Simultaneous conversion of methane and methanol into gasoline over bifunctional Ga-, Zn-, In-, and/or Mo-modified ZSM-5 zeolites

Article abstract of DOI:10.1002/anie.200500694

(Chemical Equation Presented) Gasoline from methane: Methane can be activated nonoxidatively and converted into higher hydrocarbons over bifunctional zeolite catalysts at low temperatures (≤ 600°C; see scheme). The amount of methane converted per mole of methanol by this process can meet or even exceed 1 mol. This approach could potentially lead to an economically feasible technology for the commercial conversion of methane into liquid hydrocarbons.

Full text of DOI:10.1002/anie.200500694

Angewandte
Chemie
Hydrocarbon Chemistry
Simultaneous Conversion of Methane and
Methanol into Gasoline over Bifunctional Ga-,
Zn-, In-, and/or Mo-Modified ZSM-5 Zeolites**
Vasant R. Choudhary,* Kartick C. Mondal, and
Shafeek A. R. Mulla
In response to increased energy demands, worldwide efforts
have been made over the past 2–3 decades to develop
alternative methods for the conversion of methane (a main
constituent of natural gas, coal-bed gas, and biogas) into
value-added products. These include ethylene (by oxidative
coupling of methane),[1–3] syngas (by partial oxidation with or
without simultaneous steam- and/or CO2-mediated methane
reformation),[4–8] and gasoline/liquid hydrocarbon fuels[9–14]
that involve either oxidative[1–10] or nonoxidative[11–15] meth-
ane activation. As methane and carbon dioxide are key
greenhouse gases responsible for global warming, the current
industrial practice of emission and/or flaring of produced
methane will be banned in the near future. The methane
produced in remote places must therefore be converted into
easily transportable energy sources like liquid hydrocarbon
fuels at the sites of methane production. The most important
approach for the transformation of methane into liquid
hydrocarbons is the well-established and already commer-
cially proven route:[16] methane!syngas!methanol!gas-
oline, based on the methanol-to-gasoline (MTG) process
developed by Mobil.[16–18] In 1985, Mobil successfully oper-
ated a commercial plant in New Zealand for MTG con-
versions. However, its operation had to be discontinued as a
result of unfavorable process economics.[18–20] The cost of the
syngas production step (by steam reforming) is about twice
the cost of the MTG production step.
The MTG process could become economically viable if it
were possible to convert a significant portion of the methane
into gasoline without the intermediate steps of conversion
into syngas and methanol. Herein, we show that this very
difficult goal can be met through the nonoxidative activation
of methane and its simultaneous conversion with methanol
into gasoline-range hydrocarbons over bifunctional Ga-, In-,
Zn-, and/or Mo-modified ZSM-5 type zeolites that have both
dehydrogenation and acid functions. We also show that the
amount of methane converted can be equimolar to the
amount of methanol converted in this novel process, depend-
[*] Dr. V. R. Choudhary, K. C. Mondal, S. A. R. Mulla
Chemical Engineering and Process Development Division
National Chemical Laboratory
Pune - 411 008 (India)
Fax: (+91)20-2589-3041
E-mail: vrc@ems.ncl.res.in
[**] K.C.M. thanks the University Grant Commission, New Delhi for a
Senior Research Fellowship. The authors are grateful to Dr. O. G. B.
Nambiar and his research groupfor their helpwith GC–MS analysis.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200500694
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ing on the specific conditions. The conversion of methane to
practical interest that the amount of methane converted per
mole of methanol in the feed could meet or exceed 1 mol,
depending on the process temperature and methane/meth-
anol ratio in the feed (Figure 2). The required methanol itself
can be produced from methane through a well-established
technology (CH4!syngas!methanol). Similarly, methane
can also be activated and converted over bifunctional zeolite
catalysts during the aromatization of other organic oxygen-
ates, such as ethanol (which can be produced from renewable
sources) and dimethyl ether (produced directly from syngas).
Notably, Figure 1 illustrates that the hydrocarbon product
distribution in the aromatization process presented herein is
strongly influenced by the bifunctional zeolite, the oxygenate
(methanol, ethanol, or dimethyl ether) used in the process,
and the presence of methane in the feed. However, such
strong influences are not observed in the aromatization of
oxygenates over the monofunctional (acidic) zeolite (H-
ZSM-5).
higher hydrocarbons was confirmed by using 13C-labeled
methanol and analysis of the reaction products by GC–MS.
This work has the potential to revolutionize the methane-to-
liquid hydrocarbons (MTL) conversion technology through
the modified MTG process of Mobil.
The results in Figure 1, Figure 2, and Table 1 clearly
indicate that methane is nonoxidatively activated and con-
verted at low temperature (< 6008C) during methanol con-
version over the bifunctional zeolite catalysts. However,
methane is not activated/converted over H-ZSM-5; instead, it
is produced in appreciable quantity (Figure 2). The simulta-
neous conversion of methane results in a significant change in
the hydrocarbon product distribution relative to that of the
conventional MTG process. Furthermore, the formation of
individual Cꢀ2 hydrocarbons is increased markedly, as shown
in Figure 1 and Table 1. The liquid hydrocarbons formed are
mainly C7–12 aromatic compounds. The formation of benzene
(the presence of which is undesirable because of its toxicity)
was very small. In the absence of methane, an appreciable
amount of methane is produced in the aromatization of
methanol. Therefore, the presence of methane as a co-
reactant in our process is beneficial, as the aromatization of
methanol results in no net formation of methane. It is of great
The H-GaAl-ZSM-5 (which is highly acidic and contains
finely dispersed gallium oxide species) performs much better
than the catalyst prepared through external impregnation of
either the gallium, zinc, indium, or molybdenum oxide species
on H-ZSM-5. However, the performance of the molybdenum,
gallium, and indium oxide-impregnated H-ZSM-5 was
Figure 1. Distribution of hydrocarbons formed in the aromatization of a) methanol over H-GaAl-ZSM-5 at 5508C, b) methanol over Mo-Zn/H-
ZSM-5 at 5008C, c) ethanol over H-GaAl-ZSM-5 at 6258C, and d) dimethyl ether (DME) over H-GaAl-ZSM-5 at 5508C in the presence (solid bars)
and absence (open bars) of methane. CH4/(CH3OH or DME)=15.0:1, CH4/C2H5OH=10:1, CH4/N2 =8.5:1, and gas hourly space velocity
(GHSV), measured at 08C and 1 atm=1050 cm3 gÀ1 hÀ1. Values in round brackets correspond to the percentage increase in the formation of a
particular hydrocarbon as a result of the presence of methane. The conversion of methanol, ethanol, or DME was 100%. In the presence of oxy-
genate, the amount of methane converted was 0.96 and 1.19 mol/mol CH3OH for H-GaAl-ZSM-5 and Mo-Zn/H-ZSM-5, respectively, and 0.8 and
0.94 mol/mol CH3CH2OH and DME, respectively. In continuous operation for 7 h, both catalysts showed no significant change in catalytic activity
for methane conversion.
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sites, it becomes thermodynamically favorable as a result of
the simultaneous conversion of methane and methanol. (For
example, DG = À8.8, À15.5, and À22.2 kJmolÀ1 at 400, 500,
and 6008C, respectively, for one mole of methane converted
per mole of methanol.)
In the absence of methanol and/or the absence of
bifunctional sites on the zeolite catalyst, there was no
conversion of methane. The negative values observed for
the ratio of (methane converted)/(methanol converted) with
the H-ZSM-5 zeolite (Figure 2a) and in the absence of
methane (Figure 2b) indicates the net formation of methane
to a significant extent. An temperature optimum is observed
for obtaining the highest ratio of (methane converted)/
(methanol converted), which depends on the bifunctional
zeolite used (Figure 2a). This can be explained by the
expected competition between the formation of methane
from methanol by cracking reactions (which have a higher
rate at higher temperature) and the activation/conversion of
methane, which has a lower rate at lower temperature. The
ratio of (methane converted)/(methanol converted) is, how-
ever, increased markedly with an increase in the methane/
methanol ratio in the feed (Figure 2b).
The activation and conversion of methane in the process
was confirmed by using 13C-labeled methanol and analysis of
the aromatic hydrocarbons formed by GC–MS. The MS data
of toluene and xylene produced in the presence and absence
of methane in the feed are compared in Figure 3. The mass
spectra clearly reveal the formation of unlabeled toluene
(formed from methane alone) and partially labeled aromatic
compounds (formed from both methane and methanol) in the
aromatization of 13CH3OH over
Figure 2. a) Moles of methane converted per mole of methanol
(XCH /CH OH) in the simultaneous aromatization of methane and metha-
4
3
nol over H-GaAl-ZSM-5 and H-ZSM-5 (CH4/CH3OH=15:1, CH4/
N2 =8.5:1, GHSV=1050 cm3 gÀ1 hÀ1); Mo-Zn/H-ZSM-5 (CH4/
CH3OH=14.1:1, CH4/N2 =8.5:1, GHSV=1050 cm3 gÀ1 hÀ1); Zn-In/H-
ZSM-5 (CH4/CH3OH=12.72, CH4/N2 =8.5:1,
GHSV=1200 cm3 gÀ1 hÀ1); Mo-Ga/H-ZSM-5 (CH4/CH3OH=13.75:1,
CH4/N2 =7.3:1, GHSV=1200 cm3 gÀ1 hÀ1); and Zn-Ga/H-ZSM-5 (CH4/
CH3OH=5.9:1, CH4/N2 =7.2:1, GHSV=1200 cm3 gÀ1 hÀ1). b) Effect of
the ratio of CH4/CH3OH in the feed on moles of methane converted
per mole of methanol in the simultaneous aromatization of methane
and methanol over H-GaAl-ZSM-5 at 5508C (CH4/N2 =8.5:1,
GHSV=1050 cm3 gÀ1 hÀ1). Methanol conversion in all cases was
100%.
the H-GaAl-ZSM-5 and Zn-In/H-
Table 1: Influence of methane on the process formation of C2–12 hydrocarbons with H-GaAl-ZSM-5 and
ZSM-5 catalysts in the presence of
methane. Methane appears to act
as an alkylating agent in the for-
mation of methylbenzenes and
methylnaphthalenes. Indeed, upon
passing a mixture of methane and
benzene (CH4/C6H6 = 12.5:1) over
the H-GaAl-ZSM-5 zeolite at
5508C, the alkylation of benzene
was observed in the formation of
toluene and xylenes in significant
quantity (with 1.0% conversion of
benzene).
We propose the following
mechanism for the activation and
conversion of methane during the
aromatization of methanol over
the bifunctional zeolite catalysts
Z, which have strong Brønsted
Mo-Zn/H-ZSM-5 zeolite catalysts.
D (C2–C12 hydrocarbons) [%][b]
[a]
Reaction conditions
CH4/CH3OH
[mol ratio]
XCH /CH OH
4
3
T [8C]
GHSV
[cm3 gÀ1 hÀ1
]
catalyst: H-GaAl-ZSM-5
500
550
550
550
15.0:1
15.0:1
7.0:1
1050
1050
1050
2100
0.52
0.96
0.27
0.79
49
108
31.8
83
15.0:1
catalyst: Mo-Zn/H-ZSM-5
450
500
550
500
500
14.1:1
14.1:1
14.1:1
7.1:1
1050
0.55
1.19
0.78
0.29
0.55
53
126
77
30
47
1050
1050
1050
2100
14.1:1
[a] Moles methane converted per mole methanol. [b] Increase due to the presence of methane.
improved appreciably after the addition of zinc oxide to the
catalyst. The catalytically active Mo-containing species may
be Mo2C and/or MoOxCy.[21]
acid sites and nonframework gallium, zinc, and/or indium
oxide species, with or without Mo2C or MoOxCy species as
dehydrogenation sites in the zeolite channels.
Low-temperature (ꢁ 6008C) conversion of methane to
aromatic hydrocarbons is not thermodynamically feasible, as
DG > 33.4 kJmolÀca1rbon. However, if this conversion is carried
out in the presence of methanol over the bifunctional zeolite
catalysts, which have strong protic acid and dehydrogenation
Methanol aromatization [Eq. (1)]:
þ
ZÀHþ þ CH3OH Ð ZÀ½CH3OH2Š I !
ð1Þ
ZÀHþ þ H2O þ C1À12 hydrocarbons
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X þ ZÀCH3 ! XCH2 þ ZÀHþ
ð6Þ
ð7Þ
þ
2 XCH2 ! 2 X þ C2H4
Methane is activated mostly as a result of the combined
action of the metal oxide species and zeolitic protons, both
present at the channel intersections in close proximity to each
other. Our earlier studies on the alkane and alkene aroma-
tization over Ga-modified ZSM-5 zeolites also revealed that
the zeolitic protons and nonframework gallium oxide species
work in cooperation with each other.[22] Methyl cations are
likely to be produced through the formation of pentacoordi-
nated carbocations II, similar to that proposed earlier by
Olah.[23] As no methane conversion is observed in the absence
À
of methanol and/or the bifunctional sites, activation of the H
CH3 bond and formation of the pentacoordinated carboca-
tions II are expected to be facilitated by the nonframework
metal oxide species in the presence of oxonium cations I and/
or carbenium ions formed from the olefins produced from
methanol.
The highly unstable methyl cations are expected to be
stabilized by the zeolitic framework negative charge (ZÀ) as
ZÀCH3 . The role of the metal oxide/carbide species (gallium,
+
indium, and zinc oxides, MoC2 or MoOxCy species) present at
the zeolite channel intersections in close proximity to the
zeolite protons is not only to facilitate the methane activation
but also to facilitate recombination of hydrogen adatoms[24]
À
formed during the C H bond activation [Eq. (2)]. One mole
Figure 3. GC–MS spectra of toluene and xylene formed in the aromati-
of H2 (which appears in the products) is produced per mole of
methane activated. The mechanism explains the observed
direct conversion of methane into toluene and incorporation
of 12C from methane into the aromatics produced from
13CH3OH. A very high concentration of methane relative to
that of higher hydrocarbons (and consequently relative to
methanol) is essential for the formation of a significant
amount of methyl cations [by Eq. (2)] and/or pentacoordi-
nated carbocations II. This is consistent with the observations;
the methane converted per mole of methanol converted is
increased markedly with an increase in the methane/methanol
ratio in the feed (Table 1, Figure 2b).
zation of 13C-labeled methanol (13CH3OH) over the H-GaAl-ZSM-5 zeo-
lite in the presence and absence of methane under identical conditions
(at 5508C and GHSV=1050 cm3 gÀ1 hÀ1). Reaction conditions:
a) CH4/13CH3OH=14.7:1, CH4/N2 =7.3:1, toluene, tret =4.56 min;
b) CH4/13CH3OH=0:1, N2/13CH3OH=16.7:1 toluene, tret =4.56 min;
c) CH4/13CH3OH=14.7:1, CH4/N2 =7.3:1, xylene, tret =7.68 min;
d) CH4/13CH3OH=0:1, N2/13CH3OH=16.7:1 xylene, tret =7.68 min.
tret =retention time.
Methane activation and conversion (M = Ga, Zn, or In;
X = Mo2C or MoOxCy species) [Eq. (2)–(7)]:
Methanol aromatization is highly exothermic (DHr ꢂ
À264 kJmolÀ1 (m-xylene)), and this is one of the major
concerns about the MTG process of Mobil. However, in our
process, the simultaneous endothermic conversion of meth-
ane and exothermic aromatization of methanol over the same
catalyst are coupled, which renders the process highly energy
efficient. It also eliminates the possibility of process runaway.
The other important advantages of this process are: 1) the
oxygenate additive (methanol or dimethyl ether) in the
methane feed can be produced from methane itself through a
well-established technology (methane!syngas!methanol
or dimethyl ether); 2) the conversion of the additive is
ZÀCH3 ! ZÀHþ þ CH2
ð3Þ
ð4Þ
þ
À
Z
ÀHþþMnÀO2
aliphatics
aromatics and H2
2 CH2 ! C2H4
ƒƒƒƒƒƒƒƒ!
oligomerization
aromatization
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temperature (> 7008C) methane aromatization over Mo/H-
ZSM-5 catalysts[21] is not technically feasible, as the catalyst is
deactivated very rapidly. Because of the above highly
attractive features, the process presented herein is expected
to be much more feasible commercially. Further development
of the bifunctional zeolite catalysts and optimization of the
process conditions should lead to an economically feasible
MTL technology which would not only eliminate the flaring
of methane and associated natural gases but which would also
satisfy increasing energy demands.
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Experimental Section
A mixture of methanol (or 13C-labeled methanol) vapors and
premixed methane–N2 (molar ratio CH4/N2 = 8.5) gas from a cylinder
was passed over bifunctional Ga-, In-, Zn- and/or Mo-modified ZSM-
5 (H-GaAl-ZSM-5 (Si/Al = 49.6, Si/Ga = 15.3, nonframework Ga =
0.65 mmolgÀ1), Mo (2%)–Zn (2%)/H-ZSM-5, Zn (2%)–In (2%)/H-
ZSM-5, Mo (2%)–Ga (2%)/H-ZSM-5, and Zn (2.3%)–Ga (2.4%)/
H-ZSM-5) zeolite catalysts[25,26] in a tubular quartz reactor[11,26] at
different reaction conditions. Before reaction, the Mo-containing
catalyst was pretreated by passing a mixture of CH4–H2 (6% H2) over
it at 5508C for 4 h, whereas the other catalysts were pretreated in the
presence of N2 under similar conditions. The reaction products were
analyzed by GC with thermal conductivity and flame ionization
detectors connected in series, or by GC–MS. Methane conversion was
determined by using nitrogen as an internal standard: (% CH4
converted) = {[(CH4/N2)feedÀ(CH4/N2)products]/(CH4/N2)feed} ” 100. In
all runs, 100% conversion of methanol to hydrocarbons and water
was observed. The formation of carbon monoxide in the methanol
conversion in the presence or absence of methane was negligible.
Furthermore, no formation of carbon monoxide was detected upon
passing a mixture of methane and steam over the bifunctional zeolite
catalyst at 5508C.
[24] E. Iglesia, D. G. Burton, J. A. Biscardi, M. J. L. Gines, S. L.
Soled, Catal. Today 1997, 38, 339.
[25] The H-GaAl-ZSM-5, Mo (2%)–Zn (2%)/H-ZSM-5, Zn (2%)–
In (2%)/H-ZSM-5,
Mo (2%)–Ga (2%)/H-ZSM-5,
and
Zn (2.3%)–Ga (2.4%)/H-ZSM-5 zeolite catalysts were pre-
pared by impregnating H-ZSM-5 (Si/Al = 20, Na/Al < 0.01)
with required quantities of the respective metal nitrates or
ammonium heptamolybdate, and calcination at 5508C for 4 h.
[26] V. R. Choudhary, A. K. Kinage, T. V. Choudhary, Appl. Catal.
1997, 162, 239.
Received: February 24, 2005
Published online: June 13, 2005
Keywords: gallium · heterogeneuos catalysis ·
.
methane activation · molybdenum · zeolites
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