The siting of Cu(II) in mordenite: a theoretical spectroscopic study Annelies Delabie,* a Kristine Pierloot, a Marijke H. Groothaert, b Bert M. Weckhuysen b and Robert A. Schoonheydt b a Department of Chemistry, University of L euven, Celestijnenlaan 200F, B-3001 Heverlee-L euven, Belgium. E-mail: annelies.delabie@chem.kuleuven.ac.be b Center for Surface Chemistry and Catalysis, University of L euven, Kasteelpark Arenberg 23, B-3001 Heverlee-L euven, Belgium Receiv v ed 2nd July 2001, Accepted 11th October 2001 First published as an Adv v ance Article on the web 18th December 2001 ThesitingofCu(II)inmordenitehasbeenstudiedby ab initio calculationsonlargeclustermodels,representing the cation exchange sites in mordenite. Partial geometry optimizations, based on density functional theory (DFT),wereperformedtoobtainthestructureofthecoordinationenvironmentofCu(II)atthedifferentsites. The ligand field spectra and EPR g-tensors of these clusters were then calculated by means of multiconfigurational perturbation theory (CASPT2). The calculated results were compared with experimental information,obtainedbydiffusereflectancespectroscopy(DRS)andEPR.Thecalculationsindicatethatatlow exchange levels Cu(II) is coordinated to oxygen six-rings in the main channel of mordenite, in the presence of two aluminiums. At higher loadings, six- or five-rings containing only one aluminium also become occupied, where Cu(II) is coordinated as a single ion, not as (Cu–OH) þ . The calculations indicate also that in fully dehydrated mordenite, the twisted eight-ring (site A) is not occupied by Cu(II). 1 Introduction Zeolites containing transition metal ions are widely studied, becauseoftheirinterestingadsorptiveandcatalyticproperties. 1–3 It has been found that pentasil zeolites (ZSM-5, ferrierite and mordenite) loaded with transition metal ions may display high activities towards the decomposition and reduction of NO. 4–6 Despitethelargeamountofavailableexperimentalandtheoretical studies, the exact nature of the active site and the reaction mechanism are still under debate. Detailed knowledge of the location and coordination of the transition metal ions in these zeolitesisanimportantfirststeptoabetterunderstandingofthese materials. Over the years, X-ray diffraction (XRD), EPR and electronicspectroscopyhaveproventooffervaluableinformation concerningthelocationandcoordinationofisolatedCu(II)ionsin dehydratedzeolitematrices. According to our knowledge, an XRD structure of fully dehydrated Cu(II) exchanged mordenite is not yet available. MostoftheinterpretationsofthesitingofCu(II)inmordenite arethereforebasedontheXRDstructureofdehydratedCa 2þ mordenite. 7 The structure of mordenite (MOR), with indica- tion of the possible exchangeable cation sites, is shown in Fig.1.FoursitesareoccupiedbyCa 2þ .Firstly,Ca 2þ ionsare found in site A, a twisted eight-ring located in the elliptical eight-ring channels of mordenite. The Ca 2þ ion in this site is coordinated to six lattice oxygens in C 2h symmetry. A second occupied site is the six-ring in the twelve-membered ring channel (site E), where Ca 2þ is coordinated to four oxygens. A smaller portion of the Ca 2þ ions is present in the boat- shaped site, located in the eight-ring channel of the zeolite. This site is composed of a non-planar six-ring (site C) in between two bent five-rings (see Fig. 1). Finally, Ca 2þ ions werealsofoundinthecirculareight-rings,situatedinthewalls of the large twelve-ring channel (site D). Fig. 1 Structure of mordenite with indication of the cation sites (A,B,C,D,andE).ThestructureofsiteE,AandCareindicatedin light-grey. 134 Phys. Chem. Chem. Phys., 2002, 4, 134–145 DOI: 10.1039/b105799p This journal is # The Owner Societies 2002