Oral 23rd International Society of Magnetic Resonance Conference 2023

Heterogeneous parahydrogen induced-polarization on Rh-containing Silicalite-1 zeolite: effect of catalyst structure on signal enhancement (#46)

Weiyu Wang 1 , Jun Xu 1 , Feng Deng 1
  1. State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan, HUBEI, China

    Parahydrogen-induced polarization (PHIP) is a nuclear spin hyperpolarization method that can enhance NMR sensitivity by 4-5 orders. Heterogeneous parahydrogen-induced polarization (HET-PHIP) generates polarization by incorporating heterogeneous hydrogenation systems and has an advantage in catalyst separation to produce a catalyst-free polarized mixture. However, HET-PHIP has low efficiency in catalyzing pairwise hydrogenation to obtain an intensive polarized signal. Metal-supported zeolites are important heterogeneous catalysts but their utilization in the HET-HPIP study is limited due to low polarization levels. This work investigates propene hydrogenation over different rhodium-containing Silicalite-1 zeolites using the HET-PHIP technique to explore the effect of catalyst structure on signal enhancement. Both liquid-state and in situ magic angle spinning (MAS) NMR experiments are performed to explore the PHIP effect on propene hydrogenation over different Rh-containing zeolites.

    The supported Rh species and zeolite structure determine molecule polarization and signal enhancement. Intensive polarized signals are generated on self-pillared Silicalite-1 nanosheets supported Rh catalyst (Rh/SP-S-1, Figure 1a and 1d), which can be correlated to ultrasmall Rh nanoparticles and enhanced mass transport efficiency of the zeolite support. Silicalite-1 nanocrystals supporting large Rh nanoparticles on the external surface (Rh/Nano-S-1, Figure 1b and 1e) are less active for PHIP generation. Single-atom Rh encaged in Silicalite-1 nanocrystals (Rh@S-1, Figure 1c and 1f) demonstrates high activity for hydrogenation but low observed polarization generation efficiency. This provides evidence about the critical role of interactions between zeolite-confined polarized molecules and micropore structures in attenuating polarization.

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Fig.1 Polarized 1H NMR spectra of propene hydrogenation over different catalysts in time on stream experiments (a-c), and the calculated signal enhancement factor (SEF), propene conversion and propane selectivity (d-f).

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