Palladium supported on magnetic core-shell nanoparticles: An efficient and reusable catalyst for the oxidation of alcohols into aldehydes and ketones by molecular oxygen

Article abstract of DOI:10.1002/cplu.201300353

In continuing our efforts to develop economical and ecofriendly synthetic protocols, we present a proline-palladium catalyst supported on magnetic nanoparticles (0.5 mol %) for oxidation by molecular oxygen. The supported Pd catalyst shows high activity in the oxidation of alcohols into the corresponding carbonyl compounds where easy catalyst recovery and excellent recycling efficiency are observed. It is possible to recover and reuse the grafted palladium catalysts at least eight times without significant loss of its catalytic activity. Copyright

Full text of DOI:10.1002/cplu.201300353

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DOI: 10.1002/cplu.201300353
Palladium Supported on Magnetic Core–Shell
Nanoparticles: An Efficient and Reusable Catalyst for the
Oxidation of Alcohols into Aldehydes and Ketones by
Molecular Oxygen
Lei Zhang,[a] Pinhua Li,*[a] Jin Yang,[a] Min Wang,[a] and Lei Wang*[a, b]
In continuing our efforts to develop economical and ecofriend-
ly synthetic protocols, we present a proline–palladium catalyst
supported on magnetic nanoparticles (0.5 mol%) for oxidation
by molecular oxygen. The supported Pd catalyst shows high
activity in the oxidation of alcohols into the corresponding car-
bonyl compounds where easy catalyst recovery and excellent
recycling efficiency are observed. It is possible to recover and
reuse the grafted palladium catalysts at least eight times with-
out significant loss of its catalytic activity.
catalyst separation and recycle, and heavy-metal contamina-
tion of the product, and are therefore of particular environ-
mental and economic concerns in large scale preparation. To
overcome these problems, the development of efficient sup-
ported catalysts, which can be recycled and reused while keep-
ing their inherent catalytic activity, is highly desirable, and is
still a hot topic in current organic chemistry. Various inorganic
and organic supports, such as mesoporous silica, ionic liquids,
and polymers have been explored to support Pd for the oxida-
tion of alcohols.[7] For example, Uozumi and Nakao have re-
ported an efficient amphiphilic resin-supported Pd nanoparti-
cles for the oxidation of benzylic alcohols in water.[8] After that,
Karimi et al. have also prepared a highly ordered mesoporous
silica (SBA-15) supported bipyridyl PdII complex and used it for
the oxidation of benzyl alcohols into the corresponding car-
bonyl compounds in excellent yields under atmospheric pres-
sure of molecular oxygen.[9] These two approaches represent
the most highly efficient strategies in this field, while the sepa-
ration of the catalyst is not convenient, and developing more
efficient and practical catalysts for the oxidation of alcohols are
still demanded.
The selective oxidation of alcohols into the corresponding car-
bonyl compounds is a fundamental organic transformation,
and has attracted much attention both in organic synthesis
and in industrial processes, owing to the wide-ranging utility
of these compounds as important precursors and intermedi-
ates for drugs, fragrances, and fine chemicals.[1] Traditionally, al-
cohols are oxidized by toxic, corrosive, and expensive oxidants
such as hypervalent iodines,[2] DMSO-coupled reagents,[3] CrVI,[4]
permanganate,[1a–c] and other heavy-metal reagents.[1,4] These
protocols usually require stoichiometric amounts of oxidants
or additives, which in many cases lead to the generation of
a large amount of chemical wastes. Hydrogen peroxide is
a clean oxidant, while it is relatively less practical because of
its safety problems in terms of transportation and storage.[5]
Owing to increasing environmental concerns, transition-metal-
catalyzed oxidation of alcohols using molecular oxygen as oxi-
dant is particularly attractive because it is atom economic and
only produces water as the by-product. In the past decades,
many metal catalysts, such as Pd, Ru, Co, Cu, Au, Pt, and Rh
have been developed for the oxidation of alcohols using mo-
lecular oxygen.[6] However, homogeneous catalysis suffers from
Magnetic nanoparticles have been studied widely for various
applications,[10] and have emerged as smart and promising
supports because catalysts supported on magnetic nanoparti-
cles can be easily and simply recovered by magnetic separa-
tion and without filtration.[11] Recently, several palladium cata-
lysts supported on magnetic nanoparticles have been devel-
oped for the construction of carbon–carbon and carbon–heter-
oatom bonds, however, there are few reports on the oxidation
of alcohols by molecular oxygen in the presence of this type
of supported palladium catalysts.[12] It should be noted that
the hypervalent iodine reagent TEMPO and Au catalysts each
supported on magnetic nanoparticles have also been used for
the oxidation of alcohols, along with the regeneration of the
oxidant, addition of co-catalyst, expensive precursor, or low re-
usability of the catalyst have occurred in some cases.[13]
[a] L. Zhang, Prof. P. Li, Dr. J. Yang, Dr. M. Wang, Prof. L. Wang
Department of Chemistry
Huaibei Normal University
Huaibei, Anhui 235000 (P. R. China)
Fax: (+86)561-380-3233
In continuing our efforts to develop economical and eco-
friendly synthetic protocols for organic transformations from
the view of sustainable chemistry,[14] herein we report a pro-
line–palladium catalyst supported on magnetic nanoparticles
for the oxidation of alcohols by molecular oxygen. The sup-
ported Pd catalyst shows high activity in the oxidation of alco-
hols (Scheme 1). In particular, easy catalyst recovery and excel-
lent recycling efficiency of the catalyst make it an ideal proto-
col for the oxidation of alcohols. It is possible to recover and
E-mail: pphuali@126.com
[b] Prof. L. Wang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences
Shanghai 200032 (P. R. China)
Fax: (+86)561-309-0518
E-mail: leiwang@chnu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300353.
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Scheme 1. Preparation of the palladium catalyst supported on magnetic
nanoparticles and its application in the oxidation of alcohols by molecular
oxygen. a) Si(OEt)4, EtOH; b) (4-chloromethyl)phenylethyltrimethoxysilane,
toluene; c) tert-butyl-N-boc-4-hydroxyprolinate, NaH, THF; d) TFA, CH2Cl2;
e) Pd(OAc)2, THF. Pro=proline, TFA=trifluoroacetic acid.
reuse the grafted palladium catalysts at least eight times with-
out significant loss of its catalytic activity.
Figure 1. TEM images: (a) SiO2@Fe3O4–Pro–Pd catalyst, (b) recycled
SiO2@Fe3O4–Pro–Pd catalyst.
The palladium catalyst supported on magnetic nanoparticles
was synthesized according to the procedure shown in
Scheme 1. The silica-coated Fe3O4 (SiO2@Fe3O4) was prepared
according to the literature.[15] The prepared Fe3O4 nanoparti-
cles, with an average diameter of 140 nm, were coated with
a thin layer of silica using a sol-gel process to give silica-
coated Fe3O4. The TEM images of the SiO2@Fe3O4 indicate the
core–shell structure of the particles, and the silica coating,
which has a uniform thickness of 20 nm (Figure 1). The benzyl
chloride could be anchored easily onto the surface of the
SiO2@Fe3O4 using (4-chloromethyl)phenylethyl trimethoxysilane
at reflux in toluene for 24 hours, with a loading of 1.22 mmol
of benzyl chloride per gram, which was quantified by CHN mi-
croanalysis based on carbon content determination. Immobili-
zation of the proline moieties was carried out by the nucleo-
philic substitution reaction of tert-butyl-N-boc-4-hydroxyproli-
nate with the functionalized magnetic core–shell nanoparticles
in the presence of NaH in anhydrous THF. Then the above
nanoparticles were further treated with TFA in dichlorome-
thane to remove the Boc and tert-butyl groups to form the
proline-loaded magnetic nanoparticles, with a loading of
1.05 mmol of proline per gram, which was quantified by CHN
microanalysis on the basis of nitrogen content determination.
The supported Pd catalyst was obtained by dissolving
Pd(OAc)2 in THF and treating it with the above proline-func-
tionalized SiO2@Fe3O4, with a loading of 0.106 mmol of palladi-
um per gram, which was determined by inductively coupled
plasma atomic emission spectrometry (ICP-AES). The X-ray dif-
fraction (XRD) measurements of the supported palladium cata-
lyst exhibits diffraction peaks corresponding to the typical
Figure 2. XRD of the samples: (a) SiO2@Fe3O4–benzyl chloride, (b) SiO2@
Fe3O4–Pro, (c) SiO2@Fe3O4–Pro–Pd.
spinel maghemite structure, meanwhile the diffraction peak of
the layered amorphous silica was not obvious and no peaks
characteristic for Pd0 nanoparticles were observed (Figure 2).
The obtained supported palladium catalyst was also verified by
X-ray photoelectron spectroscopy (XPS) determination. The
electron binding energy analysis indicated that the oxidation
state of palladium in freshly prepared catalyst was PdII, which
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Table 1. Optimization of reaction conditions.[a]
Entry
1
Base
Solvent
toluene
Yield [%][b]
K2CO3
96
51[c]
0[d]
90[e]
94
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
K3PO4
tBuOK
Na2CO3
Cs2CO3
KOAc
DBU
toluene
toluene
toluene
toluene
toluene
toluene
toluene
benzene
DMSO
n-hexane
DMF
CH3CN
dioxane
THF
92
57
49
47
37
19
81
69
61
57
40
37
34
29
84[f]
96[g]
56[h]
96[i]
61[j]
96[k]
Et3N
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Figure 3. XPS of the supported catalysts: (a) SiO2@Fe3O4–Pro–Pd; (b) recycled
SiO2@Fe3O4–Pro–Pd.
coordinated to nitrogen and oxygen atoms. However, the oxi-
dation state of palladium in the used catalyst was partly
changed from PdII to Pd0 and its other oxidation state
(Figure 3).
H2O
toluene
18
19
K2CO3
K2CO3
toluene
toluene
The activity of the supported palladium catalyst was first in-
vestigated for the oxidation of benzyl alcohol by molecular
oxygen in toluene at 908C in the presence of K2CO3, and 96%
yield of benzaldehyde was isolated after 10 hours with more
than 99% selectivity (Table 1, entry 1). No formation of the
overoxidation product benzoic acid and its corresponding
ester was observed. However, when the reaction was conduct-
ed in air, only 51% yield of benzaldehyde was obtained. Mean-
while, when the model reaction was carried out under nitro-
gen, no desired product was observed. On the other hand,
when Pd(OAc)2 (0.5 mol%) was used in the reaction instead of
SiO2@Fe3O4–Pro–Pd catalyst, 90% yield of benzaldehyde was
isolated. Different bases and solvents for the oxidation of
benzyl alcohol using the supported palladium catalyst
(0.5 mol%) were screened. Bases screening showed that K2CO3,
K3PO4, and tBuOK gave 96%, 94% and 92% yields of benzalde-
hyde, respectively (Table 1, entries 1–3). Meanwhile Na2CO3,
Cs2CO3, KOAc, DBU, and NEt3 were less effective bases (Table 1,
entries 4–8). On the other hand, toluene was the best solvent
among the tested solvents, including benzene, DMSO, n-
hexane, DMF, CH3CN, dioxane, THF, and H2O (Table 1, entries 9–
16). As for the reaction temperature, 908C was optimal and
the reaction was generally completed in 10 hours. The amount
of supported palladium catalyst was also screened, and
0.5 mol% loading of palladium was found to be optimal
(Table 1, entries 17–19). In addition, the model reaction carried
out under the model reaction conditions without substrate
(benzyl alcohol) indicated that there was no oxidation of tolu-
ene (solvent) to benzyl alcohol or benzaldehyde formation.
With the optimized reaction conditions in hand, a series of
alcohols were then oxidized under atmospheric oxygen in the
presence of 0.5 mol% of the supported palladium catalyst. The
results were listed in Scheme 2. The substituted benzyl alco-
[a] Reaction conditions: benzyl alcohol (1.0 mmol), SiO2@Fe3O4–Pro–Pd
catalyst (50 mg, contained Pd 0.005 mmol, 0.5 mol%), K2CO3 (1.0 mmol),
solvent (2.0 mL) at 908C under oxygen and stirred for 10 h. [b] Yield of
isolated product. [c] Under air for 10 h. [d] Under nitrogen for 10 h.
[e] Pd(OAc)2 (0.5 mol%) instead of SiO2@Fe3O4–Pro–Pd catalyst. [f] 808C
for 10 h. [g] 1008C for 10 h. [h] 0.25 mol% of SiO2@Fe3O4–Pro–Pd catalyst
was used. [i] 1.0 mol% of SiO2@Fe3O4–Pro–Pd catalyst was used. [j] 908C
for 5 h. [k] 908C for 15 h. DBU=1,8-diazabicyclo[5.4.0]undec-7-ene,
DMF=N,N-dimethylformamide, DMSO=dimethyl sulfoxide.
hols, except for 4-dimethylaminobenzyl alcohol, with both
electron-donating and electron-withdrawing groups at the
para or meta positions, were oxidized smoothly under the
standard reaction conditions and afforded the corresponding
aldehydes 2a–r in excellent yields without the formation of
carboxylic acids. Meanwhile, the oxidation of 2-substituted
benzylic alcohols gave the corresponding aldehydes 2s–v in
72–80% yields. It was worthy noting that 1-naphthalenemetha-
nol, 2-naphthalenemethanol, 9-anthracenemethanol, 2-thio-
phenemethanol, and cinnamyl alcohol could also be oxidized
under the present reaction conditions and generated the cor-
responding aldehydes 2v–aa in high yields (81–97%). In addi-
tion, aliphatic alcohols, such as 3-methyl-1-butanol, 1-butanol,
(E)-2-hexen-1-ol, 4-methylpentan-2-ol, and cyclohexanol could
also participate in the reaction smoothly and gave the corre-
sponding products 2ab–af in moderate yields (51–57%). It is
important to note that this catalyst was also effective for the
oxidation of secondary benzylic alcohols and the correspond-
ing ketones 2ag–ai were obtained in good yields.
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Table 2. The reuse of the recycledSiO2@Fe3O4–Pro–Pd catalyst.[a]
Run
Yield [%][b]
Run
Yield [%][b]
1
2
3
4
96
95
94
94
5
6
7
8
93
93
92
90
[a] Reaction conditions: benzyl alcohol (1.0 mmol), reused SiO2@Fe3O4–
Pro–Pd catalyst (50 mg, contained Pd 0.005 mmol, 0.5 mol%), K2CO3
(1.0 mmol) in toluene (2.0 mL) at 908C under oxygen and stirred for 10 h.
[b] Yield of isolated product.
activity of the supported catalyst was almost maintained and it
could be recycled and reused for eight consecutive trials with-
out significant loss of its catalytic activity (Table 2). Moreover,
no formation of agglomerated palladium was detected after
the second cycle as seen by the TEM image (Figure 1). The
amount of palladium leaching in the supported catalyst was
also determined by ICP-AES analysis, and the result indicated
that Pd concentration was less than 0.20 ppm in the reaction
solution (2.0 mL). Furthermore, to ensure that the reaction was
truly heterogeneous, a hot filtration test (at 908C, ca. 50% con-
version of substrate) was performed. However, no additional
amount of the desired product was obtained for the model re-
action by adding substrates to a filtrate obtained from the hot
filtration of the supported palladium catalyst. This result con-
firmed the heterogeneous character of the catalytically active
species in this reaction.
In conclusion, a novel and highly efficient palladium catalyst
supported on magnetic nanoparticles was developed for the
oxidation of alcohols. Substituted primary and secondary
benzyl alcohols could be oxidized smoothly to afford the corre-
sponding aldehydes and ketones in good to excellent yields in
the presence of supported palladium catalyst (0.5 mol%) in tol-
uene by molecular oxygen. Importantly the catalyst is readily
prepared, easily recoverable, and reusable for at least eight
cycles without significant loss of its activity.
Scheme 2. SiO2@Fe3O4–Pro–Pd catalyzed oxidation of alcohols. [a] Reaction
conditions: alcohol (1.0 mmol), SiO2@Fe3O4–Pro–Pd catalyst (50 mg, con-
tained Pd 0.005 mmol, 0.5 mol%), K2CO3 (1.0 mmol), toluene (2.0 mL) at
908C under oxygen for 10 h. [b] Yield of isolated product. [c] 1.0 mol% of
SiO2@Fe3O4–Pro–Pd catalyst was used.
Experimental Section
General procedure for the aerobic oxidation of alcohols
An alcohol (1.0 mmol), base (1.0 mmol), and SiO2@Fe3O4–Pd cata-
lyst (50 mg, contained Pd 0.005 mmol) were added to a Schlenk
tube. Anhydrous solvent (5.0 mL) was added by syringe and the re-
sulting solution was degassed twice and refilled with O2 (1.0 atm).
The mixture was then stirring (rate 500 rpm) at 908C for 10 h. Next
the catalyst was separated by using an external magnet, the cata-
lyst was washed three times with H2O (10.0 mL), C2H5OH (10.0 mL),
and C2H5OC2H5 (5.0 mL), respectively and used directly for the next
run. The organic phase was evaporated under reduced pressure
and the product was purified by column chromatography on silica
gel.
The recovery and reuse of the developed supported palladi-
um catalyst was examined. Pleasingly the catalyst could be effi-
ciently and simply recovered by external magnetic separation
(Figure S1 in the Supporting Information). After seperation, the
catalyst was washed with water, ethanol, and then diethyl
ether, dried in air and reused for the next run. For the model
reaction of oxidation of benzylic alcohol, it was found that the
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Synthesis of magnetic nanoparticles (Fe3O4)
Synthesis of SiO2@Fe3O4–Pd
The magnetic microspheres were synthesized according to the lit-
erature.[15] FeCl3·6H2O (5.0 g) was dissolved in ethylene glycol
(100 mL) with stirring for 1.0 h and then NaOAc (11.0 g) was added
to the solution. After stirring at a rate of 500 rpm for 1.0 h, the re-
sultant solution was transferred into a Teflon-lined stainless-steel
autoclave (250 mL). The autoclave was sealed and heated at 1808C
for 12 h, then cooled to RT. The magnetic microspheres were sepa-
rated by using an external magnet, washed three times with
C2H5OH (15.0 mL) and C2H5OC2H5 (10.0 mL), respectively and dried
under vacuum at 508C for 10 h.
Palladium acetate (22.4 mg, 0.10 mmol) and THF (15.0 mL) were
added to a sealable reaction tube. The solution was shaking at RT
for 10 min, and then the above functionalized magnetic nanoparti-
cles (SiO2@Fe3O4-proline, 1.0 g) were added. The mixture was shak-
ing at RT for 5 h, then the catalyst was magnetically separated by
using an external magnate, and the solid was washed three times
with THF (10.0 mL) and dried under vacuum at 508C for 5 h. The
SiO2@Fe3O4–Pd was obtained with a loading of 0.106 mmol of pal-
ladium per gram as determined by ICP-AES analysis.
Acknowledgements
Synthesis of silica-coated magnetic nanoparticles
(SiO2@Fe3O4)
We gratefully acknowledge the financial support from the Na-
tional Natural Science Foundation of China (nos. 21372095,
21172092, 21002039), and the Science and Technology Project of
the Department of Anhui Provincial Education (nos. KJ2010ZD09,
KJ2012A253).
The magnetic nanoparticles (0.20 g) were first treated with an
aqueous solution HCl of (5.0 mL, 2m) under ultrasound for
5 min.[15] Then the particles were washed with deionized H2O
(25.0 mL) until the solution was neutral. Immediately the magnetic
nanoparticles and NH3·H2O (2.0 mL, 28% aq.) were added to a mix-
ture of deionized H2O (20.0 mL) and C2H5OH (90.0 mL), then the
mixture was sonicated for approximately 1.0 h. To this well-dis-
persed solution of magnetic nanoparticles, Si(OEt)4 (1.0 g) was
added slowly. Then the obtained mixture was stirred (rate
500 rpm) at RT for 10 h. Finally, the product was washed with de-
ionized H2O (25.0 mL) until the filtrate was neutral, then washed
three times with C2H5OH (15.0 mL) and C2H5OC2H5 (10.0 mL), re-
spectively and dried under vacuum at 508C for 10 h.
Keywords: homogeneous catalysis · magnetic nanoparticles ·
oxidation · palladium · sustainable chemistry
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a suspension of
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Received: October 16, 2013
Published online on November 22, 2013
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