Low temperature methane coupling in a Pd-based membrane reactor with UV activation

Article abstract of DOI:10.1016/j.mencom.2013.03.003

The nonoxidative coupling of methane to hydrocarbons C₂ and above at 475-555 K by UV activation with hydrogen removing from the reaction zone through a palladium membrane increased CH₄ conversion.

Full text of DOI:10.1016/j.mencom.2013.03.003

Mendeleev
Communications
Mendeleev Commun., 2013, 23, 69–70
Low temperature methane coupling in a Pd-based
membrane reactor with UV activation
Nikolay L. Basov,*a Margarita M. Ermilova,a Natalia V. Orekhova,a
Ivan A. Oreshkina and Andrei B. Yaroslavtseva,b
a A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 495 955 4387; e-mail: basov@ips.ac.ru
b N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russian Federation
DOI: 10.1016/j.mencom.2013.03.003
The nonoxidative coupling of methane to hydrocarbons C2 and above at 475–555 K by UV activation with hydrogen removing from
the reaction zone through a palladium membrane increased CH4 conversion.
The production of hydrocarbons C2 and above from methane
(methane coupling) is of interest because it allows one to consider
natural gas as the alternative source of these compounds for fuel
production and chemical synthesis. Because of the very high
stability of methane,1 all the processes with its participation
except the oxidation require power expenses.
Hydrogen formed during the process partially dissolved in the
wall of the membrane tube, and, as a rule, in an hour after the
experience beginning, in the absence of a stream through a tube,
a dynamic balance between sorbed and desorbed hydrogen in reac-
tion zone was established.After 1 h from the beginning of experi-
ment, the argon stream started to pass through a tube, which
caused the removal of hydrogen from the reactor in most cases.
The products were analyzed by gas chromatography. Sampling
for the analysis was provided every hour. The quantitative analysis
of hydrogen concentration in a reaction zone and on the other
side of a membrane was not performed because of a small volume
of the reactor and, accordingly, a small volume of the formed
hydrogen, whose concentration after dilution by argon stream
on the other side of a membrane was already insufficient for
authentic identification though the hydrogen presence was fixed
at the bottom level of sensitivity of the detector.
As expected, the temperature growth leads to an increase
in methane conversion (Table 1). The maximum conversion of
14.1% was received at 538 K with the argon flow inside the
membrane tube with a length of 1.7 m. The main products of
methane transformation were ethane, propane and ethylene. The
chromatography–mass spectrometry analysis showed the presence
of a very small amount of butane, butene, propane, isopropylenes
and trace amount of benzene. The changes of the composition of
the reaction mixture in the reactor with a membrane tube 3.8 m
in length are shown in Table 2. The conversion increases with the
membrane tube length. It allowed reaching the considerable con-
version at smaller temperatures due to a rise of a portion of
removing hydrogen from a reaction zone.
The basic temporary method to produce hydrocarbons C2 and
above from methane is the thermal activation of CH4 molecules
that breaks a C–H bond. The most obvious way of methane pro-
cessing allowing such transformations nowadays is the conver-
sion of methane to synthesis gas with the subsequent trans-
formation into desirable products. However, different steps of
the process require different conditions, that makes impossible
to perform them in one reactor with the use of heat liberating in
the reaction. Moreover, the process occurs at 1000–1150 K and
higher. For these reasons, the problem of the alternative ways for
hydrocarbons production based on methane remains actual.
The example of one-stage process is the oxidative dehydro-
genation that may be the result of interactions between methane
and oxygen to produce an ethane–ethylene mixture. This process
requires a temperature of 950–1150 K and oxide catalysts. Another
instance of this process is based on the control of oxygen supplying
through an oxygen penetrable membrane to a reaction zone to
exclude the deep oxidation of methane.2,3 Such membranes work
with high productivity at 850 K and above.4
The energy required for the removal of the first hydrogen atom
from the methane molecule is 435.4 kJ mol–1.1 It corresponds to
energy saturation of UV radiation. The use of UV activation allows
conducting the process at a low temperature, but the methane
conversion remains low because of the reversibility of reactions.
The aim of this work was to study methane coupling in a
reactor with a palladium alloy membrane and the activation of
CH4 molecules by UV irradiation.
A mercury lamp and an optical quartz glass reactor were placed
on the axises of the elliptical cylinder that was used as the reflector
for UV beams. The experiments were performed in the semi-
continues regime under atmospheric pressure of pure methane
(99.99%) in a temperature range of 480–550 K. Twisted in a
screw spiral from Pd alloy with 6 wt% Ru tube with varied length
and a wall thickness of 50 mm was placed inside the reactor. The
dense alloy structure membranes showed high mechanical strength
with permeability for hydrogen only, which was 9% higher than
for pure palladium one.5 The hydrogen formed in the reaction
could penetrate through the wall of this tube to its inner volume.
The growth of an argon flow rate through the membrane tube
led to increase in the portion of removing hydrogen from the
Table 1 Effect of temperature on methane conversion in a reactor with a
1 m membrane tube.
Conversion of methane (%)
Time from the
beginning/h
210°C
220°C
240°C
250°C
1
2
3
4
5
6
1.41
4.19
5.61
6.99
8.28
9.61
1.83
5.62
2.74
4.14
6.35
7.85
9.25
11.12
3.57
5.88
6.90
8.34
8.97
9.77
10.69
10.20
10.88
11.57
© 2013 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2013, 23, 69–70
Table 2 Dependence of the reaction mixture composition on the reaction
according to literature data, its probability in the test temperature
range is small. The close position of starting points of curves in
Figure 2 corresponding to the reactor with a membrane and
without it specifies that palladium alloy presence does almost
not influence the conversion of methane within the first hour of
UV activation.
Rather low temperature not only makes improbable display of
catalytic properties of palladium alloy, but also prevents the deep
conversion of methane leading to the formation of free carbon,
whose traces were not revealed in experiments, neither on a
membrane, nor in the reactor. Thus, the UV activation of methane
coupling under these conditions allows one to avoid the unpro-
ductive expense of methane for the formation of free carbon
or carbon oxides that takes place in processes of an oxidative
coupling of methane.3,4
There are no reasons to assert that observed transformations
are the result of photocatalytic processes on a palladium surface
that is possible with the use of hard UV irradiation on oxide and
metallic catalytic systems.8,9 All observed transformations are
resulted from methane photodissociation under UV radiation.
The adding of an ordinary glass screen between the reactor and
mercury lamp afforded zero conversion of methane.
duration.
Composition (vol%)
Time from the
beginning/h
CH4
C2H4
C2H6
C3H8
1
2
3
4
5
99.63
99.55
90.39
88.75
86.71
0.11
0.11
0.23
0.27
0.29
0.26
0.34
5.13
6.44
8.03
0
0
4.25
4.54
4.96
reaction zone as well. The influence of an argon flow rate on
methane conversion at 523 K is shown in Figure 1. The curve has
a maximum in the area with a flow rate of about 40 cm3 min–1.
It is unexpected fact that the replacement of argon by air at a
flow rate of 37 cm3 min–1 resulted in lower degree of conversion
though hydrogen oxidation, as a rule, leads to growth of the
degree of hydrogen extraction from a zone of reaction and to
increase in methane conversion. This phenomenon can probably
be explained by the interaction between the reaction products and
the membrane surface. The adsorbed molecules occupy a part of
the surface and decrease the surface area necessary for hydrogen
adsorption with further transport through membrane. The similar
phenomena were observed earlier6,7 when full hydrogen removing
led to maximum occurrence on the curve of the dehydrogenated
substances conversion vs. the degree of hydrogen removing.
However, it does not mean that the effect of hydrogen removing
does not promote an increase in the conversion of methane. The
results of experiments without the Pd tube are shown in Figure 2.
The methane conversion after 5 h was at least four times lower
than that with a membrane. Moreover, even the presence of a
membrane tube without argon flow has provided the conversion
growth that may be explained by a partial hydrogen removal.
It is reasonable to assume that the catalytic dehydrogenation
of methane on a palladium–ruthenium alloy is possible. However,
The following formation of products via methyl radical can
be proposed:
·
·
CH4 ® Me + H
·
2Me ® C2H6
·
·
Me + CH4 ® C2H6 + H
·
·
Me + C2H6 ® C3H8 + H
·
2H ® H2.
Meantime, the presence of ethylene allows us also to assume
the formation of carbene:
:
CH4 ® CH2 + H2
:
2 CH2 ® C2H4
:
CH2 + C2H6 ® C3H8.
Thus, we demonstrated the occurrence of nonoxidative methane
coupling due to UV activation at low temperatures. It allows
carrying out the process with smaller power inputs and without
the unproductive expense of raw materials. Process performing in
the membrane reactor made it possible to increase methane con-
version at the expense of formed in reaction hydrogen removing
through a palladium membrane. The maximum methane conver-
sion was 14.1% at 538 K.
11
10
9
8
7
6
5
4
3
2
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2
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Figure 2 Methane conversion (1) without and (2) with a Pd-membrane tube.
Received: 9th November 2012; Com. 12/4012
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