Ž . Journal of Molecular Catalysis A: Chemical 162 2000 147–157 www.elsevier.comrlocatermolcata The importance of acid site locations for n-butene skeletal isomerization on ferrierite L. Domokos a , L. Lefferts a , K. Seshan a , J.A. Lercher b, ) a Catalytic Processes and Materials, Faculty of Chemical Technology, UniÕersity of Twente, P.O. Box 217, 7500 AE, The Netherlands b Technische UniÕersitat Munchen, Institut fur Technische Chemie, D-85749 Garching, Germany ¨ ¨ ¨ Dedicated to Prof. H. Knozinger on the occasion of his 65th birthday. ¨ Abstract Ž . The role of the acid site location in H-ferrierite H-FER on the skeletal isomerization of linear butenes was studied. Assignment of OH groups observed in FTIR analysis is addressed taking into account previous NMR and computational studies regarding the FER structure. Possible locations of alkali ions in the zeolite framework are discussed. Close structure Ž . activity relationship has been observed between Brønsted acid sites located in the 10 member ring MR channels and Ž . selective isobutene formation. Molecular dynamics MD calculations of possible products observed during n-butene transformation at reaction temperatures support the experimental findings. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ion exchange; Butene isomerization; Ferrierite; Brønsted acidity 1. Introduction Over the past decade, skeletal isomerization of wx linear butenes has been intensively investigated 1. Ž . Catalysts based on halogenated alumina, silica, sil- ica–alumina and related materials were reported to have limited lifetime during butene isomerization w x 2,3 . Substantial coke formation and butene dimer- wx ization were observed with these catalysts 4. In many cases, water has been cofed to increase catalyst lifetime and isobutene selectivity. Medium pore zeolites, however, show remarkable performance allowing yields of isobutene close to the thermodynamic limits even under severe operat- ) Corresponding author. Tel.: q 49-89-289-13540; fax: q 49-89- 289-13544. Ž . E-mail address: johannes.lercher@ch.tum.de J.A. Lercher . w x ing conditions 5–9 . At partial pressures of n-butene close to 1 bar and relatively low temperatures, where oligomerization of olefins is a favored side reaction of skeletal isomerization, it was possible to achieve close to 50 mol% yield of isobutene at 3508C in a Ž . w x single pass over H-ferrierite H-FER 10 . Other type of medium pores zeolites such as TON, AEL were also reported to be good catalysts for this w x reaction 11–13 . In contrast to TON and AEL, FER contains a two-dimensional pore system, where 8 Ž . and 10 member ring MR channels are perpendicu- larly interconnected. Although it was early realized that pore dimensions are crucial, it is unclear to present, whether or not the additional eight MR Ž . channels side channels in FER contribute to the w x selective isomerization reaction 14 . It has been thoroughly demonstrated that skeletal isomerization of linear butene is catalyzed by w x Brønsted acid sites 15 . It was also shown that 1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S1381-1169 00 00286-7