[VO(acac)₂] hybrid catalyst: from complex immobilization onto silica nanoparticles to catalytic application in the epoxidation of geraniol

Abstract

This work reports the preparation of a novel hybrid nanocatalyst through the immobilization of oxidovanadium^IV acetylacetonate ([VO(acac)₂]) onto silica nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTES) and its catalytic application in the allylic epoxidation of geraniol. All the nanomaterials were characterized by chemical analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The parent and amine-functionalized silica nanoparticles were also analyzed by ²⁹Si and ¹³C solid-state nuclear magnetic resonance and the supported [VO(acac)₂] nanomaterial was characterized by magnetic susceptibility measurement. The APTES organosilane was grafted onto the silica nanoparticles with an efficiency of 90%, through mono- and bidentate covalent grafting. The [VO(acac)₂] complex was covalently anchored on the surface of the APTES-functionalized silica nanoparticles with 25% of efficiency. Magnetic susceptibility measurements in combination with XPS indicated that the oxidation state of the metal center (VO²⁺) was preserved upon the immobilization reaction. The catalytic performance of the novel hybrid [VO(acac)₂] nanocatalyst was evaluated in the epoxidation of geraniol, at room temperature, using tert-butyl hydroperoxide as an oxygen source. The nanocatalyst presented 100% of substrate conversion, 99% of selectivity towards the 2,3-epoxygeraniol product and was stable upon reuse in further four cycles.

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1 Introduction

Over the years, catalysis has been playing a crucial role in the chemical industry with significant economic and environmental impact. Transition metal complexes are of paramount importance in oxidative catalysis for the production of valuable intermediates for organic synthesis such as epoxides.^1–3 In particular, oxidovanadium^IV acetylacetonate ([VO(acac)₂]) is a very efficient homogeneous catalyst in the epoxidation of allylic alcohols into the corresponding epoxides, using tert-butyl hydroperoxide (t-BuOOH) as an oxidant, presenting very high activity, stereo- and regioselectivity.^4–9 Despite the advantages of homogeneous catalysts, their high cost, low chemical and thermal stability and difficulties associated with their separation from the reaction media are major drawbacks towards the goals of Green Chemistry.^1,10–13 In this framework, the immobilization of transition metal complexes onto solid supports such as inorganic materials and organic polymers constitutes a potential strategy to overcome these problems,^1,10–14 leading to the development of more sustainable catalytic processes.

The [VO(acac)₂] complex has already been anchored onto several types of bulk materials by covalent bonding (either directly or through a linker between the complex and support)^15–18 and by encapsulation,¹⁹ for the heterogeneous epoxidation of allylic alcohols. This complex has been immobilized onto amine-functionalized smectite-type clays,¹⁵ porous clay heterostructure,¹⁶ mesoporous silicas^16,17 and activated carbon¹⁸ by Schiff condensation between the carbonyl group of the acetylacetonate ligand and the free amine groups previously grafted on the supports surface. It has also been directly anchored onto clays¹⁵ and onto a hexagonal mesoporous silica¹⁷ by reaction between the complex and the free silanol groups of the support through one of two possible mechanisms: (i) hydrogen bonding between the pseudo π system of the acetylacetonate ligand and the silanol protons of the support or (ii) ligand-exchange with the formation of a covalent bond between the vanadium centre and an oxygen atom from the support.^15,20–23 Furthermore, a different immobilization route has been the complex microencapsulation in polystyrene, i.e., the physical envelopment of the complex by a thin polymer film involving interactions between the complex vacant orbitals and the π electrons of the polymer benzene rings.¹⁹

Concerning the catalytic performance, the [VO(acac)₂] microencapsulated in polystyrene¹⁹ and anchored onto amine-functionalized K10-montmorillonite clay¹⁵ and activated carbon¹⁸ were the most active heterogeneous catalysts in the epoxidation of allylic alcohols, presenting similar substrate conversion as the homogeneous counterpart and good/moderate stability upon recycling/reuse in further cycles without significant metal leaching. Nevertheless, in most cases an increase of the reaction time relative to the homogeneous phase reaction was acknowledged, due to diffusion constraints of the substrate and oxidant to the catalytic active centers.

In this context, the quest for novel recyclable catalysts that combine high substrate conversion/selectivity with low reaction time continues to be a challenging milestone in catalysis.

In the last decades, silica nanoparticles have conquered new horizons in a myriad of research areas including Materials Science and Engineering, Medicine and Biology.^24–27 The nanometre size, large surface area to volume ratio and presence of silanol groups on the surface, which allow the incorporation of a wide variety of functionalities, are some of the outstanding features^24,25,27 that make silica nanoparticles a promising support for the fabrication of the so-called “quasi-homogeneous” catalysts. Moreover, they can be easily dispersed in several solvents (aqueous and organic) which facilitates the accessibility of the reactants to the catalytic active sites, thus avoiding the diffusion problems typically associated with heterogeneous bulk catalytic systems.²⁸ Despite these potentialities, until now only a few publications have reported the catalytic activity of transition metal complexes anchored onto silica nanoparticles;^29–32 for instance, hybrid nanocatalysts containing N-heterocyclic carbene–palladium complexes for Suzuki and Heck coupling reactions,²⁹ chiral RuCl₂–diphosphine–diamine complexes for the asymmetric hydrogenation of aromatic ketones³⁰ and Co(III) salen complexes for the hydrolytic kinetic resolution of epichlorohydrin.³¹ All these nanocatalysts presented remarkable results in terms of activity, selectivity and stability upon reuse.

Very recently, we prepared a [VO(acac)₂] magnetic nanocatalyst by immobilization of the complex onto superparamagnetic silica-coated maghemite (γ-Fe₂O₃) nanoparticles functionalized with amine groups (240 nm particle size).³³ In a preliminary study, the magnetic nanocatalyst was tested in the epoxidation of geraniol, leading to similar activity and selectivity as the homogeneous counterpart,³³ which highlighted the importance of nanosize supports for the immobilization of the [VO(acac)₂] catalyst. Nevertheless, the reaction time was still much higher than that of the homogeneous phase reaction (30 h vs. 15 min), despite the smaller particle size of the nanocatalyst (240 nm). Consequently, the immobilization of this complex onto colloidal nanoparticles with sizes below 100 nm may provide additional advantages namely increasing the surface area to volume ratio and the concentration of catalytic active sites in the reaction medium.

In this context, we report the preparation of a hybrid [VO(acac)₂] nanocatalyst through the immobilization of the complex onto silica nanoparticles of approximately 45 nm size which were previously functionalized with 3-aminopropyltriethoxysilane (APTES). The catalytic performance of the supported complex is evaluated in the epoxidation of geraniol, at room temperature, using t-BuOOH as an oxidant and dichloromethane as a solvent, and compared to that of the free complex. We endeavor to examine the influence of the nanometre size support on the complex immobilization efficiency, location of the grafted complex and anchoring mechanism. Additionally insights into the effect of this type of support on the catalyst activity, stability and recyclability will be provided.

2 Experimental

2.1 Materials, reagents and solvents

[VO(acac)₂] (95%), APTES (99%), silica nanoparticles (spherical, 99.5% trace metals basis, according to the supplier specifications), dry toluene (99.8%), chlorobenzene (≥99.5%), geraniol (98%) and t-BuOOH solution 5.0–6.0 M in decane were purchased from Aldrich. The dichloromethane solvent was supplied by Romil (HPLC grade) and chloroform (analytical grade) was from Merck. All reagents and solvents were used as received without further purification.

2.2 Catalyst preparation

2.2.1 Functionalization of silica nanoparticles with APTES. Prior to the functionalization procedure, the nanosilica (SiO₂ sample) was dried in an oven at 120 °C overnight, under vacuum, to remove any physisorbed water. Afterwards, a mixture of 4.50 g of nanosilica and 2.66 cm³ of APTES (11.2 mmol) in 220 cm³ of dry toluene was refluxed for 24 h under an argon atmosphere. The resulting nanomaterial (APTES@SiO₂) was separated by centrifugation, refluxed with 220 cm³ of dry toluene for 2 h and dried at 120 °C, under vacuum, overnight.

2.2.2 Immobilization of the [VO(acac)₂] complex onto APTES-functionalized silica nanoparticles. A solution of 0.32 g of [VO(acac)₂] complex (1.16 mmol, 25.0 wt%) in 120 cm³ of dry toluene/chloroform (5/1 v/v) was refluxed with 1.30 g of APTES@SiO₂ for 24 h. The resulting nanomaterial ([VO(acac)₂]APTES@SiO₂) was separated by centrifugation, refluxed with 120 cm³ of chloroform for 2 h and dried at 120 °C, under vacuum, overnight.

2.3 Physicochemical characterization

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) studies were performed at “Centro de Materiais da Universidade do Porto” (CEMUP, Porto, Portugal), using a high-resolution environmental scanning electron microscope (FEI Quanta 400 FEG ESEM) equipped with an energy-dispersive X-ray spectrometer (EDAX Genesis X4M). The [VO(acac)₂]APTES@SiO₂ sample was analyzed as powder both uncoated and coated with a thin gold–palladium film.

The amount of silanol groups in the silica nanoparticles was determined through the alkali neutralization method reported in the literature.³⁴ The qualitative identification of free amine groups in APTES@SiO₂ was performed by the ninhydrin test;³⁵ the appearance of a blue/purple color indicates the presence of those groups.

Vanadium contents, obtained by inductively coupled plasma emission spectrometry (ICP-AES) and nitrogen elemental analysis (EA) were performed at “Laboratório de Análises do Instituto Superior Técnico” (Lisboa, Portugal).

Magnetic susceptibility experiments were performed in a Quantum Design MPMS instrument with a Reciprocal Sample Option, by measuring the field-cooled magnetization as a function of temperature in the range of 5–300 K with an applied magnetic field of 0.05 T.

X-Ray photoelectron spectroscopy (XPS) was performed in a Kratos Axis Ultra DLD spectrometer using monochromatic Al Kα radiation (1486.6 eV). The powdered samples were pressed into pellets prior to the XPS studies. To correct possible deviations caused by the electric charge of the samples, the calibration of the binding energies was performed by using the C 1s band at 284.6 eV (C–H/C–C) as internal reference. The XPS spectra were deconvoluted with the XPSPEAK 4.1 software, using non-linear least squares fitting routine after a Shirley-type background subtraction. The surface atomic percentages were calculated from the corresponding peak areas and by using the sensitivity factors provided by the manufacturer.

Solid state nuclear magnetic resonance (NMR) spectra of carbon 13 and silicon 29, ¹³C CPMAS (cross-polarization magic angle spinning) and ²⁹Si CPMAS were recorded at “Departamento de Química, Universidade de Aveiro” (Aveiro, Portugal) on a 9.4 T Bruker Avance 400 spectrometer, at a spinning rate of 7 kHz for carbon and 5 kHz for silicon. ¹³C CPMAS NMR spectra were recorded with 4.4 μs ¹H 90° pulses, 1.5 ms contact time and 5 s recycle delays. ²⁹Si CPMAS NMR spectra were recorded with 4.25 μs ¹H 90° pulses, 8 ms contact time and 5 s recycle delays. The ¹³C and ²⁹Si chemical shifts (δ) were referenced to glycine and caulinite, respectively.

The Fourier transform infrared (FTIR) spectra of the compounds were collected with a Jasco FT/IR-460 Plus spectrophotometer in the 400–4000 cm⁻¹ range, using a resolution of 4 cm⁻¹ and 32 scans. The spectra of the samples were obtained in KBr pellets (Merck, spectroscopic grade) containing 0.6 wt% of silica-based nanomaterial. All samples and KBr were dried at 100 °C overnight before KBr pellets preparation.

GC-FID chromatograms were obtained with a Varian CP-3380 gas chromatograph equipped with a FID detector, using helium as carrier gas and a fused silica Varian Chrompack capillary column CP-Sil 8 CB Low Bleed/MS (30 m × 0.25 mm id; 0.25 μm film thickness). Temperature program for geraniol epoxidation reactions: 40 °C (1 min), 25 °C min⁻¹, 150 °C, 5 °C min⁻¹, 200 °C (1 min); injector temperature, 200 °C; detector temperature, 250 °C. GC-FID was used to determine the catalytic parameters (geraniol conversions, products selectivities and 2,3-epoxygeraniol yields), since this technique provided the same results as those obtained from ¹H NMR and 2,3-epoxygeraniol isolated yield (based on the results reported in our previous work¹⁶). In this context, the reaction parameters geraniol conversion (Conv.), 2,3-epoxygeraniol selectivity (S_2,3-EG), geranial selectivity (S_geranial) and 2,3-epoxygeraniol yield (η_2,3-EG) were calculated as follows, where A stands for chromatographic peak area:

where n(2,3-EG) was determined by GC-FID using the expression:

2.4 Catalytic studies

The geraniol epoxidation was carried out at room temperature using 1.00 mmol of geraniol (substrate), 0.50 mmol of chlorobenzene (GC internal standard) and 0.10 g of [VO(acac)₂]APTES@SiO₂ (catalyst) in 5.00 cm³ of dichloromethane, with continuous stirring. The oxygen source, t-BuOOH, 1.50 mmol, was progressively added to the reaction using a Bioblock Scientific Syringe Pump at a rate of 3.05 cm³ h⁻¹. To monitor the reaction progress, during the catalytic experiment, 0.05 cm³ aliquots were withdrawn from the reaction mixture with a hypodermic syringe, filtered and analyzed by gas chromatography. At the end of the reaction, the heterogeneous nanocatalyst was recovered by centrifugation, refluxed/centrifuged with 50 cm³ of CH₂Cl₂ for 2 h, dried under vacuum at 120 °C during 2 h, and reused four times. The catalytic reaction was also performed under similar conditions (a) in the homogeneous phase using the same [VO(acac)₂] loading as that of the heterogeneous nanomaterial and (b) in the heterogeneous phase using the parent SiO₂ and APTES@SiO₂ supports.

3 Results and discussion

3.1 Morphology and chemical composition

The SEM micrograph of [VO(acac)₂]APTES@SiO₂ (Fig. 1(A)) shows that the hybrid nanomaterial presents the same morphology as the parent silica nanosupport (∼45 nm particle size, not shown).

Fig. 1 (A) SEM micrograph and (B) EDS spectrum of [VO(acac)₂]APTES@SiO₂ nanomaterial (inset: atomic percentages of C, O, Si and V).

The EDS analysis (Fig. 1(B)) reveals the presence of carbon (27.9%) and vanadium (0.6%), in addition to oxygen and silicon from the silica framework, providing a first evidence for the complex immobilization onto SiO₂; nevertheless, the carbon contribution from the double-sided carbon tape used in the sample preparation cannot be disregarded.

The amount of silanol groups on the surface of the parent SiO₂ nanomaterial was determined through the alkali neutralization method. Based on this method, each gram of nanosilica contains 1.51 mmol of OH groups.

The ninhydrin test was performed to detect qualitatively the existence of free amine species on APTES@SiO₂ (Fig. S1, ESI†). The colour of the nanomaterial changed from white to blue when the ninhydrin solution was added, confirming the presence of free amine groups. In contrast, the test was negative for the parent SiO₂ (no colour change; Fig. S1, ESI†).

The nitrogen and vanadium analyses allowed the determination of the amount of APTES and [VO(acac)₂] grafted onto APTES@SiO₂ and [VO(acac)₂]APTES@SiO₂, respectively, and the calculation of their anchoring efficiencies (Table 1).

Table 1 Chemical composition of the silica-based nanomaterials

Material	N/mmol g^−1		V/μmol g^−1
	EA	XPS ^a	ICP	χ(T) ^b	XPS ^a
a Surface contents determined from XPS data in Table 2: mmol N/weight of material = atomic% N/Σ[atomic% (i) × Ar(i)]; μmol V/weight of material = atomic% V/Σ[atomic% (i) × Ar(i)]. b Vanadium bulk content estimated by magnetic susceptibility measurements: μmol V/weight of material = Canc/[Cfree × M([VO(acac)2]).
SiO2	—	—	—	—	—
APTES@SiO2	1.4	2.4	—	—	—
[VO(acac)2]APTES@SiO2	1.4	2.3	295	204	681

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Table 2 Surface atomic percentages for silica-based nanomaterials obtained by XPS^a

Material	Atomic%
	C 1s	N 1s	O 1s	Si 2p	V 2p
a Determined by the areas of the respective bands in the high-resolution XPS spectra.
SiO2	4.1	—	66.8	29.1	—
APTES@SiO2	18.9	4.5	50.8	25.8	—
[VO(acac)2]APTES@SiO2	21.4	4.2	50.7	22.5	1.2

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The nitrogen loading of APTES@SiO₂ is 1.4 mmol g⁻¹, which corresponds to an APTES functionalization efficiency of 56%, estimated by the expression: amount of grafted APTES/amount used in the functionalization reaction × 100. Considering that: (i) the amount of APTES used during the functionalization was higher than the amount of silanol groups on the surface of the parent SiO₂, (ii) the amount of grafted APTES is almost similar to the amount of silanol groups on the parent SiO₂, and finally (iii) APTES is grafted through bidentate (T²) and monodentate (T¹) mechanisms (see NMR data below), the occurrence of a small extent of APTES lateral polymerization may not be disregarded.

In [VO(acac)₂]APTES@SiO₂ the nitrogen content is similar to that of APTES@SiO₂, indicating that no APTES leaching occurred during the complex immobilization. Additionally, the vanadium content is 295 μmol g⁻¹, which corresponds to an anchoring efficiency (amount of anchored complex/amount in original solution × 100) of 25%. If the anchoring efficiency of the metal complex is calculated relative to the amount of grafted APTES (amount of anchored complex/amount of APTES grafted in APTES@SiO₂ × 100), the value obtained is 22%. The complex loading in this nanomaterial is higher than those reported for [VO(acac)₂] grafted onto amine-functionalized clays, mesoporous silicas (bulk supports) and silica-coated magnetic nanoparticles (nanosupport), which ranged from 35 to 137 μmol g⁻¹; ^15–17,33 this improvement is probably due to the nanometre size of SiO₂.

The magnetic properties of [VO(acac)₂] and of [VO(acac)₂]APTES@SiO₂ were studied in order to determine the oxidation state of the vanadium centers and estimate the complex loading in [VO(acac)₂]APTES@SiO₂. In Fig. S2, ESI†, are presented the corresponding temperature-dependent magnetic susceptibility (χ(T)) curves.

The χ(T) curve of the free [VO(acac)₂] complex (Fig. S2(a), ESI†) follows the Curie–Weiss law, typical of a paramagnetic state, with an additional small diamagnetic contribution (arising from the acetylacetonate ligands). In order to determine the Curie temperature (ϑ_P), the χ(T) curve of [VO(acac)₂] was fitted using the Curie–Weiss law:

where C is the Curie–Weiss constant, ϑ_P is the Curie temperature and χ_d is the diamagnetic contribution. The Curie temperature obtained is ϑ_P = 0.157 K, indicating the absence of magnetic interactions between the vanadyl (VO²⁺) ions. From the Curie–Weiss constant of the free complex (C_free = 1.23 × 10⁻³ emu K g⁻¹Oe⁻¹), an effective paramagnetic moment μ_eff of ∼1.66 μ_B (considering g = 2) is obtained, which is reasonably close to the spin-only value expected for V⁴⁺ (∼1.73 μ_B), since vanadium in the +4 oxidation state has a d¹ (S = 1/2) configuration and a d_xy¹ ground state. ^36–39 Furthermore, the diamagnetic contribution χ_d = −3.77 × 10⁻⁷ emu g⁻¹Oe⁻¹ is of the same order of magnitude of the values obtained by Costa Pessoa et al.⁴⁰

The χ(T) of the anchored [VO(acac)₂] (Fig. S2(b), ESI†) also exhibits a behavior characteristic of a paramagnetic state, revealing that the vanadyl cations remain in the same oxidation state after the immobilization of the complex on the nanosupport. From the Curie–Weiss law (eqn (6)) a Curie constant C_anc ≈ 6.65 × 10⁻⁵ emu K g⁻¹Oe⁻¹, a diamagnetic component χ_d ≈ −5.22 × 10⁻⁷ emu g⁻¹Oe⁻¹ and a Curie temperature ϑ_P ≈ −1.2 K for the anchored complex are obtained, which indicate the presence of very weak antiferromagnetic interactions between the VO²⁺ cations.⁴¹

The vanadium content in [VO(acac)₂]APTES@SiO₂, estimated from the ratio between the Curie–Weiss constants of [VO(acac)₂]APTES@SiO₂ and [VO(acac)₂], is ∼204 μmol g⁻¹ (Table 1), which is comparable to the V loading determined by ICP-AES (295 μmol g⁻¹).

All the nanomaterials were characterized by XPS to gather information regarding the surface chemical composition and the functionalization/immobilization mechanisms. The XPS surface atomic percentages and the core-level binding energies (BEs) are summarized in Tables 2 and 3.

Table 3 Core-level binding energies and area of each component for silica-based nanomaterials obtained by curve fitting of XPS spectra

Material	C 1s		O 1s		N 1s		Si 2p3/2		Si 2p1/2		V 2p3/2		V 2p1/2
	BE ^a/eV	Area^b/%	BE ^a/eV	Area^b/%	BE ^a/eV	Area^b/%	BE ^a/eV	Area^b/%	BE ^a/eV	Area^b/%	BE ^a/eV	Area^b/%	BE ^a/eV	Area^b/%
a The values between brackets refer to the full width at half-maximum of the bands. b Area of each component relative to the total core-level peak area.
SiO2	284.6 (1.7)	67.2	532.7 (1.6)	100	—	—	103.4 (1.4)	66.7	104.0 (1.4)	33.3	—	—	—	—
	286.3 (1.7)	21.8
	288.7 (1.7)	11.0
APTES@SiO2	284.6 (1.5)	45.7	532.4 (1.5)	100	399.5 (1.9)	89.1	103.1 (1.5)	66.7	103.7 (1.5)	33.3	—	—	—	—
	285.4 (1.5)	28.8			401.6 (1.9)	10.9
	286.2 (1.5)	19.5
	288.0 (1.5)	6.0
[VO(acac)2] APTES@SiO2	284.6 (1.6)	41.9	530.3 (1.7)	9.5	399.9 (2.2)	94.8	103.0 (1.7)	66.7	103.6 (1.7)	33.3	516.5 (2.5)	73.6	523.9 (2.5)	26.4
	285.5 (1.6)	27.7	532.3 (1.7)	90.5	402.1 (2.2)	5.2
	286.4 (1.6)	17.0
	288.2 (1.6)	11.4
	292.0 (1.6)	2.0
[VO(acac)2]	284.6 (1.7)	70.7	530.7 (1.9)	100	—	—	—	—	—	—	517.1 (2.2)	73.7	524.5 (2.2)	26.3
	286.9 (1.7)	27.4
	289.4 (1.7)	1.9

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The parent SiO₂ nanomaterial is mainly composed of oxygen and silicon (Table 2) in the expected O/Si ratio. A small amount of carbon is also present probably due to some organic impurities. In the O 1s and Si 2p_3/2 high-resolution XPS spectra are detected bands with BEs of 532.7 and 103.4 eV, respectively (Table 3), which are characteristic of oxygen and silicon in a silica framework.^16,33,42

The functionalization of SiO₂ with APTES (APTES@SiO₂ sample) leads to the appearance of nitrogen as well as to the increase of the carbon surface content and to the decrease of the oxygen and silicon atomic percentages. The N 1s high-resolution spectrum exhibits a peak at 399.5 eV, which is assigned to free amine groups and confirms the grafting of the organosilane on the surface of SiO₂,^{15,16,33,43–46} in accordance with the data from EA and the ninhydrin test; the small shoulder around 401.6 eV is probably associated with some protonated amine species.^16,44–46 Additionally, the bands in the O 1s and Si 2p regions are shifted to lower BEs, suggesting that the organosilane was anchored onto the surface of SiO₂ by reaction between the APTES ethoxy moieties and the silanol groups from the silica surface.^15,16,33,43 The C 1s spectrum exhibits three main components: the bands at 284.6 and 285.4 eV are assigned to C–H/C–C and C–N bonds from the grafted APTES and the band at 286.2 eV is related to C–O bonds probably from unreacted ethoxy groups of the organosilane.^42,47

Upon the anchorage of [VO(acac)₂] onto APTES@SiO₂, a new peak is detected in the V 2p_3/2 region with a BE (516.5 eV) characteristic of vanadyl (VO²⁺) ions, in accordance with magnetic susceptibility measurements, thus confirming the successful immobilization of the complex. Furthermore, the BE of the V 2p_3/2 peak is lower than that of the free complex (517.1 eV), indicating a change in the chemical environment of the vanadium centre, which can be interpreted as a consequence of covalent immobilization of the complex to the APTES@SiO₂ support. Similarly, both N 1s components are slightly shifted to higher BEs also revealing a change in the nitrogen chemical environment. These features, combined with the high APTES loading which prevents the direct interaction between the complex and the silanol groups, suggest that [VO(acac)₂] is covalently bonded to the nanosupport surface through the grafted APTES. There are two major mechanisms for the covalent immobilization of the complex onto the support via APTES spacers (Scheme 1): (i) axial coordination of –NH₂ to the vanadium center, or (ii) Schiff condensation between the NH₂ group from the grafted APTES and the C=O group of the acac ligand, leading to the formation of an imine bond coordinated to the metal center: this latter mechanism was proposed in our previous works on the immobilization of [VO(acac)₂] onto APTES-functionalized bulk materials (clays, porous clay heterostructure, mesoporous silicas and activated carbon) and silica-coated magnetic nanoparticles.^15,16,18,33 However, XPS data per se cannot discriminate unambiguously the two anchoring mechanisms, since both pathways can lead to similar shifts of the V 2p_3/2 and N 1s BEs.

Scheme 1 Schematic representation (not drawn to scale) of complex immobilization onto silica nanoparticles.

The C 1s high-resolution spectrum exhibits four peaks assigned to C–H/C–C (284.6 eV), C–N (285.5 eV), C–O (286.4 eV) and C=N/C=O (288.2 eV) bonds from the anchored complex and APTES and a weak shoulder at 292.0 eV ascribed to the π–π* shake-up satellite from the π system of the chelate rings.^42,48,49

In the O 1s region, besides the band from lattice oxygen, an additional peak at 530.3 eV is observed, which may be assigned to the oxygen-based groups from the anchored complex (O–V/O=V and O=C bonds).^42,50,51 It is also noteworthy that the C 1s and O 1s peaks related to the free [VO(acac)₂] complex show a shift to higher and lower BEs, respectively, upon its immobilization onto the nanomaterial.

The comparison between the surface and bulk contents, for N and V elements, provides information concerning the location of the grafted APTES and [VO(acac)₂] on the support, respectively. In APTES@SiO₂, the nitrogen surface loading determined by XPS is higher than the bulk value obtained by EA (Table 1), revealing that APTES functionalities are preferentially located on the most external surface of the nanomaterial. Similarly, in the [VO(acac)₂]APTES@SiO₂ nanomaterial the vanadium surface content (681 μmol g⁻¹) is higher than the bulk value (295 μmol g⁻¹), pointing out that [VO(acac)₂] is anchored onto the most external surface of the nanosupport. This is an indirect indication of the complex immobilization via APTES spacers, since these groups are also located on the SiO₂ outer surface.

3.2 NMR and FTIR studies

²⁹Si and ¹³C solid-state NMR experiments were performed to unveil the grafting mechanism involved in the functionalization of SiO₂ with APTES (Fig. 2).

Fig. 2 (a) ²⁹Si and (b) ¹³C CPMAS solid state NMR spectra of SiO₂ and APTES@SiO₂ and peaks assignments.

The ²⁹Si CPMAS spectrum of SiO₂ (Fig. 2(a)) exhibits three resonance peaks at δ = −109, −100 and −91 ppm, which correspond to Q⁴ (fully condensed silica units), Q³ (terminal silanol groups) and Q² (geminal silanol groups) silicon sites, respectively, where Qⁿ = (≡SiO)_nSi(OH)_4−n, with n = 2–4.^23,52–54

The grafting of APTES onto SiO₂ (APTES@SiO₂, Fig. 2(a)) leads to a significant reduction of the intensity of the Q³ and Q² resonances relative to that of Q⁴, to the broadening of the Q⁴ resonance peak and to the appearance of a broad band around −52 ppm assigned to T² and T¹silicon environments (T^m = (SiO)_mSi(OH)_3−mR, where m = 1–2 and R = –(CH₂)₃NH₂).^23,52–54 These features indicate that the organosilane moieties were covalently anchored to the silica backbone through the reaction between the terminal and geminal surface silanol groups of SiO₂ and two or one ethoxy groups of APTES (bidentate and monodentate covalent grafting).

The ¹³C CPMAS NMR spectrum of APTES@SiO₂ (Fig. 2(b)) exhibits three resonance peaks at δ = 9.9, 21.8 and 43.0 ppm, which are associated to the three carbon atoms from 3-aminopropyl groups, C1, C2 and C3, respectively,^{44,47,52–54} confirming that the grafted APTES functionalities preserved their structural integrity. The broadening of the C2 peak at δ = 21.8 ppm suggests the presence of a small fraction of protonated amine groups,⁵⁵ in accordance with XPS. The low intensity peaks at δ = 16.8 and 58.1 ppm are assigned to a small amount of unreacted ethoxy groups from APTES (C4 and C5 carbon atoms in Fig. 2(b)),^47,53,54 since the organosilane is covalently attached to the silica network through 1- or 2-fold covalent grafting.

The silica-based nanomaterials were characterized by FTIR spectroscopy and the corresponding spectra are depicted in Fig. 3.

Fig. 3 FTIR spectra of samples (A) SiO₂, (B) APTES@SiO₂, (C) [VO(acac)₂]APTES@SiO₂ and (D) [VO(acac)₂], in the (a) 4000–400 cm⁻¹ range and (b) amplified 1750–650 cm⁻¹ region. The red and blue lines highlight the characteristic peaks of the grafted APTES and anchored [VO(acac)₂] respectively.

The FTIR spectrum of the SiO₂ sample exhibits the characteristic bands of the silica framework related to Si–O–Si asymmetric stretching (1090 cm⁻¹ with a shoulder around 1220 cm⁻¹), Si–O stretching of Si–OH and Si–O⁻groups on the nanoparticles surface (948 cm⁻¹), Si–O–Si symmetric stretching (803 cm⁻¹) and Si–O–Si bending vibrations (469 cm⁻¹).^{16,33,56–59} Additionally, the spectrum presents a band at 1629 cm⁻¹ associated with H–O–H bending vibrations of physically adsorbed water and a broad band centered around 3344 cm⁻¹ due to O–H stretching vibrations of hydrogen-bonded surface silanol groups and physically adsorbed water; the weak shoulders around 3735 and 3650 cm⁻¹ are ascribed to isolated and internal silanol groups, respectively.^16,33,56,60

The post-grafting of APTES onto SiO₂ is confirmed by the appearance of several new peaks (with low intensity) in all the frequency range, namely in the ranges of 2973–2880 cm⁻¹ and 1472–1389 cm⁻¹ due to C–H stretching and bending vibrations, respectively, and in the range of 1595–1541 cm⁻¹ and at 695 cm⁻¹ assigned to N–H bending vibrations of the aminopropyl groups.^{15,16,44,45,57,61} Furthermore, ongoing from SiO₂ to APTES@SiO₂, a significant reduction of the intensity of the O–H stretching and bending vibrations bands (∼3344 and 1629 cm⁻¹, respectively) and of the peak from Si–OH stretching modes (at 948 cm⁻¹) is observed which sustains the grafting mechanism illustrated in Scheme 1, catalyzed by the physisorbed water, in accordance with XPS and ²⁹Si CPMAS NMR results and with the literature.^15,16,33,43

The IR bands related to the anchored complex are very weak when compared to those associated with the support and grafted APTES, due to the low complex loading. Nevertheless, the immobilization of [VO(acac)₂] onto APTES@SiO₂ leads to changes in the C–H stretching vibration region (2979–2880 cm⁻¹) which indirectly indicate the presence of [VO(acac)₂] in the nanomaterial, since the free complex exhibits bands in this range.^16,62 In the low energy region, the most prominent features are the appearance of an asymmetric and broad band centered around 1653 cm⁻¹ and a peak at 1447 cm⁻¹ that can be unambiguously assigned to the immobilized complex since they are different from those of the free complex and are not detected in the APTES@SiO₂ spectrum.^62,63 These unique bands are not sufficient to make reliable assignments of the type of vibrations/functionalities involved, preventing the clear discrimination between the two covalent binding mechanisms (i) and (ii) depicted in Scheme 1. In one previous work, we performed theoretical studies at the DFT level to predict the IR spectra of the model compounds representing the two different immobilized V species.¹⁶ In that work, the theoretical calculations showed that the C=N stretching vibration coupled with the vibration of the acac chelate ring occurred, in the IR spectrum of the complex immobilized through mechanism (ii), at higher wavenumber (1634 cm⁻¹) than the C=O/C–O stretching vibrations of the acac chelate ring (1570 cm⁻¹) in the IR spectrum of the complex immobilized by mechanism (i). Therefore, the band at 1653 cm⁻¹ detected in the experimental FTIR spectrum of [VO(acac)₂]APTES@SiO₂ (Fig. 3) can be tentatively assigned to C=N stretching vibrations associated to the formation of an imine bond, suggesting that the complex may be anchored through mechanism (ii). But, as stated before, a full discrimination of the two anchoring mechanisms requires more experimental support.

3.3 Catalytic epoxidation of geraniol

The [VO(acac)₂]APTES@SiO₂ nanomaterial was tested as a catalyst in the epoxidation of geraniol, at room temperature, in dichloromethane, using t-BuOOH as an oxidant and the results are summarized in Table 4. Data from the homogeneous phase reaction and from the blank experiments (with the parent SiO₂ and APTES@SiO₂ nanosupports) under similar conditions are also included.

Table 4 Epoxidation of geraniol, at room temperature, catalyzed by [VO(acac)₂] in homogeneous and heterogeneous phases^a

Sample	Run	t/h	Conv.^b/%	S^c/%		2,3-EG yield^d/%
				2,3-EG	Geranial
a Reaction conditions: 1.00 mmol of geraniol, 0.50 mmol of chlorobenzene (internal standard), 0.10 g of heterogeneous material, 1.50 mmol of t-BuOOH; solvent: dichloromethane. The SiO2 and APTES@SiO2 parent materials led to 0% of geraniol conversion after 2 h. b Based on geraniol consumption. c Products selectivity: 2,3-EG = 2,3-epoxygeraniol. d Determined by GC-FID using chlorobenzene as internal standard and isolated 2,3-EG product. e Homogeneous phase reaction using the same V loading as [VO(acac)2]APTES@SiO2.
[VO(acac)2]APTES@SiO2	1st	2	100	99	1	98
	2nd	2	100	99	1	98
	3rd	2	100	98	2	98
	4th	2	100	98	2	98
	5th	2	100	98	2	98
[VO(acac)2]^e	1st	0.25	100	100	0	100

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The [VO(acac)₂]APTES@SiO₂ hybrid nanocatalyst is very active and regioselective towards the 2,3-epoxygeraniol product, leading to 100% of geraniol conversion and 98% of 2,3-epoxygeraniol yield. Furthermore, it exhibits similar activity and regioselectivity as the homogeneous counterpart (100% of geraniol conversion and of 2,3-epoxygeraniol selectivity/yield), albeit an increase in the reaction time from 15 min to 2 h, which may be in part attributed to the modification of the vanadium coordination sphere upon the complex immobilization. A small amount of geranial (1–2%) was also formed.^16,33

The lack of activity of the parent nanosupports SiO₂ and APTES@SiO₂ (0% of geraniol conversion) confirms that the catalytic efficiency of [VO(acac)₂]APTES@SiO₂ is due to the anchored complex.

The [VO(acac)₂]APTES@SiO₂ nanomaterial was recycled and reused in further four cycles, preserving its catalytic performance, in terms of reaction time, substrate conversion and 2,3-epoxygeraniol selectivity. Nevertheless, a very small amount of vanadium was detected (by AAS) in the filtered reaction medium after each catalytic cycle, which decreased from cycle to cycle–2.8, 1.9, 1.5, 1.2 and 1.1 μmol for the 1st, 2nd, 3rd, 4th and 5th cycle respectively. These values correspond to 9.5, 6.4, 5.0, 3.9 and 3.8% of leaching and to a total leaching percentage of 28.6. After the five consecutive cycles, the nanocatalyst was also characterized by chemical analyses and the N and V bulk contents were 1.1 mmol g⁻¹ and 216 μmol g⁻¹, respectively, corresponding to leaching percentages of 16 and 27%. The latter value is almost equal to that obtained from the sum of the partial leaching percentages. As neither the reaction time nor the substrate conversion of the [VO(acac)₂]APTES@SiO₂ nanocatalyst decreased in the consecutive cycles, this may be taken as evidence that the homogeneous component (due to the leached V species) is not significant for the global catalytic activity, although it may exist.

The novel hybrid nanocatalyst prepared herein presents promising results when compared to [VO(acac)₂] anchored onto bulk materials (clays, porous clay heterostructure and mesoporous silicas), especially in terms of reaction time (2 h vs. 48 h) and geraniol conversion (100% vs. 34–100%).^15–17 The decrease of the reaction time can be assigned to the nanometre size of the support which improves the dispersion of the catalytic active sites in the reaction medium and leads to a higher complex loading.

In fact, very recently, this complex was anchored onto magnetic silica-coated γ-Fe₂O₃ nanoparticles with ∼240 nm size and the preliminary test of its catalytic performance (100% of geraniol conversion and 96% of 2,3-epoxygeraniol yield) highlighted the potentialities of designing hybrid nanocatalysts for epoxidation reactions.³³ However, the reaction time (30 h), although being lower than those of the bulk counterparts, was still significantly higher than the value observed in the homogeneous reaction (15 min). The main differences between the silica-coated magnetic nanocatalyst and the one prepared in this work are the particles size, ∼240 vs. ∼45 nm, and the amount of anchored complex, 35 vs. 295 μmol g⁻¹. From our point of view, the higher complex loading in [VO(acac)₂]APTES@SiO₂ may be the factor that justifies the significant reduction of the reaction time to 2 h.

4 Conclusions

The [VO(acac)₂] complex was covalently anchored onto the surface of APTES-functionalized silica nanoparticles. The novel [VO(acac)₂] hybrid nanocatalyst was very active and regioselective in the epoxidation of geraniol at room temperature using t-BuOOH as an oxidant, with 100% of substrate conversion and 98% of 2,3-epoxygeraniol yield after only 2 h, which are comparable to the values obtained with the homogeneous counterpart (100% of geraniol conversion and epoxide yield after 15 min). Furthermore, it could be recycled and reused four times, with similar catalytic activity and regioselectivity. The improved catalytic performance of this hybrid nanocatalyst relative to bulk [VO(acac)₂] heterogeneous catalysts, mostly in terms of reaction time, was assigned to the reduced particles size of the support (average particles size below 100 nm) which allowed the immobilization of a higher amount of complex and enhanced the dispersion of the catalytic active sites in the reaction medium, thus improving their accessibility to the substrate and oxidant species.

Hence, the immobilization of transition metal complexes onto colloidal nanoparticles with sub-100 nm size opens new perspectives for the design of eco-friendly hybrid catalysts that can bridge the gap between homogeneous and heterogeneous catalysis.

Acknowledgements

This work was funded by Fundação para a Ciência e a Tecnologia (FCT) and FEDER, through projects refs. PTDC/CTM/65718/2006 and PTDC/QUI-QUI/105304/2008 and the Portuguese-Spanish Bilateral Cooperation through Projects REF E-72/10 (Portugal) and PT2009-0126 (Spain). C.P. and A.M.P. thank FCT for their grants.

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Footnotes

† Electronic supplementary information (ESI) available: Ninhydrin test for SiO₂ and APTES@SiO₂ nanomaterials and temperature-dependent magnetic susceptibility curves of [VO(acac)₂] and [VO(acac)₂]APTES@SiO₂. See DOI: 10.1039/c1cy00090j

‡ Present address: CeNTI-Centro de Nanotecnologia e Materiais Técnicos, Funcionais e Inteligentes, Rua Fernando Mesquita 2785, 4760-034 Vila Nova de Famalicão, Portugal.

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