In Situ Preparation of Au-SH@SO3H-SBA-15 Catalyst with Enhanced Activity and Durability in Alkyne Hydration

Article abstract of DOI:10.1002/cjoc.201400408

A facile approach was developed for synthesizing Au-SH@SO₃H-SBA-15 with ordered mesoporous channels by reducing Au³⁺ to Au nanoparticles with SH-group bonded to silica support, followed by in situ coordinating Au with the unreacted SH-groups. This catalyst exhibited high efficiency in alkyne hydration owing to the high activity of uniformly dispersed ultrasmall Au nanoparticles, the diminished diffusion limit due to the mesoporous structure, and the promoting effect of acidic SO₃H-groups resulting from oxidation of the SH-group by Au³⁺. Meanwhile, the catalyst could be easily recycled and displayed strong durability owing to the strong hydrothermal stability of mesoporous structure and the enhanced stability against Au leaching due to the Au-SH coordination bond.

Full text of DOI:10.1002/cjoc.201400408

FULL PAPER
DOI: 10.1002/cjoc.201400408
In Situ Preparation of Au-SH@SO3H-SBA-15 Catalyst with
Enhanced Activity and Durability in Alkyne Hydration†
Fengxia Zhua and Hexing Li*,b
a Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and
Chemical Engineering, Huaiyin Normal University, Huaian, Jiangsu 223300, China
b The Education Ministry Key Lab of Resource Chemistry, Shanghai University of Electric Power,
Shanghai 200234, China
A facile approach was developed for synthesizing Au-SH@SO3H-SBA-15 with ordered mesoporous chan-
＋
nels by reducing Au3 to Au nanoparticles with SH-group bonded to silica support, followed by in situ coordinat-
ing Au with the unreacted SH-groups. This catalyst exhibited high efficiency in alkyne hydration owing to the high
activity of uniformly dispersed ultrasmall Au nanoparticles, the diminished diffusion limit due to the mesoporous
＋
structure, and the promoting effect of acidic SO3H-groups resulting from oxidation of the SH-group by Au3 . Mean-
while, the catalyst could be easily recycled and displayed strong durability owing to the strong hydrothermal stabil-
ity of mesoporous structure and the enhanced stability against Au leaching due to the Au-SH coordination bond.
Keywords gold (Au) nanoparticles, hydration of alkynes, water-medium clean reaction, SH-functionalized
SBA-15
Introduction
efficient hydration reactions have been reported.[17]
However, these processes are often carried out in the
mixture of water and organic solvent as the reaction
media, together with the addition of strong acids in-
cluding H2SO4 and CF3SO3H or additive (AgSbF6) to
facilitate the reactions under N2 protected atmosphere,
which may cause environmental pollution. Moreover,
these homogeneous catalysts are difficult to be recycled
and reused, which limits their industrial application due
to high cost.
Au catalysts deposited on periodic mesoporous or-
ganosilicas could be easily recycled and also exhibit
high activity owing to the high dispersion of Au
nanoparticles on the support with high surface areas
and convenient diffusion/adsorption due to the dimin-
ished steric hindrance.[18,19] Meanwhile, the hydration
reactions catalyzed by those supported Au cata-
lysts could be carried out in pure water medium under
mild conditions, representing an important aspect of
green chemistry.[20]
Supported gold catalysts have attracted great atten-
tion owing to their high activity and selectivity in vari-
ous reactions including oxidation and hydration etc.[1-4]
Both experimental results and theoretical predictions
demonstrated that the ultrasmall size of gold nanoparti-
cles plays a crucial role in determining the catalytic
performance.[5-7] However, very small Au nanoparticles
are difficult for separation from the reaction prod-
ucts, corresponding to the Au leaching. Moreover, they
are easily aggregated during the reactions owing to the
high surface energy, leading to the gradual decrease in
the catalytic efficiency. A possible method to solve the
problem is to develop the supported Au catalyst to en-
sure the high dispersion of tiny Au nanoparticles with
diminished agglomeration and leaching owing to the
strong metal-support interaction and facilitate the cata-
lyst recovery.[8,9]
Alkyne hydration is a viable process for the prepara-
tion of synthetic versatile carbonyl compounds.[10]
However, their applications in organic synthesis and
industry are limited due to the use of highly toxic mer-
cury salts as catalysts.[12-14] To date, many studies are
focused on the design of non-toxic catalysts.[15] A pos-
sible breakthrough is the Au-based catalysts.[16] So far,
many homogeneous Au(I) and (III) catalyzed highly
Previously, we reported an Au catalyst immobilized
＋
onto ethyl-bridged PMO by in situ reduction of Au3
with thiol in mercaptopropyl (SH) terminally bonded to
PMO support.[21] However, besides the tedious prepara-
tion procedure and the poor stability of ethyl, the ethyl-
functionalization was possibly harmful for hydration
reactions due to strong surface hydrophobility. Herein,
*
E-mail: HeXing-Li@shnu.edu.cn
Received June 23, 2014; accepted August 23, 2014; published online September 15, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400408 or from the author.
Dedicated to Professor Chengye Yuan and Professor Li-Xin Dai on the occasion of their 90th birthdays.
†
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Chin. J. Chem. 2014, 32, 1072—1076

In Situ Preparation of Au-SH@SO3H-SBA-15 Catalyst with Enhanced Activity and Durability
we reported a facile in situ approach to prepare
Au-SH@SO3H-SBA-15 with ordered mesoporous chan-
nels and 1－2 nm Au nanoparticles coordinated with
HS-groups bonded to SBA-15. This catalyst exhibited
high activity and strong durability during alkyne hydra-
tion owing to the high reactivity of ultrasmall Au
nanoparticles, the diminished diffusion limit in the or-
tion of the original SH-SBA-15 with 30 wt% H2O2
aqueous solution.
Characterization
Fourier transform infrared (FT-IR) spectra were col-
lected on a Nicolet Magna 550 spectrometer. X-ray
powder diffraction (XRD) data were acquired on a
SCINTAG PADX diffractometer using Cu Kα radiation.
N2 adsorption-desorption isotherms were measured at
−203 ℃ on a Quantachrome NOVA 4000e analyzer,
from which the specific surface area (SBET), the pore
volume (VP) and the average pore diameter (DP)
were calculated by applying multiple-point Brunauer-
Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)
models on adsorption branches, respectively. Morphol-
ogy and Au particle size were observed through a
transmission electron microscopy (TEM, JEOL
JEM2011). Solid NMR spectra were recorded on a
Bruker DRX-400 NMR spectrometer. The surface elec-
tronic states were analyzed by X-ray photoelectron
spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA).
All the binding energy values are calibrated by using C1s
284.6 eV as a reference. The Au loading was deter-
mined by inductively coupled plasma optical emission
spectrometer (ICP-OES, Varian VISTA-MPX).
dered mesopore, the co-catalysis from acidic SO3H-
＋
goup due to SH-group oxidation by Au3 , and strong
stability against both the structure damage and Au
leaching due to the Au-SH coordination bond.
Experimental
Catalyst preparation
Firstly, SH-functionalized SBA-15 (SH-SBA-15)
was synthesized by P123 surfactant self-assembly as-
sisted co-condensation of tetraethoxysilane (TEOS) and
(3-mercaptopropyl) trimethoxysilane (MPTMS). Briefly,
1.0 g P123, 3.0 g KCl and 2.1 mL TEOS were mixed
in 7.6 mL of 2.0 mol/L HCl aqueous solution. After be-
ing prehydrolyzed for 1.5 h at 40 ℃, 0.013 mL of
MPTMS was added into the solution, followed by stir-
ring at 40 ℃ for 24 h and aging at 100 ℃ for 24 h.
The precipitate was filtrated and dried at vacuum over-
night. Then, the surfactants and other organic sub-
stances were extracted and washed away by refluxing
the sample in ethanol solution at 80 ℃ for 24 h, fol-
lowed by drying at 80 ℃ under vacuum overnight.
Secondary, a certain amount of SH-SBA-15 was added
into 7.0 mL of 0.017 mol/L HAuCl4 ethanol solution
and stirred at room temperature for 24 h. After thor-
oughly washed with distilled water and ethanol, the
powder was vacuum dried at 80 ℃ overnight, which
was denoted as Au-SH@HSO3-SBA-15, where HSO3
Activity test
Alkyne hydration (Scheme 1) was carried out
at 70 ℃ in a 10 mL round-bottomed flask by adding
Au-SH@HSO3-SBA-15 catalyst containing 0.020 mmol
Au, 0.50 mmol alkyne, 4.0 mL distilled water and
n-decane as an internal standard. After stirring for 1.5 h,
the products were extracted by acetic ether, followed by
analysis on a gas chromatograph (SHIMADZU, GC-
17A) equipped with a JWDB-5, 95% dimethyl 1-(5%)-
diphenylpolysiloxane column and a FID detector.
The column temperature was programmed from 80 to
250 ℃ at a ramp speed of 10 ℃/min. N2 was used
as carrier gas. The reproducibility was checked by re-
peating each result at least three times and was found
to be within ±5%.
refers to sulfonic acid group resulting from the oxida-
＋
tion of HS by Au3 . According to the ICP analysis, the
Au loading was determined as 0.092 mmol/g.
For comparison, the Au-SH-SBA-15 was also syn-
thesized by depositing Au nanoparticles onto SH-SBA-
15 according to the following procedure. 10 mL tetralin,
10 mL oleylamine, and 0.10 g HAuCl4•3H2O were
mixed at 40 ℃ and stirred under N2 flow for 10 min,
followed by adding a solution containing 0.50 mmol
tert-butylamine-borane complex, 1.0 mL tetralin, and
1.0 mL oleylamine. After reaction at 40 ℃ for 1 h, 60
mL acetone was added to settle down the precipitate,
followed by washing thoroughly with acetone, leading
to Au nanoparticles with the average size around 3.0 nm.
Then, they were dispersed in hexane, followed by add-
ing SH-SBA-15 and stirring at 40 ℃ under N2 flow for
4 h. The solid product was washed thoroughly with
hexane and ethanol, followed by vacuum drying at
80 ℃ overnight. The Au loading was determined as
0.12 mmol/g by ICP analysis. Meanwhile, the Au/
HSO3-SBA-15 with Au loading of 0.10 mmol/g was
also prepared by depositing the as-received Au nanopar-
ticles onto the HSO3-SBA-15 pre-synthesized by oxida-
Scheme 1 Alkyne hydration equation
O
H
H2 O
R
R
A u-S H @ H S O3 -S B A -15
CH3
In order to determine the catalyst durability,
the catalyst was allowed to centrifuge after each run of
reactions and the clear supernatant liquid was decanted
slowly. The catalyst was washed thoroughly with dis-
tilled water and ethanol, followed by drying at 80 ℃
for 8 h under vacuum condition. Then, the catalyst was
reused with fresh charge of reactants for subsequent
recycle under the identical reaction conditions.
Results and Discussion
The XPS spectra (Figure 1) displayed two principal
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FULL PAPER
peaks at binding energy (BE) around 164.1 and 169.1
eV in S2p3/2 level, corresponding to −2 and ＋6 oxida-
tion states of S species in －SH and －SO3H groups,
respectively. All the Au in Au-SH@SO3H-SBA-15
were present in the metallic state, corresponding to the
BE of 84.7 eV in the Au4f7/2 level,[22] which could be
further confirmed by the free Au particles obtained by
reduction of HAuCl4 with NaBH4 alkaline solution
(Figure S1, Supporting Information). In comparison
with the free Au nanoparticles, the slight positive Au
BE shift indicated the formation of Au-SH coordina-
of the mercaptopropyl moiety in the Au-SH@SO3H-
SBA-15, which could be further confirmed by solid
NMR spectra (Figure 4). The Si CP MAS NMR spec-
29
trum displayed three resonance peaks upfield corre-
sponding to Q4 (δ＝−112), Q3 (δ＝−102), and Q2 (δ＝
−92), and two peaks downfield corresponding to T3 (δ＝
−67) and T2 (δ ＝ −56). The presence of Qn
peaks could be assigned to the condensation of TEOS
and the Tm peaks indicated the condensation of the
trimethoxysilane and TEOS.[25] According to the 13C CP
MAS NMR spectrum, the Au-SH@SO3H-SBA-15 sam-
ple displayed three weak signals around δ 10, 16,
and 56, corresponding to three carbon atoms in the
HO3SCH2CH2CH2-group.[26] A weak peak around δ 27
was assigned to the C atoms in the HSCH2CH2CH2-
group connecting with silicon due to the incomplete
oxidation.[27,28]
tion bond.[23] Therefore, it could be concluded that the
＋
Au3 ions were reduced into Au nanoparticles by
SH-groups together with formation of SO3H-groups
resulting from the SH oxidation, followed by coordi-
nating the Au with the unreacted SH-ligand, as illus-
trated in Scheme 2.
Figure 1 XPS spectra of the S and the free Au obtained by
Figure 3 FTIR spectra of the Au-SH@SO3H-SBA-15 sample.
KBH4 reduction (a) and the Au in Au-SH@SO3H-SBA-15 (b).
Scheme 2 Preparation of Au-SH@SO3H-SBA-15 from SH-
SBA-15 and HAuCl4
Figure 4 Solid NMR of the Au-SH@SO3H-SBA-15 sample.
As shown in Figure 5, both the SH-SBA-15 and the
Au-SH@SO3H-SBA-15 displayed typical IV type N2
adsorption-desorption isotherms with an H1-type hys-
teresis loop indicative of the mesoporous structure.[29,30]
The low-angle XRD patterns showed an intense diffrac-
tion around 2θ ＝0.9°－1.0° characteristic of the or-
dered 2D P6mm hexagonal mesoporous structures,[31]
which could be further confirmed by the TEM images
(Figure 6), which clearly showed the ordered mesopor-
ous channels in the Au-HS@SO3H-SBA-15 catalyst.
The Au-SH@SO3H-SBA-15 exhibited lower peak in-
tensity than the SH-SBA-15, suggesting that the immo-
bilization of Au nanoparticle catalyst by grafting
method disturbed the ordering degree of the mesoporous
structure. The positive shift of peak position implied the
increase of wall thickness since Au nanoparticles were
mainly anchored on the pore surface in the Au-SH@
Figure 2 TEM image (a) and particle size distribution (b) of the
Au obtained by HF corrosion of the Au-SH@SO3H-SBA-15.
As shown in Figure 2, the TEM image and the parti-
cle size distribution curve displayed ultrasmall Au
nanoparticles with average size around 1－2 nm. The
FT-IR spectrum in Figure 3 displayed no significant
signals indicative of P123 molecule, implying the com-
plete removal of the surfactant after extraction in etha-
nol. Two peaks around 2926 and 2564 cm−1 belonged to
the C－H and S－H vibration,[24] showing the presence
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In Situ Preparation of Au-SH@SO3H-SBA-15 Catalyst with Enhanced Activity and Durability
SO3H-SBA-15, leading to the decrease in both the pore
temperature and longer reaction time than that used for
diameter (DP) and surface area (SBET), as shown in Table
1.
the Au-SH@SO3H-SBA-15 heterogeneous catalyst),
suggesting that the metallic Au(0) rather than the Au(I)
was the real active sites in the present reaction. A plau-
sible reaction mechanism based on the related precedent
studies is proposed (see Scheme S1, Supporting Infor-
mation). The C≡H bond of alkynes is activated by Au
active phases to form a gold complex. And then the wa-
ter reacts with gold complex to give product.
Table 2 Performances of different catalysts in hydration of
phenylacetylenea
Entry Catalyst
H2SO4/mmol
Yield/%
Figure 5 N2 adsorption-desorption isotherms (a) and low-angle
XRD patterns (b) of SH-SBA-15 and Au-SH@SO3H-SBA-15.
1
2
3
4
5
6
Au(PPh3)Cl
0.50
－
30b
SO3H-SBA-15
0
Au-SH@SO3H-SBA-15
Au@SO3H-SBA-15
Au@SH-SBA-15
Au@SH-SBA-15
－
99
29
0
－
－
0.25
36
a Reaction conditions: a catalyst containing 0.020 mmol Au, 0.50
mmol phenylacetylene, 4.0 mL H2O, 80 ℃, 1.5 h. b 100 ℃ for 7
h.
Figure 6 TEM images of the Au-SH@SO3H-SBA-15.
Table 1 Structural parameters of different samples
Table 3 further demonstrated that the Au-SH@
SO3H-SBA-15 also displayed high activity in various
hydration reactions with different alkynes as substrates.
Terminal aromatic acetylenes bearing electron-neutral
(entry 1) and electron-donating groups exhibited high
reaction efficiencies (entries 2 and 3) and those bearing
electron-withdrawing groups showed slightly low reac-
tion efficiencies (entries 4 and 5). Meanwhile, the posi-
tion of substitutes on the aromatic ring played a key role
in determining the reaction efficiency since the hydra-
tion of o-CF3-Ph and o-CH3-Ph gave much lower yields
than those of p-CF3-Ph and p-CH3-Ph even with longer
SBET
/
Vp/
Entry Sample
DP/nm
(m2•g−1) (cm3•g−1)
1
2
3
SH-SBA-15
697
688
595
0.61
0.60
0.46
8.2
7.3
6.4
SO3H-SBA-15
Au-SH@SO3H-SBA-15
Table 2 summarized the catalytic performances of
different catalysts in phenylacetylene hydration reaction.
Besides the acetophenone, no other products were iden-
tified, suggesting all these catalysts were exclusively
selective. The Au-SH@SO3H-SBA-15 displayed much
higher activity than either the Au@SH-SBA-15 or the
Au@SO3H-SBA-15. The SO3H-SBA-15 was inactive,
implying that the Au active sites were essential for the
present reactions. No reaction occurred by using
Au@SH-SBA-15, suggesting the acid was essential for
the alkyne hydration, which could be further con-
firmed by the fact that the alkyne hydration proceeded
on the Au@SH-SBA-15 catalyst when little amount of
H2SO4 was added. Meanwhile, the Au@SO3H-SBA-15
also showed very poor activity in the absence of
SH-groups possibly due to the absence of Au-SH coor-
dination, which might supply suitable chemical micro-
environment of the Au active sites and also inhibit the
leaching of Au active sites from the catalyst. These re-
sults demonstrated the synergetic effect of both the
SH-groups and the SO3H-groups for promoting the al-
kyne hydration. It was also found that the Au(PPh3)Cl
homogeneous catalyst exhibited extremely activity in
the present alkyne hydration (see the higher reaction
Table 3 Hydration of alkynes catalyzed by Au@SH/SO3H-
SBA-15a
O
H
H2 O
R
R
A u-S H @ H S O3 -S B A -15
CH3
Entry R
Reaction time/h
1.5
Conv./% Yield/%
1
2
Ph
99
＞99
99
94
74
67
99
p-CH3O-Ph 1.5
＞99
99
94
74
67
3
p-CH3-Ph
p-Br-Ph
p-CF3-Ph
o-CF3-Ph
o-CH3-Ph
n-C4H9
1.5
1.5
1.5
5
4
5
6
7
4
75
75
8
2
95
95
9
n-C6H13
2
98
98
10
t-Bu
1.5
96
96
a Reaction conditions: a catalyst containing Au(0) (0.020 mmol),
alkyne (0.50 mmol); H2O (4.0 mL), 80 ℃; b determined by GC
analysis.
Chin. J. Chem. 2014, 32, 1072—1076
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FULL PAPER
reaction periods (entries 6 and 7), obviously due to the
steric hindrance. In addition, this Au-SH@SO3H-SBA-
15 catalyst was also applicable to the hydration reac-
tions of various aliphatic alkynes with excellent effi-
ciencies (entries 8－10), indicating the universal appli-
cations.
nation, and surface-modification with acidic SO3H-
group. This new protocol offered a valuable platform to
design high active supported metal catalysts for indus-
trial applications.
Acknowledgement
An important merit of heterogeneous catalysts was
their convenient recycle and reuse. As shown in Fig-
ure 7, during hydration reaction of phenylacetylene, the
Au-SH@SO3H-SBA-15 catalyst could be easily recy-
cled and used repetitively for 9 times without significant
decrease in activity. Both the XRD pattern and the TEM
image in Figure 8 demonstrated that the Au-SH@
SO3H-SBA-15 still remained the ordered mesoporous
structure after being reused for 9 times, showing the
excellent hydrothermal stability. Meanwhile, the ICP
analysis revealed that the Au-SH@SO3H-SBA-15 dis-
played no significant decrease in the Au loading, show-
ing strong stability against the leaching of Au active
sites, which could be mainly attributed to the Au－
SH coordination bond.
This work was supported by the National Natural
Science Foundation of China (21237003, 21261140333),
the Project from College Natural Science Research Plan
of Jiangsu Province (13KJB150008) and Project of
Huaian City (HAG2013077).
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In conclusion, we have developed a novel Au-SH@
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displayed high activity and strong durability in alkyne
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(Pan, B.)
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