Performance of phase-pure M1 MoVNbTeOx catalysts by hydrothermal synthesis with different post-treatments for the oxidative dehydrogenation of ethane

Article abstract of DOI:10.1016/j.apcata.2015.03.039

Phase-pure M1 catalysts with different post-treatments have been prepared from the same precursor slurry by hydrothermal synthesis and used to study the key factors influencing their performance in the oxidative dehydrogenation of ethane (ODHE) process. Different purification processes (i.e. steam treatment and hydrogen peroxide treatment) result in different tellurium (Te) contents and V⁵⁺ concentration in the catalysts. Catalytic tests reveal that there is a direct correlation between the amount of V⁵⁺ present in the catalysts and the catalytic activity. A hydrogen peroxide treatment increases the V⁵⁺concentration and decreases the Te content which can improve the catalytic activity and stability in comparison with the steam treatment. A post-treatment with oxalic acid improves the catalyst surface area (54 m²/g) but causes some vanadium leaching. The phase-pure M1 catalyst calcined at 650 °C and purified by H₂O₂ shows the best catalyst productivity of 0.77 kg_C2H4/kg_cat/h at 73% ethane conversion and 85% ethylene selectivity in ODHE process. The formation of reduced Te(0) aggregates blocking the active sites is identified as a main reason for catalyst deactivation. A low Te content favors a stable catalyst with less risk of Te aggregation. However, at harsh operating conditions (i.e. high oxygen concentration and high reactor temperature) the performance of phase-pure M1 catalysts with a low Te content can also reduce obviously due to the formation of a new (V, Nb)-substituted θ-Mo₅O₁₄ and MoO₂ phases.

Full text of DOI:10.1016/j.apcata.2015.03.039

Accepted Manuscript
Title: Performance of Phase-pure M1 MoVNbTeOx Catalysts
by Hydrothermal Synthesis with Different Post-treatments for
the Oxidative Dehydrogenation of Ethane
Author: Bozhao Chu Lara Truter T.A. Nijhuis Yi Cheng
PII:
S0926-860X(15)00225-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.03.039
APCATA 15329
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
30-11-2014
13-3-2015
26-3-2015
Please cite this article as: B. Chu, L. Truter, T.A. Nijhuis, Y. Cheng, Performance of
Phase-pure M1 MoVNbTeOx Catalysts by Hydrothermal Synthesis with Different Post-
treatments for the Oxidative Dehydrogenation of Ethane, Applied Catalysis A, General
(2015), http://dx.doi.org/10.1016/j.apcata.2015.03.039
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Performance of Phase-pure M1 MoVNbTeOx Catalysts by
Hydrothermal Synthesis with Different Post-treatments for the
Oxidative Dehydrogenation of Ethane
Bozhao Chua, Lara Truterb, T. A. Nijhuisb*, Yi Cheng a*
a Department of Chemical Engineering, Beijing Key Laboratory of Green Chemical
Reaction Engineering and Technology, Tsinghua University, Beijing 100084, PR China
b Department of Chemical Engineering and Chemistry, Laboratory of Chemical Reactor
Engineering, Eindhoven University of Technology, Eindhoven 5600 MB, The Netherlands
Abstract
Phase-pure M1 catalysts with different post-treatments have been prepared from the same
precursor slurry by hydrothermal synthesis and used to study the key factors influencing
their performance in the oxidative dehydrogenation of ethane (ODHE) process. Different
purification processes (i.e. steam treatment and hydrogen peroxide treatment) result in
different tellurium (Te) contents and V5+ concentration in the catalysts. Catalytic tests
reveal that there is a direct correlation between the amount of V5+ present in the catalysts
and the catalytic activity
A hydrogen peroxide treatment increases the V5+concentration and
.
decreases the Te content which can improve the catalytic activity and stability in comparison
* Corresponding Author. Dr. T.A. Nijhuis, Tel: 31-40-2473671; Fax: 0041-26-3009624;
E-mail address: t.a.nijhuis@tue.nl
* Corresponding Author. Dr. Yi Cheng, Tel: 86-10-62794468; Fax: 86-10-62772051;
E-mail address: yicheng@tsinghua.edu.cn
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with the steam treatment. A post-treatment with oxalic acid improves the catalyst surface
area (54 m2/g) but causes some vanadium leaching. The phase-pure M1 catalyst calcined at
⁰
650 C and purified by H2O2 shows the best catalyst productivity of 0.77 kgC2H4/kgcat/h at
73% ethane conversion and 85% ethylene selectivity in ODHE process. The formation of
reduced Te(0) aggregates blocking the active sites is identified as a main reason for catalyst
deactivation. A low Te content favors a stable catalyst with less risk of Te aggregation.
However, at harsh operating conditions (i.e. high oxygen concentration and high reactor
temperature) the performance of phase-pure M1 catalysts with a low Te content can also
reduce obviously due to the formation of a new (V, Nb)-substituted θ-Mo5O14 and MoO2
phases.
Key words: oxidative dehydrogenation, ethane, ethylene, mixed-metal oxide
1. Introduction
Over the years, ethylene has been the largest consumed worldwide chemical commodity due
to it being an important building block in the plastics industry. The ethylene production
keeps increasing and is expected to reach 150 million tons in 2015 which is 50% more than
that in 2005 [1]. Nowadays, ethylene production is mainly based on the petroleum industry.
It is mostly produced by steam cracking of various hydrocarbon streams while smaller
amounts are obtained by the direct catalytic dehydrogenation of ethane during the
fluid-catalytic cracking of gas oils [2]. However, the oxidative dehydrogenation of ethane
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(ODHE) has attracted more interest due to the advantages in process performance and
energy efficiency [3]. Moreover, ethane, as the only carbon source in the ODHE process, is
abundant in main consumer of shale and associated gas. Compared to the limited traditional
fossil energy, shale gas has been demonstrated as a new and increasingly important source
of natural gas from the start of the 21st century. The global shale gas potential is estimated at
about 206.7 trillion cubic meters in 2013 and shale gas development is expected to continue
at an accelerated rate around the globe [4]. As a result, abundant ethane with a continuously
decreasing price further expands the advantage of the ODHE process.
Catalyst development and research in the ODHE process can be traced back to the 1970’s
[5]. Some of the catalysts used in the reaction of methane coupling are directly applied in
the ODHE process and also some entirely new catalytic systems have been developed
during the last 40 years. The catalysts of the ODHE process can be classified based on the
active components (i.e. unreducible metal oxides and reducible metal oxides). The
unreducible metal oxide catalysts are composed of an alkali metal or alkaline earth metal
(i.e. Li, Na, Mg),which is active for both methane coupling and the ODHE process above
600 °C. The reducible metal oxide catalysts consist of transition metals (i.e. Ni, Mo, V),
which are active for the oxidative dehydrogenation of alkanes between 300 and 500 °C. The
most successful non-reducible catalysts (i.e. Li-O/MgO) can only give a 38% ethane
conversion and 80% ethylene selectivity [6] while that of the reducible catalysts (i.e.
MoVNbTeOx) shows a much higher activity and selectivity at a relatively low temperature
[7-17]. At 400 °C and 1 atm, the ethane conversion and ethylene selectivity can respectively
reach 88.5% and 80.0% with a space time yield of 0.29 kgC2H4/kgcat/h [7]. However, the
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catalyst performance is still far from the commercial requirement. It has been concluded that
the ethane conversion should be in the range of 60-80%, ethylene selectivity higher than
90%, reaction temperature below 500 °C, and productivity of more than 1.00 kgC2H4/kgcat/h
to realize an economically viable industrial scale production of ethylene based on the ODHE
process [18-21].
In the aim of enhancing the catalytic performance (i.e. C2H4 conversion, selectivity and
productivity), properties of the MoVNbTeOx catalyst (i.e. chemical and phase composition
[7-17], crystal structure [22-27], reaction mechanism and kinetics [28-32]) have been
studied in detail.
The MoVNbTeOx catalyst usually consists of M1 and M2 crystalline phases and minor
amounts of other phases such as Mo5O14-type structures ordinary MoV and MoTe oxides
[25]. M1 is an orthorhombic phase with a Pba2 space group. M2 is a pseudo-hexagonal
phase with a P6mm space group. The M1 phase is built by center occupied pentagonal rings
that are connected by corner sharing MO6 octahedrons (M = Mo, V), which are assembled
in the (001) plane to form characteristic hexagonal and heptagonal rings hosting Te-O units.
Only M2 has characteristic hexagonal rings hosting the Te-O units without any pentagonal
or heptagonal rings in the (001) plane [24]. It is thought that the M1 phase evolves from the
M2 phase by the introduction of Nb5+ as a pentagonal template leading to the formation of
the pentagonal and heptagonal rings [16, 17]. A bronze-like channel structure grows along
the (001) direction and a needle-like crystal morphology finally forms in which the (001)
planes are perpendicular to the axis length of the needle. It has been demonstrated that
terminating (001) planes of the M1 phase cannot supply more activity or selectivity than
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other surfaces in the selective oxidation processes [26], however, recently the heptagonal
micropores (~ 0.4 nm) of the (001) planes were found to be responsible for the catalytic
activity for the ethane selective oxidation [27]. The M1 phase possesses the only V5+ and
heptagonal channel in the MoVNbTeOx catalysts [24]. The V5+ ions are considered as the
active sites for alkane activation [15,24,40]. The M2 phase has more Te4+ than the M1 phase,
which is considered as the active sites for the activation of α-H in the alkene [24]. As a
result, the M2 phase has no activity in the ODHE process and the catalyst performance
increases with the M1 phase content in the MoVNbTeOx catalyst. The phase-pure M1
catalyst is therefore ideal for obtaining a high productivity in the ODHE process.
Commonly, the MoVNbTeOx catalyst is prepared by either precipitation or hydrothermal
synthesis [7-17]. Hydrothermal synthesis is more suitable to prevent the formation of more
impure phases than the precipitation method. Although it has been reported that phase-pure
M1 catalysts can be directly obtained by hydrothermal synthesis under certain conditions
[33], a certain amount of M2 often exists in the catalyst which requires a further
post-treatment procedure. According to the different properties of the M1 and M2 phase,
some post-treatment methods (i.e. H2O2 treatment and steam treatment) are effective in the
M1 phase purification [34-36]. However, different post-treatments may not only influence
the purity but also the chemical properties and morphology of the catalyst due to its
sensitivity to the preparation procedure [37]. Few papers pay attention to the influence of
post-treatment on the performance and stability of phase-pure M1 catalyst [38].
In the present work, different post-treatment processes were applied to produce phase-pure
M1 catalysts which contain different chemical compositions and surface areas. These four
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catalysts were evaluated at different temperatures (340-460 °C) with different feed
composition to investigate the effect of different post-treatments on the catalyst performance
(ethylene selectivity, productivity and stability) in the ODHE process. All of the catalyst
evaluations were carried out in a lab-scale fixed-bed reactor and the properties of the
catalyst before and after reaction were characterized by XRD, BET, ICP, XPS, SEM and
TEM.
2. Experimental
2.1 Catalyst Preparation
MoVNbTeOx catalysts were prepared by hydrothermal synthesis [33] and, thereafter,
various post-treatment methods were applied.
Ammonium heptamolybdate (Sigma-Aldrich, 99.0%), vanadyl sulfate (Sigma-Aldrich,
97%), telluric acid (Sigma-Aldrich, 98%) and ammonium niobium oxalate (Sigma-Aldrich,
99.99%) were used as starting materials to prepare the initial precursor slurry with a molar
ratio of 1 Mo:0.25 V:0.12 Nb:0.23 Te. After preparation of the precursor slurry, the slurry
was inserted into 100ml Teflon-lined autoclaves and the residual air was replaced by
nitrogen. Hydrothermal synthesis was carried out at 175 °C for 48 h. The resulting powder
was filtered and washed thoroughly with distilled water, followed by drying for 16 h at 80
C in air, and calcination for 2 h at 600 C. The MoVNbTeOx catalyst was further purified
by removal of the M2 phase to form the phase-pure M1 catalyst using various
post-treatment procedures. Catalyst A was purified during the calcination step where steam
was introduced into the tube-oven to accelerate the decomposition of the M2 phase under
hydrothermal synthesis conditions. Catalyst B was obtained by the dissolution of the M2
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phase in a 7.5% H2O2 solution at 60 C for 3 h, followed by filtration, washing and drying
overnight at 110 C [35]. Other post-treatment methods were also used to generate
phase-pure M1 catalyst with different morphology and chemical composition. Catalyst C
was obtained by calcination at 650 C for 2 h, followed by the purification in a 7.5% H2O2
solution as described previously. Catalyst D was obtained after an initial oxalic acid
post-treatment followed by calcination 600 C for 2 h and the 7.5% H2O2 purification steps.
The oxalic acid post-treatment was carried out in 60 mmol/ml oxalic acid solution at 60 C
for 2 h for the catalyst only after hydrothermal synthesis, filtration, washing and drying. The
oxalic acid treatment was originally used in the purification of orthorhombic Mo3VOx
oxides [27].
2.2 Catalyst Testing
The M1 catalysts were prepared for catalytic testing by diluting with SiC powder in a 1:4
ratio to improve the heat distribution during the reaction and, thereafter, pelletized to a 100 –
200 µm particle size. 0.625 g catalyst was loaded in a quartz tube (6 mm i.d. and 750 mm
length) and operated isothermally at atmospheric pressure. The gas feed consisted of ethane,
oxygen and helium with the total flow rate of 30 Nml/min and a C2H6/O2/He molar ratio of
30/20/50. The space time W/FC2H6 (W is the catalyst mass and FC2H6 is the ethane molar flow
rate) was maintained at 20.74 gcat∙h/molC2H6 and the reaction temperature varied between
360 and 460 °C. During the heating up of the fixed-bed reactor, helium is the only gas fed
through the reactor tube. Once the reaction temperature was reached, the reagent and the gas
feed with the appropriate composition were introduced into the system. The steady-state
data of catalyst performance was determined 6h after the beginning of the reaction because
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there is usually an induction period for the catalysts in oxidative dehydrogenation of
alkanes.
The reactor effluent was analyzed by an online gas chromatography (GC) equipped with two
columns. A PorapakQ column was used to separate the CO2, C2H4 and C2H6 and
5A molecular sieve column was used to separate the O2, N2, CH4 and CO. A blank run was
conducted by loading the reactor with only SiC and using the same operating conditions. No
ethane conversion was observed indicating that the effect of the homogeneous gas phase
reaction can be neglected.
The conversion and selectivity for ODHE process are defined as follows:


2 f
C2H6
XC H  1
100%
(1)




2
6

2 fC H  2 fC H  fCO  fCO

2
6
2
4
2


2 f
C2H4
SC H

100%
(2)






2
4
2 fC H  fCO  fCO
2
4
2


f
CO2
SCO

100%
(3)
(4)






2
2 fC H  fCO  fCO
2
4
2


fCO
2 fC H  fCO  fCO
SCO

100%






2
4
2
Where, X is the ethane conversion, S is the selectivity to a certain product, and f is the molar
fraction in the effluent gas.
2.3 Catalyst Characterization
X-ray diffraction (XRD) patterns of samples were recorded on a Bruker D8 Advance
equipment with Cu Kα radiation in the 2θ range of 5-50°. Phase identification of the
catalysts was carried out by comparing the collected spectra with those listed in the JCPDS
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database (Joint Committee of Powder Diffraction Standards) and the ICSD database
(Inorganic Crystal Structure Database).According to the ICSD database, M1 phase (ICSD
55097) has characteristic diffraction lines located at 2θ = 6.6, 7.7, 8.9, 22.1, and 27.1° and
the M2 phase (ICSD 55098) at 2θ = 22.1, 28.1and 36.2°. The average particle diameter
along a certain direction was calculated with the Scherrer equation.
The specific surface area was calculated based on the multipoint Brunauer–Emmett–Teller
method (BET) in the p/p0 = 0.05-0.30 pressure range. Nitrogen adsorption was carried out at
77 K on a Quantachrome Autosorb-6B analyzer. Prior to the measurement, the samples were
degassed in vacuum at 110 C for 2 h. Inductively coupled plasma-optical emission
spectrometry (ICP-OES, Varian Vista RL spectrometer) was used to analyze the metal
content of the MoVNbTeOx catalyst.
The morphology and particle size distribution was characterized using a scanning electron
microscopy (SEM, JEOL, JSM-7401F) and a JEOL JEM2010 high-resolution transmission
electron microscopy (HR-TEM).TEM samples were prepared by dispersing 0.01 g catalyst
powder in 5ml ethanol by ultrasonic agitation. Thereafter, the suspension was dropped onto
a carbon-coated copper grid and dried in air at 110 C for 2 h.
X-ray photoelectron spectra (XPS) was acquired using a PHI Quantera SXM system,
equipped with Al Kα X-ray source. The binding energy data of reference material is
obtained from NIST X-ray Photoelectron Spectroscopy Database. Survey scans (0-1200 eV)
and high-resolution Mo (3d), V (2p), Te (3d), Nb (3d) and C (1s) spectra were obtained. To
obtain accurate results of valance distribution by de-convolution, the dwell time was
increased from 300 ms to 500 ms and the scan times were increased from 6 to 10 during the
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high-resolution scanning for V and Te. The uncertainty of the peak positions was estimated
to be 0.2 eV for all spectra and the binding energy (BE) scale was calibrated by setting the C
(1s) transition to 284.8 eV. The analysis of the measured high-resolution spectra was
performed using XPSPEAK 4.1 software.
The hydrogen peroxide concentration was verified by a hydrogen peroxide titration with a
standard solution of cerium (IV) sulfate.
3. Results and Discussion
3.1 General Properties of MoVNbTeOx Catalysts
The chemical and structural details of the MoVNbTeOx catalysts are summarized in Table 1
and the XRD patterns of the respective catalysts are presented in Figure 1.
[Table 1]
[Figure 1]
Without applying a purification treatment, the hydrothermally synthesized, Catalyst E,
contains small amounts (~20 wt %) of the M2 phase. The removal of the M2 phase was
made possible by applying either a steam treatment (Catalyst A) or a H2O2 treatment
(Catalyst B, C& D) which resulted in the phase-pure M1 catalysts.
However, despite these catalysts all containing only the M1 phase, the different
post-treatment processes were seen to greatly influence the final chemical composition of
the MoVNbTeOx catalysts (despite being prepared from the same precursor slurry). Catalyst
A, which was calcined in the presence of steam, produced the M1 phase catalyst with a high
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tellurium content. The presence of the steam was thought to inhibit the volatilization of
tellurium during the calcination step. However, despite the higher amount of Te in Catalyst
A, no Te metal is seen in the XRD patterns or XPS measurements (Figure 2 &3). Previously,
it has been shown that a steam post-treatment enables the formation of phase-pure M1 but
with the additional formation of metallic Te [35]. However, this was not observed which
could be due to the milder post-treatment conditions. Meanwhile, the post-treatment with
oxalic acid resulted in the vanadium content of Catalyst D to be lower. This loss in
vanadium could be caused from the vanadium being leached from the M1 crystalline
structure due to it being more soluble in comparison to other elements. In addition, the
crystal structure of all the catalyst was comparable with the pure M1 phase [39].
The morphology of catalysts also indicated obvious differences after the various treatments.
TEM and SEM images of the respective catalysts are shown in Figure 2.
[Figure 2]
The particle size and surface area of the catalysts were seen to decrease in the following
order: Catalyst A > B > C > D. Catalyst D, prepared from the oxalic acid post-treatment
resulted in the largest decrease in particle size to approximately 22 nm. The surface area was
approximately 54 m2/g which is the largest among the MoVNbTeOx catalysts reported in
literature. It is thought that the precursor material of long-range ordering in the [001]
direction formed during the hydrothermal synthesis [33] are tapered by oxalic acid resulting
in the formation of smaller needle-like crystals. Catalyst C which was calcined at a higher
temperature (650 °C) was found to give a smaller particle size and larger surface area in
comparison to Catalyst B (calcined at 600 °C).
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3.2 XPS Analysis
Figure 3 shows XPS spectra of the key elements in the MoVNbTeOx catalysts which were
measured to investigate the element valence state on the catalyst surface.
[Figure 3]
For all catalysts, the binding energy (BE) of molybdenum (232.8 eV) and niobium (207.2
eV) corresponds well to the typically reported binding energies for MoO3 and Nb2O5 which
suggests a single molybdenum and niobium valence state of +6 and +5, respectively.
Tellurium has a binding energy between 576.5±0.1 eV for TeO2 and 577.3±0.1 eV for TeO3
which implies the presence of both the Te4+ and Te6+ species. The molar ratio of Te6+ and
Te4+ maintains the similar value between 0.49 and 0.53 by the de-convolution of the Te
3d5/2 core level spectra [40]. No formation of Te0 (573.2±0.1 eV) was detected. It indicates
that the valances distribution of Te on the surface of catalysts almost make no difference to
the catalyst performance.
The binding energy of vanadium is between the value of 516.3±0.1 eV for VO2 and
517.2±0.1 eV for V2O5, inferring that vanadium has two valence states of +4 and +5 [40].
Since V5+ is considered the only active site for ethane activity, the abundance of these V5+
species remains an important factor affecting the catalyst performance (Section 3.3). The
abundance of the V5+ species on the respective catalyst surfaces was calculated by
combination of the elemental composition and de-convolution of the vanadium 2p3/2 peak
(Figure 3).
Catalysts B, C and D show a higher percentage of V5+ content in comparison to Catalyst A
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which could be attributed to the H2O2 treatment where the oxidative property of the H2O2
may favor the formation of V5+ from V4+. Catalyst D shows the lowest quantity of total
vanadium due to the leaching of vanadium during the oxalic acid post-treatment step.
However, a high portion of the vanadium is in the V5+ valance state which, similarly, can be
attributed to the H2O2 treatment. The effect of the V5+ quantity on the catalytic activity of
the respective catalysts is further discussed in section 3.3.
3.3 Catalytic Evaluation of MoVNbTeOx Catalysts
Figure 4 shows the catalytic performance of the respective catalysts in the ODHE process.
[Figure 4]
The catalytic performance of the respective catalysts shows great variations in productivity
depending on the post-treatment method used. Catalyst B and Catalyst C show good
performance at 400 °C with ethane conversions of 74.2% and 76.9%.This is consistent with
Catalysts B and C having the highest V5+ concentration. Catalyst D has a catalytic activity
which follows the same trend of the V5+ concentration in the respective catalysts. Although,
the total V content was lower for Catalyst A, the higher V5+ content still ensured a greater
catalytic activity. The smaller particle size and larger surface area of Catalyst D is seen to
not affect the catalytic performance significantly where Catalyst B and C with larger particle
size and smaller surface area have a higher activity. It was similarly found by Kolen’ko et al.
[36] that the particle size and surface area do not influence the catalytic performance for the
selective oxidation of propane. Catalyst A shows the lowest productivity due to the low
ethane conversion which can be attributed to the low V5+ abundance. Since the surface
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oxidation state of vanadium varies in the reaction conditions and a re-equilibration takes
place under reaction conditions[40], the V5+ abundance of the catalysts after a 6 h catalytic
test was also measured as illustrated in Figure S1. Although the total vanadium and V5+
abundance was slightly higher after the reaction, the sequence of the catalyst based on the
V5+ abundance did not change. Moreover, the high Te content could also affect the activity
by increasing the occupancy of Te-O unit located in the heptagonal channel of M1 phase
which has been recently demonstrated by Ishikawa et al. [27] as the necessary path way for
ethane to be selectively oxidized to ethylene.
It can therefore be concluded that the V5+ concentration, not the total amount of V [10] , is
the main factor affecting the catalytic activity of the catalyst where the catalyst morphology,
particle size and surface area do not significantly affect the catalytic activity. It is thought
that the H2O2 treatment is important in the oxidation of the MoVNbTeOx catalyst due to its
ability to increase the V5+ content. Since the V content of Catalyst A, B and C is similar and
all are phase-pure M1, the higher V5+ content can be ascribed to the H2O2 increasing the
formation of V5+ from V4+ sites. The extent of the M2 removal during the H2O2 treatment
could also affect the V5+ concentration since V5+ is not present in the M2 phase [34].
However, as seen in the XRD patterns (Figure 2), the respective catalysts are all pure M1
phase and contain similar quantities of total V, so the differences in V5+ concentration is
thought to be due to the oxidative properties of the H2O2.
To further investigate this, the H2O2 concentration was varied (2.5 - 10%) and the effect on
the V5+ abundance in the catalyst and catalytic activity was determined (Figure 5).
[Figure 5]
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As the H2O2 concentration increases, the V5+ concentration increases together with the
ethane conversion. This indicates that the H2O2 treatment step is an effective method for
increasing the V5+ concentration in the catalyst. All the catalysts used in this experiment
have been demonstrated as phase-pure M1 by XRD and similar Te content by ICP. So it is
sufficient to remove the M2 phase and decrease Te content even by using the H2O2 solution
with the concentration as low as 2.5%.
The ethylene selectivity determines the atom economy of the ODHE process. The selectivity
of propane selective oxidation on MoVNbTeOx catalyst has been discussed extensively in
literature but few focus on the selectivity of the ODHE process [41-46]. In this work, at the
same ethane conversion level of approximately 45%, Catalyst A, B and C show a similar
ethylene selectivity of 85% whilst Catalyst D gives a lower ethylene selectivity of 80%. As
proposed by Grasselli [44], the deep oxidation of ethane and ethylene to COx on
MoVNbTeOx catalysts can be suppressed by spatially isolating the active sites. According to
Ueda et al.[45-46], the Nb species can promote rapid desorption of the desired products so
as to prevent further oxidation and increase the selectivity. But this theory cannot be used to
explain the low selectivity of Catalyst D since there is no obvious loss of Nb during the
oxalic acid treatment based on the XPS and ICP chemical composition analyses. It is
deduced that oxalic acid treatment may destroy the crystal structure of the M1 phase which
affects the ethylene selectivity. The oxalic acid treatment could therefore provide an
opportunity to detect the key factors of the M1 phase’s crystal structure in the selectivity of
the ODHE process.
15
Page 15 of 37

3.4 Stability of Phase-pure M1 Catalysts
In addition to the activity and selectivity, the stability of the catalyst is also a significant
factor in the realization of an industrial-scale production process. Time-on-stream and harsh
operating conditions (i.e. high temperature and high oxygen concentration)experiments of
the respective catalysts were performed in order to investigate the stability and main
deactivation factors of the phase-pure M1 catalyst.
3.4.1 Stability of Phase-pure M1 Catalysts with Different Te Content
The time-on-stream as a function of ethane conversion and ethylene selectivity at standard
operating conditions is shown in Figure 6 for Catalyst A (high Te content) and Catalyst B
(low Te content). Catalyst C and D show similar Te content and stability in comparison to
Catalyst B and are thus not included. Different flow rates of feed gas are used to keep
Catalyst A and B at similar ethane conversion levels at the start of the experiment in order to
compare the stability.
[Figure 6]
Over a 36 h time-on-stream period, Catalyst A is seen to deactivate from 32% to 28% ethane
conversion whilst Catalyst B remains stable at 31%. The reason for this difference in
deactivation can be provided by XRD and XPS measurements of the used catalyst (Figure
7).
[Figure 7]
XRD results demonstrate that Te exists in the used Catalyst A while XPS results similarly
indicate that Te0 is produced by the reduction of Te4+. Moreover, ICP chemical analysis
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indicates that the Te content of Catalyst A does not significantly change after the reaction. It
is therefore thought that Te is reduced from Te4+ to form Te0 on the catalyst surface which
blocks the active sites or heptagonal channels resulting in the higher rate of deactivation
with time for Catalyst A, containing a higher Te content. This conclusion is consistent with
the research of Valente et al. [38] in the ODHE process over the MoVNbTeOx catalyst
composed of both the M1 phase and M2 phase. For Catalyst B, containing a lower Te
content, there is no detectable formation of metallic Te from the XRD and XPS results. The
avoidance of the formation of metallic Te is thus important for obtaining a highly stable
catalyst. Catalyst preparation should keep Te content at moderate levels.
3.4.2 Stability of Phase-pure M1 Catalysts with low Te content under harsh condition
The stability of Catalyst B, with a low Te content, was evaluated at harsh operating
conditions of high oxygen concentration in the feed stream (Figure 8A) and high reactor
operating temperature (Figure 8B).
[Figure 8]
After a 24h time-on-stream period there is a 45% deactivation for the higher oxygen
concentration feed conditions and a 10% deactivation for the catalyst operating at a high
reactor temperature. XRD analyses of the fresh and used Catalyst B are shown in Figure 10.
At high oxygen concentration conditions, a (V, Nb)-substituted θ-Mo5O14 (JCPDS 31-1437)
phase forms in addition to the M1 phase. Whereas, at high reactor temperature conditions, a
MoO2 (JCPDS 32-0671) forms. The (V, Nb)-substituted θ-Mo5O14phase has little activity in
the ODHE process whilst the MoO2 phase has no activity in the ODHE reaction [10]. It is
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verified by ICP analysis that there is approximately 25% weight loss of Te in Catalyst B
after the reaction under high temperature conditions. The formation of these new phases
with less or no ODHE activity by the loss of Te under harsh conditions is thus the main
reason for the deactivation. The leaching of Te was also observed in MoVNbTeOx catalyst
containing both M1 and M2 phases [38]. The precise control of oxygen concentration and
reactor temperature is a prerequisite to maintain a high stability and phase-pure M1 catalyst
in the ODHE process.
[Figure 9]
4. Conclusion
The influence of post-treatment steps in the preparation of phase-pure M1 MoVNbTeOx
catalysts has been studied in this work. Excellent catalytic performance in the ODHE
process with ethane conversion of 73%, ethylene selectivity of 85% and space-time yield of
0.77 kgC2H4/kgcat/h was obtained by a combination of high calcination temperature (650 °C)
and H2O2 treatment. The V5+ concentration was found to be the main factor influencing the
phase-pure M1 catalyst activity and its stability was dependent on the Te content. Besides
the function of purification for M1 phase, H2O2 treatment was demonstrated as an effective
method to increase the V5+ abundance on the catalyst surface and decrease the Te content in
the catalyst bulk which can improve the phase-pure M1catalyst activity and stability in
ODHE process. Other post-treatment procedures, such as treatment with oxalic acid,
resulted in a significantly lower particle size (23 nm) and higher surface area (54 m2/g).
However, loss of vanadium during this treatment reduced its catalytic activity despite the
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improvements in surface morphology. Moreover, harsh operating conditions such as high
oxygen concentration in the feed gas and high reactor operating temperature can also result
in the deactivation even in catalysts with low Te content. It is, therefore, recommended that
catalysts with high vanadium content and moderate Te content be synthesized and operated
in the ODHE process at moderate oxygen feed concentrations and reactor temperatures (<
460 °C).
Acknowledgements
Financial supports from joint project supported by KNAW and Ministry of Education
(China) are acknowledged.
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Figures Captions
Figure 1 XRD patterns (above) and enlargement of region from 2θ = 26-30° (below) of the
MoVNbTeOx catalysts. (A) Catalyst A; (B) Catalyst B; (C) Catalyst C; (D) Catalyst D (E)
Catalyst without post-treatment
Figure 2 TEM and SEM images of the fresh MoVNbTeOx catalysts
Figure 3 XPS spectra of the (A) Catalyst A, (B) Catalyst B, (C) Catalyst C and (D) Catalyst
D for the main transitions elements and V5+ abundance in fresh catalyst A-D
Figure 4 MoVNbTeOx catalyst performance in the ODHE process as a function of reaction
temperature for (A) ethane conversion, (B) oxygen conversion,(C) ethylene selectivity, (D)
CO selectivity and (E) CO2 selectivity, at space time of 20.74 gcat·h/molC2H6 and feed molar
ratio of C2H6/O2/He = 30/20/50.
Figure 5 Ethane conversion and V5+/V4+ molar ratio as a function of H2O2 concentration
during post-treatment for catalyst B. Catalytic experiments were performed at 420 °C with a
space time of 20.74 gcat·h/molC2H6 and a reactor inlet C2H6/O2/He molar ratio of 30/20/50
Figure 6 Ethane conversion and ethylene selectivity as functions of time-on-stream for
catalyst A and B. Experiments were performed at 420 °C with a space time of 20.74
gcat.h/molC2H6for Catalyst A and 6.91 gcat·h/molC2H6 for Catalyst B and a reactor inlet
C2H6/O2/He molar ratio of 30/20/50
(■) ethane conversion of catalyst A (□) ethylene selectivity of Catalyst A
(●) ethane conversion of catalyst B (○) ethylene selectivity of Catalyst B
Figure 7 XRD patterns (above) and XPS spectra (below) for used catalyst (A) Catalyst A
and (B) Catalyst B
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Figure 8 Ethane conversion (●) and ethylene selectivity (○) as functions of time-on-stream
for catalyst B under harsh conditions. Experiments were performed at (A) 420 °C with a
space time of 20.74gcat.h/molC2H6and a reactor inlet C2H6/O2/He molar ratio of 30/40/30 and
(B) 460 °C with a space time of 20.74 gcat·h/molC2H6 and a reactor inlet C2H6/O2/He molar
ratio of 30/20/50
Figure 9 XRD patterns of (A) fresh catalyst B (B) used catalyst B after ODHE in high O2
concentration and (C) used catalyst B after ODHE at high temperature
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Table Captions
Table 1 General properties of the MoVNbTeOx catalysts
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Figures
Figure 1 XRD patterns (above) and enlargement of region from 2θ = 26-30° (below) of the
MoVNbTeOx catalysts. (A) Catalyst A; (B) Catalyst B; (C) Catalyst C; (D) Catalyst D (E)
Catalyst without post-treatment
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Figure 2 TEM and SEM images of the fresh MoVNbTeOx catalysts
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Figure 3 XPS spectra of the (A) Catalyst A, (B) Catalyst B, (C) Catalyst C and (D) Catalyst
D for the main transitions elements and V5+ abundance in fresh catalyst A-D
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Figure 4 MoVNbTeOx catalyst performance in the ODHE process as a function of reaction
temperature for (A) ethane conversion, (B) oxygen conversion,(C) ethylene space-time yield
(D) ethylene selectivity, (E) CO selectivity and (F) CO2 selectivity, at space time of 20.74
gcat·h/molC2H6 and feed molar ratio of C2H6/O2/He = 30/20/50.
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