Study of the deactivation of copper-based catalysts for dehydrogenation of cyclohexanol to cyclohexanone

Article abstract of DOI:10.1016/j.cattod.2011.10.010

Catalytic dehydrogenation of cyclohexanol was carried out in the gas phase in a continuous fixed bed reactor under atmospheric pressure. Two commercial catalysts composed by copper chromite and copper zinc oxide were tested. The activity of the catalysts

Full text of DOI:10.1016/j.cattod.2011.10.010

Catalysis Today 187 (2012) 150–158
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Study of the deactivation of copper-based catalysts for dehydrogenation of
cyclohexanol to cyclohexanone
Ernesto Simón, Juana María Rosas, Aurora Santos, Arturo Romero∗
Dpto Ingenieria Quimica, Facultad de Ciencias Químicas, Universidad Complutense Madrid, Ciudad Universitaria S/N, 28040 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2011
Received in revised form 5 October 2011
Accepted 11 October 2011
Available online 9 November 2011
Catalytic dehydrogenation of cyclohexanol was carried out in the gas phase in a continuous fixed bed
reactor under atmospheric pressure. Two commercial catalysts composed by copper chromite and cop-
per zinc oxide were tested. The activity of the catalysts was evaluated at 250 ◦C, with a weight hourly
space velocity of 2.89 h−1 and at times on stream (TOS) higher than 400 h. A slow deactivation of the
catalysts was observed during the reaction, obtaining an activity reduction of 50% at TOS = 350 h, but
maintaining 30% conversion at these conditions. Yield higher than 97% to cyclohexanone was obtained
with both catalysts, in all the range of TOS studied. The main impurity observed was phenol, obtained
from dehydrogenation reaction, and cyclohexene, only obtained with the copper chromite catalysts, and
derived from cyclohexanol dehydration. The results of stability and selectivity with these catalysts are
better than others reported in the literature. The main causes of the activity loss are associated to the
coke deposition over the copper active sites and the increase of the copper metallic crystallite size. The
copper zinc oxide also presents a slight blockage of the porous structure due to coke deposition, what
can also decrease the catalyst activity.
Keywords:
Cyclohexanol
Cyclohexanone
Dehydrogenation
Copper
Deactivation
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
stable metal oxides (Al, Si, and Ti) are added to the copper cata-
lysts to enhance their properties. Among the different additives,
Cyclohexanone is an important intermediate of the chemical
industry. Among its different applications, cyclohexanone is used as
raw material for caprolactam manufacture, which is the monomer
of nylon 6. Cyclohexanone is mainly produced through the cat-
alytic dehydrogenation of cyclohexanol. From an industrial point
of view, the heterogeneous catalytic gas-phase dehydrogenation, at
atmospheric pressure, is severely restricted by highly endothermic
reaction (ꢀH = 65 kJ/mol) and thermodynamic equilibrium [1,2].
Two different conditions for the catalytic dehydrogenation of
cyclohexanol can be considered depending on the reaction tem-
perature used: at low temperature, from 200 to 300 ◦C; and at high
temperature, from 350 to 450 ◦C. Furthermore, two different kinds
of catalysts can be used to produce this reaction: copper-based
and zinc calcium oxide catalysts, respectively. Recently, most of
the studies are carried out with catalysts based on copper formu-
lations. They present a highly dispersed copper phase, when they
are carefully reduced; and they usually operate under mild con-
ditions (200–300 ◦C) [3–7]. These copper catalysts are not used
at high temperature due to copper sintering [8]. Metals such as
Zn, Cr, Fe, Ni, alkali metals, alkaline earth metals, and thermally
chromium oxide is frequently used, because it acts as a structural
promoter, increasing the BET surface area and also inhibiting the
sintering of copper particles [9,10]. The addition of ZnO is also
very useful because it promoted the dispersion and stability of the
resulting copper catalysts [11,12].
The cyclohexanol dehydrogenation reaction has been widely
studied in our research group. In previous papers, we have focused
on the identification of the different impurities obtained in this
reaction, formed in the transformation stages of the reagents (oxi-
dation, hydrogenation, dehydrogenation, oximation, Beckmann
rearrangement, etc.), which affect seriously the final fibber proper-
ties; and on the proposal of a kinetic model that describes both the
cyclohexanol conversion to cyclohexanone and the phenol produc-
tion (main impurity of this reaction) [13,14].
Other important factor governing an adequate catalyst selection
for an industrial application is deactivation resistance during a long
run test. To our best knowledge little works have been published
on the deactivation of the copper catalysts for cyclohexanol dehy-
drogenation reaction. In these works, some of the catalysts tested
suffered deactivation, although the authors did not carry out a deep
study of the deactivation causes [4,15].
In the present work, we have evaluated the behaviour of two
different commercial copper based catalysts on the catalytic dehy-
drogenation of cyclohexanol, to establish correlations between
∗
Corresponding author.
E-mail address: aromeros@quim.ucm.es (A. Romero).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.10.010

E. Simón et al. / Catalysis Today 187 (2012) 150–158
151
at 4 cm−1 resolution. Pressed KBr pellets at a sample/KBr ratio of
around 1:250 were used.
The surface texture of the samples was characterized by scan-
ning electron microscopy (SEM). Scanning electron micrographs
were obtained using a JEOL JSM-6400 instrument, working at a high
voltage of 20–25 kV.
physicochemical properties and the catalytic activity of calcined,
reduced and used catalysts. Furthermore, changes on the cata-
lyst during cyclohexanol dehydrogenation at different times on
stream have been also analyzed with the objective to determine the
possible deactivation causes (coke depositions, active phase modi-
fications, thermal sintering or physical modifications) [16], as well
as the possibility of the catalyst regeneration.
The solid state 13C NMR experiments were recorded on Bruker
Avance-400-WB spectrometers at 100.62 MHz for 13C. Glycine was
used as a reference for 13C chemical shifts. 13C CP/MAS NMR spec-
tra were recorded with a spinning rate of 12 kHz, a pulse width of
2.5 ␮s, a contact time of 5 ␮s and a recycle delay of 5 s. 13C NMR
experiments were conducted with proton decoupling.
2. Experimental
2.1. Chemical and catalysts
The copper metallic surface and dispersion were measured
using a nitrous oxide titration method. The sample was in situ
reduced with 95/5% of a nitrogen/hydrogen mixture at 180 ◦C for
18 h, prior feeding N2O. Afterwards, sample was exposed to pulses
of 6.4% N2O balance with helium at 90 ◦C. N2 and N2O in the exit gas
were separated using a PLOT Q column and determined by thermal
conductivity detector (Agilent 3000A Micro GC analyzer). A molar
Cyclohexanol (Sigma–Aldrich, 105899), cyclohexanone (Fluka,
29135), phenol (Riedel-de Haën, 33517) and cyclohexene (Fluka,
29230) have been used as reactants or standards. Two commercial
catalysts were used: copper chromite catalysts, Cu-0230 (tablets,
3.1 mm × 3.1 mm) supplied by Engelhard, with a weight composi-
tion of 72% CuO, 26% CuCr2O4 and 2% graphite; and copper zinc
oxide catalyst, T-2130 (tablets, 3 mm × 3 mm), provided by Süd-
Chemie, with a weight composition of 33% CuO, 66% ZnO and 1%
graphite. For the sake of simplicity, Cu-0203 is denoted as C1, while
T-2130 as C2.
0
stoichiometry Cusup0/N2O = 2 was assumed, where Cusup implies
a copper atom on surface.
2.4. Analytical methods
2.2. Catalytic activity
Cyclohexanol and cyclohexanone were analyzed by GC/FID
(HP 6890 GC-FID). For this analysis a HP-50+ 19095L-021 (50%
phenyl–50% dimethylpolysiloxane) 15 m × 0.53 mm ØI × 1 ␮m col-
umn were used. An undecane (Aldrich, U407) was used as ISTD
for calibration. Cyclohexene and phenol were analyzed by GC/MS
(HP 6890 GC-FID). For this analysis a HP-INNOWAX 19091N-116
(crosslinked PEG) 60 m × 0.25 mm ØI × 0.25 ␮m column were used.
A 1,4-benzodioxan (Aldrich, 179000) was used as ISTD for calibra-
tion.
Catalytic dehydrogenation of cyclohexanol to cyclohexanone in
the gas phase was carried out at atmospheric pressure in a contin-
uous fixed-bed reactor made of a stainless-steel tube. 10 grams of
catalyst are loading in each test. The bed volume was completed
with nonporous glass spheres, inert glass wool and stainless-steel
wire mesh. As pretreatment, the catalysts were reduced with 95%
nitrogen–5% hydrogen at 180 ◦C for 18 h (gas hourly space velocity,
GHSV = 1100 h−1). The reaction temperature was 250 ◦C. Before the
reaction starting, the catalyst was stabilized with N2 at the reaction
temperature. Cyclohexanol was fed with a 5 wt.% cyclohexanone
to avoid the cyclohexanol solidification (melting point, m.p. 22 ◦C).
The liquid flow rate was 0.5 mL min−1 (weight hourly space veloc-
ity, WHSV = 2.89 h−1). The vapour effluent from the reactor was
cooled at 20 ◦C and liquid and gas phase were separated and col-
lected. Liquid phase was subsequently analyzed by GC/FID and
GC/MS.
3. Results and discussion
3.1. Catalytic activity
Fig. 1 shows the cyclohexanol conversion for C1 (a) and C2 (b)
catalysts, at 250 ◦C and a WHSV = 2.98 h−1. Both copper catalysts
presented high initial cyclohexanol conversions. These cyclohex-
anol conversion values obtained with both catalysts seem to be
higher than others reported in the literature with different cop-
per catalysts, however, the experimental conditions and the copper
content were not exactly the same [3,15,17]. C1 catalyst showed
higher activities than C2, obtaining with both catalysts very high
yield (>97%) to cyclohexanone production, as it will be shown in a
next section of this work. As it was previously reported, the high
catalytic activity observed in the dehydrogenation for both cata-
lysts could be ascribed to the small copper crystallite sizes and
the high copper dispersions. Furthermore, the reaction has been
also evaluated at 220 and 290 ◦C, observing steady-state increasing
cyclohexanol conversions with the temperature at the same space
time [14].
2.3. Catalyst characterization
BET surface area (SBET) and pore volume (Vp) of the catalysts
were determined using the N2 adsorption–desorption technique at
−196 ◦C performed by a Beckman Coulter SA3100 analyzer. Before
each measurement, the sample was outgassed at 563 K for 120 min.
The total carbon (TC) was determined by catalytically aided
combustion oxidation at 900 ◦C, with a Co3O4 catalyst, using
500 mL min−1 of O2 (99.9 vol.%) as carrier gas, performed by a Shi-
madzu solid sample combustion unit SSM-5000A.
XRD patterns were recorded on a Philips X’Pert diffractometer,
using monochromated Cu K␣ radiation (ꢁ = 1.5418 A), operating at
˚
45 kV and 40 mA. The measurements were recorded in steps of
The catalytic activity of both catalysts was evaluated to times on
stream (TOS) higher than 400 h. As can be seen in Fig. 1, a reduction
of the cyclohexanol conversion, about 50%, is observed for both
catalysts at very high TOS (350 h), mainly associated to a partial
deactivation of the catalysts. After the observed decreased in the
activity, steady state conversions higher than 30% were detected for
both catalysts at these high TOS. It is important to point out that,
to our best knowledge, the most of the copper catalysts used for
cyclohexanol dehydrogenation are only tested at TOS lower than
4 h and only a few authors reported data at higher TOS. Among
these studies, the most relevance ones are the works of Krishna
0.04◦ with a count time of 1 s in the 2Â range of 5–90◦.
The surface chemistry of the samples was analyzed by X-
ray photoelectron spectroscopy (XPS) analyses using a 5700C
model Physical Electronics spectrometer with Mg K␣ radiation
(1253.6 eV). For the analysis of the XPS peaks, the C 1s peak posi-
tion was set at 284.5 eV and used as internal reference to locate the
other peaks. Fitting of the XPS peaks was done by the least-squares
method using Gaussian–Lorentzian peak shapes.
Infrared (FTIR) spectra were obtained using a Nicolet 5700 spec-
trometer by adding 256 scans in the 4000–400 cm−1 spectral range

152
E. Simón et al. / Catalysis Today 187 (2012) 150–158
the catalysts. These data are summarized in Table 1. A more
detailed explanation of the characterization of the fresh catalysts
was reported in a previous work [14].
C1: The three peaks observed at 43.4◦, 50.3◦, and 74.1◦ are char-
acteristic peaks of metal copper crystallite. Wang et al. observed
that when the chromium content is below 40%, Cu species mainly
exist, in the catalysts, as Cu2O and Cu0 [18]. However, the pres-
ence of other crystallographic phases of copper, which could be
associated to the existence of Cu2O crystallites, was not detected.
In this sense, no signal associated to chromium species has either
been observed, suggesting that the species containing chromium
and/or Cu2O could be highly dispersed or exist in amorphous
phase.
The XRD profiles of the reduced and used catalysts are very
similar, suggesting that the cyclohexanol dehydrogenation reac-
tion does not produce any significant change on the crystalline
phases. However, the ratio between the intensities of the peaks
associated to the copper crystallite and the intensities of the charac-
teristic peak of carbonaceous species decreases during the reaction,
suggesting the formation of coke. Table 1 shows that the copper
crystallite size of the catalyst increases approximately a 30% after
the reaction at TOS = 395 h.
C2: The XRD pattern of this catalyst shows the presence of metal-
lic copper and ZnO phases. The XRD profile has not been notably
affected by the cyclohexanol reaction, in the same way as was
observed with C1. However, the crystallite size of metallic copper
increases to a great extent with the TOS, from 9 to approximately
27 nm at TOS = 349 h (see Table 1). The ratio between the intensities
of the peaks associated to the copper crystallite and the intensities
of the characteristic peak of carbonaceous species also decreases
during the reaction, probably, as a result of the coke deposition.
Copper dispersion. The copper metal areas are measured by
N2O titration according to the following surface stoichiometry:
N2O(g) + 2Cus → Cus–O–Cus + N2(g). The sizes of copper crystallites
and the Cu metal areas of the reduced and used catalysts are also
summarized in Table 1. The metallic dispersion of C1 and C2 cat-
alysts determined by N2O decomposition gave a value of 3.0 and
9.1%, respectively. This value is indicating that metallic phase is
formed by large particles of metal copper.
The copper dispersion of the C1 catalyst is slightly decreased
after the reaction, due to an increase of the copper crystallite size.
However, an important decrease of the surface coverage, about
a 45%, is observed at TOS = 395 h. This decrease is related to an
increase of the apparent surface area (as shown in Table 3), used
for the calculation of the surface coverage. The copper dispersion
of the C2 catalyst decreases almost three times after the reaction
(TOS = 349 h), increasing significantly the copper crystallite size. It
can be seen that the copper crystallite sizes calculated from N2O
chemisorption are higher than those obtained from the XRD data
in all cases. In view of these results, the sintering of copper par-
ticles seems to be one of the most likely causes of the observed
deactivation, mainly in C2 catalyst.
Fig. 1. Evolution of cyclohexanol (XOL) conversion and total carbon content (TC) as
a function of TOS for C1 (a) and C2 (b) catalysts. T = 250 ◦C; WHSV = 2.98 h−1
.
Reddy et al. [12], using gamma alumina supported Cu catalysts,
they observed stable cyclohexanol conversion at 10 h; Cesar et al.
[4], using bimetallic Cu–Co/SiO2 with different metal loading, they
evaluated the stability of their catalysts up to 48 h; and Fridman
et al. [7], with Cu–Mg and Cu–Zn–Al catalysts, although they did
not show any data at TOS higher than 2 h, they stated that each
catalyst was tested for about 170 h, without changes in catalyst
activity.
To summarize, an initial deactivation can be observed on both
catalysts, obtaining steady state conversions at TOS higher than
200 h. From this point, both catalysts showed a high stability for
the cyclohexanol dehydrogenation reaction even at very high TOS.
The initial activity decreases can be associated to poisoning, coking,
sintering, and/or solid phase transformations. The analysis of the
possible causes of deactivation results necessary to take place the
study of the regeneration process.
3.2. Chemical properties
XRD. The XRD patterns of the reduced and used catalysts, C1
and C2, are shown in Fig. 2. The copper crystallite size was esti-
mated from Debye–Scherrer equation from the XRD patterns of
Table 1
Physicochemical properties of the reduced and used catalysts.
Catalyst
Cu surface aread (m2/g)
Cu dispersiona (%)
Surface coverageb
Cu crystallite sizec,d (nm)
Cu crystallite sizee (nm)
C1 reduced
C1 used TOS 395 h
12.6
11.4
3.00
2.71
0.937
0.511
34.5
38.2
27.2
36.2
C2 reduced
C2 used TOS 349 h
15.3
5.6
9.06
3.33
0.353
0.151
11.4
31.3
9.0
24.9
a
Calculated as the ratio of surface Cu atoms to bulk Cu atoms.
Calulated by dividing the Cu surface area by the BET surface area.
Calculated as: 6000 × (Cu content)/(Cu metal area) × (Cu density).
Determined by N2O chemisorptions.
b
c
d
e
Determined by XRD.

E. Simón et al. / Catalysis Today 187 (2012) 150–158
153
Fig. 2. XRD patterns of the C1 and C2 reduced, used and regenerated catalysts. T = 250 ◦C; WHSV = 2.98 h−1
.
3.3. Coke formation
for the C2 catalyst. This signal can correspond to the formation of
carbon atoms with a sp2 hybridized state, which can be associated
to polynuclear aromatic compounds. Specifically, the broader line
with the greatest intensity at 127.3 ppm can be attributed to non-
substituted aromatic carbons of the aromatic rings. While, the weak
line at 144.0 ppm, can be assigned to substituted aromatic carbons
[21]. Other broad resonance signal was observed between 30 and
50 ppm, which can corresponds to aliphatic carbons.
Based on the XRD data, the possible formation of coke was ana-
lyzed. Fig. 1 also shows the total carbon content (TC) of the used
catalyst as a function of TOS. For the sake of simplicity, this TC
corresponds to the total carbon measured after subtracting the ini-
tial TC of the catalysts, thus meaning the carbon formed during
cyclohexanol dehydrogenation. As can be seen, the carbon con-
tent increases with TOS. About 7.67 wt.% coke on C1 catalyst has
deposited at 395 h, while about 5.64 wt.% coke on C2 catalyst has
been observed at 349 h. This TC increasing tendency of C1 takes
place simultaneously to an exponential decay of the cyclohexanol
conversion (see Fig. 1), suggesting that the formation of new coke
deposits does not produce any significant effect on the catalyst
activity. On the other hand, a low coke deposition rate is obtained
for C2 catalysts and a pseudo steady state conversion value was
obtained at high TOS (>300 h), what suggests that, the decrease on
the catalyst activity in C2 can be very influenced by the forma-
tion of coke. The coke deposited was analyzed by NMR and FTIR
techniques.
FTIR. No significant differences were observed between the
reduced and used C1 catalyst, only limited to weak and wide bands
between 3000 and 2800 cm−1, attributed to C–H stretching bands.
NMR. Any significant signal was not observed for the used C1
catalyst at the different TOS studied, probably due to the forma-
tion of coke in form of graphite-like or different kind of carbons
close to the paramagnetic centres, which are “invisible carbons” by
NMR [19]. In fact, some authors proposed that the coke invisible to
NMR could be highly condensed polycyclic aromatics or “graphitic”
structures [20]. The solid state 13C CP/MAS NME spectra of the
reduced and used C2 catalyst at TOS 349 h are shown in Fig. 3. A
resonance signal centred between 110 and 150 ppm was observed
Fig. 3. Solid state 13C CP/MAS NME spectrum of the C2 reduced and used catalyst.
T = 250 ◦C; WHSV = 2.98 h−1
.

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E. Simón et al. / Catalysis Today 187 (2012) 150–158
Table 2
Atomic surface concentration of the reduced and used catalysts determined by XPS analysis.
Catalyst
C (%)
O (%)
Cr (%)
Zn (%)
Cu (%)
Cu+ (%)
Cu2+ (%)
C1 reduced
49.93
74.65
74.00
72.37
34.04
19.81
20.11
21.01
4.45
2.61
3.48
3.64
–
–
–
–
11.58
2.93
2.41
2.98
54.34
76.5
88.43
73.81
45.66
23.5
11.57
26.19
C1 used TOS 34 h
C1 used TOS 217 h
C1 used TOS 395 h
C2 reduced
38.72
43.07
48.11
52.56
36.72
34.09
30.84
28.71
–
–
–
–
18.77
16.77
16.39
14.96
5.8
54.99
79.42
93.46
87.19
45.01
20.58
6.54
C2 used TOS 51 h
C2 used TOS 292 h
C2 used TOS 349 h
6.08
4.66
3.77
12.81
The used C2 catalyst showed the typically absorption bands asso-
ciated to polycyclic aromatic structures.
deposition does not seem as drastic as the one observed with the
C1 catalyst.
At this point, it is important to mention that coke deposition
can produce a decrease in the activity of the catalysts by two
mechanisms: active site suppression or pore blocking. Both of the
two coking modes can occur simultaneously, although one of the
two mechanisms is usually predominant. In this sense, the surface
chemistry and the textural properties of the catalysts were also
evaluated.
The chemical oxidation states of Cu in C1 and C2 catalysts at var-
ious stages of reaction were also studied by X-ray photoelectron
spectroscopy (XPS). The XPS spectra of Cu2p3/2 of the catalysts
are shown in Fig. 4(a) and (b), respectively. For the sake of com-
parison, the XPS spectra of the catalysts before the reduction step
were also included (calcined catalyst). The C1 catalyst, before the
reduction step, presents a well-separated main peak at 934.5 eV
and a satellite peak at 941.5 eV, characteristic of Cu2+. A signifi-
cant decrease of the satellite peak and a simultaneous shift of the
Cu2p3/2 peak towards lower binding energy (BE) are observed after
the reduction step, indicating the co-existence of non-reduced and
reduced copper species, probably due to an ineffective reduction of
the catalyst. A further decrease of the satellite peak was observed
during the reaction, suggesting that the reductant atmosphere pro-
duced during the cyclohexanol dehydrogenation can provide an
additional and more effective reduction of the copper species. As
can be seen, the Cu2p3/2 XPS spectra of the catalyst at the dif-
ferent TOS are very similar, what can indicate that steady-state
conditions are reached beyond a certain time on stream. For the
C1 used catalyst, the Cu2p3/2 band can be deconvoluted into two
different peaks. The most important peak is obtained at 932.4 eV,
the KE is at 917.3 eV and the modified Auger parameter is about
1849.7 eV. According to the copper chemical state plot, the peak
can be attributed to Cu2O, which means that Cu+ is the main cop-
per species (approximately 75%) on the catalyst during the reaction
[23]. The relative percentages of the different copper species are
also included in Table 2.
3.3.1. Surface chemistry
XPS. Reduced and used catalysts were analyzed by XPS. Table 2
shows the relative atomic surface concentration determined by XPS
analysis of these catalysts. As can be seen, the Cu/Cr atomic surface
ratio of the reduced C1 catalyst is approximately 2.6. This ratio is
lower than the corresponding bulk ratio provided by the manufac-
turer. In this sense, Guoyi Bai et al. proposed that this is due to Cr is
present in a highly dispersed phase on the catalyst surface, which
is in agreement with the XRD results [22]. The Cu/Cr atomic surface
ratio decreases to values lower than 1 during the reaction, due to an
important reduction of the copper content (about a 75%), while the
chromium content decreases only slightly. The decrease of the cop-
per content can be observed simultaneously to the increase of the
carbon content. These results also suggest the formation of coke, in
agreement with the XRD and TC analysis. As aforementioned, this
coke deposited can produce the blockage of the pores or maybe the
poisoning of the copper active sites. In this sense, the results shown
in Table 2 suggest that the partial deactivation of the catalysts can
be mainly related to the presence of coke deposited over the copper
active sites.
The atomic copper content on the surface of the reduced C2 cat-
alyst is lower than the one observed for the reduced C1 catalyst. The
Cu/Zn ratio of the reduced C2 catalyst is approximately 0.3, decreas-
ing to 0.25 at high TOS. In this case, the copper content decreases
about a 35%, while the carbon content increases the same 35%, sug-
gesting that coke deposition takes places mainly over the copper
active sites in the same way as it occurs with the other catalyst.
Although the reduction of the copper active sites due to the coke
The Cu2p3/2 XPS spectrum of the calcined C2 catalyst presents
an only peak associated to the presence of Cu2+. After the reduction,
the band can be deconvoluted into two peaks, attributed to Cu2O
(76.5%) and CuO (23.5%). In this case, the C2 catalyst also undergoes,
during the reaction, an additional reduction with the H2 produced,
obtaining approximately a 90% of Cu+. This percentage remains very
similar at the different TOS studied.
The XPS results of both catalysts initially indicate the presence
of Cu+ as the only non-reduced specie. These results seem to be in
Fig. 4. XPS spectra of Cu2p3/2 of the reduced and used C1 and C2 catalysts. T = 250 ◦C; WHSV = 2.98 h−1
.

E. Simón et al. / Catalysis Today 187 (2012) 150–158
155
Fig. 5. . SEM micrographs of the C1 reduced (a), C1 used at TOS 395 h (b), C2 reduced (c) and C2 used at TOS 349 h (d) catalysts. T = 250 ◦C; WHSV = 2.98 h−1
.
contrast of the XRD data. However, as aforementioned, XRD data
shows the existence or not of crystallographic phases of the differ-
ent compounds. The results of XRD can indicate that Cu2O could
be present in an amorphous state or highly dispersed. It is also
noteworthy that XRD can only calculate crystalline sizes that are
larger than 2–5 nm [24]. On the other hand, it is important to men-
tion that Auger spectra, used for the assignment of the chemical
oxidation states, have distinctive features for the different copper
species, CuO, Cu2O and Cu. But since they are very broad and close
in energy, these Auger spectra have a limited utility when trying
to quantify the amount of Cu2+, Cu+ and Cu0 present in a partially
reduced sample. In this respect, the Cu 2p3/2 peak position neither
does allow a clear distinction of Cu2O and Cu [25].
With regard to the active chemical oxidation state of copper
for this reaction can be found many works, the most relevant are
the works of Fridman and Davydov [5–7], Bai et al. [22] and Wang
et al. [23]. Fridman and Davydov [5–7] studied this reaction with
Cu–Zn–Al or Cu–Mg catalysts and they suggested that Cu+ is more
active than Cu0. However, Wang et al. [23] reported a different
result, namely that Cu0 is the active site for the dehydrogenation
of cyclohexanol to cyclohexanone, and Cu+ is the active site for
the aromatization of cyclohexanol over Cu–Zn–Si catalysts. Based
on our own results, the high activity of these catalysts and the
low amount of aromatization products obtained seem to be more
related to the presence of metallic copper, clearly observed by XRD
analyses than the Cu+, as suggested Wang et al. [23].
C2 catalysts, respectively. The reduced C1 catalyst presents a very
low value of SBET, indicative of a poor development of the porous
structure. After the C1 catalyst was used for 395 h, its catalytic
activity decreased (Fig. 1), however, its BET surface area and pore
volume slightly increases. The data derived from the N2 adsorption-
desorption isotherms (not shown) suggest that coke is deposited in
the macropores of the catalysts, leading to the formation of wide
micropores and mesopores. In this case, the partial deactivation of
the catalyst due to the partial blockage of the pores must be ruled
out, suggesting that the coke deposition mainly takes place over
the copper particles, decreasing the number of active sites.
The reduced C2 catalyst presents higher values of SBET and Vp
than C1. These values decrease approximately a 10% during the
reaction. The N2 adsorption–desorption isotherm data indicates
that this decrease is due to the reduction of the micro and meso-
pores. In this case, the coke deposition can be the responsible of the
partial blockage of the porous structure. Other authors also pro-
posed that sintering of the Cu and ZnO particles may also lead to
Table 3
Structural parameters of the reduced and used catalysts.
a
Catalyst
SBET (m2/g)
Vp (cm3/g)
rp (nm)
C1 reduced
C1 used TOS 217 h
C1 used TOS 395 h
14
20
22
0.040
0.047
0.064
5.7
4.7
5.8
C2 reduced
43
39
37
43
0.223
0.198
0.175
0.242
10.4
10.2
9.5
C2 used TOS 292 h
C2 used TOS 349 h
C2 regenerated
3.3.2. Textural properties
Table 3 shows the apparent surface area (SBET), the pore volume
(Vp) and the mean pore radius of the reduced and used, C1 and
11.3
a
Determined by Wheeler model.

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E. Simón et al. / Catalysis Today 187 (2012) 150–158
Fig. 6. Yield to cyclohexanone (YONE), phenol (YPhOH) and cyclohexene (YCXEN), if produced, as a function of TOS for the C1 (a) and C2 (b). T = 250 ◦C; WHSV = 2.98 h−1
.
a decrease in the pore size, and eventually pores close making the
active species inaccessible to the reactants [26].
the yield to the desirable product (cyclohexanone) has not been
significantly affected by both effects.
Fig. 5 shows the SEM micrographs of the reduced and used
C1 and C2 catalysts, respectively. The morphology of the reduced
catalysts is somewhat different from the used catalysts. A non-
structured coke seems to be formed on both catalysts after
the reaction. The carbonaceous structure deposited on C1 cata-
lyst seems to be heterogeneously deposited, what could slightly
increase the porosity of the catalyst. However, in the case of
the C2 catalyst, the coke appears more uniformly deposited on
the catalysts, decreasing their porosity, in agreement of the N2
adsorption–desorption data.
3.5. Regeneration
The reactivation of the catalysts was carried out using a regen-
eration method similar to that described by Marchi et al. [27],
when the partial deactivation has been caused by the sintering of
the copper, could be carried out by different steps: (i) recovery of
the original CuO phase by oxidation of the sintered copper parti-
cles, and (ii) regeneration of the Cu+ phase by reduction with the
hydrogen-containing mixture. The treatment in air and hydrogen
would therefore redisperse the sintered Cu+ phase, leading to a
recovery of the initial catalytic activity. When the main causes of
deactivation are derived from the coke deposition, the regenera-
tion of the catalysts could be performed by the carbon burn-off.
As the coke deposition was relatively high in these catalysts, the
regeneration by gasification of the carbonaceous deposits was eval-
uated. However, the gasification of the carbon at high temperatures
can produce the copper sintering in these catalysts. Therefore it is
advisable to carry out this regeneration at temperatures lower than
350 ◦C.
3.4. Study of the selectivities
More than 97% of the reacted cyclohexanol produced cyclo-
hexanone. However, other compounds can be obtained by the
dehydrogenation and dehydration reaction of cyclohexanol. When
cyclohexanone is used as raw material for the production of nylon,
these compounds or impurities can strongly affect the fibber prop-
erties. Phenol is the most important impurity obtained from the
dehydrogenation reaction and it is generated in a slightly major
amount by the C2 catalyst. While, cyclohexene is obtained from
cyclohexanol dehydration reaction and it can be only observed
with the C1 catalyst. The other minority impurities obtained in
this reaction for C1 catalyst were water, (directly formed by both
dehydration from cyclohexanol to cyclohexene) and a mixture
of the isomers 2-cyclohexylidene-cyclohexanone and cyclohexyl-
cyclohexanone (from self-condensation of cyclohexanone), and for
C2 catalyst was 2-cyclohexen-1-one [14].
These results can be due to the low amount of surface acidic sites
on these catalysts, so the dehydration of cyclohexanol is restrained,
and the yield of cyclohexanone is high, as can be also deduced by the
similarities between cyclohexanol conversion and yield to hydro-
gen. In this sense, Nagaraja et al. [15] found that the largest amount
of chrome in copper chromite catalysts (as this catalyst presents
(26%)), can induce dehydrating effects and therefore, it can generate
cyclohexene as one of the main impurities.
As aforementioned, these catalysts are very stable under the
experimental conditions. In this point, the analysis of the yield with
the TOS makes necessary. Fig. 6 reports the yields to cyclohexanone,
phenol and cyclohexene (if produced) as a function of TOS for the
C1 and C2 catalysts. As can be seen, the yields to the main reaction
products remain stable with the TOS, despite the decrease of the
activity observed with both catalysts. Even at relatively low activ-
ity values, the yield to cyclohexanone is higher than 97%. These
results suggest that both the coke deposition and the copper sinter-
ing can reduce considerably the cyclohexanol conversion. However,
In order to determine if the observed deactivation is reversible
or irreversible, we tried one regeneration procedure based on the
following steps: treatment in air at 300 ◦C for 8 h, followed by
reduction in H2 (100%) at 300 ◦C for 8 h. Afterwards, the regenerated
catalysts were tested in the gas-phase cyclohexanol dehydrogena-
tion at 250 ◦C and WHSV = 2.98 h−1
.
With this treatment, the carbon content was notably decreased
from 7.67 to 0.47% for C1 and from 5.64 to 0.11% for C2, respectively.
Therefore, the gasification of the catalysts produced an almost
complete removal of the carbonaceous deposits. In this sense,
the structural parameters of the regenerated C2 catalysts were
analyzed, obtaining the same apparent surface area of the fresh
catalyst, what suggests that the initial porosity is totally released.
As can be seen in Fig. 2, the XRD profiles of the regenerated cat-
alysts were not noticeable affected by the gasification reaction.
However, a 20% increase of the copper crystallite size to 43.5 nm
was observed in C1. In the case of C2, the copper crystallite size
was not significantly influenced by the regeneration process.
Fig. 7 shows a comparison of the cyclohexanol conversion before
and after the regeneration step for C1 (a) and C2 (b) catalysts, at
250 ◦C and WSHV = 2.98 h−1. The results indicate that the regenera-
tion process was successfully applied and the recovery of the initial
cyclohexanol conversions was observed in both catalysts. Never-
theless, a significant decay on the activity was obtained for both
catalysts. The cyclohexanol conversion decreases very fast in C1 to
finally reach a steady-state conversion value of 30% at TOS higher

E. Simón et al. / Catalysis Today 187 (2012) 150–158
157
Fig. 7. Evolution of cyclohexanol (XOL) conversion of C1 (a) and C2 (b) and total carbon content of C1 (c) and C2 (d) as a function of TOS, before and after regeneration, at
250 ◦C and a WHSV = 2.98 h−1
.
than 50 h (450 h, if considered the time previous to the regener-
ation process). This value corresponds to the same one observed
previous to the gasification reaction. The decrease on the conver-
sion observed takes place simultaneously to an important increase
of the carbon content (to 2.5%). The decay on the activity observed
in C2 takes place more slowly, being the cyclohexanol conversions
in the range of TOS evaluated similar to the ones obtained before the
regeneration process. In this case, the carbon content also increases
to 3.3%.
The yield to cyclohexanone for both catalysts remains the same
after the regeneration process, indicating that the nature of the
active sites is not appreciably modified during the gasification reac-
tion. These results suggest that a regeneration of the catalysts is
initially possible due to the removal of the carbonaceous deposits.
However, a fast decay on the activity takes place in both cata-
lysts, suggesting that the increase of the copper crystallite size can
present some influence on the conversion decrease.
on stream. While, the simultaneous decrease of the relative cop-
per concentration and the increase of the carbon concentration
(determined by XPS analysis) suggest that coke deposition takes
places preferentially over the copper active sites. Therefore, the
main causes of the activity loss observed for these catalysts can
be associated to the coke deposition over the copper active sites
and the increase of the copper metallic crystallite size. Besides, in
the case of the copper zinc oxide catalyst, a slight blockage of the
porous structure after the reaction can be observed, probably due
to the coke deposition, which can, in addition, diminish the cata-
lyst activity, limiting the accessibility of the reactants to the pores.
The regeneration of the catalysts by carbon burn-off removes the
carbonaceous deposits. This gasification treatment does not pro-
duce any significant modification on the nature of the active sites,
because the same yield to cyclohexanone was obtained after this
regeneration treatment.
Acknowledgements
4. Conclusions
The authors acknowledge financial support for this research
from the Spanish Ministry of Science and Innovation under projects
CTM 2006-00317 and PET 2008-0130. The authors would like to
express their gratitude to UBE Corporation Europe S.A. for its sup-
port, as well as Süd-Chemie AG for providing the catalyst.
Both catalysts tested present high cyclohexanol conversion. A
slow deactivation of the catalysts was observed during the reaction,
obtaining an activity reduction of the 50% at 400 h. Yield higher than
97% for the cyclohexanol reaction to cyclohexanone was obtained
for both catalysts, in all the range of time on stream studied. The
main impurities observed were phenol, obtained from the dehydro-
genation reaction, and cyclohexene, only observed with the copper
chromite catalysts, and derived from the cyclohexanol dehydration.
The values of stability and yield for the cyclohexanol dehydrogena-
tion are much higher than others reported in the literature; what
could make them very suitable for an industrial application.
The characterization of the catalysts indicated the presence of
crystallographic phases of metallic copper, with copper dispersion
between 3 and 9%. However, an increase of the crystallite size of
metallic copper was observed after the reaction. XRD and SEM
analysis evidenced the presence of coke deposited at high time
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