Deactivation of a mixed oxide catalyst of Mo–V–Te–Nb–O composition in the reaction of oxidative ethane dehydrogenation

Article abstract of DOI:10.1134/S0036024416060133

The operational stability of a mixed oxide catalyst of Mo–V–Te–Nb–O composition in the oxidative dehydrogenation of ethane (ratio of C₂H₆: O₂ = 3: 1) is studied in a flow reactor at temperatures of 340–400°C, a pressure of 1 atm, and a WHSV of the feed mixture of 800 h^?1. It is found that the selectivity toward ethylene is 98% at 340°C, but the conversion of ethane at this temperature is only 6%; when the temperature is raised to 400°C, the conversion of ethane is increased to 37%, while the selectivity toward ethylene is reduced to 85%. Using physical and chemical means (XPS, SEM), it is found that the lack of oxidant in the reaction mixture leads to irreversible changes in the catalyst, i.e., reduced selectivity and activity. Raising the reaction temperature to 400°C allows the reduction of tellurium by ethane, from the +6 oxidation state to the zerovalent state, with its subsequent sublimation and the destruction of the catalytically active and selective phase; in its characteristics, the catalyst becomes similar to the Mo–V–Nb–O system containing no tellurium.

Full text of DOI:10.1134/S0036024416060133

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 6, pp. 1132–1136. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © I.I. Mishanin, A.N. Kalenchuk, K.I. Maslakov, V.V. Lunin, A.E. Koklin, E.D. Finashina, V.I. Bogdan, 2016, published in Zhurnal Fizicheskoi Khimii,
2016, Vol. 90, No. 6, pp. 855–859.
CHEMICAL KINETICS
AND CATALYSIS
Deactivation of a Mixed Oxide Catalyst of Mo–V–Te–Nb–O
Composition in the Reaction of Oxidative Ethane Dehydrogenation
I. I. Mishanina, A. N. Kalenchuka,b, K. I. Maslakova, V. V. Lunina,b,
A. E. Koklinb, E. D. Finashinab, and V. I. Bogdana,b
a Department of Chemistry, Moscow State University, Moscow, 119991 Russia
b Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
e-mail: arnochem@yandex.ru
Received June 24, 2015
Abstract—The operational stability of a mixed oxide catalyst of Mo–V–Te–Nb–O composition in the oxida-
tive dehydrogenation of ethane (ratio of C2H6 : O2 = 3 : 1) is studied in a flow reactor at temperatures of 340–
400°C, a pressure of 1 atm, and a WHSV of the feed mixture of 800 h−1. It is found that the selectivity toward
ethylene is 98% at 340°C, but the conversion of ethane at this temperature is only 6%; when the temperature
is raised to 400°C, the conversion of ethane is increased to 37%, while the selectivity toward ethylene is
reduced to 85%. Using physical and chemical means (XPS, SEM), it is found that the lack of oxidant in the
reaction mixture leads to irreversible changes in the catalyst, i.e., reduced selectivity and activity. Raising the
reaction temperature to 400°C allows the reduction of tellurium by ethane, from the +6 oxidation state to the
zerovalent state, with its subsequent sublimation and the destruction of the catalytically active and selective
phase; in its characteristics, the catalyst becomes similar to the Mo–V–Nb–O system containing no tellu-
rium.
Keywords: oxidative dehydrogenation of ethane, mixed oxide catalyst, Mo–V–Te–Nb–O.
DOI: 10.1134/S0036024416060133
INTRODUCTION
reduction of the catalyst. One of the key elements
responsible for the high selectivity toward ethylene in
the ODE reaction is tellurium [5]. It is responsible for
the formation of the highly selective orthorhombic M1
phase of the Te2M20O57 composition (M = Mo, V, Nb)
and prevents the formation of nonselective Mo- and
V-containing crystalline phases [5].
Due to the growing demand for the deep, energy-
efficient processing of natural, associated, and refin-
ery gases, there is a strong need to develop processes
for the dehydrogenation of such light hydrocarbons as
ethane. One ways of solving this problem could be the
oxidative dehydrogenation (OD) of lower alkanes,
which has several advantages over direct dehydrogena-
tion. First, OD is an exothermic process; this should
certainly serve as an incentive to use it in the produc-
tion of olefins. Second, the use of OD allows us to
cope with the coking process, a cause of rapid catalyst
deactivation.
There are currently a great many catalysts known
for the oxidative dehydrogenation of ethane (ODE)
[1]. It is known that the best results in ODE are shown
by mixed oxide catalysts Mo–V–Te–Nb–O [2],
which at a fairly low temperature (400°C) ensure 90%
selectivity for ethylene at an ethane conversion of 60%.
The activity of Mo–V–Te–Nb–O catalysts is largely
determined by the ratio of metals in them, the tem-
perature of treatment, the initial precursors for syn-
thesis, and the methods of preparation [2–9].
The aim of this work was to study the stability of
Mo–V–Te–Nb–O catalyst in the ODE reaction.
EXPERIMENTAL
Mixed oxide catalysts Mo1.0V0.37Te0.17Nb0.15Ох
(referred to below as Mo–V–Te–Nb–O) and
Mo1.0V0.29Nb0.15Ox (below, Mo–V–Nb–O) were pre-
pared via hydrothermal synthesis as described in [9].
The following substances were used as precursors:
ammonium
heptamolybdate
tetrahydrate
(NH4)6Mo7O24 · 4H2O (Acros Organics, 99+%), tellu-
ric acid H6TeO6 · 2H2O (Alfa Aesar, 99+%), vanadyl
sulfate VOSO4 · xH2O (Alfa Aesar, 99.9+%), and
tris(oxalato)niobic acid H3[NbO(C2O4)3] · 7.5H2O
(Reakhim, special purity).
The ODE reaction over the oxide catalysts pro-
The crystallinity and phase composition of the
ceeds according to the Mars–Van Krevelen step fresh and spent catalysts were determined via X-ray
mechanism involving the alternating oxidation and diffraction on a DRON-2 unit (2θ range of 5° to 35°).
1132

DEACTIVATION OF A MIXED OXIDE CATALYST
1133
X-ray photoelectron spectra were recorded on a Kra- acetic acid were detected. At 380°C, its content was
tos Axis Ultra DLD instrument using monochromatic hundredths of a percent.
AlKα radiation (1486.6 eV). The pass energy of the
analyzer was 160 eV for the survey spectra and 40 eV for
the high resolution spectra. The spectra were recorded
using a neutralizer. Energy calibration of the spectra
was performed based on the position of Mo3d5/2 line
(233.2 eV).
Electron micrographs of the samples surfaces were
obtained on a JEOL JSM-6390LA scanning electron
microscope. The accelerating voltage (0.5 to 30 kV)
was selected depending on the structure and material
of a sample, as was the working distance (8–25 mm).
Energy dispersive X-ray microanalysis spectra were
recorded using EDS module at an accelerating voltage
of 20 kV.
Our study of the catalytic oxidative dehydrogena-
tion reaction was conducted using a flow unit (a tita-
nium reactor with an inner diameter of 0.55 cm and a
length of 58 cm) at atmospheric pressure in the tem-
perature range of 340–400°C. To eliminate local over-
heating, the catalyst (in powder form) was mixed with
titanium oxide TiO2 at a ratio of 1 : 1 by weight. It was
then compressed and a fraction of 0.25–0.5 mm was
collected. The sample loading was 4 g. The mixture of
C2H6 : O2 = 3 : 1 was prepared in the gas cylinder by
mixing ethane (purity, 99.9%) with technical oxygen
(98%). The feed rate of the reaction mixture was
50 cm3/min (space velocity, 800 h−1).
The reaction products were analyzed by means of
gas chromatography on an LKhM-80 chromatograph
equipped with a thermal conductivity detector, using
two packed columns (3 m × 0.3 mm) with different
adsorbents: Porapak Q for analyzing hydrocarbons
and carbon dioxide, and a CaA molecular sieve for
analyzing oxygen, nitrogen, and carbon monoxide.
It was found that raising the temperature of the
reaction favored the complete oxidation of ethane to
carbon oxides. According to our data, high selectivity
toward ethylene (98%) was achieved on the Mo–V–
Te–Nb–O catalyst at 340°C. The conversion of eth-
ane rose to 30% when the temperature was raised from
340 to 400°C, but the selectivity toward the desired
product (ethylene) was reduced to 85%. The slowing
of conversion upon moving from 380 to 400°C was due
to around 95% of the oxygen present in the mixture
being consumed at 380°C, and complete conversion
took place at 400°C. A further increase in temperature
(above 400°C) in the presence of a C2H6 : O2 = 3 : 1
mixture did not affect the conversion of ethane, which
is limited by the amount of oxygen in the feed gas
stream. An increase in the conversion of ethane would
thus be possible if gas mixtures with large molar con-
tents of oxygen were prepared, but such mixtures are
explosive.
The sample containing no tellurium displayed not
only low selectivity toward ethylene, but also exhibited
lower (by nearly 50%) activity, confirming the key role
of tellurium in the formation of the active and selective
phase in the ODE reaction, the presence of which has
been shown in studies of samples using X-ray diffrac-
tion (XRD).
Mo–V–Nb–Ta–O catalyst stability in ODE was
studied by varying the reaction temperature. After
exposing the sample at 360°C for 3 h, the temperature
was raised to 400°C, kept there for 0.5 h, and returned
to the initial temperature (360°C) (Fig. 1).
The studied catalyst operated stably at 360°C, and
the conversion of ethane was 18%. After raising the
temperature to 400°C, conversion was as high as 40%.
After the reaction temperature was lowered from 400
to 360°C, however, the conversion of ethane fell by
50%, compared to the original value at the given tem-
perature. The selectivity toward ethylene was also
reduced. To find the reasons for the observed reduc-
tion in conversion and selectivity, the catalyst samples
were studied via XRD, X-ray photoelectron spectros-
copy (XPS), and scanning electron microscopy
(SEM).
Analysis of literature data shows that the complex
oxide Mo–V–Te–Nb–O systems contained M1 and
M2 phases [10–12]. The XRD patterns of the catalysts
used in this work are shown in Fig. 2. Three-compo-
nent mixed oxides without tellurium form different
phase compositions. The amount of the orthorhombic
phase of M1 responsible for the selective ODE to eth-
ylene was thus lower, affecting sample activity during
the experiments described above.
RESULTS AND DISCUSSION
Key findings from our study of three- and four-
components oxide catalysts in the ODE reaction are
shown in Table 1. The reaction products were eth-
ylene, carbon oxides, and water. Trace amounts of
Table 1. Oxidative dehydrogenation of ethane over Mo–
V–Te–Nb–O and Mo–V–Nb–O catalysts
Ethane
Т, °С conversion,
wt %
Selectivity,%
Catalyst
C2H4 СО2
СО
Mo–V–Te–Nb–O 340
6
20
34
37
9
98
92
88
85
68
70
2
3
3
3
16
9
0
5
9
12
16
21
360
380
400
To determine the stability of the studied Mo–V–
Te–Nb–O catalysts, the samples were kept under high
vacuum in the spectrometer chamber for 12 h. No
changes in the composition of oxide systems following
Mo–V–Nb–O
360
400
21
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MISHANIN et al.
360°C
400°C
360°C
C2H4
96
94
92
90
40
30
20
10
0
~
~
6
4
2
0
CO
K
CO2
0
100
200
300
400
Time, min
Fig. 1. Dependences of ethane conversion (K) and selectivity toward products in the reaction of OD in an oxygen–ethane mixture
on time over the Mo–V–Te–Nb–O catalyst.
such exposure were noted by XPS data. The initial
oxide systems and the samples pretreated in a stream
of ethane and in a mixture of ethane and oxygen (in a
ratio of 3 : 1) at a temperature of 400°C for 0.5 h were
investigated via XPS. As was noted above, the oxygen
of the gas phase was completely consumed at 400°C
and there was a lack of oxidant in the system. The data
for the Mo–V–Te–Nb–O sample treated with pure
ethane thus represent the limiting case when there is
absolutely no oxidant in the gas phase. The concentra-
tions of elements on the catalyst’s surface were calcu-
lated per 1 atom of molybdenum:
Mo1.00V0.27Nb0.23Te0.12O3.88C0.28 (after С2H6),
Mo1.00V0.29Nb0.24Te0.36O4.48C0.24 (after С2H6/O2).
It follows from the gross formulas that after treatment
with pure ethane, a strong reduction in the tellurium
content was observed on the catalyst’s surface. The
concentrations of other metals vary slightly. There was
a slight increase in the content of tellurium in the sam-
ple treated with a mixture of С2H6/O2, due probably to
redistribution of the element between the surface and
the bulk phase. These changes in the concentration of
tellurium are illustrated by Te3d XPS spectra in Fig. 3.
In addition to a substantial reduction in the signal
intensity of tellurium at the higher Te+6 oxidation
state, Te3d5/2 state with an electron binding energy
equal to 573.8 eV emerged in the spectrum of Te3d
electrons after treating the sample with pure ethane.
This binding energy is slightly higher than that of
metallic tellurium (573.1 eV) and is characteristic of
organotellurium compounds in which tellurium has a
+2 oxidation state [13, 14]. In addition, it could be that
tellurium surface structures have no direct analogy
with the bulk phases.
Mo1.00V0.29Nb0.24Te0.32O4.20C0.51 (initial),
Note that the observed reduction in the concentra-
tion of tellurium in the sample after the reaction could
be due to the presence of reduced metallic tellurium
on its surface that sublimates upon sample evacuation
in the spectrometer chamber under ultrahigh vacuum.
The spectra of the other metals (V, Nb, Mo) are virtu-
ally the same for all the samples.
1
2
Our results from X-ray microanalysis also con-
firmed the reduction of tellurium content in samples
of the mixed oxide catalysts (Table 2).
5
10
15
20
25
30
35
2θ, deg
Our study of mixed oxide systems via XPS and
SEM showed that treating Mo–V–Te–Nb–O sam-
Fig. 2. XRD patterns of samples (1) Mo–V–Te–Nb–O,
(2) Mo–V–Nb–O; (▼) phase M1, (■) phase M2.
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DEACTIVATION OF A MIXED OXIDE CATALYST
Te6+
1135
C2H6/O2
Original
Te6+
Te3d5/2
Te3d3/2
C2H6
Te2+
Te2+
596
592
588
584
580
576
572
Binding energy, eV
Fig. 3. Te3d XPS spectrum of the Mo–V–Te–Nb–O sample before and after treatment with C H and C H /O .
2
6
2
6
2
ples in ethane with no oxygen varied their surface mor- which proceeds in the reaction mixture with the com-
phology considerably. The reduction of reactive tellu- plete consumption of the oxidizer. Catalyst deacti-
rium species in the surface layer of the catalyst occurs vated in this manner exhibits lower conversion and
as a result of its reduction with hydrocarbon with sub- selectivity toward ethylene in the ODE reaction that
sequent sublimation.
are similar to those obtained for a three-component
oxide catalyst containing no tellurium.
CONCLUSIONS
ACKNOWLEGMENTS
Under optimum reaction conditions (temperature,
360°C), the conversion of ethane on a four-compo-
nent mixed oxide catalyst of Mo–V–Te–Nb–O is as
high as 20% with a selectivity toward ethylene of 92%.
Raising the reaction temperature to 400°C increases
the total conversion of ethane with full engagement of
the oxidant, but the selectivity toward ethylene is
reduced to 85%. The catalyst is irreversibly deactivated
under such operating conditions. It was shown that the
reason for the observed catalyst deactivation was the
partial reduction of tellurium to the metallic state,
This work was supported by the Russian Founda-
tion for Basic Research, project no. 13-03-12266-
ofi_m; and by the ENVAIROKET Company. It was
performed using equipment purchased under the Pro-
gram for the Development of Moscow State Univer-
sity.
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