Palladium Nanoparticles Supported on Titanate Nanobelts for Solvent-Free Aerobic Oxidation of Alcohols

Article abstract of DOI:10.1002/cctc.201500491

The solvent-free selective oxidation of alcohols by O₂ without any additives has been studied using palladium nanoparticles supported on a titanate nanobelt. The catalysts were prepared by polyol reduction and impregnation with different kinds of supports. They were evaluated in oxidation reactions of benzyl alcohol. The calcined titanate nanobelt was found to be the best support for the solvent-free Pd-catalyzed oxidation of alcohols and excellent catalytic activity as well as high selectivity to the corresponding benzaldehyde could be obtained. Moreover, palladium nanoparticles immobilized on the calcined titanate nanobelt could completely suppress the formation of toluene and benzoic acid, an effect that might be caused by the enhanced basic nature and morphology of the support. The catalyst was not only active and selective towards carbonyl compounds, but also recyclable under the oxidation conditions. This catalyst was also applicable for the oxidation of other alcohols. Nanobelt-supported nanocatalyst: A titanate nanobelt has been found to be an excellent support for solvent-free Pd-catalyzed alcohol oxidation, which could completely suppress the formation of toluene and benzoic acid. The basic nature and morphology of the support played a crucial role for the reaction performance.

Full text of DOI:10.1002/cctc.201500491

DOI: 10.1002/cctc.201500491
Full Papers
Palladium Nanoparticles Supported on Titanate Nanobelts
for Solvent-Free Aerobic Oxidation of Alcohols
Yong-Ming Lu,*[a, b] Hai-Zhou Zhu,[a] Jian-Wei Liu,[a] and Shu-Hong Yu*[a]
The solvent-free selective oxidation of alcohols by O2 without
any additives has been studied using palladium nanoparticles
supported on a titanate nanobelt. The catalysts were prepared
by polyol reduction and impregnation with different kinds of
supports. They were evaluated in oxidation reactions of benzyl
alcohol. The calcined titanate nanobelt was found to be the
best support for the solvent-free Pd-catalyzed oxidation of al-
cohols and excellent catalytic activity as well as high selectivity
to the corresponding benzaldehyde could be obtained. More-
over, palladium nanoparticles immobilized on the calcined tita-
nate nanobelt could completely suppress the formation of tol-
uene and benzoic acid, an effect that might be caused by the
enhanced basic nature and morphology of the support. The
catalyst was not only active and selective towards carbonyl
compounds, but also recyclable under the oxidation condi-
tions. This catalyst was also applicable for the oxidation of
other alcohols.
Introduction
The oxidation of alcohols to the corresponding carbonyl com-
pounds without forming over-oxidized or undesired products
is of great importance in both laboratory and industrial appli-
cations. Traditionally, the transformation was carried out by
using stoichiometric, toxic, and expensive oxidants such as
permanganate, chromate, bromate, chromium, or peroxy acids.
More recently, numerous transition-metal-based heterogene-
ous and homogenous catalysts have been widely developed in
an aerobic or anaerobic oxidation manner.[1] However, there
are still some drawbacks in these protocols. Employing a high
concentration of oxidant, base additive, or organic solvent may
lead to environmental problems. The harsh reaction conditions
of high oxygen pressure and/or temperature can cause opera-
tional difficulty and security issues that hinder practical appli-
cation. For an ideal oxidation process, the reaction should be
performed under mild conditions using oxygen or air as the
oxidant in the absence of any solvent or additives.
hols in the absence of solvent.[2] Among the reported catalysts,
palladium catalysts have attracted extensive interest owing to
their excellent activity and selectivity. It is well known that the
composition of the catalyst support plays a crucial role in the
performance of the heterogeneous catalysis. Support selection
and modification has been widely studied to show its great im-
portance for alcohol oxidation. Mori reported hydroxyapatite-
supported palladium nanoclusters,[3,4] which exhibited excel-
lent activity in the solvent-free oxidation of 1-phenylethanol to
give a high turnover number (TON) of up to 236000 with an
excellent turnover frequency (TOF) of 9800 hÀ1. PdO/Al2O3 pre-
pared by wet impregnation[5] and Pd/Al2O3 prepared by ad-
sorption[6] both showed superior activity and selectivity to-
wards solvent-free benzyl alcohol oxidation. Moreover, a re-
markable TOF of 18800 hÀ1 for 1-phenylethanol oxidation was
achieved by zeolite-supported Pd nanoparticles with a mean
diameter of 2.8 nm.[7] Mesoporous SBA-16,[8] surface-modified
TUD-1,[9] and amorphous MnCeOx[10] also showed extraordinary
activity in Pd-catalyzed solvent-free benzyl alcohol oxidation
reactions, with TOF values of 14438, 18571, and 7858 hÀ1, re-
spectively. In addition to the above-mentioned catalyst sup-
ports, carbonaceous materials are also promising palladium
nanoparticles carriers. Pd nanoparticles supported on gra-
phene[11] and functionalized carbon nanotubes[12] showed ex-
cellent activities for solvent-free alcohol oxidation with TOF
values of 30137 and 288755 hÀ1. Doping mesoporous carbon
with nitrogen[13] or boron[14] both gave activity and selectivity
improvement compared with the original carbonaceous mate-
rials. It was suggested that the surface acido-basicity of the
supports played a crucial role.
Over the past few decades, a wide variety of heterogeneous
catalysts have been developed for aerobic oxidation of alco-
[a] Dr. Y.-M. Lu, H.-Z. Zhu, Dr. J.-W. Liu, Prof. Dr. S.-H. Yu
Division of Nanomaterials and Chemistry
Hefei National Laboratory for Physical Sciences at the Microscale
Collaborative Innovation Center of Suzhou Nano Science and Technology
Department of Chemistry
University of Science and Technology of China
Hefei 230026 (P.R. China)
Fax: (+86)551-63603040
E-mail: shyu@ustc.edu.cn
[b] Dr. Y.-M. Lu
School of Life Sciences
Anhui University
Hefei 230601 (PR China)
E-mail: ymlu@ahu.edu.cn
Extensive research on TiO2 materials has resulted in TiO2-de-
rived one-dimension (1D) nanostructures prepared by a hydro-
thermal alkali treatment attracting great attention since the
pioneering work by Kasuga et al.[15,16] Among various potential
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500491.
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applications, efforts have also been made to employ these
structures as supports for catalytic transformations. Mostly as
a result of their high specific surface area and crystallinity, tita-
nate nanotubes have been extensively used to immobilize
metal nanoparticles for catalytic reactions such as benzyl alco-
hol oxidation,[17] reduction of NO,[18] phenol hydrogenation,[19]
double-bond migration,[20] and tetralin hydrogenation.[21]
Chang et al. investigated the morphology-dependent CO oxi-
dation by titanate nanotubes, nanosheets, and their derived
Pd catalysts.[22] The Pd-loaded sheet-like catalyst showed
higher activity in spite of smaller specific surface area. TiO2-
nanobelt-supported noble metal catalysts for aerobic oxidation
of alcohols has been largely explored;[23,24] however, there are
few reports about using titanate nanosheets or nanobelts as
catalyst supports. For example, Shen et al. reported NOx stor-
age and reduction over a potassium titanate nanobelt-based
catalyst.[25] To the best of our knowledge, there are no reports
about solvent-free alcohol oxidation with palladium nanoparti-
cles immobilized on titanate nanobelts.
intercalated water and partial conversion into hexatitanate.[28,29]
Thermogravimetric analysis (TGA) of the titanate nanobelt
(TNB) is also presented (see the Supporting Information, Fig-
ure S1). About 5.3% weight loss below 1258C was seen, which
could be caused by loss of physically adsorbed water. A further
weight loss of about 6.2% between 125 and 4008C could be
attributed to the loss of interlayer and structural water, which
was consistent with the XRD results. The morphology of the ti-
tanate nanobelt was discovered to be uniform, several micro-
metres in length, approximately 30–400 nm in width, and
ꢀ10 nm in thickness (Figure 2A). The morphology and struc-
Herein, we report that titanate and TiO2 nanobelts hydro-
thermally produced in alkali solution can be loaded with Pd
nanoparticles by polyol reduction. The catalytic performance
for aerobic alcohol oxidation in the absence of solvent and ad-
ditives was evaluated to show that titanate nanobelts are
a highly efficient support for Pd-catalyzed solvent-free oxida-
tion of various alcohols.
Results and Discussion
Figure 1 shows the XRD patterns of the as-synthesized nano-
belt support. Complete conversion of commercial TiO2 to a lay-
ered titanate structure was observed with the typical strong
diffraction peak around 2q=108, which indicated a interlayer
distance of approximately 8.85 Š. The synthesized titanates
could be assigned to mainly sodium trititanate (JCPDs No. 31-
1329) with the general formula Na2Ti3O7·nH2O with small ad-
mixtures of Na2Ti4O9(JCPDs No. 33-1294) and Na2Ti9O19 (JCPDs
No. 33-1293).[26,27] After calcination at 5008C, the interlayer dis-
tance was shortened to approximately 7.24 Š with a diffraction
peak at about 2q=128. This could be ascribed to the loss of
Figure 2. A) SEM and B) HR-TEM images of Pd nanoparticles deposited on
calcined titanate nanobelts. C) EDX spectrum and D) size distribution profile
of the catalyst.
ture were found to be well maintained after calcination (see
the Supporting Information, Figure S2). The nitrogen adsorp-
tion–desorption analysis showed that the specific surface areas
of the as-prepared titanate nanobelt and calcined sample were
13.6 and 15.3 m2 gÀ1, respectively (see the Supporting Informa-
tion, Figure S3).
Palladium nanoparticles could be easily deposited on the ti-
tanate nanobelts by the polyol reduction. However, no clear ar-
tifacts assignable to any palladium compounds were observed
in the XRD pattern after the reduction. A mean size of 2.43 nm
was observed in the high-resolution transmission electron mi-
croscopy (HR-TEM) image (Figure 2B,D). From the HR-TEM
image, a lattice fringe of approximately 0.22 nm was clearly
displayed, which is consistent with the (111) plane of palladi-
um nanocrystals. The energy dispersive X-ray spectroscopy
(EDX) pattern is shown in Figure 2C, which confirms the exis-
tence of Pd in the sample. Moreover, Na, Ti, and O signals
were also found, which verifies the formation of the titanate
nanostructure. This was further confirmed by the XPS survey
spectrum (Supporting Information, Figure S4), which clearly
showed the characteristic peaks of Na1s (ꢀ1071 eV), O1s
(ꢀ530 eV), Ti2p (ꢀ458 and 464 eV), and Pd3d (ꢀ337 and
Figure 1. X-ray diffraction patterns of: a) commercial TiO2, b) titanate nano-
belts, and c) titanate nanobelt sample calcined at 5008C.
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342 eV). The deposited palladium species were investigated by
XPS spectroscopy and found to be metallic Pd.
comparison, Pd and PdO nanoparticles were immobilized on ti-
tanate nanobelts by impregnation and they both showed high
benzaldehyde selectivity with no formation of toluene (Table 1,
entries 5 and 6). However, the conversion of benzyl alcohol de-
creased significantly as a result of the larger metal particle
sizes of about 5.04 nm (Figure S5 in the Supporting
Information).
The catalytic activities of different Pd catalysts were evaluat-
ed in the solvent-free oxidation of benzyl alcohol and the re-
sults are listed in Table 1. All Pd catalysts exhibited excellent
Table 1. Catalytic effect of Pd catalysts on different supports in the solvent-free
Blank experiments were also carried out. Only
8.3% conversion, with 81.9% benzylaldehyde selec-
tivity, was achieved in the absence of any catalyst
(Table 1, entry 8). The main byproducts were benzyl
benzoate and hydrobenzoin. When only the titanate
nanobelts were added, the conversion was very slow
but benzaldehyde was found to be the sole product
(Table 1, entry 7). Even if reacted for 16 h, the conver-
sion was still as low as 0.3%. This phenomenon has
also been observed with porous carbon supports.[13]
It indicated that without the Pd active phase, the tita-
nate support showed a similar negative effect on the
oxidation of benzyl alcohol, which might be caused
by the enhanced oxygen adsorption on the supports
and limited oxygen diffusion in the solution.
benzyl alcohol oxidation.[a]
Entry Catalyst[b]
Pd content Conv.[c]
Selectivity [%][c]
TOF[e]
[wt%]
[%]
Benzaldehyde Toluene Others[d] [hÀ1
]
1
2
3
4
5
6
7
8
Pd/TiO2-NB 2.88
44.5
45.5
30.2
45.2
25.5
27.7
0.1
71.7
67.7
88.4
98.5
99.2
96.8
20.7
30.2
5.0
n.d.[f]
n.d.
n.d.
n.d.
n.d.
7.6
2.1
6.6
1.5
0.8
3.2
n.d.
18.1
5563
Pd/TNT
Pd/TNB-u
Pd/TNB
3.12
2.66
2.17
5688
3775
5650
3188
3463
–
Pd/TNB-im 3.72
PdO/TNB
TNB
3.53
–
100
81.9
no catalyst
–
8.3
–
[a] Reaction conditions: Benzyl alcohol (50 mmol), Pd (4”10À3 mmol), O2
(20 mLminÀ1), 1208C, 1 h. [b] Titanate nanobelts with or without calcination, and tita-
nate nanotubes are denoted as TNB, TNB-u, and TNT, respectively. Pd/TNB-im repre-
sented calcined TNB-supported Pd catalyst by impregnation. [c] Determined by GC
analysis. [d] Including benzyl benzoate and hydrobenzoin. Benzoic acid was not de-
tected in the specified reaction time. [e] The turnover frequency was calculated by
the moles of substrate converted per mole of Pd in the whole catalyst per hour.
[f] Not detected.
A recycling study was also carried out with Pd/
TNB. After each reaction the catalyst was easily recov-
ered by centrifugation and used again directly. In this
case, the conversion of benzyl alcohol was 45.2, 40.3,
and 36.8% for the three cycles with a reaction time
activities towards oxidation of benzyl alcohol and benzalde-
hyde was revealed to be the main product. Pd supported on
the TiO2 nanobelt showed conversion of 44.5% and TOF of
5563 hÀ1 at 1208C for benzyl alcohol oxidation (Table 1,
entry 1). However, the selectivity of benzaldehyde was rather
low (71.7%), which was mainly due to the dehydrogenation re-
action. Pd supported on titanate nanotube showed similar
conversion and selectivity (45.5% and 67.7%, respectively;
Table 1, entry 2). When the uncalcined titanate nanobelt pre-
cursor of the TiO2 nanobelt was employed as the catalyst sup-
port, the oxidation of benzyl alcohol was reduced, but the se-
lectivity for benzaldehyde improved and the amount of tolu-
ene as byproduct was inhibited from 20.7% to 5.0% (Table 1,
entry 3). After calcination at 5008C, high activity with a TOF
value of 5650 hÀ1could be obtained over Pd/TNB catalyst
(Table 1, entry 4). In addition, 98.5% benzaldehyde selectivity
was achieved and the production of toluene was completely
suppressed even after reaction for 5 h. It is well known that an
alkaline environment is beneficial to achieve high selectivity in
the oxidation of benzyl alcohol. For example, Au–Pd bimetallic
catalysts supported on ZnO and MgO resulted in no formation
of toluene.[30] The basic nature of the titanate nanobelts makes
the catalyst particularly useful, especially compared with the
extensive and more complicated efforts involving base addi-
tion[31–33] and support modification.[8–10,34] After full water re-
moval by calcination, partial hydrolysis and Na+ exchange in
the titanate nanobelts could be completely inhibited to
strengthen the basicity of the support. Based on these results
Pd/TNB was shown to be the most active and selective catalyst
of the as-prepared samples for benzyl alcohol oxidation. For
of 1 h each, conversion values that indicated that the catalyst
remained active under the reaction conditions. Inductively cou-
pled plasma-atomic emission spectrometry (ICP-AES) analysis
of the reaction solution revealed that 5.4% Pd content was
leached each cycle. Moreover, the tendency of Pd nanoparti-
cles to grow larger was also observed in the TEM image of re-
covered catalyst (Figure S6 in the Supporting Information).
The temperature effect on the solvent-free oxidation of
benzyl alcohol with Pd/TNB catalyst was also examined.
Figure 3 shows the time-dependent conversion, selectivity, and
TOF of the reaction at different temperatures (100, 120, 140,
and 1608C). Both conversion and TOF were expected to in-
crease with elevated reaction temperature. In spite of the
higher catalytic activity at higher temperature, the selectivity
for the corresponding benzaldehyde was found to be worse.
For example, selectivity dramatically dropped to about
85%with a conversion of more than 90% at 140 or 1608C.
However, when the reaction was performed at 1208C, greater
than 95% selectivity was observed throughout the oxidation
process. After 5 h, a selectivity of 95.6% was achieved with an
alcohol conversion of about 92.8%. Notably, benzoic acid was
not detected at any point during the required reaction time at
any of the tested temperatures. This suggested that titanate
nanobelt could suppress the over-oxidation of benzyl alcohol.
The origin of this phenomenon is still under investigation.
Several aromatic alcohols were selected to be tested in the
catalysis and the results are listed in Table 2. All the alcohols
could be oxidized to their corresponding carbonyl compounds
smoothly in the absence of any solvent and additive under an
atmosphere of oxygen. However, the catalytic activity differed
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Table 2. Catalytic performance of various alcohols over Pd catalyst with
molecular O2.[a]
Entry
R1
R2
T
[h]
Conv.
[%]
Selectivity
[%]
TOF
[hÀ1
]
0.5
5
0.5
6
0.5
6
24.2
92.8
42.2
82.6
30.8
73.6
25.9
83.5
2.9
31.1
2.4
9.9
15.1
100
46.5
88.4
47.8
100
99.6
95.6
98.3
93.5
90.9
94.2
86.5
77.4
82.8
47.6
100
75.8
87.8
84.1
100
100
100
6050
1
Ph
H
2320
10550
1721
7700
1533
6475
435
725
162
600
62
3775
2083
11 625
1842
23900
4167
2
4-MePh
4-MeOPh
3-MeOPh
2-MeOPh
4-NO2Ph
PhCH=CH
Ph
H
3
H
0.5
4
H
24
0.5
24
0.5
20
0.5
6
0.5
6
5
H
6[b]
7
H
H
8
Me
Me
0.5
6
9[c]
Ph
100
[a] Reaction conditions: Aromatic alcohol (50 mmol), Pd (4”10À3 mmol),
O2 (20 mLminÀ1), 1208C. Conversion and selectivity was determined by
GC analysis. [b] 1408C. [c] 1608C, Pd (2”10À3 mmol).
The catalyst was also effective for the solvent-free oxidation
of cinnamyl alcohol (Table 2, entry 7). The conversion reached
15.1% with an initial TOF value of 3775 hÀ1 after half an hour
and complete conversion with selectivity of 84.1%could be ob-
tained after 6 h. The main byproduct was found to be benzal-
dehyde, with a selectivity of 10.2%. Enhanced catalytic activity
was also observed in the oxidation of 1-phenylethanol. Quanti-
tative transformation to acetophenone was observed after 6 h
at 1608C with a high initial TOF value of 23900 hÀ1 (Table 2,
entry 9).
Figure 3. Time-dependent catalytic activity at different temperature: squares,
1008C; circles, 1208C; triangles, 1408C; diamonds, 1608C.
Conclusions
depending on the type and position of the substituent. Gener-
ally, aromatic alcohols bearing an electron-donating group ex-
hibited higher activity than those with an electron-withdraw-
ing group. p-Methylbenzyl alcohol and p-methoxylbenzyl alco-
hol presented high activity with initial TOF values of 10550
and 7700 hÀ1, respectively, and both could be oxidized to their
corresponding aldehydes with good conversion after a short
period of reaction (Table 2, entries 2 and 3). However, the Pd
catalyst showed very poor catalytic activity in the oxidation of
p-nitrobenzyl alcohol even after 20 h at 1408C (Table 2,
entry 6). As can be seen from Table 2, m-methoxylbenzyl alco-
hol exhibited almost the same catalytic activity as p-methoxyl-
benzyl alcohol (Table 2, entry 4). However, significant activity
decrease was observed in the oxidation of o-methoxylbenzyl
alcohol (Table 2, entry 5). It is thought that the oxidation path-
way might undergo a chemisorption of alcohol on the metal-
support surface. Hence, increasing steric hindrance might
hinder the interaction of reactant with the active sites.
We have demonstrated that palladium nanoparticles immobi-
lized on titanate nanobelts are highly efficient nanocatalysts in
the solvent-free oxidation of various alcohols. The polyol re-
duction resulted in small nanoparticles with a size of 2.4 nm,
which led to good catalytic performance. The selectivity could
be greatly strengthened by removal of water from the titanate
nanobelts. The enhanced basic nature and morphology of the
support are probably responsible for this effect. Titanate nano-
belts act as a superior support for Pd-catalyzed alcohol oxida-
tion compared with other TiO2 derivatives.
Experimental Section
Characterization
X-Ray powder diffraction patterns (XRD) of the products were ob-
tained on a Japan Rigaku DMax-gA rotation anode X-ray diffrac-
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tometer equipped with graphite-monochromated CuKR radiation
(l=1.54178 Š). X-Ray photoelectron spectroscopy (XPS) was con-
ducted on an ESCALab MKII X-ray photo electron spectrometer
using MgKa radiation exciting source. Scanning electron micro-
scope (SEM) measurements were performed using a Zeiss Supra 40
microscope. Transmission electron microscope (TEM) photographs
were taken on a Hitachi model H-800 transmission electron micro-
scope at an accelerating voltage of 200 kV. Inductively coupled
plasma-atomic emission spectroscopy (ICP-AES) was recorded
using an Atomscan Advantage (Thermo Jarrell Ash Corporation,
U.S.A.) instrument. TGA was measured on Q500IR. Nitrogen physi-
sorption isotherms were measured at À1968C on Tristar II 3020m.
Prior to each measurement, the sample was degassed at 2508C for
12 h under high vacuum. The specific surface area was calculated
by the Brunauer–Emmett–Teller method.
was collected and fresh reactant was added. To analyze Pd leach-
ing during the reaction, the mixed solvent of centrifugate was
evaporated under reduced pressure and the residue was dissolved
in aqua regia (conc. HCl:conc. HNO3 =3:1, v/v) and H2O. This solu-
tion was then analyzed by ICP-AES.
Acknowledgements
We acknowledge funding support from the National Natural Sci-
ence Foundation of China (Grants 21301165, 21431006,
91227103) and the National Basic Research Program of China
(Grants 2013CB933900, 2014CB931800).
Keywords: alcohols · nanobelts · oxidation · palladium ·
titanate
Preparation of the catalysts
Titanate and TiO2 nanobelts were synthesized as reported in the lit-
erature[35,36] with slight modification. Typically, commercially avail-
able TiO2 powder (mainly anatase, 1 g) was suspended in NaOH
aqueous solution (50 mL, 10m) and stirred for 30 min. Then, the re-
sulted milky solution was transferred to a Teflon-lined stainless
steel autoclave with a volume of 80 mL. The autoclave was sealed
and subsequently heated at 2008C for 24 h. After cooling to room
temperature, the slurry was centrifuged and washed thoroughly
with deionized water. The titanate nanobelts were obtained by
drying the collected precipitate at 608C and calcination in air at
5008C for 3 h. The as-synthesized uncalcined and calcined titanate
nanobelt supports are designated as TNB-u and TNB, respectively.
For comparison, the TiO2 nanobelts were prepared by further sus-
pending in HCl (0.1m) overnight followed by drying and calcina-
tion at 7008C. This support was denoted as TiO2-NB.
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polyol reduction. The nanobelt supports (0.2 g) were immersed in
ethylene glycol (30 mL) and stirred at 110 8C in an oil bath for
30 min. Then, ethylene glycol solution (6 mL) containing Na2PdCl4
(18.0 mg) was added dropwise. The solution was further heated for
another 4 h. After cooling to room temperature the products were
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Catalytic alcohol oxidation
The selective alcohol oxidation over supported Pd catalysts in the
absence of solvent was performed under atmospheric conditions.
Typically, a 25 mL three-necked glass flask was precharged with
the catalyst sample (20 mg) and benzyl alcohol (50 mmol) and
fitted with a reflux condenser. The mixture was heated and stirred
vigorously (800 rpm) at 1208C in an oil bath with a continuous
oxygen stream bubbled in at a flow-rate of 20 mLminÀ1. After the
desired reaction time, the catalyst sample was removed by centri-
fugation and the organic products were analyzed by gas chroma-
tography (Agilent 7890A series) equipped with a FID detector and
a HP-5 column (30 mm”0.32 mm”0.25 mm). The structure of the
product was confirmed by comparison with a standard sample and
by GC-MS (Agilent GC-MS 6890/5973).
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