Effect of the conditions of thermal pretreatment on the properties of Mo/ZSM-5 catalyst of the nonoxidative conversion of methane

Article abstract of DOI:10.1134/S0036024416120281

Mo-containing zeolite catalysts for the nonoxidative conversion of methane are produced via solid-phase synthesis, followed by thermal treatment at different temperatures. The state of nanosized Mo powders is investigated by means of X-ray diffraction and

Full text of DOI:10.1134/S0036024416120281

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 12, pp. 2364–2369. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © A.A. Stepanov, L.L. Korobitsyna, Ya.E. Barbashin, A.V. Vosmerikov, 2016, published in Zhurnal Fizicheskoi Khimii, 2016, Vol. 90, No. 12, pp. 1797–1803.
CHEMICAL KINETICS
AND CATALYSIS
Effect of the Conditions of Thermal Pretreatment
on the Properties of Мо/ZSM-5 Catalyst
of the Nonoxidative Conversion of Methane
A. A. Stepanov, L. L. Korobitsyna, Ya. E. Barbashin, and A. V. Vosmerikov*
Institute of Petroleum Chemistry, Siberian Branch, Russian Academy of Sciences, Tomsk, 634055 Russia
*e-mail: pika@ipc.tsc.ru
Received February 2, 2016
Abstract—Mo-containing zeolite catalysts for the nonoxidative conversion of methane are produced via
solid-phase synthesis, followed by thermal treatment at different temperatures. The state of nanosized Mo
powders is investigated by means of X-ray diffraction and thermal analysis. The effect the temperature of
Мо/ZSM-5 catalyst annealing has on its acidic characteristics and activity in the process of methane dehy-
droaromatization is established.
Keywords: methane, zeolite, Мо/ZSM-5 catalyst, nanopowder, methane dehydroaromatization, conversion
DOI: 10.1134/S0036024416120281
INTRODUCTION
EXPERIMENTAL
The nonoxidative conversion of methane into aro-
matic hydrocarbons on zeolite-containing catalysts
modified with transition metals is of great interest as
an effective way of utilizing natural gas, which is gen-
erally more than 90% methane. High-silica ZSM-5
zeolites modified with molybdenum are the most
promising catalysts of nonoxidative methane conver-
sion. Мо-containing zeolite catalysts are usually pro-
duced by impregnating zeolite with solutions of
ammonium paramolybdate (heptamolybdate), i.e.,
(NH4)6Мо7O24 · 4H2O. The catalyst is dried after
impregnation and annealed in air at temperatures of
500–720°С. The duration of annealing ranges from 4
to 30 h [1–3]. In addition to impregnation, solid-
phase synthesis based on the mechanical mixing of a
zeolite with molybdenum salt [4] or oxide [5] is used to
produce Mo-containing zeolite catalysts. It was shown
in [6] that Мо/ZSM-5 catalysts produced by mechan-
ically mixing zeolite with molybdenum nanosized
powder (NSP) exhibits higher activity and operating
stability in the nonoxidative methane conversion into
aromatic hydrocarbons than catalysts produced via
impregnation.
High-silica zeolite with molar ratio SiO2/Al2O3 =
40 was synthesized by hydrothermal crystallization
using hexamethylenediamine (HMDA) as a struc-
ture-forming additive. Nanosized molybdenum
powder was produced by electrically exploding a con-
ductor in argon. Мо/ZSM-5 catalyst was prepared
via the dry mechanical mixing of H-form zeolite and
Mo nanopowder in a KM-1 ball vibromill. The con-
tent of Mo in the zeolite was 4.0 wt %. The prepared
Мо/ZSM-5 catalyst was annealed in air in a muffle
furnace at different temperatures in the range of 250
to 650°С for 1 h.
Thermal analysis was performed on a Q-1500 D
derivatograph (МОМ, Hungary) in the temperature
range of 20–800°C. A sample (60 mg) in an alundum
crucible was heated in air at a rate of 10 K/min. Alu-
minum oxide (α-Аl2О3) was used as the reference
sample. X-ray diffraction analysis was performed on a
DISCOVER D8 diffractometer (Bruker) using mono-
chromatic CuKα-radiation and a LynxEye-detector.
Scanning was done in the range of 2θ = 10°–90° with
a step of 0.02° and an accumulation time for each
point of 3 s.
In this work, we investigate the effect the tem-
perature of annealing of mechanical mixtures of zeo-
The acidic properties of the samples were investi-
lite and Mo NSP has on the acidic properties and gated via the temperature-programmed desorption
catalytic activity and stability of Мо/ZSM-5 catalyst (TPD) of ammonia, allowing the distribution of acidic
during methane dehydroaromatization, and present sites to be determined according to their strength and
the results from investigating the oxidation of molyb- total number [7]. The concentration of acidic centers
denum nanopowder used in preparing Мо/ZSM-5 was determined from the amount of ammonia
catalyst.
desorbed at the moment of desorption peak fixing and
2364

EFFECT OF THE CONDITIONS OF THERMAL PRETREATMENT
2365
150
140
130
120
110
100
100
80
60
40
20
0
1
2
3
20
120
220
320
T, °C
420
520
620
720
Fig. 1. Thermogram of Mo nanopowders: (1) TG, (2) DTG, (3) DTA.
presented as micromoles per 1 g of catalyst. Мо/ZSM-
As can be seen from the data presented in Fig. 1,
5 catalyst samples were tested in a flow unit at 750°С, the process of Mo oxidation is nonlinear as the tem-
perature rises, and four stages can be identified in the
process. The first stage is the slow oxidation of Mo
observed from 155 to 375°С, accompanied by a 1.7%
increase in the sample’s mass and the release of a
moderate amount of heat. Raising the temperature
further sharply increases the rate of Mo oxidation. A
shoulder is observed in the DTG curve at 470°С, along
with peaks having maxima at 514 and 565°С; this is in
good agreement with the exo effects in the DTA curves
at 470, 516, and 567°С. The rate of Mo oxidation slows
gradually as the temperature rises from 565 to 730°С,
and the sublimation of the formed МоО3 starts at tem-
peratures above 730°С and is accompanied by a reduc-
tion in the sample’s mass. At the same time, it is likely
that heating under nonisothermal conditions does not
result in the complete oxidation of molybdenum to
stoichiometric МоО3.
atmospheric pressure, and a volume rate of feed sup-
ply of 1000 h–1. A 1.0 cm3 volume of each sample was
placed into a tubular quartz reactor 12 mm in diame-
ter, heated to the reaction temperature in an argon
flow. Methane (99.99% purity) was then fed into the
reactor. Reaction products were analyzed via GLC on
a Khromatek-Kristall 5000.2 chromatograph every 40
min of the process. The degree of methane conversion
and the yield of the reaction products were determined
to assess the catalytic activity of the samples.
RESULTS AND DISCUSSION
To evaluate the effect the conditions of Мо/ZSM-5
catalyst thermal treatment had on its activity in the
process of methane dehydroaromatization, it was nec-
essary first to investigate the change in the state of
molybdenum NSP itself in its oxidation during
annealing. The behavior of Mo NSP during its oxida-
tion in air in the temperature range of 20–800°C was
investigated by means of thermal analysis (Fig. 1).
A 0.4% reduction of the sample mass was observed
upon heating it from room temperature to 155°C, due
to removal of adsorbed compounds (mainly water).
Raising the temperature further increases the sample
mass, due to the oxidation of molybdenum nanopow-
der. The data from X-ray diffraction analysis (Fig. 2)
The sample mass grew by 17.5% when the tempera-
ture reached 526°С (the inflection point on the DTG
curve). According to the equation (Мо + (1/2)О2 =
МоО), the mass increase must be 16.7%. The similar-
ity of these values indicates that intermediate oxide
MoO forms during the first stage of Mo oxidation, and
is then rapidly oxidized to МоО3. The total increase of
the sample mass as a result of heating to 730°С was
48.4%. From the stoichiometry (2Мо + 3О2
=
2МоО3), it follows that the mass increase should be
of the samples annealed in air indicate that molybde- 50% for the complete transformation of Мо to МоО3.
num oxide МоО3 formed; in addition, no nitrides
were detected in the products.
Deviation from the stoichiometric value is likely
explained by the abovementioned incomplete oxida-
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STEPANOV et al.
4
3
2
1
8
7
6
5
0
20
40
2θ, deg
60
80
100
0
20
40
2θ, deg
60
80
100
Fig. 2. X-ray diffractograms of Mo NSP annealed at different temperatures: (1) 250, (2) 300, (3) 350, (4) 400, (5) 450, (6) 500,
(7) 550, and (8) 650°С.
tion of Mo under the nonisothermal conditions of annealed at 350 and 450°С it reaches 26 and 88%,
thermal analysis, and by the partial loss of Mo oxide respectively. When treated at 550°С, Мо is virtually
due to its sublimation at high temperatures. In addi- 100% oxidized to МоО3.
tion, the observed difference could be associated with
the partial oxidation of the initial molybdenum nano-
powder, meaning that it already had certain amount of
molybdenum oxide in its composition.
It is known that the channel structure and acidity
of zeolite, along with the localization and state of
molybdenum on its surface, substantially affect the
physicochemical and catalytic properties of
Мо/ZSM-5 catalysts. The results from investigating
the acidic properties of Мо/ZSM-5 catalyst samples
produced at different annealing temperatures are pre-
sented in the table. It can be seen that introducing Mo
NSP into zeolite reduces the strength and concentra-
tion of acidic centers of both types, due likely to the
localization of Mo nanopowder on the outer surface of
the zeolite and the partial blocking of its channels by
metal particles. A further drop in the concentration of
strong acidic centers is observed for the sample
annealed at 250°С. Raising the annealing temperature
of the modified zeolite from 300 to 650°C increased
the concentration of weak acidic centers, while the
content of strong acidic centers in the catalyst passed
through its maximum at a treatment temperature of
450°С. The increased concentration of weak acidic
centers was probably due to the formation of МоО3
phase over the outer surface of the zeolite, the amount
of which grew along with the annealing temperature
[9]. At the same, raising the annealing temperature
from 250 to 450°С reduced the strength of the high-
temperature acidic centers, as is apparent from the
shift of the temperature maxima in the TD spectrum
of the investigated samples toward lower temperatures.
Annealing the Мо/ZSM-5 catalyst at temperatures
above 450°C reduced the concentration and increased
Thermal analysis of 4% Мо/ZSM-5 catalyst
showed that the stepwise oxidation of Mo occurred in
a manner similar to that of Mo NSP. However, the
peaks were observed to shift toward lower tempera-
tures during the process, and oxidation ceased at 630
instead of 730°С, as was the case with the Mo nano-
powder. Hence, the stepwise oxidation of Mo occurs
in the presence of zeolite, but at lower temperatures,
due apparently to the participation of water adsorbed
by zeolite during Mo oxidation (since, as is well
known, nanopowders of various metals can interact
with water [8]).
X-ray diffractograms of the Mo NSP annealed at
different temperatures are presented in Fig. 2. The
X-ray diffractogram of the sample annealed at 250°С
(1) exhibits the pattern typical of Mo, indicating that
no Mo oxidation occurred at this temperature. Raising
the temperature of Mo treatment to 300°С (2) results
in the emergence of peaks at 22.514°, 25.784°, 27.435°,
and 33.383° that correspond to MoO3 (PDF 00-047-
1320). Raising the temperature of Mo treatment fur-
ther reduces the number of major peaks at 40.750°,
58.774°, and 73.780° that correspond to Mo (PDF 01-
076-9187), and increases the number of peaks attrib-
utable to MoO3. Quantitative calculations of the
change in the Mo fraction as the annealing tempera- the strength of high-temperature acidic centers, due to
ture changes show that the fraction of МоО3 in the the considerable increase in the МоО3 content on the
sample annealed at 250°С is 2%, while in the sample zeolite surface (Mo NSP is completely oxidized to
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EFFECT OF THE CONDITIONS OF THERMAL PRETREATMENT
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Acidic properties of the initial zeolite and the Mo/ZSM-5 catalyst annealed at different temperatures (annealing tempera-
tures are given in parentheses)
Тmax, °С
Concentration, μmol/g
Sample
ТI
ТII
СI
СII
СΣ
Initial ZSM-5
185
180
170
170
175
180
180
190
195
205
440
420
420
415
410
405
400
415
425
425
559
525
552
608
624
653
658
728
740
744
303
285
274
317
331
343
394
296
256
145
862
810
Mo/ZSM-5 (no annealing)
Mo/ZSM-5 (250°С)
Mo/ZSM-5 (300°С)
Mo/ZSM-5 (350°С)
Mo/ZSM-5 (400°С)
Mo/ZSM-5 (450°С)
Mo/ZSM-5 (500°С)
Mo/ZSM-5 (550°С)
Mo/ZSM-5 (650°С)
826
925
955
996
1052
1024
996
889
Т , T are the temperatures of the maxima of low- and high-temperature peaks, respectively; С is the concentration of weak (C ) and
I
II
i
I
strong (С ) acidic centers and their total concentration (С ), respectively.
II
Σ
МоО3 at 550°С) and its growing diffusion into the molybdenum oxide concentration in the zeolite and
the higher annealing temperature result in the emer-
gence of out-of-lattice Al and the formation of non-
active Al2(MoO4)3 crystal phase.
zeolite channels where it interacts with Brønsted
acidic centers.
Different types of interaction between different Mo
forms and zeolite are thus possible as a result of
The results from investigating the effect the tem-
Мо/ZSM-5 catalyst annealing at different tempera- perature of Мо/ZSM catalyst annealing has on its
tures during catalyst preparation. Molybdenum can activity and stability in the nonoxidative conversion of
interact with either the acidic centers of the zeolite or methane are presented in Fig. 3. The sample not sub-
the lattice aluminum. In addition, the increased jected to thermal treatment exhibits very low activity
Conversion, %
20
1
2
16
12
8
3
4
5
6
7
8
9
4
0
20
100
180
260
340
τ, min
Fig. 3. Change in the degree of methane conversion over time τ of the operation of Mo/ZSM-5 catalyst annealed at different tem-
peratures: (1) no annealing; (2) 250, (3) 300, (4) 350, (5) 400, (6) 450, (7) 500, (8) 550, and (9) 650°С.
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STEPANOV et al.
Ethane yield, %
0.16
(а)
1
2
3
4
5
6
7
8
0.12
0.08
0.04
0
20
100
180
260
340
τ, min
Ethylene yield, %
(b)
1.2
0.8
0.4
0
20
100
180
260
340
τ, min
Benzene yield, %
(c)
8
6
4
2
0
20
100
180
260
340
τ, min
Naphthalene yield, %
8
(d)
6
4
2
0
20
100
180
260
340
τ, min
Fig. 4. Change in the yield of (a) ethane, (b) ethylene, (c) benzene, and (d) naphthalene during methane conversion over time τ
of the operation for Mo/ZSM-5 catalysts annealed at different temperatures; (1–8), see Fig. 2.
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EFFECT OF THE CONDITIONS OF THERMAL PRETREATMENT
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in this process; the degree of methane conversion on it est amount of naphthalene is in the reaction products
formed in the first 100 min of the reaction on the sam-
ples annealed at 300–350°С (Fig. 4d). The ben-
zene/naphthalene ratio in the reaction products
formed on all of the investigated samples changes over
a wide range during the reaction, but it is close to 2 in
most cases.
is no greater than 2%. Methane conversion is 7.8%
after the first 20 min of the reaction in the presence of
the catalyst sample annealed at 250°С, and it falls
gradually during the process. The maximum conver-
sion of methane (17.2 and 15.9%) is reached after the
first 20 and 60 min, respectively, of the process con-
ducted with the sample annealed at 350°С. After that,
a sharp drop in the degree of conversion is observed.
The samples annealed at higher temperatures are less
active at the beginning of the process, but over time
their activity reaches levels above those for the samples
annealed at temperatures below 400°С. The growth in
the activity of the Мо/ZSM-5 catalyst as the anneal-
ing temperature rises during its production is due to
the increase in the fraction of molybdenum oxide and
its migration into the zeolite channels. The highest
activity and operating stability after 60 min of the reac-
tion was displayed by the sample annealed at 650°С.
Raising the temperature of Мо/ZSM-5 catalyst treat-
ment further reduced its activity over the investigated
time interval. It was shown in [10, 11] that the zeolite
crystal lattice disintegrates completely when the
annealing temperature of the Mo-containing catalysts
is above 800°С, due to the formation of aluminum
molybdate Al2(MoO4)3.
CONCLUSIONS
The conditions of the thermal treatment of Mo-
containing zeolite affect the content of МоО3 in the
catalyst, and consequently its activity in nonoxidative
conversion of methane. In order to produce Mo-con-
taining zeolite catalyst that is active in the process of
methane dehydroaromatization, it is sufficient to
anneal it at 350°С. This indicates that the amount of
active phase of МоО3 formed in annealing the cata-
lysts is sufficient for methane activation and the subse-
quent conversion of intermediate products on the zeo-
lite acidic centers. Raising the annealing temperature
of Мо/ZSM-5 catalyst to 650°С slightly reduces its
activity and operating stability. This is likely due to the
increased amount of Mo penetrating into the zeolite
channels and interacting with its strong Brønsted
acidic centers, which in turn lowers the rate of
Мо/ZSM-5 catalyst carbonization.
Our results from investigating the effect the tem-
perature of Мо/ZSM catalyst annealing has on the
composition and yield of the products of methane
conversion are presented in Fig. 4. Ethane and eth-
ylene are the main components of the gaseous prod-
ucts; their qualitative composition does not depend on
the conditions of catalyst production (Figs. 4a, 4b).
The least amount of С2-hydrocarbons forms on cata-
lysts annealed at 250–350°С. The yield of gaseous
hydrocarbons increases in reactions with the catalysts
annealed at higher temperatures. The patterns of the
changes in the С2-hydrocarbon yield during the reac-
tion are the same for all investigated samples. The eth-
ane concentration increases with the time of reaction,
reaching its maximum level for the samples treated at
400–450°С (Fig. 4a). The greatest amount of ethylene
forms on the samples annealed at 450–500°С
(Fig. 4b). The amount of ethylene of the reaction
products is higher than that of ethane over the dura-
tion of the process, and its maximum value does not
exceed 1.25%.
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Translated by L. Brovko
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