MCM-41 anchored manganese salen complexes as catalysts for limonene oxidation P. Oliveira a , A. Machado a , A.M. Ramos a , I. Fonseca a , F.M. Braz Fernandes b , A.M. Botelho do Rego c , J. Vital a, * a REQUIMTE, CQFB, DQ, FCT-UNL, 2829-516 Caparica, Portugal b DCM, CENIMAT, FCT-UNL, 2829-516 Caparica, Portugal c CQFM-CI, IST, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal article info Article history: Received 20 July 2007 Received in revised form 15 December 2008 Accepted 17 December 2008 Available online 1 January 2009 Keywords: Manganese Schiff base MCM-41 Anchoring Epoxidation Limonene Diisocyanate abstract This work reports the covalent attachment of three different salen-based complexes on MCM-41, using two different methods. In both methods, a diisocyanate is used as a binder. All the prepared catalysts were tested on the liquid phase limonene oxidation reaction, using diluted t-butyl hydroperoxide as oxidant. Limonene oxide, carveol, carvone and a polymer were the main products obtained. The preser- vation of the MCM-41 channel system was checked by X-ray diffraction, nitrogen adsorption analysis and transmission electron microscopy (TEM). Catalytic activity seems to increase in the following order: Mn(4-OHsalen) < Mn(4-OHsalhd) < Mn(4-OHsalophen). The most active catalysts were used in four consecutive experiments without any activity loss, confirm- ing the success of the anchoring process and the catalysts stability. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Epoxidation is among the most useful oxidation reactions, since the epoxides may be transformed, by ring opening, into highly func- tionalized products [1,2]. Oxyfunctionalized terpenes are key raw materials of great importance, used in the synthesis of fine chemi- cals, pharmaceuticals and flavours [3]. Schiff base complexes have been widely described as excellent catalysts for oxygen transfer reactions, such as epoxidation [4–7]. They are easy to synthesize and, as well as porphyrins and phthalocyanines, mimic the enzy- matic catalysts present in the biological systems [8,9]. Also, their electronic and steric effects can be finely tuned by introducing suit- able groups in the phenolic and/or in the diimine bridge [10–12]. However, in homogeneous liquid solution, these catalysts easily deactivate, due to ligand oxidation and/or to formation of l-oxo di- mers and other polymeric species [13,14]. Many attempts have been reported concerning immobilization of the homogeneous Schiff bases onto solid supports, combining their high activity and selectiv- ity with the advantages of the heterogeneous catalysis [8,13,15,16]. MCM-41 materials are very attractive supports, due to their very high surface areas and a regular array of large pore channels. In addi- tion, the high concentration of silanol groups allows the develop- ment of different strategies to covalently attach metal complexes. Many published works report the anchoring of salen complexes on the MCM-41 surface. Different coupling agents have been used, such as organosilanes bearing specific functional groups, e.g. chloride, carboxylic acid, thiol, amine, which are available for binding to the complex ligand or to the central metal itself [17–24]. Zhang et al. [25] have also reported a different strategy for the chiral Mn(salen) complexes immobilization into MCM-41, via a phenoxy group. In re- cent work Kumar et al. [26] reported the anchoring of a Schiff base complex on a carbamate modified silica gel. Recently, we have re- ported a new method to covalently attach Schiff base complexes, bearing hydroxyl groups in the salen ligand, on the surface of the MCM-41, using 1,4-butanediisocyanate as a linking agent [27]. Isocyanates are known to be very reactive with AOH groups, forming urethane bonds [28,29], which are, depending on the diis- ocyanate used, at least as strong as those obtained with the orga- nosilane binders [29]. In this work, we describe the anchoring of three Schiff base com- plexes, with different diimine bridges, bearing hydroxyl groups in the phenolic moiety, onto MCM-41, using: (a) a linking agent con- sisting on an organosilane (3-chloropropyltrimethoxysilane - ClPTS) and a diisocyanate (DIC-4) (Scheme 1, route a), further designated as OHPTS/DIC series; (b) a diisocyanate as a binder (Scheme 1, route b), outlined as DIC series. Both methods use 1,4-butanediisocyanate as a coupling agent, being one of the isocyanate endings attached to the organosilane AOH ending (OHPTS/DIC-method), or to the silica sur- face (DIC-method), while the other isocyanate ending remains avail- able for reaction with the complex. The catalysts obtained by these methods were tested in the lim- onene liquid phase oxidation, using diluted t-butyl hydroperoxide (t-BHP), as oxidant. 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.018 * Corresponding author. Tel./fax: +351 212948385. E-mail address: jmv@dq.fct.unl.pt (J. Vital). Microporous and Mesoporous Materials 120 (2009) 432–440 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso