak alignment of the magnetic nuclei in an applied magnetic field. These NMR signals tend to be small and difficult to detect. “Hyperpolarization” techniques, like parahydrogen induced polarization (PHIP), overcome this by introducing a high degree of alignment of the nuclei, yielding a signal amplification that can exceed 10,000. Parahydrogen Induced Polarization (PHIP) is a robust, scalable and inexpensive method for the rapid preparation of bulk quantities of nuclear spin hyperpolarized fluids. The pure symmetrization order inherent in parahydrogen is transformed via pair-wise symmetry breaking chemical hydrogenation reaction into spin hyperpolarization for sensitivity enhanced nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging. Parahydrogen induced polarization using heterogeneous catalysis can produce impurity-free hyperpolarized gases and liquids, but the comparatively low signal enhancements and limited scope of substrates that can be polarized pose significant challenges to this approach. These challenges were addressed by this ACS PRF project.

In this project, the PHIP effect was studied using oxide-supported metal nanoparticles. The catalysts were synthesized by either of two methods: incipient wetness impregnation or precipitation-deposition. The goal of the project was to study the mechanism of hydrogenation reaction on the surface, allowing the conditions that mitigate the processes that reduce pairwise hydrogenation to be identified and selectivity to be maximized. Pairwise selectivity obtained with oxide supported metal particles was found to be limited to a few percent due to loss of spin-correlation that occurs following chemisorption of H₂on the metal surface. The proposed studies also explored a new type of catalytic process, which we refer to as “pairwise replacement catalysis,” whereby the singlet spin order of parahydrogen is incorporated into a substrate molecule with no change in its molecular structure.

Firstly, the processes affecting the disposition of the bilinear spin order derived from parahydrogen in the hydrogenation of propyne over TiO₂-supported Pt nanoparticles were examined. The hyperpolarized adducts formed at low magnetic field are adiabatically transported to high field for analysis by proton NMR spectroscopy at 400 MHz. For the first time, the stereoselectivity of pairwise addition to propyne was measured as a function of reaction conditions. The correlation between partial reduction selectivity and stereoselectivity of pairwise addition is revealed. The systematic trends were rationalized in terms of a hybrid mechanism incorporating non-traditional concerted addition steps and well-established reversible step-wise addition involving the formation of a surface bound 2-propyl intermediate.

Pairwise and random addition processes are ordinarily indistinguishable in hydrogenation reactions. The distinction becomes important only when the fate of spin correlation matters, as in PHIP. Supported metal catalysts were not expected to yield PHIP signals given the rapid diffusion of H atoms on the catalyst surface and in view of the sequential stepwise nature of the H atom addition in the Horiuti–Polanyi mechanism. Thus, it seems surprising that supported metal hydrogenation catalysts can yield detectable PHIP NMR signals. Even more remarkably, supported Pt and Ir nanoparticles were shown in this project to catalyze pairwise replacement on propene and 3,3,3-trifluoropropene. By simply flowing a mixture of parahydrogen and alkene over the catalyst, the scalar symmetrization order of the former is incorporated into the latter without a change in molecular structure, producing intense PHIP NMR signals on the alkene. An important indicator of the mechanism of the pairwise replacement is its stereoselectivity, which was revealed with the aid of density matrix spectral simulations. PHIP by pairwise replacement has the potential to significantly diversify the substrates that can be hyperpolarized by PHIP for biomedical utilization.

Finally, PHIP signals were observed in the hydrogenation of propene and propyne over ceria nanocubes, nano-octahedra, and nanorods. The well-defined ceria shapes, synthesized by a hydrothermal method, expose different crystalline facets with various oxygen vacancy densities, which are known to play a role in hydrogenation and oxidation catalysis. While the catalytic activity of the hydrogenation of propene over ceria is strongly facet-dependent, the pairwise selectivity is low (2.4% at 375°C), which is consistent with stepwise H atom transfer, and it was the same for all three nanocrystal shapes. Selective semi-hydrogenation of propyne over ceria nanocubes yields hyperpolarized propene with a similar pairwise selectivity of (2.7% at 300°C), indicating product formation predominantly by a non-pairwise addition. Ceria was also shown to be an efficient pairwise replacement catalyst for propene.