Polyhedron 28 (2009) 994–1000 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly Grafting of vanadyl acetylacetonate onto organo-hexagonal mesoporous silica and catalytic activity in the allylic epoxidation of geraniol Bruno Jarrais a,1, Clara Pereira a, Ana Rosa Silva a,2, Ana P. Carvalho b, João Pires b,*, Cristina Freire a,* a REQUIMTE, Departamento de Química, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal b Departamento de Química e Bioquímica and CQB, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal article info abstract Article history: Vanadyl(IV) acetylacetonate ([VO(acac)2]) was grafted onto a hexagonal mesoporous silica (HMS) using Received 28 November 2008 three different methodologies: method A – direct complex immobilisation; method B – functionalisation Accepted 31 December 2008 of the HMS with 3-aminopropyltriethoxysilane (APTES) followed by the complex immobilisation; and Available online 4 February 2009 method C – treatment of the APTES functionalised support prepared by method B with trimethylethox- ysilane (TMS) to deactivate eventually unreacted surface silanol groups, followed by complex grafting. Keywords: All the materials were characterised by nitrogen elemental analysis, XPS, FTIR, N2 adsorption isotherms Vanadyl acetylacetonate at À196 °C and the [VO(acac) ] based materials were also characterised by vanadium ICP-AES analysis. Hexagonal mesoporous silica 2 The results indicated that, in method B, APTES was successfully grafted onto the HMS with 90% of effi- Complex immobilisation Allylic epoxidation ciency and allowed the covalent attachment of [VO(acac)2] complex mainly in the inner pores with an efficiency of 65%. In method C, a lower complex immobilisation efficiency was obtained, c.a. 25%, but the complex was covalently bonded throughout the functionalised material. In the case of method A, the parent HMS material immobilised a very low quantity of vanadium complex (2% of efficiency), mainly in the external surface through non-covalent interactions. The catalytic activity of [VO(acac)2] based materials in the epoxidation of geraniol using tert-butyl hydroperoxide (t-BuOOH) as oxygen source was assessed. The selectivities of the two epoxides formed in the heterogeneous phase reactions were similar to those observed in the homogeneous phase and the major reaction product was always 2,3-epoxygeraniol. The catalyst which allowed higher substrate conversion was obtained by method B but, when considering the leaching of the active phase, method C produced the most efficient catalyst. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction solvents such as toluene or dichloroethane. Since then, the homo- geneous epoxidation of allylic alcohols by [VO(acac)2] under mild There is a growing interest in using epoxides as building blocks conditions and using t-BuOOH as the oxygen source has been in the synthesis of organic compounds since they act as excellent widely studied due to its high activity, selectivity and regioselec- intermediates which can yield a great variety of products [1].In tivity [2–7]. It was found that a catalytic active oxo-peroxo inter- the epoxidation of allylic alcohols by electrophilic oxidants, the mediate was formed in situ by oxidation of V(IV) to V(V) with presence of a –CH2OH group in the alcohol substrate reduces the excess of t-BuOOH, yielding a tert-butyl hydroperoxo vanadium(V) nucleophilic character of the allylic double bond. For a long time, complex. A good example of the high regioselectivity of this [VO(a- the reagents of choice for such reaction were strong oxidants such cac)2]-t-BuOOH system is in the epoxidation of geraniol, an allylic as organic peracids [2]. Then, List and Kuhnen, in 1967 [2], Sheng alcohol containing an isolated double bond, where the allylic dou- and Zajacek, in 1970 [3], and Sharpless and Michaelson, in 1973 ble bond is selectively oxidised, whereas peracid oxidants prefer- [4], reported that the combination of t-BuOOH and [VO(acac)2] entially epoxidise the isolated double bond [2]. was a powerful epoxidising agent of allylic alcohols in non-polar In recent years, the heterogenisation of transition metal com- plexes through immobilisation onto solid supports has received great attention due to the inherent advantages of shape selectivity, * Corresponding authors. Tel.: +351 220402590; fax: +351 220402695 (C. Freire), easy separation and recycling of catalysts, products purification tel.: +351 217500898; fax: +351 217500088 (J. Pires). and better handling properties [8–11]. Complex encapsulation E-mail addresses: [email protected] (J. Pires), [email protected] (C. Freire). and methodologies involving grafting and tethering procedures 1 Present address: CeNTI – Centro de Nanotecnologia e Materiais Técnicos, have shown several advantages over methods based on non-cova- Funcionais e Inteligentes, Rua Fernando Mesquita, 2785, 4760-034 Vila Nova de Famalicão, Portugal. lent interactions, such as electrostatic, p–p and hydrophobic/ 2 Present address: Unilever R&D, Port Sunlight, Bebington, United Kingdom. hydrophilic interactions and hydrogen bonding [9,12–15]. 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.12.049 B. Jarrais et al. / Polyhedron 28 (2009) 994–1000 995 3 The [VO(acac)2] complex has already been directly immobilised mol) was added to a stirred solution of ethanol (88.1 cm , 1.51 on several solid matrixes, namely alumina, silica gel, mesoporous mol), water (88.5 cm3, 4.91 mol) and 1-dodecylamine (10.3 cm3, silicas and clays [16–19]. It has also been immobilised by microen- 0.0448 mol). The mixture was stirred at room temperature for 24 capsulation in polystyrene [20] and by covalent grafting onto h. The white precipitate obtained was vacuum-filtered and washed amine functionalised activated carbon [21] and clays [16] through with deionised water (100 cm3) and ethanol (100 cm3). In order to Schiff base condensation between surface amine groups and the remove the template (1-dodecylamine), the precipitate was cal- carbonyl groups of the acetylacetonate ligand. cined at 600 °C for 24 h. Hexagonal mesoporous silicas are ordered mesoporous materi- Immobilisation of [VO(acac)2]: The immobilisation of [VO(acac)2] als with wormlike pore structure [22,23]. Unlike the well-known was performed by three different methodologies described below MCM-41 materials, which are prepared using an anionic template, and summarised in Scheme 1. the HMS type materials are synthesised using a neutral template Method A: A mixture of HMS (0.5 g) in dry toluene (50.0 cm3) (long-chained alkylamines) [23,24]. The surface functionalisation and [VO(acac)2] (0.0261 g, 0.09843 mmol) was refluxed for 24 h. of HMS materials with amine groups via reaction with APTES has The material was vacuum-filtered, washed by reflux in dichloro- been previously reported in the literature [25–29]. Both unmodi- methane (50.0 cm3) and methanol (50.0 cm3) and dried overnight fied and amine functionalised HMS materials have already been in an oven at 120 °C; the resulting material is denoted as A1. used as supports for several transition metal catalysts such as man- Method B: A mixture of HMS (1.6 g) in dry toluene (100.0 cm3) ganese, copper and titanium [26–30]. and APTES (0.80 mmol) was refluxed for 24 h and the resulting Herein we report the immobilisation of [VO(acac)2] onto a syn- material (B1) was vacuum-filtered, washed with toluene (2 Â 100 thetic HMS using three different methodologies, as depicted in cm3) and dried overnight in an oven at 120 °C. Afterwards, a mix- Scheme 1: method A, direct complex immobilisation onto the par- ture of B1 (0.6 g) in dry toluene and [VO(acac)2] (0.0322 g, 0.1214 ent material; method B, functionalisation of HMS with APTES fol- mmol) was refluxed for 24 h and the resulting material was vac- lowed by covalent complex grafting; and method C, reaction uum-filtered, washed by reflux in dichloromethane (50.0 cm3) between the APTES modified HMS prepared in method B and TMS and methanol (50.0 cm3) and dried overnight in an oven at to deactivate eventually unreacted surface silanol groups and sub- 120 °C; the resulting material is denoted as B2. sequent complex grafting. The novel [VO(acac)2] based materials Method C: A mixture of B1 (0.6 g) and TMS (1 mmol) was re- were tested as heterogeneous catalysts in the epoxidation of gera- fluxed in dry toluene for 24 h and the resulting material (C1) niol, at room temperature, in dichloromethane, using t-BuOOH as was vacuum-filtered, washed with toluene (2 Â 50 cm3) and dried the oxygen source; furthermore, the reusability of these heteroge- overnight in an oven at 120 °C. Afterwards, a mixture of C1 (0.4 g) neous catalysts was tested for four times. The main purpose of this and [VO(acac)2] (0.0233 g, 0.0879 mmol) was refluxed in dry tolu- work was to correlate different grafting methodologies for the ene for 24 h and the final material was vacuum-filtered, washed by 3 3 immobilisation of [VO(acac)2] catalyst onto HMS with the corre- reflux in dichloromethane (50.0 cm ) and methanol (50.0 cm ) and sponding catalytic activity and stability in the reaction media. dried overnight in an oven at 120 °C; the resulting material is de- noted as C2. 2. Experimental 2.3. Physico-chemical measurements 2.1. Materials and solvents Vanadium bulk contents obtained by inductively coupled plas- ma emission spectrometry (ICP-AES) and nitrogen elemental anal- All the reagents and solvents used in the preparation and mod- ysis (EA) were performed at ‘Laboratório de Análises’, IST, Lisbon ification of the HMS material were used as received. [VO(acac)2], (Portugal). X-ray photoelectron spectroscopy (XPS) was performed À3 APTES, TMS, t-BuOOH solution in decane (5 mol dm ) and chloro- at ‘‘Centro de Materiais da Universidade do Porto” (Portugal), in a benzene were from Aldrich, tetraethoxysilane was from Lancaster, VG Scientific ESCALAB 200A spectrometer using non-monochro- dichloromethane was from Romil (HPLC grade) and toluene and matised Al Ka radiation (1486.6 eV).