Polyhedron 28 (2009) 994–1000

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Polyhedron

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Grafting of vanadyl acetylacetonate onto organo-hexagonal mesoporous silica and catalytic activity in the allylic epoxidation of

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 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 (t-BuOOH) as oxygen source was assessed. The selectivities of the two 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 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 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 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). The materials were com- other solvents used in the synthesis procedures were from Fischer pressed into pellets prior to the XPS studies. To correct possible Scientific (analytical reagent grade). deviations caused by electric charge of the samples, the C 1s band at 285.0 eV was taken as internal standard. The surface atomic per- 2.2. Preparation of the heterogeneous catalysts centages were calculated from the corresponding peak areas and using the sensitivity factors provided by the manufacturer. Synthesis of HMS: The HMS material was synthesised following a The nitrogen adsorption isotherms at À196 °C were measured reported procedure [23]. Briefly, tetraethoxysilane (37.0 cm3, 0.166 in an automatic apparatus (ASAP 2010; Micromeritics). Before

OH O O O V O O O O Si N O O O O EtO EtO O V V O EtO Si NH O OH O O 2 OH B2 O O OH EtO V O OH O O Si NH O O Method A OH Method B 2 CH OH EtO 3 SiEtO CH3 O O O A1 HMS B1 Method CCH3 CH3 V CH3 SiO CH3 O O SiO CH3 CH3 CH3 O O O Si NH2 O Si N O EtO EtO O V O 1C 2C O

Scheme 1. Methodologies for [VO(acac)2] immobilisation onto HMS materials. 996 B. Jarrais et al. / Polyhedron 28 (2009) 994–1000

the adsorption experiments the samples were outgassed under a 18 vacuum during 2.5 h at 150 °C. HMS The FTIR spectra of the materials were obtained with a Jasco FT/ 16 IR Plus spectrophotometer in the 400–4000 cmÀ1 region, with a 14 A1 resolution of 4 cmÀ1 and 32 scans, using pellets of the materials di- luted with KBr. 12 B1

Gas chromatography (GC) experiments were performed with a -1 B2 Varian CP-3380 chromatograph equipped with a FID detector and 10 using helium as carrier gas and a fused silica Varian Chrompack cap- /mmolg 8 a illary column CP-Sil 8 CB Low Bleed/MS (30 m  0.25 mm i.d.; 0.25 n lm film thickness). The chromatographic conditions were: 40 °C(3 6 min), 5 °C minÀ1, 170 °C (2 min), 20 °C minÀ1, 200 °C (10 min); 4 injector temperature, 200 °C; detector temperature, 300 °C. The reaction parameters geraniol conversion (%C), 2,3-epoxygeraniol selectivity 2 (%S2,3-epoxygeraniol) and 6,7-epoxygeraniol selectivity (%S6,7-epoxygeraniol) were calculated using the following formula, where A stands for 0 0 0.2 0.4 0.6 0.8 1 chromatographic peak area: %C = {[A(geraniol)/A(chlorobenzene)]t=0h p/p 0 À [A(geraniol)/A(chlorobenzene)]t=xh}  100/[A(geraniol)/A(chloro- benzene)]t=0h,%S2,3-epoxygeraniol = A(2,3-epoxygeraniol)  100/[A(2, 18 b HMS 3-epoxygeraniol) + A(6,7-epoxygeraniol)], %S6,7-epoxygeraniol = A(6,7- epoxygeraniol)  100/[A(2,3-epoxygeraniol) + A(6,7-epoxygeraniol)]. 16

14 2.4. Catalysis experiments 12 B1 -1 3 C1, C2 The reactions were carried out in 5.00 cm of dichloromethane, 10 at room temperature with constant stirring, and the composition of /mmolg a 8 the reaction medium was 1.00 mmol of geraniol, 0.50 mmol of n chlorobenzene (internal standard) and 0.10 g of heterogeneous cat- 6 alyst. The oxidant, 1.50 mmol of t-BuOOH, was progressively added to the reaction medium using a Bioblock Scientific Syringe Pump at 4 3 À1 a rate of 0.5 cm h . For each material the %C was determined by 2 withdrawing periodically 0.1 cm3 aliquots from the reaction mix- ture. The reaction media composition was determined by GC, with 0 geraniol, t-BuOOH, 2,3-epoxygeraniol, 6,7-epoxigeraniol and dec- 0 0.2 0.4 0.6 0.8 1 0 ane being quantified by the internal standard method (chloroben- p/p zene). After the reaction, the catalysts were washed by reflux with 3 Fig. 1. Nitrogen adsorption–desorption isotherms at À196 °C of the materials 100 cm of dichloromethane, and dried in an oven at 120 °C for prepared by: (a) methods A and B and (b) method C. Closed points are desorption 13 h, under vacuum. The catalysts were then reused for four times points. using the same experimental conditions. Control experiments using HMS, B1 and C1 materials as well as with [VO(acac)2] com- plex in the homogeneous phase were also performed under identi- the supports even the lowest ABET value (obtained for the B2 sam- cal reaction conditions. 2 À1 ple) is as high as 792 m g. In Fig. 2 the mesopore size distributions, obtained by the Broek- hoff–de Boer method [35], are presented. As noticed from the HMS 3. Results and discussion sample distribution, the mesopores present widths in the range of 2.4 and 3 nm. The direct complex immobilisation (A1) leads to an 3.1. Textural and chemical characterisation increase of the intensity of the peaks at the lowest pore diameter values (near 2.4 nm) and this effect is also observed for the remain- The nitrogen adsorption–desorption isotherms at À196 °C for ing materials containing the vanadium complex (B2 and C2). In the the different materials are depicted in Fig. 1. The curves do not case of C2, that is, the [VO(acac)2] based material where TMS was show any hysteresis cycle, but the data analysis by the t-method added, besides the peaks shift to lower values, the distribution [31] confirms the inexistence of microporosity. The isotherms seems to be more regular, that is, the different maxima become can be considered of type IVc according to the IUPAC classification closer, suggesting that the TMS molecules, after reacting with the [31], being characteristic of mesoporous materials (pore widths be- remaining silica OH groups, produce more uniform porous tween 2 and 50 nm). As described in more detailed elsewhere [32], materials. type IVc isotherms, although they are obtained in mesoporous In Table 1 are also collected the N and V contents obtained by materials, they are completely reversible, that is, they do not pres- elemental and ICP-AES analysis, respectively, and by XPS. From ent hysteresis. This type of isotherms was found by other authors the N bulk analysis and taking into account the initial amount of in materials that are similar to those studied in this work, such alkoxysilane used, it was possible to calculate the APTES grafting as some samples of MCM-41 [32,33] and HMS type materials efficiency (amount of reacted alkoxysilane/amount in original [34]. The specific surface areas (ABET) and mesoporous volumes of solution  100) for B1, which is 90%. Furthermore, the bulk (EA) the samples are given in Table 1. From this table it can be seen that and surface nitrogen (XPS) contents are similar, suggesting that the subsequent modifications performed on the parent HMS mate- APTES is uniformly grafted onto this material. rial lead to a concomitant decrease of the ABET values. However, it is Upon complex immobilisation, the ICP-AES vanadium analysis rather noticeable that after the immobilisation of the complex onto in association with the peaks observation in the V 2p3/2 region of B. Jarrais et al. / Polyhedron 28 (2009) 994–1000 997

Table 1 Chemical analysis and textural properties of HMS based materials.

Material N(lmol gÀ1)V(lmol gÀ1) Textural properties

a b c 2 À1 3 À1 EA XPS ICP XPS ICP after catalysis ABET (m g ) Vmeso (cm g ) HMS n.d. 59 – – – 1138 0.52 A1 n.d. 194 4 84 – 1020 0.41 B1 450 446 – – – 908 0.36 B2 364 170 128 25 35 792 0.31 C1 378 248 – – – 793 0.32 C2 293 237 51 51 41 808 0.32

a lmol N/weight of sample = at% N/[at% C  Ar(C) + at% N  Ar(N) + at% O  Ar(O) + at% Si  Ar(Si) + at% V  Ar(V)]. b lmol V/weight of sample = at% V/[at% C  Ar(C) + at% N  Ar(N) + at% O  Ar(O) + at% Si  Ar(Si) + at% V  Ar(V)]. c Vanadium ICP-AES content after five catalytic cycles.

HMS A1 the VO complex is mainly grafted in the inner pores, since the bulk content is higher than the surface loading and finally, in C2, the metal complex appears to be homogeneously distributed through- out the HMS pores, since the vanadium bulk and surface contents

a. u. are similar. a. u. Analysis of ABET values as a function of vanadium surface (XPS) and bulk (ICP-AES) contents for A1, B2 and C2 materials, Table 1, can give some interesting relations concerning the functionalised 1.5 2.5 3.5 4.5 1.5 2.5 3.5 4.5 materials. In the case of the vanadium surface contents, the sample pore diameter (nm) pore diameter (nm) with higher complex loading (A1) is also the one which presents

higher ABET value since the surface inside the pores remains unoc- B2 C2 cupied to a large extent. More relevant, however, is the relation be-

tween the ABET values and the vanadium bulk contents: in this case, as can be expected, the ABET decreases with the increase in the vanadium total content: ABET(A1) > ABET(C2) > ABET(B2). However, À1

a. u. for vanadium values of 51 and 128 lmol g (for C2 and B2, respec- a. u. 2 À1 tively), the ABET values only decrease from 808 to 792 m g , that is, in this vanadium content range, when the vanadium bulk amount increases 2.5 times the decrease in the surface area is only 1.5 2.5 3.5 4.5 1.5 2.5 3.5 4.5 of 2%. pore diameter (nm) pore diameter (nm) 3.2. FTIR measurements Fig. 2. Mesopore size distributions for HMS and for samples with anchored complex, obtained from the low temperature nitrogen adsorption data using the Broekhoff–de Boer method. The infrared spectrum of the HMS parent material, Fig. 3, shows the typical Si–O lattice vibrations: two low intensity broad bands between 2100 and 1750 cmÀ1, a broad and strong band in the the high resolution XPS spectra of A1, B2 and C2 materials, con- 1390–900 cmÀ1 region and two strong bands between 900 and firm the presence of the metal complex in the HMS materials. 400 cmÀ1 [26–29]. Moreover, it can also be observed a broad band From the vanadium contents determined by ICP-AES it was pos- at 3450 cmÀ1 assigned to O–H stretching vibrations and a very sible to determine the complex loadings for A1, B2 and C2 mate- sharp peak at 3745 cmÀ1, with medium intensity, due to stretching rials, which are 4, 128 and 51 lmol gÀ1, respectively (Table 1), vibrations of isolated silanol groups [26–29]. A band at 1630 cmÀ1 corresponding to complex immobilisation efficiencies (amount is also observed in the spectrum and is attributed to H–O–H bend- of immobilised complex/amount in original solution  100) of ing vibrations of physisorbed water [26–29]. 2%, 65% and 25%, respectively. As can be seen, the direct immo- Upon HMS functionalisation with APTES (B1), Fig. 3b, several bilisation of the complex onto the parent HMS (method A) leads changes can be observed in all the frequency range. The peak at to the lowest complex immobilisation efficiency, whereas the 3745 cmÀ1 disappears, indicating the reaction between the surface highest value is achieved for complex immobilisation onto the silanol groups and the ethoxyl functionalities of APTES. A decrease APTES functionalised HMS (method B). This is a consequence in the broad band centred at 3450 cmÀ1 can also be observed due of the absence of chemical adsorption sites for the complex to a decrease in the adsorbed water content, as a consequence of immobilisation in the parent HMS in contrast with the NH2- the APTES grafting reaction. Furthermore, new bands due to N–H functionalised HMS (B1), where the grafted NH2 groups can act and C–H stretching vibrations can be observed in the 2970–2850 as specific sites for complex covalent anchorage. C1 material cmÀ1 region and the H–N–H and H–C–H bending vibrations are shows lower V content than B1, probably because the grafted also observed in the range of 1560–1430 cmÀ1 [26–29]. TMS molecules hinder the reaction between APTES and the com- Upon treatment of the APTES modified HMS with TMS (C1), plex molecules. Fig. 3c, no significant changes are detected; nevertheless a small Moreover, the comparison between the vanadium ICP-AES and decrease in the broad band centred at 3450 cmÀ1 due to OH XPS values can also provide some insights about the complex local- stretching vibrations can be observed, suggesting the reaction be- isation within the HMS materials. In A1 material, [VO(acac)2]is tween the remaining free silanol groups of the support and the eth- mainly immobilised in the outer pores of the parent HMS, since oxyl groups of the TMS molecules. The presence of TMS in the the total amount of vanadium per weight of material is lower than material is also demonstrated by a small increase in the peaks cor- the vanadium surface content. On the other hand, in B2 material responding to C–H stretching vibrations. 998 B. Jarrais et al. / Polyhedron 28 (2009) 994–1000

a b HMS

HMS B1

A1 B2

[VO(acac) ] [VO(acac) ] 2 2

4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1) Wavenumber (cm -1)

c HMS

B1

C1 C2

[VO(acac) ] 2

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1)

Fig. 3. FTIR spectra of the free [VO(acac)2] complex and of parent and modified HMS materials prepared by methods: (a) A, (b) B and (c) C.

The FTIR spectra of the materials after complex immobilisation 2970–2850 cmÀ1 region and a change in their profile, (ii) increase (A1, B2 and C2), Fig. 3a–c, present some changes in the ranges of in the intensity and broadness of the band at 1630 cmÀ1 and (iii) 2970–2850 cmÀ1 and of 1780–1350 cmÀ1, that can be unambigu- the appearance of a shoulder on the latter band in the high energy ously assigned to the immobilised complex; however, in all these side, at approximately 1710 cmÀ1. The broadening of the band at spectra, the band due to the V@O stretching vibration, which ap- 1630 cmÀ1 can be due to the presence of overlapped bands as- pears at 997 cmÀ1 in the free complex could not be observed prob- signed to the grafted complex, as already seen in the grafting of ably because it is masked by the strong and broad Si–O–Si band the same VO(IV) complex and Cu(II) analogue in Laponite and positioned in the 1390–900 cmÀ1 range. In fact, the free complex K10-montmorillonite, using the same immobilisation procedure spectrum exhibits the most relevant bands in the ranges 3000– [16,37]. These new bands are assigned to chelate stretching vibra- 2920 cmÀ1 (C–H stretching vibrations), 1560–1350 cmÀ1 (stretch- tions, now containing the new C@N bond formed by Schiff conden- ing vibrations associated with the chelate ring) and at 997 cmÀ1 sation reaction between the amine groups of the grafted APTES and corresponding to the VO stretching vibrations [36]. the C@O group from the coordinated acetylacetonate ligand, The FTIR spectrum of A1, Fig. 3a, presents three new bands Scheme 1 [16,37]. Due to the high APTES grafting efficiency onto in the C–H stretching vibrations range, at 2960, 2932 and 2857 the parent HMS, no significant direct immobilisation of [VO(acac)2] cmÀ1, which are consistent with a band shift of the free complex may occur in materials B1 and C1. bands situated at 3000, 2971 and 2923 cmÀ1. This result, combined with the observed decreases in both the intensity of the broad 3.3. Epoxidation of geraniol catalysed by [VO(acac)2] heterogeneous band centred at 3450 cmÀ1 and the sharp peak at 3745 cmÀ1, sug- catalysts gest that the HMS hydroxyl surface groups may be involved in the complex immobilisation through hydrogen bonds with the deloca- The results obtained in the epoxidation of geraniol at room tem- lised p-system of the acetylacetonate ligand, Scheme 1, as ob- perature using [VO(acac)2] based materials as catalysts and t- served in the direct immobilisation of [VO(acac)2] onto alumina, BuOOH as the oxygen source, in dichloromethane, are summarised clays, silica gel and other mesoporous silicas [16,18,19]. in Table 2. Data from homogeneous phase and blank experiments When comparing the spectra of B2 and C2 with the correspon- ran under identical conditions are also included. dent parent materials (B1 and C1, respectively) it is possible to see The geraniol conversions for 48 h of reaction are 100%, 35% and several changes which are attributed to the grafted VO complex: (i) 17% using B2, C2 and A1 materials, respectively (Table 2). This increase in the intensity of the C–H stretching vibrations in the sequence follows the order of vanadium loadings obtained by B. Jarrais et al. / Polyhedron 28 (2009) 994–1000 999

Table 2 a Epoxidation of geraniol catalysed by [VO(acac)2] in homogeneous and heterogeneous phases.

[VO(acac)2] 1-2 % mol O O OH OH OH t-BuOOH 1.5 eq. / CH2Cl2 / r.t.

geraniol 2,3-epoxygeraniol 6,7-epoxygeraniol

Catalyst t (h) Cycle %Cb % Selectivity (% yield)c 2,3-Epoxygeraniol 6,7-Epoxygeraniol

[VO(acac)2] 1 first 100 99 (99) 1 (0.7) HMS 48 first 7 49 (3) 51 (4) A1 48 first 17 92 (16) 8 (1) B1 48 first 4 50 (2) 50 (2) B2 48 first 100 98 (98) 2 (2) 48 second 100 98 (98) 2 (2) 48 third 99 99 (98) 1 (1) 48 fourth 100 99 (98) 1 (1) 48 fifth 100 99 (99) 1 (1) C1 48 first 4 57 (2) 43 (2) C2 48 first 35 99 (35) 1 (0) 48 second 93 100 (93) 0 (0) 48 third 66 99 (66) 1 (0) 48 fourth 77 99 (76) 1 (1) 48 fifth 34 99 (34) 1 (0)

a Typical reaction conditions: 1.00 mmol geraniol, 0.50 mmol chlorobenzene (internal standard), 0.10 g heterogeneous catalyst and 1.50 mmol t-BuOOH. b Based on geraniol consumption. c Selectivity of the geraniol epoxides; yield calculated as: (%C Â %S)/100.

ICP-AES (Table 1) onto the HMS materials: B2  C2 > A1. The reg- leading to a homogeneous distribution of the complex through- ioselectivity towards the allylic double bond remains identical to out the support. the homogeneous counterpart for all the heterogeneous [VO(a- cac)2] based catalysts, being nearly 100% for 2,3-epoxygeraniol (Ta- 4. Conclusions ble 2). From the control experiments with the parent materials it is possible to conclude that the silica surface both unmodified and New heterogeneous catalysts were prepared by immobilisation functionalised with APTES/TMS does not have relevant catalytic of [VO(acac)2] onto HMS supports both directly and after function- activity on its own, as can be confirmed by the geraniol conversion alisation with amine groups. In the case of APTES functionalised percentages of HMS, B1 and C1 materials, of only 7% and 4%, supports, the effect of TMS addition before complex grafting was respectively. also studied. All the reactions in the heterogeneous phase have higher reac- The ICP-AES and XPS results of the final materials showed that tion times when compared to the homogeneous reaction. This ef- the [VO(acac)2] immobilisation was achieved in all cases, with B2 fect is due to diffusion constraints imposed to the substrate and presenting the highest grafting efficiency (highest complex load- reagents by the specific localisation of the active catalytic centres ing). In the case of A1 (method A), the metal complex was mainly within the porous matrix [16,21,27]. immobilised on the outer silica surface by hydrogen bonds be- With the exception of A1 which presents high catalyst leaching tween the HMS hydroxyl surface groups and the delocalised p-sys- after the first cycle, detected by visual inspection of the colour of tem of the acetylacetonate ligand. Complex immobilisation by the reaction medium, the new heterogeneous catalysts were re- methods B and C (materials B2 and C2) occurred by covalent bond used for further four catalytic cycles. During the catalytic cycles, formed by the reaction between the surface NH2 grafted groups B2 shows similar %C and %S2,3-epoxygeraniol to those exhibited in and the C@O group of the acetylacetonate ligand. In B2 the VO the first cycle, but C2 shows some change in %C. In order to under- complex was mainly grafted in the inner pores of the solid matrix, stand these different behaviours, leaching of the catalytic active but in the case of C2, the complex distribution in HMS pores was phase was determined by measurement of vanadium contents by more homogeneous. ICP-AES after the five catalytic cycles; the values are presented in In the geraniol epoxidation, the material which led to the high- Table 1. Materials B2 and C2 have complex leaching percentages est geraniol conversion was B2. One of the reasons that could jus- of 73% and 20%, respectively, and thus it can be concluded that tify the highest values for this material, in particular when the constant high catalytic activity of B2 in all cycles may have a compared with C2, could be its highest vanadium content for com- significant homogeneous component. In this context, C2 can be parable specific surface area values. However, B2 material also re- considered the most stable catalyst upon reuse, presenting a satis- sulted in a higher leaching percentage after five catalytic cycles factory geraniol conversion, but showing high selectivity for the when compared with C2. It could be concluded that the grafting desired product even after four reuse cycles. process leading to C2 catalyst, although resulting in lower [VO-

As a summary, in terms of catalyst stability, the most effec- (acac)2] content, was the most efficient one, giving rise to a heter- tive immobilisation method is the one which uses TMS in com- ogeneous catalyst with satisfactory substrate conversion, high reg- bination with APTES (method C: covalent grafting, Scheme 1), ioselectivity until the fourth catalytic cycle and high stability, 1000 B. Jarrais et al. / Polyhedron 28 (2009) 994–1000 corresponding to a leaching percentage of 20% after five consecu- [13] D.E. de Vos, I.F.J. Vankelecom, P.A. Jacobs (Eds.), Chiral Catalyst Immobilisation tive catalytic cycles. Material A1 showed the lowest %C and the and Recycling, Wiley-VCH Verlag, Weinheim, 2000, p. 19. [14] D.E. de Vos, M. Dams, B.F. Sels, P.A. Jacobs, Chem. Rev. 102 (2002) 3615. highest active phase leaching; this was a consequence of the low- [15] Q.-H. Xia, H.-Q. Ge, C.-P. Ye, Z.-M. Liu, K.-X. Su, Chem. Rev. 105 (2005) 1603. est V content, determined by the absence of specific immobilisa- [16] C. Pereira, A.R. Silva, A.P. Carvalho, J. Pires, C. Freire, J. Mol. Catal. A: Chem. 283 tion sites and by weak non-covalent support interactions through (2008) 5. [17] S. Shylesh, A.P. Singh, J. Catal. 244 (2006) 52. hydrogen bonds, which can easily be disrupted by the oxidative [18] A.-M. Hanu, S. Liu, V. Meynen, P. Cool, E. Popovici, E.F. Vansant, Micropor. nature of the catalytic reaction medium. Mesopor. Mater. 95 (2006) 31. [19] M. Baltes, O. Collart, P. Van der Voort, E.F. Vansant, Langmuir 15 (1999) 5841. [20] A. Lattanzi, N.E. Leadbeater, Org. Lett. 4 (2002) 1519. Acknowledgements [21] B. Jarrais, A.R. Silva, C. Freire, Eur. J. Inorg. Chem. (2005) 4582. [22] X.S. Zhao, F. Su, Q. Yan, W. Guo, X.Y. Bao, L. Lv, Z. Zhou, J. Mater. Chem. 16 This work was funded by FCT Fundação para a Ciência e a Tecn- (2006) 637. [23] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865. ologia (FCT) and FEDER, through the Project Refs. POCI/CTM/ [24] K. Wilson, J.H. Clark, Pure Appl. Chem. 72 (2000) 1313. 56192/2004 and PTDC/CTM/65718/2006. C.P. thanks FCT for a [25] P.M. Price, J.H. Clark, D.J. Macquarrie, J. Chem. Soc., Dalton Trans. (2000) 101. Ph.D. fellowship. [26] A.R. Silva, K. Wilson, J.H. Clark, C. Freire, Micropor. Mesopor. Mater. 91 (2006) 128. [27] A.R. Silva, K. Wilson, A.C. Whitwood, J.H. Clark, C. Freire, Eur. J. Inorg. Chem. References (2006) 1275. [28] A.R. Silva, K. Wilson, J.H. Clark, C. Freire, Stud. Surf. Sci. Catal. 158 (2005) [1] T. Katsuki, Coord. Chem. Rev. 140 (1995) 189. 1525. [2] V. Conte, F. Di Furia, G. Licini, Appl. Catal. A 157 (1997) 335. and references [29] Z. Fu, D. Yin, W. Zhao, Y. Chen, D. Yin, J. Guo, L. Zhang, Catal. Lett. 90 (2003) therein. 205. [3] M.N. Sheng, J.G. Zajacek, J. Org. Chem. 35 (1970) 1839. [30] Z. Fu, D. Yin, Q. Xie, W. Zhao, A. Lv, D. Yin, Y. Xu, L. Zhang, J. Mol. Catal. A: Chem. [4] (a) K.B. Sharpless, R.C. Michaelson, J. Am. Chem. Soc. 95 (1973) 6136; 208 (2004) 159. (b) S. Tanaka, H. Yamamoto, H. Nozaki, K.B. Sharpless, R.C. Michaelson, J.D. [31] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids, Cutting, J. Am. Chem. Soc. 96 (1974) 5254; Academic Press, 1999. (c) K.B. Sharpless, T.R. Verhoeven, Aldrichim. Acta 12 (1979) 63. [32] M. Grün, K.K. Unger, A. Matsumoto, K. Tsutsumi, in: B. McEnaney, T.J. Mays, J. [5] C. Bolm, Coord. Chem. Rev. 237 (2003) 245. Rouquerol, F. Rodríguez-Reinoso, K. Sing, K.K. Unger (Eds.), Characterisation of [6] B.E. Rossiter, T.R. Verhoeven, K.B. Sharpless, Tetrahedron Lett. 20 (1979) 4733. Porous Solids IV, The Royal Society of Chemistry, London, 1997. [7] A.G.J. Ligtenbarg, R. Hage, B.L. Feringa, Coord. Chem. Rev. 237 (2003) 89. [33] R. Schmidt, M. Stöcker, E. Hansen, D. Akporiaye, O.H. Ellestad, Micropor. Mater. [8] L. Canali, D.C. Sherrington, Chem. Soc. Rev. 28 (1999) 85. 3 (1995) 443. [9] Q.-H. Fan, Y.-M. Li, A.S.C. Chan, Chem. Rev. 102 (2002) 3385. [34] A.P. Fotopoulos, K.S. Triantafyllidis, Catal. Today 127 (2007) 148–156. [10] N.E. Leadbeater, M. Marco, Chem. Rev. 102 (2002) 3217. [35] W.J. Lukens Jr., P. Schmidt-Winkel, D. Zhao, J. Feng, G.D. Stucky, Langmuir 15 [11] D. Brunel, N. Belloq, P. Sutra, A. Cauvel, M. Laspéras, P. Moreau, F. Di Renzo, A. (1999) 5403. Galarneau, F. Fajula, Coord. Chem. Rev. 178–180 (1998) 1085. [36] P. Van der Voort, I.V. Babitch, P.J. Grobet, A.A. Verbeckmoes, E.F. Vansant, J. [12] C. Freire, C. Pereira, in: J.L. Figueiredo, M.M. Pereira, J. Faria (Eds.), Catalysis Chem. Soc., Faraday Trans. 92 (1996) 3635. from Theory to Application – An Integrated Course, Imprensa da Universidade [37] C. Pereira, S. Patrício, A.R. Silva, A.L. Magalhães, A.P. Carvalho, J. Pires, C. Freire, de Coimbra, Portugal, 2008, p. 213 (Chapter A.9). J. Colloid Interf. Sci. 316 (2007) 570. 本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

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