Magnetic Resonance Imaging (MRI) was proven to be a powerful tool for studying catalytic reactions in operando due to its non-invasive nature and versatility. However, MRI application for gas-phase reactions studies is particularly challenging because of low spin density of reactants and products, and it’s further hampered by inhomogeneity of the magnetic field, associated with the presence of solid catalyst beads or even catalytic reactors inside the scanner. The problem of low signal sensitivity can be solved by the implementation of hyperpolarization techniques, such as parahydrogen-induced polarization (PHIP) or spin-exchange optical pumping (SEOP).
In this work we’ve studied a new type of model catalytic reactors that minimize the perturbation of the magnetic field homogeneity and are thus suitable for MRI investigations of a working catalytic reactor. These reactors are glass tubes with a thin layer of titania, that was impregnated with the Rh precursor solution, and in situ reductive treatment of impregnated reactors yields catalytically active porous Rh/TiO2 coating. The reactors were found to be active in hydrogenation of propylene, and, moreover, they were also selective to the pairwise addition of hydrogen. Developed MRI procedure allowed to achieve the maximum possible separation of signals from hyperpolarized and thermally polarized propane molecules, and 1H MRI visualization of a working catalytic reactor during propylene hydrogenation with parahydrogen was performed. It was shown that the developed protocol can be applied in case of normal hydrogen. Moreover, we explored hyperpolarized Xe as a sensitive internal temperature probe for propylene hydrogenation reaction. It’s expected that combination of two hyperpolarization methods would provide new useful information about catalytic reactions in situ.