Nobel metal free, oxidant free, solvent free catalytic transformation of alcohol to aldehyde over ZnO-CeO2 mixed oxide catalyst

Article abstract of DOI:10.1016/j.apcata.2016.05.003

The catalytic transformation of alcohols to aldehydes under oxidant-free condition has drawn significant attention from the perspective of green chemistry. In this work, we designed noble metal free ZnO-CeO₂ mixed oxide catalyst in four different ratios and tested for vapor phase benzyl alcohol dehydrogenation reaction as a model reaction under oxidant free condition. The ZnO-CeO₂ mixed oxide catalyst having ratio Zn/Ce?=?30/70 composition showed highest selectivity towards formation of benzaldehyde. Interestingly in addition to benzaldehyde, toluene was formed in the reaction due to hydrogenolysis of benzyl alcohol. The lowest Ce³⁺/Ce⁴⁺ ratio was observed from the XPS analysis of Ce(3d) core electron for the catalyst having Zn/Ce?=?30/70 composition compared to others. CO₂-TPD results proved that mostly the medium strength basic sites were responsible for hydrogen abstraction from benzyl alcohol producing benzaldehyde. H₂-TPR results showed that ZnO-CeO₂ catalyst (Zn/Ce?=?30:70) had lowest reduction temperature which is in the 673?K to 573?K temperature range. The amount of toluene was higher for the ZnO-CeO₂ catalyst having Zn/Ce?=?40:60 ratio which had less basic sites and higher fraction of Ce³⁺ ion. The ZnO-CeO₂ catalyst (Zn/Ce?=?30:70) did not deactivate for a reaction time up to 2?h. While successive regenerations of the catalyst, toluene selectivity were increased. This may be due to the reduction of Ce⁴⁺ to Ce³⁺ by adsorbed hydrogen species. Also the ZnO-CeO₂ catalyst (Zn/Ce?=?30/70) showed activity for the formation of ethanal, propanal, butanal and octanal along with corresponding alkanes from ethanol, 1-propanol, 1-butanol and 1-octanol in oxidant free condition demonstrating the in-situ generation of hydrogen. The micro kinetic analysis showed that there is no external and internal mass transfer limitation in the present case.

Full text of DOI:10.1016/j.apcata.2016.05.003

Applied Catalysis A: General 523 (2016) 21–30
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Nobel metal free, oxidant free, solvent free catalytic transformation of
alcohol to aldehyde over ZnO-CeO mixed oxide catalyst
2
Nagasuresh Enjamuri , Shahid Hassan , Aline Aurouxb,∗, Jai Krishna Pandeyc,
Biswajit Chowdhury
a
a
a
a,∗
Department of Applied Chemistry, Indian School of Mines, Dhanbad, India
Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS-UCB Lyon 1, 2 av. Albert Einstein, 69626 Villeurbanne Cedex, France
Central Institute of Mining and Fuel research (CIMFR) Dhanbad, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic transformation of alcohols to aldehydes under oxidant-free condition has drawn significant
attention from the perspective of green chemistry. In this work, we designed noble metal free ZnO-CeO2
mixed oxide catalyst in four different ratios and tested for vapor phase benzyl alcohol dehydrogenation
reaction as a model reaction under oxidant free condition. The ZnO-CeO2 mixed oxide catalyst having
ratio Zn/Ce = 30/70 composition showed highest selectivity towards formation of benzaldehyde. Inter-
estingly in addition to benzaldehyde, toluene was formed in the reaction due to hydrogenolysis of benzyl
Received 19 February 2016
Received in revised form 23 April 2016
Accepted 3 May 2016
Available online 11 May 2016
Keywords:
Dehydrogenation
Benzyl alcohol
Propane
Zinc
Ceria
3
+
4+
alcohol. The lowest Ce /Ce ratio was observed from the XPS analysis of Ce(3d) core electron for the
catalyst having Zn/Ce = 30/70 composition compared to others. CO2-TPD results proved that mostly the
medium strength basic sites were responsible for hydrogen abstraction from benzyl alcohol producing
benzaldehyde. H2-TPR results showed that ZnO-CeO2 catalyst (Zn/Ce = 30:70) had lowest reduction tem-
perature which is in the 673 K to 573 K temperature range. The amount of toluene was higher for the
ZnO-CeO2 catalyst having Zn/Ce = 40:60 ratio which had less basic sites and higher fraction of Ce3+ ion.
The ZnO-CeO2 catalyst (Zn/Ce = 30:70) did not deactivate for a reaction time up to 2 h. While successive
regenerations of the catalyst, toluene selectivity were increased. This may be due to the reduction of
Toluene
4
+
3+
Ce to Ce by adsorbed hydrogen species. Also the ZnO-CeO2 catalyst (Zn/Ce = 30/70) showed activ-
ity for the formation of ethanal, propanal, butanal and octanal along with corresponding alkanes from
ethanol, 1-propanol, 1-butanol and 1-octanol in oxidant free condition demonstrating the in-situ gener-
ation of hydrogen. The micro kinetic analysis showed that there is no external and internal mass transfer
limitation in the present case.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
its wider applications [7]. Traditionally, benzaldehyde is produced
through stoichiometric oxidation of manganese and chromium
salts or by the hydrolysis of benzyl chloride and the oxidation of
toluene in industrial processes [8]. Although there are many meth-
ods for the oxidation of benzyl alcohol to benzaldehyde by using
molecular oxygen catalyzed by Au nanoparticles supported on
CeO , TiO , MnO , hydrotalcite and bimetallic Au–Pd/TiO [9,10].
In the catalytic thermal decomposition of alcohols, it is com-
monly known that dehydrogenation reaction proceeds on metal
surfaces [1]. A previous study shows that decomposition of alcohols
takes place on reduced copper powders [2]. During dehydrogena-
tion process hydrogen was chemisorbed on the metal surface [3].
Recent reports reveals that Ag and Cu nanoparticles supported on
TiO and hydrotalcite materials were effectually catalyze the dehy-
drogenation of various alcohols[4–6], particularly benzyl alcohol
to benzaldehyde is one of the most important reaction because of
2
2
2
2
Alternative designs for the development of promising oxidant-free
methodology are particularly interesting due to inhibition towards
over oxidation of the substrate to form carboxylic acid [11]. In this
scenario the ceria based catalysts have drawn attention recently,
due to Ce3+/Ce redox couple it alters the ionicity and covalent
nature of the metal-oxygen bond by creation of the acidic/basic
sites on its surface [12]. An amphoteric nature of ceria are widely
used either as a support or active component in versatile reactions
e.g. water-gas shift reaction [13], CO oxidation [14], allylic oxidation
2
4+
∗ Corresponding authors.
E-mail addresses: aline.auroux@ircelyon.univ-lyon1.fr (A. Auroux),
biswajit chem2003@yahoo.com (B. Chowdhury).
http://dx.doi.org/10.1016/j.apcata.2016.05.003
0
926-860X/© 2016 Elsevier B.V. All rights reserved.

2
2
N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
Fig. 1. BET S.A of (A) CeO2, (B) ZnO, (C) ZnO-CeO2 (Zn/Ce = 20:80), (D) ZnO-CeO2 (Zn/Ce = 30:70), (E) ZnO-CeO2 (Zn/Ce = 40:60) & (F) ZnO-CeO2 (Zn/Ce = 50:50) inset shows
the pore size distribution of the respective catalysts.
Table 1
Summary of surface area and porosity results of Zinc-ceria mixed oxide catalysts.
2
Catalyst
Surface Area (m /g)
Total pore volume (cc/g)
Av. Pore diameter (nm)
CeO2
8.84
3.39
4.68
5.84
4.15
2.38
0.029
0.016
0.021
0.027
0.022
0.007
3.8
3.7
3.8
12.8
11.2
3.0
ZnO-CeO2(Zn/Ce = 20:80)
ZnO-CeO2 (Zn/Ce = 30:70)
ZnO-CeO2 (Zn/Ce = 40:60)
ZnO-CeO2 (Zn/Ce = 50:50)
ZnO
[
15], alcohol oxidation [29], knoevenagel condensation [16] etc.
In this work, it is hypothesized that substitution of lower valent
Zn2+ ion to the ceria may increase the ionicity of the M-O-M’ bond
leads to form acidic/basic properties which can abstract hydrogen
from the alcohols. The four different Zinc-ceria mixed oxide cat-
alysts with different Zn/Ce ratio 20:80; 30:70; 40:60; 50:50 were
CeO2 has face-centered cubic crystal structure, into which various
cation dopants can be introduced in order to increase the physico-
chemical properties of ceria [17–19]. the modification of CeO with
Zn ions leads to creation of oxygen ion vacancy around the Zn
ion in the CeO2 based mixed oxide catalysts.
2
2+
2+
synthesized and characterized by XRD, N -Physisorption, UV–vis,
2
H. Imai et al. [20] have reported little activity towards methanol
dehydrogenation over zinc and zinc alloy catalyst system. Zinc sup-
ported on ceria catalyst has been reported by yongjun He et al.
XPS, CO -TPD and H -TPR techniques. The several reaction param-
2
2
eters like reaction temperature, pretreatment temperature, and
pretreatment time were varied for benzyl alcohol reaction to find an
optimum catalytic activity. The catalytic activity was evaluated for
the benzyl alcohol, Ethanol, 1-propanol, 1-butanol and 1-octanol
dehydrogenation reaction by using fixed-bed catalytic reactor in
an inert atmosphere condition. Interestingly toluene was formed
[
21] for the oxidative coupling of methane with carbon dioxide.
In another study B. G. Mishra et al. [22] reported the CeO -ZnO
2
composite oxide for cyclohexanol dehydrogenation reaction.

N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
23
Fig. 2. X-ray diffraction patterns of (A) CeO2, (B) ZnO-CeO2 (Zn/Ce = 20:80), (C) ZnO-
CeO2 (Zn/Ce = 30:70), (D) ZnO-CeO2 (Zn/Ce = 40:60) & (E) ZnO-CeO2 (Zn/Ce = 50:50).
Fig. 3. UV–vis spectra of (A) CeO2, (B) ZnO-CeO2 (Zn/Ce = 20/80), (C) ZnO-CeO2
(Zn/Ce = 30/70), (D) ZnO-CeO2 (Zn/Ce = 40/60), (E) ZnO-CeO2 (Zn/Ce = 50/50) & (F)
ZnO catalysts.
along with the benzaldehyde in the reaction of benzyl alcohol
whereas ethane, propane, butane and octane were formed in the
dehydrogenation of ethanol, 1-propanol, 1-butanol and 1-octanol.
The micro-kinetic analysis was done for the reaction of benzyl alco-
hol. The special surface property of zinc-ceria mixed oxide catalyst
has been discussed in the following section.
2
. Experimental
2.1. Catalyst preparation
Zinc-ceria mixed oxide was prepared by the non-hydrothermal
sol–gel method as described earlier [16]. In this procedure the solu-
tion of 15.2 g (0.07 mol) Ce(NO ) ·6H O (Aldrich) was added to
3
3
2
1
4.9 g (0.2 mol) triethanolamine (Acros)/water mixture under stir-
ring condition at room temperature. After complete addition of
cerium nitrate solution, 4.5 g (0.03 mol) zinc nitrate solution was
added to it with a constant stirring. This total content was stirred
for 10 min and after that 7.3 g (0.1 mol) tetraethyl ammonium
hydroxide (Merck, Germany) was added drop wise in the result-
ing solution. The mixture was kept in constant stirring condition
at room temperature which forms a gel of composition; tri-
ethanolamine: Ce (NO ) ·6H O: H O: Zn (NO ) ·6H O: tetraethyl
Fig. 4. CO2-TPD traces of the catalysts (A) ZnO-CeO2 (Zn/Ce = 20/80), (B) CeO2,
(
(
C) ZnO-CeO2 (Zn/Ce = 50/50), (D) ZnO-CeO2 (Zn/Ce = 40/60)
Zn/Ce = 30:70).
& (E) ZnO-CeO2
3
3
2
2
3
2
2
ammonium hydroxide 0.2: x: 1.1: (0.1-x): 0.1 molar ratio (where
x = 0.08-0.05). The gel material was dried at 383 K for 24 h and it
was calcined at 973 K for 10 h in a furnace (Thermocraft; USA) with
◦
temperature ramp of 1 C/min.
2.2. Catalyst characterization
2.2.1. BET surface area and porosity measurements
The BET surface area and pore size analysis of the samples were
measured at liquid nitrogen temperature (77 K) with a Nova 3200e
instrument (Quantachrome; USA). Before the measurement of sur-
face area, the pretreatment of the samples was done at 473 K for
5
h under high vacuum condition. The surface area was calculated
Fig. 5. Quantitative CO2 desorption profile of the catalysts (A) CeO2, (B) ZnO-CeO2
Zn/Ce = 50/50), (C) ZnO-CeO2 (Zn/Ce = 40/60), (D) ZnO-CeO2 (Zn/Ce = 30:70), (E)
ZnO-CeO2 (Zn/Ce = 20/80).
by the Brunauer–Emmett–Teller (BET) equation. The pore size dis-
tributions were calculated using the BJH method.
(
2.2.2. X-ray diffraction (XRD)
XRD patterns and catalyst crystalline phases were identi-
fied at ambient temperature in an X-ray diffractometer (Rigaku,
sizes were estimated by considering the most intense CeO diffrac-
tion peak (111) located at 2␪ = 28.55 through Scherrer’s formula
D = 0.94 K/␤COS␪ where D, K, ␤, and ␪ are the average crystallite
size, X-ray wavelength, peak width and Bragg’s angle respectively.
2
ꢀ
Ultima IV) using Cu K␣ radiation as the X-ray source (40 kV,
0
2
0 mA), wavelength (K) = 1.54056A ). The XRD results were ana-
lyzed quantitatively using X’pert software. The average crystallite

2
4
N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
Fig. 6. Ce(3d) core level XPS spectra of (A) CeO2, (B) ZnO-CeO2 (Zn/Ce = 20/80), (C) ZnO-CeO2 (Zn/Ce = 30:70), (D) ZnO-CeO2 (Zn/Ce = 40/60) & (E) ZnO-CeO2 (Zn/Ce = 50/50)
catalyst O(1s) core level XPS spectra (F) CeO2, (G) ZnO-CeO2 (Zn/Ce = 20/80), (H) ZnO-CeO2 (Zn/Ce = 30:70), (I) ZnO-CeO2 (Zn/Ce = 40/60) & (J) ZnO-CeO2 (Zn/Ce = 50/50).
Table 2
Quantitative analysis of X-ray diffraction patterns of Zinc-ceria mixed oxide catalysts.
Catalyst
Phase
d spacing (nm)
Crystallize sizea (nm)
Cell parameterb (nm)
CeO2
Cerianite
Cerianite
Cerianite
Cerianite
Cerianite
0.312375
0.312268
0.310998
0.309166
0.311767
89.18
50.96
14.86
14.87
14.86
0.5410335
0.5408481
0.5386485
0.5354755
0.5399804
ZnO- CeO2 (Zn/Ce = 20:80)
ZnO- CeO2 (Zn/Ce = 30:70)
ZnO- CeO2 (Zn/Ce = 40:60)
ZnO- CeO2 (Zn/Ce = 50:50)
a
Estimated by Scherrer equation.
2
+k2+l2
b
1
h
by using the equation
=
.
d2
a
Table 3
Quantitative analysis of Ce (3d) XPS spectra of Zinc-ceria mixed oxides.
2
.2.5. X-ray photoelectron spectroscopy (XPS)
The XPS analyses were carried out on a Kratos Ultra DLD spec-
Catalyst
CeO2
Ce [III] Ce [IV] Ce[III]/Ce[IV] Ce+3 fraction %Ce+3
trometer using monochromatic Al K␣ radiation (10 mA, 15 kV). All
spectra were taken in the hybrid (combined electrostatic and mag-
netic lens) mode. Survey scans were conducted between 1200 eV
and 0 binding energies with a step size of 1 eV, analyser pass energy
22.44 29.40 0.76
0.43
0.39
0.29
0.37
0.35
43.28
39.22
29.88
37.18
35.21
ZnO- CeO2(Zn/Ce = 20:80) 19.92 30.86 0.65
ZnO- CeO2(Zn/Ce = 30:70) 12.70 29.80 0.43
ZnO- CeO2(Zn/Ce = 40:60) 17.25 29.14 0.59
ZnO- CeO2(Zn/Ce = 50:50) 17.85 32.84 0.54
1
60 eV, and dwell time per step100 ms, whereas scans over specific
photoelectron regions were carried out at 20 eV pass energy and
00 ms dwell time. The specific photoelectron scans covered appro-
2
priate energy ranges, viz. 20 eV for O(1s) and C(1s), with 0.1 eV step
size, 50 eV for Zn(2p) and 80 eV for Ce (3d), with 0.2 eV step size for
the latter two. All spectra were referenced to the C(1s) line at bind-
ing energy 284.6 eV, characteristic of the ever-present adventitious
carbon (C C and C H), and its exact peak position was obtained
after Shirley background subtraction and decomposition of the
C(1 s) peak envelope using a Gaussian-Lorentzien (70%–30%) curve
fit. All other photoelectron peaks were background-subtracted and
curve-fit in the same manner. Quantification was carried out with
the VISION software supplied. The relative sensitivity factors (RSF)
applied here is inherent to this software and incorporate Wagner
photoelectron cross-sections and analyser transmission correction.
2
.2.3. Ultraviolet–visible spectroscopy (UV–vis)
DRUV–vis measurement was carried out by using Var-ian Cary
00 (Shimadzu) spectrophotometer. The spectra were recorded in
5
the range of 200–800 nm wavelengths.
2.2.4. CO -Temperature programmed desorption (TPD)
2
The TPD was carried out in a Micrometrics Chemisorb 2750
instrument (USA) equipped with a TCD detector. Prior to the
analysis all the catalyst samples were purged with a He flow of
3
0 cc/min at 623 K temperature for 60 min to remove adsorbed
water. The purged catalyst was cooled to ambient temperature.
CO chemisorption was done passing 9.95% of CO /He mixture. The
2
2
amounts of CO desorbed were evaluated by heating the sample up
2
to 1273 K with ramp rate of 10 K/min in presence of He.

N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
25
Table 4
Quantitative XPS elemental analysis of Zinc-ceria mixed oxides.
Catalyst
Atom (%) Ce
Atom (%) Zn
Atom (%) O
Ce/Zn
O/(Ce + Zn)
ZnO- CeO2(Zn/Ce = 20:80)
ZnO- CeO2(Zn/Ce = 30:70)
ZnO- CeO2(Zn/Ce = 40:60)
ZnO- CeO2(Zn/Ce = 50:50)
18.2
21.1
17.4
16.5
9.6
9.6
12.3
12.0
53.4
51.6
50.8
51.3
1.89
2.19
1.41
1.375
1.92
1.68
1.71
1.8
Table 5
Binding energy and% of Peak area of O (1s) component of Zinc-ceria mixed oxides.
O (1s) Component →
a
b
c
d
Sample
B.E
% of Peak
B.E
% of Peak
B.E
% of Peak
B.E
% of Peak
CeO2
528.5
528.8
528.5
528.4
528.7
–
78.9
59.6
70.5
56.9
53.2
–
–
–
–
–
–
–
28.6
27.4
70.9
530.3
530.2
530.7
530.8
–
16.2
24.7
29.5
5.4
–
–
–
8.4
–
9.1
12.9
29.2
ZnO- CeO2(Zn/Ce = 20:80)
ZnO- CeO2(Zn/Ce = 30:70)
ZnO- CeO2(Zn/Ce = 40:60)
ZnO- CeO2(Zn/Ce = 50:50)
ZnO
531.4
–
531.7
531.3
531.2
529.7
529.9
529.8
–
–
Table 6
Catalytic activity data for oxidant free dehydrogenation reaction of benzyl alcohol over Zinc-ceria mixed oxide catalyst.
Catalyst
Conversion
%)
Selectivity (%)
Benzaldehyde
STY (mol g cat h
−
1
−1
)
Mass balance (%)
Mears
criteria *
Cwp$
(
Toluene
−8
−6
−6
−6
−6
−4
−2
−2
−3
−3
CeO2
0.77
20.4
43.6
30.6
29.0
46.8
82.3
94.8
72.7
81.6
53.2
17.7
5.2
27.3
18.4
2.08
99.8
97.3
94.8
93.6
96.3
4.34 × 10
2.02 × 10
4.97 × 10
2.68 × 10
2.85 × 10
3.65 × 10
1.75 × 10
4.19 × 10
6.70 × 10
8.15 × 10
ZnO- CeO2 (Zn/Ce = 20:80)
ZnO- CeO2 (Zn/Ce = 30:70)
ZnO- CeO2 (Zn/Ce = 40:60)
ZnO- CeO2 (Zn/Ce = 50:50)
96.88
238.50
128.37
136.55
Reaction conditions: pretreatment temperature: 723 K; duration of pretreatment: 4 h; reaction temperature: 673 K; carrier gas (N2) flow: 30 ml/min, amount of catalyst:0.5 g; particle
−1
−4
size: 50 ꢀm; reaction time: 60 min; WHSV: 4000 h ; Reynold number (Re): 6.89 × 10 ; Schmidt number (Sc): 0.65; Sherwood number (Sh): 2.01; *calculated by Diwedi
method; $: Weisz criterion for internal diffusion.
2.2.6. Hydrogen temperature programmed reduction (H2-TPR)
The TPR measurements were carried out using Micromeritics
Chemsorb 2750 apparatus. The pretreatment under high purity
Argon flow was carried out at 573 K for half an hour. Then the
catalyst was cooled to ambient temperature. Finally, furnace tem-
◦
perature was raised at 10 C/min to 1273 K under 20 mL/min flow
rate of H /Ar mixture containing 10 vol.% of H . The H consump-
2
2
2
tion was monitored by a thermal conductivity detector (TCD).
2.3. General procedure for alcohol dehydrogenation reaction
The catalytic experiments were carried out in a fixed bed reac-
tor (1.5 cm id) at atmospheric pressure under N2 flow. About 0.5 g
of the catalyst was loaded and pretreatment was done in N2 gas
flow (purity 99.99%, 30 mL/min) at 723 K for 4 h. A thermocouple
was inserted in the catalyst bed to measure the catalyst bed tem-
perature. After pretreatment, the reactor was cooled to the desired
reaction temperature and the reactant (benzyl alcohol) was fed at
−
1
a flow rate of 3.0 mL h along with the N gas flow at a rate of
3
2
−1
0 mL/min to keep the WHSV at 8.56 h . The product was col-
lected in an ice cold trap periodically for every 60 min and analyzed
by a gas chromatograph (CIC, India) equipped with a flame ioniza-
tion detector and SE-30 column. The oven temperature was kept
at 483 K, with ramp rate 10 K/min whereas injector and detector
temperature were kept at 503 K during the analysis.
The regeneration of the catalyst was done by treating the spent
catalyst with O /N mixture (1:9 vol ratios) in the same flow as
2
2
reaction condition at 673 K temperature. Then reactor was flushed
with N2 flow for one hour and feed mixture was introduced and
product was collected and analyzed as described earlier.
Fig. 7. Zn(2p) core level XPS spectra (K) ZnO, (L) ZnO-CeO2 (Zn/Ce = 20/80), (M) ZnO-
CeO2 (Zn/Ce = 30:70), (N) ZnO-CeO2 (Zn/Ce = 40/60) & (O) ZnO-CeO2 (Zn/Ce = 50/50).
In the case of dehydrogenation of primary alcohol substrates
(
ethanol, 1-Propanol, 1-butanol and 1-octanol) the reactor efflu-
1-octanol. We used a gas chromatograph (CIC, India) equipped with
a flame ionization detector and SE-30 column. All transfer lines
ent was analyzed online for first three substrates and offline for

2
6
N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
Table 7
Optimization of reaction temperature for oxidant free dehydrogenation reaction of benzyl alcohol over Zinc-ceria (Zn/Ce = 30/70) catalyst.
Catalyst
Reaction
temperature K
Conversion (%)
Selectivity (%)
Benzaldehyde
STY (mol g cat h
−
1
−1
)
Mass balance (%)
Toluene
ZnO- CeO2 (Zn/Ce = 30:70)
ZnO- CeO2 (Zn/Ce = 30:70)
ZnO- CeO2 (Zn/Ce = 30:70)
573
673
773
11.7
43.6
40.3
100
94.8
90.2
0
5.1
9.9
67.51
238.50
209.75
98.5
94.3
94.2
Reaction conditions: pretreatment temperature: 723 K; duration of pretreatment: 4 h; carrier gas (N2) flow: 30 ml/min, amount of catalyst:0.5 g; particle size: 100 ꢀm; reaction time:
−1
60 min; WHSV: 4000 h
.
Table 8
Optimization of pretreatment temperature for oxidant free dehydrogenation reaction of benzyl alcohol over Zinc-ceria (Zn/Ce = 30/70) catalyst.
o
STY (mol g-1 cat h-1
Catalyst
Pretreatment Temperature ( C)
Conversion (%)
Selectivity (%)
Benzaldehyde
)
Mass balance (%)
Toluene
ZnO-CeO2 (Zn/Ce = 30:70)
ZnO-CeO2 (Zn/Ce = 30:70)
ZnO-CeO2 (Zn/Ce = 30:70)
350
450
550
24.3
43.6
24.2
73.7
94.8
82.5
26.2
5.1
17.4
103.34
238.5
115.2
>99
94.8
>99
Reaction conditions: duration of pretreatment: 4 h; reaction temperature: 673 K; carrier gas (N2) flow: 30 ml/min, amount of catalyst:0.5 g; particle size: 100 ꢀm; reaction time:
−1
60 min; WHSV: 4000 h
.
from the vaporizer to the reactor and gas chromatograph (GC) were
heated to above 383 K to avoid condensation of the reactants and
products. The oven temperature was kept at 383 K with ramp rate
the zinc loading, there is an increase in intensity of ZnO diffraction
peaks observed in all cases. Table 2. Lists the crystallize size and the
unit cell parameter of the Zinc-ceria catalysts. The Crystal sizes of
both the Ceria and Zinc-ceria catalysts calculated by using Scherer’s
formula from the powder XRD data. The decrease in size of Zinc-
ceria mixed oxides may be attributed to decrease in concentration
of surface hydroxyls as low valent zinc substitutes the higher valent
cerium in the lattice [26,27]
5
4
K/min whereas injector and detector temperature were kept at
03 K during the online analysis. For 1-octanol dehydrogenation
product, the G.C conditions were kept same as for benzyl alcohol.
−
1
−1
Product i formation rate [C-mol kg cat , h ] = (benzyl alcohol
−1
−1
SV in C-mole kg cat
h
) x (C-moles in product i)/(total C −moles
in products and un-reacted alcohol)
The absence of external limitation was confirmed by the Mears
−
06
3.1.3. UV–vis spectra
criterion [23] (-rA
ꢁ
␳bR␩/kcCAb) < 0.15 whose value was 4.97 × 10
From the UV–vis study (Fig. 3.), it is found that in the pure CeO2
and Zinc-ceria mixed oxides, the constant band at 280 nm can be
ascribed to the intrinsic character of CeO2 accordingly [28], while
absorption edge at 413 nm in CeO2 is blue shifted for Zinc-ceria
mixed oxides, may indicate formation of Ce-O-Zn solid solution.
for ZnO-CeO (Zn/Ce = 30/70) catalyst. The absence of internal mass
2
transfer was ascertained by means of the Weisz-Prater criterion
2
−2
in our
(
CWP = −r ’ ␳cR /DeC ) < 1 whose value was 4.19 × 10
A
AS
case. The details of the calculations are provided in the supplemen-
tary material (Calculation S2).
3
.1.4. Temperature programmed desorption (TPD) of CO2
3
3
3
. Results and discussion
The acid-base properties of zinc-ceria mixed oxide catalysts
were measured by TPD using CO2 as the acidic gas probe molecule
Fig. 4. On metal oxide surface, basic sites are primarily associated
to surface oxygen ions bonded to positively charge metal centers
.1. Catalyst characterization
.1.1. BET S.A and porosity results
The adsorption/desorption isotherms and the corresponding
(Lewis acid sites) [29].The strength of the basic sites has been esti-
mated from the area under the corresponding TPD curve Fig. 5.
At two desorption temperature ranges: for example, 323–523 K
and 523–773 K. The basic sites at these temperature ranges have
been designated as weak, medium and strong basic sites respec-
tively [29,30]. As the reaction temperature was 673 K, so we chose
pore size distributions of the studied catalysts are reported in
Fig. 1. The isotherms can be observed in all cases characterized
by the presence of a hysteresis loop indicating that the Zinc-ceria
mixed oxide catalysts are mesoporous in nature [16]. The hystere-
sis loops which do not level off at relative pressures close to the
saturation vapor pressure were reported for materials comprised
of aggregates (loose assemblages) of plate like particles forming
slit like pore [24]. The textural features of the samples have been
investigated by BET surface area measurements and the results are
reported in Table 1. Zinc-ceria mixed oxides changed the BET sur-
face area slightly, but it did not affect the catalytic activity or the
texture significantly [25]. The decrease in surface area after zinc
incorporation has been observed. This may be due to pore blocking.
the temperature till 773 K to observe the desorption of CO . It is
2
observed from CO -TPD results that distribution of weak, medium
2
and strong basic sites altered significantly by the incorporation
of the zinc ion to the ceria matrix in different molar ratios. The
total amount of desorbed carbon dioxide was highest for ZnO-CeO2
(Zn/Ce = 50/50) catalyst as shown in Fig. 5 and Table S2.
3.1.5. Cerium X-ray photoelectron spectroscopy (XPS)
The core level binding energies of CeO , ZnO and ZnO-CeO2
2
3
.1.2. X-ray diffraction (XRD)
Fig. 2. displays X-ray diffractions of the Zinc-ceria catalysts. It
mixed oxides are displayed in Fig. 6 and 7. Which exhibits the char-
acteristic binding energy peaks of Ce(3d), O(1s), and Zn(2p). All
the binding energies are referenced to the adventitious elemental
carbon (C1s) spectrum at 284.6 eV. As depicted in Table S1 (sup-
porting information) the core level binding energy for Ce 3d5/2 and
Ce 3d3/2 of the samples as well as quantitative analysis show some
interesting features.
can be observed that diffraction peaks are attributable to the flu-
orite phase of ceria in all cases. XRD peaks are corresponding to
ZnO crystal phases at (JCPDS card number 00-036-1451) 2␪ values
of 31.7, 34.4, 36.2 and 63.0 are observed for the Zinc-ceria cata-
lysts with different ZnO contents. Fig. 2. shows that as increasing

N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
27
Table 9
Optimization of pretreatment time for oxidant free dehydrogenation reaction of benzyl alcohol over Zinc-ceria (Zn/Ce = 30/70) catalyst.
−
1
−1
Catalyst
Pretreatment Time (hours)
Conversion (%)
Selectivity (%)
Benzaldehyde
STY (mol g cat h )
Mass Balance(%)
Toluene
ZnO- CeO2 (Zn/Ce = 30:70)
ZnO- CeO2 (Zn/Ce = 30:70)
ZnO- CeO2 (Zn/Ce = 30:70)
3 h
4 h
5 h
10.2
43.6
11.64
93.0
94.8
89.02
6.9
5.1
10.98
54.74
238.5
59.79
>99
94.8
>99
Reaction conditions: pretreatment temperature: 723 K; reaction temperature: 673 K; carrier gas (N2) flow: 30 ml/min, amount of catalyst:0.5 g; particle size: 100 ꢀm; reaction time:
−1
60 min; WHSV: 4000 h
.
Table 10
Time on stream study for the oxidant free dehydrogenation of benzyl alcohol over Zinc-ceria (Zn/Ce = 30/70) catalyst.
−
1
−1
Catalyst
Time on Stream (TOS) (min.)
Conversion (%)
Selectivity (%)
Benzaldehyde
STY (mol g cat h )
Mass balance (%)
Toluene
ZnO-CeO2 (Zn/Ce = 30:70)
ZnO-CeO2 (Zn/Ce = 30:70)
ZnO-CeO2 (Zn/Ce = 30:70)
60
120
180
43.6
41.1
40.3
94.8
96.3
95.2
5.1
3.6
4.8
238.5
228.38
221.38
94.8
95.1
95.4
Reaction conditions: pretreatment temperature: 723 K; duration of pretreatment: 4 h; reaction temperature: 673 K; carrier gas (N2) flow: 30 ml/min, amount of catalyst:0.5 g; particle
−
1
size: 100 ꢀm; reaction time: 60 min; WHSV: 4000 h
.
The core level binding energies of Ce (3d5/2) and Ce (3d3/2
)
cerium compounds are known to exhibit rather complex features
due to hybridization with ligand orbitals and fractional occupancy
of the valence 4f orbital. Peaks around 881.4, 883.3, 887.9 and
1
11
111
8
97.3 eV are denoted as V, V , V , V
(corresponding to Ce (3d5/2)
states) whereas peaks at 901.7, 906.6 and 915.8 eV are denoted as
1
11
111
U , U and U
(corresponding to Ce (3d3/2) states) respectively,
11
11
as shown in Fig. 6. and Table S1, U /V doublet having the spin-
orbit splitting of two states of 18.6 eV [31]. The U /V
1
11 111
doublet
9
is due to the primary photoemission from Ce(IV)O , that is, Ce 3d
2
0
6
11 11
4
f
O 2p Ce (IV). The U /V doublets are shakedown features
resulting from the transfer of one or two electrons from a filled O
9
2
4
2
3
p orbital to an empty Ce 4f orbital, that is, Ce 3d 4f O 2p and Ce
d
9
1
5
1
1
4f O 2p of Ce(IV) in the final states [32] whereas the U /V
doublet has been assigned to photoemission from Ce cations, that
is, Ce 3d 4f O 2p of Ce(III). The presence of the U1 peak in the
spectrum indicates that the sample contains some oxygen vacan-
cies and is in a partially reduced state, namely, Ce in +3 state. The
Ce(3d) spectrum basically denotes a mixture of Ce3+/Ce oxida-
tion states giving rise to a large number of peaks indicating that
the surface of the sample is not fully oxidized [33]. The reduction is
mainly due to the progressive elimination of surface hydroxyls and
oxygen ions from the CeO2 surface upon vacuum treatment. In the
case of higher zinc loadings (Table S1) there is a shift in the binding
energy of the peak due to Ce3+ (U ) towards higher values which
indicates interaction between zinc oxide and cerium oxide species,
i.e. Ce-O-Zn bond formation.
9
1
6
Fig. 8. H2-TPR of different catalysts (A) CeO2, (B) ZnO, (C) ZnO-CeO2 (Zn/Ce = 20/80),
(
(
D) ZnO-CeO2 (Zn/Ce = 30/70), (E) ZnO-CeO2 (Zn/Ce = 40/60)
Zn/Ce = 50/50).
& (F) ZnO-CeO2
4+
maximum ionicity is also associated with ZnO-CeO (Zn/Ce = 30/70)
2
mixed oxide [36]. When percentage of ZnO increases in CeO lat-
2
+3
tice above than 30%, Ce fraction again increases which indicates
replacement of Ce+4 by Zn+2 chiefly in higher zinc doping and the
Ce-O bond become more covalent. (Table 5)
1
3.1.7. Zinc X-ray photoelectron spectroscopy
The XPS Zn(2p) Zinc-ceria mixed oxides is shown in Fig. 7.
Quantitative XPS elemental analysis of Zinc-ceria mixed oxides are
As found in quantitative analysis (Tables 3 and 4), especially
in ZnO-CeO2 (Zn/Ce = 30/70), the Ce+3/Ce+4 ratio is minimum 0.43
presented in Table 4 . The XPS Zn(2p) spectrum of ZnO-CeO mixed
2
that means among all cerium content of surface, 70% of total ceria
ions are Ce+ and only 30% are Ce . This may be attributed to the
4
+3
oxide shows the band chiefly around 1021 eV that may be Zn or
ZnO [37–39]. To circumvent this difficulty, we have also measured
the kinetic energy of the principal Auger peak (Zn L3M45M45) of
zinc and we found a value of K.E. = 987.9 eV for the latter and this
is more consistent with the ZnO (K.E.lit ∼988 eV) rather than the
metallic state (K.E.lit ∼992 eV) [38]. It is found that Zn/Ce = 30/70
has less surface oxygen species compared to other Zn/Ce catalyst.
replacement of Ce+4 (ionic radii 101pm) as well as Ce3+ (ionic radii
2
+
1
15) by divalent Zn (ionic radii 88pm).
3.1.6. Oxygen X-ray photoelectron spectroscopy
It is observed that O(1s) core level peaks as shown in Fig. 6. and
Table 5 For the M-O-M’ bonding where M and M’ denote the metal
with different electro negativities the O(1s) gives multiple peaks as
reported in literature [34]. The peak at 528.6 eV can be assigned to
ionic oxygen present in the mixed oxide. The peak at 529.8 eV can be
assigned to either Ce O or ZnO as reported in literature. The O(1s)
3.1.8. H - temperature programmed reduction (TPR)
To know the reducibility of Ceria and Zinc-ceria mixed oxide
catalysts of the hydrogen TPR was carried out and H consumption
vs Temperature plot is shown in Fig. 8. As reported in literature two
reduction peaks appeared at 723 K and 1073 K temperature in the
2
2
2
3
peak at 530.5 eV and 531.4 eV are mostly due to covalent oxygen
+
4
species as also found in our earlier study [35]. Replacement of Ce
case of pure CeO . These two peaks are mostly assigned to the sur-
2
or Ce+ by lower valent Zn increases the ionicity of Ce-O bond so
3
+2
face capping oxygen ions and bulk oxygen [40]. Interestingly due

2
8
N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
Table 11
Regeneration study for the oxidant free dehydrogenation of benzyl alcohol over Zinc-ceria (Zn/Ce = 30/70) catalyst.
Regeneration No
Substrate
Conversion
Selectivity of BzOH
Selectivity of Toluene
Fresh run
1
2
Benzyl alcohol
43.6%
35.7%
29.34%
94.8%
70.56%
59.42%
5.2%
29.43%
40.57%
.
Reaction conditions: pretreatment temperature: 723 K; duration of pretreatment: 4 h; reaction temperature: 673 K; carrier gas (N2) flow: 30 ml/min, amount of catalyst:0.5 g; particle
−1
size: 100 ꢀm; reaction time: 60 min; WHSV: 4000 h
.
Table 12
Substrate variation study for the oxidant free dehydrogenation of different alcohol over Zinc-ceria (Zn/Ce = 30/70) catalyst.
Reactant Product 1(P1) Product 2(P2) Conversion (%)
Selectivity of P1(%)
Selectivity of P2(%)
1
1
2.3
9.9
84.7
15.3
68.9
31.0
6
1
3.1
4.4
87.1
99.3
12.9
0.7
.
Reaction conditions: pretreatment temperature: 723 K; duration of pretreatment: 4 h; reaction temperature: 673 K; carrier gas (N2) flow: 30 ml/min, amount of catalyst:0.5 g; particle
size: 100 ꢀm; reaction time: 60 min.
to doping by zinc in the mixed oxide becomes easily reducible at
low temperature only. The Zn/Ce = 30/70 sample has lowest reduc-
tion temperature in comparison with other Zinc-ceria mixed oxide
catalysts. As observed from XPS and TPR results the Zn/Ce = 30/70
3.2.2. Effect of reaction temperature
Effect of reaction temperature on the catalytic activity was stud-
ied over ZnO-CeO2 catalyst (Zn/Ce = 30/70) and shown in Table 7.
At 573 K temperature, benzyl alcohol conversion is 12% and ben-
zaldehyde selectivity is 100%. As temperature rises to 673 K, the
best catalytic conversion 44% having 95% benzaldehyde selectivity
is noticed. After further increase of reaction temperature to 773 K,
catalytic conversion was decreases to 40%. As the reaction temper-
ature rises, benzaldehyde selectivity was decreased and toluene
selectivity was increased. As described earlier the basic sites with
medium strength having CO2 desorption temperature at 673 K are
responsible for abstracting the hydrogen atom from benzyl alcohol
during the initial step of the reaction.
had lowest Ce+ /Ce ratio i.e. more Ce species so it can be easily
3
4+
4+
reducible compared to other catalyst. (Table 4)
3
.2. Catalytic activity results
.2.1. Effect of Ce-Zn ratio
3
The catalytic activities obtained over CeO , ZnO and ZnO-CeO
2
2
mixed oxide catalytic system towards benzyl alcohol dehydrogena-
tion in vapor phase are shown in Table 6. It is found that CeO2
or ZnO catalyst system showed very little catalytic activity. How-
ever, ZnO-CeO2 mixed oxide systems shows enhanced catalytic
3.2.3. Effect of pretreatment temperature and pretreatment time
The effect of pretreatment temperature and pretreatment time
on benzyl alcohol conversion and benzaldehyde selectivity over
ZnO-CeO2 (Zn/Ce = 30/70) catalysts are shown in Tables 8 and 9.
Respectively. At 723 K pretreatment temperature and 4 h pretreat-
ment time, ZnO- CeO2 (Zn/Ce = 30/70) catalyst shows optimum
performance. It should be also noticed that both at lower and higher
pretreatment temperatures, conversion of benzyl alcohol is almost
equal but toluene selectivity increased with the expense of ben-
zaldehyde selectivity.
activity. Among all, ZnO-CeO (Zn/Ce = 30/70) catalyst showed best
2
catalytic activity. From the XPS Ce(3d) spectra, it is found that ZnO-
+3
CeO (Zn/Ce = 30/70) has least Ce % fraction and more ionic oxygen
2
present in the catalyst surface. From the CO -TPD results it is found
2
that ZnO-CeO2 (Zn/Ce = 30/70) catalyst has more medium strength
sites ranging temperature 673–723 K temperature. The TPR profile
shows that ZnO- CeO2 (Zn/Ce = 30/70) is highly reducible at lower
temperature (673–723 K) compared to other catalysts. So it can be
said that medium strength basic sites are responsible for higher cat-
alytic activity, ZnO-CeO2 catalyst with Zn/Ce = 30/70 ratio towards
benzyl alcohol dehydrogenation reaction. The formation of toluene
as a side product is observed after dehydrogenation reaction. The
toluene formation was more for the catalyst which has more frac-
3.2.4. Time on stream (TOS) study
In order to find out whether the catalyst was deactivated in
long run the catalyst activity of ZnO- CeO2 (Zn/Ce = 30/70) cata-
lyst was evaluated for till 3 h time on stream as shown in Table 10.
On increasing time on stream up to six hours, benzyl alcohol con-
version, benzaldehyde selectivity and toluene selectivity varied in
3
+
tions of Ce species i.e. ZnO-CeO catalyst with Zn/Ce = 40/60 ratio.
2

N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
29
Fig. 9. The proposed reaction mechanism of benzyl alcohol transformation over ZnO-CeO2 catalyst surface.
small amount (supporting information; Fig. S1). Overall, it can be
said that the zinc containing ceria catalyst system showed bet-
ter activity and slower deactivation. Previously, C.Yu. et al. have
shown that zinc addition in catalytic system enhances the dehy-
drogenation activity of propane and retard the coke formation [41].
Similarly, we found that ZnO-CeO2 (Zn/Ce = 30/70) is also most
active towards the benzyl alcohol dehydrogenation reaction as well
as it has least deactivation.
ysis of benzyl alcohol takes place in presence of hydrogen resulting
formation of toluene occurs (Fig. 9).
C H5CH OH → C H5CHO + H
6
2
6
2
C H5CH OH + H → C H5CH + H O
6
2
2
6
3
2
In the case Zn/Ce (30/70 ratio) catalyst, the percentage of ionic-
ity is more compared to other catalysts in the series. The CO -TPD
2
study shows that medium strength basic sites (in the desorption
temperature 673–773 K are highly populated for the catalyst Zn/Ce
ratio (30/70). In general benzaldehyde after hydrogenation gives
benzyl alcohol but surprisingly we got toluene due to hydrogenol-
ysis of benzyl alcohol. The selectivity of toluene is found to increase
with increasing surface coverage of Ce+3. So, similarly ethane,
propane, butane and octane were obtained due to hydrogenolysis of
ethanol, 1-propanol, 1-butanol and 1-octanol respectively. We have
3
.2.5. Effect of catalyst regeneration
The spent catalyst was regenerated using a mixture of O and N2
2
gas (1:9 vol ratios) at 723 K temperature for two hours (Table 11). It
is observed that both the benzyl alcohol conversion and benzalde-
hyde selectivity decreased in recycling whereas toluene selectivity
was increased.
measured the XPS of the used catalyst ZnO-CeO (Zn/Ce = 30/70).
2
3
.2.6. Substrate variation
After obtaining satisfactory result we have carried out the oxi-
The quantitative analysis shows Ce 3+/Ce ratio has been increased
which indicates that catalyst was reduced during reaction (support-
ing information Table S1, S2, S3, S4 and Fig. S2). The TPR profile of
4+
dant free dehydrogenation of various primary alcohols e.g. ethanol,
-propanol, 1-butanol and 1-octanol. Interestingly we found
1
fresh and used ZnO-CeO (Zn/Ce = 30/70) catalyst showed that peak
2
high selectivity towards dehydrogenated products as depicted
in Table 12. In the case of ethanol, 1-propanol, 1-butanol and
1
carbons i.e. ethane, propane, butane and octane were also obtained
which evidenced in-situ generation of hydrogen.
due to ceria reduction shifted to higher temperature (supporting
information; Fig. S3). It implies that used catalyst was difficult to
reduce. The UV–vis spectra of fresh and used ZnO-CeO2 catalyst
-octanol along with the corresponding aldehydes saturate hydro-
(
Zn/Ce = 30/70) are recorded. The absorption edges of the mixed
oxides (412 nm for ZnO-CeO2 (Zn/Ce = 30/70) fresh catalyst and
38 nm benzyl alcohol adsorbed ZnO-CeO2 (Zn/Ce = 30/70) shifts
4
4
. Proposed mechanism
to visible range remarkably. The band gap of the samples is calcu-
lated by using the formula Eg (eV) = 1240/␭g (nm), where ␭g stands
for the wavelength value corresponding to the intersection point
of the vertical and horizontal part of the spectra, the red shift of
absorption edges and narrowing of band gaps (3.00 eV for ZnO-
CeO2 (Zn/Ce = 30/70) fresh catalyst and 2.83 eV for benzyl alcohol
adsorbed ZnO-CeO2 (Zn/Ce = 30/70) observed in the UV–vis spec-
tra [42]. IR spectra of used catalyst showed the adsorbed benzyl
From the experimental results and literature reports it can be
said that initially hydrogen atom is abstracted by oxygen radi-
cal species from hydroxyl group of benzyl alcohol and oxo-metal
species formed. Further, Ce4+ species converted to Ce species.
Then zinc site cleaved the C-H bond of Ph-CH2 group formation of
benzaldehyde occurs by liberation of hydrogen. Later hydrogenol-
+3

3
0
N. Enjamuri et al. / Applied Catalysis A: General 523 (2016) 21–30
alcohol and benzaldehyde species present in the catalyst surface
supporting information; Fig. S4). From the FTIR spectra of used
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−
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ZnO-CeO (Zn/Ce = 30/70) catalyst the band appeared at 1029 cm
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− −1 −1 −1
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1
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[
[
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