Copper oxide as efficient catalyst for oxidative dehydrogenation of alcohols with air

Article abstract of DOI:10.1039/c4cy01622j

The oxidative dehydrogenation of alcohols to carbonyl compounds was studied using CuO nanoparticle catalysts prepared by solution synthesis in buffered media. CuO nanoparticles synthesized in N-cyclohexyl-3-aminopropanesulfonic acid buffer showed high catalytic activity for the oxidation of benzylic, alicyclic and unsaturated alcohols to their corresponding carbonyl compounds with excellent selectivities. The observed trend in activity for conversion of substituted alcohols suggested a β-H elimination step to be involved, thus enabling a possible reaction mechanism for oxidative dehydrogenation of benzyl alcohols to be proposed. The use of CuO as an inexpensive and efficient heterogeneous catalyst under aerobic conditions provides a new noble metal-free and green reaction protocol for carbonyl compound synthesis. This journal is

Full text of DOI:10.1039/c4cy01622j

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Copper oxide as efficient catalyst for oxidative
dehydrogenation of alcohols with air†
Cite this: DOI: 10.1039/c4cy01622j
a
b
a
Raju Poreddy, Christian Engelbrekt and Anders Riisager*
The oxidative dehydrogenation of alcohols to carbonyl compounds was studied using CuO nanoparticle
catalysts prepared by solution synthesis in buffered media. CuO nanoparticles synthesized in N-cyclohexyl-
3-aminopropanesulfonic acid buffer showed high catalytic activity for the oxidation of benzylic, alicyclic
Received 6th December 2014,
Accepted 10th February 2015
and unsaturated alcohols to their corresponding carbonyl compounds with excellent selectivities. The
observed trend in activity for conversion of substituted alcohols suggested a β-H elimination step to be
involved, thus enabling a possible reaction mechanism for oxidative dehydrogenation of benzyl alcohols to
be proposed. The use of CuO as an inexpensive and efficient heterogeneous catalyst under aerobic condi-
tions provides a new noble metal-free and green reaction protocol for carbonyl compound synthesis.
DOI: 10.1039/c4cy01622j
www.rsc.org/catalysis
source for the oxidation of alcohols to carbonyl com-
1
. Introduction
1
0
pounds. Although remarkable advances have been made in
heterogeneous catalysis for the selective oxidation of alco-
hols, particularly with transition metals, only a limited num-
ber of catalysts are effective under mild reaction conditions.
Heterogeneous catalytic systems are kinetically constrained
The oxidative dehydrogenation of primary and secondary alco-
hols to their corresponding carbonyl compounds is a funda-
mental reaction in nature and synthetic organic chemistry.
Synthetic chemistry relies heavily on stoichiometric quantities
of inorganic oxidants, whereas nature uses metal catalysis
and oxygen as the terminal oxidant. Notably palladium,
1
12
by surface availability and metal–support interactions.
2
3
Therefore, most research on aerobic oxidations using hetero-
geneous catalysts has focused mainly on highly active noble
4
,5
chromiumIJVI) reagents
and manganese or ruthenium
6
,7
13
salts have attracted much attention as stoichiometric oxi-
dants. However, the application of one or more equivalents of
such relatively expensive and rather toxic oxidizing agents is
an important factor limiting their usage in industry today. In
addition, problems relating to corrosion and plating on reac-
tor walls, handling, recovery and reuse of the catalyst repre-
sent serious process limitations. Therefore, the desire to
replace stoichiometric oxidants with methodologies more
resembling nature has gained much interest.
metals such as platinum, palladium and gold. Despite a
handful of reports on platinum and palladium as potential
metals to catalyze oxidative dehydrogenation of alcohols, an
overshadowing focus has emerged on gold chemistry since
1
4
15
the pioneering work by Bond et al., Hutchings, Haruta
1
6
17
et al. and Prati and Rossi utilizing supported gold nano-
particles. This abrupt interest is not only because gold is
cheaper than its noble metal counterparts, but also that it cir-
cumvents most of the drawbacks associated with Pt and Pd.
One example is the fast deactivation rates attributed to aggre-
8
In the last few years numerous methods for oxidation of
alcohols using heterogeneous catalysts have emerged, espe-
cially metal nanoparticles.
gation of particles or leaching of noble metals during the oxi-
9
–11
18,19
Their use of either molecular
dation and/or catalyst regeneration processes.
This essen-
oxygen or oxygen donating agents like hydrogen peroxide as
the ultimate stoichiometric oxidant makes these methods
attractive and eco-friendly. In particular, molecular oxygen –
or even air – has been used as a cheap and abundant oxygen
tially holds true for the liquid-phase oxidation of alcohols
under aerobic conditions. Apart from cost considerations, the
residues of noble metals (e.g. Pt and Pd) in pharmaceutical
and nutritional products are prone to be highly harmful and
2
0,21
problematic.
2
2
Wang et al. recently studied the activity of copper oxide
supported gold nanoparticles and the influence of synthesis
parameters on the transformation of alcohols to carbonyl
compounds via oxidative dehydrogenation. They demon-
strated that the catalytic performance of gold strongly
depended on the preparation method, pH value and stirring
rate by presumably influencing the electronic state and
a
Centre for Catalysis and Sustainable Chemistry, Department of Chemistry,
Technical University of Denmark, Kemitorvet, Building 207, DK-2800 Kgs. Lyngby,
Denmark. E-mail: ar@kemi.dtu.dk; Tel: +45 45252233
b
NanoChemistry, Department of Chemistry, Technical University of Denmark,
Kemitorvet, Building 207, DK-2800 Kgs. Lyngby, Denmark
†
Electronic supplementary information (ESI) available: Supplementary data are
supplied in the Supporting Information. See DOI: 10.1039/c4cy01622j
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particle size of the gold nanoparticles as well as the structure
of the support. With this system there was no need for addi-
tional base to promote the reaction, which is otherwise often
the case. This is good in terms of waste minimization and
product recovery, but higher loading of the rather expensive
noble metal gold was required to obtain moderate conver-
sion. Hence, more sustainable solutions based on earth
abundant, cheap, harmless and stable metals to replace
noble metals would be desirable. In this connection, applica-
tion of catalysts based on the relatively inexpensive metals
Ag, Cu and Fe has increasingly been explored for aerobic and
about 11 is optimal for aqueous CuO synthesis. Therefore,
CAPS (pK = 10.4) was chosen to replace MES (pK = 6.15) as
a
a
the buffer. Preparation was done by first dissolving 4.432 g of
CAPS (nanoparticles denoted “CAPS-CuO”) or 3.905 g of MES
(nanoparticles denoted “MES-CuO”) in 950 mL of Millipore
water and adjusting the pH to 11 for CAPS-CuO and 12 for
MES-CuO with KOH. The buffer was preheated to 95 °C
under stirring before rapid addition of a 50 mL solution of
2
0.682 g of CuCl in Millipore water. Final concentrations of
buffer and CuCl2 were 20 and 4 mM, respectively. The ini-
tially bright blue CuCl2 solution immediately turned green
indicating intermediate formation of hydrated copper hydrox-
ide and after few seconds the solution turned dark brown as
CuO was formed. Heating was maintained for 30 min after
which the solution was allowed to cool to room temperature
with stirring. Stirring was terminated and the particles were
left to sediment overnight. The CuO nanoparticles were fil-
tered under vacuum with a 0.2 μm Nylon membrane filter
from Gelman Sciences (New York, USA) and washed several
times with water. The remaining water was expelled from the
product by washing with ethanol, and then dried under vac-
uum to remove excess ethanol leaving a brittle film that
could easily be recovered from the membrane. The dry pow-
der was lightly crushed and left under vacuum overnight.
Two different samples of Cu IJOH) Cl were tested as refer-
23
non-aerobic oxidations under ambient conditions. Thus,
silver nanoparticles or clusters supported on different metal
oxides have been reported to be potential catalysts for the
oxidative dehydrogenation of alcohols to carbonyl com-
1
0,24
25,26
27,28
pounds.
Likewise, copper
and iron
have gained
importance in recent years and iron oxides – although harm-
less, less toxic, and cheaper – are not better catalysts than
copper for oxidation reactions of alcohols at low tempera-
2
9
30
tures. Other homogeneous copper catalysts and metallic
3
1
copper catalysts with styrene as the hydrogen acceptor are
also reported for alcohol dehydrogenations.
2
2
As mentioned above, Wang et al. have showed that CuO
is an effective support material for Au in the catalytic oxida-
tion of alcohols. However, they also reported that CuO alone
cannot catalyze the reaction being nearly inactive, since the
adsorption of benzyl alcohol onto the CuO surface was too
weak to be activating and that pure CuO could not activate
2
3
2 3
ence systems. A-Cu IJOH) Cl was prepared in a similar proce-
dure to the CuO nanoparticles but in 1.5 L batches and final
concentrations of MES and CuCl of 50 and 10 mM, respec-
2
the O
2
molecule. Copper, because of its characteristic pro-
tively. The buffer was adjusted to pH 11 and preheated to 80
moting ability, was also involved in bi-metallic catalysis par-
°C before CuCl
2 2 3
addition. The synthesis of B-Cu IJOH) Cl was
3
2
33
ticularly with gold and platinum. Our interest in the syn-
thesis of carbonyl compounds as well as non-noble metal
catalysis prompted us to examine a range of supported
copper-based catalysts for alcohol oxidation.
In this work, we have elaborated our previous synthesis
procedure for CuO nanoparticles and the structure and mor-
phology of the CuO catalysts have been thoroughly character-
ized. We further demonstrate that CuO as a non-noble metal
oxide is a highly efficient heterogeneous catalyst in the oxida-
tion of alcohols under mild reaction conditions using oxygen
as a green terminal oxidant.
achieved by heating a mixture of Cu IJOH) CO and CuCl as
described previously.
2
2
3
2
3
2
2
.3 Catalyst characterization
The textural properties of the samples were determined by
nitrogen physisorption analysis using Micromeritics
a
(Norcross, GA, USA) ASAP 2020 with an Automated Gas
Adsorption Surface area and Porosimetry Analyzer at liquid
nitrogen temperature. The samples were outgassed in vac-
uum at 200 °C prior to measurement. The surface areas were
determined by the Brunauer–Emmett–Teller (BET) method.
X-ray powder diffraction (XRD) spectra were measured on
a Huber (Rimsting, Germany) G670 diffractometer with a
Guinier imaging plate camera operated in transmission mode
with CuKα1 (λ = 1.54 Å) irradiation from a focusing quartz
monochromator. The samples were fixed between two pieces
of Scotch tape and rotated during data collection. The diffrac-
tion patterns of the samples were recorded at room tempera-
ture in the 2θ range of 3–100° in steps of 0.005°.
Transmission electron microscopy (TEM) studies were
performed using a Tecnai T20 G2 and Titan instrument from
FEI Company (Hillsboro, OR, USA) operated at 200 and 120
kV, respectively. The Titan was fitted with a field-emission
electron source and a spherical aberration corrector on the
condenser lens system. The samples were redispersed in
2
. Experimental section
2
.1 Chemicals
CuCl Ĵ2H O ( p.a., Riedel-de Haën), N-cyclohexyl-3-amino-
2
2
propanesulfonic acid (CAPS) (≥98%, Sigma-Aldrich), 2-IJN-
morpholino)ethanesulfonic acid (MES) hydrate (99.5%,
Sigma-Aldrich), and KOH (≥85%, Sigma-Aldrich) were used
as received for the synthesis of CuO nanoparticles. All the
substrates for the catalytic oxidations were purchased from
Sigma Aldrich and used without any further purification.
2
.2 Catalyst preparation
CuO nanoparticles were synthesized following a modification
3
4
of our previously reported procedure. It was found that pH
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ethanol by ultrasonication and deposited on plain or holey
carbon-coated Cu grids from Agar Scientific (Stansted, UK).
X-ray photoelectron spectroscopy (XPS) was performed
internal standard. The reported conversions were calculated
from the conversion of alcohol.
with a Thermo Scientific XPS using Al-Kα (1486 eV) as the 3. Results and discussion
excitation X-ray source. The pressure of the analysis chamber
3
.1 Catalyst preparation
In our previous work, copper mineral nanoparticles (CuO
and Cu IJOH) Cl) were selectively synthesized by controlling
−
10
was maintained at 2 × 10
mbar during measurement. The
XPS measurements were performed in the electron binding
energy ranges corresponding to copper 2p and oxygen 1s core
excitations.
2
3
pH with a buffered reaction medium. The optimal conditions
for CuO formation were determined to be 95 °C and pH 11.
For the current work, the synthesis was further optimized for
the preparation of the CuO catalysts. A Good's buffer with a
pK close to pH 11 was chosen (CAPS, pK = 10.4) and the
a a
reactant concentrations increased providing well-defined
CuO nanoparticles in less than 30 min.
The reducibility of the copper catalysts was investigated by
temperature programmed reduction using H (H -TPR). The
2
2
−
1
samples were pretreated in 20 mL min 10% O
2
–He mixture
gas flow at 500 °C for 1 h and then cooled down to 30 °C and
flushed out with He for 1 h. Then the samples were heated
−
1
−1
from 30 to 600 °C at a ramp of 10 °C min in 60 mL min
% H –Ar mixture gas flow. The water formed during reduc-
4
2
3.2 Catalyst screening
tion with H was trapped using a dry ice cold trap and the
2
hydrogen consumption was continuously monitored with a
TCD detector.
The oxidation of benzyl alcohol was investigated in the pres-
ence of a range of commercial and synthesized copper cata-
lysts in toluene at 100 °C and atmospheric pressure of air.
The reaction proceeded smoothly to afford the corresponding
aldehydes with good to excellent yields. Table 1 compiles the
catalytic performance obtained with CuIJI) and CuIJII) oxides,
chlorides and hydroxychlorides (Table 1, entries 1–10) for
aldehyde formation under the given reaction conditions.
The CuO synthesized in different buffered media (i.e.
CAPS-CuO and MES-CuO) showed activity and selectivity
superior to other tested catalysts for benzaldehyde formation
(Table 1, entries 1 and 2). Hence, MES-CuO with an excellent
selectivity of >99% yielded 71% benzaldehyde whereas CAPS-
CuO afforded >99% yield of benzaldehyde. Over-oxidation or
aldehyde disproportionation products such as acids, esters,
and alcohols were not observed during the reaction.
The amount of copper leached into the reaction liquor
after the catalytic reactions was determined by inductively
coupled plasma mass spectrometry (ICP-MS). Toluene was
removed from 1 mL of filtrate after 24 h reaction by drying
and the remnant, containing any leached copper, was
dissolved in 5 mL of 0.1 M nitric acid for analysis. 4 mg of
fresh com-CuO was similarly dissolved in 5 mL of 0.1 M
nitric acid.
Thermogravimetric analysis of catalysts was performed
using a TGA/DSC 1 STAR system from Mettler Toledo in 80
−
1
−1
mL min
2
N flow and a heating rate of 10 °C min .
Attenuated total reflectance Fourier transform infrared
spectroscopy (ATR-FTIR) measurements were performed
using a Thermo Scientific Nicolet iS5 and Specac Golden Gate
(
Waltham, MA, USA) with a diamond plate. The spectra were
The commercial copper oxide nanopowder (com-CuO)
with a particle size of <50 nm and a surface area of 13.1
m g (Table 1, entry 3) performed best of the commercial
catalysts (entries 7–10). The lack of significant activity of the
bulk CuO (entry 7) may be ascribed to the low BET surface
−
1
recorded in the range of 4000–400 cm with a resolution of 2
cm .
−
1
2
−1
2
.4 General oxidation procedure
In a typical oxidation reaction, the copper catalyst (60 mg,
.75 mmol, 75 mol% with respect to the substrate) was dis-
Table 1 Catalyst screening for aerobic oxidation of benzyl alcohola
0
persed in a glass tube in 3 mL of toluene facilitated by
ultrasonication followed by the addition of substrate (1
mmol) and internal standard anisole (0.1 mmol). The tube
containing the reaction mixture was connected to a reaction
station which provided stirring and heating at 100 °C for 22–
2
−1
b
Entry Catalyst
SBET [m g ] Conversion [%] Selectivity [%]
1
2
3
CAPS-CuO
MES-CuO
com-CuOc
com-CuO
46.2
37.4
>99
71
69
4
23
29
3
2
5
16.7
2
>99
>99
91
>99
>99
>99
>99
>99
>99
89
13.1d
4
n.d.
5
6
7
8
A-Cu
B-Cu
CuO bulk
Cu O bulk
2
IJOH)
3
Cl 20.1
2
4 h. The reaction was carried out under atmospheric pres-
2
IJOH)
3
Cl 5.1 d
sure of air with oxygen as the oxidant.
n.d.
2
0.10d
n.d.
5.3
9
10
1
CuCl
CuCl
—
2
.5 Product analysis
2
e
1
—
>99
During the reactions samples were periodically collected, fil-
tered and analyzed by GC-FID (Agilent, 6890N) and GC-MS
a
Reaction conditions: benzyl alcohol (2 mmol), anisole (0.2 mmol),
toluene (6 mL), 120 mg catalyst, 24 h, 100 °C, atmospheric air.
Conversion was evaluated by GC and GC-MS using anisole as inter-
nal standard. Reduced com-CuO using H (10% in N ). Not detect-
(Agilent, 6850N) with an HP-5 capillary column (Agilent, J&W)
b
using N as the carrier gas. The amounts of substrates and
c
d
2
2
2
e
reaction products were quantified using anisole as an
able. Control experiment without catalyst.
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area. Although com-CuO was more active than the other com-
mercial catalysts, it was less selective. Thus, over-oxidation of
benzaldehyde to benzoic acid followed by esterification
seemed unavoidable with com-CuO resulting in lower selec-
tivity (91%) than CAPS- and MES-CuO.
displays the fitted XRD patterns of CAPS-CuO and MES-CuO,
respectively. Both catalysts showed clear tenorite CuO struc-
ture and no other detectable crystalline phases. This is
supported by Rietveld refinement of the spectra confirming
previous findings that no redox process was involved in the
3
2
The formation of the over-oxidation product benzyl benzo-
ate can also be obtained by oxidation of the hemiacetal (a
product resulting from the reaction of benzyl alcohol and
benzaldehyde). CopperIJII) chloride (entry 10) showed some
activity for benzaldehyde formation, but catalysed also the
nucleophilic exchange reaction with benzyl alcohol yielding
benzyl chloride and alkylation of toluene (solvent) with ben-
zyl alcohol in significant amounts. The catalysis of side reac-
tions was not observed for the synthesized hydroxychlorides
synthesis. The refinement provided crystallite sizes of 17.5
and 18.8 nm for CAPS-CuO and MES-CuO, respectively. Apart
from the small variation in crystallite size, no differences
were detected with XRD. The purity of the as-synthesized
CAPS-CuO was further confirmed by ICP-MS measurements
of the dissolved catalyst showing no other metal impurities
(ESI† Fig. S1).
The morphology, i.e. shape, size and roughness, of the
CuO nanoparticles was investigated with TEM which is sup-
plementary to the crystallite information from XRD. High
and low magnification TEM micrographs of CAPS- and MES-
CuO are presented in Fig. 2. Both catalysts consist of flat,
elongated nanoparticles up to 200 nm long, 100 nm wide and
roughly 10 nm thick (Fig. 2a and e). The particles are seem-
ingly assembled by rods via side-to-side attachment and over-
all smaller in CAPS-CuO than MES-CuO. The rod-like struc-
tures and rough surfaces are evident in Fig. 2b and f. Due to
the aggregating nature of the nanoparticles on the TEM grids
it was difficult to obtain electron diffraction from single par-
ticles. Instead, the nanoscale crystal structure was studied by
high-resolution (HR) TEM. HR micrographs of several single
nanoparticles from CAPS- and MES-CuO were obtained and
the corresponding FFTs indexed (Fig. 3 and ESI† Table S1).
The facets making up the main surface of the crystals were
determined from the zone axis with the assumption that the
flat structures were lying roughly perpendicular to the beam.
The crystal directions corresponding to the long, intermedi-
ate and short axes of the sheets were identified as [010], [100]
(entries 5 and 6). Their activity was improved compared to
CuCl2 but still not comparable to the synthesized oxides.
com-CuO was reduced at 300 °C by H2 (10% in N ) to its
2
metallic state and studied for the same reaction (entry 4).
The reduction of com-CuO did not improve the activity but
instead reduced it to only 4%. In fact, only copper in the oxi-
dation state +2 showed any significant activity for the oxida-
tion of benzyl alcohol. A control experiment without added
catalyst was further performed and, as expected, no substan-
tial amount of benzaldehyde was formed under the given
reaction conditions (entry 11).
3
.3 Characterization of CAPS-, MES- and com-CuO
Detailed characterization of the catalysts (especially CAPS-
CuO which exhibited superior performance) was undertaken
to correlate their activity and structural properties. The crys-
tal structure and morphology of the synthesized nanostruc-
tures were characterized by XRD and TEM. Fig. 1a and c
Fig. 1 Rietveld refinements of fresh (a) CAPS-CuO and (b) MES-CuO catalysts and the corresponding profiles for spent (c) CAPS-CuO and (d)
MES-CuO after 22 h of oxidation of benzyl alcohol in toluene at 100 °C in air. Each graph includes the refined pattern (top line), the difference
between the refinement and raw data (bottom line) and Bragg positions of crystallographic phases of CuO, Cu O and metallic copper (black bars).
2
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Fig. 2 TEM micrographs of the as-synthesized (a–b) CAPS-CuO and (e–f) MES-CuO, and the corresponding spent (c–d) CAPS-CuO and (g–h) MES-
CuO after 24 h of benzyl alcohol conversion at 100 °C in air. TEM images of as-received com-CuO are shown in the ESI† Fig. S2.
Fig. 3 Representative HR-TEM micrographs of (a, c) CAPS- and (e, g) MES-CuO with corresponding indexed FFTs for (b, d) CAPS-CuO and (f, h)
MES-CuO.
and [001] for CAPS-CuO and [01−1], [100] and [011] for MES-
CuO, respectively. These observations indicate that while the
overall crystallite size and nanoparticle shape were similar
for the two structures the different chemical environments
during synthesis may have led to changes in the relative sur-
face energies of the main facets and oriented attachment
mechanisms. The slight differences in exposed facets may
contribute to the differences in observed activity (Table 1) as
the main facets may represent different inherent activities
towards the dehydrogenation of benzyl alcohol. Similar aniso-
is allocated to CuIJII) species with different environment or
coordination. Full range spectra of both CAPS- and com-
4
0
CuO are shown in the ESI† Fig. S3 with spin–orbit coupled
1
/2
Cu 2p transition (953.0 eV) between two well defined satel-
lite structures at around 945.1 and 956 eV.
4
1
3/2
McIntyre et al. have reported that the main Cu 2p
transition for metallic copper and Cu O appears at energy
levels lower than 933 eV, while it shifts to values higher than
2
3
/2
933 eV for CuIJII) species. The Cu 2p
transition for Cu(0)
and CuIJI) was reported to occur at the same binding energy
−
39,42
tropic growth and [OH ] dependency have previously been
of 932.5 eV, about 1.3 eV below the main core level line.
3
5–38
observed.
According to this assignment, the profiles shown in Fig. 4
reveal neither Cu(0) nor CuIJI) in synthesized and commercial
copper oxide. A satellite peak located above the core level line
(945.1 eV) is prominent in both CuO samples (ESI† Fig. S3),
Fig. 4 shows the XPS spectra corresponding to the Cu 2p
transition for CAPS- and com-CuO. The peaks at binding
energies of about 934.2 and 953.0 eV (full spectrum, ESI†
Fig. S3) are due to the spin–orbit doublet of the Cu 2p core
4
0
but hardly visible in CuIJI) XPS. This observation confirms
that the oxidation state of Cu in both catalysts is +2 and is
consistent with the XRD results. However, the energy of the
3
/2
level transition. The Cu 2p transition included in Fig. 4 of
both CAPS- and com-CuO can be deconvoluted giving two
main contributions located around 934.2 and 937.5 eV. The
relatively sharp band located at 934.2 eV is tentatively
3
/2
Cu 2p transition does not allow unequivocally determina-
tion of the oxidation state of copper unless the Auger param-
3
9
38
assigned to CuO, while the second peak located at 937.5 eV
eter (α) is determined. Furthermore, a relatively narrow line
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resulting in a shift in reduction peak to higher temperatures
without exhibiting true bulk behavior. At this point, it is
worth noting that the specific surface area of highly dis-
2
−1
persed CAPS-CuO (46.2 m g ) was substantially higher than
2
−1
that of com-CuO (13.1 m g ). Hence, more active CuIJII) spe-
cies were available providing increased catalytic activity.
3.4 Optimization of reaction conditions
CAPS-CuO as the best performing catalyst was used to opti-
mize the reaction conditions. In order to investigate the
effect of the atmosphere, the reaction was tested under air,
argon, pure oxygen and wet oxygen conditions (Fig. 6).
Reaction in atmospheric air resulted in higher yields
Fig. 4 High resolution XPS core level spectra of the Cu 2p transition
for CAPS-CuO (top) and com-CuO (bottom).
(
(
>99%) than inert (10%), pure oxygen (21%) and wet oxygen
28%) conditions. These results underlined that oxygen was
1
/2
(
FWHM = 1.8 eV) at around 532.5 eV representing O 1s
4
1
transition was observed for both of the catalysts (ESI†
Fig. S4). This transition appears to arise mainly from one
type of oxygen. It has been reported that the CuO phase likely
has three types of oxygen components, namely O–Cu, HO–Cu,
and surface oxygen (O-surf) with binding energies around
29.1, 530.6, and 531.5 eV, respectively. This clearly sug-
gests that both catalysts (CAPS- and com-CuO) have a high
degree of surface oxygen.
essential for the reaction, but also suggested that only a mod-
erate amount of oxygen was beneficial for the reaction.
Increased oxygen concentration could lead to saturation of
the surface by oxygen. As a result, the rate of aldehyde forma-
tion could be lowered. Furthermore, the wet oxygen experi-
ment confirmed the activity to increase by almost 50% com-
pared to dry oxygen conditions. XRD results of the four spent
catalysts revealed an interesting observation that copper
oxide was partially reduced only in air, while no reduction
apparently occurred under the conditions where only low or
insignificant conversions were obtained (ESI† Fig. S5a).
The catalytic activity of the prepared CAPS-CuO was tested
using atmospheric air at different temperatures to further
optimize the reaction conditions. As shown in Table 2, the
conversion of benzyl alcohol to benzaldehyde after 24 h
increased from 11 to >99% when the temperature was
increased from 40 to 100 °C. The catalyst was observed to be
more selective for benzaldehyde at higher temperatures with
a change in selectivity from 95 to 99% in 24 h.
4
3
5
Additional information about the copper species in the
CAPS- and com-CuO catalysts was obtained by H
(
2
-TPR
Fig. 5). The H consumption was also studied for bulk CuO
and Cu O to elucidate how bulk copper species behave differ-
ently from highly dispersed surface Cu species on CAPS- and
com-CuO. Interestingly, the reduction temperatures for CAPS-
and com-CuO were very low compared with those of bulk
2
2
2
CuO and Cu O, suggesting that bulk species were reduced at
higher temperatures than CuIJII) on the surface. The low tem-
perature peaks of TPR due to highly dispersed CuO and/or
CuIJII) species have also been observed for supported
44,45
46
47
CuO.
Van der Grift et al. and Robertson et al. further
The effect of catalyst loading was further studied with
CAPS-CuO at 100 °C in air with catalyst mass between 10 and
supported this observation concluding that the highly dis-
persed copper oxide species are more easily reduced than
bulk CuO. CAPS-CuO seemed to have no bulk copper oxide
and was therefore reduced at very low temperatures, while
com-CuO comprises both dispersed and bulk-like CuO
6
0 mg (Fig. 7). Not surprisingly, the overall reaction rate was
found to increase with the amount of catalyst and even
0 mg (25 mol%) of the catalyst gave 75% yield in 24 h.
2
Fig. 6 Effect of atmosphere on benzyl alcohol oxidation. Reaction
conditions: benzyl alcohol (1 mmol), anisole (0.1 mmol), toluene (3 mL),
60 mg CAPS-CuO catalyst, 24 h, 100 °C.
Fig. 5
2
H -TPR profiles of different copper catalysts.
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Table 2 Influence of temperature on the activity of catalyst CAPS-CuOa
b
b
b
Entry Temp. [°C] Conversion [%] Selectivity [%] Yield [%]
1
2
3
4
40
60
80
100
11 (5)
21 (9)
49 (18)
>99 (36 )
95 (>99)
97 (99)
98 (98)
10 (5)
20 (9)
48 (18)
>99 (35)
>99 (97)
a
Reaction conditions: benzyl alcohol (1 mmol), anisole (0.1 mmol),
b
toluene (3 mL), 60 mg catalyst, 24 h, atmospheric oxygen. After
4 h and after 6 h in parentheses.
2
Fig. 8 Conversion of benzyl alcohol with time over CAPS-CuO,
MES-CuO, and com-CuO (commercial) catalysts. Reaction conditions:
benzyl alcohol (1 mmol), anisole (0.1 mmol), toluene (3 mL), catalyst
(60 mg) 24 h, 100 °C, atmospheric air.
MES-CuO and the distribution of CuO, Cu O and Cu metal
2
were monitored by XRD of the catalysts after 24 h of reaction
(
Fig. 1b and d). Catalyst CAPS-CuO was reduced partially with
time (ESI† Fig. S5b) to Cu O, but not to an extent that could
2
Fig. 7 Effect of CAPS-CuO catalyst loading on oxidation of benzyl
alcohol showing conversion (black) and yield (grey). Reaction condi-
tions: benzyl alcohol (1 mmol), anisole (0.1 mmol), toluene (3 mL),
affect the reaction rate. The reduction was more pronounced
in the case of the MES-CuO catalyst, where only about 5 wt%
of the copper remained as CuO.
24 h, 100 °C, atmospheric air.
The partial reduction of the CuO nanoparticles led to a
change in morphology. The spent MES-CuO catalyst showed
a large number of spherical, hollow structures around 100
nm in size (Fig. 2g). These were not present in the spent
CAPS-CuO sample where only a few 10 nm spherical struc-
tures were found. It is clear that the slight difference in the
synthesis procedure of the two copper oxides had great influ-
ence on the stability resulting in the superior performance of
CAPS-CuO. This agrees with the HR-TEM results showing that
the surface of CAPS-CuO is dominated by highly stable [001]
Furthermore, the selectivity of the reactions seemed indepen-
dent of the catalyst loading with only insignificant amounts
(1%) of esterification product (benzyl benzoate) observed for
the reaction run with 40 mg of catalyst. In terms of reaction
time, conversion and selectivity a loading of 60 mg of CAPS-
CuO catalyst was considered optimal and thus maintained
for further experiments. This corresponds to 75 mol% and
approaches stoichiometric amounts. However, this is less
important since it is a non-precious metal and can be suc-
cessfully regenerated (see section 3.8).
Fig. 8 shows the time-dependent catalytic performance of
commercial CuO (com-CuO) in comparison with the prepared
CuO (CAPS- and MES-CuO). The initial reaction rate was low
and similar for the three CuO catalysts, indicating that an
induction period prevailed before reaching full activity. Thus,
after 4 h of reaction the conversion of benzyl alcohol had
reached only about 11% for all of the catalysts. However, at
prolonged reaction time of 24 h the CAPS-CuO provided
3
4,35
facets.
com-CuO resulted most likely in lower catalytic
activity than the CAPS-CuO catalyst, because of its substan-
tially lower surface area and the absence of well-defined crys-
tallinity (ESI† Fig. S2).
3.5 Versatility of the reaction
With the optimized reaction conditions established, the
scope of aldehyde and/or ketone formation was examined
with the CAPS-CuO catalyst. The substrates were carefully
chosen to demonstrate the versatility of the reaction between
saturated and unsaturated alcohols, aromatic and alicyclic
alcohols, and substituted and unsubstituted alcohols
(Table 3). The results confirmed that conversion of a broad
range of alcohols was achieved with excellent ≥98% selectiv-
ity except for cinnamyl alcohol which only gave 61% selectiv-
ity at full conversion (Table 3, entry 6) due to benzopyran for-
mation (cyclization product). Thus, importantly the
unsaturated alcohols, allyl alcohol and cinnamyl alcohol
(entries 6 and 13), yielded the desired aldehydes in good to
excellent yields without loss of the carbon double bonds. The
>99% yield of benzaldehyde, whereas the reaction rate was
lower for the MES- and com-CuO yielding only 69 and 70%
benzaldehyde in 24 h, respectively. The origin of the induc-
tion period could be related to low initial surface–substrate
interaction, heating of the solvent from room temperature to
the target temperature, or catalyst wetting processes although
this is not presently disclosed.
The stability of the catalysts can be closely related to the
ability to maintain a high amount of CuIJII) avoiding transfor-
mation into CuIJI) and especially metallic Cu, both of which
have shown poor activity. The phase transition in CAPS- and
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Table 3 Catalytic oxidation of various alcohols over CAPS-CuO with aira
Entry
1
Substrate
Product
Conversion [%]
Selectivity [%]
99
>99
2
3
75
>99
98
98
4
5
100
61
98
>99
6
7
100
6
61
>99
8
9
98
27
99
>99
1
0
1
3
7
98
1
>99
12
>99
>99
b
13
14
15
>99
7
96
>99
>99
6
a
b
Reaction conditions: alcohol (1 mmol), anisole (0.1 mmol), toluene (3 mL), catalyst (60 mg), 24 h, 100 °C, atmospheric air. 80 °C.
saturated aliphatic alcohols (entries 14 and 15) afforded very
low yields of aldehyde but in excellent selectivity, suggesting
that longer reaction time (or higher temperature) than com-
pared to benzyl alcohol was needed to reach high yields.
To study the effect of ring strain on the activity cyclo-
pentanol, cyclohexanol and cycloheptanol (entries 10–12)
were also included as substrates. Interestingly, the activity
towards these alcohols increased markedly with ring size, i.e.
the number of carbon atoms in the ring. This was most pro-
nounced in the case of cycloheptanol where >99% yield of
cycloheptanal was obtained. These results are well-matched
with previously reported results by Wang et al., that tension
in the ring increases with the size leaving the molecule more
reactive. Furthermore, Wang et al. also reported that para-
substituents had no obvious effect on the catalytic oxidation
rates of benzyl alcohols with Au/CuO under the given condi-
tions. This is understandable if benzyl carbocation
intermediates are not generated of which stability (and thus
reactivity) would strongly depend on the para substituent.
Arguably, a reaction mechanism including β-H elimination is
still proposed in the work. In our work, different para-
substituted benzyl alcohols were also tested as substrates in
the CuO catalyzed oxidation reaction (Table 3, entries 2–5).
A
significant decrease in substrate conversion was
observed with the substituents in the following order:
p-OCH (100%) > p-CH (98%) > p-Cl (75%) > p-CF (61%).
3
3
3
This suggests a linear free energy relationship between the
+
activity and the Brown–Okamoto (σ ) parameter, where sub-
2
2
strates with electron donating groups IJp-OCH
are more reactive than those with electron withdrawing
groups (p-Cl and p-CF ). These results clearly indicate that
3 3
and p-CH )
3
the reaction proceeded via a rate determining β-H elimina-
tion step resulting in the formation of an intermediate benzyl
carbocation. Such an elimination step is also generally found
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4
8,49
to be the rate determining step.
The slight decrease
3.7 Reaction heterogeneity
(
1–2%) found in selectivity to benzylic aldehydes (Table 3,
To assess the heterogeneity of the reaction, a hot filtration
test was performed after 8 and 24 h of the reaction with the
CAPS-CuO catalyst (Fig. 9). After removal of the catalyst after
entries 1–5) was due to the formation of the corresponding
esters. Such esters may be formed by condensation of the
benzyl alcohols with benzoic acids (from over-oxidation of
aldehyde or aldehyde disproportionation) or by oxidation of
the hemiacetals (from reaction of benzyl alcohols with
benzaldehydes).
8
h of reaction, no further conversion was observed indicat-
ing that active species did not leach to the reaction liquor.
However, after 24 h of reaction a trace amount of copper was
indeed observed by ICP-MS in the reaction phase (ESI†
Fig. S1). Interestingly, the leached copper species did not cat-
alyze the reaction homogeneously when excess benzyl alcohol
was added to the filtrate, and the yield remained on the same
level (2–3%) as found in the blank experiment after 48 h of
reaction (Table 1, entry 11). These findings clearly ruled out
that leached copper species contributed to the oxidation of
benzyl alcohol.
3
.6 Mechanism of oxidation
The XRD measurements showed that part of the CuO catalyst
was reduced during reaction initially forming Cu O and even-
2
tually metallic Cu. The amount of reduced copper increased
with the reaction time (ESI† Fig. S5b). Copper was reoxidized
to CuO after regeneration by calcining at 300 °C for 30 min
(ESI† Fig. S6). The observation of reduced copper in experi-
3.8 Reusability studies
ments with high conversion and the lack of Cu(0) in reac-
tions with low conversion (i.e. under an atmosphere other
than air) suggests that redox cycling of active copper centers
may be involved in the reaction mechanism.
An imperative feature of heterogeneous catalysts is their
recovery from the reaction medium and reuse. The prepared
CAPS-CuO catalyst could easily be separated from the reac-
tion mixture by filtration followed by drying (120 °C, 1 h) and
reused in the oxidation of benzyl alcohol as shown in Fig. 10.
In the second reaction run the catalytic performance remained
unchanged with conversion and yield of >99%, showing the
apparent stability of the prepared catalyst. However, during
further reuse the catalyst lost activity leading to a gradual
decrease in conversions to 53, 7 and 2% in the successive third,
fourth and fifth reaction run, respectively. Notably, the conver-
sion after the fifth run (2%) was equal to the conversion
obtained without the presence of the catalyst (Table 1, entry 11).
From the findings of TGA (ESI† Fig. S7), it was evident
that the spent catalyst lost 11 wt% when treated at 250 °C in
air. This suggested that carboxylate deposits had accumu-
lated on the active sites of Cu restricting the accessibility to
A simplified mechanism for CuO catalyzed benzaldehyde
formation under aerobic conditions is proposed in Scheme 1
4
9
in agreement with reports by Abad et al. and Fristrup et al.
In the cycle, benzyl alcohol adsorbs on a copper site forming
metal–alkoxide intermediate. The hydroxy proton is
a
abstracted by a neighbouring surface oxygen. Subsequently,
β-H elimination facilitated by CuIJII) generates CuIJII) hydride
and a benzyl carbocation intermediate followed by aldehyde
formation. In the final step, atmospheric oxygen reacts with
the hydride forming the peroxide anion and regenerating the
catalytic CuIJII) site. Reaction of the peroxide anion with a sec-
ond hydride reduces it further to hydroxide anions. These
hydroxide anions react in an off-cycle mechanism with the
protonated surface oxygen to produce water as the only by-
product. Further studies have been initiated to elucidate the
mechanistic pathway. The observed reduction of the CuO
may arise from deprotonation of the hydride intermediate
forming Cu(0) and subsequent conproportionation of Cu(0)
and adjacent CuIJII) to from CuIJI).
Fig. 9 Yields from separate hot filtration experiments where the
CAPS-CuO catalyst was removed by filtration and the reaction filtrate
was allowed to react further. (●): Catalyst removed after 8 h. (▼): Cata-
lyst removed after 24 h followed by addition of 1 mmol substrate and
continued reaction for another 24 h. (■): Results with catalyst from Fig. 8
(for comparison). Reaction conditions: benzyl alcohol (1 mmol), anisole
(0.1 mmol), toluene (3 mL), catalyst (60 mg), 100 °C, atmospheric air.
Scheme
1 Simplified reaction mechanism for the CuO catalyzed
oxidation of benzyl alcohol with atmospheric oxygen.
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4. Conclusions
Two types of copper oxide nanoparticles (CAPS-CuO and
MES-CuO) were synthesized using the different buffers CAPS
and MES, respectively, thoroughly characterized by imaging
spectroscopy and diffraction techniques, and tested for the
oxidation of benzyl alcohol to benzaldehyde with air at atmo-
spheric pressure. Synthesis with the two buffers had a signifi-
cant effect on the catalytic performance of the copper nano-
particles, and CAPS-CuO (CAPS buffer) gave better product
yields than MES-CuO (MES buffer). HR-TEM characterization
of the catalysts revealed a difference in exposed crystal facets
with CAPS-CuO exposing mainly stable [001] facets possibly
limiting its degradation during reaction. In addition, Rietveld
refinements of XRD data showed a clear difference in CuO
reduction between the catalysts, which likely influenced their
catalytic performance.
Fig. 10 Reuse of the CAPS-CuO catalyst in the oxidation of benzyl
alcohol with intermediate drying at 120 °C for 1 h (until run 5). Reac-
tion conditions: benzyl alcohol (1 mmol), anisole (0.1 mmol), toluene
(
3 mL), catalyst (60 mg), 24 h, 100 °C, atmospheric air.
The versatility of the oxidation reaction with CAPS-CuO
was demonstrated by transforming benzylic, alicyclic and
unsaturated alcohols to the desired products with exceptional
selectivity (>99%). A significant increase in conversion was
observed with benzyl alcohols containing electron donating
the substrate. This was further supported by ATR-FTIR where
−
1
a weak absorbance between 1300 and 1650 cm
was
observed (ESI† Fig. S8). Hence, if the catalyst was pre-heated
at 600 °C for 30 min before being reused again in a sixth
reaction run the catalyst regained a significant part of its
original activity and 76% conversion was obtained (Fig. 10).
However, a significant decrease in activity to about 60% con-
version (99% selectivity) was again obtained after yet another
reaction run (seventh run), possibly due to agglomeration of
copper oxide particles facilitated by the thermal regeneration
process.
groups IJp-OCH3 and p-CH ) compared to those containing
3
electron withdrawing groups (p-Cl and p-CF ), suggesting a
3
+
linear free energy relationship between activity and σ .
Results from competitive oxidation of benzyl alcohols sup-
port that α-C–H dissociation results in the formation of a sta-
ble benzylic carbocation intermediate. Moreover, the activity
towards alicyclic alcohols was shown to follow the ring ten-
sion leaving larger rings more reactive. Importantly, CAPS-
CuO was further found to be stable and reusable if thermally
regenerated. Further studies, including kinetic isotopic stud-
ies and Hammett correlations, are in progress to elucidate
the reaction mechanism in more detail.
The use of CuO as an efficient, inexpensive and selective
heterogeneous catalyst for the oxidative dehydrogenation of
alcohols proposes a new sustainable reaction protocol for car-
bonyl compound synthesis.
Another reusability study was performed with fresh cata-
lyst and intermediate re-activation at 300 °C for 45 min
(instead of 600 °C for 30 min) (Fig. 11). As expected, the cata-
lyst demonstrated here almost consistent performance for
four consecutive catalytic runs with consistent conversion
and selectivity of ≥97% (the slight decrease in conversion in
the third run is attributed to an experimental error).
Acknowledgements
Financial support from the UNIK research initiative Catalysis
for Sustainable Energy and Lundbeck (R45-A3878) for the
PhD study of R.P and C.E. is acknowledged. Authors would
like to express their gratitude to Jonas Andersen for work on
Rietveld refinements and Dr. Leonhard Schill for assistance
with XPS acquisition.
References
1
G. Tojo and M. Fernandez, in Oxidation of Alcohols to
Aldehydes and Ketones: A Guide to Current Common Practice,
Springer, New York, 2006.
2
(a) M. A. Halcrow, P. F. Knowles and S. E. V. Phillips, in
Handbook on Metalloproteins, ed. I. Bertini, A. Sigel and H. Sigel,
Marcel Dekker Inc, New York, 2001, p. 709; (b) Q. Jr. Lawrence
and B. T. William, Nature, 2008, 455, 333.
Fig. 11 Reuse of the CAPS-CuO catalyst in the oxidation of benzyl
alcohol with intermediate thermal re-activation at 300 °C for 45 min.
Reaction conditions: benzyl alcohol (1 mmol), anisole (0.1 mmol), tolu-
ene (3 mL), catalyst (60 mg), 24 h, 100 °C, atmospheric air.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
3
(a) S. S. Shannon, Science, 2005, 309, 1824; (b) J. Muzart,
Tetrahedron, 2003, 59, 5789; (c) J. Muzart, Chem. – Asian J.,
and C. W. Tornøe, Chem. Rev., 2008, 108, 295; (d) F. Collet,
C. Lescot and P. Dauban, Chem. Soc. Rev., 2011, 40, 1926.
26 (a) D. Yu and Y. G. Zhang, Proc. Natl. Acad. Sci. U. S. A.,
2010, 107, 20184; (b) D. Yu and Y. G. Zhang, Adv. Synth.
Catal., 2011, 353, 163; (c) Y. G. Zhang, L. W. Chen and T. Lu,
Adv. Synth. Catal., 2011, 353, 1055; (d) A. Martínez-Asencio,
D. J. Ramon and M. Yus, Tetrahedron, 2011, 67, 3140.
27 (a) C. Bolm, J. Legros, J. Le Paih and L. Zani, Chem. Rev.,
2004, 104, 6217; (b) B. D. Sherry and A. Fuerstner, Acc. Chem.
Res., 2008, 41, 1500; (c) A. Correa, M. O. Garcia and C. Bolm,
Chem. Soc. Rev., 2008, 37, 1108; (d) R. H. Morris, Chem. Soc.
Rev., 2009, 38, 2282; (e) C. L. Sun, B. J. Li and Z. J. Shi,
Chem. Rev., 2011, 111, 1293; ( f ) M. O. Garcia, Angew. Chem.,
Int. Ed., 2011, 50, 2216; (g) A. C. Gonzalez, K. Yoshida,
R. Luque and P. L. Gai, Green Chem., 2010, 12, 1281.
28 O. David, M. B. Alina, A. R. Antonia, G. Walter, L. Rafael and
K. C. Oliver, Green Chem., 2013, 15, 1530.
2
006, 1, 508; (d) T. F. Blackburn and J. Schwartz, J. Chem.
Soc., Chem. Commun., 1977, 157; (e) C. Liu, S. Tang and
A. Lei, Chem. Commun., 2013, 49, 1324.
G. Cainelli and G. Cardillo, in Chromium Oxidations in
Organic Chemistry, Springer, Berlin, 1984.
S. Zhang, L. Xu and M. L. Trudell, Synthesis, 2005, 11, 1757.
G. B. Shul'Pin, Y. N. Kozlov, L. S. Shul'Pina, T. V. Strelkova
and D. Mandelli, Catal. Lett., 2010, 138, 193.
R. A. Sheldon and J. K. Kochi, in Metal-Catalysed Oxidations
of Organic Compounds, Academic Press, New York, 1981.
T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037.
D. L. Pradeep, R. W. Smita, G. Harsh and P. P. Hankare,
J. Korean Chem. Soc., 2012, 56, 539.
4
5
6
7
8
9
1
1
1
0 J. Mielby, R. Poreddy, C. Engelbrekt and S. Kegnæs, Chin. J.
Catal., 2014, 35, 670.
1 Z. Junjiang, L. F. Joaquim, L. F. Jose and T. Arne, Chem. –
Eur. J., 2011, 17, 7112.
2 S. J. Tauster, S. C. Fung, R. T. K. Baker and J. A. Horsley,
Science, 1981, 211, 1121.
29 K. Qiang and Z. Yugen, Green Chem., 2012, 14, 1016.
30 I. E. Marko, M. Tsukazaki, P. R. Giles, S. M. Brown and
C. J. Urch, Angew. Chem., Int. Ed. Engl., 1997, 36, 2208.
31 F. Zaccheria, N. Ravasio, R. Psaro and A. Fusi, Chem. – Eur.
J., 2006, 12, 6426.
1
1
3 C. Parmeggiani and F. Cardona, Green Chem., 2012, 14, 547.
4 G. C. Bond, P. A. Sermon, G. Webb, D. A. Buchanan and
P. B. Well, J. Chem. Soc., Chem. Commun., 1973, 444.
5 G. J. Hutchings, J. Catal., 1985, 96, 292.
32 D. P. Cristina, F. Ermelinda and R. Michele, J. Catal., 2008,
260, 384.
1
1
33 R. M. Rioux and M. A. Vannice, J. Catal., 2005, 233, 147.
34 E. Christian, M. Philip, A. Jonas, Z. Lijuan, S. Kenny, L. Bin,
H. Jun and Z. Jingdong, J. Nanopart. Res., 2014, 16, 2562.
35 Z. Zhongping, S. Haiping, S. Xiaoqiong, L. Dongfei,
Y. Haidong and H. Mingyong, Adv. Mater., 2005, 17(1), 42.
36 Y. Li-Xia, J. Z. Ying, T. Hau, L. Liang and Z. Ling, Mater.
Chem. Phys., 2008, 112, 442.
37 L. Jinping, H. Xintang, L. Yuanyuan, K. M. Sulieman,
H. Xiang and S. Fenglou, Cryst. Growth Des., 2006, 6, 1690.
38 L. Jing, J. Jun, D. Zhao, Z. H. Shao, Y. H. Zhi, W. Li,
W. Chao, H. C. Li, L. Yu, T. G. Van and L. S. Bao,
J. Colloid Interface Sci., 2012, 384, 1.
6 M. Haruta, N. Yamada, T. Kobayashi and S. Ijima, J. Catal.,
1
989, 115, 301.
1
1
7 L. Prati and M. Rossi, J. Catal., 1998, 176, 552.
8 Z. Hou, N. Theyssen, A. Brinkmann, K. V. Klementiev,
W. Grünert, M. Bühl, W. Schmidt, B. Spliethoff, B. Tesche,
C. Weidenthaler and W. Leitner, J. Catal., 2008, 258, 315.
9 A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180.
1
2
0 Guideline on the specification limits for residues of metal
catalysts from the European Agency for the Evaluation of
Medicinal Products (EMEA), 2007 (http://www.emea.europa.eu).
According to the guideline, only iron residues are
considered entirely unproblematic. The limit for the total
39 J. F. Xu, W. Ji, Z. X. Shen, S. H. Tang, X. R. Ye, D. Z. Jia and
X. Q. Xin, J. Solid State Chem., 1999, 147, 516.
40 A. B. Pereda, L. T. U. De, G. N. J. Illan, L. A. Bueno and
V. J. R. Gonzales, Appl. Catal., B, 2014, 147, 420.
−
1
amount of platinoid-group metals is 5 mg kg (for dosages
of up to 10 g per day). Compliance to this limit is usually
assured by post-reaction scavenging/purification.
2
1 (a) C. E. Garrett and K. Prasad, Adv. Synth. Catal., 2004, 346,
41 N. S. McIntyre and M. G. Cook, Anal. Chem., 1975, 47, 2208.
42 http://www.lasurface.com, accessed on 13th May 2014.
43 P. Juyun, L. Kyounga, D. R. Rex and C. K. Yong, Bull. Korean
Chem. Soc., 2011, 32, 3395.
44 M. Shimokawabe, H. Asakawa and N. Takezawa, Appl. Catal.,
1990, 59, 45.
8
89; (b) C. J. Welch, W. J. Albaneze, W. R. Leonard, M. Biba,
J. DaSilva, D. Henderson, B. Laing, D. J. Mathre, S. Spencer,
X. Bu and T. Wang, Org. Process Res. Dev., 2005, 9, 198; (c)
J. T. Bien, G. C. Lane and M. R. Oberholzer, Top. Organomet.
Chem., 2004, 6, 263.
2
2
2 H. Wang, F. Weibin, H. Yue, W. Jianguo, N. K. Junko and
T. Takshi, J. Catal., 2013, 299, 10.
3 (a) S. Enthaler, K. Junge and M. Beller, Angew. Chem., Int.
Ed., 2008, 47, 3317; (b) E. Nakamura and K. Sato, Nat.
Mater., 2011, 10, 158.
45 W. P. Dow, Y. P. Wang and T. J. Huang, J. Catal., 1996, 160, 155.
46 C. J. G. Van der Grift, A. Mulder and J. W. Geus, Appl. Catal.,
1990, 60, 181.
47 S. D. Robertson, B. D. Mcnicol, J. H. Debaos, S. C. Kloet and
J. W. Jenkins, J. Catal., 1975, 37, 424.
2
2
4 R. Poreddy, E. J. Garcia-Suarez, A. Riisager and S. Kegnæs,
Dalton Trans., 2014, 43, 4255.
5 (a) A. Alexakis, J. E. Backvall, N. Krause, O. Pamies and
M. Dieguez, Chem. Rev., 2008, 108, 2796; (b) L. M. Stanley
and M. P. Sibi, Chem. Rev., 2008, 108, 2887; (c) M. Meldal
48 K. I. Shimizu, S. Kenji, S. Kyoichi and S. Atsushi, Chem. –
Eur. J., 2009, 15, 2341.
49 (a) A. Abad, A. Corma and H. Garcia, Chem. – Eur. J., 2008,
14, 212; (b) P. Fristrup, L. B. Johansen and C. H. Christensen,
Catal. Lett., 2008, 120, 184.
This journal is © The Royal Society of Chemistry 2015
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