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
Enjamuri, Nagasuresh
