Applied Catalysis A: General 206 (2001) 57–66 Investigations on the reaction mechanism of the skeletal isomerization of n-butenes to isobutene Part I. Reaction mechanism on H-ZSM-5 zeolites D. Rutenbeck a , H. Papp a,* , D. Freude b , W. Schwieger c a Universität Leipzig, Institut für Technische Chemie, Linnéstr. 3, 04103 Leipzig, Germany b Universität Leipzig, Institut für Experimentelle Physik I, Linnéstr. 5, 04103 Leipzig, Germany c Friedrich-Alexander-Universität Erlangen-Nürnberg, Lehrstuhl für Technische Chemie I, Egerlandstr. 3, 91058 Erlangen, Germany Received 22 May 1999; received in revised form 18 April 2000; accepted 18 April 2000 Abstract A series of H-ZSM-5 zeolites with Si/Al ratios between 17 and 170 were tested in the skeletal isomerization of n-butenes to isobutene in the temperature range of 573–773 K. A selectivity to isobutene of ca. 20% was obtained with all zeolites at 573 K independent of the conversion of n-butenes. With increasing temperature the conversion of n-butenes increased on the zeolites with Si/Al ratios up to 70 (high number of acid sites), whereas the yield of isobutene and the selectivity to isobutene decreased. On the zeolites with higher Si/Al ratios (160 and 170) an opposite behaviour was observed; the conversion of n-butenes decreased while the yield of isobutene and the selectivity to isobutene increased with increasing temperature. A selectivity to isobutene of nearly 90% could be obtained with the latter zeolites at 773 K. These observations are interpreted in such a way that on H-ZSM-5 zeolites with a high number of acid sites, isobutene is formed via the bimolecular mechanism independent of the reaction temperature. On H-ZSM-5 zeolites with a low number of acid sites, a change from the bimolecular to the monomolecular mechanism takes place with increasing temperature. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Skeletal isomerization; Isobutene; ZSM-5; Reaction mechanism 1. Introduction Stringent limits for air pollutants enacted in many countries have led to an increased demand for isobutene, since this compound is used for the production of methyl tert-butyl ether (MTBE), an oxygenate with a high octane number. Since the high demand cannot be supplied by the traditional cracking processes, the catalytic isomerization of the surplus n-butene fractions to isobutene has been the subject of recent investigations. Especially a * Corresponding author. E-mail address: papp@sonne.tachemie.uni-leipzig.de (H. Papp). number of 10-membered ring molecular sieves like ferrierite [1–19], ZSM-22 [8,10,17,20–24] and the iso-structural Theta-1 [1], ZSM-23 [25], MeAPO-11 [13,18,26–28], and SAPO-11 [8,10,13,26,28–30] have been proven to be efficient catalysts for this reaction. A high selectivity to isobutene and yields of isobutene close to the thermodynamic equilibrium have been obtained with them. H-ZSM-5 zeolites, on the other hand, have been described as non-selective cata- lysts for the isomerization of n-butenes to isobutene [1,2,4,7,8,10,11,13,14,29,31,32]. Even after isomor- phous substitution of framework aluminium by iron, leading to a reduced acid strength, the selectivity to isobutene was lower than the selectivity obtained with 0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0926-860X(00)00583-4