EPR investigation, before and after adsorption of naphtalene, of mordenite containing Fe 3+ and Cr 5+ ions as impurities
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Catalysis Letters Vol. 111, Nos. 1–2, October 2006 (Ó 2006) DOI: 10.1007/s10562-006-0134-7
EPR investigation, before and after adsorption of naphtalene, of mordenite containing Fe3+ and Cr5+ ions as impurities A. Aboukaı¨ s,a,* E. A. Zhilinskaya,a I. N. Filimonov,b N. S. Nesterenko,b S. E. Timoshin,b and I. I. Ivanovab a
Laboratoire de Catalyse et Environnement E.A. 2598, Universite´ du Littoral Coˆte d’Opale, MREID, 145 Avenue Maurice Shumann, Dunkerque, 59140, France b Laboratory of Kinetics and Catalysis, Department of Chemistry, Moscow State University, Lenin Hills 1, bld. 3, Moscow, 119992, Russia
Received 6 June 2006; accepted 29 June 2006
During the study of mordenite with the EPR technique, three EPR signals centered at g = 4.3; g = 2.4 and g = 1.98 were evidenced. The first two were attributed to Fe3+ ions (signal at g = 4.3 typical to isolated framework iron species, signal at g = 2.4 characteristic to agglomerated non-framework ones). The signal at g = 1.98 was assigned to Cr5+ ions. All these cations are present in the solid as impurities. Furthermore, under certain conditions the Cr5+ ions can be in interaction with two equivalent 27 Al nuclei according to a formation of a superhyperfine structure in the EPR signals. After adsorption of naphtalene on the mordenite samples, organic p-radicals are formed via secondary reactions of primary cation-radicals nearby the aluminum atoms of the mordenite framework. The formation of radicals seems to be correlated to the presence of impurities in the solids. Thus not only acidity but also content of transition metals may be an important factor in the formation of radicals. KEY WORDS: EPR; mordenite; aluminum; iron; chromium; naphtalene.
1. Introduction The growing demand for 2,6-diisopropylnaphtalene, an important monomer in the synthesis of advanced polymer materials, promoted intense investigation of shape selective alkylation of naphthalene over zeolite catalysts. Among the systems studied, mordenites seem to be the most promising microporous catalysts due to the optimal pore size dimension for para-selective alkylation of condensed aromatics. Corresponding referencing on this subject may be found e.g., in a recent review [1]. Nevertheless, deactivation by accumulation of alkylated naphthalene derivatives and coke is in the pores a major drawback. Coking occurs comparatively fast under the reaction conditions e.g., Kim et al. shown coking of mordenites after 1 h of catalytic run [2]. Positive effect of mordenite dealumination is reported for naphthalene alkylation with olefin. Dealumination improves catalyst longevity due to the lower density of acid sites, creation of a secondary (meso)porous network and thus lowering the probability of coking [3]. Formation of coke deposits proceeds via non-selective oligomerization and dehydrogenation-cracking reactions of adsorbed organic molecules on external and internal surface of zeolites. Non-selective side-reactions are often initiated by the oxidative centers existing on zeolites (and in particular, mordenites [4])
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