Longitudinal Relaxation (T ₁) of Methane/Hydrogen Mixtures for Operando Characterization of Gas-Phase Reactions

Catalytic hydrogenation reactions are important in a modern hydrogen-based society. To optimize these gas-phase reactions, a deep understanding of heat, mass, and momentum transfer inside chemical reactors is required. Nuclear magnetic resonance (NMR) measurements can be used to obtain spatially resolved values of temperature, gas composition, and velocity in the usually opaque catalytic macrostructures. For this, the desired values are calculated from measured NMR parameters like signal amplitude, T ₁, or T ₂. However, information on how to calculate target values from these NMR parameters in gases is scarce, especially for mixtures of gases. To enable detailed NMR studies of hydrogenation reactions, we investigated the T ₁ relaxation of methane and hydrogen, which are two gases commonly present in hydrogenation reactions. To achieve industrially relevant conditions, the temperatures are varied from 290 to 600 K and the pressure from 1 bara to 5 bara, using different mixtures of methane and hydrogen. The results show that hydrogen, which is usually considered to be nondetectable in standard MRI sequences, can be measured at high concentrations, starting at a pressure of 3 bara even at temperatures above 400 K. In the investigated parameter range, the absolute T ₁ values of hydrogen show only small dependence on temperature, pressure, and composition, while T ₁ of methane is highly dependent on all three parameters. At a pressure of 5 bara, the measured values of T ₁ for methane agree very well with theoretical predictions, so that they can also be used for temperature calculations. Further, it can be shown that the same measurement technique can be used to accurately calculate gas ratios inside each voxel. In conclusion, this study covers important aspects of spatially resolved operando NMR measurements of gas-phase properties during hydrogenation reactions at industrially relevant conditions to help improve chemical processes in the gas phase.