Product Selectivity as a Function of ZrO2 Phase in Cu/ZrO2 Catalysts in the Conversion of Cyclohexanol

Article abstract of DOI:10.1007/s10562-018-2473-6

The influence of ZrO₂ phase on the product selectivity arising from the preparation method of Cu/ZrO₂ is studied in the gas phase conversion of cyclohexanol. This study results are supported by NH₃-TPD, XRD, pyridine-FTIR and N₂O pulse chemisorptions measurements. However, N₂O pulse chemisorptions studies did not reveal significant differences between the two catalysts. The product selectivity is completely dependent on the ZrO₂ phase which ultimately led to the differences in the acidic properties observed through NH₃-TPD and pyridine-FTIR experiments. Catalyst poisoning experiments using NH₃ co-feeding brought a reversal in the product selectivity. Graphical Abstract: Cu/ZrO₂ catalyst prepared by impregnation method containing monoclinic ZrO₂ yields cyclohexanone and Cu/ZrO₂ catalyst prepared by coprecipitation method with tetragonal ZrO₂ phase possessing strong acidic sites yields benzene when cyclohexanol is contacted in vapour phase conditions [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2473-6

Catalysis Letters
https://doi.org/10.1007/s10562-018-2473-6
Product Selectivity as a Function of ZrO Phase in Cu/ZrO Catalysts
2
2
in the Conversion of Cyclohexanol
1
1
1
1
Peddinti Nagaiah · Chodimella Venkata Pramod · Madhuluri Venkata Rao · Burri David Raju ·
Kamaraju Seetha Rama Rao1
Received: 28 March 2018 / Accepted: 22 June 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
The influence of ZrO phase on the product selectivity arising from the preparation method of Cu/ZrO is studied in the
2
2
gas phase conversion of cyclohexanol. This study results are supported by NH -TPD, XRD, pyridine-FTIR and N O pulse
3
2
chemisorptions measurements. However, N O pulse chemisorptions studies did not reveal significant differences between
2
the two catalysts. The product selectivity is completely dependent on the ZrO phase which ultimately led to the differences
2
in the acidic properties observed through NH -TPD and pyridine-FTIR experiments. Catalyst poisoning experiments using
3
NH co-feeding brought a reversal in the product selectivity.
3
Graphical Abstract
Cu/ZrO catalyst prepared by impregnation method containing monoclinic ZrO yields cyclohexanone and Cu/ZrO catalyst
2
2
2
prepared by coprecipitation method with tetragonal ZrO phase possessing strong acidic sites yields benzene when cyclohex-
2
anol is contacted in vapour phase conditions
Keywords Copper · Zirconia · Monoclinic · Tetragonal · Cyclohexanol
1
Introduction
Zirconia is reported to be a good catalyst support for vari-
ous reactions by reason of its amphoteric character having
acid–base properties, high thermal and chemical stabil-
ity [1–9]. Zirconia exhibits three crystalline structures
*
Kamaraju Seetha Rama Rao
ksramaraoiict@gmail.com
1
Inorganic & Physical Chemistry Division, Indian Institute
of Chemical Technology, Hyderabad 500007, India
Vol.:(0
1
123456789)
3

P. Nagaiah et al.
depending on the preparation method i.e., cubic, tetrag-
onal and monoclinic phases [10, 11]. Only cubic phase
possesses low thermal stability and hence less studied in
catalysis. Tetragonal and monoclinic phases of zirconia
exhibit different types of acidic and basic properties that
may result in difference in activity depending on the reac-
2 Experimental Section
2.1 Preparation of Catalysts
Two catalysts were prepared using ZrO as a support and
2
Cu as an active metal (10 wt% Cu loading in each catalyst)
via impregnation, designated as CZI and co-precipitation
method, designated as CZP. In the co-precipitation method,
aqueous solutions of Cu(NO ) ·3H O (M/s. SD Fine, India)
tion. Effect of the ZrO phase on the structure and perfor-
2
mance of supported Cu catalysts in ethanol conversion was
studied [12] in which it was observed that the Cu/m-ZrO
2
3 2
2
catalyst showed higher activity of ethyl acetate formation
while the tetragonal phase directed the reaction towards
aldol condensation products that is catalyzed by Lewis
acid sites. Ding and co-workers examined the synthesis of
higher alcohols over different zirconia phases and claimed
that tetragonal phase is responsible for ethanol formation
while the monoclinic phase exhibited isobutanol as major
product [13]. Another study by Shin and co-workers on the
and ZrOCl ·8H O (M/s. SD Fine, India) in required amounts
2 2
were mixed together and precipitated with 5% NH OH solu-
4
tion. Completion of precipitation was assured after reaching
a pH of 9.0 by the addition of few drops of dil. NH OH
4
to the supernant liquid. Later, the precipitate has been fil-
tered and washed several times with de-ionized water to
remove chlorides completely (using AgNO test). Further,
3
the solid mass was kept for overnight drying at 393 K and
acidities of monoclinic and tetragonal phases of ZrO in
then subjected to calcination at 773 K for 5 h in air [20].
2
the production of H from decomposition of formic acid
In the impregnation method, firstly ZrO2 support was pre-
2
showed that t-ZrO has higher acidic strength than m-ZrO
pared following the above procedure mentioned in the co-
precipitation method without the use of Cu precursor in the
solution. Later, required amounts of Cu(NO ) ·3H O solu-
2
2
[
14]. The role of ZrO phase widely affected the hydrode-
2
oxygenation process of stearic acid on Ni/ZrO catalysts
2
3 2
2
[
15] where threefold increase in activity is observed on
tion was used to impregnate ZrO . Heating on a hot plate
2
Ni/m-ZrO compared to Ni/t-ZrO which was claimed to
was continued to get dried solid mass. After further drying
for 12 h at 393 K, the solid was subjected to calcination at
773 K for 5 h in air.
2
2,
be related to the synergistic interaction between Ni and
m-ZrO support. Another study by Witoon et al. on the
2
synthesis of methanol from hydrogenation of CO over dif-
2
ferent zirconia phase supported Cu catalysts [16] showed
2.2 Catalyst Characterization Techniques
that Cu/t-ZrO is four times higher active than the Cu/m-
2
ZrO . The reason they claimed was that the Cu/t-ZrO
X-ray diffractometer (Ultima- IV, supplied by M/s. Rigaku
2
2
catalyst has large copper surface area, strong metal support
Corporation, Japan) with Cu K radiation [2θ range from 10°
α
−
1
(
t-ZrO ) interaction and a high surface concentration ratio
to 80° and 4° min scan rate (40 kV, 30 mA)] was used to
get the XRD patterns of catalysts in calcined, reduced and
spent forms. A single point surface area analyzer (SMART
SORB 92/93 supplied by M/s. SMART Instruments, India)
was used to get the surface area of catalysts. The reducible
behaviour of active components and other precursors were
obtained by performing temperature programmed reduction
(TPR) experiments on a homemade set up. Briefly, about
0.1 g of catalyst was taken in a quartz reactor of 200 mm
long and 6 mm inner diameter and pretreated at 473 K in
Ar for 2 h to remove any water vapor present on the cata-
2
of atomic hydrogen to CO . Therefore, it is understood
2
from the above examples that the phase of ZrO plays a
2
decisive role in tuning the catalyst surface acid-base prop-
erties in nature and/or strength and the degree of interac-
tion of the impregnated metal with ZrO which ultimately
2
will lead to the change in reaction rates and/or product
selectivity. Furthermore, the literature showed that the
studies on dehydrogenation of cyclohexanol [17–19] were
mainly focused on achieving higher conversion (equilib-
rium reaction) and no one has dedicated the study towards
the effect of catalytic properties on product selectivity. In
this context, we attempted a study to understand the role
of catalyst support on the product selectivity in the con-
version of cyclohexanol. Therefore, we synthesized Cu/
3
−1
lyst. 5% H –Ar mixture at a flow rate of 30 cm min was
2
−
1
maintained while heating the sample linearly at 10 K min
from ambient temperature to 923 K. The consumption of
H has been observed by a thermal conductivity detector
2
ZrO catalyst in two different methods namely impreg-
(TCD) equipped gas chromatograph (GC), connected with a
standard GC software. The Cu characteristics like, its metal
surface area, particle size and dispersion were obtained by
2
nation and co-precipitation and applied them in the con-
version of cyclohexanol. We found that cyclohexanone is
predominant over the catalyst prepared via impregnation
method while benzene is the major product on the catalyst
prepared by co-precipitation method.
carrying out N O pulse chemisorption experiments on a
2
homemade pulse chemisorption setup. An automatic gas
sampling valve and a GC-17A (M/s. Shimadzu Instruments
1
3

Product Selectivity as a Function of ZrO Phase in Cu/ZrO Catalysts in the Conversion…
2
2
Corporation, Japan) with TCD detector and porapak-T col-
umn for the separation of N O and N have been connected
Silica beads were placed above the catalyst bed which acts
as the preheating zone. The reaction was performed in a
temperature range of 498–573 K with an interval of 25 K.
2
2
at the reactor inlet and outlets respectively. Before the exper-
iment, the catalyst was subjected to reduction in H flow at
Prior to the reaction, the catalyst was reduced in H flow
2
2
3
−1
5
53 K for 3 h. Later, H flow was replaced with He flow
(30 cm min ) at 553 K for 3 h. After setting the required
2
which was also used as GC carrier gas and the temperature
of the reactor was maintained at ambient temperature and
N O–He mixture (6% N O) was introduced onto the catalyst
temperature of the reaction, H flow was replaced with N
2
2
3
−1
gas flow (1200 cm h ) to remove any physisorbed H .
2
Later, cyclohexanol feed was injected continuously at a rate
2
2
3
−1
sample in pulses using a 6-port valve. It is assumed that
of 1 cm h using a Syringe pump (M/s. B. Braun, Ger-
many). The effect of time on the activity for this reaction
was carried out at 523 K. The products were collected in
a trap maintained at 273 K, placed below the reactor. The
obtained product components were confirmed by GCMS
QP-5050 (M/s. Shimadzu instruments, Japan) and quantified
by a flame ionization detector (FID) equipped gas chroma-
tograph, using ZB WAX column (30 m length 0.53 mm i.d.
and 3 µm film thickness). In a separate experiment cyclohex-
surface Cu metallic sites react with N O as per the following
2
stoichiometry [21, 22].
2
Cu + N O → Cu O + N
2 2 2
−
20
2
Copper atomic cross sectional area of 6.8×10 m and
−
3
Cu density of 8.92 g cm were used to calculate the Cu
dispersion and its particle size. AUTOSORB-iQ, a chem-
isorption analyzer (M/s. Quantachrome Instruments, USA)
was used for ammonia temperature programmed desorption
anol conversion over CZP catalyst with NH co-feeding was
3
(
NH -TPD) experiments to get an insight into the amount
3
carried out.
and strength of acid sites present on the catalyst surface.
Firstly, the catalyst (100 mg) was prereduced in H gas
2
3
−1
(
30 cm min ) at 553 K for 3 h and subsequently cooled
3
Results and Discussion
to 353 K and then saturated with 10% NH /He mixture gas
3
3
−1
(
30 cm min ) for half an hour. Later, the sample is flushed
3.1 XRD Studies
3
−1
with He gas (30 cm min ) at the same temperature for half
an hour to remove any physisorbed NH present on the sur-
3
XRD studies of catalysts in reduced and spent forms were
displayed in Figs. 1, 2 respectively. XRD pattern of CZI
face. Later, while maintaining the He flow, the temperature
−
1
was increased linearly to 1173 K at 10 K min from 353 K
catalyst shows monoclinic phase of ZrO (JCPDS Number:
2
and the desorbed NH was monitored with on-line connected
3
88-2390) at 2θ values of 28.1°, 31.5°, 49.25° and 50.1°.
TCD equipped GC. The desorbed NH concentration from
3
On the other hand, tetragonal phase of ZrO was observed
2
acid sites as a function of temperature was obtained which
gives the amount and the strength of acid sites present.
FTIR spectra of pyridine adsorbed catalysts were recorded
by ex situ method on a Spectrum GX spectrometer (M/s.
Perkin–Elmer, Germany) at room temperature. 10 mg of
the each catalyst was oven dried at 373 K for 1 h. To this
oven dried sample, 0.1 ml of pyridine was added, followed
by vacuum drying at 393 K for 1 h to remove the phys-
isorbed pyridine. Later, the samples were finely ground and
dispersed in KBr (1:10 ratio) and made into a pellet. The
−1
spectra were collected using 4 cm resolution and averaging
−
1
five scans in the spectral range of 400–4000 cm . Copper
composition in the catalyst was obtained by ICP-OES (M/s.
Thermo Scientific ICAP 6500 DU) by dissolving the sample
in aquaregia.
2.3 Activity Evaluation
The gas phase activity experiments were conducted in a
tubular down flow glass reactor (14 mm inner diameter with
3
00 mm length). The catalyst (1 g) is sandwiched between
two quartz wool plugs placed at the centre of the reac-
tor and within the isothermal zone of the heated furnace.
Fig. 1 XRD patterns of reduced catalysts (a) CZI (b) CZP
1
3

P. Nagaiah et al.
Fig. 2 XRD patterns of spent catalysts (a) CZI (b) CZP
in CZP catalyst with major peaks centered at 2θ values of
3
0.24°, 50.36° and 60.26° (JCPDS Number: 88-1007). How-
ever, the monoclinic phase appears as a shoulder prior to the
tetragonal phase with the shoulder peaks (inset of Fig. 1) at
2
θ values of 49.25 and 50.1. Presence of low intense metal-
lic Cu phase (JCPDS Number: 80-0836) at 2θ values of
4
3.3° (111 plane) and 74.13° (220 plane) can be witnessed
from the XRD patterns of CZI and CZP (reduced) catalysts.
In the XRD pattern of reduced CZP catalyst, high intense
reflections due to tetragonal phase of ZrO are visible while
2
a relatively less crystalline monoclinic phase is observed in
CZI catalyst. The crystallinity of both forms of zirconia is
calculated by the formula given below and the results are pre-
sented in Table 1. Stabilization of tetragonal phase of ZrO
2
was reported in literature by many researchers [23, 24] and
this is what happened in the CZP catalyst. XRD diffracto-
gram of spent catalysts collected after the time on stream
reaction (Fig. 2) are similar to those of reduced catalysts
indicating that there is no phase transformation either with
respect to Cu or ZrO .
2
Crystallinity(%)
ꢀ
ꢁ
2
Total area of crstalline peaks of m − ZrO t − ZrO
2
=
×
Total area of all peaks
100
3.2 Physicochemical Properties
BET surface area results of both CZI and CZP catalysts
along with ZrO are shown in Table 1 which indicates slight
2
1
3

Product Selectivity as a Function of ZrO Phase in Cu/ZrO Catalysts in the Conversion…
2
2
decline in the surface area after deposition of Cu. However,
similar values are observed for both CZI and CZP catalysts.
reducible CuO species that are weakly bound to ZrO sup-
2
port. This result revealed that the interaction between copper
N O pulse chemisorption studies (Table 1) revealed that
oxide species and support is weaker in CZP method than
2
a very slight difference persists in both the catalysts with
CZI method. We calculated the amount of H consumption
2
respect to Cu dispersion (D %), particle size (CPS) and
for the catalysts based on the area of the corresponding H
Cu
2
metal surface area (S ). It was reported that Cu dispersion
consumption peaks and the values are presented in Table 1.
However, it is worth to mention here that in both the cata-
lysts, the Cu dispersion is more or less same.
Cu
is slightly higher on tetragonal ZrO than on monoclinic
2
ZrO [25]. Cu dispersion in the present study is in line with
2
the literature values [25]. It seems that the smaller Cu parti-
cle size (~15 nm) in both the catalysts would be enough to
convert cyclohexanol efficiently. The metal content analyzed
by ICP showed similar Cu content which is close to the theo-
retical value (10% by weight).
3.4 Pyridine IR Studies
To understand the type of acidic centers present on the cat-
alyst surface, Pyridine-FTIR analysis was performed and
the obtained spectra were depicted in Fig. 4. Pyridine-FTIR
studies are useful to distinguish between Bronsted (B) and
Lewis acid (L) sites from their peak positions. The corre-
3.3 TPR Studies
−
1
The reduction profile of a catalyst provides information on
the reducibility of the active component in oxidic form and
its interaction with the support [18]. The reducibility of cop-
per species in the present catalysts is obtained from the TPR
profiles (Fig. 3). From the figure it is clear that the prepara-
tion method has an impact on the reduction behavior of CuO
species. In the TPR patterns of both the catalysts, the signal
in the low temperature region (~450 K) corresponds to the
conversion of highly dispersed CuO particles to metallic Cu
species and the high temperature (~530 K) signal indicates
the reduction of relatively bigger copper oxide particles
sponding peaks are located at 1490, 1545 and 1590 cm
[26, 27]. As shown from Fig. 4, the pure zirconia possesses
−
1
−1
two bands at 1490 and 1545 cm . The peak at 1490 cm
represents either Bronsted (BAS) or Lewis acid sites (LAS)
−
1
or both. The peaks at 1545 and 1590 cm are due to the
presence of BAS and LAS respectively. The peak corre-
sponds to BAS are absent from the FTIR pattern of CZI
which contains monoclinic ZrO phase while these sites are
2
present in CZP catalyst. In the impregnation method, BAS
−1
(1545 cm ) disappeared or covered by copper bonded to the
surface of zirconia and in CZP method, copper is embedded
into the zirconia matrix (Scheme 1) [28]. It is also possible
[
18]. The composition of these species is quite opposite in
both the catalysts indicating the difficult reduction of larger
copper oxide species in CZI catalyst interacting strongly
with the support. On the other hand, TPR pattern of CZP
catalyst witnesses the presence of larger number of easily
Fig. 4 Pyridine FTIR patterns of the catalysts (a) ZrO , (b) CZI, (c)
2
Fig. 3 TPR profile of catalysts (a) CZI (b) CZP
CZP. B BAS, L LAS
1
3

P. Nagaiah et al.
Scheme 1 Plausible reaction
mechanism
that the some of Zr4+ are replaced by Cu2+ which lead to
increase in the acidity.
3
.5 NH ‑TPD Study
3
After understanding the type of acid sites present in the cata-
lysts, we have analysed the strength of the acidic sites pre-
sent in the catalyst using NH -TPD technique. The results are
3
presented in Fig. 5. The acid site distributions can be mainly
categorized by temperature ranges as weak (< 523 K),
medium (523–673 K) and strong (>673 K) acidic sites [29,
3
0] in the NH TPD patterns. The relationship between the
3
zirconia structure and acidity is debatable in the literature.
de Souza et al. reported that tetragonal zirconia (t-ZrO ) has
2
higher acidity compare to monoclinic zirconia (m-ZrO ) [31]
2
but Elise et al. reported higher acidity for monoclinic phase
[
32]. TPD pattern of ZrO catalyst possesses three desorp-
2
tion peaks centered at T
of 450, 600 and 890 K respec-
max
tively representing weak, medium and strong acidic sites.
The TPD pattern of CZI catalyst witnesses two desorption
Fig. 5 NH -TPD of the catalysts (a) ZrO (b) CZI (c) CZP
3
2
1
3

Product Selectivity as a Function of ZrO Phase in Cu/ZrO Catalysts in the Conversion…
2
2
peaks centered at a Tmax of 500 and 750 K suggesting the
existence of both weak and medium acidic sites. The TPD
pattern of CZP catalyst indicate three desorption peaks cen-
of t-ZrO was already reported by many researchers [23, 24,
2
34–39], including Cu as a stabilizing species [40].
tered at a T
of 473, 750 and ~ 1000 K suggesting the
3.6.1 Time on Stream Study
max
existence of weak, medium and strong acidic sites and the
strong acid sites are an indicative of Bronsted acidity (also
evident from Pyridine-FTIR) which would promote dehy-
dration reaction and follow dehydrogenation because of the
At a reaction temperature of 523 K, time on stream activ-
ity on both the catalysts was conducted for 10 h and the
results are shown in Fig. 6. On CZI catalyst, CHO conver-
sion slowly increases with time, reaches a maximum at 3 h
and thereafter stays around 62%. Cyclohexanone selectivity
robustly progresses at around 90%. On CZP catalyst, for the
initial 2 h, the CHO conversion increases and attains stabili-
zation point of ~80% with BZN selectivity sturdily progress-
ing at 90%. The 10% selectivity to others includes phenol
over CZI and cyclohexene over CZP catalysts. The slight
decrease in the activity after 6 h is due to the formation of
coke as the species like cyclohexene and benzene are known
to be coke yielding precursors. The CHN elemental analysis
shows higher carbon content over CZP catalyst (4.04%) than
0
presence of Cu on catalyst surface to yield benzene as a
major product. On the other hand, CZI catalyst which pos-
sesses medium acidic sites (mostly Lewis) would promote
dehydrogenation reaction yielding cyclohexanone as a major
product.
3.6 Activity Studies
The relationship between the activity and reaction tempera-
ture can be found from Table 2. The conversion of cyclohex-
anol (CHO) on the both catalysts followed a similar trend
except in their magnitudes. It is known that the equilibrium
conversion of CHO at 523 K is ~70% for the reaction form-
ing cyclohexanone [33]. The conversion of CHO on CZI
catalyst is ~ 62% (4th hour) while the conversion of CHO
to form benzene (BZN) over CZP catalyst was initially
~
80% (4th hour) and dropped down with rise in reaction
temperature. There is a distinct difference between the prod-
uct selectivity on both the catalysts. CZI catalyst produced
cyclohexanone while the CZP catalyst showed benzene pre-
dominantly. The significant difference between the zirconia
polymorphs promotes different products when cyclohexanol
is contacted. Phenol and cyclohexene as by-products respec-
tively were observed on CZI and CZP catalysts. Pyridine-IR,
NH -TPD results reveal that the tetragonal zirconia (CZP)
3
contains more acidic sites (majorly Bronsted) than that on
monoclinic zirconia (CZI). These Bronsted acid sites assist
in dehydration of cyclohexanol to form cyclohexene and
0
the Cu sites on CZP catalyst promotes further cyclohexene
dehydrogenation to get benzene. A difference in acidity of
ZrO was also reported on Pd/t-ZrO which promotes hydro-
2
2
deoxygenation of phenol reaction [31]. Formation of BZN
on CZP catalyst is due to the existence of tetragonal phase of
the zirconia which is stabilized by Cu particles. Stabilization
Fig. 6 Effect of time on stream studies. Reaction conditions: catalyst
3
−1
weight—1 g, temperature—523 K, N flow—1200 cm h and reac-
2
3
−1
tant feed—1 cm h
Table 2 Temperature effect of
Tempera- Conversion
CZI selectivity Selectivity to Conversion
CZP selectiv- Selectivity to
cyclohexanol conversion over
CZI and CZP catalysts
ture (K) of CHO (%) to CHN (%)
phenol (%)
of CHO (%) ity to BZN (%) cyclohexene
(
%)
498
523
548
573
39.2
62
52.3
41.3
90
93
87
82
10
7
13
18
69
83
81
79
92
96
93
94
8
4
7
6
3
−1
3
−1
Reaction conditions: catalyst weight—1 g, N flow—1200 cm h , reactant feed—1 cm h
2
1
3

P. Nagaiah et al.
to higher selectivity towards styrene [42] by pulling out
the formed hydrogen in the equilibrium reaction. Another
report showed that the conversion of cyclohexanol can
be increased by pumping CO which will consume H to
x
2
form methanol [43]. Therefore, in the present case, CZP
catalyst was studied by co-feeding 10% NH –He along
3
with cyclohexanol and the results are shown in Fig. 7. Co-
feeding 10% NH –He has dramatic influence on product
3
selectivity. Product reversal from benzene to cyclohex-
anone is observed because of the fact that ammonia neu-
tralizes strong acidic sites present on the surface of the
catalyst. This is an additional evidence of the presence of
large concentration of strong acidic centers on CZP cata-
lyst, which is also observed from the NH -TPD analysis.
3
Also it is worth to note that conversion of CHO decreased
over CZP when the product formed is altered therefore, co-
precipitated catalyst with larger number of strong acidic
centers promotes benzene formation. Furthermore, we
have compared the efficiency of the present catalysts with
the systems that were reported in the literature so far. The
statistics are mentioned in Table 3.
Fig. 7 Time on stream studies of NH co-feeding on CZP catalyst.
3
3
−1
Reaction conditions: catalyst weight—1 g, gas flow—1200 cm h
,
3
−1
temperature—523 K and feed flow—1 cm h
on CZI catalyst (2.85%) which is due to the formation of
unsaturated/aromatic compounds.
4
Conclusions
3
.6.2 Product Reversal Experiment with NH Co‑feed Over
3
CZP Catalyst
ZrO upon Cu deposition via impregnation method lead
2
to monoclinic phase of ZrO and co-precipitation method
2
Earlier study of co-feeding of CO /NH over isophorone
yields tetragonal phase. Tetragonal phase in co-precip-
2
3
reaction indicates that CO co-feeding decreases the selec-
itated catalyst contains majorly BAS. CZI catalyst with
2
tivity of aromatization (3, 5-dimethyl phenol) products due
to enhancement of acidity in the catalyst and therefore
results in getting decomposition products in major compo-
monoclinic ZrO phase yields selectively cyclohexanone
2
via dehydrogenation of cyclohexanol while CZP catalyst
yields cyclohexene via dehydration followed of dehydro-
genation to form benzene. Arresting the strong acidic
sition. However co-feeding of NH increases the aromati-
3
zation products due to neutralization of strong acidic sites
in the catalyst [41]. Similar results were obtained due to
in-situ formation of ammonia over nitride based catalysts
sites in co-precipitated catalyst by co-feeding NH has
3
dramatic change in the product reversal from benzene to
cyclohexanone.
with N co-feeding along with ethyl benzene which led
2
Table 3 Comparison of catalyst efficiencies with the present system
Catalyst
Conversion of
cyclohexanol
Selectivity to
Year
Condition
References
cyclohexanone
1
Cu/Al O3
Cu/MgO
Cu/ZnO
69
59
30
57
35
70
62
83
79.4
89
97
81
87
99
93
2013
2004
2012
2006
2005
2017
–
523 K, N flow=50 ml min− , catalyst weight—3 g
[44]
[45]
[46]
[18]
[47]
[48]
Present
Present
2
2
−1
523 K, N flow=30 ml min , catalyst weight—1 g
2
−1
523 K, GHSV=1100 h , catalyst weight—10 g
−1
Cu/Al O –ZrO
523 K, N flow=50 ml min , catalyst weight—0.5 g
2
3
2
2
−1
Cu/TiO –ZrO2
523 K, N flow=50 ml min , catalyst weight—0.5 g
2
2
−1
Cu-SBA-15
CZI
CZP
523 K, N flow=30 ml min , catalyst weight—1 g
2
−1
523 K, N flow=30 ml min , catalyst weight—1 g
2
−1
96 (benzene)
–
523 K, N flow=30 ml min , catalyst weight—1 g
2
1
3

Product Selectivity as a Function of ZrO Phase in Cu/ZrO Catalysts in the Conversion…
2
2
Acknowledgements The authors P.N and M.V thank University Grant
26. Varkolu M, Velpula V, Ganji S, David Raju B, Rama Rao KS
(2015) RSC Adv 5:57201–57210
Commission and CSIR, New Delhi, for the financial support.
2
7. Samson K, Sliwa M, Socha RP, Gora-Marek K, Mucha D, Rut-
kowska-Zbik D, Paul J-F, Ruggiero-Mikolajczyk M, Grabowski
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