orbital is out of plane whereas the dxy orbital is in
plane. The first-principles calculations show a considerably enhanced magnetic moment for single-atom-thick Fe membranes (3.08 mB) as compared
with bulk BCC Fe (2.1 mB), in good agreement
with previously calculated values (8). The total
magnetic moment is slightly decreased by the
Fe-C boundary effect but is still much larger the
bulk value (fig. S17).
In summary, the existence of free-standing
monoatomic suspended Fe membranes is demonstrated. These 2D Fe nanomembranes are directly imaged and are shown to have a square
lattice with a 2.65 Å lattice constant at room temperature. These studies provide valuable data for
further more accurate and in-depth theoretical investigations. The potential of perforated graphene
as a support for 2D membranes is shown, and
one can anticipate new 2D structures from a variety of elements to emerge.
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Acknowledgments: J.Z. thanks the Deutscher Akademischer
Austausch Dienst DAAD Foundation, and S.G. thanks the
Deutsche Forschungsgemeinschaft (RU 1540/8-1). thanks
the DFG (project PO 1602/1-1 to A.P.). This work was
supported by the Institute of Basic Science (IBS) Korea.
We also thank Y.-h. Luo (East China University of Science
and Technology) for providing us with the Dmol3 code.
We thank S. Melkhanova and J. Pang for the fabrication of
the CVD-grown graphene. We thank the IFW Dresden for
granting us use of their microscopy facilities.
Materials and Methods
Figs. S1 to S17
Tables S1 and S2
28 August 2013; accepted 19 February 2014
Main-Group Compounds Selectively
Oxidize Mixtures of Methane, Ethane,
and Propane to Alcohol Esters
Brian G. Hashiguchi,1 Michael M. Konnick,1 Steven M. Bischof,1 Samantha J. Gustafson,2
Deepa Devarajan,2 Niles Gunsalus,1 Daniel H. Ess,2 Roy A. Periana1*
Much of the recent research on homogeneous alkane oxidation has focused on the use of
transition metal catalysts. Here, we report that the electrophilic main-group cations thallium(III)
and lead(IV) stoichiometrically oxidize methane, ethane, and propane, separately or as a one-pot
mixture, to corresponding alcohol esters in trifluoroacetic acid solvent. Esters of methanol, ethanol,
ethylene glycol, isopropanol, and propylene glycol are obtained with greater than 95% selectivity
in concentrations up to 1.48 molar within 3 hours at 180°C. Experiment and theory support a
mechanism involving electrophilic carbon-hydrogen bond activation to generate metal alkyl
intermediates. We posit that the comparatively high reactivity of these d10 main-group cations
relative to transition metals stems from facile alkane coordination at vacant sites, enabled by
the overall lability of the ligand sphere and the absence of ligand field stabilization energies in
systems with filled d-orbitals.
The world is undergoing a revolution in raw hydrocarbon feedstock supply with the dis- covery of increasingly abundant sources
of natural gas in shale and offshore gas fields (1).
Although natural gas is primarily methane, natu-
ral gas—particularly from shale—also has sub-
stantial amounts of ethane and propane (2). The
conversion of methane, as well as these higher
alkanes in natural gas, into liquid fuels and com-
modity chemicals such as methanol, ethylene, etha-
nol, ethylene glycol, isopropanol, and propylene
glycol at lower costs than the current multistep,
capital-intensive processes could reduce emissions
and our dependence on petroleum, as well as in-
crease the value of natural gas.
An important approach that has emerged in
the past few decades is the design of molecular
(homogeneous) catalysts for the oxidative func-
tionalization of alkanes based on the CH acti-
vation reaction. This involves selective reaction
of an M-X catalyst with a hydrocarbon CH bond
(R-H) under relatively mild conditions without
the involvement of radicals in order to generate a
M-R intermediate that is converted to the desired
R-X product with regeneration of M-X (Eq. 1).
There has been considerable effort in this area of
research with homogeneous (3–23) as well as
heterogeneous catalysts (24–26), and substantial
progress has been made in recent years. Most of
the work on the homogeneous systems have been
primarily based on transition metals (with un-
filled d-shells, d<10) such as Pt (3, 4, 16), Pd
(14, 17–19, 23), Rh (20–22), and Ir (7–10). In
contrast, relatively few studies have been directed
toward the classic main-group elements with a
filled d-shell (d10). In 1993, we reported an ex-
ample of a main-group metal cation, HgII, in the
superacid concentrated H2SO4 for direct conver-
sion of methane to methanol esters (15). In spite
of the simplicity of the HgII system, it was not
further developed because of lack of reaction
in more practical, weaker acid media such as
CF3CO2H (TFAH), CH3CO2H (HOAc), or aqueous
acids with which product separation could be prac-
tical. Another key issue was that the reactions of
ethane and propane were unselective with the
HgII system. We originally proposed an electro-
philic CH activation mechanism for the HgII sys-
tem. However, later work by Sen, based on the
observation of products resulting from C-C cleav-
age reactions with higher alkanes, suggested that
Hg(II) in superacid media was sufficiently oxi-
dizing to initiate free-radical reactions (5)
M-X + R-H → M-R → M-X + R-X (1)
This possibility for unselective radical reactions
with higher alkanes was also considered by Moiseev
and co-workers in the early 1990s on the reaction
of alkanes in TFAH with strongly oxidizing salts
that were known to be effective for oxidizing hydrocarbons through free-radical mechanisms
(27, 28). The initial report showed high yield and
selectivity for the stoichiometric reactions of
CoIII with methane to Me-TFA (27). Carrying out
1 The Scripps Energy and Materials Center, Department of Chemistry, The Scripps Research Institute, Jupiter, FL 33458, USA.
2Department of Chemistry and Biochemistry, Brigham Young
University (BYU), Provo, UT 84602, USA.
*Corresponding author. E-mail: email@example.com (R.A. P.);
firstname.lastname@example.org (D.H.E.); email@example.com (B.G.H.)