Enhanced propane dehydrogenation to propylene over zinc-promoted chromium catalysts

Article abstract of DOI:10.1039/c9cy01921a

Direct propane dehydrogenation (PDH) is an attractive technology for propylene production. We show here that propane conversion is significantly enhanced by the addition of ZnO to Cr₂O₃. Furthermore, its activity is strongly dependent on the Zn/Cr molar ratio and one with Zn/Cr = 0.3 gives the highest propane conversion and propylene selectivity among the studied Zn_xCr catalysts (x denoting the molar ratio, 0-0.5). Characterization with X-ray diffraction, nitrogen physisorption, X-ray fluorescence spectroscopy, temperature-programmed reduction, temperature-programmed desorption, transmission electron microscopy, and X-ray photoelectron spectroscopy indicates that addition of zinc to Cr₂O₃ leads to formation of a spinel phase and reduction of the particle size, and hence forms more defect sites. The Arrhenius plots suggest that the apparent activation energy of the PDH reaction is significantly lowered over Zn_0.3Cr.

Full text of DOI:10.1039/c9cy01921a

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21 January 2017
Pages 313-534
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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/C9CY01921A
ARTICLE
Enhanced Propane Dehydrogenation to Propylene over Zinc-
Promoted Chromium Catalysts
Jie Liu a,b,c+, Yong Liu a,b+, Youming Ni a,b, Hongchao Liu a,b, Wenliang Zhu a,b* and Zhongmin Liu a,b*
Received 00th January 20xx,
Accepted 00th January 20xx
Direct propane dehydrogenation (PDH) is an attractive technology for propylene production. We show here that propane
conversion is significantly enhanced by the addition of ZnO to Cr2O3. Furthermore, the activity is strongly dependent on
the Zn/Cr molar ratio and the one with Zn/Cr = 0.3 gives a highest propane conversion and propylene selectivity among
the studied ZnxCr catalysts (x denoting the molar ratio, 0 - 0.5). Characterizations with X-ray diffraction, nitrogen
physisorption, X-ray fluorescence spectrometer, temperature-programmed reduction, temperature-programmed
desorption, transmission electron microscopy, X-ray photoelectron spectroscopy indicate that addition of zinc to Cr2O3
leads to formation of spinel phase and reduces the particle size, and hence forms more defective sites. The Arrhenius plots
suggest that the apparent activation energy of PDH reaction is significantly lowered over Zn0.3Cr.
DOI: 10.1039/x0xx00000x
exhibiting propane conversion above 35% and propylene
selectivity greater than 95% at 723 K in PDH reaction.13 And in
the typical Catofin process, the γ-Al2O3-supported catalyst
Introduction
Propylene is an important and basic building block in the
chemical industry, which is employed for the production of
polymers, oxygenates and other chemical compounds.1-3
Currently, propylene is commercially manufactured by steam
cracking, fluid catalytic cracking (FCC) and methanol-to-olefins
(MTO).4-6 With the rapidly growing demand for propylene and
the availability of large reserves of shale gas, direct propane
dehydrogenation to propylene (PDH) becomes an attractive
alternative process for production of propylene.7,8
Cr-based catalysts have been extensively studied for PDH
reaction since the first report by Frey and Huppke.9 A wide
range of Cr-based catalysts have been examined including
Cr/Al2O3, Cr/SiO2, Cr/ZrO2.10-12 Rossi et al. reported highly
selective Cr/Al2O3 and Cr/SiO2 catalysts for PDH in the
temperature range of 723-823 K. However, these catalysts
underwent a deactivation process with time on stream and the
activity was still low.11 For the most frequently studied
Cr/Al2O3 catalyst, its selectivity needs to be improved due to
the side reactions such as cracking and coking over the surface
acid sites of Al2O3, and hence leading to reduced propane
conversion and propylene selectivity. Therefore, great efforts
have been made to improve the performance of Cr/Al2O3
catalyst by use of different promoters, preparation methods
and support materials.13,14 Fujdala et al. synthesized Cr/Si/Al/O
and Cr/Si/Zr/O catalysts via a cothermolysis method capable of
contained 18-20wt% CrOx promoted by 1-2wt% Na or K.15 In
addition, other catalysts based on V, Ga, Zr and Zn have also
been investigated for PDH reaction.16-19 Among them, Zn-
based catalysts were thought to be promising due to its low
cost and high catalytic performance in PDH.20 Chen et al.
reported that small ZnO nanoclusters supported on
dealuminated ß zeolite showed high catalytic activity and
propylene selectivity in PDH raction.20 Schweitzer et al. found
that coordinatively unsaturated Zn2+ supported on silica
exhibited Lewis acidic properties and could catalyze PDH
reaction with a high propylene selectivity more than 95%.21
Therefore, the issue whether zinc oxide is an effective
promoter for Cr-based catalysts in PDH reaction is worthy to
be explored.
Zn-Cr based oxides are typical methanol synthesis catalysts
from syngas derived from coal, natural gas and biomass at
relative high reaction temperature and were also employed for
other reactions. Tian et al. reported Zn-Cr catalyst with a Zn/Cr
element ratio being 1:1 exhibited an outstanding isobutanol
synthesis activity from syngas.22 Wadaani et al. studied Zn-Cr
catalyst with a wide range of Zn/Cr ratio (20/1-1/30) for
dehydroisomerisation of α-pinene to p-cymene and the one
with Zn/Cr ratio of 1:1 showed the highest yield.23 According to
X-ray diffraction (XRD) measurements of Zn-Cr mixed oxides
used in these reactions, the catalysts display Zn–Cr spinel
phase with an excess of Zn2+ cations. Bertoldi et al. proposed
that the excess Zn2+ located in the octahedral sites, which were
coordinatively unsaturated, had a remarkable adsorptive
capacity towards CO and other molecules, resulting in high
catalytic performance.24 However, little research has been
done on Zn-Cr mixed oxides with excess Cr3+ and Zn-Cr based
oxides are not yet studied for PDH reaction.
a. National Engineering Laboratory for Methanol to Olefins, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Email: wlzhu@dicp.ac.cn, liuzm@dicp.ac.cn
b. Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian 116023, China
c. University of Chinese Academy of Sciences, Beijing 100049, China
† These authors contributed equally to this work.
DOI: 10.1039/x0xx00000x
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Therefore, we prepared a series of Zn-Cr mixed oxides with sample at 480 °C, before and after which the XPS spectra of
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DOI: 10.1039/C9CY01921A
different Zn/Cr molar ratios (0-0.5) as catalysts for PDH sample were measured.
reaction. Interestingly, these bi-component oxides are active in
The reducibility was characterized by H2-TPR, which was
PDH reaction and the catalyst Zn0.3Cr shows the highest performed on a Micromeritics AutoChem 2920 analyzer. The
activity with 31.3% of propane conversion and around 94% of samples were firstly treated in a flowing Ar at 150 °C for 1 h
propylene selectivity. Detailed characterization studies were before the test. After that, the samples were reduced in 10%
carried out and reveal that excellent performance of Zn-Cr H2/Ar flow at a heating rate 10 °C/min. NH3-TPD was also
mixed oxides in PDH reaction could be attributed to the carried out on the Micromeritics AutoChem 2920 analyzer.
formation of small-size Zn-Cr spinel with defective sites.
Before the measurement, the sample was firstly treated in a
flowing He at 450 °C for 1 h. Then 10% NH3/He was introduced
into the sample at 100 °C for 30 min followed by flushing with
He. Subsequently, the sample was heated in He at a rate of 10
°C /min while the NH3 desorption was monitored by a TCD
detector.
Experimental
Catalyst preparation
ZnxCr oxides (x = 0.1, 0.3, 0.5) were prepared by co-
precipitation method, with x standing for the molar ratio of
Zn/Cr. Taking Zn0.3Cr sample as an example, the following
synthesis procedure was used. 0.03 mole Zn(NO3)2·6H2O and
0.10 mole Cr(NO3)3·9H2O were dissolved in a certain amount of
deionized water to form a solution having a concentration of 1
mol/L. 200 ml aqueous solution with 0.2 mole (NH4)2CO3 was
used as the precipitant. The aforementioned solutions were
simultaneously added into a beaker containing 200 ml
deionized water under stirring at 70 °C. During the process, the
pH of the suspension was controlled between 7.0 and 7.1. The
suspension was then aged for 3 h at 70 °C under stirring,
followed by filtration and washing for three times with
deionized water. The resulting sample was dried at 100 °C for
12 h and then calcined at 500 °C in air for 4 h. ZnO, Cr2O3 and
other ZnxCr samples were prepared following the same
procedure under the same conditions.
Catalytic activity measurement
PDH reactions were performed in a fixed-bed stainless steel
reactor fitted with a quartz lining. 0.3 g catalyst was loaded
and heated to 400 °C for 1 h and 480 °C for 30 min in N2 (30
ml/min). A mixture of 5% C3H8 and 95% Ar was fed into the
reactor at a flow rate of 25 ml/min. The reaction was
monitored after 5 min from the start and was allowed for 5 h.
The effluents were analysed by an on-line Agilent 7890B gas
chromatograph, which was equipped with
a thermal
conductivity detector (TCD) and a flame ionization detector
(FID). Propane conversion (XC3H8) and propylene selectivity
(SC3H6) were calculated on a carbon atom basis, which were
expressed as the following:
∑
Fi ∗ Ai
=
∑
Fi ∗ Ai +FC3H8 ∗ AC3H8
XC3H8
SC3H6
For comparison, CrOx/Al2O3 was prepared by incipient
wetness impregnation of Al2O3 with Cr(NO3)3·9H2O aqueous
solution.25 After impregnation, the sample was dried at 120 °C
overnight and then calcined at 550 °C in air for 4 h. The actual
concentration of Cr2O3 in the prepared catalyst is 17wt%.
Fi ∗ Ai
=
∑
Fi ∗ Ai
Fi = fi*ni,
Among those, fi stands for the molar correction factor of the
product i, ni for the carbon number of the product i, and Ai
refers to the peak areas of the product i measured by FID.
Catalyst characterization
The BET surface areas were obtained by nitrogen
physisorption on a Micromeritics ASAP2020 system at 77 K.
Prior to measurements, the samples were degassed at 625 K
for 4 h. The chemical composition of all the samples was
determined by XRF on PhilipsMagix-601 XRF.
The crystal phases of the samples were characterized by
XRD, which were performed on a PANalytical X’Pert PRO X-ray
diffraction meter with Cu Kα radiation. The crystallite sizes
were calculated by Scherrer equation. The particle size was
also estimated from TEM, which was performed on a JEM-
2100 microscope operating at 200 kV. Energy dispersive
spectroscopy (EDS) spectra were obtained on a JEM-2100F
microscope operating at 200 kV.
Results and discussion
The performance of all catalysts for PDH reaction was
conducted at 480 °C, 0.1 MPa, 5% C3H8/Ar and space velocity
of 5000 ml/h·gcat. Upon addition of Zn, the activity improves
obviously. However, there appears to be an optimum range of
Zn addition. Among all the catalysts tested, Zn0.3Cr exhibits the
optimal activity. As shown in Fig. 1, an initial conversion of 21%
over Zn0.3Cr is obtained and further increases to 31.3% with
time on stream, which is much higher than that of 7% for
Zn0.1Cr and 11% for Zn0.5Cr. Meanwhile, The Zn0.3Cr catalyst
also presents a highest selectivity to propylene, with SC3H6
retaining around 94% along time on stream up to 300 min.
Methane and C4 hydrocarbons are the main by products for
Zn0.3Cr with very little ethylene and ethane (Fig. 1c). For
comparison, the single component Cr2O3 as well as ZnO were
used as catalyst for PDH reaction at the same reaction
XPS was performed on ESCALAB 250Xi spectrometer with a
monochromatized Al Kα as the exciting radiation. The binding
energy of contaminant carbon C1s at 284.8 eV was used as the
reference. In situ XPS was performed on SPECS EnviroESCA
spectrometer. 10 mbar 5% C3H8/Ar was introduced into the
conditions. Fig. 1a shows that Cr2O3 gives
a propane
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conversion of about 10% initially and drops rather quickly to
around 5% and then levels off with a propylene selectivity of
around 88%, which is lower than Zn0.3Cr. By contrast, ZnO is
much less active, with XC3H8 lower than 2% demonstrating the
weak ability of ZnO to activate propane. In addition,
Cr2O3/Al2O3, which was frequently studied, was synthesized for
comparison. The impregnation method was employed and the
Cr2O3 loading was 17wt%. Fig. 1d shows that
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DOI: 10.1039/C9CY01921A
reaching a steady state.
To understand the origin of the high activity of Zn0.3Cr, we
prepared a physically mixed ZnO-Cr2O3 catalyst with the single
its PDH activity under the same conditions is much lower than
the Zn0.3Cr catalyst as propane conversion is only 9% after
Fig. 2 XRD patterns of (a) Cr-containing catalysts and (b) Zn0.3Cr sample before and
after reaction
component ZnO and Cr2O3 with the same molar ratio of Zn/Cr
=
0.3 (denoted as ZnO-Cr2O3). One sees that XC3H8 is only 3%,
which is similar to that obtained over ZnO, lower than that
obtained over Cr2O3, and one order of magnitude lower than
that on Zn0.3Cr (Fig. 1d). These results suggest that the high
activity of the Zn0.3Cr catalyst is not just the coexisting of ZnO
and Cr2O3. Therefore, we turned to understanding of the
physicochemical properties and structure of the ZnxCr catalysts
in comparison to references ZnO and Cr2O3.
XRD was carried out in order to identify the crystal phase
structure of the catalysts and the results are shown in Fig. 2a.
The single component catalyst Cr2O3 exhibits only the crystal
phase of Cr2O3 with an average crystal size of 32.8 nm,
estimated according to Scherrer equation, as listed in Table 1.
With the addition of Zn to Cr2O3 at Zn/Cr = 0.1, ZnCr2O4
diffraction peaks appear in addition to those of Cr2O3 and the
crystal size of ZnCr2O4 is estimated to be 7.9 nm. At Zn/Cr =
0.3, the intensity of Cr2O3 reduces further and the diffraction
peaks of ZnCr2O4 phase become more prominent but still
rather broad. Interestingly, the estimated crystal size of
ZnCr2O4 phase becomes smaller (4.8 nm) with respect to that
over Zn0.1Cr. But the remaining Cr2O3 crystals still retain a large
size. At Zn/Cr = 0.5, there is only ZnCr2O4 spinel phase
detected and its crystal size increases to 8.8 nm compared to
that over Zn0.3Cr. Note that the crystal structure does not
change after the reaction,
Fig. 1 Catalytic performance of different catalysts in propane dehydrogenation
reaction. (a) Propane conversion; (b) Selectivity of propylene; (c) Selectivity of
byproducts for Zn0.3Cr; (d) The activity of Zn Cr in comparison with a physical
This journal is © The Royal Society of Ch0e.3mistry 20xx
J. Name., 2013, 00, 1-3 | 3
mixture ZnO-Cr2O3 and 17% Cr2O3/Al2O3. Reaction conditions: T = 480 °C, P = 0.1
MPa, GHSV = 5000 ml/gcath, C3H8/Ar = 5/95.
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Table 1 Textural properties of different catalysts
Zn/Cr
Average diameter b (nm)
Average diameter c (nm)
BET surface area
(m2/g)
Pore volume
(cm3/g)
Catalysts
atomic
ratio a
Cr2O3
ZnCr2O4
Cr2O3
ZnCr2O4
Cr2O3
Zn0.1Cr
Zn0.3Cr
Zn0.5Cr
ZnO
–
9.5
43.0
0.03
0.11
0.29
0.19
0.03
32.8
53.6
61.7
–
–
75.7
–
7.3
6.2
11.6
–
0.11
0.29
0.54
–
7.9
4.8
8.8
–
104.0
148.4
85.0
–
–
–
11.8
–
a
b
c
Zn/Cr atomic ratio was measured by XRF. Crystallite size was calculated by Scherrer equation based on XRD. Average particle size was determined by TEM
Fig. 3 TEM images of the Cr-containing catalysts with the images a referring to the ZnCr2O4 phase, and images b referring to the Cr2O3 phase.
as evidenced by the XRD pattern of the spent Zn0.3Cr catalyst 1). The specific surface areas and pore volumes of the bi-
(Fig. 2b).
component catalysts ZnxCr are much higher. It is 43.0 m2/g and
Smaller particles generally result in a higher surface area, as 0.11 cm3/g for Zn0.1Cr, and 148.4 m2/g and 0.29 cm3/g for
evidenced by the textual properties of these catalysts (Table Zn0.3Cr. However, further increasing the Zn/Cr ratio to 0.5
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leads to a reduced specific surface area and pore volume (85.0 show two different phases, i. e. Cr2O3 and ZnCr2O4, consistent
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m2/g and 0.19 cm3/g), in consistence with the crystal size.
with XRD. Interestingly, the particles sizeDoOfI:t1h0e.10Z3n9C/rC29OC4Y0p1h9a2s1Ae
Further characterization with HRTEM (Fig. 3) shows that the are rather uniform with a size around 7.3 nm whereas the
particle size of the Cr2O3 catalyst is not uniform, with an Cr2O3 phase is
average size around 75.7 nm. The Zn0.1Cr and Zn0.3Cr catalysts
composed of large particles around 100 nm for Zn0.1Cr. The from the decomposition of the catalyst itself since it was
particle size of ZnCr2O4 decreases into 6.2 nm for Zn0.3Cr. There calcined at 500 oC, as shown in Fig. 5b. NH3-TPD further shows
are only few Cr2O3 particles observed. In comparison, HRTEM that the amount of NH3 desorbing from Zn0.3Cr is considerably
validates that Zn0.5Cr exhibits merely the ZnCr2O4 crystal phase higher than the other two, as listed in Table 2. Tian et al.
and the particles are rather uniform but the size increases studied Zn excessive spinels (Zn0.8Cr and Zn1.2Cr) for isobutanol
further to around 11.6 nm with respect to that over Zn0.3Cr. synthesis from syngas
The above results indicate that the addition of zinc to Cr2O3
leads to formation of ZnCr2O4 spinel with a size lower than 10
nm, whereas the Cr2O3 phase in the bi-component catalysts
ZnxCr still retains a bulky particle size larger than 50 nm.
Among them, the ZnCr2O4 phase in Zn0.3Cr has the smallest size
with 4.8 nm. Generally, smaller particles give a higher surface
area and thus it is understandable that Zn0.3Cr exhibits a
highest surface area. Furthermore, the EDS mapping on
ZnCr2O4 crystal in the Zn0.3Cr catalyst in Fig. 4 shows that the
Zn and Cr elements are evenly distributed. The Zn/Cr molar
ratio is 0.32 by EDS analysis, in agreement with the value
measured by XRF (0.29). Therefore, the remarkably enhanced
PDH activity is likely related to the formation of small sized Zn-
Cr spinel species because smaller particles are frequently
accompanied by defects.
Defect sites on oxides usually exhibit Lewis acidic properties
and NH3-TPD is frequently employed to characterize the
properties.18,26 It is generally accepted that the position and
the area of ammonia desorption peak are directly associated
with the acidic strength and the acid amount, respectively.26 As
shown in Fig. 5a, there are several NH3 desorption peaks for
ZnxCr mixed oxides, implying the presence of multiple types of
defect sites with different binding strength to NH3. These
peaks fall in a similar temperature range for the three ZnxCr
samples. It appears that the Zn0.3Cr sample has another strong
peak coming up beyond 400 oC. A blank temperature
Fig.5 Temperature-programmed-desorption profiles of catalysts with different
Zn/Cr molar ratios. (a) NH3-TPD, (b) Blank experiment.
programmed experiment in He under the same conditions
suggests that the signal after 500 oC has some contribution
and they pointed out that the acidity character over the Zn-Cr
spinel catalyst reflected by NH3-TPD was closely related to its
surface property.27-28 The higher peak area reflects the
presence of more defect sites. Although Zn excessive non-
stoichiometric spinels have been widely studied for methanol
and higher alcohol synthesis, 24,27-28 Zn deficient spinel is rarely
studied. Elucidation of the actual structures of these defects
will require more sophisticated experimental and theoretical
studies.
Gao et al. reported that the metastable structure of non-
stoichiometric Zn–Cr spinels with cation disorder and many
defects could be stabilized by hydroxyl groups.29 Therefore, we
Fig. 4 EDS mapping of Zn-Cr spinel phase in Zn0.3Cr catalyst
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studied the surface hydroxyl by XPS. As presented in Fig. 6, the increases from 0.1 to 0.3, and then decreases toV2ie9w.A6r%ticlewOhnleinne
signal at a low binding energy (530.3-530.7 eV) is assigned to Zn/Cr molar ratio further increases to 0.D5O. IT: h10e.s10e39d/aCt9aCfYu0r1t9h2e1Ar
lattice oxygen (labeled as Olatt) and the signal at a higher validate the presence of more defect sites in the small-size Zn–
binding energy (531.9-532.2 eV) is derived from surface Cr spinel of the Zn0.3Cr catalyst.
hydroxyl groups (labeled as OOH), as reported previously.29,30
The ratio of (OOH)/(Olatt + OOH) is estimated and summarized in on Cr-based catalyst for PDH reaction, the apparent activation
Table 2. For the Zn-Cr oxides, the ratio of (OOH)/(Olatt + OOH energy over ZnxCr catalysts is estimated according to the
In order to further understand the promotion effect of zinc
)
increases from 27.6% to 33.9% when Zn/Cr molar ratio formation rate of propylene in a temperature range between
Table 2 The results of NH3-TPD and the O species composition of ZnxCr catalysts
Amount of acid sites
(mmol/g)
Total acidity
(mmol/g)
Catalysts
Peaks of acid sites (°C)
OOH/(Olatt + OOH)
Zn0.1Cr
Zn0.3Cr
Zn0.5Cr
222, 342
163
0.07, 0.06
0.36
0.13
0.36
0.25
27.6%
33.9%
29.6%
203, 234
0.14, 0.11
o
480-540 C. As presented in Fig. 7, Zn0.3Cr catalyst exhibits a
much lower apparent activation energy than the other two
catalysts. It indicates that the addition of zinc to Cr2O3
promotes the active sites in addition to generating ZnCr2O4
spinel phase and reducing the particle size.
H2-TPR was conducted to investigate the reducibility of the
catalysts and the profiles of the ZnxCr catalysts are presented
in Fig. 8. For comparison, the H2-TPR profiles of single
component Cr2O3 and ZnO catalysts are also presented. The
results show that no obvious reduction peak is observed for
ZnO below 650 °C. There is an obvious reduction peak at
around 441 °C for Cr2O3. It was reported that there existed a
small amount of Cr6+ species over the freshly calcined Cr2O3
oxides.31 The reduction peak around 441 °C over Cr2O3 was
likely attributed to the
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Fig. 7 Arrhenius plots for estimating the apparent activation energy of PDH reaction
over Zn/Cr catalysts with different ratio.
Fig. 6 O 1s XPS spectra of ZnxCr with different Zn/Cr molar ratios
reduction of Cr6+ to Cr3+, as reported by Akula et al..32
Interestingly, there are two reduction peaks observed for ZnxCr
oxides and the reduction temperatures (< 361 °C) are much
lower than that for the single component Cr2O3. This implies
the existence of more than one type of chromium species over
ZnxCr oxides. For Zn0.1Cr, there are two reduction peaks
observed at around 252 and 361 °C, which could be assigned
to the reduction of Cr6+, from the Zn-Cr spinel phase or
partially oxidized Cr2O3 phase, to Cr3+ as reported by Simard et
al.33 The Zn and Cr cations within the stoichiometric Zn–Cr
spinel (normally ZnCr2O4) are very difficult to reduce.34
Therefore the broad reduction peak in the temperature range
of 100-350 °C for Zn0.3Cr and the reduction peaks at around
256 and 300 °C for Zn0.5Cr could be assigned to the surface
Cr2O3 species and/or Zn-Cr spinel phase with defects.34
Interestingly, the Zn0.3Cr catalyst exhibits the highest hydrogen
consumption among all the ZnxCr catalysts, which is consistent
with its highest concentration of
The spectra of Cr 2p are shown in Fig. 9b. Deconvolution of
the Cr 2p3/2 spectrum indicates the presence of both Cr6+ and
Cr3+ species over the catalyst before reaction, represented by
the signals at the binding energy of 576.8 and 579.3 eV,
respectively.37 However, after reaction for 5min at 480°C, the
surface Cr6+ species disappear and there are only Cr3+ species
left. The Cr 2p spectrum of Zn0.3Cr after reaction for 30 min
exhibits a similar feature and there is not much change with
respect to that for 5 min. These results indicate that some Cr6+
species on the fresh catalyst surface are reduced to Cr3+ during
the initial stage of PDH reaction and remain as Cr3+ during the
reaction. The reduction of Cr6+ to Cr3+ species during the initial
defect sites Cr3+.22 Lin et al. stated that there was a direct
relation between catalytic activity and the reducibility of
catalyst, i.e. the higher the reducibility of supported Chromium
oxide catalysts, the higher catalytic activity in the
dehydrogenation of C2H6 to C2H4.35 Consequently, it is
understandable that the Zn0.3Cr catalyst exhibits a highest PDH
activity among the studied catalysts.
The Zn0.3Cr catalyst was further characterized by in situ XPS.
Spectra were taken before propane was fed in and after the
reaction had been allowed for 5 and 30 min, respectively. As
shown in Fig. 9a, the peaks at 1021.4 and 1044.6 eV are
attributed to Zn2+,36 which do not change during the reaction.
Fig. 8 H2-TPR profile of ZnxCr along with those of single component catalysts.
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stage of PDH reaction was also observed by in situ XANES and catalysts of Cr2O3 and ZnO. XRD, TEM, NH3-TPD, XPS and H2-
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UV−vis spectroscopy previously.38,39 Rossi et al. reported a TPR indicate that Zn0.3Cr contains small spinel particles, which
DOI: 10.1039/C9CY01921A
correlation of the concentration of the coordination could lead to formation of more defect sites around ZnCr2O4
unsaturated Cr3+ species with the PDH activity over Cr/Al2O3 particles and/or their interfaces with Cr2O3 though the latter
and
phase still retains a large particle size. Furthermore, a lowest
apparent activation energy over Zn0.3Cr implies that the Zn
additive may have also modified the properties of the active
sites.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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Conclusions
We report here that Cr2O3 promoted with zinc remarkably
enhances its catalytic activity in PDH reaction. Furthermore,
this enhancement is dependent on the molar ratio of Zn/Cr
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propylene. For instance, propane conversion reaches 31.3%
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DOI: 10.1039/C9CY01921A
Small-size Zn-Cr spinel with defective sites formed by adding Zn to Cr2O3 is favorable
for PDH reaction.