Direct, nonoxidative conversion of methane to ethylene, aromatics, and hydrogen

Article abstract of DOI:10.1126/science.1253150

The efficient use of natural gas will require catalysts that can activate the first C-H bond of methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At 1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The lattice-confined single iron sites delivered stable performance, with no deactivation observed during a 60-hour test.

Full text of DOI:10.1126/science.1253150

Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics,
and Hydrogen
Xiaoguang Guo et al.
Science 344, 616 (2014);
DOI: 10.1126/science.1253150
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REPORTS
K. M. Shen for helpful discussions and communications.
DE-AC02-98CH10886. J.L. acknowledges support from the
Institute for Basic Science, Korea. I.A.F. acknowledges support
from Fundação para a Ciência e a Tecnologia, Portugal, under
fellowship number SFRH/BD/60952/2009. S.M. acknowledges
support from NSF grant DMR-1120296 to the Cornell Center
for Materials Research. Theoretical studies at Cornell University
were supported by NSF grant DMR-1120296 to Cornell
Center for Materials Research and by NSF grant DMR-0955822.
The original data are archived by Davis Group, BNL, and
Cornell University.
Supplementary Materials
www.sciencemag.org/content/344/6184/612/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S9
Experimental studies were supported by the Center for
Emergent Superconductivity, an Energy Frontier Research
Center, headquartered at Brookhaven National Laboratory
(BNL) and funded by the U.S. Department of Energy under
grant DE-2009-BNL-PM015, as well as by a Grant-in-Aid for
Scientific Research from the Ministry of Science and Education
References (42–45)
Movies S1 and S2
(
Japan) and the Global Centers of Excellence Program for
Japan Society for the Promotion of Science. C.K.K. acknowledges
support from the FlucTeam program at BNL under contract
21 November 2013; accepted 20 March 2014
10.1126/science.1248783
naphthalene) nonoxidatively, thereby avoiding CO
2
Direct, Nonoxidative Conversion of
Methane to Ethylene, Aromatics,
and Hydrogen
formation (15–18). CH is activated on the metal
4
sites forming CH
x
species, which dimerize to
C H . Subsequent oligomerization on the acidic
2
y
sites located inside the zeolite pores yields ben-
zene and naphthalene, as well as copious amounts
of coke (19–21). Commercial prospects for this
process are further hampered by the instability of
zeolites at the very high reaction temperatures.
1
1
2,3
1
2
1
4
5
Xiaoguang Guo, Guangzong Fang, Gang Li, Hao Ma, Hongjun Fan, Liang Yu, Chao Ma,
5
1
1
1
6
6
4
Xing Wu, Dehui Deng, Mingming Wei, Dali Tan, Rui Si, Shuo Zhang, Jianqi Li, Litao Sun,
To achieve direct conversion of CH efficient-
4
2
1
1
Zichao Tang, Xiulian Pan, Xinhe Bao *
ly, the challenges lie in cleaving the first C–H
bond while suppressing further catalytic dehy-
The efficient use of natural gas will require catalysts that can activate the first C–H bond of
2
drogenation, avoiding both CO generation and
methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that coke deposition. We report that these conditions
single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of
can be met using lattice-confined single iron sites
methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of embedded in a silica matrix. These sites activate
methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron
sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At
4
CH in the absence of oxidants, generating methyl
radicals, which desorb from the catalyst surface
and then undergo a series of gas-phase reactions to
yield ethylene, benzene, and naphthalene as the
only products (with ethylene dominating at short
space-times for a selectivity of ~52.7% at 1293
K). A methane conversion as high as 48.1% is
1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity
peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an
atom-economical transformation process of methane. The lattice-confined single iron sites
delivered stable performance, with no deactivation observed during a 60-hour test.
he challenge of converting natural gas into syngas routes dominate current and near-term in- achieved at 1363 K.
transportable fuels and chemicals (1) has dustrial practices for natural gas conversion (6, 7).
The catalysts were obtained by fusing ferrous
been spurred by several emerging indus-
Direct conversion of CH is potentially more metasilicate with SiO at 1973 K in air and from
trial trends, including rapidly rising demand for economical and environmentally friendly but is commercial quartz, followed by leaching with
H (for upgrading lower-quality oils) and a global challenging because CH exhibits high C–H bond aqueous HNO and drying at 353 K (22). The
shortage of aromatics caused by shifting refinery strength (434 kJ/mol), negligible electron affinity, resulting catalyst was designated 0.5% Fe©SiO
T
2
4
2
4
3
2
targets toward gasoline. Light olefins, which are large ionization energy, and low polarizability (8). (© denotes confinement and here represents a cat-
key chemical feedstocks, are currently made from In the pioneering work of Keller and Bhasin in the alyst characterized by the lattice-confined single
methanol, which itself is made through multistage early 1980s, CH was activated with the assistance iron sites embedded within a silica matrix). It con-
4
catalytic transformations via syngas (a mixture of of oxygen (9). This finding initiated a worldwide tained 0.5 weight percent (wt %) Fe and had a
2
H and CO) (2, 3), although there is also ongoing research surge to explore the high-temperature Brunauer–Emmett–Teller surface area of <1 m /g.
2
research to convert syngas directly to light olefins (>1073 K) oxidative coupling of methane (OCM) The catalyst was activated in a fixed-bed micro-
(
4, 5). However, in all such approaches, either CO to C
or H
is needed to remove oxygen from CO, result- materials have since been synthesized and tested, percent (vol %) CH
ing in a carbon-atom utilization efficiency below principally during the 1990s, as well as in recent ent was analyzed by online gas chromatography
0%. Despite their low efficiency, high capital and years. Unfortunately, the presence of O
leads (GC). At 1223 K, CH conversion was 8.1% (Fig.
2
hydrocarbons (10, 11). Hundreds of catalytic reactor in the reaction atmosphere [90 volume
/N ] at 1173 K. The efflu-
2
2
4
5
2
4
production costs, and enormous CO
2
emissions, irreversibly to overoxidation, resulting in a large 1A) and increased with temperature, exceeding
amount of the thermodynamically stable end- 48.1% at 1363 K (Fig. 1B). Only ethylene, ben-
1
State Key Laboratory of Catalysis, Dalian Institute of Chemical
products CO
zation efficiency of OCM remains relatively low coke nor CO
12, 13). Slow progress in discovering new cata- ly high reaction temperature. A single-pass yield
2
and H
2
O. Thus, the carbon utili- zene, and naphthalene were produced; neither
Physics, Chinese Academy of Sciences, Dalian 116023, Peo-
2
2
was detected, despite the relative-
ple’s Republic of China. State Key Laboratory of Molecular
Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese
(
Academy of Sciences, Dalian 116023, People’s Republic of lysts to circumvent this problem has hindered of 48% hydrocarbons is achieved at 1363 K and
3
China. State Key Laboratory of Fine Chemicals, Institute of
further development, and no economically viable 21.4 liters per gram of catalyst (gcat) per hour.
Coal Chemical Engineering, School of Chemical Engineering,
Dalian University of Technology, Dalian 116012, People’s
Republic of China. Beijing National Laboratory for Condensed
process has been put into practice so far.
Selectivities vary from 40.9 to 52.1% for ethylene,
4
In a recent report, elemental sulfur was used 21.0 to 29.1 for benzene, and 23.6 to 38.2% for
Matter Physics, Institute of Physics, Chinese Academy of Sciences, as a softer oxidant than O (14): For a 5% CH /Ar naphthalene, over the investigated temperature
2
4
5
Beijing 100190, People’s Republic of China. Nano-Pico Center,
Key Laboratory of Micro-Electro-Mechanical System (MEMS) of
Ministry of Education, Southeast University, Nanjing 210096,
range (1223 to 1363 K).
mixture at 1323 K, the best catalyst, PdS/ZrO
2
,
gave a CH conversion of ~16% and ethylene
By comparison, a blank experiment (an empty
4
6
selectivity near 20%, albeit at the expense of the reactor with no catalyst) under the same conditions
conversion of only 2.5%, and 95%
People’s Republic of China. Shanghai Synchrotron Radiation
Facility, Shanghai Institute of Applied Physics, Chinese Academy by-products CS and H S (14). In contrast, the showed a CH
2
2
4
of Sciences, Shanghai 201204, People’s Republic of China. bifunctional catalysts based on Mo/zeolites cata- of the product was coke (Fig. 1A). A test with
*Corresponding author. E-mail: xhbao@dicp.ac.cn
lyze CH conversion to aromatics (benzene and unmodified SiO
2
as the catalyst yielded virtually
4
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REPORTS
the same result (table S1) (22). Most notably, the during a 60-hour test at 1293 K (Fig. 1C). Meth- unsaturated iron sites toward the C-H bond of
use of 0.5 wt % Fe/SiO as the catalyst (prepared ane conversion remained at ~32% throughout CH (28, 29). The isolated nature of these sites, as
2
4
with wet impregnation on amorphous SiO
2
with this long run. Selectivities to ethylene (52.7%), evidenced by sub-angstrom–resolution high-angle
2
a high surface area, 348 m /g) (22) also led to benzene (21.5%), and naphthalene (25.8%) were annular-dark field (HAADF) scanning transmis-
high coke formation (>98%). We have varied the constant, and the total selectivity to these pro- sion electron microscopy (STEM) and in situ x-ray
iron loadings, support materials, and preparation ducts remained >99%. The combination of atom- absorption near-edge spectroscopy (XANES), pre-
methods, which unfortunately do not preclude economy, high selectivity, and high conversion is cludes surface C-C coupling and, hence, coke
coke on iron nanoparticles (NPs). For example, notable, considering the rather low loading of Fe formation. Transmission electron microscopy in-
2
coke remained the principal product (>50%) over (0.5 wt %) and very low surface area (<1 m /g) of dicates that in the fresh 0.5% Fe©SiO
0
2
catalyst,
iron oxide NPs with a size of ~3 to 4 nm are
method and 0.2% Fe/SiC (22) enhanced selective has been extensively studied for light hydrocarbon distributed homogeneously throughout the SiO
.5% Fe/ZSM. 0.8% Fe/SiO prepared by sol-gel the SiO . Although noncatalytic pyrolysis of CH
2
2
4
2
formation of hydrocarbons to some extent, but still synthesis, the product is dominated with acetylene matrix (fig. S3) (22). A STEM-HAADF image of
with a considerable amount of coke (Fig. 1A). accompanied by high coke formation (25–27). By the catalyst after reaction reveals many bright
Figure S1 (22) demonstrates that the reac- comparison, the catalytically initiated reaction de- dots of atomic size scattered across the SiO2
tivity of 0.5% Fe©SiO was very reproducible. scribed here compares very favorably with other matrix, highlighted by the red circles in Fig. 2A.
2
All mass balances are above 99%. At 1363 K, reported direct-conversion processes, including Each dot represents an individual Fe atom, con-
the space-time yields for ethylene, benzene, and pyrolysis, OCM (13), and nonoxidative aromati- sidering the much lower contrast of Si and O in the
–1
–1
naphthalene were 91, 18, and 9 mol kgcat hour , zation (21). Finally, because natural gas usually HAADF image. This result suggests that the iron
respectively. Moreover, the process represents a contains some ethane, we added 1 and 5 vol % species are redistributed from the original oxide
new and sustainable approach to H
because the by-products are high–value-added ethane substantially enhanced methane conver-
hydrocarbons (ethylene and aromatics) instead of sion (fig. S2) (22), and ethane is almost completely XANES during activation. The near-edge spec-
coke and CO (23, 24). The yield of H varies converted, although a small amount of coke trum of the catalyst is similar to that of Fe foil
with the reaction conditions, and the concentra- is formed at 1173 K and a space velocity of (Fig. 2B). In Fig. 2C, the Fourier-transformed
2
production ethane to the reactant stream. The presence of NPs to isolated atoms during catalyst activation.
This hypothesis is validated by the in situ
2
2
–
1
–1
3
tions in the effluent range from 10.9 to 51.2% 4.84 liter gcat h .
fig. 1D) (22). The unprecedented efficiency of the catalyt- number) (30) shows that, after activation, the
Furthermore, the 0.5% Fe©SiO catalyst was ically initiated CH conversion process is attri- Fe-O scattering paths apparent in the spectrum
k -weighted c(k) function (where k is wave
(
2
4
very stable, and no deactivation was observed buted to the high activity of the coordinatively of the fresh catalyst (line 1) have disappeared,
–1
–1
Fig. 1. Reaction performance. (A) Comparison of different catalysts at 1223 K
test of 0.5% Fe©SiO
Hydrogen contents of the reactor effluent (open circles) and the calculated
values (solid circles); (bottom) H peaks in GC analysis normalized by the
internal standard N (22). a. u., arbitrary units.
2
at 1293 K and 14.5 liter gcat hour . (D) (Top)
–1
–1
and 4.84 liter gcat hour . (B) Effect of reaction temperatures and space
velocities on the 0.5% Fe©SiO catalyst. Blue circles denote CH con-
version, whereas bars represent product selectivities. (C) Long-term stability
2
4
2
2
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617

REPORTS
whereas new scattering paths appear (line 2).
They are assigned to Fe-C and Fe-Si paths, by
comparison to the spectra of reference materials
such as Fe O , FeSi , and iron carbides (31). In
2
3
2
4
the presence of CH above 1173 K, iron oxide
species in the fresh 0.5% Fe©SiO interact ex-
2
tensively with the support, becoming embed-
ded in the silica matrix through bonding to Si
and C atoms. Thus, these otherwise extremely
reactive, coordinatively unsaturated iron atoms
are stabilized and persist under the very harsh
reaction conditions. No aggregation was observed,
even after prolonged reaction for 60 hours.
In contrast, the 2- to 5-nm-sized iron NPs in
0
.5% Fe/SiO
der the same conditions exhibit only a Fe–Fe bond
line 3 in Fig. 2, B and C). This result explains the
extensive carbon deposition observed for 0.5%
Fe/SiO , considering that iron NPs are widely
2
(fig. S5a) (22) after activation un-
(
2
used for the synthesis of carbon nanotubes (32).
That process involves catalytic cleavage of C–H
bonds and dissolution of carbon species into the
iron lattice. Subsequent C-C coupling on an iron
NP surface and crystallization from the super-
saturated carbide solid solution drive the growth
of nanotubes (33). However, under the harsh re-
action conditions in the current reaction, 0.5%
2
Fig. 2. Structural features of 0.5%Fe©SiO . (A) STEM-HAADF image of the catalyst after reaction, with
the inset showing the computational model of the single iron atom bonded to two C atoms and one Si
3
atom within silica matrix. (B) In situ XANES upon activation and (C) Fourier transformed (FT) k -weighted
Fe/SiO
aggregated and grew to 20 to 30 nm after reaction
fig. S4b) (22). These results again highlight the
crucial role played by the site isolation of the iron
species in 0.5% Fe©SiO in achieving high sel-
ectivity toward hydrocarbons and preventing coke represented by the signal at mass/charge ratio actor favors formation of C
formation.
(m/z) = 15, are clearly observed. Additional sig- lower flow rates (corresponding to longer resi-
Furthermore, density functional theory (DFT) nals at m/z = 28, 40, 42, 78, 92, and 128 are assigned dence times) promote cyclization of intermediates
calculations suggest that the most stable struc- to ethylene (C ), propyne or propadiene (C
), leading to aromatics, which are consistent with the
ture in the reactive atmosphere is an iron atom propylene (C H ), benzene (C H ), toluene GC analysis obtained in the microreactor. These
2
deactivates very rapidly, and iron NPs
c(k)-function of the EXAFS spectra. Solid lines denote reference samples of Fe foil, FeSi , and Fe O . Line
2
2 3
1
2 2 2
denotes the fresh 0.5% Fe©SiO . Line 2 stands for 0.5% Fe©SiO and line 3 for 0.5% Fe/SiO upon
(
activation in 10% CH /N at 1173 K for 2 hours, respectively. R(Å), distance in angstroms.
4
2
2
2 4
H (Fig. 3D), whereas
2
H
4
3 4
H
3
6
6 6
coordinated by one Si and two C atoms and is (C
thus embedded within the SiO matrix, as de-
7
H
8
), and naphthalene (C10
H
8
), respectively.
results lend further support to the hypothesis that
To further elucidate the mechanism, the reac- the reaction is initiated by the catalytic generation
2
picted in Fig. 3A and fig. S5. The calculated Fe–C tion profile of methyl radicals at 1225 K was sim- of methyl radicals, which subsequently undergo a
and Fe–Si bond lengths are 1.6 and 2.4 Å, respec- ulated with DFT (Fig. 3C and fig. S7) (22). Two series of gas-phase reactions. Thus, the conversion
tively, which are consistent with those estimated •CH radicals combine to form C H
3 2 6
via a strongly efficiency is high, despite the very limited number
from extended x-ray absorption fine structure exothermic process. C H undergoes dehydrogen- of surface iron sites and the extremely low surface
2
6
(
EXAFS) (table S2) (22). This lattice-confined ation readily, giving C
2
H
4
and H atoms with an area of the catalyst.
Heterogeneous systems for CH
radical tends to erally still suffer from poor carbon utilization,
releases from the surface with an energy barrier react with additional C H molecules. Further de- caused in part by low selectivity. Here, we dem-
single iron site initiates CH dehydrogenation by energy barrier of 1.58 eV. By abstraction of H
4
activation gen-
4
3 2 4 2 3
generating a •CH radical, which subsequently from C H , the resulting •C H
2
4
of 2.32 eV instead of undergoing further dehydro- hydrogenation and cyclization leads to benzene, onstrate an atom-economical direct CH
genation or C-C coupling (fig. S5) (22). The Fe with an energy barrier of 2.85 eV. C
is also sion process, enabled by the lattice-confined
site is then exposed and becomes active for adsorp- readily dehydrogenated by •H and, after further single iron sites embedded within a silica matrix,
tion of a second methane molecule and release of chain growth and cyclization, yields the thermody- which activate CH and generate methyl radicals.
another methyl radical, with energy barriers of namically more stable naphthalene. The low bar- A conversion as high as 48.1% was obtained at
conver-
4
6 6
H
4
–
1
–1
3
.07 and 2.19 eV, respectively (Fig. 3A). Migra- rier for transformation of C
tion of H from C in Fe-C-Si sites to Fe involves a the absence of C
among the experimentally with a selectivity to C
barrier of 0.58 eV. The resulting surface H species observed products under steady-state reaction con- mainder being aromatics). No deactivation was
desorbs as H with an energy barrier of 1.61 eV.
ditions, whereas the thermodynamically more observed even after reaction for 60 hours, and the
The intermediacy of methyl radicals was stable hydrocarbons C
verified by online vacuum ultraviolet soft photoioni- cumulated and were detected.
zation molecular-beam mass spectrometry (VUV-
SPI-MBMS) (fig. S6) (22). Molecules were ionized pressure, the yields of C H , C H , and C H
2
H
6
to C
H
2 4
explains 1363 K and a space velocity of 21.4 liter gcat hour ,
of >48.4% (the re-
H
2 6
2 4
H
2
2
H
4
6
, C H
6
, and C10H
8
ac- total carbon selectivity to the three products re-
mained >99%. Although the dehydrogenation
At equilibrium at 1225 K and atmospheric itself is endothermic, high selectivity to ethylene
in this process substantially reduces the heat
were estimated to be 9.0, 34.0, and input (estimated to be about half that of a typical
2
4
6
6
10 8
with a 10.6-eV VUV lamp, which has an energy from CH
4
lower than the CH ionization energy (12.6 eV). 57.0% (22), respectively. The relative ratios of thermal pyrolysis process with dominating acet-
4
This allows detection of intermediate radicals and these products could be manipulated by changing ylene in product), as shown in table S3 (22). These
products (34, 35). Figure 3B and its inset display the reaction conditions. For example, increasing findings open up new possibilities for fundamental
all species detected at 1193 K. Methyl radicals, the CH
4
flow rate in the VUV-SPI-MBMS re- studies of direct, nonoxidative activation of CH .
4
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REPORTS
Fig. 3. Investigation of the reaction mechanism over 0.5% Fe©SiO2. atomic mass units. (C) DFT simulated reaction profile of methyl radicals in the
A) DFT calculations on catalytic generation of methyl radicals at 1223 K. gas phase at 1225 K. DG, Gibbs free energy. (D) Relative intensity of VUV-SPI-
flow rate at 1223 K.
(
(B) Species in the reactor effluent at 1193 K, detected by VUV-SPI-MBMS. amu, MBMS signals of major products as a function of CH
4
It is anticipated that combining a catalyst such as
this one with an efficient reactor technology may
enable the development of non–syngas-based
routes to transform light hydrocarbons into high–
value-added chemicals.
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Acknowledgments: This work was financially supported by
the “Strategic Priority Research Program” of the Chinese
Academy of Sciences (grant XDA09030101), the National
Natural Science Foundation of China (grants 21321002,
11079005, 21033009, and 21103181), and the Ministry
of Science and Technology of China (grants 2011CBA00503
and 2013CB933100). We thank S. L. Scott and H. Metiu
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Supplementary Materials
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Materials and Methods
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