Palladium supported on an acidic metal-organic framework as an efficient catalyst in selective aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c2gc36618e

Highly dispersed palladium nanoparticles were deposited on an acidic MOF (MIL-101) by using a simple colloidal method. The resulting Pd/MIL-101 catalyst was highly active in the liquid-phase aerobic oxidation of a wide range of alcohols including benzyl, allylic, aliphatic and heterocyclic alcohols as well as diols, affording the desired oxidation products in high yields under base-free and mild conditions. The catalyst was shown to be able to efficiently catalyze aerobic oxidation even at ambient temperature using air instead of pure O₂. The solvent-free oxidation of benzyl alcohol gave a remarkably high turnover frequency (TOF) of approximately 16900 h^-1. However, the catalytic activity was significantly suppressed when ethylenediamine was grafted on the uncoordinated Cr sites of the MIL-101 support, which suggests that the open Cr sites might play an important role in promoting the oxidation of alcohols in the present catalytic system.

Full text of DOI:10.1039/c2gc36618e

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Palladium supported on an acidic metal–organic
framework as an efficient catalyst in selective aerobic
Cite this: DOI: 10.1039/c2gc36618e
oxidation of alcohols
Gongzhou Chen, Shijian Wu, Hongli Liu, Huanfeng Jiang and Yingwei Li*
Highly dispersed palladium nanoparticles were deposited on an acidic MOF (MIL-101) by using a simple
colloidal method. The resulting Pd/MIL-101 catalyst was highly active in the liquid-phase aerobic oxi-
dation of a wide range of alcohols including benzyl, allylic, aliphatic and heterocyclic alcohols as well as
diols, affording the desired oxidation products in high yields under base-free and mild conditions. The
catalyst was shown to be able to efficiently catalyze aerobic oxidation even at ambient temperature
using air instead of pure O2. The solvent-free oxidation of benzyl alcohol gave a remarkably high turn-
over frequency (TOF) of approximately 16 900 h−1. However, the catalytic activity was significantly sup-
pressed when ethylenediamine was grafted on the uncoordinated Cr sites of the MIL-101 support, which
suggests that the open Cr sites might play an important role in promoting the oxidation of alcohols in
the present catalytic system.
Received 14th October 2012,
Accepted 14th November 2012
DOI: 10.1039/c2gc36618e
www.rsc.org/greenchem
Metal–organic frameworks (MOFs) are a new class of
porous materials assembled with metal ions and organic
Introduction
Selective oxidation of alcohols is an important transformation linkers.27 Owing to their high surface area, porosity, and
in the chemical industry.1,2 The traditional oxidizing reagents, chemical tunability, the applications of MOFs in hetero-
which have usually required stoichiometric amounts of metal geneous catalysis have recently attracted tremendous atten-
oxidants such as chromate or permanganate, are expensive tion, especially, those using MOFs as a support for metal
and have serious toxicity issues associated with them.3,4 With (e.g., Pd, Au, Ru, and Pt) nanoparticles.28–41 A few MOF-
ever-increasing environmental concerns, there is substantial supported noble-metal (e.g., Au, Pt) nanoparticles have also
interest in the development of heterogeneous catalytic systems been investigated as active heterogeneous catalysts in the
that use molecular oxygen as the oxidant.5 Accordingly, a aerobic oxidation of alcohols,42,43 even under base-free
remarkable number of transition-metal nanoparticles such as conditions.35
palladium,6–8 gold,9–17 platinum,18 and rhodium,19 have been
Here we report a new and highly active heterogeneous Pd
used as catalysts for the aerobic alcohol oxidation processes. catalyst, which was deposited on a zeolite-type MOF (i.e.
Although some of the metal catalysts have been shown to be MIL-101) using a simple colloid method. The supported Pd
effective for the aerobic oxidation reactions, in most cases they nanoparticles have been shown to be highly efficient in the
have been applied at temperatures above 100 °C or in the pres- liquid-phase aerobic oxidation of a variety of alcohols in the
ence of a large excess of base additives.20–26 Moreover, most of absence of base under mild conditions. We have chosen
the catalysts can be applied only to a very limited range of sub- MIL-101, Cr3F(H2O)2O[(O2C)–C6H4–(CO2)]3,44 because it has
strates under base-free conditions.20–22 Therefore, the develop- a large surface area and pore size as well as a good water
ment of excellent reusable catalysts for the liquid-phase stability, which jointly make it an attractive support for
aerobic alcohol oxidation that exhibit a wide range of substrate depositing small palladium nanoparticles. On the other
tolerance under mild and base-free conditions is still challen- hand, MIL-101 possesses numerous potentially unsaturated
ging in both green chemistry and organic synthesis.
chromium sites (up to 3.0 mmol g−1) upon removal of the
terminal water molecules.30,45,46 The significant Lewis acidity
of the MIL-101 support has been demonstrated to play sig-
nificant roles in promoting the reactivity of aromatic sub-
strates.40,46 The effect of the open chromium sites of the
support on the reactivity of alcohol oxidation was also investi-
gated in this work.
Key Laboratory of Fuel Cell Technology of Guangdong Province and The Key Lab of
Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of
Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou 510640, China. E-mail: liyw@scut.edu.cn
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and Table 1. The appreciable decreases in nitrogen adsorption
amount and surface area indicate that the cavities of MIL-101
Results and discussion
The powder XRD pattern of the as-synthesized MIL- are occupied by highly dispersed Pd nanoparticles or/and
101 matches well with the already published XRD patterns blocked by Pd nanoparticles that are deposited at the pore
(Fig. 1). No apparent loss of crystallinity in the XRD patterns surface. In addition, the possible residual PVA (added as a pro-
was found upon ethylenediamine grafting on MIL-101. After tecting agent in the course of preparation) in the samples
the incorporation of Pd nanoparticles, the structure of could also have some impact on the gas adsorption capacities
MIL-101 is mostly preserved regardless of the metal content in of the catalysts.
the materials. Moreover, no diffractions for Pd nanoparticles
The XPS spectra of the 0.35% Pd/MIL-101 and 0.35% Pd/
were detected, which imply that the Pd content is too low or En-MIL-101 samples are shown in Fig. 4. The two samples
Pd particles are too small in all samples. As indicated in the
TEM images, Pd nanoparticles were well dispersed on MIL-101
with a uniform size of 2.5 0.5 nm (Fig. 2).
The specific surface areas of the samples were measured by
N2 adsorption at 77 K and the results are presented in Fig. 3
◆
Fig. 3 Nitrogen adsorption isotherms at 77 K of the as-synthesized MIL-101 ( ),
■
●
▲
0.35% Pd/MIL-101 ( ), 0.59% Pd/MIL-101 ( ), and 0.35% Pd/En-MIL-101 ( ).
Table 1 Surface areas and pore volumes of the Pd/MIL-101 samples
Sample
SBET (m2 g−1) SLangmuir (m2 g−1) Vpore (cm3 g−1
)
Fig. 1 Powder XRD patterns of MIL-101 samples: (a) MIL-101; (b) 0.15% Pd/
MIL-101; (c) 0.35% Pd/MIL-101; (d) 0.59% Pd/MIL-101; (e) 0.94% Pd/MIL-101;
(f) 0.35% Pd/MIL-101 after five runs; (g) 0.35% Pd/En-MIL-101.
MIL-101
2869
2506
2412
2084
2029
4874
3947
3875
2648
2581
2836
1.59
1.36
1.25
1.11
1.07
1.18
0.15% Pd/MIL-101
0.35% Pd/MIL-101
0.59% Pd/MIL-101
0.94% Pd/MIL-101
0.35% Pd/En-MIL-101 2173
Fig. 2 TEM images (a–c) of 0.35% Pd/MIL-101 and corresponding size distri-
Fig. 4 XPS spectra of 0.35% Pd/MIL-101 and 0.35% Pd/En-MIL-101 catalysts:
bution of Pd nanoparticles (d). The inset in (c) is the EDX pattern.
(a) Pd3d; (b) Cr2p; (c) survey spectra of 0.35% Pd/En-MIL-101.
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Table 2 Results of the oxidation of cinnamyl alcohola
both exhibit Pd 3d5/2 bands at ca. 335–336 eV (Fig. 4a), typical
of Pd metal,47 indicating that the Pd(II) cations were trans-
formed to Pd(0) after reduction by sodium borohydride. No
appreciable differences were observed in the binding energies
and intensities of the Pd(3d) spectra for the two materials. The
results suggest that the ethylenediamine molecules were not
grafted on the Pd nanoparticles nor had a strong interaction
with Pd.
Entry Catalyst
Solvent
Time (h) Conv./sel. (%)
1
2
3
4
5
6
7
8
9
10
MIL-101
Toluene
Toluene
Toluene
Toluene
Toluene
DMF
o-Xylene
DMSO
Isooctane 0.5
Isooctane 0.5
+ toluene
3
<3/98
75/97
99/99
93/99
90/99
74/99
97/99
6/95
0.15% Pd/MIL-101
0.35% Pd/MIL-101
0.59% Pd/MIL-101
0.94% Pd/MIL-101
0.35% Pd/MIL-101
0.35% Pd/MIL-101
0.35% Pd/MIL-101
0.35% Pd/MIL-101
0.35% Pd/MIL-101
0.5
0.5
0.75
0.75
1
0.5
1
Fig. 4b presents the Cr(2p) spectra of Pd/MIL-101 and
Pd/En-MIL-101. It can be seen that the Cr(2p) spectrum of the
amine-grafted material is less intense compared to Pd/
MIL-101. In addition, the Cr(2p) peaks of Pd/En-MIL-101 are
shifted by ca. 0.6 eV toward lower binding energies, compared
to those of the pristine Pd/MIL-101. Such shifts reflect an
increase in the electron density of Cr that may be attributed to
the metal–ethylenediamine interactions on the Pd/En-MIL-101
sample. The incorporation of ethylenediamine in the sample
was also confirmed by the presence of a well-defined band
corresponding to N(1s) in the survey spectrum (Fig. 4c).
The infrared spectra of the 0.35% Pd/MIL-101 and Pd/En-
MIL-101 samples are shown in Fig. 5. The γ(N–H) and γ(C–H)
stretching regions observed for Pd/En-MIL-101 indicate the
presence of ethylenediamine by comparing with the spectrum
of ethylenediamine in the liquid phase (not shown here). The
aliphatic C–H stretching vibrations (at 2800–3000 cm−1) are
shifted to higher wavenumbers (from 2848 and 2924 cm−1 to
2860 and 2931 cm−1, respectively), as observed when the mole-
cule is coordinated to a Lewis acid center,48 further demon-
strating the successful grafting of ethylenediamine onto the
coordinately unsaturated Cr sites of the MIL-101.
The aerobic oxidations of alcohols were carried out under
atmospheric O2 pressure and the prepared Pd/MIL-101
samples with different Pd loadings were used as catalysts. The
results of the oxidation of cinnamyl alcohol are summarized
in Table 2. The use of the parent MIL-101 gave essentially no
conversion in this reaction system (Table 2, entry 1), confirm-
ing as expected the need of a metal to perform the oxidation
of alcohol. The incorporation of Pd nanoparticles significantly
promoted the reaction. Results of the aerobic oxidation
61/98
78/99
(1 : 1)
11
0.35% Pd/En-MIL-101 Toluene
0.35% Pd/En-MIL-101 Toluene
3
3
2
45/99
95/99
23/99
12b
13
0.35% Pd/AC
Toluene
a Reaction conditions: cinnamyl alcohol (1 mmol), catalyst (Pd
1 mol%), solvent (10 mL), 80 °C, 1 atm, O2 bubbling rate (20 mL
min−1). b 1 eq. NaOH was added.
pointed to an optimized performance of 0.35% Pd/MIL-101
(Table 2, entry 3), which provided a complete conversion of
cinnamyl alcohol to cinnamyl aldehyde within 0.5 h at 80 °C.
With the 0.35% Pd/MIL-101 catalyst, various solvents were sub-
sequently screened, which indicated that toluene was the best
solvent for this transformation under the conditions (Table 2,
entries 6–10). In principle, the conversion increased with a
decrease in the polarity of solvent. The relatively low activity of
the reaction in isooctane (with a low polarity) may be attribu-
ted to the poor solubility of cinnamyl alcohol in this solvent.
The activity could be enhanced remarkably when toluene was
added in this system to improve the solubility of cinnamyl
alcohol in the mixed solvents (Table 2, entry 10).
For comparison, the amine grafted sample 0.35% Pd/En-
MIL-101 was also tested in the aerobic oxidation of cinnamyl
alcohol under the optimized reaction conditions. The use of
En-grafted MIL-101 as support led to inferior activity in the oxi-
dation (Table 2, entry 11), giving only 45% yield of cinnamyl
aldehyde even though the reaction time was prolonged to 3 h.
Nevertheless, when a small amount of NaOH was added to the
reaction system, the activity was restored significantly, furnish-
ing 95% conversion of alcohol within 3 h (Table 2, entry 12).
As is well known, the addition of base additives may promote
the oxidation of alcohols by facilitating the dehydrogenation of
alcohols.43 Therefore, this result implies that most of the Pd
active sites were not covered by the ethylenediamine mole-
cules, in agreement with the XPS data (Fig. 4a). The remark-
ably reduced activity of Pd/MIL-101 upon amine grafting on
the unsaturated chromium sites of MIL-101 suggests that the
uncoordinated Lewis acidic Cr sites have played an important
role in the alcohol oxidation, which correlates well with the
low activity observed for Pd/AC under the same reaction con-
ditions (Table 2, entry 13). These findings also support pre-
vious findings of the group related to a promoting effect
Fig. 5 Infrared spectra of (a) Pd/En-MIL-101 and (b) Pd/MIL-101.
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Table 3 Aerobic oxidation of various alcohols catalyzed by Pd/MIL-101a
observed in MIL-101-supported metal nanoparticles for the
conversions of aromatics.40,46
Entry
1
Substrate
Time (h)
1.5
Conv./sel. (%)
99/99
The scope of the presented catalyst system was then sub-
sequently extended to the aerobic oxidation of a large variety
of alcohols under base-free conditions using the optimum
0.35% Pd/MIL-101 catalyst. The results summarized in Table 3
show that Pd/MIL-101 was highly active and extremely selective
for the oxidation of all substrates, indicating a high versatility
of the MOF-supported Pd catalyst. Substituted benzyl alcohols
containing electron-donating groups such as CH3, OCH3, or
OH, (Table 3, entries 1–4) are more easily oxidized than those
containing electron-withdrawing groups (Table 3, entry 5). Sec-
ondary benzylic alcohols were also converted to the corres-
ponding ketones in quantitative yields (Table 3, entry 6). A
complete conversion of allylic alcohols, for example, cinnamyl
alcohol and trans-2-hexen-1-ol, to the desired aldehydes was
achieved within 1 h (Table 3, entries 7 and 8). The CvC
double bonds remained intact without an intramolecular
hydrogen transfer over Pd/MIL-101 under the investigated
conditions.
2
3
4
0.75
1
99/99
99/99
99/99
1.5
5
6
4.5
1.5
97/99
99/99
7
0.5
99/99
Aliphatic alcohols have been reported to be difficult com-
pounds to undergo oxidation.9,35 Our Pd/MIL-101 catalyst was
also applicable to the oxidation of various aliphatic alcohols
including primary and secondary linear, and cyclic aliphatic
alcohols, to afford the corresponding aldehydes or ketones in
excellent yields within short reaction times under mild and
base-free conditions (Table 3, entries 9–11). Heterocyclic alco-
hols, such as 2-pyridinemethanol and furfuryl alcohol, were
also smoothly oxidized into the corresponding aldehydes in excel-
lent yields (Table 3, entries 12 and 13). The Pd/MIL-101 catalyst
was also active in the oxidation of diols, furnishing the corres-
ponding lactones in high yields (Table 3, entries 14 and 15).
Encouraged by these promising results, we further exam-
ined the catalytic activity of Pd/MIL-101 for the aerobic oxi-
dation of cinnamyl alcohol using air instead of pure O2 at
ambient temperature. Cinnamyl alcohol was selectively oxi-
dized into cinnamyl aldehyde in the absence of water or base,
furnishing a complete conversion with 99% selectivity after
24 h of reaction (Table 3, entry 16). We also investigated the
oxidation of cinnamyl alcohol on the 10 mmol scale, which
gave a complete conversion with 98% selectivity for cinnamyl
aldehyde (isolated yield: 94%) within 3 h (Table 3, entry 17).
The recyclability of the Pd/MIL-101 catalyst was examined
in the aerobic oxidation of cinnamyl alcohol under the opti-
mized conditions. After the catalytic reaction, the catalyst was
isolated from the liquid phase by centrifugation, thoroughly
washed with ethanol, and then reutilized as catalyst in sub-
sequent runs under identical reaction conditions. The results
8
1
99/99
95/96
99/98
9b
10b
10
5
11b
12b
6
4
96/99
99/98
13b
14c
4
4
99/98
99/90
15c
16d
6
99/88
99/99
24
17e
3
99/98
a Reaction conditions: alcohol (1 mmol), 0.35% Pd/MIL-101 (Pd 1 mol%),
toluene (10 mL), 80 °C, 1 atm, O2 bubbling rate (20 mL min−1).
b Pd 1.5 mol %, 100 °C. c Lactones were the predominant oxidation
products. d Pd 3.0 mol %, 25 °C, air was used as the oxidant instead of
pure O2. e Alcohol (10 mmol), toluene (30 mL), O2 bubbling rate
(30 mL min−1).
The high reactivity of the present catalyst system was
included in Table 4 indicate that no efficiency loss was further highlighted by a 50 mmol scale oxidation of benzyl
observed in the aerobic oxidation of alcohol for up to five alcohol employing 1 × 10−3 mol% Pd under base- and solvent-
runs. These findings were in accordance with AAS experi- free conditions. The Pd/MIL-101 catalyst provided an extremely
ments for which no Pd traces (below the detection limit) high turnover frequency (TOF) of 16 900 h−1, with greater than
were detected in the reaction solution as well as with the 99% selectivity for the desired product. It is noteworthy that
PXRD experiments of the catalyst after the fifth run which the TOF value achieved by Pd/MIL-101 is comparable to those
showed no appreciable changes in the crystallinity of the reported for most other active catalysts, e.g., Pd/7Mn3Ce-C that
MIL-101 material (Fig. 1).
afforded a TOF of 15 235 h−1 for the conversion of benzyl
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Table 4 Reuse of Pd/MIL-101a
followed by washing thoroughly with deionized water. The
sample was finally dried at 100 °C under vacuum for 2 h. The
Pd loading on the sample was 0.35 wt% based on atomic
absorption spectroscopy (AAS) analysis. An identical protocol
was applied for the preparation of other Pd/MIL-101 catalysts
except using different amounts of Pd reagents (calculated load-
ings: 0.16, 0.63, and 0.97 wt%). The actual Pd contents
measured by AAS in these Pd/MIL-101 catalysts were 0.15, 0.59,
and 0.94 wt%, respectively.
Use
First
99
Second
99
Third
98
Fourth
97
Fifth
99
Yield (%)
a Reaction conditions: alcohol (1 mmol), 0.35% Pd/MIL-101, toluene
(10 mL), 1 atm, O2 bubbling rate (20 mL min−1).
Synthesis of Pd/En-MIL-101: amine-functionalized MIL-101
was prepared according to the reported protocol.45 Identical
procedures were followed to deposit Pd nanoparticles onto the
En-MIL-101 as for the synthesis of Pd/MIL-101. The Pd content
in the sample was ca. 0.35 wt% based on AAS analysis.
Synthesis of Pd/AC: prior to use, the activated carbon
Scheme 1 TOF for the aerobic oxidation of benzyl alcohol for the initial 1 h of
reaction (ratio of moles of benzaldehyde per mole of Pd per hour). Reaction
conditions: 0.35% Pd/MIL-101 (1 × 10−3 mol%), benzyl alcohol (50 mmol).
(DARCO®, SIGMA-ALDRICH, BET surface area: 981 m2 g−1
)
was washed with acetone and dehydrated at 150 °C for 12 h.
Then, identical procedures were followed to deposit Pd nano-
particles onto the AC as for the synthesis of Pd/MIL-101. The
Pd content in the sample was ca. 0.36 wt% based on AAS
analysis.
alcohol under similar conditions as used in this study
(Scheme 1).49
Conclusions
Characterization
Surface area measurements were performed with N2 adsorp-
tion/desorption isotherms at 77 K on a Micromeritics ASAP
2020 instrument. Before the analysis, the samples were evacu-
ated at 150 °C for 12 h. Powder X-ray diffraction patterns
of the samples were obtained by a Rigaku diffractometer
(D/MAX-IIIA, 3 kW) using Cu Kα radiation (40 kV, 40 mA, λ =
0.1542 nm). The palladium contents of the samples were deter-
mined quantitatively by atomic absorption spectroscopy (AAS)
analysis on a HITACHI Z-2300 instrument. X-Ray photoelectron
spectroscopy (XPS) measurements were performed on a Kratos
Axis Ultra DLD system with a base pressure of 10−9 Torr.
Infrared spectra of the samples were performed in a Thermo
Scientific Nicolet iS10 spectrometer using a Smart OMNI-
Transmission accessory.
In summary, we have developed a highly efficient hetero-
geneous catalyst system for liquid-phase aerobic oxidation of a
variety of alcohols using MIL-101-supported Pd nanoparticles
as the catalyst. The catalyst can even efficiently catalyze the
aerobic oxidation under ambient conditions in the absence of
any basic additives. Moreover, the catalyst is recyclable. The
combination of high activity and selectivity, wide substrate tol-
erance, and good stability and recyclability makes Pd/MIL-101
a potential material for practical applications in the aerobic
oxidation of alcohols. Work is underway to investigate the cata-
lytic mechanism and understand the acidic effect of MIL-101
on oxidations in our laboratories.
Experimental
Catalyst preparation
Catalytic reactions
Synthesis of Pd/MIL-101: a series of supported palladium cata- Typical procedures for aerobic oxidation of alcohol: Alcohol
lysts were synthesized using a sol–gel method reported in our (1 mmol) and supported palladium catalyst (1 mol%) were
previous work for the preparation of Au nanoparticles on added to 10 mL of toluene. The reaction mixture was stirred at
MIL-101.35 Using the synthesis of 0.35 wt% Pd/MIL-101 as an 80 °C under 1 atm of O2 (O2 bubbling rate, 20 mL min−1).
example, an aqueous solution of PdCl2 (200 mg L−1, 15.4 mL) Upon reaction completion, the catalyst particles were removed
was first prepared, and PVA was added as a protecting agent from the solution by filtration and washed with toluene. The
(PVA monomer/metal = 10 : 1 molar ratio, 7.6 mg). The mixed liquid phase was subsequently analyzed by GC/MS (Shimadzu
solution was placed in an ice bath and vigorously stirred for GCMS-QP5050A equipped with a 0.25 mm × 30 m DB-WAX
1 h. Then, a freshly prepared aqueous solution of NaBH4 capillary column).
(NaBH4/metal = 5 : 1 (molar ratio), 0.1 M, 0.87 mL) was added
For the recyclability tests, the reactions were performed
under vigorous stirring to obtain a dark colloidal dispersion. under the same reaction conditions as described above, except
The pH value of the solution was adjusted to 7–8 with diluted using the recovered catalyst. Each time, the catalyst was iso-
hydrochloric acid. After the Pd-PVA colloids were formed, the lated from the reaction solution at the end of the reaction,
activated MIL-101 (0.5 g, dispersed in 5 mL of water) was washed with ethanol, and then heated at 150 °C under
immediately added. The suspension was stirred at 0 °C for 4 h, vacuum.
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25 H. Tsunoyama, H. Sakurai, Y. Negishi and T. Tsukuda,
Acknowledgements
J. Am. Chem. Soc., 2005, 127, 9374.
This work was supported by the NSF of China (20803024, 26 H. Miyamura, R. Matsubara, Y. Miyazaki and S. Kobayashi,
20936001, and 21073065), the NSF of Guangdong Angew. Chem., Int. Ed., 2007, 46, 4151.
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and the Fundamental Research Funds for the Central Univer-
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29 B. Z. Yuan, Y. Y. Pan, Y. W. Li, B. L. Yin and H. F. Jiang,
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