Pyrrolidone-modified SBA-15 supported Au nanoparticles with superior catalytic properties in aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c000226g

Au nanoparticles with small sizes (2-3 nm) were effectively stabilized in the mesopores of (S)-(-)-2-pyrrolidinone-5-carboxylic acid-modified SBA-15, which showed superior catalytic properties in aerobic oxidation of alcohols, compared with pyrrolidone-free SBA-15 supported Au catalysts.

Full text of DOI:10.1039/c000226g

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Pyrrolidone-modified SBA-15 supported Au nanoparticles with superior
catalytic properties in aerobic oxidation of alcoholsw
Liang Wang,b Xiangju Meng,*a Bin Wang,c Wenyang Chib and Feng-Shou Xiao*a
Received 6th January 2010, Accepted 13th May 2010
First published as an Advance Article on the web 3rd June 2010
DOI: 10.1039/c000226g
Au nanoparticles with small sizes (2-3 nm) were effectively
stabilized in the mesopores of (S)-(ꢀ)-2-pyrrolidinone-5-
carboxylic acid-modified SBA-15, which showed superior
catalytic properties in aerobic oxidation of alcohols, compared
with pyrrolidone-free SBA-15 supported Au catalysts.
(SBA-15-Py). After treating with HAuCl4 solution and reducing
by NaBH4, Au/SBA-15-N (2.6wt% Au) and Au/SBA-15-Py
(2.7wt% Au) samples were finally obtained.
The organic species supported on SBA-15 were character-
ized by 13C NMR and IR techniques (Fig. 1). The 13C NMR
Since the pioneer work of Hutchings and Haruta in the field of
Au cluster catalysis,1 Au catalysts have been paid much
attention in both homogeneous and heterogeneous systems
recently.2–8 One of the most exciting findings in this field is
that small-size Au nanoparticles are able to promote the
selective oxidation of alcohols,7–9 which is of great importance
in the production of fine chemicals and pharmaceuticals.10
Traditionally, the alcohol oxidations were performed by using
organic solvents (benzene) or inorganic oxidants (KMnO4),
which produce a great deal of environmentally undesirable
wastes.11 Recently, a series of metal (Pt, Pd, Co) compounds
dissolved in solvents were selected as active homogeneous
catalysts for alcohol oxidations.12 Particularly, monodispersed
Au nanoclusters (NCs) stablized by poly(N-vinyl-2-pyrrolidone)
(PVP) dissolved in water exhibit excellent activities in oxida-
tions of alcohols with molecular oxygen in a homogeneous
system.9 This is typically a green process because the use of
water as solvent and molecular oxygen as oxidant has obvious
advantages including high atom efficiency and little pollution.
However, the wide applications of water-soluble Au NCs are
still challenging by the separation and regeneration of the
catalysts from the homogeneous system.9a Heterogeneous Au
catalysts have a good feature for the separation and recycle,
but their catalytic activities are much lower than those of
homogeneous ones due to the limitation of exposed Au sites
on the solid surface. Herein, toward the goals of high activity
and good recyclability, a new Au catalyst (Au/SBA-15-Py) is
successfully designed and prepared by supporting Au nano-
particles in the mesopores of pyrrolidone-modified SBA-15.
As proposed in the scheme of Fig. 1, after refluxing in
toluene, N-[3-(trimethoxysilyl)-propyl]diethylenetriamine (DETA)
was successfully coated on the surface of mespores in SBA-15
(SBA-15-N). Furthermore, (s)-(ꢀ)-2-pyrrolidinone-5-carboxylic
acid (Py) was linked by amidation on the surface of SBA-15-N
a Key Lab of Applied Chemistry of Zhejiang Province, Department of
Chemistry, Zhejiang University (XiXi Campus), Hangzhou 310028,
China. E-mail: mengxj@zju.edu.cn, fsxiao@mail.jlu.edu.cn
b State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun 130012, China
c State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, Changchun 130012, China
Fig. 1 Scheme, 13C NMR, IR, and Au4f XPS spectra of (a) SBA-15,
w Electronic supplementary information (ESI) available: 13C NMR
spectra, UV-vis spectra, and XRD patterns. See DOI: 10.1039/c000226g
(b) SBA-15-N, (c) Au/SBA-15-Py and (d) Au/SBA-15-N samples.
ꢁc
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spectrum of SBA-15-N shows the signals at 10.3, 23.0, 41.1,
49.5, and 56.9 ppm, which are reasonably associated with DETA
species on SBA-15 (Fig. S1w). Interestingly, Au/SBA-15-Py
exhibits three additional signals at 29.0, 167.0, and 180.8 ppm,
which are typically assigned to the presence of pyrrolidone
species in the sample (Fig. S2w).13 Additionally, IR spectra of
SBA-15-N and Au/SBA-15-Py give a band at about 1559 cmꢀ1
,
which is characteristic of the C–N bond.14 Au/SBA-15-Py has
two additional bands at about 1668 and 1743 cmꢀ1, which are
attributed to CQO bonds of pyrrolidone species.14b The
results of IR spectra are in good agreement with the results
obtained by NMR technique, confirming that organic species
of DETA and Py have been successfully grafted on the surface
of mesopores in SBA-15.
X-Ray diffraction (XRD) patternsz of Au nanoparticles
supported on SBA-15-Py and SBA-15-N (Fig. S3w) show three
well-resolved peaks in the region of 0.6–21 indexed to the
(110), (200) and (211) reflections of hexagonal mesoporous
arrays, indicating that the hexagonal mesostructure of SBA-15
remains well in both samples. A very broad diffraction peak at
38.71 is observed in the wide angle, which is attributed to the
corresponding (111) incidence of Au nanoparticles. However,
the diffraction peaks at 44.5, 64.7 and 77.71, associated with
(200), (220) and (311) incidences of larger Au nanoparticles are
not observed, suggesting that Au nanoparticles in the
mesopores of SBA-15 are highly dispersed. According to the
Scherrer equation,15 the average sizes of Au nanoparticles in
Au/SBA-15-Py and Au/SBA-15-N are estimated to be 2.1 nm
and 2.2 nm, respectively.
Fig.
2
TEM images and Au nanoparticle size distribution of
(A) Au/SBA-15-Py and (B) Au/SBA-15-N samples.
TEM images give direct observation of particle sizes for Au
nanoparticles (Fig. 2). Both Au/SBA-15-N and Au/SBA-15-Py
have a high dispersion of Au nanoparticles with a size of
2–3 nm in the mesoporous channels. UV-vis spectra (Fig. S4w)
of Au/SBA-15-Py and Au/SBA-15-N samples show the
same peak position at about 525 nm, suggesting that Au
sizes of the two samples are very similar because the peak
of the Au nanoparticles is sensitive to changes in the
size.15,16
alcohols, Au/SBA-15-Py still exhibits 50.7% conversion, while
Au/SBA-15-N gives a conversion of 41.0%. The dependence
of activity on time confirms that Au/SBA-15-Py is much more
active than Au/SBA-15-N (Fig. S3w). Considering the very
similar size distribution of Au nanoparticles and the same
mesoporous support (SBA-15), much better catalytic perfor-
mance in aerobic oxidations over Au/SBA-15-Py than that
over Au/SBA-15-N should be reasonably assigned to the
presence of pyrrolidone species, which strongly interact with
Au nanoparticles, resulting in the formation of negatively
charged surface Au. Possibly, the negatively charged surface
Au nanoparticles could promote catalytic conversion of
alcohols over Au catalysts.
Furthermore, XPS analysis of Au/SBA-15-N (Fig. 1) shows
the binding energy of Au4f7/2 at 84.0 eV, which is typically
assigned to the metallic Au.17 Interestingly, Au4f7/2 of
Au/SBA-15-Py (Fig. 1) shows the binding energy at 83.4 eV,
with 0.6 eV of downshift from the metallic Au, indicating that
there is an interaction between the Au nanoparticles and Py
species. Clearly, the surface of the Au nanoparticles over
Au/SBA-15-Py are negatively charged, compared with
Au/SBA-15-N.
It is worth noting that Au/SBA-15-Py has good recyclability.
After 3 recycles, Au/SBA-15-Py still exhibits 93.9% conver-
sion and >99.5% selectivity in oxidation of phenylethanol. In
contrast, the Au/PVP catalyst dissolved in water is not easy to
reuse due to the difficulty in catalyst separation and aggrega-
tion of Au nanoclusters in the homogeneous system.9a It is
also emphasized that aerobic oxidation of alcohols over
Au/SBA-15-Py is performed using water as solvent. Water is
usually regarded as a safe, cheap, and environmentally friendly
solvent.
Table 1 presents catalytic activities in aerobic oxidation of
primary and secondary aliphatic, alicyclic and aromatic
alcohols including phenylethanol, benzyl alcohol, 2-butanol,
1,4-butanediol, cyclohexanol, and 1-butanol over Au/SBA-15-Py
and Au/SBA-15-N using water as a solvent (Scheme S1w).
Very interestingly, Au/SBA-15-Py always has better activities
than Au/SBA-15-N. For example, in phenylethanol oxidation,
Au/SBA-15-Py shows 94.0% conversion with over 99.5%
selectivity for acetophenone, which are comparable with those
in homogenous catalysis.12a In contrast, Au/SBA-15-N
only gives 80.0% conversion. In the oxidation of a primary
aliphatic alcohol (1-butanol), one of the most inactive
In summary, the Au/SBA-15-Py catalyst shows high
catalytic performance and good recyclability in aerobic
oxidation of a series of alcohols using water as solvent, which
synergistically combines the advantages of both homogeneous
catalysts with high activity and heterogeneous catalysts with
good recyclability. This kind of Au catalyst is a good example
of green chemistry.
ꢁc
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Table 1 Catalytic activities in aerobic oxidation of alcohols over Au/SBA-15-Py and Au/SBA-15-N catalystsa
Substrate
Product
Catalyst
Time/h
Conv. (%)
Sel. (%)
Phenylethanol
Acetophenone
Au/SBA-15-Py
Au/SBA-15-Pyb
Au/SBA-15-Pyc
Au/SBA-15-N
Au/SBA-15-Py
Au/SBA-15-N
Au/SBA-15-Py
Au/SBA-15-N
Au/SBA-15-Py
Au/SBA-15-N
Au/SBA-15-Py
Au/SBA-15-N
Au/SBA-15-Py
Au/SBA-15-N
10
10
10
10
12
12
24
24
20
20
24
24
24
24
94.0
94.0
93.9
80.0
90.0
76.9
74.3
56.2
70.1
64.0
66.5
60.0
50.7
41.0
>99.5
>99.5
>99.5
>99.5
88.0
Benzyl Alcohol
2-Butanol
Benzoic Acid
2-Butanone
90.0
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
98.0
1,4-Butanediol
Cyclohexanol
1-Butanol
Butyrolactone
Cyclohexanone
1-Butyric Acid
99.0
a
b
c
Reaction conditions: 130 1C, 5.0 MP of O2, 10 mg of catalyst, 3 mmol of substrate, 0.8 mmol of NaAc, 5 ml of water. Reuse. Recycles for
3 times.
This work is supported by the National Natural Science
Foundation of China (20973079) and the State Basic Research
Project of China (2009CB623501).
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Notes and references
z X-Ray diffraction patterns (XRD) were obtained with a Siemens
D5005 diffractometer and Rigaku D/MAX 2550 diffractometer with
Cu-Ka radiation. The average sizes of Au nanoparticles estimated by
XRD were calculated using the half-width of the gold peak at 2y =
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 5003–5005 | 5005