CHEMICAL ENGINEERING

A Hot Spot of Activity

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Science  18 Nov 2005:
Vol. 310, Issue 5751, pp. 1093
DOI: 10.1126/science.310.5751.1093a

The kinetics of heat flow during chemical reactions usually becomes a concern only for large-scale industrial manufacturing. However, Hyde et al. show that even during small-scale studies in the laboratory, local heating can lead to surprising results. Previously, the authors had found that attempts to reduce vinyl cyclohexene by palladium-catalyzed hydrogenation in supercritical carbon dioxide yielded instead a dehydrogenated product, ethyl benzene. To explore this puzzling observation, they monitored reactivity in the absence of H2 by gas chromatography/mass spectrometry, analyzing the data using two-dimensional correlation techniques. The results suggested that an initial burst of hydrogenation generates intense heat locally, which degrades the catalyst and ignites the self-sustaining and exothermic dehydrogenation process. Thermocouple measurements confirmed that H2 addition produces hot spots of 200°C in a catalyst column that is otherwise near room temperature. — JSY

Angew. Chem. Int. Ed.10.1002/anie.200502049 (2005).

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