Parahydrogen-Induced Polarization Detected with Continuous Flow Magic Angle Spinning NMR

Parahydrogen-induced polarization (PHIP) detected as an enhancement of NMR signal at pairwise addition of hydrogen atoms from the H-2 molecule to olefin is an effective tool for discriminating among the possible mechanisms of hydrogenation reaction. Previously reported PHIP effects for heterogeneous hydrogenation were observed only for gas-phase molecules, that is, for those desorbed from the catalyst surface. In this case, valuable information on the possible intermediates and the reaction products on the catalyst surface could not be provided. In this paper, a magic angle spinning (MAS) NMR technique was applied for the first time for registration of PHIP produced in hydrogenation of propene with para-H-2 on the Pt-supported on metal oxide catalyst (Pt/TiO2). We demonstrate that the use of the H-1 MAS NMR technique under the condition of a continuous flow of the reagents (CF MAS NMR) enables registration of polarization in both the gas phase and the adsorbed species. Application of CF MAS NMR could be useful for detecting PHIP effects on heterogeneous catalysts and for the study of the mechanisms of heterogeneously catalyzed hydrogenation reactions.