Catalytic behavior of 1-(2-pyridylazo)-2-naphthol transition metal complexes encapsulated in Y zeolite I. Kuz ´ niarska-Biernacka a , K. Biernacki b , A.L. Magalhães b , A.M. Fonseca a , I.C. Neves a,⇑ a Departamento de Química, Centro de Química, Universidade do Minho, Campus de Gualtar, 4170-057 Braga, Portugal b REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal article info Article history: Received 18 October 2010 Revised 29 November 2010 Accepted 29 November 2010 Available online 31 December 2010 Keywords: NaY Heterocyclic ligand Metal complexes Encapsulation Oxidation abstract The goal of this study is the preparation of new heterogeneous catalysts to be used in the oxidation of organic molecules under mild conditions. The metals in zeolite (M–Y) were prepared by the ion exchange method. The in situ encapsulation of selected transition metal complexes as a guest, namely cobalt(II), nickel(II), copper(II), and zinc(II), with 1-(2-pyridylazo)-2-naphthol (PAN) ligand in supercages of Y zeo- lite (host) was accomplished by the flexible ligand method. The coordination geometry of neat and Y- encapsulated metal complexes has been determined by molecular simulations. The resulting catalysts were fully characterized by different techniques (FTIR, SEM, and chemical analysis), and the results indi- cated that complexes were encapsulated in supercages of Y zeolite. Catalytic studies were performed in liquid phase for cyclohexene and cyclohexanol oxidations, by using tert-butylhydroperoxide as the oxi- dizing agent at 40 and 60 °C, respectively. All prepared catalysts exhibited catalytic activity for the oxi- dation reactions. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high tonnage commodities, high-value fine chemicals, agrochemicals, and pharmaceuticals, but oxidation is often an inefficient process. The introduction of heterogeneous catalysts offers obvious advantages: ease of product and catalyst recovery and suitability for continuous processing. Consequently, heterogenization of homogeneous catalysts has be- come an important strategy for obtaining supported catalysts that retain the active catalytic sites of the homogeneous analog while providing the advantages of a heterogeneous catalyst [1,2]. Zeolite encapsulation of transition metal complexes with oxida- tion catalytic activity is one example of these heterogenization strategies and also is a theme of current research. Several ap- proaches have been used for the encapsulation of transition metal complexes in zeolite supercages, the choice of any specific method being dictated by the size of the ligand relative to the free diameter of the zeolite channels. Complexes or ligands that are smaller than the channels can be adsorbed from a solution phase into the zeo- lite, or they can be synthesised via diffusion of the ligand by the flexible ligand method into a metal-exchanged zeolite [3–6]. These new host–guest materials are interesting for application as biomi- metic heterogeneous catalysts for the oxidation of alkanes, alkenes, and alcohols [1–10]. The oxyfunctionalization of inexpensive hydrocarbons to pro- duce more valuable organic compounds such as alcohols, alde- hydes, and ketones requires the selective oxidation of strong C–H bonds. Alcohols and ketones are important intermediate materials for the manufacture of many important products, such as fiber, drugs, and fragrance. Maurya et al. have encapsulated oxovana- dium(IV) and copper(II) complexes with amino acid as well as Schiff base ligands in the zeolite Y and studied their catalytic potentialities for the oxidation of unsaturated hydrocarbons [11,12]. Alumina-supported manganese(II) complexes with nitro- gen and oxygen donor ligands catalyze, in presence of peroxides, the oxidation of cyclohexene in moderate yield [13], and also tran- sition metal complexes with tetradentate Schiff base ligand en- trapped in the cavity of Y zeolite have also been used as catalysts for this reaction [14]. It was also found that transition metal com- plexes selectively catalyzed the oxidation of cyclohexanol into ke- tone. Manganese(II) complexes both in homogeneous and in heterogeneous phases can act as selective catalysts in this reaction [15,16], as chromium immobilized on silica-based materials [17] as well as tungsten-based catalysts [18]. The objective of the present work is to evaluate the catalytic behavior of the transition metal complexes with 1-(2-pyridylazo)- 2-naphthol (PAN) ligand encapsulated in Y zeolite. The guest–host catalysts, [M(PAN)]@Y where M represents metal ion (Co(II), Ni(II), Cu(II), and Zn(II)), were evaluated in oxidation of cyclohexene and 0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2010.11.022 ⇑ Corresponding author. E-mail address: ineves@quimica.uminho.pt (I.C. Neves). Journal of Catalysis 278 (2011) 102–110 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat