Palladium nanoparticles confined in the nanocages of SBA-16: Enhanced recyclability for the aerobic oxidation of alcohols in water

Article abstract of DOI:10.1016/j.molcata.2010.08.006

Via modification of mesoporous cage-like material SBA-16 followed by adsorption of Pd(OAc)₂ and reduction with NaBH₄, Pd nanoparticles with a uniform size distribution were successfully confined in the nanocages of SBA-16, leading to a new solid catalyst for the aerobic oxidation of alcohols. The solid catalyst was characterized with N₂ sorption, XRD, TEM, FT-IR and XPS. Such a catalyst showed a high activity for the oxidation of benzylic alcohols, 1-phenylethanol and allylic alcohols without the presence of bases under air or O₂ atmosphere in water even at room temperature. The selectivities for the corresponding aldehydes and ketones were more than 99% in all the cases investigated. The developed catalyst could be facilely recovered and reused twelve times without significant decreases in activity and selectivity. Its recyclability was much better than that of the catalyst derived from amorphous silica under the same conditions.

Full text of DOI:10.1016/j.molcata.2010.08.006

Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Palladium nanoparticles confined in the nanocages of SBA-16: Enhanced
recyclability for the aerobic oxidation of alcohols in water
Zhancheng Maa, Hengquan Yanga,∗, Yong Qinb, Yajuan Haoa, Guang Lia
a School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
b School of Chemistry and Chemical Engineering, Changzhou University, Changzhou 213164, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Via modification of mesoporous cage-like material SBA-16 followed by adsorption of Pd(OAc)2 and reduc-
tion with NaBH4, Pd nanoparticles with a uniform size distribution were successfully confined in the
nanocages of SBA-16, leading to a new solid catalyst for the aerobic oxidation of alcohols. The solid cata-
lyst was characterized with N2 sorption, XRD, TEM, FT-IR and XPS. Such a catalyst showed a high activity
for the oxidation of benzylic alcohols, 1-phenylethanol and allylic alcohols without the presence of bases
under air or O2 atmosphere in water even at room temperature. The selectivities for the corresponding
aldehydes and ketones were more than 99% in all the cases investigated. The developed catalyst could
be facilely recovered and reused twelve times without significant decreases in activity and selectivity.
Its recyclability was much better than that of the catalyst derived from amorphous silica under the same
conditions.
Received 16 March 2010
Received in revised form 12 July 2010
Accepted 3 August 2010
Available online 12 August 2010
Keywords:
Mesoporous material
Alcohol oxidation
Palladium nanoparticle
Water
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
the catalytic activity for the aerobic alcohol oxidation depended
on the Pd particle sizes and Pd particles of a few nanometers
The selective oxidation of alcohols to carbonyl compounds is one
of the most important transformations in the synthesis of the fine
chemicals [1–5]. Currently, of particular interest in this field is the
use of molecular oxygen as oxidant because it represents a green
process in contrast to the use of toxic and expensive stoichiometric
metal oxidants [6–12]. When molecular oxygen is used as oxidant,
water may be the most ideal solvent for the industrial applications
because it avoids explosions and the hazards associated with the
use of oxidisable organic solvents under oxygen pressure [13,14].
In this context, the development of the efficient catalytic systems
for the aerobic oxidation of alcohols in water medium is currently
an important topic.
Recently, various supported metal catalysts (Pd, Au, Pt, Ru) were
found to be active in the aerobic oxidation of alcohols in water
[15–23]. Among these supported catalysts, palladium-based cat-
alysts show promising catalytic performances because they can
selectively catalyze the reaction even under the mild (neutral pH)
conditions [24–26]. The progress along this line is considerably
notable. However, the highly recyclable catalysts that could work
in pure water are still rather limited.
exhibited the highest activity for the aerobic oxidation of benzyl
alcohol [27,9]. However, due to the Ostwald ripening, the palla-
dium nanoparticles of such sizes are prone to growth into the larger
particles even Pd black during the catalyst preparation and the fol-
lowed catalytic reaction. Thus, it is difficult to control the particle
sizes in a desired range during the catalyst preparation and main-
tain the sizes in the sequent catalytic reactions, As reported, the Pd
nanoparticles supported on silica evolved into the particles with
dozens of nanometers in size after the first reaction cycle [19]. The
growth of Pd nanoparticles causes a substantial deactivation of the
catalyst.
Among various supports for efficiently preventing the growth of
metal nanoparticles, the mesoporous materials are very promising
because of their regularly arranged pore structure on a mesoscopic
scale. More interestingly, different to the widely used channel-like
mesoporous materials MCM-41 and SBA-15, the recently synthe-
sized SBA-16 (cubic, Im3m) is one of new ordered mesoporous
materials with cage-like structures [28–30]. This mesoporous cage-
like material has tunable cage size (4–10 nm) and pore entrance
size (generally less than 4 nm). One isolated nanocage is three-
dimensionally interconnected by eight neighboring pore entrances,
which are more resistant to pore blocking and allow a fast trans-
port of reactants and products. Such isolated nanocages of SBA-16
can not only accommodate metal complexes and metal particles
formed in situ, but also limit the growth of metal nanoparticles
by the spatial restrictions [31–34]. Meanwhile, the smaller pore
One of the key factors that limit the recyclability of the catalysts,
is related to the Pd particle sizes. The previous studies showed that
∗
Corresponding author. Tel.: +86 351 7011588; fax: +86 351 7011688.
E-mail address: hqyang@sxu.edu.cn (H. Yang).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.08.006

Z. Ma et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
79
Scheme 1. Schematic description for the preparation of the solid catalyst Pd/SBA-16.
entrances may inhibit the motion of metal nanoparticles, thereby
preventing the undesired aggregation and agglomeration. Further-
more, SBA-16 has a good stability which is a prerequisite of the
applications involving high temperature and the presence of water
[35]. However, to our knowledge, the mesoporous cage-like mate-
rial SBA-16 owning such unique nano-architectures and properties
was not explored to support metal particles for the aerobic oxida-
tion of alcohols in water.
In this context, mainly aiming to improve the recyclability of
the Pd-supported catalyst for the aerobic oxidation of alcohols in
water, we chose the cage-like mesoporous material SBA-16 as a
support. Through modification of SBA-16 surface with moderate
polarity groups (aminopropyl), sequential adsorption of Pd(OAc)2
and reduction, Pd particles of a few nanometers were successfully
confined inside the nanocages of SBA-16, yielding an active catalyst
for the aerobic oxidation of alcohols in water. The recyclability of
the catalyst is significantly enhanced, compared with the catalyst
derived from amorphous silica.
and the mesoporous cage-like material SBA-16 was eventually
obtained.
2.2. Introduction of Pd nanoparticles into the nanocages of
SBA-16
6 g of SBA-16 (evacuated at 125 ◦C for 6 h) was dispersed
in 50 mL of dry toluene. Into this system 9.6 mmol of 3-
aminopropyltriethoxysilane was added. After stirring for 12 h at
100 ◦C under a N2 atmosphere, the resulting solid was isolated by
a filtration, and repeatedly washed with toluene and then dried
under vacuum overnight to give SBA-16 modified with amino
groups.
1.2 g of amino-modified SBA-16 was dispersed into 12 mL of
toluene containing a given amount of Pd(OAc)2. After the mixed
system was then stirred at room temperature for 6 h, the solid
was isolated by a filtration and dried under vacuum. The solid was
treated with NaBH4 in a mixture of toluene and ethanol (20/1, V/V),
washed with ethanol and then dried under vacuum, yielding the
catalyst Pd/SBA-16.
2. Experimental
2.3. Typical procedures for the aerobic oxidation of alcohols
2.1. Reagents and materials
Benzyl alcohol (1 mmol), water (2 mL) and the catalyst Pd/SBA-
16 were combined in a dry flask, and the mixture was stirred at
given temperature in air or O2 for a given time. Oxygen gas was
introduced into the flask from an O2 balloon under atmospheric
pressure. After the reaction, the liquid was extracted with diethyl
ether. The resulting organic layer was dried with Na2SO4 and then
analyzed with GC to determine the conversion and selectivity.
Recycling test for the aerobic oxidation of alcohol is as follows:
for the first reaction cycle, the procedure was the same to above
descriptions. After the first reaction cycle, the solid catalyst was
recovered by a centrifugation, and extracted repeatedly with ether
and ethanol. The recovered catalyst was dried under vacuum and
was directly used for the next reaction cycle.
Pluronic P123 copolymer (EO20PO70EO20), Pluronic copoly-
mer F127 (EO106PO70EO106
)
were purchased from Sigma
Company. Tetraethyl orthosilicate and benzyl alcohol were
obtained from Shanghai Chemical Reagent Company of Chi-
nese Medicine Group. 3-Aminopropyltriethoxysilane and
other alcohols as reactants were purchased from Alfa Aesar.
Pd(OAc)2 was obtained from Hangzhou Kaida and purified
before use. Amorphous silica gel with specific surface area
of 345 m2/g was obtained from Qidao Haiyang Chemical
Plant.
Mesoporous cage-like material SBA-16 was synthesized
according to the modified method [29].
A mixture of F127
(EO106PO70EO106, 7.42 g) and P123 (EO20PO70EO20, 1.19 g) were
used as the templates. After the mixed templates were completely
dissolved in a solution of 300 mL of distilled water and 52.5 g of
concentrated hydrochloric acid (36%), and the solution was fur-
ther stirred at 308 K. After 4 h, 28 mL of tetraethyl orthosilicate was
dropwise added to the solution. After stirring for 40 min, the resul-
tant suspension was transferred into autoclaves. The autoclaves
were placed under static conditions at 35 ◦C for 24 h. Afterward,
the temperature of autoclaves was raised up to 100 ◦C and fur-
ther kept at this temperature for 32 h. After the hydrothermal
treatment, the precipitated solid was isolated by a filtration and
dried at 100 ◦C for 24 h, yielding white solid powders. This pow-
der sample was then subjected to a calcination at 550 ◦C for 10 h
2.4. Characterization and analysis
Small-angle X-ray powder diffraction was performed on Rigaku
(Cu K␣, 40 kV, 30 mA). N2 physical adsorption was carried out on
micromeritics ASAP2020 volumetric adsorption analyzer (before
the measurements, samples were out gassed at 120 ◦C for 6 h). The
Brunauer–Emmett–Teller (BET) surface area was evaluated from
data in the relative pressure range from 0.05 to 0.20. The total pore
volume of each sample was estimated from the amount adsorbed
at the highest P/P0 (above 0.99). Pore diameters were determined
from the adsorption branch using Barrett–Joyner–Halenda (BJH)

80
Z. Ma et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
Fig. 2. N2 sorption isotherms of (1) SBA-16, (2) amino-modified SBA-16 and (3)
Pd/SBA-16.
Fig. 1. FT-IR spectra of SBA-16 and amino-modified SBA-16.
method. FT-IR spectra were collected on Thermo-Nicolet-Nexus
470 infrared spectrometer. X-ray photoelectron spectroscopy (XPS)
was recorded on Perkin-Elmer 5400 ESCA and C1S line at 284.9 eV
was used as the binding energy reference. TEM micrographs were
taken using a JEM-2000EX transmission electron microscope at
120 kV.
3. Results and discussion
3.1. Catalyst preparation and characterization
The process for the preparation of the catalyst Pd/SBA-16 is
schematically depicted in Scheme 1. In order to introduce Pd pre-
cursor into the nanocages of SBA-16, 3-aminopropyltriethoxysilane
was employed to modify the surface of SBA-16. Owing to the strong
coordination capacity of amino groups, Pd(OAc)2 even with a high
loading can be easily adsorbed by SBA-16 modified with amino
groups [36]. After a careful reduction of Pd(OAc)2 with NaBH4, Pd(0)
was in situ yielded and then evolved to nanoclusters. Due to the
amino coordination and the spatial restriction by the nanocages of
SBA-16, the sizes of Pd(0) clusters were expected to be limited in
the range of cage sizes of SBA-16.
Fig. 3. Pore size distributions of (1) SBA-16, (2) amino-modified SBA-16 and (3)
Pd/SBA-16.
16 and Pd/SBA-16, are displayed in Figs. 2 and 3, respectively.
The determined textural parameters are listed in Table 1. SBA-16
exhibited a type-IV isotherm pattern with an H2 hysteresis loop
in the range of P/P0 = 0.4–0.75, which is characteristic of a meso-
porous cage-like structure. Similar to the parent material SBA-16,
amino-modified SBA-16 also showed a type-IV isotherm with an
H2 hysteresis loop, indicating that SBA-16 after modification still
maintained a good mesoporous cage-like structure. It is worth
noting that the specific surface area, pore volume and pore size
underwent significant decreases after modification (Table 1). Such
decreases in these textural parameters are in line with the success-
ful introduction of amino groups onto the internal surface. After
adsorption of Pd(OAc)2 and the followed reduction, the specific sur-
The success in grafting amino group on SBA-16 was confirmed
with FT-IR spectroscopy. The FT-IR spectra of SBA-16 before and
after grafting amino group are shown in Fig. 1. Compared with
the FT-IR spectrum of the parent material SBA-16, amino-modified
SBA-16 clearly exhibits two new peaks at 2970 and 1560 cm−1
,
which correspond to the stretching vibrations of the C–H, and the
bending vibrations of N–H, respectively. These results indicate that
amino groups were successfully grafted on SBA-16. Meanwhile, the
intensity of the peak at 960 cm−1 (related to the bending vibra-
tion of Si–OH on SBA-16) significantly decreased after modification.
This also suggests that amino groups were covalently linked with
SBA-16 via a silylation reaction.
Table 1
Amino-modified SBA-16 exhibited a good adsorption capacity
towards Pd (OAc)2 in toluene. 1–10 wt.% Pd with respect to amino-
modified SBA-16 could be completely adsorbed by the materials,
resulting in a clear solution. Elemental analysis with ICP-AES
confirmed that Pd (OAc)2 was completely introduced onto amino-
modified SBA-16. After a treatment with NaBH4, Pd nanoparticles
were formed inside the nanocages of SBA-16.
Textural parameters of SBA-16, SBA-16 modified with NH2 and Pd/SBA-16.
Samples
Surface
areaa
Pore
Cage
sizec
(nm)
volumeb
(cm3/g)
(m2/g)
SBA-16
Amino-modified SBA-16
Pd/SBA-16
763
240
163
0.80
0.46
0.33
6.7
5.6
5.4
The locations of Pd nanoparticles inside the nanocages of SBA-16
were confirmed by N2 sorption and TEM. The N2 sorption isotherms
and pore size distribution plots for SBA-16, amino-modified SBA-
a
BET surface area.
b
Single point pore volume calculated at relative pressure P/P0 of 0.99.
BJH method from adsorption branch.
c

Z. Ma et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
81
Fig. 4. TEM images for SBA-16 (A) and Pd/SBA-16 (B). The bar is 50 nm.
face area, pore volume and pore size further decreased, suggesting
that Pd nanoparticles were located inside the nanocages of SBA-16.
Transmission electron microscopic investigation (TEM) pro-
vides
a direct evidence for the locations and dimensional
information of Pd nanoparticles. The TEM images for SBA-16 and
Pd/SBA-16 are presented in Fig. 4A and B, respectively. In Fig. 4A,
the (1 0 0) projection corresponding to a cubic Im3m structure was
clearly observed, XRD pattern of Pd/SBA-16 showed an ordered
mesoporous structure similar to the parent SBA-16 (Fig. 5). The
TEM and XRD results suggest that the mesoporous cage-like struc-
ture of SBA-16 was maintained during the course of preparing the
catalyst. After introduction of Pd nanoparticles, the cage-like struc-
ture of the catalyst was still observed and fine Pd particles appeared
in the nanocages of SBA-16 (Fig. 4B). Interestingly, most of Pd parti-
cles with sizes of only a few nanometers were uniformly dispersed
in the nanocages of SBA-16.
The Pd(0) on the solid catalyst was further confirmed by
XPS. Fig. 6 presents the XPS elemental survey scans of surface
elementals of the solid catalyst. The peaks corresponding to sili-
con, oxygen, carbon, nitrogen, palladium elementals were clearly
observed. Fig. 7 displays Pd binding energy for Pd(OAc)2 and
Pd/SBA-16. Pd(OAc)2 mainly exhibited two peaks centered at 343.9
and 338.7 eV, which are assigned to Pd 3d3/2 and Pd 3d5/2 sig-
Fig. 6. XPS spectrum for the elemental survey scan of Pd/SBA-16.
Fig. 7. XPS spectra of Pd binding energy for Pd(OAc)2 and Pd/SBA-16.
Fig. 5. XRD patterns of SBA-16 and Pd/SBA-16.

82
Z. Ma et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
nals, respectively. Compared with Pd (OAc)2, Pd 3d3/2 and Pd 3d5/2
peaks for Pd/SBA-16 significantly shift down to 340.7 and 335.4 eV,
respectively, indicating that the Pd(II) was transformed to Pd(0)
after reduction.
Above characterizations confirm that the prepared catalyst
keeps the mesoporous cage-like structure of the parent SBA-16 and
the fine Pd(0) nanoparticles were uniformly distributed inside the
nanocages of SBA-16.
3.2. The aerobic oxidation of alcohols
Although several excellent catalysts for the aerobic oxidation
of alcohols to the corresponding aldehydes or ketones have been
disclosed, most of them required organic solvents, heating and
addition of base, which violate the requirements of the “green
chemistry”. Water is the most ideal solvent because it is not only
a green solvent but also can avoid the hazards associated with the
use of oxidisable organic solvents under oxygen pressure. Base-free
systemsare also highlydesiredin that thepresence ofbaseprobably
leads to the unwanted by-products. In line with the requirements of
“green chemistry”, we chose water as a reaction medium to conduct
the aerobic oxidation in the absence of base.
Wang’s studies revealed that the catalytic activity of the Pd-
based catalyst for the aerobic oxidation of alcohols in organic
solvents was dependent on the Pd particle sizes, and Pd particles
of 2–4 nm in size were the most active for the aerobic oxidation of
benzyl alcohol [27]. Inspired by these findings, we examined the
catalytic activities of Pd/SBA-16 with different Pd contents for the
aerobic oxidation of alcohols in water because the Pd particle sizes
on the support can be associated with the Pd content present on the
support. By varying the amount of adsorbed Pd(OAc), five catalysts
Pd/SBA-16 with Pd contents from 1 wt.% to 2.5, 4, 5.5, and 10 wt.%
were prepared.
The results for the aerobic oxidation of benzyl alcohol in the
presence of these five catalysts were presented in Fig. 8. All the
tests were conducted in the presence of 1 mol% Pd with respect
to alcohols at room temperature under base-free conditions. The
catalyst Pd/SBA-16 with a Pd content of 1 wt.% afforded a conver-
sion of 20.5% within 6 h. No by-products such as benzoic acid were
detected under the investigated conditions. When the Pd content
of the solid catalyst increased up to 2.5 wt.%, the conversion had
an apparent increase and a 68.6% conversion was achieved. A fur-
ther increase in the Pd content up to 4 wt.% led to a increase in
the conversion (77.5%). However, when the Pd content continu-
ously increased up to 5.5 wt.%, the conversion began to decrease (a
69.1% conversion). A further increase in the Pd content led to con-
tinuous reduces in conversion and a 57.6% conversion was afforded
for the catalyst with a Pd content of 10 wt.%. These findings indi-
cate that the catalytic activity of the aerobic oxidation of alcohols
is highly dependent on the Pd content present on the solid catalyst.
The underlying reason may be related to the Pd particle sizes, as
reported by Wang and co-workers [27].
Fig. 8. The aerobic oxidations of benzyl alcohol over the catalysts with variations of
the Pd content.
Above examinations revealed that Pd/SBA-16 with a Pd content
of 4 wt.% exhibited the highest activity for the aerobic oxidation of
benzyl alcohol in water under the base-free conditions. In our previ-
ous investigations, it was found that the catalytic reaction occurring
in the nanopores had different solvent effects with that in homoge-
neous solution [32]. In order to test the possibility that Pd/SBA-16
was more active in other reaction media, we examined the activity
of the catalyst in a set of solvents under the same conditions. The
results are summarized in Table 2. Using acetonitrile, 1,4-dioxane,
DMF and toluene as solvents, the conversions of benzyl alcohol
were low (less than 16%, Table 2, entries 1–4). When the solvent was
changed to alcohols, the conversion was found to increase (Table 2,
entries 5–8) and the maximum conversion (34%) was obtained for
the case of methanol as solvent. Impressively, using water as the
reaction medium led to the fastest conversion of benzyl alcohol.
These comparative results show that in the case of our catalyst
water is not only a green solvent but also can promote the aerobic
oxidation of benzyl alcohol under base-free conditions. This can
probably be partially explained that water (with a high polarity)
forces the organic substrates to be enriched on the surface (with
a moderate polarity) of the solid catalyst and the accessibility of
substrates to Pd is thus improved, leading to a high reaction rate.
To further test the performances of the catalyst Pd/SBA-16, the
oxidations of other benzylic alcohols with air or O2 at room tem-
perature was carried out. The results are summarized in Table 3. At
Pd loading of 1 mol%, a complete conversion of benzyl alcohol was
observed under O2 atmosphere over 8 h. Under air, although the
reaction time needed to be prolonged to 12 h, a complete conver-
sion of benzyl alcohol was still afforded. Whether under O2 or in air,
the selectivity for benzaldehyde exceeded 99%, and no by-product
(benzoic acid) was detected within a given time. If the reaction time
was prolonged under air or O2 atmosphere, the by-product ben-
zoic acid (further oxidation of benzaldehyde) was observed. When
Table 2
The aerobic oxidation of benzyl alcohol over Pd/SBA-16 in different solvents.
.
Entry
Solvents
Conv. (%)
Select. (%)
Entry
Solvents
Conv. (%)
Select. (%)
1
2
3
4
5
Acetonitrile
1,4-Dioxane
DMF
Toluene
n-Butanol
4.8
7.4
10
15.7
13.7
>99
>99
>99
>99
>99
6
7
8
9
n-Propanol
Ethanol
Methanol
H2O
15.8
20.2
34.0
>99
>99
>99
>99
>99

Z. Ma et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
83
Table 3
The aerobic oxidation of benzylic alcohols over Pd/SBA-16.a.
Entry
Substrates
Product
Air (12 h)
Conv. (%)
O2 (8 h)
Select. (%)
>99
Conv. (%)
Select. (%)
>99
1
>99
–
>99
>99
>99
>99
2b
3
–
>99
>99
>99
>99
>99
>99
>99
4
5
–
–
76.3
>99
a
The reaction was carried out in air or O2 atmosphere in the presence of 1 mol% Pd.
The reaction was conducted at 50 ◦C for 4 h.
b
Table 4
The aerobic oxidation of other alcohols over Pd/SBA-16.a.
Entry
Substrates
Product
Pd (mol%)
Temp. (◦C)
Time (h)
Conv. (%)
Select. (%)
>99
1
1.0
1.0
50
10
24
96.9
87.6
2
60
>99
3
4
1.0
1.0
50
90
10
24
90.8
97.8
>99
>99
5
6
7
1.0
1.0
1.0
25
50
60
12
12
12
>99
>99
>99
>99
89.5
16.2
8
2.5
60
12
22.6
>99
a
The reaction was carried out in O2 atmosphere.

84
Z. Ma et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
the reaction temperature was raised up to 50 ◦C, it took only 4 h
to complete the reaction (Table 3, entry 2). For CH3- and CH3O-
substituted benzylic alcohols, over 99% conversions with more than
99% of selectivities were also achieved under O2 atmosphere over
8 h (Table 3, entries 3 and 4). Under air, it also took a longer time
(12 h) to obtain complete conversions. For heterocyclic alcohols,
the catalyst Pd/SBA-16 showed a moderate activity with excellent
selectivity (Table 3, entry 5).
The aerobic oxidation of other alcohols in water was inves-
tigated to further examine the application range of the catalyst
Pd/SBA-16. Representative results are summarized in Table 4. For 1-
phenylethanol, at Pd loading of 1 mol% the catalyst Pd/SBA-16 gave
a 96.9% conversion within 10 h (Table 4, entry 1). The selectivity for
acetophenone was observed to be more than 99%. For methoxy-
substituted 1-phenylethanol, it needed 24 h to achieve a good
conversion (Table 4, entry 2). Compared with 1-phenylethanol, the
electron-rich methoxy-substituted 1-phenylethanol was less reac-
tive. The selectivity was still up to 99%. For 1-phenylpropanol, a
90.8% conversion with 99% selectivity was obtained within 10 h
(Table 4, entry 3). A 97.8% conversion of diphenyl methanol could
be achieved over Pd/SBA-16 although it required a relatively higher
temperature and a longer reaction time (Table 4, entry 4). Pd/SBA-
16 also showed high activity for allylic alcohols (Table 4, entries 5
and 6). Cinnamyl alcohol was completely converted to cinnamalde-
hyde at room temperature within 12 h. For 3-methyl-2-butan-1-ol,
an 89.5% conversion was obtained at 50 ◦C over 12 h. The selectivity
for ␣, ␤-unsaturated aldehydes was morethan 99%forthe two cases
investigated. For the secondary cyclic alcohols, Pd/SBA-16 shows a
relatively low activity. A 16.2% conversion of cyclopentanol and a
22.6% conversion of cyclohexanol were afforded (Table 4, entries
7 and 8). The above investigations showed that the developed cat-
alyst Pd/SBA-16 had a moderate substrate scope in water under
base-free conditions.
Fig. 9. The recyclability tests of Pd/SBA-16 and Pd/silica for the aerobic oxidation of
benzyl alcohol.
for the next cycles. For the second reaction cycle, the fresh benzyl
alcohol was added and other conditions were the same as the first
reaction cycle. A 99% conversion was still obtained over 4 h. From
the third to 12th reaction, 99% conversions could be achieved. The
selectivity for benzaldehyde was kept at >99% throughout the 12
reaction cycles. Such a recyclability of Pd/SBA-16 represents one of
the best results for the alcohol oxidation in water.
The recyclability of Pd/SBA-16 is impressive in view of water
as a solvent. In order to further understand the high recyclability,
Pd nanoparticles were supported on amorphous silica through the
same procedures, resulting in a catalyst Pd/silica (the Pd loading
was the same to the Pd/SBA-16). Comparative tests for the recycla-
bility were conducted under the same conditions. The results for
Pd/silica are also included in Fig. 9. From the first to the third reac-
tion cycle, the catalyst Pd/silica could afford a complete conversion
of benzyl alcohol and the selectivity also exceeded 99%. But from
the forth reaction cycle, the conversion began to decrease and a
Further experiments were performed to verify the recyclabil-
ity of the developed solid catalyst. The consecutive oxidations of
benzyl alcohol in O2 atmosphere were also carried out at 50 ◦C
under base-free conditions using water as solvent. The results are
reflected in Fig. 9. At Pd loading of 1 mol%, Pd/SBA-16 afforded a
complete conversion under O2 atmosphere over 4 h. After the first
reaction cycle, the resulting product was extracted with ether three
times. The catalyst was recovered by a centrifugation and dried
Fig. 10. TEM images for (A) Pd/SBA-16 used four times and (B) Pd/silica used four times. The bar is 20 nm.

Z. Ma et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 78–85
85
conversion of only 76.5% was obtained. For the followed fifth and
Acknowledgments
sixth reaction cycles, the conversions further decreased to 66.7%
and 26.4%, respectively. Obviously, the recyclability of Pd/SBA-16
was much better than that of Pd/silica.
We acknowledge New Teacher Foundation from Education Min-
istry of China (200801081035), the Natural Science Foundation of
China (20903064), Shanxi Natural Science Foundation for Youths
(2009021009), Shanxi University Innovative Experimental Project
for Undergraduates and Jiangsu Key lab for fine petrochemistry for
financial supports (KF0802).
The significantly enhanced recyclability of the catalyst may be
partially attributable to the unique structure of SBA-16. To confirm
the role of the pore structure, TEM was employed to characterize
Pd/SBA-16 used four times and Pd/silica used four times. Their TEM
images are presented in Fig. 10A and B, respectively. For the cat-
alyst Pd/SBA-16 used four times, the cage-like structure of parent
material SBA-16 could be still observed although the structure was
subjected to collapse to some extent. It is worthwhile to note that
most of fine Pd particles with sizes of less then 5 nm were dispersed
in the cages of SBA-16 although a small potion of larger Pd particles
are located on the external surface of SBA-16. While for the catalyst
Pd/silica after being used four times, it was observed that Pd par-
ticles were relatively larger and not uniform in size. Additionally,
the structure of the parent silica underwent a severe collapse. The
size differences of Pd particles between Pd/SBA-16 and Pd/silica
partially accounted for their differences in the activities of the
recovered catalysts.
The smaller Pd particles with a relatively uniform distribution
in size may be attributed to the mesoporous cage-like structure
of SBA-16. Due to the high surface energy, the unstable nanoclus-
ters are prone to growing into larger particles through aggregation
and agglomeration. Owing to the spatial restriction of the isolated
nanocages and smaller pore entrances of SBA-16, the aggregation
and agglomeration of Pd nanoclusters were efficiently prevented.
However, for the Pd/Silica, it was unable to prevent the growth of
Pd nanoparticles because of lacking the ordered cage-like pores.
Additionally, a low stability probably led to the structure collapse
of silica and a portion of Pd particles were thus buried.
References
[1] T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037.
[2] G.J. Brink, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000) 1636.
[3] M.S. Sigman, D.R. Jensen, Acc. Chem. Res. 39 (2006) 221.
[4] S.S. Stahl, Angew. Chem. Int. Ed. 43 (2004) 3400.
[5] T. Matsumoto, M. Ueno, N.W. Wang, S. Kobayashi, Chem. Asian J. 3 (2008) 196.
[6] Y.H. Ng, S. Ikeda, T. Harada, Y. Morita, M. Matsumura, Chem. Commun. (2008)
3181.
[7] Y.M.A. Yamada, T. Arakawa, H. Hocke, Y. Uozumi, Angew. Chem. Int. Ed. 46
(2007) 704.
[8] A. Abad, A. Corma, H. García, Chem. Eur. J. 14 (2008) 212.
[9] J. Chen, Q.H. Zhang, Y. Wang, H.L. Wan, Adv. Synth. Catal. 350 (2008)
453.
[10] B.A. Steinhoff, A.E. King, S.S. Stahl, J. Org. Chem. 71 (2006) 1861.
[11] B. Karimi, A. Biglari, J.H. Clark, V. Budarin, Angew. Chem. Int. Ed. 46 (2007) 7210.
[12] I.W.C.E. Arends, G.J. Brink, R.A. Sheldon, J. Mol. Catal. A: Chem. 251 (2006)
246.
[13] A. Biffis, L. Minati, J. Catal. 236 (2005) 405.
[14] W. Hou, N.A. Dehm, R.W.J. Scott, J. Catal. 253 (2008) 22.
[15] T. Wang, H. Shou, Y. Kou, H.C. Liu, Green Chem. 11 (2009) 562.
[16] J.H. Liu, F. Wang, K.P. Sun, X.L. Xu, Adv. Synth. Catal. 349 (2007) 2439.
[17] R.L. Oliveira, P.K. Kiyohara, L.M. Rossi, Green Chem. 12 (2010) 144.
[18] H.R. Li, B.T. Guan, W.J. Wang, D. Xing, Z. Fang, X.B. Wan, L.P. Yang, Z.J. Shi,
Tetrahedron 63 (2007) 8430.
[19] B. Karimi, A. Zamani, S. Abedi, J.H. Clark, Green Chem. 11 (2009) 109.
[20] J. Yang, Y.J. Guan, T. Verhoeven, R. Van Santen, C. Li, E.J.M. Hensen, Green Chem.
11 (2009) 322.
[21] P. Paraskevopoulou, N. Psaroudakis, S. Koinis, P. Stavropoulos, K. Mertis, J. Mol.
Catal. A: Chem. 240 (2005) 27.
[22] Y. Uozumi, R. Nakao, Angew. Chem. Int. Ed. 42 (2003) 194.
[23] Y.M.A. Yamada, Y. Uozumi, Org. Lett. 8 (2006) 1375.
[24] H. Wang, S.X. Deng, Z.R. Shen, J.G. Wang, D.T. Ding, T.H. Chen, Green Chem. 11
(2009) 1499.
[25] Z.S. Hou, N. Theyssen, A. Brinkmann, K.V. Klementiev, W. Grünert, M. Bühl,
W. Schmidt, B. Spliethoff, B. Tesche, C. Weidenthaler, W. Leitner, J. Catal. 258
(2008) 315.
[26] K.M. Gligorich, M.S. Sigman, Chem. Commun. (2009) 3854.
[27] F. Li, Q.H. Zhang, Y. Wang, Appl. Catal. A: Gen. 334 (2008) 217.
[28] H. Skaff, T. Emrick, Chem. Commun. (2003) 52.
[29] T.W. Kim, R. Ryoo, M. Kruk, K.P. Gierszal, M. Jaroniec, S. Kamiya, O. Terasaki, J.
Phys. Chem. B 108 (2004) 11480.
[30] O.C. Gobin, Y. Wan, D.Y. Zhao, F. Kleitz, S. Kaliaguine, J. Phys. Chem. C 111 (2007)
3053.
[31] H.Q. Yang, J. Li, J. Yang, Z.M. Liu, Q.H. Yang, C. Li, Chem. Commun. (2007) 1086.
[32] H.Q. Yang, X.J. Han, G. Li, Y.W. Wang, Green Chem. 11 (2009) 1184.
[33] H.Q. Yang, L. Zhang, L. Zhong, Q.H. Yang, Can Li, Angew. Chem. In. Ed. 46 (2007)
6861.
[34] H.Q. Yang, L. Zhang, P. Wang, Q.H. Yang, Can Li, Green Chem. 11 (2009) 257.
[35] M. Kruk, C.M. Hui, J. Am. Chem. Soc. 130 (2008) 1528.
[36] R. Xing, Y.M. Liu, H.H. Wu, X.H. Li, M.Y. He, P. Wu, Chem. Commun. (2008) 6297.
4. Conclusion
By confining Pd nanoparticles in the nanocages of the modified
SBA-16, a new solid catalyst for the aerobic oxidations of alcohol
was prepared. The catalyst showed high activity for the oxidation of
benzylic alcohols, 1-phenylethanol and allylic alcohols under air or
O2 atmosphere in water even at room temperature. The selectivities
for the corresponding aldehydes and ketones were more than 99%
in all the cases investigated. The catalyst can be facilely recovered
and reused twelve times without the changes in the conversion
and selectivity, representing the most durable solid catalysts for
the alcohol oxidation in water. Its recyclability was much better
than that of the catalyst derived from amorphous silica under the
same conditions. The significantly enhanced recyclability may be
attributed to the high stability and the isolated nanocages of SBA-
16 which could efficiently prevent the growth of Pd nanoparticles
during the catalytic reaction.