Volume 02 Issue 10-2022
47
International Journal of Advance Scientific Research
(ISSN
–
2750-1396)
VOLUME
02
I
SSUE
10
Pages:
47-55
SJIF
I
MPACT
FACTOR
(2021:
5.478
)
(2022:
5.636
)
METADATA
IF
–
7.356
A
BSTRACT
In this article, the textural properties of catalysts and the process of catalytic dimerization of methane using
catalysts based on salts of alkali and alkaline-earth metals containing molybdenum and manganese oxides
were studied. It was found that the catalytic conversion of methane takes place on the surface of a catalyst
that adsorbs oxygen to form an active complex [ZOCH3]. It has been shown that the catalytic conversion of
methane can be carried out simultaneously with the formation of ethane and ethylene. At the same time,
the percentage of ethylene in the free volume of the reactor increases compared to ethane, which helps the
reaction of ethylene formation without oxygen and catalyst. Phenomenological equations for the kinetics
of the catalytic reaction of methane dimerization are formulated and rate constants and Arrhenius
parameters are calculated.
Journal
Website:
http://sciencebring.co
m/index.php/ijasr
Copyright:
Original
content from this work
may be used under the
terms of the creative
commons
attributes
4.0 licence.
Research Article
OXIDATIVE DIMERIZATION OF METHANE: KINETICS OF THE
REACTION
Submission Date:
October 01, 2022,
Accepted Date:
October 05, 2022,
Published Date:
October 14, 2022
Crossref doi:
https://doi.org/10.37547/ijasr-02-10-08
Safarova Mavjuda Jomurodovna
Samarkand State University, 15 University Boulevard, Samarkand, 140104, Uzbekistan
Normurot Ibodullayevich Fayzullaev
Samarkand State University, 15 University Boulevard, Samarkand, 140104, Uzbekistan
Volume 02 Issue 10-2022
48
International Journal of Advance Scientific Research
(ISSN
–
2750-1396)
VOLUME
02
I
SSUE
10
Pages:
47-55
SJIF
I
MPACT
FACTOR
(2021:
5.478
)
(2022:
5.636
)
METADATA
IF
–
7.356
K
EYWORDS
Methane, ethylene, catalyst, differential reactor, X-ray, micrograph, velocity constants, Arrhenius
parameters.
I
NTRODUCTION
One of the promising methods for producing
ethylene is the reaction of oxidative dimerization
of methane.
Currently, in the vast majority of cases, ethylene
continues to be obtained by thermal
decomposition of gasoline. However, the dynamic
growth in the demand for gasoline and other
motor fuels encourages the search for another
innovative way to produce organic synthesis
products, in particular, ethylene. Catalytic
dimerization of methane, which leads to the
production of ethylene, can be considered as such
a method. However, this method has not yet left
the scope of the laboratory experiment, which is
largely due to the lack of special equipment and
an adequate theoretical description of the process
itself, which allows optimizing the technological
regime as a whole.
A necessary condition for solving the problem is
to study the kinetic regularities of the methane
dimerization reaction and the thermodynamics of
this process.
A number of papers [1-4] presented the results of
research related to this topic, as well as proposed
various catalysts for the conversion of methane to
ethane and ethylene. The specificity of the
methane oxidative condensation reaction is that
with all known catalysts, the reaction occurs at
high temperatures, and the process is highly
exothermic. At the same time, the gas mixture of
methane and oxygen in a wide range of their
concentrations is particularly explosive.
Kinetic processes of dimerization of methane in
order to obtain the target product
–
ethylene, even
using such effective catalysts as Bi
2
O
3
; 9%
K
2
CO
3
/Al
2
O
3
; 34% PbO/Al
2
O
3
; 4% Na
2
MoO
4
·10%
Mn-O/SiO
2
, have not been sufficiently studied and
require additional research [5-6 ].
Previously, we found [7-15] that the catalysts of
the methane dimerization reaction containing
Mn, Mo, Zr and oxides of rare earth elements have
a high catalytic activity.
In this regard, the purpose of this work is to
further study the influence of physical-chemical
and textural characteristics on the activity of the
catalyst
of
the
composition
(Mo
2
O
3
)
x
·
(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
in the reaction of
oxidative dimerization of methane and to
establish appropriate kinetic models.
EXPERIMENTAL PART
The catalytic activity was studied in the
differential flow of a quartz reactor (P=0.1 MPa,
V
cat
=0.5 ml÷2ml, CH
4
: O
2
=2 ÷ 4 contact time 0.1-
0.09 seconds) in the temperature range from
750º to 850ºC.
The phase composition of the samples was
determined by X-ray phase analysis (XRF) on a
Bruker D8 diffractometer (Germany) using CuK
α
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International Journal of Advance Scientific Research
(ISSN
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VOLUME
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SJIF
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)
(2022:
5.636
)
METADATA
IF
–
7.356
radiation (λ = 1.5418 Ǻ) when scanning at points
with an interval of 0.05° in the range 2θ from 20
to 80°. The stability of the phase composition at
high temperatures (up to 900°C) was studied
using high-temperature RF in the HTK-16 X-ray
chamber (Anton Paar, Austria). Experiments
were carried out in the air. The textural
characteristics of the samples were studied using
N
2
adsorption isotherms at a temperature of -
196°C at the ASAP-2400 facility (Micromeritics,
USA).
The morphology of the catalysts was studied
using vector electron microscopy (ZEM) by Vegall
LMU (Czech Republic). The porous structure was
corrected based on the analysis of the obtained
adsorption
curves
by
the
nitrogen
thermosorption method. The specific surface area
S
spec
of the catalyst was determined by the BET
method, and the volume of micro-and meso-
porosity was determined by the BJH method[16].
Analysis of the gas mixture at the inlet and outlet
of the reactor was performed using the gas
chromatographic method [17-19].
RESULTS AND DISCUSSION
Considering the catalytic processes, it should be
noted that a non-additive synergistic effect can be
observed when other elements are combined as a
promoter in a single system. To study the nature
of this effect, the physicochemical, textural, and
structural characteristics of the catalysts were
studied.
According to the results of powder XRD, when
active components are added to the catalyst, solid
solutions containing defective oxygen voids are
formed, which causes an increase in the catalytic
activity of the system [20-23].
Another reason for the increase in the catalytic
activity of the system is an increase in the
proportion of amorphous mesoporicity.
Mesoporous amorphous catalytic systems
containing Mn and Mo were obtained by the
method of sol-gel - synthesis of the catalyst
(Mo
2
O
3
)
x
·
(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
.
Figure 1 shows an XRD spectrum of mesoporous
amorphous catalyst of the composition
(Mo
2
O
3
)
x
·
(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
.
Figure 1. XRD spectrum amorphous catalyst composition (Mo
2
O
3
)
x
·(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
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VOLUME
02
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SJIF
I
MPACT
FACTOR
(2021:
5.478
)
(2022:
5.636
)
METADATA
IF
–
7.356
The diffractogram of a mesoporous amorphous catalyst with the composition (Mo
2
O
3
)
x
·
(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
agrees with the theoretical radiographs calculated from the results of XRD analysis.
Figure 2. Experimental diffractogram of the burned (black, bottom) and calculated from the
structure data (top) of the catalyst composition (Mo
2
O
3
)
x
·(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
Figure 3. SEM micrograph of the catalyst (Mo
2
O
3
)
x
·(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
Volume 02 Issue 10-2022
51
International Journal of Advance Scientific Research
(ISSN
–
2750-1396)
VOLUME
02
I
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10
Pages:
47-55
SJIF
I
MPACT
FACTOR
(2021:
5.478
)
(2022:
5.636
)
METADATA
IF
–
7.356
To establish the kinetic regularities of the
catalytic conversion of CH
4
to C
2
-hydrocarbons,
the content of ethylene and ethane in the gas
mixture at the reactor outlet was quantitatively
determined using catalysts of different
compositions. Based on the obtained data, the
catalytic efficiency of the catalyst and the
conversion of methane were evaluated.
It is proved that the greatest catalytic activity,
manifested in the efficiency of methane
conversion, was shown by the proposed catalyst
of the composition (Mo
2
O
3
)
x
·
(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
.
When CO
2
is introduced into the CH
4
:O
2
: CO
2
system at molar ratios of the mixture components
from 3:1: 0 to 3:1:2 moles, respectively, the
conversion of methane increases from 52.6% to
63.1%, the selectivity increases with respect to
ethylene from 62.4% to 66.2%, with respect to
ethane, the selectivity increases from 19% to
20.7%; with respect to hydrocarbon C
2
, the
selectivity increases from 81.4% to 86.9%.
Table 1. Effect of carbon dioxide on the reaction of catalytic dimerization of methane and the yield
of the target product
CH
4
:O
2
:CO
2
Conversion (%)
Selectivity (%)
CH
4
CO
2
H
2
CO
2
CO
C
2
H
4
C
2
H
6
3:1:0
52,6
-
6,2
13,4
16,8
62,4
19,0
3:1:1
57,2
-33,2
1,9
7,6
20,6
64,0
19,8
3:1:2
63,1
-14,0
2,6
4,1
21,0
66,2
20,7
3:1:2.5
57,1
-9,6
2,7
10,0
26,5
55,9
17,6
3:1:2.8
44,7
-1,6
3,3
3,1
70,4
45,0
11,5
3:1:3
42,7
2,0
3,1
-
92,6
32,2
5,2
As can be seen from table 1, the conversion of methane in the absence of CO
2
is 52.6%, and the selectivity
for ethylene is 62.4%.
A further increase in the amount of CO
2
leads to a decrease in the above values, and the selectivity increases
slightly in relation to carbon monoxide.
Under constant conditions of the experiment, but when replacing carbon dioxide with argon along with the
growth of the latter, there is a decrease in the yield of C
2
-hydrocarbons decreases.
Table 2. Effect of argon on the yield of the target product
CH
4
:O
2
:Ar
CH
4
Selectivity (%)
CO
2
H
2
CO
C
2
H
4
C
2
H
6
3:1:0
52,6
13,4
6,2
16,8
52,4
19,0
3:1:1
56,5
16,1
5,6
17,2
50,5
17,5
Volume 02 Issue 10-2022
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International Journal of Advance Scientific Research
(ISSN
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VOLUME
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Pages:
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SJIF
I
MPACT
FACTOR
(2021:
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)
(2022:
5.636
)
METADATA
IF
–
7.356
3:1:2
55,5
17,1
7,1
19,5
48,1
17,3
3:1:3
29,7
18,7
9,4
22,3
45,8
15,5
3:1:4
12,5
28,2
18,6
26,4
33,9
7,7
The dependence of oxygen conversion on contact time and temperature in the methane dimerization
reaction with (Mo
2
O
3
)
x
·
(Na
2
MoO
4
)
y
·
(ZrO
2
)
z
catalysis was investigated. The effect of temperature on oxygen
conversion at a volume ratio of CH
4
: air = 2:1 was studied at temperatures of 700-800
o
C and a pressure of
P = 0.1 MPa
Figure 4. The relationship between the reactant contact time and the oxygen conversion
temperature at the CH
4
:air ratio=2:1 and a pressure of 0.1 MPa.
As can be seen from the figure, deep oxygen
conversion is achieved at 750 ºC in 0.6 seconds,
and at 800ºC in 0.3 seconds.
When studying the effect of pressure on the ratio
of reagents, the selectivity of the ethane-ethylene
mixture decreases with increasing pressure.
When the volume ratios of reagents increase, the
selectivity of the process increases slightly. Thus,
in the case of a 2:1 ratio of reagents, the minimum
selectivity is 56.4%, and in the case of a 6:1 ratio,
the selectivity reaches 76.2%.
To determine the activation energy of the process
in different temperature ranges, the least square
method was used. Methane conversion and
selectivity of C
2
hydrocarbons were calculated
using the obtained kinetic parameters.
Table 3. Arrhenius parameters calculated on the
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VOLUME
02
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SJIF
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)
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)
METADATA
IF
–
7.356
basis of kinetic regularities of the methane
dimerization reaction and experimental data
№
Reaction of the equation
Kinetic equation
E
a
, kJ/mol
1
2CH
4
+1/2O
2
→ C
2
H
6
+H
2
O
W
1
=k
1
∙
𝑷
𝑪𝑯
𝟒
𝟐
∙
𝑷
𝑶
𝟐
49,86
2
C
2
H
6
+1/2O
2
→C
2
H
4
+H
2
O
W
2
=k
2
∙
𝑷
𝑪
𝟐
𝑯
𝟔
∙ 𝑷
𝑶
𝟐
41,42
3
2CH
4
+O
2
→C
2
H
4
+2H
2
O
W
3
=k
3
∙
𝑷
𝑪𝑯
𝟒
∙ 𝑷
𝑶
𝟐
51,35
4
C
2
H
6
→C
2
H
4
+H
2
W
4
=k
4
∙
𝑷
𝑪
𝟐
𝑯
𝟔
41,46
5
CH
4
+2O
2
→CO
2
+ 2H
2
O
W
5
=k
5
∙
𝑷
𝑪𝑯
𝟒
∙ 𝑷
𝑶
𝟐
44,85
6
C
2
H
4
+2O
2
→2CO+2H
2
O
W
6
=k
6∙
𝑷
𝑪
𝟐
𝑯
𝟒
∙
𝑷
𝑶
𝟐
71,45
7
CH
4
+O
2
→CO +H
2
O+H
2
W
7
=k
7
∙
𝑷
𝑪𝑯
𝟒
∙ 𝑷
𝑶
𝟐
92,11
8
C
2
H
4
+2H
2
O →2CO +4H
2
W
8
=k
8
∙ 𝑷
𝑪
𝟐
𝑯
𝟒
∙ 𝑷
𝑯
𝟐
𝑶
74,6
9
CO
2
+H
2
→ CO+4H
2
O
W
9
=k
9
∙ 𝑷
𝑪𝑶
𝟐
∙ 𝑷
𝑯
𝟐
26,47
10
C
2
H
4
+3O
2
→2CO
2
+2H
2
O
W
10
=k
10
∙ 𝑷
𝑪
𝟐
𝑯
𝟒
∙ 𝑷
𝑶
𝟐
57,59
11
2CO + O
2
→2CO
2
W
11
=k
11
∙ 𝑷
𝑪𝑶
∙ 𝑷
𝑶
𝟐
27,23
12
C
2
H
6
+2H
2
O → 2CO + 5H
2
W
12
=k
12
∙ 𝑷
𝑪
𝟐
𝑯
𝟔
∙ 𝑷
𝑯
𝟐
𝑶
74,6
As can be seen from table 3, the formation of
ethane from methane in relation to methane is a
2nd-order reaction (see reaction 1).
C
ONCLUSIONS
1.
The textural characteristics of catalysts
and the process of catalytic dimerization of
methane using catalysts based on salts of alkaline
and
alkaline-earth
metals
containing
molybdenum and manganese oxides in their
composition are studied.
2.
It is shown that the catalytic conversion of
methane can be accompanied by both the
formation of ethane and ethylene. At the same
time, the proportion of ethylene in the free
volume of the reactor increases in comparison
with ethane, which allows the ethylene formation
reaction to proceed without oxygen and a
catalyst.
3.
Optimization of the catalytic process of
dimerization of methane was performed, showing
that the process should proceed at a temperature
of 750-800ºC at a controlled rate of supply of the
reaction mixture of gases to the reaction zone of
the catalyst of 0.3 mol/sec CH
4
and 0.12 mol/sec
O
2
.
4.
Phenomenological equations for the
kinetics of the catalytic reaction of methane
dimerization are built and the rate constants and
Arrhenius parameters are calculated.
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02
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Pages:
47-55
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5.478
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