Solvent-free aerobic oxidation of alcohols over palladium supported on MCM-41

Article abstract of DOI:10.1016/j.molcata.2013.01.007

In present work, the [PdCl₄]^2- ions were successfully supported on as-synthesized [CTA⁺]MCM-41 materials through adsorption method. Subsequently, the composite materials were reduced by sodium borohydride, followed by being extracted of [CTA⁺] surfactants with acidified ethanol. The resulted solid (denoted as Pd-M41-AE) could be used as an efficient catalyst for the solvent-free aerobic oxidation of benzylic alcohol under atmospheric pressure. The structures of the samples were characterized by X-ray diffraction (XRD), N₂ adsorption-desorption, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetry (TG) techniques. 0.4%Pd-M41-AE exhibited high activity (turnover frequency of 4344 h^-1) for the oxidation of benzyl alcohol, remarkably superior than other kinds of Pd-M41 at 100 °C. Moreover, the superior turnover frequencies (69,162 h^-1 for benzyl alcohol, 69,920 h^-1 for 1-phenylethanol and 117,130 h^-1 for cinnamyl alcohol, respectively) were achieved over 0.05%Pd-M41-AE at 160 °C. Recycling studies proved the recyclability of Pd-M41-AE as a heterogeneous catalyst, which did not lose the catalytic activity after four recycles appreciably.

Full text of DOI:10.1016/j.molcata.2013.01.007

Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Solvent-free aerobic oxidation of alcohols over palladium supported on MCM-41
Ben Qi, Yanbing Wang, Lan-Lan Lou, Linyue Huang, Ying Yang, Shuangxi Liu∗
Institute of New Catalytic Materials Science and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry,
Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
In present work, the [PdCl4]2− ions were successfully supported on as-synthesized [CTA+]MCM-41
materials through adsorption method. Subsequently, the composite materials were reduced by sodium
borohydride, followed by being extracted of [CTA+] surfactants with acidified ethanol. The resulted solid
(denoted as Pd-M41-AE) could be used as an efficient catalyst for the solvent-free aerobic oxidation of
benzylic alcohol under atmospheric pressure. The structures of the samples were characterized by X-ray
diffraction (XRD), N2 adsorption–desorption, transmission electron microscopy (TEM), X-ray photoelec-
tron spectroscopy (XPS) and thermogravimetry (TG) techniques. 0.4%Pd-M41-AE exhibited high activity
(turnover frequency of 4344 h−1) for the oxidation of benzyl alcohol, remarkably superior than other
kinds of Pd-M41 at 100 ◦C. Moreover, the superior turnover frequencies (69,162 h−1 for benzyl alcohol,
69,920 h−1 for 1-phenylethanol and 117,130 h−1 for cinnamyl alcohol, respectively) were achieved over
0.05%Pd-M41-AE at 160 ◦C. Recycling studies proved the recyclability of Pd-M41-AE as a heterogeneous
catalyst, which did not lose the catalytic activity after four recycles appreciably.
Article history:
Received 27 August 2012
Received in revised form
29 December 2012
Accepted 11 January 2013
Available online 20 January 2013
Keywords:
Aerobic oxidation of alcohol
[CTA+]MCM-41
Palladium nanoparticle
Solvent-free
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
nanoparticles on mesoporous molecular sieves functionalized
by organic groups (e.g., (CH2)3NH2 or (CH2)3SH) or to graft
Catalytic oxidation of alcohols is one of the most important
transformations in organic synthesis [1,2]. Conventionally, stoi-
chiometric oxidants (e.g., chromate or permanganate) have been
used for oxidation of alcohols. However, lots of toxic wastes
produced from the processes are environmentally undesirable.
Molecular oxygen as a clean and cheap oxidant in the catalytic
oxidation of alcohols has been extensively investigated in chem-
ical industrial and academic fields. In recent years, noble metal
catalysts, such as Pd- [3–21], Au- [22–32], Pd–Au [33–38], and
Ru- [39–41] based catalysts and non-metallic catalysts, e.g., nitric
acid-assisted carbon [42,43] and TEMPO [44] were found to be
highly active for selective aerobic oxidation of alcohol. Especially,
Pd-based catalysts exhibited high activity and selectivity even in
solvent-free and base-free system under atmospheric pressure con-
dition [8–19].
Pd-complexes on the surface of mesoporous materials [16,17].
However, the Pd nanoparticles loaded inside the mesoporous chan-
nels could often generate pore diffusion resistance for the reactants
[17].
As is known to all, cetyltrimethylammonium ions ([CTA+]) have
been usually used as templates for the synthesis of mesoporous
materials. Recently, the [CTA+] ions were used as surface-stabilizing
agents and morphological control reagents for the preparation
of noble metal nanoparticles [45–47]. However, the Pd nanopar-
ticles were hardly reused compared with the Pd supported
catalysts. Wang et al. reported preparation of supported noble
metal nanoparticle catalysts with the aid of [CTA+] in as-synthesis
[CTA+]MCM-41 [48]. Because of occlusion of the channels by the
surfactants [CTA+] in [CTA+]MCM-41 materials, the noble metal
precursors could be loaded on the pore mouths of mesopores,
and the size of noble metal particles retained small after reduc-
tion. Unfortunately, continuous leaching of the [CTA+] species into
the reaction system would contaminate the products. Mokhonoana
et al. used calcination method to remove the [CTA+] species in Au-
[CTA+]MCM-41 at 500 ◦C. However, the Au species sintered into
about 12 nm nanoparticles [49]. Very recently, Hutchings et al.
found that the stabilizer agents (PVA) could be removed from
Pd–Au/TiO2 by the water reflux method, and the resulting cat-
alyst showed remarkably higher activity for CO oxidation and
alcohol oxidation than that by calcination method [50]. However,
Pd-based heterogeneous catalysts applied for the solvent-free
aerobic oxidation of alcohol have attracted much attention. Tra-
ditionally, heterogeneous Pd-based catalysts were prepared by
2−
2+
impregnation [10], adsorption of PdCl4 or Pd(NH3)4 by adjust-
ment of pH [11–13], ion-exchange [14], and sol-immobilization
methods [31–38]. Another method was to support palladium
∗
Corresponding author. Tel.: +86 22 2350 9005; fax: +86 22 2350 9005.
E-mail address: sxliu@nankai.edu.cn (S. Liu).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.01.007

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B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
to the best of our knowledge, removal of [CTA+] species in
Pd-[CTA+]MCM-41 through extraction method has rarely been
reported [51].
In present study, the size of Pd particles retained small through
extraction with acidified ethanol to remove the [CTA+] species in
Pd-[CTA+]MCM-41. Experiment results showed that Pd-M41-AE
exhibited high activity for solvent-free oxidation of alcohols with
molecular oxygen. Furthermore, the influences of Pd loading and
reaction temperature on the activities of catalysts were studied.
2.2. Characterizations of catalysts
XRD (X-ray diffraction) patterns of powder samples were taken
on a Bruker D8 FOCUS diffractometer in the 2Â range of 1–10◦ (scan-
ning rate of 0.5◦/min in steps of 0.01◦) and 10–80◦ (scanning rate
of 6◦/min in steps of 0.02◦) using Cu K␣ (ꢀ = 0.15406 nm) radia-
tion at 40 kV and 40 mA. N2 adsorption–desorption analysis was
carried out at −196 ◦C on a Micromeritics TriStar 3000 appara-
tus. The analytical data were processed by the BET equation for
surface areas and by the BJH model for pore size distributions.
Transmission electron microscopy (TEM) measurement was car-
ried out on a Philips Tecnai G2F20 electron microscope operating
at 200 kV. Samples for TEM measurements were suspended in
ethanol and ultrasonically dispersed. Drops of suspensions were
applied on a copper grid coated with carbon. The content of Pd in
the sample was determined by the inductively coupled plasma-
atomic emission spectrometry (ICP-AES, ICP-9000(N+M)). X-ray
photoelectron spectroscopy (XPS) was recorded on a Kratos Axis
Ultra DLD multitechnique X-ray photoelectron spectrometer using
Mg K␣ radiation. Thermogravimetry (TG) was performed with a
Rigaku thermogravimetry–differential thermal analyzer in air with
the heating rate of 10 ◦C/min. FT-IR spectra were recorded on KBr
pellets in a BRUKER VECTOR 22 spectrometer in the region of
2. Experimental
2.1. Preparation of catalysts
MCM-41 was prepared by hydrothermal synthesis at
110 ◦C for 52 h using tetraethyl orthosilicate (TEOS) and
cetyltrimethyl ammonium bromide (CTAB) as the silicon
source and template, respectively [52]. A gel composition of
1TEOS:0.12CTAB:8NH3·H2O:114H2O was used. In a typical syn-
thesis, 2.7 g of CTAB was dissolved in 125 mL of deionized water,
and then 33 mL of 25% ammonia was dripped to the solution under
vigorous stirring. Then 14 mL of TEOS was slowly dripped into
the above solution. After 0.5 h of stirring at 35 ◦C, the resulting
gel was allowed to crystallize at 110 ◦C for 52 h in a Teflon-lined
autoclave. Finally, the sample was filtered, washed with 5 L of
deionized water up to pH ≈ 7, and dried at 60 ◦C for 24 h to obtain
as-MCM-41. The resultant powder was calcined at 550 ◦C in air
atmosphere for 6 h to remove the template, to obtain MCM-41.
1.5 g of as-MCM-41 was added to 30 mL of aqueous H2PdCl4
solution of desired concentration. The mixture was stirred vigor-
ously for 10 min at room temperature. After the Pd species was
adsorbed to as-MCM-41, the resulted precipitate was filtered and
washed with distilled water for several times. Then, the light orange
powder was reduced with 0.1 M NaBH4 solution at room tem-
perature for 30 min. The gray precipitate was filtered, washed
with distilled water, and dried at 100 ◦C for 24 h. The sample was
denoted as Pd-as-M41. Finally, Pd-as-M41 was Soxhlet-extracted
with 200 mL of acidified ethanol solution (HCl/EtOH = 5/200 (V/V))
for 72 h to remove the template, and dried at 80 ◦C for 24 h. The
obtained products were marked as x-Pd-M41-AE (x varies from 0.05
to 0.85%), where x represents the mass fraction of Pd. On the other
hand, Pd-as-M41 was calcined at 550 ◦C in air atmosphere for 6 h
to remove the template, to obtain Pd-M41-AC.
For comparison, MCM-41 surface functionalized with 3-
aminopropyltrimethoxysilane was prepared via post-synthesis
grafting method. MCM-41 (1.0 g) vacuum dried at 100 ◦C for 8 h was
suspended in anhydrous toluene (60 mL). To this suspension, 0.5 mL
of 3-aminopropyltrimethoxysilane was added and the mixture was
stirred under reflux for 10 h. The resulted powder was filtered,
washed thoroughly with ethanol and acetone, and then extracted
with dichloromethane for 24 h. Finally, the solid was dried at 60 ◦C
for 12 h to obtain AP-MCM-41. AP-MCM-41 was added to 30 mL of
H2PdCl4 solution, and stirred vigorously at 60 ◦C for 3 h. The solid
product was recovered by filtration, washed with deionized water,
and dried at 110 ◦C for 12 h. Similarly, the powder was reduced
with 0.1 M NaBH4 solution at room temperature for 30 min. The
gray precipitate was filtered, washed with distilled water, and dried
at 100 ◦C for 24 h. The catalyst named as Pd-AP-M41. In addition,
Pd-M41-I was prepared through impregnation method. MCM-41
(1.0 g) was added to an aqueous solution of H2PdCl4 (30 mL). The
mixture was stirred vigorously at 50 ◦C for 4 h. The liquid phase was
removed by rotary evaporator at 50 ◦C, followed by drying at 120 ◦C
overnight and calcination at 550 ◦C for 6 h in air. The resulted pow-
der was reduced with 0.1 M NaBH4 solution at room temperature
for 30 min, to obtain Pd-M41-I.
400–4000 cm−1
.
2.3. Catalytic oxidation reaction
The solvent-free aerobic oxidation of benzyl alcohol using
molecular O2 was carried out in a bath-type reactor operated under
atmospheric conditions. Typically, 0.1 g of the catalyst and 5 mL of
benzyl alcohol were added into a 25-mL two-necked flask equipped
with a cooling condenser under magnetic stirring and heated to the
desired temperature. Then, molecular oxygen (O2, 25 mL min−1
)
was introduced by bubbling into the bottom of the reactor at
atmospheric pressure. After the required time, the reaction was
terminated and the solid catalyst was separated by filtration. The
liquid products were analyzed by a GC-7970 gas chromatograph
equipped with a FID detector and a SGE AC20 capillary column
(Polyethylene Glycol, 30 m × 0.32 mm × 0.50 ␮m) using N2 as the
carrier gas. The conversion of alcohol (%) was calculated based
on the moles of alcohol converted to aldehyde and by-products.
The selectivity of aldehyde (%) was the molar ratio of aldehyde to
alcohol converted. The calculated turnover frequency (TOF, h−1
)
equaled the molar ratio of converted alcohol to Pd content of the
catalyst per unit time. In order to investigate the stability and recy-
clability of the catalyst, after each cycle the catalyst was washed
with acetone and ethanol, and used for the next cycle.
3. Results and discussion
3.1. Structures of catalysts
The small-angle XRD patterns of the Pd-M41-AE samples with
different Pd loadings are shown in Fig. 1(A). The XRD patterns
showed three distinct peaks corresponding to the (1 0 0), (1 1 0) and
(2 0 0) reflections. The XRD reflections became less intense with the
increase of Pd loading, indicating that the mesoporous ordering is
decreased to some extent. Fig. 1(B) shows the wide-angle XRD pat-
terns (10–80◦) of the samples. The Pd-M41-AE samples exhibit only
a broad diffraction peak at 23◦, which is ascribed to the amorphous
framework of MCM-41, and the peaks characteristic of metallic Pd
are not detected.
Fig. 2 shows the low-temperature N2 adsorption–desorption
isotherms and pore size distributions of various Pd-M41 cata-
lysts. It can be observed that all the samples exhibited type IV

B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
97
A
A
(100)
(i)
(110)
(200)
0.85%
(h)
(g)
0.6%
(f)
0.4%
0.2%
(e)
(d)
(c)
(b)
0.1%
(a)
0.05%
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
2theta (deg.)
Relative Pressure
(
P/P0)
B
B
0.85%
0.6%
(i)
(h)
(g)
0.4%
0.2%
(f)
(e)
(d)
0.1%
(c)
0.05%
(b)
(a)
10
20
30
40
50
60
70
80
2theta (deg.)
2
3
4
5
6
7
8
9
10
Pore diameter (nm)
Fig. 1. Small-angle (A) and wide-angle (B) XRD patterns of Pd-M41-AE with differ-
ent Pd contents.
Fig. 2. N2 adsorption–desorption isotherms (A) and pore diameter distributions (B)
of catalysts (a) Pd-AP-M41, (b) Pd-M41-I, (c) Pd-M41-AC, (d) 0.05-Pd-M41-AE, (e)
0.1-Pd-M41-AE, (f) 0.2-Pd-M41-AE, (g) 0.4-Pd-M41-AE, (h) 0.6-Pd-M41-AE, and (i)
0.85-Pd-M41-AE.
isotherms typical for mesoporous materials (Fig. 2(A)). The physi-
cal properties and structural parameters of the samples calculated
by N2 adsorption–desorption isotherms are listed in Table 1, and
the pore size distributions calculated from the desorption branch
are shown in Fig. 2(B). The [CTA+] in Pd-M41-AE was almost not
detected by TG and FT-IR analysis (see Supporting information).
The surface area, pore size and pore volume for the samples except
Pd-AP-M41 became slightly lower than pure MCM-41. It is worth
mentioning that the pore size of Pd-AP-M41 was smaller than 2 nm,
which could be ascribed to the relatively large dimension of 3-
aminopropyltrimethoxysilane (ca. 0.6 nm) and the introduction of
Pd nanoparticles into the channels of mesoporous MCM-41 [17].
Fig. 3 shows the TEM images of the Pd-M41 samples. Pd particle
size distributions derived from the TEM micrographs by count-
ing more than 50 particles in each case are also plotted in Fig. 3.
The Pd nanoparticles were almost not detected by TEM for Pd-as-
M41 samples, implying that Pd nanoparticles were too small to be
clearly distinguished [48,49]. However, the sample of 0.4-Pd-M41-
AE (Fig. 3a) showed Pd nanoparticles on the surface of MCM-41. The
results indicated that Pd nanoclusters migrated to the surface and
aggregated while the [CTA+] species were extracted from the chan-
nels of the mesoporous support. Moreover, a small part of invisible
Pd nanoparticles loaded on the mouths of pores were detected

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B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
Table 1
Physicochemical characterization of various catalysts.
c
c
Sample
Pd loading (%)a
Pd size (nm)b
SBET (m2 g−1
)
Pore volume (cm3 g−1
)
Pore size (nm)c
MCM-41
Pd-AP-M41
Pd-M41-I
Pd-M41-AC
0.05-Pd-M41-AE
0.1-Pd-M41-AE
0.2-Pd-M41-AE
0.4-Pd-M41-AE
0.6-Pd-M41-AE
0.85-Pd-M41-AE
–
–
941
378
730
824
866
846
835
801
793
779
0.82
0.31
0.47
0.61
0.58
0.61
0.60
0.61
0.55
0.55
2.79
–
0.4
0.4
0.4
0.05
0.1
0.2
0.4
0.6
0.85
1.8
10.2
12.5
3.7
6.3
6.5
7.1
7.4
9.2
2.20
2.48
2.46
2.44
2.44
2.44
2.40
2.44
a
Results obtained from ICP-AES.
Average size measured by TEM.
Values obtained from N2 adsorption–desorption.
b
c
by the EDX technique (see Supporting information). Increasing
Pd loading resulted in a larger mean particle diameter of Pd,
varying between 3.7 nm for 0.05-Pd-M41-AE and 9.2 nm for 0.85-
Pd-M41-AE (see Supporting information). As the [CTA+] species
were removed by high-temperature calcination, larger Pd nanopar-
ticles (∼12.5 nm) could be seen in the sample of 0.4-Pd-M41-AC
(Fig. 3b). Notably, the shape of Pd nanoparticles in 0.4-Pd-M41-AC
is Pd black, while that for the Pd nanoparticles dispersed on the sur-
face of Pd-M41-AE cannot be clearly distinguished. The previous
report showed that the spherical Pd nanoparticles with a diam-
eter of about 7 nm were obtained in the presence of CTAB using
sonoelectrochemical method [47]. The Pd particles (∼1.8 nm) were
observed for Pd-AP-M41 sample (Fig. 3c), and the ordered structure
of mesoporous channels was retained. It can be also observed that
the Pd particles were about 10.2 nm in Pd-M41-I (Fig. 3d).
Fig. 4 shows the Pd 3d XPS spectra of Pd-M41 samples. The cal-
culated abundance of different surface species is summarized in
Table 2. The peaks at 340.7 eV (Pd 3d3/2) and 335.4 eV (Pd 3d5/2
)
Fig. 3. TEM images and Pd particle size distributions of catalysts (a) 0.4-Pd-M41-AE, (b) Pd-M41-AC, (c) Pd-AP-M41, and (d) Pd-M41-I.

B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
99
(1.4%) when it was reduced by NaBH4 again. This may be attributed
to the large Pd particles located on Pd-M41-AC (∼12.5 nm) and
Pd-M41-I (10.2 nm), which were inactive for oxidation of benzyl
alcohol. In addition, the shape of Pd particles would also have an
influence on the catalytic activities: the shape of Pd nanoparticles
in Pd-M41-AC was Pd black, while spherical Pd nanoparticles were
dispersed on the surface of Pd-M41-AE samples [53]. Pd-as-M41
exhibited 25.4% benzyl alcohol conversion with 97.6% of benzalde-
hyde selectivity, while some amount of [CTA+] species leached from
the catalyst was found (see Supporting information), which could
contaminate the products [48].
Pd2+
Pd
Pd-M41-AE
For the Pd-M41-AE series of catalysts, the conversion of ben-
zyl alcohol attained 33.8% with the Pd content increased up to
0.40%. With further increase of the Pd content, the benzyl alcohol
conversion decreased slightly (28.4–28.6%). The TOF (h−1) value
in 1 h decreased gradually in the sequence of 0.05-Pd-M41-AE
(8637) > 0.1-Pd-M41-AE (8174) > 0.2-Pd-M41-AE (5234) > 0.4-Pd-
M41-AE (4344) > Pd-as-M41 (3265) > 0.6-Pd-M41-AE (2434) > Pd-
AP-M41 (2095) > 0.85-Pd-M41-AE (1730) > Pd-M41-I (308) > Pd-
M41-AC (103). Obviously, the TOF of Pd-M41-AE catalysts
decreased with the increase of Pd content from 0.05% to 0.85%,
which was attributed to the larger particle diameter with increase
of Pd content in Pd-M41-AE, as observed from the images of TEM.
Furthermore, the conversion of benzyl alcohol for all the prepared
catalysts increased with the reaction time prolonged to 4 h. For
the Pd-M41-AE catalysts with Pd content ≥0.4%, the conversions
of benzyl alcohol reached above 64%. Meanwhile, the selectiv-
ity of benzaldehyde was maintained over 93.0% in all the cases.
Compared with the earlier reported heterogeneous catalysts, Pd-
M41-AE exhibited superior activity for the oxidation of benzyl
alcohol at 100 ◦C in solvent-free conditions. It was reported that
the catalysts Pd-APS-TUD and Au-Pd/TiO2 exhibited TOFs of 2316
and 6190 h−1, respectively [17,36].
Pd-AP-M41
Pd-M41-AC
Pd-M41-I
346
344
342
340
B.E. (eV)
338
336
334
Fig. 4. XPS spectra of Pd 3d binding energy of various catalysts.
were attributed to Pd0. The peaks around 342.4 eV (Pd 3d3/2) and
337.3 eV (Pd 3d5/2) were ascribed to Pd2+, which indicated that
some of Pd nanoparticles were oxidized on the surface of Pd-M41-
AE and Pd-AP-M41. For Pd-M41-AC, the Pd nanoparticles have been
thoroughly oxidized to PdO. And the Pd species have been abso-
lutely reduced to Pd0 for Pd-M41-I.
3.3. Effect of reaction time and catalyst amount over
0.4-Pd-M41-AE
The effect of reaction time on the catalytic performance over
0.4-Pd-M41-AE is depicted in Fig. 5(a). The conversion of benzyl
alcohol was increased from 14.0% in 0.5 h to 64.7% in 4 h, and a
slight increase to 72.5% in 6 h, which was similar to the earlier
reports [17–19]. The catalyst deactivation occurred continuously
under the reaction conditions, which may be ascribed to the pro-
nounced deposition of carbonaceous species [18]. The TOF value
increased from 3599 h−1 to 4344 h−1 by increasing reaction time
from 0.5 h to 1 h and then decreased to 1553 h−1 after 6 h. Mean-
while, the selectivity toward benzaldehyde was 94.0% in 0.5 h, rose
to 96.2% in 1 h and gradually decreased to 92.0% in 6 h.
3.2. Catalytic aerobic oxidation of benzyl alcohol over different
catalysts
The aerobic oxidation of benzyl alcohol carried out in liquid-
phase resulted in the formation of benzaldehyde, benzoic acid
and benzylbenzoate. Catalytic performances of different catalysts
are listed in Table 3. In the absence of catalyst, null product was
detected by GC. Different kinds of Pd-M41 catalysts with similar
palladium contents exhibited remarkably different catalytic activ-
ities. Pd-AP-M41 showed relatively mild activity, and 16.3% benzyl
alcohol conversion with 94.2% selectivity to benzaldehyde was
reached in 1 h, which was remarkably lower than that of 33.8%
obtained by using 0.4-Pd-M41-AE as the catalyst. This could be
explained by the fact that the Pd nanoparticles dispersed in the
channels of Pd-AP-M41 would result in pore diffusion resistance
for alcohol oxidation reaction. The catalysts Pd-M41-I and Pd-M41-
AC showed extremely low activities, i.e., only 0.8–2.4% conversions
were achieved. Moreover, Pd-M41-AC also exhibited low activity
The effect of catalyst amount on the conversion of benzyl alco-
hol at 100 ◦C over 0.4-Pd-M41-AE was studied and the results are
shown in Fig. 5(b). When the catalyst amount increased from 0.01 g
to 0.2 g, benzyl alcohol conversion was increased from 2.6% to
51.5% while the selectivity to benzaldehyde was maintained in the
range of 93.1–96.7%. The TOF of catalyst increased from 3341 h−1
to 5527 h−1 when the catalyst amount was enhanced from 0.01 g to
0.05 g. With a further increase of catalyst amount to 0.2 g, the TOF
value decreased to 3310 h−1
.
3.4. Effect of reaction temperature over Pd-M41-AE series
catalysts
Table 2
Surface Pd calculated from XPS results of various samples.
Sample
Pd0/Pd
Pd2+/Pd
The aerobic oxidation reactions over Pd-M41-AE series cata-
lysts (0.05 g) with benzyl alcohol as substrate were performed
at 120 ◦C and 140 ◦C, and the results are shown in Fig. 5(c) and
(d), respectively. At 120 ◦C, with increasing the Pd content from
0.05% to 0.4%, the conversion of benzyl alcohol increased from 8.9%
0.4-Pd-M41-AE
Pd-AP-M41
Pd-M41-AC
Pd-M41-I
74
63
0
26
37
100
0
100

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B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
Table 3
Aerobic oxidation of benzyl alcohol over various catalysts.a
Entry
Catalyst
Pd content (%)
Conv. (%)
1 h
Sel. (%)
1 h
TOFb (h−1
)
4 h
4 h
1
2
3
4
5
6
7
8
9
Pd-AP-M41
0.42
0.40
0.27
0.40
0.05
0.10
0.20
0.40
0.60
0.85
16.3
2.4
25.4
0.8
37.8
4.6
45.6
1.9
19.4
50.4
52.0
64.7
64.1
64.1
94.2
79.3
97.6
84.6
97.2
96.9
96.3
96.2
94.4
96.6
91.4
75.4
93.0
87.4
94.5
94.3
93.5
93.0
94.3
94.3
2095
308
3265
103
8637
8174
5234
4344
2434
1730
Pd-M41-I
Pd-as-M41c
Pd-M41-AC
0.05-Pd-M41-AE
0.1-Pd-M41-AE
0.2-Pd-M41-AE
0.4-Pd-M41-AE
0.6-Pd-M41-AE
0.85-Pd-M41-AE
8.4
15.9
20.4
33.8
28.4
28.6
10
a
Reaction condition: catalyst, 0.1 g; benzyl alcohol, 5 mL; reaction temp., 100 ◦C; flow rate of O2, 25 mL min−1
Calculation of TOF (h−1) after 1 h.
0.27%Pd-as-MCM-41, 0.15 g.
.
b
c
to 25.3%, and slightly increased to 29.6% with 0.85% Pd content.
140 ◦C, with increasing the Pd content from 0.05% to 0.85%, the
conversion of benzyl alcohol increased from 15.2% to 31.1%. Simi-
lar to those obtained at 100 ◦C and 120 ◦C, the TOF of the catalyst
decreased from 31,256 h−1 to 3339 h−1 with the Pd content from
Meanwhile, the selectivity toward benzaldehyde was maintained
about 95.0–97.0%. Notably, the TOF of the catalyst decreased from
18,374 to 3595 h−1 with the Pd content from 0.05% to 0.85%. At
Fig. 5. Effects of (a) reaction time, (b) catalyst amount, (c) and (d) reaction temperature over Pd-M41-AE. Reaction condition: benzyl alcohol, 5 mL; flow rate of O2, 25 mL min−1
.

B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
101
0.05% to 0.85%. When the temperature was enhanced from 120 ◦C
to 140 ◦C, the TOF of 0.05-Pd-M41-AE increased from 18,374 h−1 to
100
80
60
40
20
0
75000
60000
45000
30000
15000
(a)
31,256 h−1
.
selectivity
3.5. Aerobic oxidation of benzyl alcohol, 1-phenylethanol and
cinnamyl alcohol at 160 ◦C
For a better comparison with earlier reported catalysts, we
investigated the catalytic performance of Pd-M41-AE series cat-
alysts for aerobic oxidation of benzyl alcohol, 1-phenylethanol and
cinnamyl alcohol in solvent-free at 160 ◦C in 1 h under atmospheric
pressure, and the results are illustrated in Fig. 6. With increasing Pd
content from 0.05% to 0.4%, the benzyl alcohol conversion rapidly
increased from 13.4% to 25.3%, and then had a slight increase to
26.4% with further increasing Pd content to 0.85% (Fig. 6(a)). Simi-
lar to the tendency obtained at 100–140 ◦C, the TOF of the catalyst
decreased with the Pd content increased from 0.05% to 0.85% at
160 ◦C. These phenomena, which were similar to the earlier reports,
implied that the Pd particles aggregated and became larger with the
increase of Pd content [10]. The selectivity to benzaldehyde over all
the catalysts was well maintained above 93.3%. The catalyst 0.05-
conversion
TOF
0.8
0
0.0
0.2
0.4
0.6
Pd content (wt.%)
80
60
40
20
0
75000
Pd-M41-AE exhibited higher activity at 160 ◦C (TOF, 69,162 h−1
)
(b)
compared with Pd-APS-TUD and Pd–MnCeOx catalysts, which
possessed the TOF values of 18,571 h−1 and 15,235 h−1 [17,18],
respectively. Pd–Au/TiO2 exhibited a high TOF of 86,500 h−1; how-
ever the selectivity of benzaldehyde was only about 60% [36].
Excellent selectivity toward acetophenone (>99%) was achieved
in the aerobic oxidations of 1-phenylethanol (Fig. 6(b)). The conver-
sion rapidly increased from 13.2% to 42.0% with the Pd content from
0.05% to 0.2%, and then slightly decreased to about 40.0% with grad-
ually increasing Pd content to 0.85%. The results showed that the
catalyst 0.05-Pd-M41-AE exhibited a high TOF value of 69,960 h−1
for 1-phenylethanol oxidation, which compares favorably with
the other results reported previously except Au/MgCr-HT and
60000
45000
30000
15000
0
Pd–Au/TiO2 [30,36]. High catalytic efficiency (TOF, 117,130 h−1
)
was obtained for cinnamyl alcohol oxidation; however the selectiv-
ity to cinnamyl aldehyde was only about 58.8%, with phenylprogyl
aldehyde, phenylpropanol and allylbenzene as by-products (see
Fig. 6(c)).
Baiker et al. suggested that the comparison on TOF for different
catalysts should be made at similar conversion [29]. Moreover, the
0.0
0.2
0.4
0.6
0.8
Pd content (wt.%)
80
60
40
20
0
120000
100000
80000
60000
40000
20000
0
(c)
100
80
60
40
20
0
2000
1500
1000
500
0
selectivity
selectivity
conversion
TOF
conversion
TOF
0.0
0.2
0.4
0.6
0.8
Pd content (wt.%)
0
1
2
3
4
Fig. 6. Catalytic oxidation of (a) benzyl alcohol, (b) 1-phenylethanol and (c) cin-
namyl alcohol over Pd-M41-AE at 160 ◦C in 1 h. Reaction condition: catalyst, 0.02 g;
benzyl alcohol, 5 mL (or 1-phenylethanol and cinnamyl alcohol, 50 mmol); flow rate
Recycle of number
Fig. 7. Recycling studies of 0.4-Pd-M41-AE catalyst for 6 h. Reaction condition: cata-
lyst, 0.1 g; benzyl alcohol, 5 mL; reaction temp., 100 ◦C; flow rate of O2, 25 mL min−1
of O2, 25 mL min−1
.
.

102
B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
Table 4
Comparison of catalytic performance of 0.1-Pd-M41-AE with other catalysts for the oxidation of alcohols at 160 ◦C under atmospheric pressure.
Catalyst
Alcohol
Sub./metal
Conv. (%)
Sel. (%)
TOF (h−1
)
Time (h)
Ref.
Pd-APS-TUD
Pd/MnCeOx
Pd/2.5MnOx/CNT
Pd–Au/TiO2
107,346
83,288
126,409
161,667
257,050
22.3
18.4
15.4
26.8
17.8
95.2
98.6
100
18,571
15,325
19,467
86,500
45,936
1
1
1
0.5
1
17
18
19
36
Benzyl alcohol
0.1-Pd-M41-AE
93.3
This work
Pd/HAP
250,000
250,000
86,207
110,667
126,409
250,000
250,000
78,800
250,000
194,000
269,444
265,000
94.1
9800
32,558
18,800
29,576
21,981
12,480
25,000
15,760
81,000
26,9000
37600
24
Initial
4
1
1
Initial
0.5
1
0.5
0.5
0.5
1
8
9
14
17
19
23
26
27
30
36
36
This work
Pd/CeO2
Pd-Xa
87
27.5
17.3
95
98.2
100
>99
>99
>99
Pd-APS-TUD
Pd/2.5MnOx/CNT
Au/CeO2
Au/Ga3Al3O9
Au/MnO2-R
Au/MgCr-HT
Pd–Au/TiO2
Pd/HAP
1-Phenylethanol
5
20.0
16.2
69.3
7.0
0.1-Pd-M41-AE
21.5
>99
56,975
Reaction temp., 150 ◦C.
a
TOF for catalyst is also influenced by the reaction time (Table 4,
entries 6 and 15). In order to make a better comparison on the
activity of catalysts, the specific experimental conditions (such
as substrate to catalyst ratio, conversion and reaction time) are
listed in Table 4. The TOFs of catalysts were measured in the initial
time (0.5 h or 1 h) except Pd-X and Pd-HAP (entries 8 and 6). Fur-
thermore, the conversions of substrate for majority catalysts were
about 20%. The shape [53] and size [54] of noble metals, surface
area and mesostructure of supports [17], and metal–support inter-
actions [26,27] all had influences on the TOF of catalysts. It could be
found that the present catalytic aerobic oxidation system showed
excellent activity, as shown in Table 4.
3.6. Recycling studies
Catalyst recycling experiments were carried out with repeated
uses of 0.4-Pd-M41-AE at 100 ◦C for 6 h. It could be observed from
Fig. 7 that the activity of the catalyst showed no obvious decline
after four recycle runs. The catalyst maintained high benzaldehyde
selectivity and underwent a small decrease in the catalytic activ-
ities (conversion decreased from 72.5% to 64.1%). Furthermore, a
control experiment had been performed according to the method
described in the literature [26]. The reaction was terminated after
1 h and the solid catalyst was filtered off. The liquid reaction mix-
ture was allowed to continue reaction for another 5 h. The benzyl
alcohol conversion was found to be only 34.1%, notably lower than
72.5% conversion obtained with the normal reaction process (see
Supporting information). This study further testified the hetero-
geneity of the catalyst 0.4-Pd-M41-AE.
To confirm the size and oxidation state of Pd particles in the
catalyst reused 4 times, TEM and Pd 3d XPS characterization were
carried out. It could be found that the sample still retained the
mesoporous structure of MCM-41 and the diameter of Pd particles
was about 7.2 nm (Fig. 8(a)). The Pd 3d XPS implied that the fraction
of Pd0 increased after catalytic reaction (Fig. 8(b)). In the aerobic
oxidation of alcohol reaction, the Pd2+ could be reduced to Pd0 by
alcohol species [8,19]. During the induction period, the adsorbed
alcohol species act as a reducing agent to chemically transform Pd2+
into Pd0 phase.
4. Conclusions
A series of Pd nanoparticles supported on MCM-41 were syn-
thesized using different methods. The catalytic activities of these
catalysts were evaluated in the solvent-free selective oxidation of
benzylic alcohol using molecular oxygen under atmospheric pres-
sure. The XRD analyses implied that the framework structure of
MCM-41 remained intact after introducing Pd into MCM-41 and
no large particles were formed on the surface of Pd-M41-AE. The
Fig. 8. TEM image and Pd particle size distribution (a) and XPS spectrum of Pd 3d
binding energy (b) of 0.4-Pd-M41-AE reused 4 times.

B. Qi et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 95–103
103
TEM and EDX results illustrated that almost all of Pd nanoclus-
ters migrated and aggregated while the surfactant [CTA+] was
extracted. The catalytic results illustrated that the catalyst prepara-
tion method and Pd loading showed great impacts on the catalytic
performance. Pd-M41-AE showed higher activity for the oxidation
of benzyl alcohol, compared with Pd-AP-M41, Pd-M41-AC, and Pd-
M41-I. This could mainly be attributed to the smaller Pd particles
in Pd-M41-AE. Moreover, the Pd loading on the pore-mouth of
MCM-41 can suppress the mass diffusion resistance. The TOF of
Pd-M41-AE catalyst decreased with the Pd content from 0.05% to
0.85% and increased with the temperature from 100 ◦C to 160 ◦C. At
160 ◦C, 0.05-Pd-M41-AE exhibited high activity for benzyl alcohol
(69,162 h−1), 1-phenylethanol (TOF of 69,960 h−1) and cinnamyl
alcohol (TOF of 117,130 h−1), respectively. These catalysts could be
reused 4 times without obvious decrease in the conversion and
selectivity.
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The authors acknowledge the National Natural Science Founda-
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Appendix A. Supplementary data
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