selective oxidation of cyclohexane over gold nanoparticles supported on mesoporous silica prepared...

This journal is c The Royal Society of Chemistry 2011 Catal. Sci. Technol., 2011, 1, 285294 285

Cite this: Catal. Sci. Technol., 2011, 1, 285294

Selective oxidation of cyclohexane over gold nanoparticles supported onmesoporous silica prepared in the presence of thioether functionalityw

Pingping Wu,ab Zhigang Xiong,a Kian Ping Lohbc and X. S. Zhao*ab

Received 25th October 2010, Accepted 21st January 2011

DOI: 10.1039/c0cy00025f

Gold (Au) nanoparticles (NPs) supported on mesoporous silica were prepared using a one-pot

synthesis method in the presence of thioether functionality -(CH2)3-S-S-S-S-(CH2)3.

Bis(triethoxysilyl) propane tetrasulde was used as the source of thioether functionality. The

complexation of thioether groups with tetrachloroauric anion (AuCl4) led to a good dispersion

of Au NPs on mesoporous silica. Removal of the template and the thioether functional groups by

calcination resulted in the reduction of the Au species, leaving behind Au NPs well dispersed on

mesoporous silica. Selective oxidation of cyclohexane with molecular oxygen was used to evaluate

the catalytic properties of the Au NP catalysts prepared using the present method and compared

with that of a Au catalyst prepared using the hydrogen reduction method.

Introduction

Cyclohexanol and cyclohexanone (K/A oil) are important

intermediates for the bulk production of polyamides and

plastics. The main route to producing K/A oil is the selective

oxidation of cyclohexane catalyzed by a homogeneous

catalyst, such as transition metal salts. Such homogeneous

catalysts are undesirable in these environmentally conscious

and energy-intensive days. Solid catalysts, such as titanium

silicalite-1 (TS-1),1,2 metal-substituted aluminophosphate

molecular sieves,36 metal-incorporated mesoporous and

microporous aluminosilicate molecular sieves,710 and nano

structured catalysts (Fe2O3, Co3O4 and mixed FeCo oxide

nanoparticles)1113 have been explored as heterogeneous

catalysts for cyclohexane oxidation. These catalysts in general

exhibit a relatively low activity and/or poor selectivity.

Very recently, gold (Au) nanoparticles (NPs) supported on

mesoporous silica have been found to display a high catalytic

activity in oxidation reactions.1417 As Au NPs are unstable and

liable to aggregation, ligands such as chloro-group,18 amine,19

phenyl20 and thioether groups14 have been found to be eective

for stabilizing Au NPs. Bis(triethoxysilyl) propane tetrasulde

(TESPTS) with thioether functionality -(CH2)3-S-S-S-S-(CH2)3-

has been shown to be the most eective stabilizing ligand.14

In this work, Au NPs supported on mesoporous silica were

prepared using a one-pot synthesis method in the presence of

thioether functionality. The interaction of the Au species with

TESPTS was studied and the evolution of the Au species

was monitored using a real-time ultravioletvisible (UVvis)

technique. The Au NP catalysts exhibited an outstanding

catalytic performance. It was observed that the presence of

TESPTS in the one-pot synthesis method played an important

role in the dispersion of Au NPs on the silica support.

Results and discussion

Catalyst preparation and characterization

Fig. 1 shows the X-ray diraction (XRD) patterns of the Au

catalysts prepared with dierent amounts of TESPTS. Three

reection peaks indexed as (1 0 0), (1 1 0) and (2 0 0)

diractions (Fig. 1a and b) can be seen on the SBA-15 silica

Fig. 1 Low-angle XRD patterns of (a) SBA-15, (b) Au/0.625TESPTS

SiO2-cal, (c) Au/1.25TESPTSSiO2-cal, (d) Au/2.5TESPTSSiO2-cal, and

(e) Au/5TESPTSSiO2-cal.

aDepartment of Chemical and Biomolecular Engineering,National University of Singapore, 4 Engineering Drive, 117576,Singapore. E-mail: [email protected]; Fax: +65 6779-1936;Tel: +65 6516-4727

bNanoscience and Nanotechnology Initiative,National University of Singapore, 117576, Singapore

cDepartment of Chemistry, National University of Singapore,3 Science Drive, 117543, Singapore. E-mail: [email protected];Fax: +65 6779-1691; Tel: +65 6516-4402w Electronic supplementary information (ESI) available. See DOI:10.1039/c0cy00025f

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286 Catal. Sci. Technol., 2011, 1, 285294 This journal is c The Royal Society of Chemistry 2011

(template) and catalyst Au/0.625TESPTSSiO2-cal, indicating

a highly ordered 2D hexagonal structure of both materials.

With increasing x, the (1 1 0) and (2 0 0) diraction peaks

became less intense (Fig. 1c) and eventually disappeared

(Fig. 1d and e). Only one diraction peak can be observed from

catalysts Au/2.5TESPTSSiO2-cal and Au/5TESPTSSiO2-cal.

In addition, the intensity of the peak was lowered, revealing a

decrease in the regularity of the 2D hexagonal ordered structure.

This was probably due to the presence of TESPTS, which

aected the structure of the surfactant micelles, leading to the

formation of a poor hexagonal structure. The right shift of the

main (1 0 0) diraction peak with increasing x indicated

increased framework shrinkage after removal of the template.

The pore diameters (DBJH) shown in Table 1, estimated

from the adsorption branches using the BJH method, showed

that the pore diameters decreased with increasing x, as well as

the wall thickness. This trend can be rationalized in terms of

dierent interactions between the surfactant molecules and the

silicate and/or organosilicate precursors in the hydrothermal

system.21,22 A strong hydrophobic interaction between the

thioether group (-CH2-CH2-CH2-S-S-S-S-CH2-CH2-CH2-)

and the hydrophobic PPO blocks of P123 induced the

penetration of silicate species into the micelle core, leading to

the reduction of channel diameters. Thus, the amount of

TESPT introduced in the synthesis mixture played a crucial

role in determining the mesostructure of the resultant catalysts.

The wide-angle XRD patterns of the Au catalysts prepared

with dierent TESPTS/TEOS ratios are shown in Fig. 2 and

the Au particle sizes calculated from the Scherrer equation are

presented in Table 2. Four peaks at 38.181, 44.431, 64.551 and77.651 are the characteristic peaks of the (1 1 1), (2 0 0), (2 2 0)and (3 1 1) reections of cubic Au nanoparticles (JCPDS card

no.: 4-784). The Au nanoparticle size calculated from the

XRD data showed an optimized TEMPTS/TEOS ratio of

about 1/40 f(x = 2.5), for which well dispersed Au NPs with

an average Au particle size of about 5.5 nm were obtained.

When the TESPTS/TEOS ratio was less than 1/40, the size of

Au particles increased with the decrease of TESPTS, as shown

in the cases of catalysts Au/1.25TESPTSSiO2-cal and Au/

0.625TESPTSSiO2-cal, which exhibited average Au particle

sizes of 6.6 nm and 24.4 nm, respectively.

The Au contents in dierent catalysts were measured by

using the inductive-coupled plasma-mass spectrometer (ICP-MS)

technique and the results are shown in Table 1. It is seen that

when the amount of TESPTS was equal to or larger than

1.25%, almost all (^94%) Au precursors were introduced into

the mesoporous silica to form Au NPs, indicating a very high

Au loading eciency. However, if less than 1.25% of TESPTS

was added, only B75% of Au precursors was immobilized inthe silica.

The TEM images and Au NPs size distribution histograms

of catalysts Au/xTESPTSSiO2-cal are shown in Fig. 3. When

the amount of TESPTS was equal to 0.625%, unevenly

distributed Au NPs (with sizes in the range of 415 nm) were

obtained on the mesoporous silica support (Fig. 3a), consistent

with XRD results. With the increase of the TESPTS ratio,

uniformly dispersed Au NPs in the range of 38 nm were

observed on catalysts Au/1.25TESPTSSiO2-cal (Fig. 3b) and

Au/2.5TESPTSSiO2-cal (Fig. 3c) with a TESPTS ratio of

1.25% and 2.5%, respectively. However, Au NPs began to

aggregate and were unevenly distributed (Au NPs with sizes in

the range of 412 nm) with a further increase of the TESPTS

ratio (Fig. 3d) due to the deterioration of the mesoporous

structure.

The incorporation of thioether groups into the silica frame-

work of catalyst Au/2.5TESPTSSiO2-H2 was veried by the29Si and 13C CP MAS NMR spectra as shown in Fig. 4. From

the 29Si MAS NMR spectra, the signals in the range of 90 to150 ppm can be assigned to the silicon resonances of Si(OSi)4(Q4, d = 110 ppm), (OH)Si(OSi)3 (Q3, d = 102 ppm) and(OH)2Si(OSi)2 (Q2, d = 91.7 ppm), and the signals in therange of 50 to 70 ppm can be assigned to the siliconresonances of Tx [(SiO)x(OH)3xSiC] sites.

23 The resonances

at about 65 ppm and 56 ppm attributed to the resonancesof T3 and T2 for the organosiloxanes were observed on sample

Table 1 Textural properties of the Au catalysts prepared in this work together with SBA-15 silica

Catalysts Au loadinga/wt% d100/nm a0b/nm SBET/m

2 g1 Vtotal/cm3 g1 DBJH/nm Wall thickness/nm

SBA-15 8.7 10.0 782 1.0 7.9 2.1Au/0.625TESPTSSiO2-cal 0.75 9.5 11.0 960 1.09 9.2 1.8Au/1.25TESPTSSiO2-cal 0.94 8.8 10.2 1050 1.17 8.4 1.8Au/2.5TESPTSSiO2-cal 1.07 8.3 9.6 945 0.89 7.7 1.9Au/5TESPTSSiO2-cal 1.10 8.1 9.4 919 0.78 5.6 3.8Au/2.5TESPTSSiO2-H2 0.76 9.1 10.5 665 0.75 7.4 3.1

a Measured using the ICP-MS technique. b a0 is the lattice parameter calculated from the d100 spacing according to the equation of a0 2d100=

3p

.

SBET, surface area calculated by the BET method. Vtotal, total pore volume calculated at P/Po = 0.998. DBJH, pore diameter calculated from the

adsorption branch using BJH method. Wall thickness obtained by subtracting pore size from lattice parameter.

Fig. 2 Wide-angle XRD patterns of (a) Au/0.625TESPTSSiO2-cal,

(b) Au/1.25TESPTSSiO2-cal, (c) Au/2.5TESPTSSiO2-cal and (d)

Au/5TESPTSSiO2-cal.

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Au/2.5TESPTSSiO2-as and catalyst Au/2.5TESPTSSiO2-H2.

The predominant T3 resonance over T2 resonance indicates

the existence of cross-linked organosiloxanes in sample

Au/2.5TESPTSSiO2-as and catalyst Au/2.5TESPTSSiO2-H2.

Moreover, the enhancement of T3 resonance on catalyst

Au/2.5TESPTSSiO2-H2 was due to further dehydration of

organosiloxane [(SiO)2(OH)SiC] (T2) sites to [(SiO)3SiC] (T3)

sites during the H2 reduction at 250 1C. However, on catalyst

Table 2 Cyclohexane oxidation results on dierent catalysts

Catalysts Au NPs sizea/nm Time/h C6H12 conversion/mol%

Selectivity/mol%

TOFc/h1C6H12OH C6H12O By-productsb

SBA-15 Au/0.625TESPTSSiO2-cal 24.4 0.5 10.3 40.3 56.7 3.0 9900

1 19.4 30.7 59.5 9.82 27.8 17.7 50.4 31.8

Au/1.25TESPTSSiO2-cal 6.6 0.5 13.4 37.3 60.5 2.2 103941 24.2 34.4 60.0 5.62 31.4 20.0 57.3 22.7

Au/2.5TESPTSSiO2-cal 5.5 0.5 15.9 35.7 62.6 1.7 116531 31.7 24.0 69.5 6.52 33.4 18.5 56.6 24.8

Au/5TESPTSSiO2-cal 6.4 0.5 13.6 35.8 61.7 2.5 98861 27.3 31.0 62.7 6.32 34.5 18.8 61.6 19.6

Au/2.5TESPTSSiO2-H2 0.5 0 01 3.3 60.6 38.0 1.42 11.2 55.4 41.0 3.6

C6H12, cyclohexane. C6H12OH, cyclohexanol. C6H12O, cyclohexanone.a Au particle sizes were estimated from the XRD patterns using the

Scherrer equation, t = 0.9l/Bcos(y), where t is the crystallite size, l is the wavelength, B is the full width at the half maximum, and y is thediraction angle. b By-products are mainly ring-opened acids such as n-butyric, n-valeric, succinic, glutaric and adipic acid. c Moles of K/A oil

produced per mole of Au per hour.

Fig. 3 TEM images and Au NPs size distribution histograms of catalysts (a) Au/0.625TESPTSSiO2-cal, (b) Au/1.25TESPTSSiO2-cal,

(c) Au/2.5TESPTSSiO2-cal and (d) Au/5TESPTSSiO2-cal.

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Au/2.5TESPTSSiO2-cal, no T3 or T2 peak was observed

because of the complete removal of functional groups. The

presence of the peaks in the range of 1040 ppm on the 13C CP

MAS NMR spectra further conrmed the presence of

organosiloxane in sample Au/2.5TESPTSSiO2-as and catalyst

Au/2.5TESPTSSiO2-H2. Three dierent carbon chemical

environments with chemical shifts of d = 10 ppm, 21 ppm,and 40 ppm in the 13C CP MAS NMR spectrum can be

assigned to C1, C2 and C3 of the thioether group (Si-1CH2-2

CH2-3CH2-S-S-S-S-

3CH2-2CH2-

1CH2-Si), respectively.23 A

featureless 13C CP MAS NMR spectrum observed on catalyst

Au/2.5TESPTSSiO2-cal veried the complete removal of

functional groups.

The TEM image in Fig. 5 clearly shows the well dispersed

Au NPs on catalyst Au/2.5TESPTSSiO2-H2. The diameters

of the Au particles were estimated to be in the range of 26 nm.

But the Au NP sizes were in the range of 37 nm for catalyst

Au/2.5TESPTSSiO2-cal (see Fig. 3c). The larger Au NPs of

catalyst Au/2.5TESPTSSiO2-calmay be due to the aggregation

of Au particles during the calcination (500 1C for 6 h). The Auloadings on catalysts Au/2.5TESPTSSiO2-H2 and Au/

2.5TESPTSSiO2-cal were 0.76 wt% and 1.07 wt% (shown

in Table 1), respectively, with an expected Au loading of

1.0 wt%. The lower Au loading eciency on catalyst Au/

2.5TESPTSSiO2-H2 (76%) may be due to the leaching of Au

species during the template removal.

Fig. 6 shows the UV-vis spectra of pure SBA-15 (Fig. 6a),

catalysts Au/2.5TESPTSSiO2-H2 (Fig. 6b) and Au/

2.5TESPTSSiO2-cal (Fig. 6c). A featureless spectrum was

observed for the pure SBA-15 sample, while a strong Au

surface plasma resonance (SPR) peak at about 513 nm can

be seen on catalyst Au/2.5TESPTSSiO2-cal, which is a typical

SPR absorbance of nano-sized Au particles (510 nm).24 However,

a signicant shift of the SPR peak occurred for catalyst Au/

2.5TESPTSSiO2-H2, which exhibited an absorption peak at

ca. 423 nm. This blue shift is probably due to the interactions

between thioether groups and Au NPs.25 The dierent colours

of the catalysts (Fig. 6, inset) justied the dierent complexation

forms of Au NPs on catalysts Au/2.5TESPTSSiO2-H2 and

Au/2.5TESPTSSiO2-cal, conrming the existence of strong

interactions between Au NPs and thioether groups in the

H2-reduced catalyst.

The one-pot synthesis chemistry

Fig. 7 shows the real-time UV-vis spectra of the synthesis

mixtures in the absence and presence of TESPTS. The pure

HAuCl4 solution (Fig. 7, curve 0) with a yellow colour

exhibited two absorption bands centred at B220 nm andB310 nm, which originated from the gold(III) chloride andpartially hydrolyzed gold chloride (Au(OH)xCl4x

) species,respectively.26 After adding the AuCl4

solution into thesynthesis mixture, the UV-vis spectra of the solution are

shown in Fig. 7a and b. Without introducing the functional

precursor TESPTS, the absorption peak shifted slightly to a

longer wavelength (B325 nm) which is due to the hinderingeect of the strong acidic condition (pHo 1) on the hydrolysis

Fig. 4 29Si and 13C MAS NMR spectra of catalysts (a) Au/

2.5TESPTSSiO2-as, (b) Au/2.5TESPTSSiO2-H2 and (c) Au/2.5TESPTS

SiO2-cal.

Fig. 5 TEM image and Au NPs size distribution histogram of

catalyst Au/2.5TESPTSSiO2-H2.

Fig. 6 UV-visible spectra of (a) SBA-15, (b) Au/2.5TESPTSSiO2-H2and (c) Au/2.5TESPTSSiO2-cal.

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of AuCl4 to (Au(OH)xCl4x

). The concentration of AuCl4

in the solution exhibited a slight decrease after 1 h reaction,

which may be due to the adsorption of AuCl4 on silicate

species. On the other hand, with the presence of TESPTS, the

two absorption spectra shifted to shorter wavelengths and

developed new adsorption bands at about 210 nm and 280 nm.

The intensity of these two peaks gradually enhanced, indicating

that a certain form of complex gradually formed between

AuCl4 and functionality TESPTS.27

The evolution of this complex under the hydrothermal

treatment conditions was studied by comparing the UV-vis

spectra of the synthesis mixture with sample Au/

2.5TESPTSSiO2-as (Fig. 8a and b). Similar spectra were

observed in the range of 200300 nm due to the absorption

band of the thioetherAuCl4 complex; while an enhanced

UV-vis spectrum was seen on catalyst Au/2.5TESPTSSiO2-as

in the range of 400500 nm, indicating that Au NPs with a size

of less than 2 nm (Au clusters) may be formed during the

hydrothermal treatment.28

The XPS results shown in Fig. 9a and d supported the above

conclusion. As seen from Fig. 9a, sample Au/2.5TESPTS

SiO2-as exhibited the Au 4f7/2 and 4f5/2 doublet with binding

energies of 84.5 eV and 88.2 eV, respectively, similar to those

of thiolate-passivated Au clusters (Au:SR clusters) at

84.4 eV,29 indicating the presence of thioether-stabilized Au

clusters in the as-synthesized sample. The S 2p spectrum for

sample Au/2.5TESPTSSiO2-as showed doublet peaks with

binding energies of 163.9 eV and 165.1 eV (Fig. 9d), conrming

the existence of interaction between S and Au.30 Herein, it can

be concluded that the Au clusters began to form during the

hydrothermal process and Au species were partially present as

thioether-stabilized Au clusters in sample Au/2.5TESPTS

SiO2-as. After H2 reduction or high temperature calcination,

the UV-vis spectra showed the formation of thioether-stabilized

Au NPs on catalyst Au/2.5TESPTSSiO2-H2 (Fig. 6b) and the

formation of Au NPs on catalyst Au/2.5TESPTSSiO2-cal

(Fig. 6c). The XPS results shown in Fig. 9 veried this

conclusion. Au 4f7/2 with binding energies of 84.0 eV and

84.1 eV were observed on catalysts Au/2.5TESPTSSiO2-H2(Fig. 9b) and Au/2.5TESPTSSiO2-cal (Fig. 9c), respectively,

due to metallic Au0 (84.0 eV),31 indicating that the Au species

were completely reduced after H2 reduction or high tempera-

ture calcination. The S 2p spectrum shown in Fig. 9e con-

rmed the presence of interaction between S and Au NPs on

catalyst Au/2.5TESPTSSiO2-H2. The obvious decrease of

spectrum intensity may be due to the partial decomposition

of thioether groups during the H2 reduction. The S 2p peaks

are absent on catalyst Au/2.5TESPTSSiO2-cal (Fig. 9f). The

evolution of Au species is also veried by the colour of the

sample. The as-synthesized sample showed a light yellow

colour, indicating the presence of AuCl4 while the ruby

colour of catalyst Au/2.5TESPTSSiO2-cal indicated the

presence of Au NPs (insets in Fig. 6 and 8).

Based on the above results, a possible formation mechanism

of Au NP catalysts was proposed as illustrated in Scheme 1.

Upon mixing of the silica precursors, TEOS and TESPTS,

Fig. 7 Evolution of UV-visible spectra by introducing HAuCl4 into

the synthesis gel with and without TESPTS addition.

Fig. 8 UV-visible spectra of (a) synthesis gel after 60 min reaction

and (b) Au/2.5TESPTSSiO2-as.

Fig. 9 Au (4f) (a, b, c) and S (2p) (d, e, f) XPS spectra of catalysts

Au/2.5TESPTSSiO2-as, Au/2.5TESPTSSiO2-H2 and Au/2.5TESPTS

SiO2-cal.

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with the gold precursor (HAuCl4) and the surfactant solution,

a mesoporous silica phase was formed. At the same time,

complexation of AuCl4 with thioether groups occurred to

form AuCl4thioether complexes (Step I). With the

co-condensation of the silica precursors to form the silica

framework under the hydrothermal treatment (100 1C), partof AuCl4

thioether complexes decomposed to form Auclusters stabilized by thioether groups (Step II) as proved by

the UV-vis and XPS data of sample Au/2.5TESPTSSiO2-as.

The template was removed by ethanol extraction or high

temperature calcination. The extracted sample was reduced

by H2 (Step III). The residual AuCl4thioether complexes

were completely reduced and thioether-stabilized Au clusters

assembled to form thioether-stabilized Au NPs as proved by

the UV-vis spectrum of catalyst Au/2.5TESPTSSiO2-H2.

On the other hand, after high-temperature calcination (Step III0),the template and functional groups were completely removed

and AuCl4thioether complexes and thioether-stabilized Au

clusters decomposed and further deposited to form Au NPs.

The removal of the functional groups (e.g., -CH2-CH2-CH2-S-

S-S-S-CH2-CH2-CH2-) led to the formation of structure

defects in the silica framework, which were the favourable

sites for the formation of Au NPs, and the silica framework

prohibited the growth of Au NPs during the high temperature

treatment.14

Catalytic performance

The catalytic performance of the catalysts prepared with

dierent TESPTS/TEOS ratios or reduced by dierent methods

was investigated on selective oxidation of cyclohexane with

molecular oxygen. The catalytic reaction prole with time was

studied on catalyst Au/xTESPTSSiO2-cal and the H2-reduced

catalyst. The conversion, selectivity, turnover frequency

(TOF) and product distribution are summarized in Table 2.

It can be seen that the calcined catalysts showed a good

catalytic activity (the cyclohexane conversion ranged from

19.4% to 31.7%) and K/A oil selectivity (>90%) in the rst

hour reaction. After that, the selectivity of K/A oil declined

with a slight increase in the cyclohexane conversion

(27.834.5%). The results indicated that control over the

reaction kinetics is important in terms of avoiding further

oxidation of cyclohexanol to cyclohexanone and further

oxidation of K/A oil to by-products. During the reaction,

the formation of by-products is inevitable with the increase of

reaction time. The TOF was measured in moles of product

K/A oil produced per mole of Au per hour. The remarkably

highest TOF number (11 653 h1) for the rst half an hourreaction was observed on catalyst Au/2.5TESPTSSiO2-cal,

indicating the highest initial reaction rate of cyclohexane

oxidation on this catalyst, followed by catalysts Au/

1.25TESPTSSiO2-cal (10 394 h1), Au/5TESPTSSiO2-cal

(9886 h1) and Au/0.625TESPTSSiO2-cal (9900 h1). This

trend was closely related to the Au NP sizes of the catalysts.

Catalyst Au/2.5TESPTSSiO2-cal exhibited the smallest and

most uniform Au particles (with an average Au NPs size of

about 5.5 nm), followed by catalysts Au/1.25TESPTSSiO2-cal

(with an average Au NPs size of 6.6 nm), Au/5TESPTSSiO2-cal

(with an average Au NPs size of 6.4 nm) and Au/

0.625TESPTSSiO2-cal (with an average Au NPs size of

24.4 nm). The higher reaction activity on catalyst with smaller

Scheme 1 Evolution of Au species during the one-pot synthesis process: (I) mixing of all reaction precursors in one pot; (II) hydrothermal

treatment at 100 1C for 24 h; (III) ethanol extraction followed by H2 reduction, and (III0) high-temperature calcination.

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Au NPs size can be explained below. More surface-free Au

atoms were presented as the active sites on the catalysts with

smaller AuNPs. Judging from the characterization and reaction

results, the optimized TESPTS/TEOS ratio was obtained as

1/40 (at the value of x = 2.5) with which well-dispersed Au

NPs were obtained and a high Au loading eciency was

achieved, thus leading to a high catalytic activity.

Table 2 also shows that catalyst Au/2.5TESPTSSiO2-cal

exhibited a much higher cyclohexane conversion than catalyst

Au/2.5TESPTSSiO2-H2. The lower activity of the latter,

on the other hand, minimized the deep oxidation of K/A oil

to by-products, thus leading to a high K/A oil selectivity. The

catalytic data collected in this study showed that catalyst

Au/2.5TESPTSSiO2-H2 with Au particle sizes in the range of

26 nm exhibited a lower catalytic activity than the calcined

catalyst with Au particle sizes in the range of 37 nm. The

lower cyclohexane conversion on catalyst Au/2.5TESPTS

SiO2-H2 (0.76 wt%) is probably due to the lower Au loading

than that on catalyst Au/2.5TESPTSSiO2-cal (1.07 wt%).

But the extremely lower TOF of catalyst Au/2.5TESPTS

SiO2-H2 (0 h1) than that of catalyst Au/2.5TESPTS

SiO2-cal during the rst half an hour reaction indicated a

lower initial reaction rate of the former. This is most probably

due to the partial coordination of active Au sites with capping

agent TESPTS, which hampered the accessibility of the active

Au sites to the reactants.32

Based on above reaction results, catalyst Au/2.5TESPTS

SiO2-cal synthesized with a TESPTS/TEOS ratio of 1/40 and

reduced by calcination exhibited the best catalytic activity for

cyclohexane oxidation. Furthermore, the optimized reaction

time for this catalyst was 1 h with respect to achieving a high

cyclohexane conversion and K/A oil selectivity. The results

obtained in present work are more signicant than the previous

reported results shown in Table S1 (see ESIw).15,18,3335

Compared to these materials, the present catalyst Au/

2.5TESPTSSiO2-cal emerged as the most promising catalyst

for cyclohexane oxidation with respect to both high cyclohexane

conversion (B32%) and high TOF (11633 h1). The highreaction rate and cyclohexane conversion on catalyst Au/

2.5TESPTSSiO2-cal is because of more surface-free Au atoms

acting as active sites after complete removal of functional

groups. Correlating the above reaction results with the catalyst

properties, it was realized that the available surface-free Au

atoms determined the catalytic activity of Au nanoparticles

supported on functionalized mesoporous silica, while the

presence of functional groups on the resultant catalyst may

poison the catalytic activity.

Catalyst stability and recyclability

Recycling tests with repeated use of catalysts Au/2.5TESPTS

SiO2-cal and Au/2.5TESPTSSiO2-H2 in six consecutive reactions

were carried out. The catalyst was removed from the reaction

system by ltration after 1 h reaction and washed thoroughly

with ethanol, followed by drying at 80 1C overnight and thensubjected to the next cycle. The recycling results shown in

Fig. 10 indicated that a slight decrease in conversion occurred

after the 2nd run on catalyst Au/2.5TESPTSSiO2-cal and no

obvious activity loss was observed in the following 4 cycles,

demonstrating a good stability of catalyst Au/2.5TESPTS

SiO2-cal. The slight decrease of catalytic activity during the

rst two cycles is due to the aggregation of small Au NPs on

the surface of the silica support.

Fig. 11 shows the recyclability of catalyst Au/2.5TESPTS

SiO2-H2. It is interesting to note that the catalytic activity kept

increasing in the rst several runs and decreased after the 4th

run. This is due to the fact that part of the Au surface was

coordinated by the functional ligand in the fresh catalyst. The

functional groups would be decomposed under the high

temperature reaction. Thus, after one or two runs the active

sites were completely released from coordination, leading to

the higher activity in the 3rd and 4th cycles. However, these

free Au NPs may easily aggregate to larger Au particles;

Fig. 10 Recyclability of catalyst Au/2.5TESPTSSiO2-cal. (a) Fresh

catalyst, (b) catalyst after two reaction cycles and (c) catalyst after six

reaction cycles.

Fig. 11 Recyclability of catalyst Au/2.5TESPTSSiO2-H2. (a) Fresh

catalyst, (b) catalyst after one reaction cycle, (c) catalyst after two

reaction cycles and (d) catalyst after six reaction cycles.

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explaining that after the 4th reaction cycle, the catalytic

activity decreased again. The colours of the fresh and used

catalysts shown in Fig. 10 and 11 justied the above results.

For catalyst Au/2.5TESPTSSiO2-cal, the appearance of catalyst

became slightly dark red after two reaction cycles and no

obvious change of colour occurred after the 2nd reaction cycle

until the 6th cycle, indicating that no further aggregation

occurred after the initial aggregation of surface Au NPs. On

the other hand, the colour changed signicantly in each cycle

of catalyst Au/2.5TESPTSSiO2-H2, indicating the decomposition

of functional groups, the release of Au NPs and the further

aggregation of Au NPs.

Fig. 12 shows the S 2p XPS spectra of catalyst Au/

2.5TESPTSSiO2-H2 before and after use. It is seen that the

thioether groups with a binding energy of 163.8 eV were

oxidized to form SO3 with a binding energy of 168.8 eV after

the 1st reaction cycle, and then decomposed after the 3rd

reaction cycle. This exactly explains that the catalyst preserved

the highest catalytic activity in the 4th reaction cycle and after

that aggregation of Au nanoparticles occurred. As is seen from

the TEM images in Fig. 13, no obvious aggregation of Au NPs

was observed on catalyst Au/2.5TESPTSSiO2-cal, while

distinct aggregation of Au NPs was found on catalyst Au/

2.5TESPTSSiO2-H2. The excellent stability and recyclability

of catalyst Au/2.5TESPTSSiO2-cal is due to the connement

of Au NPs in the silica framework, while the poor stability of

catalyst Au/2.5TESPTSSiO2-H2 is attributed to the attachment

of Au NPs with functional groups, which were unstable at high

temperatures. These reaction results conrmed the validity of the

formation mechanism proposed in Scheme 1 that for catalyst Au/

2.5TESPTSSiO2-H2 the AuNPs were attached and stabilized by

thioether groups, while on catalyst Au/2.5TESPTSSiO2-cal, the

Au NPs were protected by the silica framework.

Experimental

Chemicals

HAuCl4xH2O (Aldrich), tetraethyl orthosilicate (TEOS, 98%,Acros Organics), triblock co-polymer PEO20PPO70PEO20

(P123, Aldrich),bis(triethoxysilyl) propane tetrasulde

(TESPTS, 97%, Aldrich), hydrochloric acid (37%, Merck),

cyclohexane (99.99%, Fisher) and absolute ethanol (99.98%,

Merck) were used as received without further purication.

Catalyst preparation

In a typical synthesis, 4 g of P123 was dissolved in 30 mL of

deionized water at room temperature followed by adding 120 mL

of a 0.74 M HCl solution into this solution. A mixture of

TEOS and TESPTS was slowly added. Then, 6.4 mL of 0.02 M

HAuCl4xH2O solution was added. After stirring at 40 1C for24 h, the mixture was transferred to a Teon-lined stainless

steel autoclave to undergo a static hydrothermal treatment at

100 1C for 24 h. The solid products were ltered o, washedwith deionized water till no Cl was detected using a AgNO3solution, and dried at 80 1C in a vacuum oven overnight. Toinvestigate the eect of TESPTS on the loading of Au, the

molar percentage of TESPTS/TEOS = x% was varied by

changing x from 0.625 to 5. The samples thus obtained, herein

denoted as Au/xTESPTSSiO2-as, were calcined in air at 500 1Cfor 6 h to remove the template. During the calcination process,

Au species were reduced to Au nanoparticles. The nal solid

products are designated as Au/xTESPTSSiO2-cal, which

were used directly as catalysts without further reduction.

For comparison purpose, sample Au/2.5TESPTSSiO2-as

was treated with ethanol to remove the template, followed by

hydrogen reduction at 250 1C for 2 h. This catalyst is designatedas Au/2.5TESPTSSiO2-H2.

Fig. 12 S 2p XPS spectra of fresh and used catalyst Au/

2.5TESPTSSiO2-H2. (a) Fresh catalyst, (b) after one reaction cycle,

(c) after two reaction cycles and (d) after three reaction cycles.

Fig. 13 TEM images of catalysts (a) Au/2.5TESPTSSiO2-cal and (b)

Au/2.5TESPTSSiO2-H2 after 6 cycles of reaction.

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Catalyst characterization

X-Ray powder diraction (XRD) patterns were recorded on a

XRD-6000 (Shimadzu, Japan) system with a Cu-Ka radiationof wavelength l = 0.15418 nm. N2 adsorptiondesorptionisotherms were measured at 196 1C on an automaticvolumetric sorption analyzer (Micromeritics, ASAP2020).

Prior to adsorption, the samples were degassed at 200 1C for4 h under vacuum. The solid ultravioletvisible (UV-vis)

spectra were measured on a UV-vis-NIR (UV-visible Near

Infra-Red) scanning spectrophotometer (Shimadzu, UV-3101 PC)

with an ISR-3100 integrating sphere attachment and BaSO4 as

an internal reference. Chemical analyses of Au in the catalysts

were carried out on an Agilent 7500 series inductive-coupled

plasma-mass spectrometer (ICP-MS), after dissolving the

solids by attacking with a 2 : 1 mixture of HNO3/HF.

X-Ray photoelectron spectroscopy (XPS) spectra were recorded

on an AXIS HIS 165 spectrometer (Kratos Analytical) with a

monochromatized Al-Ka X-ray source. The Au 4f signals wererecorded in a 0.05 eV step with a pass energy of 40 eV. Solid-

state magic-angle spinning (MAS) nuclear magnetic resonance

(NMR) spectra were obtained on a Bruker DRX400 FT-

NMR spectrometer. A 4 mm rotor was used. The MAS speed

was 8 kHz. The cross-polarization (CP) technique was used for13C measurements while the single-pulse method was used for29Si spectrum collections. The microstructures of the catalysts

were observed on a transmission electron microscope (JEM

2010 from JEOL) operated at 200 kV.

Measurement of catalytic properties

The selective oxidation of cyclohexane was carried out in a

200 mL Parr batch reactor with a polytetrauoroethylene

(PTFE) liner. In a typical oxidation reaction, 20 mL of

cyclohexane and 50 mg solid catalyst were added into the

reactor and the reaction was conducted under the conditions

of 150 1C and a pressure of 1 MPa controlled by O2. After thereaction, the mixture was dissolved by ethanol and an excessive

amount of triphenylphosphine (Ph3P) was added to the reaction

mixture to completely reduce the cyclohexyl hydroperoxide

(CHHP), an intermediate in the cyclohexane oxidation to

cyclohexanol. The products were analyzed using a gas chromato-

gram (HP 7890 series GC) with a mass spectrometer detector

(HP 5973 mass selective detector) and a capillary column

(HP 5MS).

Conclusions

Gold nanoparticles supported on mesoporous silica have been

prepared using a simple one-pot synthesis method in the

presence of bis(triethoxysilyl) propane tetrasulde (TESPTS)

as a ligand. The amount of TESPTS present in the synthesis

system was found to play a crucial role in the dispersion of

the nal gold nanoparticles because of the complexation eect

of thioether groups with AuCl4 species. Calcination of the

as-synthesized solids for removing the template and the organic

ligand directly yielded ruby-colored gold catalysts. Removal of

the template by using alcohol extraction followed by hydrogen

reduction yielded a light-brown-colored gold catalyst. The

catalyst prepared by direct calcination exhibited a higher

catalytic activity in selective oxidation of cyclohexane with

molecular oxygen than the catalyst prepared using the hydrogen

reduction method because of the smaller gold nanoparticle size

of the former catalyst and partial coverage of the gold particle

surface by organic functional groups in the latter catalyst. The

gold catalyst prepared using the one-pot synthesis method also

displayed an excellent stability and recyclability.

Acknowledgements

P. W. wishes to thank National University of Singapore

Nanoscience and Nanotechnology (NUSNNI) for oering a

scholarship.

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selective oxidation of cyclohexane over gold nanoparticles supported on mesoporous silica prepared...

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