III International Conference

CHEMICAL REACTIVITY OF ZINC EXCHANGED ZSM-5 ZEOLITES:A COMPREHENSIVE DFT STUDYPP-I-75Pidko E.A., van Santen R.A.Schuit Institute of Catalysis, Eindhoven University of Technology, PO Box 513, NL-5600 MBEindhoven, The NetherlandsE-mail: e.a.pidko@tue.nlPossible reaction pathways of alkane dehydrogenation over different zinc containingintrazeolite cationic species were studied by means of a density functional theory clustermodeling. It is shown that the isolated zinc ions stabilized at the charge alternating cation sitewith distantly separated anionic [AlO] 2 - framework units are the most probable active sites. Inaddition a novel mechanism of ethane dehydrogenation is proposed. The presented theoreticalresults allow the precise explanation of the recent unusual experimental data.Zinc-exchanged H-ZSM-5 zeolites (Zn/ZSM-5) are known to be effective catalysts forpromoting the selective conversion of light alkanes to aromatics [1]. The reaction mechanismis thought to consist of a complex scheme involving dehydrogenation, oligomerization, andring-closure steps [2]. The modifying cations play key roles in dehydrogenation of paraffins,whereas Brønsted acid protons catalyze the oligomerization of the olefins thus produced andpossibly their subsequent aromatization. Despite numerous experimental and theoreticalstudies have been devoted to investigation of the mechanism of catalytic dehydrogenation oflight alkanes over Zn exchanged ZSM-5 zeolite, the structure of the active intrazeolitecationic species and, accordingly, the mechanism of hydrocarbon activation have not beenfully elucidated.In this work we present a comparative cluster DFT study of the reactivity of variouscationic species such as Zn 2+ , [Zn-OH] + and [ZnOZn] 2+ stabilized in the zeolite. In addition,for isolated Zn 2+ sites we discuss the dependence of their chemical properties on thealuminum distribution in ZSM-5 zeolite. Possible reaction paths for ethane dehydrogenationover the zeolitic zinc sites are schematically shown in Scheme 1. Initial activation of lightalkanes over all of the abovementioned cationic species proceeds via heterolytic C-H bondcleavage (I→II) resulting in formation of an alkyl group grafted to the Zn ion and an acidicproton attached either to the basic lattice oxygen in case of Zn 2+ ions or to the extraframeworkO atom of the [Zn-OH] + and [ZnOZn] 2+ species.The reactivity of the isolated Zn 2+ strongly depends on the aluminum distribution in thezeolite. The most active Zn 2+ ions are those stabilized at the distant anionic sites of the zeolite141