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Thermal Maps of Gases in Heterogeneous Reactions

Over 85% of all chemical industry products are made using catalysts, with the overwhelming majority of these employing heterogeneous catalysts functioning at the gas-solid interface. Consequently, optimizing catalytic reactor design attracts much effort. Such optimization relies on heat transfer and fluid dynamics modeling coupled to surface reaction kinetics. The complexity of these systems demands many approximations, which can only be tested with experimental observations of quantities such as temperature, pressure, concentrations, flow rates, etc. One essential measurement is a map of the spatial variation in temperature throughout the catalyst bed. We present here the first non-invasive maps of gas temperatures in catalyst-filled reactors, including high spatial resolution maps in microreactors enabled by parahydrogen. The thermal maps reveal energy flux patterns whose length scale correlates with the catalyst packing. By exploiting the motional averaging under a weak applied magnetic-field gradient, the nuclear magnetic resonance (NMR) linewidths are inversely proportional to temperature. Measurements during the hydrogenation of propylene in reactors packed with metal nanoparticles and metal-organic framework catalysts yield temperature coefficients of ~0.1 Hz/K, and temperature error <4%. Temperature sensitivity increases with gradient strength, enabling tuning of precision.