A milestone in methane conversion

Direct conversion of methane, the dominating component of natural and shale gases, into key chemical feedstock and liquid fuels has long been a big challenge in chemistry and chemical engineering [1, 2]. The industrial processes for methane-resourced higher hydrocarbons (e.g. olefin and gasoline) are based on indirect methane conversion integrating several catalytic reaction technologies. The production of syngas (a mixture of H2 and CO) by catalytic methane steam reforming over a supportedNi catalyst is always the first technology. The second is methanol synthesis from syngas using a supported Cu–ZnO catalyst. The third technology in the integration is methanol-to-olefin or methanolto-gasoline, employing zeolite-based catalysts. The syngas production is energy intensive and capital costing; the breaking of C–H bonds in methane has to be substantially reversed in the subsequent processes to produce olefins and fuels [3]. When the syngas conversion is devised instead to produce higher hydrocarbons, inevitable loss of carbon as CO2 would happen, leading to methane utilization efficiency no higher than 50%. As the smallest hydrocarbonmolecule in tetrahedron symmetry, methane holds the strongest C–H bond (434 kJ/mol) and the lowest polarizability. Suitable catalysts and oxidants (such as oxygen, halogens and water) are always desirable for the initiation (activation) and product selectivity control of its reaction [1–3]. Summarized in Fig. 1A are the presently known possibilities of direct methane conversion. Catalysts are the keys to these possibilities except the long-known energy-intensive high-temperature non-catalytic methane pyrolysis process for acetylene production (a). The discovery in the early 1980s of ethylene formation via oxidative coupling of methane over redox oxide

superconducting electronics, such as superconducting interconnects, superconducting quantum interference device (SQUID), and field-effect transistor. Since the superconducting layer is experimentally confirmed only 0.55 nm thick by transmission electron microscope, transport measurements, and STM/STS experiments [4], Xue and Wang's research actually proves that the 1-UC FeSe on STO is the thinnest high-T C superconductor. This work not only paves the way to searching for new high-temperature superconductors and enhancing superconductivity by interface engineering, but also offers an ideal platform for studying and understanding the fundamental nature of unconventional and low-dimensional superconductivity.

Bo-Qing Xu
Direct conversion of methane, the dominating component of natural and shale gases, into key chemical feedstock and liquid fuels has long been a big challenge in chemistry and chemical engineering [1,2]. The industrial processes for methane-resourced higher hydrocarbons (e.g. olefin and gasoline) are based on indirect methane conversion integrating several catalytic reaction technologies. The production of syngas (a mixture of H 2 and CO) by catalytic methane steam reforming over a supported Ni catalyst is always the first technology. The second is methanol synthesis from syngas using a supported Cu-ZnO catalyst. The third technology in the integration is methanol-to-olefin or methanolto-gasoline, employing zeolite-based catalysts. The syngas production is energy intensive and capital costing; the breaking of C-H bonds in methane has to be substantially reversed in the subsequent processes to produce olefins and fuels [3]. When the syngas conversion is devised instead to produce higher hydrocarbons, inevitable loss of carbon as CO 2 would happen, leading to methane utilization efficiency no higher than 50%.
As the smallest hydrocarbon molecule in tetrahedron symmetry, methane holds the strongest C-H bond (434 kJ/mol) and the lowest polarizability. Suitable catalysts and oxidants (such as oxygen, halogens and water) are always desirable for the initiation (activation) and product selectivity control of its reaction [1][2][3]. Summarized in Fig. 1A are the presently known possibilities of direct methane conversion. Catalysts are the keys to these possibilities except the long-known energy-intensive high-temperature non-catalytic methane pyrolysis process for acetylene production (a). The discovery in the early 1980s of ethylene formation via oxidative coupling of methane over redox oxide catalyst (c) [4] had been very encouraging since it ignited the hope for formation of a carbon-carbon bond and higher hydrocarbons from methane. Another important avenue to direct methane conversion was discovered in early of the 1990s by Wang et al. at Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences, which showed that bifunctional Mo-loaded zeolites can catalyze the dehydro-aromatization of methane for selective formation of benzene and naphthalene at above 600 o C (f) [5]. Unfortunately, the yield of aromatics remained low and catalyst deactivation was fast due to surface coking.
Bao at DICP has been devoting a part of his group to searching for better catalysts for more efficient non-oxidative methane activation since the late 1990s. Their work has now advanced to a report in Science of 9 May [6], which uncovers that lattice-confined single iron sites embedded in silica matrix enable direct methane conversion exclusively to ethylene, benzene, naphthalene and hydrogen. Ethylene selectivity at the maximized methane conversion at 1363 K (48.1%) reached as high as 48.4% by carbon balance and the total selectivity to ethylene, benzene and naphthalene exceeded 99%, demonstrating an atom-economic process of direct methane conversion. The catalyst also exhibits a remarkable stability; no sign of deactivation was detected during a 60-hour reaction test (Fig. 1B). Highresolution high-angle annular dark-field scanning transmission electron microscopy and in situ X-ray absorption near edge spectroscopy/extended X-ray absorption fine structure spectroscopy measurements of the iron species, as well as theoretical calculations, suggest that the catalytic active sites would be SiO 2 -embedded isolated Fe atoms that are coordinated with one silicon and two carbon atoms (inset of Fig. 1B). The conversion and product selectivity during a long-term stability test of the Fe C SiO 2 catalyst. Adapted from [6]. (Copyright C 2014, American Association for the Advancement of Science) (C) Density function theory calculations on the catalytic generation at 1223 K of methyl radicals (Adapted from [6], Copyright C 2014, American Association for the Advancement of Science), whose following up gas-phase reactions lead to the formation of ethylene, benzene and naphthalene, as well as hydrogen.
reaction is shown to be initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions (Fig. 1C). The absence of adjacent Fe sites seems essential for avoiding a catalytic C-C coupling at the catalyst surface, assuring no deposition of coke [6].
This work marks the beginning of direct methane conversion into higher hydrocarbons without loss of carbon, setting a milestone in the history of methane conversion. Future efforts exploring how the catalytic chemistry gained in this study would behave in engineered large-scale reactors could uncover application potentials of this atom-economic methane conversion process. Though details of the surface catalysis are not completely understood, the finding that an isolated monoatomic Fe site embedded in a thermal-resistant material can serve as a methane-activation catalyst at above 1200 K will certainly motivate interest in seeking more efficient catalysts to lower the reaction temperature for methane conversion.

GEOSCIENCES
Exploring the real causes of the end-Permian mass extinction

Jiayu Rong
Of the 'big five' mass extinction events in the Earth's history [1], the end-Permian extinction was the largest. Over the past 30 years, the end-Permian extinction has attracted a lot of attention with many causes being suggested for the mass extinction. Suggestions have included asteroid impact, Siberia volcanism eruption, global warming, oceanic anoxia, massive release of methane, decrease of ozone levels, and an increase of atmospheric CO 2 levels [2][3][4][5]. However, our