Uncatalysed and catalysed vapour phase reactions

Thermal decompositions (pyrolyses) and catalysed reactions in the vapour phase are widely used large-scale industrial techniques. These vapour phase reactions often lead to more economic conversions than the smaller batchwise laboratory methods, because relatively inexpensive catalyst preparations (compared to the often expensive reagents required in laboratory procedures) may be used, and because the technique lends itself to automated continuous production. In undergraduate laboratory courses the technique has not achieved widespread use. The discussion below of the various apparatus designs, to meet a range of experimental conditions, may be regarded as an introduction to this topic. 

A number of research groups have explored vapour phase pyrolytic reactions using apparatus designs (available from, for example, Aldrich), which allow for controlled vapour flow (and hence heat contact time), under moderate to high vacuum conditions. These techniques enable preparative procedures to be developed which involve the trapping of reaction intermediates (e.g. carbenes, nitrenes, arynes, etc.), and also the study of very short-lived species by the interfacing of the pyrolysis unit with, for example, a mass spectrometer. This specialist area has been comprehensively reviewed.26 

Examples which are cited in the following experiments are the depolymerisa- 

When higher temperatures for pyrolysis are required in an uncatalysed vapour phase reaction, the apparatus illustrated in Fig. 2.65(h) could be used. This was originally designed for the pyrolysis of acetone vapour which when passed over a nichrome filament heated at 700-750 °C gives keten in yields exceeding 90 per cent. 

The construction of the filament will be apparent from the enlarged inset. 

---