Copper nanoparticles on dichromium trioxide: A highly efficient catalyst from copper chromium hydrotalcite for oxidant-free dehydrogenation of alcohols

Article abstract of DOI:10.1002/aoc.3261

Stable copper(0) nanoparticles supported on chromium (Cu(0)/Cr₂O₃) are prepared from the composite precursor copper chromium hydrotalcite. The resulting Cu(0)/Cr₂O₃ catalyst is first used in the selective dehydrogenation of alcohols to aldehydes. More impressively, these dehydrogenations are performed without oxidants and yields of products are high. The stability of Cu(0)/Cr₂O₃ is also assessed by studying its recoverability and reusability for up to five cycles.

Full text of DOI:10.1002/aoc.3261

Full Paper
Received: 28 October 2014
Revised: 12 November 2014
Accepted: 15 November 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3261
Copper nanoparticles on dichromium trioxide: a
highly efficient catalyst from copper chromium
hydrotalcite for oxidant-free dehydrogenation
of alcohols
Yaoqin Zhu, Mengnan Shen, Yonggen Xia and Ming Lu*
Stable copper(0) nanoparticles supported on chromium (Cu(0)/Cr2O3) are prepared from the composite precursor copper
chromium hydrotalcite. The resulting Cu(0)/Cr2O3 catalyst is first used in the selective dehydrogenation of alcohols to
aldehydes. More impressively, these dehydrogenations are performed without oxidants and yields of products are high.
The stability of Cu(0)/Cr2O3 is also assessed by studying its recoverability and reusability for up to five cycles. Copyright
© 2015 John Wiley & Sons, Ltd.
Keywords: copper nanoparticles; alcohols; hydrotalcite; dehydrogenation
sheet or by incorporating different anions in the interlayer of
brucite. More impressively, the materials have cation-exchange
Introduction
Catalytic transformation of alcohols to aldehydes or ketones is
perhaps the most demanding reaction in organic and green
chemistry. Many metal-based oxidants exist for these transfor-
mations, including chromium reagents,[1] ruthenium reagents,[2]
osmium(VIII) tetroxide,[3] potassium permanganate,[3] dichro-
mate,[4] permanganate ions,[5] etc. Also, hydrogen peroxide[6–13]
and/or molecular oxygen[14–19] as green oxidants have been
used for this conventional oxidation process. However, these
reactions generate enormous amounts of toxic metal salts waste,
which is undesirable from both economic and environmental
points of view, or have recovery problems for the expensive
metal catalysts. And almost all of them suffer from over-
oxidations under aerobic conditions or with hydrogen peroxide,
which constrain their utility for a wide substrate scope. In
addition, the presence of molecular oxygen may cause problems
of explosion, or flammability of organic solvents or alcohol
reactants. The safety problem becomes particularly serious for
practical large-scale production. Subsequently, organic oxidants in-
cluding 2,2,6,6-tetramethylpiperidin-1-oxyl[20–22] and phthalimide-
N-oxyl[23] were reported for these reactions, but they are too
expensive for industrial amplifications. Thus, many researchers have
made continuing attempts to develop catalysts[24–26] for achieving
dehydrogenation without oxidants. In this context, recently Kaneda
and co-workers have developed hydrotalcite-supported copper
nanoparticles to promote the oxidant-free dehydrogenation of
various alcohols.[27]
ability in the brucite layer and anion-exchange ability in the in-
terlayer space, resulting in unique organic–inorganic hybrid
characteristics. Copper aluminium hydrotalcite[29–32] has been
widely reported for a variety of reactions, but copper chromium
hydrotalcite (Cu-CrHT) has been less reported. Therefore, our
group chose Cu-CrHT as a precursor to develop a catalyst for
oxidant-free dehydrogenation of alcohols. Finally, stable copper
nanoparticles on chromium (Cu(0)/Cr2O3) were prepared by a
chemical reduction method in ethylene glycol with high effi-
ciency. Notably, the observed catalytic activity of Cu(0)/Cr2O3 in
the dehydrogenation alcohols to the corresponding aldehydes
under optimized reaction conditions was higher than that of
previously reported catalyst systems. Furthermore, the catalyst
can be recovered and reused for at least five cycles without
any deactivation.
Experimental
Materials and Instrumentation
All reagents of AR purity (analytical reagent grade) were purchased
and used as received without further purification. Instruments used
were: gas chromatograph (Agilent 7890A), X-ray diffractometer
(Bruker Advance D8), elemental analyzer (Vano MICRO), inductively
coupled plasma mass spectrometer (OPTIMA5300DV) and surface
area and particle size distribution analyzer (ASAP2020).
Our group has also investigated hydrotalcite materials. The
general formula of metal hydrotalcites is [M(II)1ÀxM(III)x
(OH)2]nÀAxn/ÀnÁyH2O, where M(II) and M(III) are divalent and trivalent
cations such as Cu2+, Mg2+ and Al3+, respectively, and OHÀ, ClÀ,
NO3À,CO32À, etc.[28] can be the compensating anion. Characteris-
tics of these inorganic compounds can be easily tuned with dif-
ferent metals M(II)/M(III) by changing their ratio in the brucite
*
Correspondence to: Ming Lu, School of Chemical Engineering, Nanjing University of
Science and Technology, Nanjing 210094, PR China. E-mail: luming302@126.com
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing 210094, PR China
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

Y. Zhu et al.
General Procedure for Synthesis of Catalyst
Cu-CrHT[29]
A mixture of solutions of Cu(NO3)2Á3H2O (0.125 mol) and Cr
(NO3)3Á9H2O (0.05 mol) in deionized and decarbonated water
(140 ml) and an aqueous solution of sodium hydroxide in deionized
and decarbonated water (140ml) were added simultaneously drop-
wise from separate burettes into a round-bottomed flask. The pH of
the reaction mixture was maintained constant (8–9) by the con-
tinuous addition of base solution. The resulting slurry was aged at
65°C for 30 min. The solid product was isolated by filtration, washed
thoroughly with deionized and decarbonated water, and dried
under vacuum. Then it was crushed into a fine powder to afford
Cu-CrHT.
[A]
[B]
[C]
0
10
20
30
40
50
60
70
80
90
Cu(0)/Cr2O3
2Theta(degree)
An amount of 5 g of Cu-CrHT was calcined at 473 K in static air for
5 h to obtain CuO supported on chromium (CuO/Cr2O3) as a grey
powder. Inductively coupled plasma atomic emission spectroscopic
analysis showed 7.4 wt% of copper on chromium. Then CuO/Cr2O3
was dispersed in 20 ml of ethylene glycol. The mixture was stirred
for 24 h at 110°C. After the mixture was cooled to room tempera-
ture, a dark brown solid was recovered by filtration, washed
thoroughly with methanol and then dried at 383 K for 12h under
vacuum to obtain a black powder of Cu(0)/Cr2O3.
Figure 1. XRD spectra of Cu(0)/Cr2O3 (A), Cu-CrHT (B) and CuO/Cr2O3 (C).
Cu-CrHT is successfully reduced to Cu(0)/Cr2O3. An X-ray photo-
electron spectroscopy (XPS) experiment was also performed.
As shown in Fig. 2, the binding energies of Cu-CrHT are at
934, 940, 956 and 962 eV while the binding energies of Cu
(0)/Cr2O3 are only observed at 932 and 952 eV. As the peaks
at 940 and 962 eV represent the +2 oxidation state of Cu,
the absence of them for Cu(0)/Cr2O3 strongly suggests the re-
duction of CuO, which supports the XRD results. Furthermore,
transmission electron microscopy (TEM) analysis of Cu(0)/
Cr2O3 shows no deformity in the shape and no agglomeration
of the nanoparticles (Fig. 3). The copper nanoparticles were
well dispersed in Cr2O3.
Catalytic dehydrogenation of primary and secondary alcohols
using transition metal catalysts usually gives aldehydes, ketones,
carboxylic acids and esters at the same time. So we attempted to
discover a catalyst that is highly efficient. As summarized in Table 2,
we examined several kinds of catalysts. Obviously, among Cu-CrHT,
CuO/Cr2O3 and Cu(0)/Cr2O3, the latter gives the best results with
almost complete conversion of benzyl alcohol without any
byproducts. Then Cr2O3 alone was tested: the conversion of benzyl
alcohol turns out to be 0%. Likewise, no product is formed when
Procedure for Catalytic Dehydrogenation of Benzyl Alcohol
Benzyl alcohol (1 mmol) and 25 mg of catalyst were added to
3 ml of DMF in a three-necked flask under nitrogen atmosphere.
The mixture was stirred for 4 h at 120°C. Then the flask was
cooled to room temperature for subsequent centrifugation of
the solid catalyst. The resulting organic products were analyzed
using gas chromatography. And the recovered catalyst was
reused in the next run.
Results and Discussion
The surface areas of Cu-CrHT, CuO/Cr2O3, fresh Cu(0)/Cr2O3 and
recovered Cu(0)/Cr2O3 (fifth run) were characterized using
Brunauer–Emmett–Teller (BET) analysis as summarized in Table 1.
Impressively, the surface area of Cu-CrHT is 19m2 gÀ1 while the
surface areas of Cu(0)/Cr2O3 and the recovered Cu(0)/Cr2O3 turn
out to be 78m2 gÀ1. And the surface area of CuO/Cr2O3 is about
45 m2 gÀ1. Apparently, a larger surface area results in better
dehydrogenation yields. The X-ray diffraction (XRD) patterns of Cu
(0)/Cr2O3, Cu-CrHT and CuO/Cr2O3 are shown in Fig. 1. Major diffrac-
tion peaks of Cu-CrHT and CuO/Cr2O3 are at 2θ =35° and 38° which
correspond to the oxidation state of copper particles, while the
characteristic peaks of the chemically reduced Cu(0)/Cr2O3 catalyst
are found at 2θ = 44°, 51° and 75°. Therefore, we can see that the
[A]
[B]
Table 1. BET results for various catalysts (amount of catalyst: 0.5 g)
Entry
Catalyst
Surface area (m2 gÀ1
)
1
2
3
4
Cu-CrHT
19
45
78
78
CuO/Cr2O3
920
930
940
950
960
970
980
Cu(0)/Cr2O3
Binding energy (eV)
Cu(0)/Cr2O3 (fifth run)
Figure 2. XPS spectra of Cu-CrHT (A) and Cu(0)/Cr2O3 (B).
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Appl. Organometal. Chem. (2015)

Nano-copper particles from hydrotalcite for alcohol dehydrogenation
As can be seen, primary alcohols are effective substrates. Un-
der these reaction conditions, various benzylic alcohols with
different substituents on the phenyl ring can be efficiently ox-
idized to the corresponding aldehydes in 31–99% conversions
(Table 3, entries 1–9). According to entries 1–9, the presence
of electron-donating and electron-withdrawing groups on
the aromatic ring is influential in the dehydrogenation of
substituted benzyl alcohols. Apparently, electron-withdrawing
groups make the substrates less easy to oxidize and the time
required for oxidation increases to achieve better conversion.
Heterocyclic primary alcohols are smoothly oxidized to give
aldehydes in good yields (Table 3, entries 10–13). These re-
sults indicate that the oxidative reactions catalyzed by Cu(0)/
Cr2O3 are highly chemo-selective without oxidizing other
groups. In addition, alkyl alcohols are also suitable substrates
giving aldehydes in 67 and 63% conversions (Table 3, entries
14 and 15). Compared to previously reported copper-based
catalysts[33,34] which always lead to low yields, using Cu(0)/
Cr2O3 can afford excellent conversions with high selectivity.
As the dehydrogenation of long-chain alkanes to their corre-
sponding aldehydes is an industrially important process, the
catalyst is crucial.
Next, we examined the dehydrogenation of secondary alco-
hols in DMF at 120°C in the presence of the catalyst. The re-
sults are summarized in Table 4. High yields are obtained for
a wide range of alcohols. Various secondary benzylic alcohols
with alkyl groups are effectively oxidized to ketones in 70–
99% conversions (Table 4, entries 1–3). The dehydrogenation
can also be extended to a variety of nonbenzylic secondary
alcohols, giving the corresponding ketones in 72–99% conver-
sions (Table 4, entries 4–13), which indicates that the catalytic
system has a wide utility range for oxidative transformations.
Cyclic alcohols with various ring sizes and substituents are
found to be effective substrates (Table 4, entries 4–10). And
acyclic, aliphatic alcohols are efficiently transformed into ke-
tones in 73–76% conversions (Table 4, entries 11–13). Com-
paring the dehydrogenations of primary and secondary
alcohols, it is obvious that the dehydrogenation of secondary
alcohols takes a much longer time, which may due to the ste-
ric effect of the secondary alcohols. Because the steric effect
of secondary alcohols is stronger than that of the primary al-
cohols, it makes the secondary alcohols more difficult to be
attacked, which results in the longer reaction times.
Stability and recyclability are key issues in catalysis. The catalyst
can be reused without loss of catalytic activity or selectivity. After
the reaction, it can be easily separated from the reaction mixture
by centrifugation. Afterwards, the recovered catalyst was reused
after washing with ethyl acetate (3× 10 ml). The results obtained
from the stability study are shown in Fig. 4. As can be seen, the yield
of benzyl aldehyde is maintained at a similar value for at least five
rounds of recycling, which indicates the excellent stability and recy-
clability of the catalyst.
Figure 3. TEM images of Cu(0)/Cr2O3 under different resolutions.
Table 2. Activity of catalysts for dehydrogenation of benzyl alcohola
Entry
Catalyst
Time
(h)
Conversion
(%)
Selectivity for
benzaldehyde (%)
1
2
3
4
5
6
7
Cu-CrHT
24
8
36
67
99
0
43
71
99
0
CuO/Cr2O3
Cu(0)/Cr2O3
Cr2O3
4
24
24
24
24
Cu powder
Cu2O
0
0
0
0
CuO
0
0
aReaction conditions: benzyl alcohol (200 mg), catalyst (25 mg), 120°C,
nitrogen atmosphere. Data obtained with GC analysis using a
normalization method. GC column: HP-5; temperature of column:
250°C; internal standard: acetone.
Moreover, we investigated the effect of Cu(0)/Cr2O3 concentra-
tion on catalytic performance. As shown in Fig. 5, a linear increase
in conversion is observed with respect to the catalyst concentration
in the range 0–25 mg. When the concentration reaches 25 mg, the
conversion is the highest. Even when the concentration is more
than 25mg, the conversion still remains stable at 99%. The effect
of temperature was also examined. As shown in Fig. 6, the reaction
at 120°C is complete and the conversion is found to be 99%. Also,
the selectivity remains at 99% with a further increase in
temperature.
using Cu powder, Cu2O and CuO. This indicates that Cu nano-
particles are the main active centers of the reaction. Cr2O3 alone
has no catalytic performance.
Representative results of the dehydrogenation of primary
alcohols in DMF at 120°C in the presence of the Cu(0)/Cr2O3
catalyst under nitrogen atmosphere are displayed in Table 3.
Appl. Organometal. Chem. (2015)
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Y. Zhu et al.
Table 3. Cu(0)/Cr2O3-catalyzed dehydrogenation of primary alcoholsa
Entry
R
Y
Product Main byproduct
Conversion (%)/selectivity (%)/time (h)
1
2
Phenyl
Phenyl
H
Benzaldehyde Benzoic acid
2-Methoxybenzaldehyde
2-Methoxybenzoic acid
p-Methoxybenzaldehyde
4-Methoxybenzoic acid
3-Methoxybenzaldehyde
3-Methoxybenzoic acid
4-Isopropylbenzaldehyde
4-Isopropylbenzoic acid
Methyl-4-formaylbenzoate
Monomethyl terephthalate
p-Chlorobenzaldehyde
4-Chlorobenzoic acid
4-Cyanobenzaldehyde
4-Cyanobenzoic acid
p-Nitrobenzaldehyde
4-Nitrobenzoic acid
99/99/4
99/97/4
o-OCH3
p-OCH3
m-OCH3
p-CH(CH3)2
p-COOCH3
p-Cl
3
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
1-Naphthyl
93/95/4
98/96/4
84/94/4
78/99/6
58/92/6
53/92/6
31/81/8
83/96/6
75/93/6
84/93/6
62/83/8
77/90/12
73/91/12
4
5
6
7
8
p-CN
p-NO2
H
9
10
11
12
13
14
15
1-Naphthaldehyde
1-Naphthoic acid
Benzo[b]thiophen-2-yl
Furfural
H
Benzo[b]thiophene-2-carboxaldehyde
Benzo(b)thiophene-2-carboxylic acid
2,5-Diformylfuran
2-CHO
H
2-Furancarboxylic acid
2-Pyridinecarboxaldehyde
2-Picolinic acid
Pyridyl
CH3(CH2)5
H
Heptanal
Heptanoic acid
CH3(CH2)6
H
n-Octylaldehyde
n-Caprylic acid
aReaction conditions: substrate (200 mg), Cu(0)/Cr2O3 (25 mg), 120°C, nitrogen atmosphere. Data obtained with GC analysis using a normalization
method. GC column: HP-5; temperature of column: 250°C; internal standard: acetone.
Table 4. Cu(0)/Cr2O3-catalyzed dehydrogenation of secondary alcoholsa
Entry
R1
R2
R3
Product
Conversion (%)/time (h)
1
2
3
4
Phenyl
Phenyl
Phenyl
—
CH3CH2
Phenyl
Acetenyl
—
—
Propiophenone
99/2
—
—
Benzophenone
92/12
70/12
72/10
1-Phenylprop-2-yn-1-one
2-Adamantanone
5
—
—
—
–(CH2)4–
Cyclopentanone
3-Methylcyclopentanone
Cyclohexanone
85/10
86/10
90/10
83/10
75/10
99/6
6
—
—
–CH2CH(CH3)CH2CH2–
7
—
–(CH2)5–
8
—
—
–(CH2)2CH(CH3)CH2CH2–
4-Methylcyclohexanone
4-Propylcyclohexanone
Cyclooctanone
9
—
—
–(CH2)2CH(CH2CH2CH3)CH2CH2–
10
11
12
13
—
—
–(CH2)7–
CH3(CH2)3–
CH3(CH2)4–
CH3(CH2)5–
CH3
CH3
CH3
—
—
—
2-Hexanone
76/12
73/12
74/12
2-Heptanone
2-Octanone
aReaction conditions: substrate (200 mg), Cu(0)/Cr2O3 (25 mg), 120°C, nitrogen atmosphere. Data obtained with GC analysis using a normalization
method. GC column: HP-5; temperature of column: 250°C; internal standard: acetone.
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Appl. Organometal. Chem. (2015)

Nano-copper particles from hydrotalcite for alcohol dehydrogenation
99 99
99 99
99 99
99
99
98
98
100
90
80
70
60
50
40
30
20
10
0
Conclusions
In summary, we developed a highly efficient catalytic system for the
dehydrogenation of alcohols. The Cu(0)/Cr2O3 catalyst is first used in
this transformation, and it is formed from Cu-CrHT by a chemical re-
duction method. Cu nanoparticles are well dispersed in Cr2O3 with a
large surface area, benefitting the dehydrogenations. And the Cu(0)/
Cr2O3 catalyst can be easily recovered and reused for at least five cy-
cles without any loss of Cu nanoparticles. This catalyst offers an attrac-
tive alternative to expensive metal-based catalysts for oxidant-free
dehydrogenation of alcohols.
1
2
3
4
5
Cycle
Conversion (%)
Selectivity for benzaldehyde(%)
Figure 4. Recycling of Cu(0)/Cr2O3 catalyst for the dehydrogenation of
benzyl alcohol. Reaction conditions: benzyl alcohol (1 mmol), Cu(0)/Cr2O3
(25 mg), 120°C, nitrogen atmosphere, 4 h.
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