Journal of the Chemical Society - Faraday Transactions p. 2101 - 2108 (1992)
Update date:2022-08-29
Topics:
Melsheimer, Joerg
Ziegler, Dietrich
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 continous gas flow of either 2 or 20 cm3 min-1 with a mixture of 5 vol.percent 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.
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