Journal of Natural Gas Chemistry 12(2003)81–89
Recent Progress in Direct Partial Oxidation of Methane
to Methanol
Qijian Zhang1,2 ,
Dehua He1∗ ,
Qiming Zhu1∗
1 State Key Laboratory of C1 Chemistry and Technology, Department of Chemistry, Tsinghua University, Beijing 100084, China;
2 Liaoning Institute of Technology, Jinzhou 121001, China
[Manuscript received April 03, 2003; revised May 22, 2003]
Abstract: The direct conversion of methane to methanol has attracted a great deal of attention for nearly
a century since it was first found possible in 1902, and it is still a challenging task. This review article
describes recent advancements in the direct partial oxidation of methane to methanol. The history of direct
oxidation of methane and the difficulties encountered in the partial oxidation of methane to methanol are
briefly summarized. Recently reported developments in gas-phase homogeneous oxidation, heterogeneous
catalytic oxidation and liquid phase homogeneous catalytic oxidation of methane are reviewed.
Key words: methane, methanol, catalytic partial oxidation, gas-phase homogeneous oxidation, catalyst
1. Introduction
Direct conversion of methane to methanol has
been attracting significant attention since it was found
possible in the early 20th century because of its
great industrial potential for the efficient utilization
of abundant natural gas reserves. Natural gas is one
of the clean and effective energy resources. However,
it is uneconomical to bring natural gas to market in
gas form because its density is too low for transportation and storage, unless there are pipelines accessible.
In order to economize transportation, the gas can be
converted into a liquid and transported as such. Liquefaction of natural gas may be a choice, but the boiling point of methane (the predominant component in
nature gas) is as low as −164 , and it requires expensive liquid nitrogen refrigeration throughout the
transportation. Therefore, the most attractive alternative is to convert the natural gas into liquid products such as methanol, which is a liquid under ambient
temperature and pressure.
∗
1.1. Conversion of natural gas
Natural gas can be converted to some chemicals
(e.g. methanol, formaldehyde, etc.) directly or via
indirect routes utilizing syngas (CO+H2 ) as an intermediate. The reactions of methane to methanol, etc.
by indirect routes (via syngas) or direct routes are as
follows:
Indirect routes:
Corresponding author. Tel: (010)62772592; Fax: (010)62792122;
E-mail: [email protected], [email protected]
CH4 + H2 O → CO + 3H2
(1)
0
∆H298
= 206 kJ/mol
CH4 + CO2 → 2CO + 2H2
(2)
0
∆H298
= 247 kJ/mol
CH4 + 1/2O2 → CO + 2H2
(3)
0
∆H298
= −35 kJ/mol
CO + 2H2 → CH3 OH
0
∆H298
= −90.7 kJ/mol
(4)
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Qijian Zhang et al./ Journal of Natural Gas Chemistry Vol. 12 No. 2 2003
Direct routes
CH4 + 1/2O2 → CH3 OH
(5)
0
∆H298
= −128 kJ/mol
CH4 + 1/2O2 → 1/2C2 H4 + H2 O
(6)
0
∆H298
= −140 kJ/mol
CH4 + 1/4O2 → 1/2C2 H6 + 1/2H2 O
increasingly popular, especially in the last 20 years
since the “energy crisis” in the 1970s. A technical
economic assessment showed that the direct process
for methanol production could compete with the conventional indirect one in terms of production costs if
an 80% selectivity of methanol could be achieved at
a single pass methane conversion of 10%.
(7)
2. Challenge of direct conversion of methane
to methanol
Conventionally, the commercialized natural gas
conversion process is an indirect process. Natural
gas is first converted to syngas by steam reforming
(1-1), and then the syngas is catalytically converted
to methanol in industry. The steam reforming of
methane is an energy intensive process, which requires
high temperature and pressure that leads to problems
in with reactor materials, operation and maintenance.
In the process of methanol production from natural
gas via syngas, about 60%-70% of the cost of the
overall process is associated with the reforming process [1]. In order to reduce the reforming cost, direct
routes have been attracting the attention of many researchers.
2.1.
Brief history of direct oxidation of
methane to methanol
0
∆H298
= −88 kJ/mol
1.2. Methanol usage and production
Methanol is one of the most important industrial
chemicals. Its major applications are as a solvent or
as an intermediate for many other chemicals that are
used as fuels or fuel additives. Demand for methanol
has recently increased because it is used to produce
methyl tertiary butyl ether (MTBE). Methanol can
also be blended with gasoline or used directly as
an automobile fuel. It has been estimated that if
methanol achieves 10% penetration into the US automotive fuel market, the demand for methanol would
increase 25 billion gallons per year [2]. Needless to
say, this would greatly expand the methanol market.
Conventionally, methanol is produced by catalytic
synthesis of syngas which is produced by steam reforming of natural gas (reaction (1) and (4)). This
process suffers from the low energy efficiency and high
capital and operating cost of steam reforming. The direct partial oxidation of methane to methanol (5) is
an exothermal reaction that is energetically more efficient than the endothermic steam reforming reaction.
Furthermore, this more simplistic process can reduce
the capital and operating cost. Therefore, the direct
partial oxidation of methane to methanol has become
The direct partial oxidation of methane to
methanol was first discovered in 1902 by Bone and
Vheeler [3,4]. In 1932, Newitt and Haffner [5] reported
the formation of methanol through high-pressure oxidation of methane in a static system. In 1934, Wiezevich and Frolich [6] began to investigate the oxidation of methane at high pressure in a flow system.
From then on, the oxidation of methane to methanol
was always carried out in the flow system. In 1937,
Boomer et al. [7–9] reported the catalytic oxidation
of methane using copper as a catalyst. However, research on the oxidation of methane was stagnant in
the following decades because of poor methanol selectivity and the rise of the petroleum industry. In the
1980s, the interest in the direct conversion of methane
to methanol was renewed by the “energy crisis” and
the demand for the efficient utilization of abundant
natural gas reserves. Lunsford [10] and Gesser [11]
separately reported good results in catalytic and noncatalytic oxidation of methane to methanol. The
reaction was extensively studied from then on, and
quite a few reviews were published [12–17]. Unfortunately, there still has been no breakthrough, and
the methanol yield is too low for commercialization.
Furthermore, good reported results have never been
reproduced.
2.2. The difficulties in the partial oxidation of
methane to methanol
The difficulty of the direct partial oxidation of
methane to methanol lies in the activation of the
methane C-H bond. The methane molecule is a
perfect, symmetrical tetrahedron, and the four C-H
bonds are completely uniform, making it the most stable hydrocarbon molecule. The first dissociation energy of its C-H bond is as high as 440 kJ/mol, and the
83
Journal of Natural Gas Chemistry Vol. 12 No. 2 2003
activation and reaction of methane therefore requires
extreme conditions. On the other hand, the required
product (methanol) is much more active under the reaction conditions. The dissociation energy of the HCH2 OH bond is 393 kJ/mol, which is 47 kJ/mol less
than the H-CH3 bond. Under the same conditions,
methanol is easier to be activated and oxidized than
methane, leading to the deep oxidation to produce
CO and CO2 . Additionally, the methanol molecule
contains an oxygen atom, which makes methanol a
polar compound (its dipole moment is 1.70 Debyes).
But the methane molecule has no polarity, therefore,
methanol is much easier to be adsorbed than methane
on the surface of catalyst or reactor metal wall and to
be activated and oxidized.
The oxidation of methane is thermodynamically
favored. The Gibbs free energies of the reaction at
different temperatures are given in Table 1. As the
data indicate, while the formation of methanol is thermodynamically feasible, production of carbon oxides
is even more favored. It is, therefore, necessary to
control the oxidation of methane to cease at the production of methanol, instead of being deeply oxidized
to the most thermodynamically favored complete oxidation product, CO2 . However, how to control the
oxidation is a serious problem, and most of the investigations on the partial oxidation of methane to
methanol were carried out to achieve this aim.
Table 1. The gibbs free energies of reaction at different temperatures
No.
R1
R2
R3
R4
Reaction
CH4 +1/2O2 → CH3 OH
CH4 +O2 → HCHO+H2 O
CH4 +1.5O2 → CO+2H2 O
∆ Gr /(kJ/mol)
298
650
700
750
800
−111
−93
−91
−88
−86
1000
−76
−288
−294
−294
−295
−296
−298
−544
−573
−578
−582
−586
−603
CH4 +2O2 → CO2 +2H2 O
−801
−800
−799
−799
−799
−798
−147
R5
CH4 +1/2O2 → 1/2C2 H4 +H2 O
−144
−147
−147
−147
−147
R6
CH4 +1/4O2 → 1/2C2 H6 +1/2H2 O
−80
−69
−67
−65
−63
−55
R7
CH4 +1/2O2 → CO+2H2
−86
−152
−162
−172
−182
−222
R8
CH4 +H2 O→ CO+3H2
142
60
48
36
23
−27
R9
CH4 +CO2 → 2CO+2H2
171
75
61
47
33
−23
3. Controlled direct oxidation of methane to
methanol
The controlled partial oxidation of methane was
mainly carried out in two directions: gas phase homogeneous oxidation and catalytic oxidation. In the
paper, the two methods will be discussed separately.
3.1.
Gas-phase homogeneous oxidation of
methane
Under certain temperatures and pressures, methane can react with oxygen in the gas phase without a
catalyst. Until now, the most promising results were
obtained with gas phase homogeneous oxidation [16].
3.1.1. Effect of the reactor wall
In early studies, the oxidation of methane was carried out in a stainless steel reactor, especially when
high pressures were employed because of the requirement of pressure resistance. The yields of methanol
obtained were very low, mainly due to the deep oxi-
dation reactions catalyzed by the metal surface of the
reactor producing CO and CO2 . A number of other
studies have been carried out and confirmed [18–23]
that methanol selectivity diminishes in the presence
of stainless steel surfaces, which is inevitable when
a stainless steel reactor is used. In order to minimize the effect of metal surface, quartz [24–30] and
Pyrex [31–39] linings have been used and improved
the methanol yield. In a quartz lined reactor, Gesser
et al. [11] reported greater than 80% methanol selectivity at over 10% methane conversion in the gas
phase oxidation of methane, and Feng et al. [40] reported ca. 80% methanol selectivity at 12% methane
conversion in a single-crystal sapphire reactor. Unfortunately, these excellent results have not been reproduced until now. A methanol selectivity of ca.
40%–50% at methane conversions of ca. 2%–5% constituted the usual reported results, and different researcher always reported different, and even opposite
results. In fact, the poor reproducibility is one of the
most serious problems in the controlled partial oxidation of methane.
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Qijian Zhang et al./ Journal of Natural Gas Chemistry Vol. 12 No. 2 2003
There is an important problem in a quartz or
Pyrex lined reactor, how to ensure the reaction take
place essentially homogeneously in the pure gas phase.
It is important that the reactants do not contact the
metal wall of the reactor under the reaction conditions. Maybe differences in the effects of the metal
wall catalytic reaction on the homogeneous reaction
are the main reason for the different results reported
by different researchers. In order to avoid the metal
wall’s influence under the reaction conditions, Zhang
et al. [41] designed a reactor in which the ringed
gap between the inner quartz line and the SS tube
was encapsulated by an O-ring pressed by a locking
nut. This design can efficiently avoid the catalytic
effect caused by the metal wall of the reactor. A
methanol yield of ca. 7%–8% (a methanol selectivity of over 60% at a methane conversion of 12%–13%)
was reported and could be steadily reproduced. It
is believed that if the metal wall effect can be eliminated, high methanol yields could be obtained in the
gas phase partial oxidation.
3.1.2. Effect of Reaction conditions
For the gas phase homogeneous oxidation of
methane, the only controllable reaction condition
parameters are the reaction temperature, pressure,
methane/oxygen ratio in the feed and total gas flow
rate (residence time). In this section, each of the parameters will be discussed separately, although they
are inextricably associated with each other.
3.1.2.1. Reaction temperature
Most studies have examined the effects of temperature on the partial oxidation of methane to methanol
between 300 and 500 . Little methane conversion occurred before the reaction temperature was increased
to a transition temperature, after which the oxygen
conversion sharply increased to almost 100% in a very
narrow temperature range [41–44]. These results indicated that the reaction was typically operated by
a free radical mechanism. The transition temperature varied depending on the other conditions, such
as pressure, methane/oxygen ratio et al. A further
increase in reaction temperature above the transition
temperature usually resulted in decreasing methane
conversion because more CO and CO2 , the deep oxidation products, were produced [42,33].
The products of the gas phase oxidation of
methane were mainly CH3 OH, CO, CO2 , and H2 O.
HCHO was usually reported in the effluence. CH3 OH
and HCHO were formed when the temperature increased to nearly the transition temperature and
passed through a maximum before decreasing as the
temperature increased further. Recently, Zhang et al.
[41] reported an interesting result. In a specially
designed reactor, the product distribution was kept
constant for a wide temperature range of ca. 40
(430–470 ) when the pressure was 5.0 MPa and
CH4 /O2 /N2 =10/1/1.
The oxidative coupling product C2 H6 was always
observed when the oxygen in the feed gas was exhausted. With increasing temperature, the production of C2 H6 increased although its selectivity remained low.
LΦdeng [45] and Chellappa [46] separately reported on the production of H2 in the gas phase oxidation of methane, but the amount of H2 produced was
quite low. Zhang et al. [41] reported much more H2
production (H2 /CO=0.4–0.5) without HCHO being
detected. It was supposed that HCHO decomposed
quickly to H2 and CO once it formed in the pure gas
phase reaction.
3.1.2.2. Pressure
Pressure is an important factor for the gas phase
oxidation of methane to methanol. Increasing the reaction pressure has been shown to shift the transition
temperature to lower temperatures. When the pressure was raised from 1.0 to 3.0 MPa, the transition
temperature dropped more than 30 . However, as
the pressure increased beyond 5.0 MPa, the effect of
pressure on the transition temperature becomes less
pronounced [42,33].
Generally, methanol selectivity has been observed
to increase with increasing pressure [18,25,34] except
that Burch reported no smooth trend related to the
effect of pressure in methanol selectivity [33]. Decreasing the pressure resulted in a marked increase in
the production of CO and CO2 .
3.1.2.3. Methane/oxygen ratio (oxygen concentration) in the feed gas
Methane/oxygen ratio (oxygen concentration) in
the feed gas is another important factor. Most studies
were carried out with a high methane /oxygen ratio in
case of an explosion and deep oxidation. Decreasing
the feed methane/oxygen ratio (increasing the feed
oxygen concentration) generally resulted in increased
Journal of Natural Gas Chemistry Vol. 12 No. 2 2003
methane conversion with a concomitant decrease in
methanol selectivity [25,28,35,38,41–44]. However,
Burch et al. [33] concluded that methanol selectivity
showed little dependence on the feed methane/oxygen
ratio. However, the effect of methane/oxygen ratio on
the methanol yield was not as dramatic, since yield is
the product of methane conversion and methanol selectivity.
3.1.2.4. Total gas flow rate (residence time)
The effect of total gas flow rate (residence time)
on the oxidation of methane was much less pronounced. Under the conditions of fixed temperature,
pressure and feed methane/oxygen ratio, increasing
the total gas flow rate did not noticeably affect the
trend in methanol selectivity and yield [41].
3.1.3. Additives (H donors and NOx )
Apart from the controllable parameters discussed
above, some additives were added to the reaction system in order to decrease reaction temperature or increase methanol selectivity. Natural gas contains certain amounts of ethane and other higher hydrocarbons, which are known to initiate the gas phase free
radical reactions at lower temperatures. It was reported that 5% ethane in the methane feed could
lower the transition temperature by approximately
50
[33]. If natural gas was substituted for pure
methane, the transition temperature could be reduced
100
[35]. Hunter and Gesser [47] have systematically examined the effect of sensitizers on the oxidation of methane. The sensitizers included hydrocarbons, ethers, aldehydes, ketones, thiols, amines,
water and peroxides; most of which were able to reduce the MTCR (Minimum Temperature when Complete Reaction occurred) in varying degrees. Some improved formaldehyde selectivity, and some increased
methanol selectivity. Omata [48] concluded that H2
and hydrocarbons can improve methanol selectivity
because they are H-donor species.
Recently, Teng et al. [49] found that when NO
or NO2 was introduced into methane-oxygen system as initiator, methane was able to be oxidized
to methanol and formaldehyde under ambient pressure. The results obtained were: X(CH4 )=10%,
S(CH3 OH)=22%, S(HCHO)=24%. It was considered
that the nitrogen atom in NOx showed higher activity for the cleavage of the C-H bond than the oxygen
atom so as to initiate the oxidation of methane at
85
ambient conditions.
3.1.4. Brief summary
The partial oxidation of methane to methanol can
take place in the gas phase, homogeneously. Assuring
that the reaction occurs without metal wall catalysis
is the most important factor to achieve high methanol
yields. While the feed oxygen is completed consumed,
low temperature and high pressure favor methanol
production. However, lower methane/oxygen ratios
result in higher CO and CO2 selectivity while the
methanol yield is not greatly affected because the drop
in methanol selectivity is mitigated by the increase in
methane conversion.
Although the gas phase partial oxidation of
methane to methanol can give quite good results, it is
operated by a free radical reaction mechanism, which
is hard to control. Furthermore, it is almost impossible to improve the methanol selectivity and yield by
merely adjusting the operational parameters. Therefore, it is expected that the participation of catalysts
could control the reaction and give better results.
3.2. Catalytic oxidation of methane
The catalytic partial oxidation of methane to
methanol has been comprehensively investigated. The
examined catalysts include metals, single-metal oxides, multi-metal oxides, zeolites and homogeneous
complex catalysts. Unfortunately, the catalytic results reported are no better than those obtained by
gas phase homogeneous reaction, and most of the
produced oxygenates are formaldehyde other than
methanol.
3.2.1. Heterogeneous catalytic oxidation
Catalysts based on MoO3 have been applied in
the heterogeneous catalytic oxidation of methane and
extensively examined. One of the earliest report using MoO3 based catalysts was published by Dowden
and Walker [50]. They developed a series of catalysts
based on MoO3 and stated that it was important that
one oxide in the catalyst be capable of catalyzing the
oxidation of hydrocarbons and the other of catalyzing the hydration of alkenes in order for the catalyst
to be successful. The most active catalyst for the
production of methanol was Fe2 O3 (MoO3 )3 . Dowden
and Walker pointed out that if the designed products
were removed from the catalyst surface and cooled to
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Qijian Zhang et al./ Journal of Natural Gas Chemistry Vol. 12 No. 2 2003
below 200
in less than 0.03 s, the selectivity of
methanol could be greatly improved at low methane
conversion.
Spencer [51] has examined partial oxidation over
silica supported MoO3 catalysts. The major reaction products were HCHO, CO and CO2 , with only
trace amounts of methanol. At low methane conversion, selectivity to HCHO could be as high as 71%.
The effects of impurities were also examined, and it
was found that a sodium level as low as 300 ppm
had a detrimental effect on both methane conversion and oxygenates selectivity. In a later publication,
Spencer [52] suggested that Na inhibits the oxidation
of methane to HCHO but accelerates the further oxidation of HCHO to CO.
Studies carried out by Barbaux et al. [53] on
SiO2 supported MoO3 catalysts revealed that there
existed three different Mo species and distribution
regions, which were dependent on the Mo loadings.
In loadings between 1wt% and 5wt%, molybdenum
strongly interacted with the support, forming silicomolybdic acid (SMA). From 5wt%–10wt% loadings,
a polymolybdate species was observed to be covering
the SMA. At a loading of 15wt%, SMA disappeared
and crystalline MoO3 was identified distributed over
the polymolybdate phase. Smith et al. [54] have
also investigated the nature of the surface species
on the MoO3 /SiO2 catalysts and identified three surface species. Having a highly dispersed silicomolybdic
phase, the lowest loading catalysts showed the best
catalytic performance. It was supposed that the silicomolybdic species have more terminal Mo=O sites,
which are responsible for selective oxidation. However, increasing the loading increased the number of
Mo-O-Mo bridging sites while decreasing the terminal Mo=O sites. It was Mo-O-Mo sites which were
responsible for the decrease in oxygenate production
and the increase in deep oxidation products.
Catalysts based on V2 O5 were also widely applied in the partial oxidation of methane. Spencer and
Pereira [55] found that on the silica supported V2 O5
catalyst, high selectivity of HCHO was observed at
low methane conversion with trace methanol under
some conditions. Compared with the Mo-based catalysts it is showed that the V-based catalysts were
more active. Kennedy et al. [56] have shown that
the yields of HCHO depend on the vanadium loading, and optimum yields were achieved in the range
1wt%-4wt%. The catalysts in their reduced state exhibited mean vanadium oxidation states between 3
and 4. Chen and Wilcox [57] suggested that increas-
ing the vanadium loading resulted in the increasing
of the size of vanadium oxide. Larger particles possessed more active oxygen, which was responsible for
the deep oxidation.
Besides Mo and V-based oxide catalysts, many
other metal oxides have also been examined [58] for
the partial oxidation of methane, but the results
were always unsatisfiable. The catalytic oxidation of
methane is also entangled by poor reproducibility.
More recently, Hodnett et al. [59,60] assessed
the limiting selectivity of active sites on oxide catalysts and stated that selectivity was determined by
the ability of the activating species to discriminate
between the target bonds in reactants and the similar but much weaker bonds in products. The conventional selective oxidation catalysts were not capable of
selectively activating a C-H bond in a reactant in the
presence of a similar C-H bond in a product when the
bond dissociation enthalpy of the product is weaker by
more than 30–40 kJ/mol. The C-H bonds in CH3 OH
and HCHO are 50 and 75 kJ/mol weaker than the
corresponding C-H bonds in methane, respectively.
Therefore, the conventional selective oxidation catalysts were not suitable for the partial oxidation of
methane to methanol or formaldehyde.
There is now a new concept in catalyst designto control the gas phase homogeneous reaction catalytically. In the homogeneous oxidation, the active
oxidizing species such as OH can oxidize CH3 O and
CH3 OH, resulting in the production of deep oxidation
products. If some catalysts can transfer these active
oxidizing free radicals into milder surface species instead of activating the reactant (methane), the violent oxidation would be greatly restrained or even
avoided, and the oxidation of methane would be controlled to give higher methanol selectivity. Zhu et al.
[61–63] have developed a multi-component catalyst,
Mo-V-Cr-Bi-Ox/SiO2 , according to this concept of
catalyst design and obtained 80% methanol selectivity at 10% methane conversion. In Mo-V-Cr-BiOx /SiO2 multi-component oxide catalysts, the crystalline phase structures of the catalysts were sensitive
to Mo, V and Bi loadings [63]. Bi could combine with
V and Mo to form BiVO4 and γ-Bi2 MoO6 , whereas
Cr seemed to form a single Cr2 O3 crystalline phase in
the presence of Bi. Mo(VI) oxide appears to favor the
formation of partial oxidation products, and Cr(III)
oxide seems to enhance the conversion of methane.
The coupling of surface reaction and gas phase reaction was supposedly responsible for the effective inhibition of deep oxidation and high methanol selectivity.
Journal of Natural Gas Chemistry Vol. 12 No. 2 2003
3.2.2. Liquid phase homogeneous catalytic oxidation
There is also a great deal of interest in systems
which could selectively activate methane at lower temperatures or preserve the selective partial oxidation
products. These alternative approaches were usually
in the liquid phase. Olah [64] reported the conversion
of methane to methanol with >95% selectivity in liquid superacidic conditions. Protonation of methanol
was suggested to prevent further oxidation. In 1987,
a two-stage process for the conversion of methane
to methanol and dimethyl ether was suggested [65].
Methane was first monohalogenated through a reaction with chlorine or bromine over either supported
solid acid catalysis (e.g. SbF5 · graphite) or supported
platinum group metal catalysts (e.g. Pt/Al2 O3 ), and
the resultant methyl halide was then catalytically hydrolyzed to yield a mixture of methanol, dimethyl
ether and hydrogen halide. A steady state conversion
of 12%–18% was obtained producing 90% methyl bromide and 10% methanol/dimethyl ether, but this process suffers from highly corrosive nature of reactants.
The most exciting results ever reported were given
by Periana et al. [66]. The electrophilic displacement of methane using concentrated sulphuric acid
catalyzed by certain metal ions for the selective oxidation of methane was developed. The required product, methanol, was produced by hydrolysis of an intermediate methyl bisulphate species. Using mercuric
ions as the catalyst, 43% methanol yield was achieved
when operating the reaction in a batch mode at 180
[67]. In a later report, methanol yield was improved to over 70% (81% selectivity at 90% conversion) using a complex of platinum as the catalyst.
3.2.3. Other processes
In the above liquid phase processes, the oxidation products are all methanol derivatives that are
more stable than methanol itself. The more reasonable methanol yields obtained demonstrated that it
is possible to achieve both methane conversion and
methanol selectivity if methanol can be protected
from further oxidation.
Supercritical fluid extraction may be another
method of protecting the produced methanol to avoid
deep oxidation, but choosing a suitable supercritical
fluid is difficult because the reaction takes place under
severe conditions. Aki et al. [68], Lee et al. [69], and
Savage [70] have separately carried out methane oxi-
87
dation using water as the supercritical fluid medium.
Unfortunately, the results were not as good as expected, and methanol yields were not greater than 1%.
A reaction-separate reactor was designed and applied to the partial oxidation of methane to methanol
by Yu et al. [71]. NO and Na2 B4 O7 were selected
as the homogeneous-heterogeneous catalysts. Cooling H2 O was used to quench the reaction mixture and
terminate the high temperature oxidation reaction.
About 20% single-pass yield of HCHO was achieved
although the concentration of the produced HCHO
was very low because of the utilization of vapour as
an additive.
The laser stimulated surface reaction (LSSR)
technique has also been applied to the partial oxidation of methane. Zhong et al. [72] reported the results
of this technique for the partial oxidation of methane
over H3 PMo12 O40 /CaF2 catalysts. The oxidation of
methane occurred at normal pressure and 200 , and
methanol was the direct product of methane oxidation, while HCHO, CH3 OCH3 and hydrocarbons were
the products of methanol continuously reacted on the
solid surface.
3.2.4. N2 O as an oxidant
In the above discussions, the common oxidant is
molecular oxygen. The molecular oxygen in the gas
phase might be transformed with the different oxygen
species in catalysts as follows:
+e
+e
+2e
2−
O2 →
− O2(ad) −−→ O−
−→ 2O−
2 −
(ad) −−→ 2O(lattice)
Different oxidation states possess different oxidizing ability, which makes it very difficult to identify
which should be responsible for the selective oxidation and which for the combustion oxidation. The
existence of a mixture of the different oxygen species
also makes it difficult to control the oxidation reaction. Therefore, some other oxidants have been applied in the oxidation of methane to methanol, of
which N2 O was the most widely studied substitute.
In the early 1980s, Liu and Lunsford [10,73] reported
methanol selectivity of 84.6% at a methane conversion of 8.1% over the MoO3 /SiO2 catalyst using N2 O
as the oxidant instead of molecular oxygen. Somojai
et al. [74] repeated Lunsford’s results and found that
V2 O5 /SiO2 was also an efficient catalyst for the oxidation of methane with N2 O as the oxidant, with a selectivity to methanol and formaldehyde of near 100%
at a conversion of approximately 0.2%. Hodnett et al.
[75] have also observed 100% formaldehyde selectivity
over a 2% Mo loaded Spherosil (porous silica) or Cab-
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Qijian Zhang et al./ Journal of Natural Gas Chemistry Vol. 12 No. 2 2003
O-Sil (fumed silica) support. ESR results confirmed
that the active oxygen species for the N2 O selective
oxidation of methane are O− . If O2− is formed from
the decomposition of N2 O, methane is more easily oxidized to CO2 [10].
Panov et al. [76,77] named the active O− species
α-oxygen, which was able to be selectively produced
by decomposition of N2 O on a properly calcined
FeZSM-5 catalyst (Fe2 O3 /ZSM-5). It was found
that the α-oxygen from N2 O could be inserted into
the methane molecule, quantitatively, to produce
methanol at ambient temperature, but the utilization of N2 O as the oxidant for the partial oxidation
of methane suffers from its high expense and violent
corrosivity to the facility.
4. Conclusions
Controlled partial oxidation of methane to
methanol through both gas phase homogeneous and
catalytic heterogeneous reactions has been studied for
a very long time. Unfortunately, there still no process that produces reasonable methanol yield, but
there has been encouraging progress. It is possible
to achieve quite good results if the reaction occurs
homogeneously in the pure gas phase. The participation of catalyst promises to improve methanol yield
by controlling the gas-phase free radical reactions by
converting the high oxidative species to less oxidative
ones or even reductive ones. Multi-component catalysts should be the obvious choice because of the potential synergetic effect and function sharing, which
are necessary to control the oxidation of methane.
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