In Situ Monitoring of Heterogeneous Catalytic Hydrogenation via 129Xe NMR Spectroscopy and Proton MRI

Dudari B Burueva,Ekaterina V Pokochueva,Igor V Koptyug,Alexandra Svyatova,Kirill V Kovtunov,Chengbo Wang,Xinpei Wang,Sean P Rigby,Thomas Meersmann,Max Filkins,Galina E Pavlovskaya

doi:10.1021/acscatal.9b05000

Abstract

The ability to use molecular hydrogen, H2, as a buffer gas in spin exchange optical pumping of noble gases enables the production of hydrogen gas containing a low percentile (5%) of hyperpolarized (HP) 129Xe as a tracer for in situ NMR spectroscopy of hydrogenation reactions. It is demonstrated that the xenon chemical shift, observed in the porous space of Pt-containing alumina pellets, can be used to monitor the temperature changes under rapidly progressing, nonsteady-state conditions during start-up of the catalytic reaction. Standard proton MR imaging was successfully applied to provide in situ evaluation of conversion for the catalyst used in this work.

Highlights

The vast majority of contemporary, industrially relevant, chemical processes are catalytic, and heterogeneous catalytic reactions are of particular interest.[1]
Even the methods that are based on direct magnetic resonance measurements of the reactants and products may impose certain restrictions on the experimental design; for instance, the use of nuclear magnetic resonance spectroscopy (NMR) line width for temperature evaluation precludes the use of a frequency encoding gradient for magnetic resonance imaging (MRI).[16]
The hyperpolarization methods are of particular interest including spin-exchange optical pumping (SEOP)[25] and parahydrogen-induced polarization (PHIP),[26,27] which provide a significant increase in NMR signals intensity, by up to several orders of magnitude

Summary

ACS Catalysis

Letter exacerbated by magnetic field inhomogeneity at the surface of solid catalysts beads and within the entire catalytic reactor.[4]. The temperature dependence of the 129Xe chemical shift and the capability to generate HP 129Xe through SEOP of a 0.05:0.95 Xe:H2 gas mixture was utilized to observe a catalytic reaction during start-up with time-resolved 129Xe NMR spectroscopy (using 99% isotopically enriched 129Xe). Taking into account the temperature dependence of xenon chemical shift obtained in the calibration experiments, the average temperature inside the catalytic reactor was about 106 °C within 30 s after the start of the reaction and increased to about 200 °C within 70 s. MR images with selective suppression of 1H signals from either propane or propene during propene hydrogenation with normal hydrogen over the 1 wt % Pt/Al2O3 catalyst pellet placed at the bottom of a 5 mm NMR tube were acquired (Figure 2).

■ ACKNOWLEDGMENTS

■ REFERENCES

Behaviour and Oligomer Accumulation within Catalyst Pores during

Findings

Situ Analysis of Gas Phase Reaction Processes within Monolithic