Ethene transformation on HZSM-5 studied by combined UV–VIS spectroscopy and on-line gas chromatography

Abstract

Combined UV–VIS spectroscopy in transmission mode and on-line gas chromatography have been performed on a HZSM-5 zeolite in a microreactor operating under a continuous gas flow of either 2 or 20 cm³ min^–1 with a mixture of 5 vol.% ethene in nitrogen. The UV–VIS experiments were carried out in situ up to 573 K and ex situ up to 673 K. Carbocations formed at temperatures up to 473 K are of the cyclopentenyl and/or cyclohexenyl type. Their absorption maxima are between 278 and 313 nm. Moreover, a strong adsorption–sorption is initially established at room temperature which then decreases with time. No appreciable spectroscopic difference could be ascertained between in situ and ex situ measurements up to 473 K. At 548 K and above the most intense absorption maximum is slightly blue shifted.

The first volatile reaction products could be identified at lower space velocity at 373 K and aromatics were detected from 573 K. The conversion is strongly dependent on the space velocity. At 523 K and above new absorption bands develop at ca. 375 and 440 nm. The former disappears above 573 K and the latter shifts towards 417 nm with increasing temperature. From the location of these bands and from reference experiments we can conclude that for toluene the alkyl-transfer mechanism seems to be valid between 523 and 573 K, and the benzylic-cation mechanism between 573 and 673 K.