Efficient sunlight-driven dehydrogenative coupling of methane to ethane over a Zn+-modified zeolite

Article abstract of DOI:10.1002/anie.201102320

Sun bath: A Zn⁺-modified zeolite catalyst showing superior photocatalytic activity for the activation of a C-H bond converted methane into ethane and hydrogen upon irradiation of sunlight (see picture). Light at wavelengths shorter than 390 nm transferred electrons from the zeolite framework to the zinc centers.

Full text of DOI:10.1002/anie.201102320

DOI: 10.1002/anie.201102320
Methane Conversion
Efficient Sunlight-Driven Dehydrogenative Coupling of Methane to
Ethane over a Zn+-Modified Zeolite**
Lu Li, Guo-Dong Li, Chang Yan, Xiao-Yue Mu, Xiao-Liang Pan, Xiao-Xin Zou, Kai-
Xue Wang, and Jie-Sheng Chen*
ꢀ
Effective conversion of methane to a mixture of more
valuable hydrocarbons and hydrogen under mild conditions
is a great scientific and practical challenge.[1–7] Up to date, the
C H activation of an alkane molecule and methane con-
version both upon high-pressure irradiation of a mercury
lamp and sunlight irradiation at room temperature. An
optimized catalyst converts 24% of methane upon irradiation
for 8 hours by a high-pressure mercury lamp with a selectivity
larger than 99% for ethane and hydrogen products. Mecha-
nistic studies suggest a two-stage photoexcitation process,
which lowers the energy threshold (l < 390 nm) needed to
power our photocatalytic system relative to that (l < 270 nm)
required by previously reported systems.
ꢀ
thermal routes for activation of the strong C H bond
(104 kcalmolꢀ1) in methane require high temperatures and
multistep processes, and therefore they are energy-consuming
and inefficient.[8,9] Compared to methods powered by thermal
energy,[10–14] techniques that use photonic energy have sub-
stantial advantages, such as the capacity to minimize coking at
room temperature. A promising approach to methane con-
version is the direct non-oxidative coupling of methane
(NOCM) to form ethane and hydrogen powered by photons
[Equation (1)].[15,16]
Interactions of zeolites with metal vapors are an effective
approach for the preparation of metal-containing zeo-
lites.[19,20] Using this approach, we have synthesized a zinc-
modified ZSM-5 catalyst with a Brunauer–Emmett–Teller
(BET) surface area of 362 m2gꢀ1 through a solid–vapor
reaction between a dehydrated HZSM-5 zeolite (protonated
ZSM-5 with a Si/Al ratio of the framework of 14.8) and
metallic zinc vapor. During the reaction, the protons of the
Brønsted acidic sites (OH groups bridging Al and Si atoms of
the framework) in the zeolite are reduced by zinc atoms to
form H2 molecules (as detected by gas chromatography, GC),
whereas the zinc atoms undergo two different oxidation
reactions, as demonstrated by our experimental observations.
One reaction involves a zinc atom that reduce two closely
positioned protons to form a Zn2+ cation; the other reaction is
based on a zinc atom that reduces one isolated proton to form
a Zn2+ cation with an extra electron delocalized on the zeolite
framework. Delocalization of electrons on a zeolite frame-
work has been reported previously,[21,22] and the delocalized
electrons do not give rise to electron paramagnetic resonance
(EPR) signals. For clarity, the corresponding as-prepared
product is designated as Zn2+-ZSM-5ꢀ. According to induc-
tively coupled plasma (ICP) elemental analysis, the compo-
sition of Zn2+-ZSM-5ꢀ is Zn0.69AlSi14.8O31.6. Because the molar
ratio of Zn/Al in the Zn2+-ZSM-5ꢀ material is 0.69, about
55% of the incorporated Zn atoms reduce one proton per Zn
atom and the rest (45%) of the Zn atoms reduce two protons
per Zn atom (see the Supporting Information). To further
demonstrate the formation of Zn2+-ZSM-5ꢀ, we have also
prepared a fully Zn2+-exchanged ZMS-5 material (designated
as Zn2+-ZSM-5) for comparison through ion exchange of
Zn(NO3)2 with NaZSM-5 in aqueous solution followed by
evacuation at 5008C to remove the adsorbed water molecules.
The composition of the reference Zn2+-ZSM-5 is
Zn0.49AlSi14.8O31.6 according to the ICP analysis.
hv
2 CH4
C H þ H
ð1Þ
ƒ!
2
6
2
The produced ethane can in turn be conveniently converted
to liquid fuels or ethene through metathesis and dehydrogen-
ation, respectively.[17] Furthermore, this NOCM reaction is
the best way to produce clean H2 energy from fossil fuels
because methane has the highest H/C ratio among all
hydrocarbons. However, the methane conversion using pho-
tocatalysts previously reported for the NOCM reaction is very
low (less than 4% upon UV irradiation for 90 hours).[18] More
importantly, the wavelength of the light used in the photo-
catalytic systems previously reported for NOCM needs to be
shorter than 270 nm, which is beyond the region of the solar
spectrum (wavelength l > 290 nm) reaching the surface of the
Earth. To achieve a substantial yield and to exploit solar
energy effectively, the development of photocatalytic systems
with a distinctly higher activity, higher selectivity, and lower
photon energy threshold is desired.
Herein, we report a Zn+-modified ZSM-5 zeolite catalyst
which exhibits superior photocatalytic activity for selective
[*] L. Li, Prof. K. X. Wang, Prof. J. S. Chen
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
Shanghai 200240 (People’s Republic of China)
E-mail: chemcj@sjtu.edu.cn
L. Li, Prof. G. D. Li, C. Yan, X. Y. Mu, X. L. Pan, X. X. Zou
State Key Laboratory of Inorganic Synthesis
and Preparative Chemistry
College of Chemistry, Jilin University
Changchun 130012 (People’s Republic of China)
[**] This work was financially supported by the NSFC of China, the
National Basic Research Program of China, and the Graduate
Innovation Fund of Jilin University.
Both Zn2+-ZSM-5ꢀ and Zn2+-ZSM-5 in vacuum are EPR-
silent, indicating that no localized unpaired electrons are
present in these two samples. However, after irradiation of
ultraviolet (UV) light from a 150 W high-pressure Hg lamp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102320.
Angew. Chem. Int. Ed. 2011, 50, 8299 –8303
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8299

Communications
spectroscopic observations further support the presence of
extra electrons on the framework of the as-prepared Zn2+-
ZSM-5ꢀ material. The transition of these extra electrons to
the 4s orbitals of the Zn2+ cations is responsible for the optical
absorption in the range of 390 and 278 nm. In fact, the
electron undergoes a dynamic equilibrium between the
zeolite framework and the 4s orbitals of Zn2+ cations upon
UV irradiation, and as a result only a very limited number of
the extra delocalized electrons are excited from the zeolite
framework to the Zn2+ 4s orbitals by UV light. This is in
agreement with the fact that the intensity of the absorption
band of (Zn+,Zn2+)-ZSM-5ꢀ between 390 and 278 nm does
not change much in comparison with that of the as-prepared
Zn2+-ZSM-5ꢀ.
Figure 1. a) Room-temperature X-band EPR spectra and b) UV/Vis
diffuse reflectance spectra of the Zn2+-ZSM-5ꢀ sample in vacuum
(curve 1), the (Zn+,Zn2+)-ZSM-5ꢀ sample (curve 2), and the reference
Zn2+-ZSM-5 sample (curve 3; K-M units=Kubelka-Munk units). The
inset of Figure 1a is the EPR spectrum of the 67(Zn+,Zn2+)-ZSM-5ꢀ
sample. The EPR spectrum for the reference Zn2+-ZSM-5 material
(curve 3) is identical to that for Zn2+-ZSM-5ꢀ (curve 1).
The photocatalytic performance of the (Zn+,Zn2+)-ZSM-
5ꢀ sample for methane conversion was tested at room
temperature upon irradiation from either a 150 W high-
pressure Hg lamp and sunlight. The catalyst was spread
evenly on the wall of an airtight quartz reactor in vacuum (see
Figure S4 in the Supporting Information), followed by
introduction of a specific quantity (1000 and 200 mmol) of
pure methane (> 99.995%). We found that the (Zn+,Zn2+)-
for one hour, the initially EPR-silent Zn2+-ZSM-5ꢀ probe
shows an intense EPR signal characteristic of the Zn+ cation
(Figure 1a), which is stable and undergoes no variations in
vacuum for at least 18 months at room temperature. The
presence of Zn+ cations in the UV-irradiated material is
confirmed by using a 67Zn(I=5/2)-enriched source (97%) to
react with HZSM-5. After irradiation, the EPR spectrum of
the 67Zn2+-ZSM-5ꢀ sample exhibits six hyperfine lines
because of the interaction of the unpaired 4s electron of
67Zn+ with the I = 5/2 nuclear spin (see Figure S1 in the
Supporting Information). The slightly negative g shift and the
large hyperfine splitting also agree with previously reported
data for the EPR spectrum of the Zn+ ion.[23] The concen-
tration of the Zn+ cations in the sample is 0.062 mmolgꢀ1
according to electron spin concentration measurements (see
Figure S2 in the Supporting Information). We attribute the
formation of the Zn+ species to a photo-induced one-electron
transfer from the zeolite framework to the 4s orbital of the
Zn2+ cation in the as-prepared Zn2+-ZSM-5ꢀ sample. The
corresponding UV-irradiated sample is designated as
(Zn+,Zn2+)-ZSM-5ꢀ accordingly. The 4s electron of the Zn+
cation falls back to the zeolite framework under thermal
treatment in the absence of UV irradiation. At above 2208C,
the photoinduced EPR signal of the Zn+ cation disappears
completely (Figure 1a), but this signal reappears after the
sample has been irradiated a second time. As expected, no
EPR signal is observed for the reference Zn2+-ZSM-5 sample
after UV-irradiation because there are no extra delocalized
electrons on the zeolite framework in this case.
ZSM-5ꢀ catalyst formed through photoirradiation of Zn2+-
ꢀ
ꢀ
ZSM-5 was rather active for selective C H bond activation
of methane and for the NOCM reaction upon irradiation with
both light sources. The catalytic reaction led to the formation
of ethane with nearly equimolar amounts of H2 (see Table S1
in the Supporting Information). Carbon mass balances during
the conversion were close to 100% and no carbon oxides were
detected by gas chromatography. Figure 2a shows the time
courses of H2 evolution from methane over 1.0 g of
(Zn+,Zn2+)-ZSM-5ꢀ upon irradiation with both light sources.
To avoid the influence of varying methane concentrations, the
amount of methane chosen for this reaction was large
(1000 mmol). According to the data, the activity of
(Zn+,Zn2+)-ZSM-5ꢀ showed no noticeable degradation after
16 h of irradiation. The total amount of H2 released upon UV
To pinpoint the nature of the zinc-containing material, a
series of further experiments were conducted. The UV/Vis
diffuse reflectance spectra of both Zn2+-ZSM-5ꢀ and
(Zn+,Zn2+)-ZSM-5ꢀ show three absorption thresholds at
390, 308, and 278 nm (Figure 1b and Figure S3 in the
Supporting Information). The intensity of the absorption
bands between 390 and 278 nm for (Zn+,Zn2+)-ZSM-5ꢀ is
slightly diminished relative to those bands for Zn2+-ZSM-5ꢀ,
Figure 2. a) Photocatalytic hydrogen evolution as a function of time
obtained at room temperature in the non-oxidative coupling of
methane (NOCM) reaction catalyzed by (Zn+,Zn2+)-ZSM-5ꢀ upon
high-pressure irradiation of the Hg lamp at an intensity of
100 mWcmꢀ2 (full circles) and sunlight irradiation at an intensity of
around 50 mWcmꢀ2 (empty circles). The activity of the reference Zn2+
ZSM-5 sample upon high-pressure irradiation of the Hg lamp is
negligible (full triangles). b) Methane conversion rate, hydrogen pro-
duction rate, and ethane selectivity obtained for the NOCM reaction
catalyzed by different photocatalysts upon direct irradiation from a
-
whereas the absorption band below 278 nm assigned to
[24]
ꢀ
framework Al O units is not affected by UV irradiation.
high-pressure Hg lamp over 8 h. An amount of 1000 mmol of methane
was used in (a) and 200 mmol in (b); 1.0 g of catalyst was used in all
For comparison, the reference Zn2+-ZSM-5 sample shows
ꢀ
only one absorption band of framework Al O units. These cases.
8300 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8299 –8303

irradiation from the high-pressure Hg lamp for 16 h was
measured by gas chromatography to be 78.3 mmol, which
corresonds to an H2 production rate of 4.9 mmolhꢀ1 gꢀ1 and an
irradiation at shorter UV wavelength (l < 270 nm), were
prepared and tested for comparison (these two materials are
more easily available than the ternary SiO2-Al2O3-TiO2
material). Using 1.0 g of the catalyst and 200 mmol of
methane (standard conditions described in Figure 2b and in
Table S2 in the Supporting Information), we obtained
conversion rates for methane of 0.58 mmolhꢀ1 gꢀ1 over
Ga2O3 and 0.17 mmolhꢀ1 gꢀ1 over MCM-41 upon UV irradi-
ation from the high-pressure Hg lamp, respectively. In
contrast, a conversion rate of methane of 6.0 mmolhꢀ1 gꢀ1
was achieved using the (Zn+,Zn2+)-ZSM-5ꢀ catalyst under
identical conditions (Figure 2b). To quantitatively evaluate
the photocatalytic activities of Ga2O3 (and MCM-41) and
(Zn+,Zn2+)-ZSM-5ꢀ upon UV irradiation within the wave-
length range of 300–400 nm, a UV-D35 filter (see Figure S12
in the Supporting Information) was carefully mounted in the
system to completely block wavelengths shorter than 300 nm
and longer than 400 nm from the high-pressure Hg lamp.
Using 1.0 g of (Zn+,Zn2+)-ZSM-5ꢀ and 200 mmol of methane,
a conversion of 17.5% and an ethane selectivity of nearly
100% were achieved after irradiation for 24 h (see Figur-
es S13 and S14 in the Supporting Information), and the
quantum efficiency was calculated to be around 0.55%. In
contrast, Ga2O3 (or MCM-41) did not show a photocatalytic
activity at all under identical reaction conditions.
Upon irradiation with visible light (l > 400 nm), the
photocatalytic activity of (Zn+,Zn2+)-ZSM-5ꢀ for NOCM
was gradually reduced, and no activity was observed after 8 h
of irradiation of visible light. Further inspection revealed that
under visible light irradiation in the presence of methane, the
(Zn+,Zn2+)-ZSM-5ꢀ material became EPR-silent within 8 h,
whereas in the absence of methane, the EPR signal persisted,
indicating that there is a chance for the electron of the Zn+
cation to fall back to the zeolite framework during the
interaction with methane. However, upon re-excitation by
UV irradiation from a 150 W high-pressure Hg lamp for 2 h,
the EPR-silent sample regains its photocatalytic activity. As
discussed earlier, the single electron transfer from the zeolite
framework to the 4s orbital of the Zn2+ cation corresponds to
UV absorption between 390 nm and 278 nm, and the energy
of the irradiated visible light is not sufficient to drive this
electron transfer. Therefore, UV irradiation (l < 390 nm)
must be used to proceed the photocatalytic reaction contin-
uously. These observations suggest a two-stage catalytic
process that requires light of wavelengths shorter than
390 nm to transfer electrons from the zeolite framework to
the Zn2+ centers, and light of visible wavelengths to promote
the Zn+ reactivity towards methane. Using visible light of
different wavelengths, we have found that the minimum
energy required to drive the electron from the Zn+ center to
activate methane corresponds to a wavelength of about
700 nm. A schematic energy diagram for the whole processes
involved in the photocatalytic reaction is given in Figure 3a.
Quantum chemical calculations give rise to the optimized
structure of the initially adsorbed methane molecule linked to
a Zn+ cation in the pore of zeolite ZSM-5 (Figure 3b).
According to the calculations, three hydrogen atoms of the
methane molecule are attracted by the Zn+ cation and the
fourth hydrogen is on the opposite side. Presumably, upon
associated
methane
conversion
rate
of
around
9.8 mmolhꢀ1 gꢀ1. On the basis of the Zn+ concentration
(0.062 mmolg), the turnover number (TON) for the
(Zn+,Zn2+)-ZSM-5ꢀ material was about 2526 for a reaction
time of 16 h, corresponding to a turnover frequency (TOF) of
158 hꢀ1. In contrast, the ternary SiO2-Al2O3-TiO2 material,
which is the most effective photocatalyst previously reported
for NOCM, exhibits
a methane conversion rate of
1.3 mmolhꢀ1 gꢀ1 under similar conditions.[15] More importantly,
if sunlight (l > 290 nm) is used as light source, (Zn+,Zn2+)-
ZSM-5ꢀ still exhibits considerable efficiency for the NOCM
reaction (Figure 2a), whereas none of the previously reported
materials shows photocatalytic activity under these condi-
tions.[16]
The reusability of (Zn+,Zn2+)-ZSM-5ꢀ was tested for four
catalytic cycles (see Figure S5 in the Supporting Information).
After the test, the crystal structure of the catalyst sample
remained intact as judged by powder X-ray diffraction (see
Figure S6 in the Supporting Information), and no carbon-
containing deposits were retained in the zeolite as demon-
strated by IR, in situ EPR spectroscopies (see Figure S7 in the
Supporting Information), and elemental analysis. The testing
result indicated that the catalyst could be used repeatedly
without noticeable deactivation in the absence of moisture. In
the presence of moisture, the photocatalytic activity of
(Zn+,Zn2+)-ZSM-5ꢀ for NOCM decreased to a certain
extent, depending on the content of water in the reaction
system (see Figure S8 in the Supporting Information).
The selectivity for ethane over alternative hydrocarbon
products (propane, butane, etc.) was measured by gas
chromatography to be 99.6% upon irradiation by the high-
pressure Hg lamp and > 99.9% upon sunlight irradiation.
When photocatalytic coupling was attempted using pure
ethane instead of methane as the reactant, neither butane nor
hydrogen was observed (see Figure S9 in the Supporting
Information). This result indicates that the formation of
butane from ethane coupling is unfavorable at room temper-
ature because the medium pores (diameter of 0.55 nm) of
ZSM-5 are not large enough for two ethane molecules to
interact with each other at the Zn+ active site in the zeolite
pore. In contrast, Zn+-modified zeolites (such as zeolite Y)
with a larger pore diameter (cage diameter of 1.0 nm and
window diameter of 0.74 nm) activate ethane (see Figure S10
in the Supporting Information), suggesting that the shape
selectivity of the zeolite framework structure plays a crucial
role in room-temperature photocatalytic conversion of hydro-
carbons. In addition, the reaction does not proceed in the dark
and in the absence of Zn+ cations (exemplified by the
reference Zn2+-ZSM-5 material), confirming that the cou-
pling of methane to ethane by (Zn+,Zn2+)-ZSM-5ꢀ is a
photocatalytic process and the Zn+ cations are the photo-
catalytic active sites.
To assess the performance of (Zn+,Zn2+)-ZSM-5ꢀ further,
[25]
Ga2O3
and the mesoporous silica MCM-41[26] (see Fig-
ure S11 in the Supporting Information), two effective photo-
catalysts reported previously for the NOCM reaction upon
Angew. Chem. Int. Ed. 2011, 50, 8299 –8303
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8301

Communications
of methane, and as a result, the solar spectrum reaching the
Earth’s surface may be exploited to power the NOCM
reaction.
Experimental Section
Preparation of Zn2+-ZSM-5ꢀ, Zn2+-bꢀ, and Zn2+-Yꢀ: The details for
the preparation of the zinc-containing photocatalysts are described in
the Supporting Information.
Figure 3. a) Schematic energy diagram for the processes of the photo-
catalytic reaction. b) The B3LYP hybrid exchange-correlation optimized
geometry of the adsorbed methane molecule attracted by the Zn+
active site (red: O, blue: Si, pink: Al, gray: C, white: H, and green: the
4s electron of Zn+).
Photocatalytic test: The (Zn+,Zn2+)-ZSM-5ꢀ catalyst was air- and
moisture-sensitive. To prevent the catalyst from deactivation by air
and moisture, all the photocatalytic tests were performed under dry
Ar atmosphere and vacuum using Schlenk glassware, glovebox, and
vacuum-line techniques. All the reactants were dried by passing the
corresponding gases through a column of MgSO4 and CuSO4 prior to
catalytic testing. After reactions, the resulting zeolite samples were
taken out from the reactor in the glove box, and for the following
characterization the samples were always kept under argon. The
photocatalytic activity for the non-oxidative coupling of methane to
ethane was evaluated in an airtight quartz reactor (25 cm3) at room
temperature. The catalyst sample (1.0 g) was spread evenly on the
wall of the closed quartz reactor in vacuum, followed by reaction with
pure methane (200 and 1000 mmol) upon UV irradiation from a
150 W high-pressure Hg lamp (and sunlight) for 8 to around 24 h. The
light intensities of the high-pressure Hg lamp and sunlight were
around 100 and 50 mWcmꢀ2, respectively. If the UV-D35 filter
(300 nm < ltrans < 400 nm; trans: transmittance) was used, the light
intensities measured at wavelengths between 300 and 400 nm were
around 2.5 mWcmꢀ2 for the high-pressure Hg lamp and around
2.0 mWcmꢀ2 for sunlight. The hydrocarbon products were thermally
desorbed by heating the catalyst gradually to 3008C, which was kept
for 60 min under evacuation, collected with a liquid-N2 trap, and
analyzed by gas chromatography (GC) with a flame ionization
detector (FID). The amount of produced hydrogen was directly
measured by GC with a thermal conductivity detector (TCD).
General characterization: The powder X-ray diffraction (XRD)
patterns were recorded on a Rigaku D/Max 2550 X-ray diffractom-
eter with Cu Ka radiation (l = 1.5418 ꢀ). The electron paramagnetic
resonance spectra were obtained on a JES-FA 200 EPR spectrometer.
The details of the instrumental parameters are as follows: scanning
frequency: 9.45 GHz, central field: 3360 G, scanning width: 8000 G,
scanning power: 0.998 mW, and scanning temperature: 258C. The
stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) and manganese
(Mn) marker were used as standards for calculation of the spin
concentration. For in situ UV/Vis diffuse reflectance spectroscopy
measurements, the powder sample was sandwiched evenly between
two quartz plates of a home-made airtight quartz cell under argon.
When the sample was evacuated and exposed to methane, it was
lowered from the preheating zone into the sample compartment for
optical measurement. All the UV/Vis diffuse reflectance spectra were
recorded on a Perkin–Elmer Lambda 20 UV/Vis spectrometer,
whereas the UV/Vis absorption spectrum of the filter was measured
with a Shimadzu UV-2450 spectrophotometer. The absorbance
spectra were obtained from the reflectance spectra through
Kubelka-Munk transformation. The FTIR spectra were recorded on
a Bruker IFS 66v/S FTIR spectrometer equipped with a deuterated
triglycine sulfate (DTGS) detector. The Raman spectra were
obtained with a Renishaw inVia confocal Raman spectrometer and
radiation at 532 nm from a solid-state laser was used as exciting
source. The power of the laser was 350 mW. The ICP elemental
analyses were performed on a Perkin–Elmer Optima 3300DV ICP
spectrometer. The 29Si magic angle spinning (MAS) NMR measure-
ments were performed on a Varian Infinity plus 400 NMR spectrom-
eter, and the Brunauer–Emmett–Teller (BET) surface areas of the
light irradiation (l < 700 nm), the 4s electron of the Zn+
ꢀ
cation is photoexcited to an empty C H s*-antibonding
ꢀ
orbital of methane to active the C H bond, followed by the
attack of a second CH4 molecule to accomplish the dehydro-
genation coupling reaction. This process must be very fast
because we did not observe a variation of the complex formed
of methane and (Zn+,Zn2+)-ZSM-5ꢀ during the photocata-
lytic reaction by in situ EPR and Raman spectroscopies (see
Figure S15 in the Supporting Information).
The photocatalytic performance of the (Zn+,Zn2+)-ZSM-
5ꢀ catalyst depends on the Si/Al ratio (14.8 to 1) of the
zeolite framework (see Table S2 entries 1 to 4 in the
Supporting Information). We found that the lower the content
of Zn+ cations is (higher Si/Al ratio), the lower is the
conversion of methane. The highest methane conversion
reached 23.8% for a catalyst sample with a Si/Al ratio of 14.8
(ZSM-5 with a lower Si/Al ratio is not available) under the
standard conditions described in Table S2 (Figure S16 and
S17) in the Supporting Information. As the Si/Al ratio for the
(Zn+,Zn2+)-ZSM-5ꢀ material is increased, not only the
methane conversion but also the TOF value diminishes. The
TOF decrease implies that when the Si/Al ratio increases in
the zeolite, the relative number of accessible Zn+ sites for
NOCM gradually decreases. Zeolite Hb and zeolite HY have
also been used as host materials for the preparation of Zn+-
containing photocatalysts (see Figures S10 and S18 in the
Supporting Information). The photocatalytic activities of the
corresponding (Zn+,Zn2+)-bꢀ and (Zn+Zn2+)-Yꢀ samples are
lower than that of (Zn+,Zn2+)-ZSM-5ꢀ (see Table S2,
entries 5 and 6 in the Supporting Information), suggesting
that the framework structure of the zeolite host, which may
influence the diffusion rate of the reactants and products,
binding energy, and transition state of the complex formed by
methane and Zn+ during the photocatalytic process, also
affect the photocatalytic conversion of methane.
The key to the performance of the (Zn+,Zn2+)-ZSM-5ꢀ
photocatalyst is the presence of univalent zinc species in the
material. Unlike conventional photocatalysts for methane
conversion, the catalytic activities of which rely on transient
photoexcited electrons and holes, the Zn+-modified zeolite
photocatalyst takes advantage of the longer lifetime of the
+
ꢀ
valence electrons of the Zn species to activate the C H bond
8302 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8299 –8303

samples were measured from the adsorption of N2 at 77 K by using a
Micromeritics ASAP 2020m system.
[11] H. Zheng, D. Ma, X. H. Bao, J. Z. Hu, J. H. Kwak, Y. Wang,
Charles H. F. Peden, J. Am. Chem. Soc. 2008, 130, 3722 – 3723.
[12] M. V. Luzgin, V. A. Rogov, S. S. Arzumanov, A. V. Toktarev,
A. G. Stepanov, V. N. Parmon, Angew. Chem. 2008, 120, 4635 –
4638; Angew. Chem. Int. Ed. 2008, 47, 4559 – 4562.
[13] D. Soulivong, S. Norsic, M. Taoufik, C. Copꢁret, J. T. Cazat, S.
Chakka, J. M. Basset, J. Am. Chem. Soc. 2008, 130, 5044 – 5045.
[14] K. C. Szeto, S. Norsic, L. Hardou, E. Le Roux, S. Chakka, J.
Thivolle-Cazat, A. Baudouin, C. Papaioannou, J. M. Basset, M.
Taoufik, Chem. Commun. 2010, 46, 3985 – 3987.
[15] L. Yuliati, H. Yoshida, Chem. Soc. Rev. 2008, 37, 1592 – 1602.
[16] K. Shimura, S. Kato, T. Yoshida, H. Itoh, T. Hattori, H. Yoshida,
J. Phys. Chem. C 2010, 114, 3493 – 3503.
[17] S. F. Hꢂkonsen, A. Holmen in Handbook of Heterogeneous
Catalysis, (Eds.: G. Ertl, H. Knçzinger, F. Schꢃth, J. Weitkamp),
Vol. 7, Wiley-VCH, Weinheim, 2008, pp. 3384 – 3400.
[18] H. Yoshida, N. Matsushita, Y. Kato, T. Hattori, J. Phys. Chem. B
2003, 107, 8355 – 8362.
Received: April 4, 2011
Revised: June 28, 2011
Published online: July 14, 2011
ꢀ
Keywords: C H activation · electron transfer · photocatalysis ·
zeolites · zinc
.
[1] J. H. Lunsford, Catal. Today 2000, 63, 165 – 174.
[2] R. A. Periana, O. Mironov, D. Taube, G. Bhalla, C. J. Jones,
Science 2003, 301, 814 – 818.
[3] A. Holmen, Catal. Today 2009, 142, 2 – 8.
[4] J. M. Basset, C. Copꢁret, D. Soulivong, M. Taoufik, J. T. Cazat,
Acc. Chem. Res. 2010, 43, 323 – 334.
[5] J. Lelieveld, S. Lechtenbçhmer, S. S. Assonov, C. A. M. Bren-
ninkmeijer, C. Dienst, M. Fischedick, T. Hanke, Nature 2005,
434, 841 – 842.
[19] Y. S. Park, Y. S. Lee, K. B. Yoon, J. Am. Chem. Soc. 1993, 115,
12220 – 12221.
[20] A. Hagen, E. Schneider, A. Kleinert, F. Roessner, J. Catal. 2004,
222, 227 – 237.
[6] R. G. Bergman, Nature 2007, 446, 391 – 393.
[7] R. Balasubramanian, S. M. Smith, S. Rawat, L. A. Yatsunyk,
T. L. Stemmler, A. C. Rosenzweig, Nature 2010, 465, 115 – 119.
[8] H. D. Gesser, N. R. Hunter, C. B. Prakash, Chem. Rev. 1985, 85,
235 – 244.
[21] H. Garcia, H. D. Roth, Chem. Rev. 2002, 102, 3947 – 4007.
[22] L. Li, X. S. Zhou, G. D. Li, X. L. Pan, J. S. Chen, Angew. Chem.
2009, 121, 6806 – 6810; Angew. Chem. Int. Ed. 2009, 48, 6678 –
6682.
[9] H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau, E. J.
Beckman, A. T. Bell, J. E. Bercaw, C. Creutz, E. Dinjus, D. A.
Dixon, K. Domen, D. L. Dubois, J. Eckert, E. Fujita, D. H.
Gibson, W. A. Goddard, D. W. Goodman, J. Keller, G. J. Kubas,
H. H. Kung, J. E. Lyons, L. E. Manzer, T. J. Marks, K. Moro-
kuma, K. M. Nicholas, R. Periana, L. Que, J. Rostrup-Nielson,
W. M. H. Sachtler, L. D. Schmidt, A. Sen, G. A. Somorjai, P. C.
Stair, B. R. Stults and W. Tumas, Chem. Rev. 2001, 101, 953 – 996.
[10] V. R. Choudhary, A. K. Kinage, T. V. Choudhary, Science 1997,
275, 1286 – 1288.
[23] F. F. Popescu, V. V. Crecu, Solid State Commun. 1973, 13, 749 –
751.
[24] E. D. Garbowski, C. Mirodatos, J. Phys. Chem. 1982, 86, 97 – 102.
[25] L. Yuliati, T. Hattori, H. Itoh, H. Yoshida, J. Catal. 2008, 257,
396 – 402.
[26] L. Yuliati, M. Tsubota, A. Satsuma, H. Itoh, H. Yoshida, J. Catal.
2006, 238, 214 – 220.
Angew. Chem. Int. Ed. 2011, 50, 8299 –8303
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8303