Dehydrogenation of cyclohexanol on copper containing catalysts: The role of the support and the preparation method

Article abstract of DOI:10.1016/j.catcom.2011.10.021

SiO₂ and Al₂O₃ supported copper catalysts were prepared by "chemisorption-hydrolysis" or incipient wetness impregnation methods and investigated by XRD, TG-TPR, UV-vis diffuse reflectance and FTIR spectroscopy. Formation of finely dispersed copper oxide species was registered for the samples prepared by "chemisorption-hydrolysis" method, while a significant amount of XRD detectable copper oxide phase is registered for the SiO₂ impregnated one. The latter materials possess higher catalytic selectivity in cyclohexanol dehydrogenation to cyclohexanone.

Full text of DOI:10.1016/j.catcom.2011.10.021

Catalysis Communications 17 (2012) 150–153
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Catalysis Communications
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Short Communication
Dehydrogenation of cyclohexanol on copper containing catalysts: The role of the
support and the preparation method
a
b
b
c
M. Popova a, , M. Dimitrov , V. Dal Santo , N. Ravasio , N. Scotti
⁎
a
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Institute of Molecular Sciences and Technologies of the National Research Council, ISTM-CNR, Milan, Italy
Dept. CIMA “L. Malatesta”, University of Milan, Via Venezian 21, 20133 Milan, Italy
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2011
Received in revised form 18 October 2011
Accepted 20 October 2011
Available online 29 October 2011
SiO2 and Al2O3 supported copper catalysts were prepared by “chemisorption–hydrolysis” or incipient wet-
ness impregnation methods and investigated by XRD, TG-TPR, UV–vis diffuse reflectance and FTIR spectros-
copy. Formation of finely dispersed copper oxide species was registered for the samples prepared by
“chemisorption–hydrolysis” method, while a significant amount of XRD detectable copper oxide phase is
registered for the SiO2 impregnated one. The latter materials possess higher catalytic selectivity in cyclo-
hexanol dehydrogenation to cyclohexanone.
Keywords:
Cyclohexanol dehydrogenation
Copper catalysts
© 2011 Elsevier B.V. All rights reserved.
“Chemisorption–hydrolysis”
Impregnation
1. Introduction
20 wt.% or less. Some of the discrepancies in the results and in the in-
terpretation can be attributed to the wide range of experimental
Cu-based catalysts are of industrial importance in the synthesis of
methanol, steam reforming of methanol, synthesis of cyclohexanone
from cyclohexanol etc. [1–3]. Dehydrogenation of cyclohexanol to cy-
clohexanone is a very important industrial process because cyclohex-
anone is an intermediate used in the production of caprolactam. The
reaction is endothermic but the increase of the reaction temperature
to above 280 °C accelerates the sintering of copper because the com-
mercial catalyst contains large amounts of copper [2–10]. To over-
come this disadvantage, catalysts with low copper loading have
been developed, which yielded cyclohexanone below a reaction tem-
perature of 280 °C. A number of investigations have been focused on
determining the influence of the support, the method of preparation,
and the metal loading on the activity and selectivity of the catalysts,
with the goal of enhancing the alcohol conversion [1–13]. Stohmeier
et al. [14] examined the surface properties of impregnated Cu/Al2O3
catalysts and found that at low copper loading (b10 wt.%) copper
ions form well-dispersed species, connected to the support, whereas
at higher copper content, segregation of bulk-like CuO occurs. How-
ever, according to [1] copper particles in silica supported catalysts
are highly dispersed in the catalyst, the Cu content of which is
conditions and catalyst preparation procedures. Both surface acid–
base characteristics and redox properties of the catalysts influence
their activity and selectivity in dehydrogenation of cyclohexanol.
In the present study, we focus on the effect of the support (SiO2
and Al2O3) and the preparation method (“chemisorption–hydrolysis”
or incipient wetness impregnation) on the catalytic behavior of copper
modified samples in cyclohexanol dehydrogenation.
2. Experimental
2.1. Synthesis
SiO2 (Grace, BET=300 m2/g) and Al2O3 (Puralox ScFa/140/L3
(La=3%), Sasol, BET=126 m2/g) were used as supports of copper
oxide which was introduced by two methods — “chemisorption–
hydrolysis” (CH) and incipient wetness impregnation (WI). The SiO2
and Al2O3 supported samples were prepared by “chemisorption–
hydrolysis” according to the procedure described in [15]. The modified
samples were calcined in air at 673 K for 4 h and they were designed
as CuO/SiO2(CH), and CuO/Al2O3(CH).
The second series was obtained by incipient wetness impregna-
tion technique with copper nitrate. The precursor was decomposed
by calcination in air at 773 K for 2 h for CuO/SiO2 and CuO/Al2O3.
The modified samples, prepared by impregnation method, were
designated as CuO/SiO2(WI), and CuO/Al2O3(WI). In all cases the
Cu content was ca. 6–8 wt.% .
⁎ Corresponding author at: Institute of Organic Chemistry with Centre of Phyto-
chemistry, Acad. G. Bonchev str., bl.9, 1113 Sofia, Bulgaria. Tel.: +359 2 979 39 61;
fax: +359 2 8700 225.
E-mail address: mpopova@orgchm.bas.bg (M. Popova).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.10.021

M. Popova et al. / Catalysis Communications 17 (2012) 150–153
151
2.2. Characterization
maximum, observed at about 250 nm, is related to charge-transfer
between mononuclear Cu2+ ion and oxygen, whereas the maxima
around 350 and 450 nm can be ascribed to the charge transfer
between Cu2+ cluster and oxygen in [Cu–O–Cu]n surface species
[16–19]. The bands characteristic for mononuclear Cu2+, as well
as for the [Cu–O–Cu]n cluster species are more intense in the spec-
trum of the CuO/Al2O3(CH) sample prepared by “chemisorption–
hydrolysis” method in comparison to its impregnated analogue.
The shift of the band to higher wavelength (>400 nm) for the
CuO/SiO2(WI), and CuO/Al2O3(CH) samples (Fig. 1) is due to the
presence of polymerized copper oxide species. At the same time,
the intensive band at 700 nm in the spectra of the CuO/SiO2(WI)
can be attributed to the presence of crystalline CuO phase, which
is in accordance with XRD data (see above). The differences in
the nature and the oxidative state of copper sites, which are formed
on the modified samples, could be ascribed to the variations in
the modification procedure and the nature of the applied support.
So, the “chemisorption–hydrolysis” method provides predominantly
the formation of copper ions and [Cu–O–Cu]n cluster species, while
the impregnation method leads to the formation of crystalline
CuO nanoparticles on the silica support as well.
More information on the state of copper oxide species is obtained
by TPR–TG experiments (Fig. 2). The reduction profiles of the copper-
containing catalysts depend on the support material used and the
method of their preparation. The copper-containing SiO2 and Al2O3
samples prepared by both methods possess clearly defined low-
temperature reduction peaks in the region of 410–500 K attributed
to Cu(II)→Cu(0) reduction. The presence of a clearly defined high-
reduction peak at 770 K is registered only for CuO/SiO2(WI) (Fig. 2).
The extent of reduction is 100% for all studied samples. The narrow
reduction peak with maximum at 450 K, registered for CuO/Al2O3
(CH) and CuO/SiO2(CH), with a small shoulder at 500 K for the latter,
is related to the more homogenous distribution of copper oxide
species prepared via CH method on the supports. The presence of
two overlapping peaks with maxima at 410–420 K and 460 K in
TPR–TG profiles of the CuO/SiO2(WI) and CuO/Al2O3(WI) samples
(Fig. 2) is due to the formation not only of finely dispersed copper
oxide species but of CuO as a separate phase as well.
X-ray diffractograms were recorded on Philips PW 1810/1870
diffractometer applying monochromatized CuKα radiation (40 kV,
35 mA).
The TPR–TG investigations were performed in a Setaram TG92
instrument.
Diffuse reflectance spectra of the samples in the UV–vis region
were registered using a Jasco V-650 UV–vis spectrophotometer
equipped with integrating sphere. All spectra were recorded under
ambient conditions.
FTIR of adsorbed pyridine (Py) were registered on a Biorad FTS-40
FTIR spectrophotometer.
2.3. Catalytic activity measurements
Prior to the catalytic test the samples were pretreated for 1 h in
nitrogen up to 673 K. Cyclohexanol dehydrogenation was studied
at atmospheric pressure using a fixed-bed flow reactor, 95% nitrogen
and 5% hydrogen as carrier gas and 30 mg of sample. The nitrogen/
hydrogen stream (20 ml/min) passed through a saturator filled with
cyclohexanol and equilibrated at 342 K (nN2+H2/ncyclohexanol=38),
WHSV of 1 h−1, P=1 atm. The activity was determined at 513 K
after 60 min time on stream. On-line analysis of the reaction
products was performed using HP-GC with a 25 m HP-35 capil-
lary column. The turnover frequency (TOF) was calculated as the
converted number of cyclohexanol molecules per metal atom per
second.
3. Results and discussion
3.1. Physico-chemical characterization of the samples
XRD patterns collected for the studied samples indicate reflections
typical of CuO only for CuO/SiO2(WI) (not shown). No reflections of
crystalline copper oxide phases are registered on CuO/Al2O3, pre-
pared by both methods due to the formation of finely dispersed
copper oxide species. Stronger copper interaction with the support
could be concluded in the case of the Al2O3 supported samples. For-
mation of CuO with particle size of about 23 nm is registered for
CuO/SiO2(WI).
Summarizing the above observations we could conclude that the
reduction of copper oxide species occurs at a low temperature
(410–500 K) and in a narrower temperature interval for the samples
prepared by “chemisorption–hydrolysis” method in comparison to
their copper analogues prepared by impregnation. Probably, the
formation of different types of copper oxide species by the two
applied methods is responsible for the different reduction behavior.
DR UV–vis spectroscopy was applied to understand the nature and
the coordination of the copper oxide species in the studied samples.
The spectra of the studied samples are presented in Fig. 1. The first
Fig. 1. UV–vis spectra of the copper samples.
Fig. 2. TPR–TG profiles of various copper modified samples.

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M. Popova et al. / Catalysis Communications 17 (2012) 150–153
This assumption is also supported by the XRD and DR UV–vis data,
where formation mainly of isolated copper ions and copper oligomers
in the case of samples prepared by “chemisorption–hydrolysis” method
and additionally presence of larger in size and amount crystalline
copper oxide particles for the samples prepared by incipient wetness
impregnation method was found.
One of the most informative techniques for the detection of Lewis
and Brönsted acid sites is FT-IR spectroscopy of adsorbed pyridine
(Py). Bands typical of Py adsorbed on Lewis acid sites are hardly de-
tectable on the initial Al2O3 support (spectra 3 in Fig. 3), there are
only very weak features at 1449 and 1611 cm−1 attributable to Py
coordinatively bound to weak Lewis sites [20]. The copper loading
of copper on Al2O3 support induces substantial changes in the FTIR
spectra (Fig. 3, spectra 1, 2). The spectra of CuO/Al2O3(CH) and
CuO/Al2O3(WI) samples show the presence of absorption bands lo-
cated at 1449/1611 cm−1 typical of Py adsorbed on Lewis acid sites
[20,21]. While CuO/Al2O3(WI) sample spectra basically behaves like
CuO/Al2O3(CH), the acidic properties of CuO/SiO2(WI) are similar to
those of the silica support, i.e. no bands of adsorbed pyridine on
Lewis acid sites are registered (Fig. 3, spectra 4–6). The bands at
1449/1611 cm−1 are observed on FTIR spectrum of CuO/SiO2(CH),
more probably because of the formation of isolated copper ions in
it, interacting as Lewis acid sites.
Fig. 4. Catalytic activity at 513 on the studied materials.
To sum up CuO/SiO2(CH), CuO/Al2O3(CH) and CuO/Al2O3(IW)
samples show the presence of Lewis acidic sites, whereas CuO/SiO2
(IW) shows no acidity and is similar to the silica support. The FTIR
results are in good accordance with the data from XRD and DR
UV–vis spectroscopy, confirming that copper oxide is formed as a
separate phase on SiO2 by incipient wetness impregnation method.
Therefore it does not influence the surface acidity, whereas the iso-
lated copper ions formed on SiO2 by “chemisorption–hydrolysis”
method are responsible for the Lewis acidity of the sample.
to cyclohexanone is favored on CuO/SiO2(CH) and CuO/SiO2(WI). The
FTIR spectra of adsorbed pyridine (Fig. 3) are in good accordance
with the catalytic data. Formation of high amounts of phenol (Fig. 4)
is observed on the SiO2 and Al2O3 supported samples prepared by
“chemisorption–hydrolysis” method. According to the literature the
Lewis acid–base pairs react with an alcohol molecule to generate an
adsorbed long-lived alkoxy intermediate [19,22]. The surface oxygen
species act as Lewis bases, which extract hydrogen from cyclohexanol
and the intermediates. At the same time, copper oxide species act as
Lewis acid sites to stabilize the adsorbed negatively charged inter-
mediates. The XRD and UV–vis data of all the spent catalysts are
presented in Fig. 5. The catalysts were purged in nitrogen to ambient
temperature to avoid their reoxidation, followed by XRD and UV–vis
measurements. The formation of crystalline Cu2O (27 nm) is registered
on CuO/SiO2(WI), whereas no copper oxide phase is detected on the
spent CuO/SiO2(CH) (Fig. 5A). The reduction of CuO to Cu2O in the
spent CuO/SiO2(WI) sample was proven by UV–vis measurement,
where the shift of the band at 700 nm to 400–500 nm range is regis-
tered (Fig. 5B). The latter is assigned to Cu2O layers formation [15].
However, the UV–vis spectrum of the CuO/SiO2(CH) sample shows
a shift of the band at 250 nm to higher wavelength (>300 nm),
indicative of the formation of copper clusters and finely dispersed
metallic copper.
3.2. Catalytic activity for cyclohexanol dehydrogenation
In Fig. 4 and Table 1 the catalytic activities of copper containing
materials in cyclohexanol dehydrogenation at 513 K are presented.
High catalytic activity is registered for CuO/SiO2(CH) and CuO/Al2O3
(CH) samples and the former one possesses higher selectivity to cyclo-
hexanone. Their analogues prepared by impregnation show lower
catalytic activity. The higher selectivity to cyclohexene observed for
CuO/Al2O3(CH) and CuO/Al2O3(WI) samples (Fig. 4) is probably
due to the acid sites of the alumina support, responsible for the
dehydration process. The dehydrogenation reaction of cyclohexanol
The UV–vis spectrum of the spent CuO/Al2O3(WI) sample also
shows the band at 400–500 nm region and a shift of the bands
above 600 nm present in the spectrum of the fresh sample to lower
wavenumbers (Fig. 5B). These effects could be ascribed to the Cu+
and metallic copper formation during the catalytic reaction. A well-
defined band at 580 nm is observed in the spectrum of the spent
CuO/Al2O3(CH) sample, indicative for metallic copper formation
(Fig. 5B). The formation of very finely dispersed metallic copper in
the spent CuO/Al2O3(CH) catalyst was proved by XRD (Fig. 5A). The
copper species in CuO/SiO2(WI) remain finely dispersed after the
catalytic test and they are not detectable by XRD (Fig. 5A).
Table 1
Catalytic activity of the samples in cyclohexanol dehydrogenation at 513 K.
Samples
TOF, s−1
CuO/SiO2(CH)
0.0969
CuO/SiO2(WI)
0.0818
CuO/Al2O3(CH)
0.0952
CuO/Al2O3(WI)
0.0634
Fig. 3. FTIR pyridine adsorption spectra of CuO/Al2O3(CH), CuO/Al2O3(WI), CuO/SiO2(CH)
and CuO/SiO2(WI) samples after evacuation at 673 K and pyridine desorption at
373 K .

M. Popova et al. / Catalysis Communications 17 (2012) 150–153
153
A
B
4. Conclusion
0
x Cu2O
245
300
* Cu
x
The method of modification and the support used (SiO2 and Al2O3)
strongly influence the state of the obtained copper oxide species.
In contrast to the incipient wetness impregnation procedure, the
“chemisorption–hydrolysis” method facilitates the formation of
higher amounts of finely dispersed and readily reducible copper
ions and copper cluster species [Cu–O–Cu]n that provide higher
catalytic activity in cyclohexanol dehydrogenation This effect is more
pronounced, when SiO2 is used as a support. The higher catalytic
selectivity to cyclohexanone of the CuO/SiO2 sample, prepared
by incipient wetness impregnation method, is due to the forma-
tion of finely dispersed Cu+ and their stabilization on the support
material.
320
CuO/SiO2(WI)
x
445
x
CuO/SiO2(WI)
CuO/SiO2(CH)
CuO/Al2O3(CH)
580
CuO/Al2O3(CH)
700
*
*
CuO/SiO2(CH)
CuO/Al2O3(WI)
CuO/Al2O3(WI)
40
50
60
70
80
200
400
600
800
Bragg angle (°2 Theta)
Wavelength, nm
Acknowledgments
Fig. 5. XRD patterns (A) and UV Vis spectra (B) of CuO/SiO2(CH) and CuO/SiO2(WI)
after the catalytic test.
Financial support by the project DO02-295_2008 and the Bulgarian-
Italian Inter-academic Exchange Agreement are greatly acknowledged.
On the basis of these observations and the catalytic data we can
conclude that Cu+ are the active sites for cyclohexanol dehydrogena-
tion to cyclohexanone. Fridman et al. [4,5] also found that Cu+ are the
most active species in Cu–Zn catalyst in cyclohexanol dehydrogena-
tion to cyclohexanone. The higher selectivity to phenol is observed
on the SiO2 and Al2O3 supported samples prepared by “chemisorp-
tion–hydrolysis” in comparison to that of the impregnated ones. The
interpretation of the phenol formation in the literature [4,5,19,22] is
ambiguous and two possible routes are suggested — directly from
cyclohexanol (direct route) and through dehydrogenation of cyclo-
hexanone (consecutive route). According to Fridman et al. [4,5] Cu+
are the active sites for cyclohexanol dehydrogenation to cyclohexa-
none, whereas zero-valent copper species are the active sites for
the formation of cyclohexanone and phenol. The applied physico-
chemical methods show that Cu+ are predominantly formed and
stabilized on SiO2 and Al2O3 catalysts prepared by impregnation.
We assume that the higher amount of the formed phenol on the sam-
ples prepared by the CH method is due to the initial presence in these
samples of a higher amount of more finely dispersed Cu2+ ions
and clusters. They can be easily reduced to finely dispersed metallic
copper under the reaction medium, whereas stabilized Cu2O species
are responsible for the higher cyclohexanol conversion to cyclohexa-
none. The results presented by us support the direct route of phenol
formation. Additional catalytic experiment with CuO/SiO2(CH), pre-
treated in hydrogen at 523 K, shows significant decrease in the selec-
tivity to phenol (not shown). This result can be explained by the
agglomeration of metallic copper formed during the reduction pre-
treatment procedure. The formation of a higher amount of phenol
was observed only on CuO/SiO2(CH) without reduction pretreat-
ment. This effect could be explained with the formation of finely
dispersed metallic copper by the reaction medium.
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