Synergistic effect on the photoactivation of the methane C-H bond over Ga3+-modified ETS-10

Article abstract of DOI:10.1002/anie.201200045

Breaking down methane: A Ga³⁺-modified microporous titanosilicate (ETS-10) exhibits distinct photoactivity for the cleavage of the methane C-H bond at room temperature. The highly enhanced activity is attributed to the synergistic effect of gallium-induced C-H bond polarization and a titania-based photoredox process. Copyright

Full text of DOI:10.1002/anie.201200045

.
Angewandte
Communications
DOI: 10.1002/anie.201200045
CꢀH Bond Activation
ꢀ
Synergistic Effect on the Photoactivation of the Methane C H Bond
over Ga -Modified ETS-10**
3+
Lu Li, Yi-Yu Cai, Guo-Dong Li, Xiao-Yue Mu, Kai-Xue Wang, and Jie-Sheng Chen*
During the past decades, many efforts have been devoted to
in a synergistic way under UV irradiation (l < 350 nm). It is
demonstrated that the combination of both oxygen-centered
and metal-centered active sites in the material significantly
enhances its photoactivity for methane CꢀH bond activation,
activation of the strong CꢀH bond in methane under mild
[
1–4]
conditions
because through suitable conversion methane
may be used to substitute for the dwindling petroleum
resources as a chemical feedstock. Among the various
strategies is a compelling approach that uses photoenergy to
drive the conversion of methane to more valuable molecules
through dehydrogenation at room temperature. Yoshida et al.
developed several effective photocatalysts, such as the ternary
leading to efficient non-oxidative coupling of methane at
room temperature. An average methane conversion rate of
ꢀ
1
ꢀ1
around 29.8 mmolh
g
was achieved after UV irradiation
+
for 5 h, which is 3 and 20 times faster than those of Zn -
modified ZSM-5 zeolite and SiO -Al O -TiO material (the
2
2
3
2
[
5]
oxide SiO –Al O –TiO , for methane conversion. Recently,
two best photocatalysts for methane conversion developed so
far), respectively.
2
2
3
2
we used zinc to modify the medium-pore ZSM-5 zeolite and
found that the resulting material exhibits substantial photo-
catalytic activity for the selective conversion of methane to
ETS-10 is a microporous titanosilicate with a framework
containing one-dimensional O-Ti-O-Ti-O semiconducting
nanowires (diameter of 0.67 nm) insulated from one another
[6]
ethane and hydrogen under ultraviolet (UV) irradiation.
[12]
However, there are still huge challenges in finding more
powerful systems to achieve a practical product yield and to
exploit solar energy more effectively for the photodriven
by the surrounding SiO matrix (Figure 1). The combina-
2
tion of quantum-confined titanate wires, uniform pore
structure, and the presence of nonframework cations renders
conversion of methane. It is now a consensus that the H CꢀH
3
bond cleavage process is a key step in the dehydrogenation
[
7,8]
and dimerization of methane.
Investigations into this
process provide insights into the essence of methane con-
version at a molecular level, and thus enable us to design
better performing systems. Nevertheless, only limited infor-
mation about the mechanism for the photoinduced cleavage
of the H CꢀH bond occurring on substrate surfaces has been
3
[
9]
revealed so far, and the nature of the photoactive species
responsible for the methane CꢀH activation has yet to be
elucidated in more detail. Previously, two models for the
photoactivation of the methane CꢀH bond were proposed.
One considers that the presence of oxygen-centered radicals
[
10]
brings about homolytic CꢀH bond cleavage, whereas in the
2
ꢀ
Figure 1. Framework structure of ETS-10. The TiO3 quantum wires
other highly dispersed metal species are believed to be the
are shown in black and the SiO tetrahedra in gray. The extraframework
4
[
6,11]
active sites of methane dehydrogenation.
+
+
cations (Na and K ) are not indicated for clarity.
3
+
Herein, we describe a Ga -modified ETS-10 zeolite
material (ETS-10 = titanosilicate) in which the photogener-
ated hydroxyl radical and the extraframework metal ion
[
13]
ETS-10 attractive for photoactivation applications.
The
interact with the methane molecule to split the H CꢀH bond
incorporation of extra metal ions into ETS-10 either through
3
[
14]
[15]
ion exchange
or through isomorphous substitution
[
*] L. Li, Y. Y. Cai, Prof. K. X. Wang, Prof. J.-S. Chen
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
Shanghai 200240 (P.R. China)
E-mail: chemcj@sjtu.edu.cn
enhances its photoactivity. The unusual attributes of ETS-10
prompted us to explore its potential to photoactivate the CꢀH
bond in methane.
Ion exchange of zeolites with metal-salt solutions is
a simple and effective route for the preparation of zeolites
containing extraframework metal cations. Through ion
exchange, we have prepared a series of M-ETS-10 (M =
L. Li, Prof. G. D. Li, X. Y. Mu
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, College of Chemistry
3
+
3+
2+
3+
2+
extraframework Ga , Al , Zn , Fe , and Cu cations)
samples with various metal contents. The ETS-10 precursor
((Na,K)-ETS-10) has a composition of (Na0.78,K0.22)2TiSi O
Jilin University, Changchun 130012 (P.R. China)
[
**] This work was financially supported by the NSFC and the National
Basic Research Program of China (2011CB808703).
5
13
and a Brunauer–Emmett–Teller (BET) surface area of
331 m g . For clarity, the corresponding metal-exchanged
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200045.
2
ꢀ1
4
702
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Angew. Chem. Int. Ed. 2012, 51, 4702 –4706

Angewandte
Chemie
ETS-10 products are designated M-ETS-10-X with M and X
standing for the particular exchanged metal and the concen-
tration of the metal solution used for the ion exchange,
respectively. As shown in Figure 2a, none of the correspond-
ing aggregates of metal oxide or metal salt impurities was
similar SPV responses ranging from 300 to 380 nm with peak
maxima at around 334 nm (Figure 2c). These observations
demonstrate that the incorporation of the extraframework
metal cations into ETS-10 through ion exchange has little
effect on the bandgap of the ETS-10 host. In contrast,
isomorphous substitution of framework Si or Ti atoms by
[15]
other metal is capable of shifting the band gap of ETS-10.
The performances of the metal-exchanged ETS-10 mate-
rials for the photodriven methane activation reaction were
tested at room temperature with UV irradiation from a 150-W
high-pressure Hg lamp (Table S1 and Figure S2). A freshly-
prepared sample (0.2 g) was spread evenly on the wall of an
air-tight quartz reactor in vacuum (see Figure S3 in the
Supporting Information), followed by introduction of pure
methane (200 mmol; > 99.995% purity). To avoid the influ-
ence of ion exchange on the nature of the ETS-10 host, the
pristine ETS-10 before the test was also subjected to ion-
exchange treatment with 0.2m sodium nitrate solution
(15 mL), and the resulting material is designated Na,K-
ETS-10-0.2. Figure 3 and Table S1 present the results of
methane conversion over various substrates after ion
Figure 2. a) Powder X-ray diffraction patterns of various metal-
exchanged ETS-10 samples and their corresponding b) UV/Vis diffuse
reflectance spectra (K-M units=Kubelka–Munk units) and c) surface
photovoltage spectra under zero external bias. From bottom to top:
pristine (Na,K)-ETS-10, Ga-ETS-10-0.2, Al-ETS-10-0.2, Zn-ETS-10-0.2,
Fe-ETS-10-0.2, and Cu-ETS-10-0.2.
Figure 3. a) Methane conversion and product distribution obtained in
the photodriven methane activation reaction over various substrates
under direct irradiation from a high-pressure Hg lamp over 5 h. From
left to right: Na,K-ETS-10-0.2, Ga-ETS-10-0.2, Al-ETS-10-0.2, Zn-ETS-10-
detected in the X-ray powder diffraction patterns of these
metal-exchanged ETS-10 samples. According to inductively
coupled plasma (ICP) elemental analysis and he powder X-
ray fluorescence (XRF) spectroscopy, the composition of Ga-
ETS-10-0.2 is (Na,K)0.29Ga0.57TiSi O . The compositions of
0.2, Fe-ETS-10-0.2, and Cu-ETS-10-0.2; b) methane conversion as
a function of time over Ga-ETS-10-0.2 under direct irradiation from
high-pressure Hg lamp. Modified zeolite material (0.2 g) and methane
5
13
(200 mmol) were used in all cases.
other metal-exchanged ETS-10 materials are presented in the
Supporting Information (Table S1). Furthermore, all the
gallium ions in Ga-ETS-10-0.2 were found to be trivalent by
using the X-ray photoelectron spectroscopy (XPS) technique
exchange. Ga-ETS-10-0.2 is the best material for methane
conversion among all the samples tested. Several hydro-
carbon products (C to C ) with a selectivity larger than 70%
[
16]
(
Figure S1).
The optical properties of the as-prepared metal-
2
4
exchanged samples were probed by UV/Vis diffuse reflec-
tance and surface photovoltage (SPV) spectroscopy. As
shown in the UV/Vis spectra (Figure 2b), the pristine ETS-
for ethane were obtained from the reaction systems. Carbon
mass balances during the reaction are close to 100%, and no
carbon oxides were detected by gas chromatography. In
addition, the reaction does not proceed at all in the dark at
room temperature, indicating that the non-oxidative methane
conversion reaction over the ETS-10 based solids is light-
driven.
3
+
3+
2+
1
0 and Ga -, Al -, and Zn -exchanged ETS-10 samples
exhibit absorption bands only in the UV range with a thresh-
old at 334 nm, which is attributed to the O(2p) to Ti(3d)
[
17]
3+
charge-transfer (CT) transitions;
whereas the Fe - and
2
+
Cu -ETS-10 materials have significant absorption bands in
the visible region other than that in the UV region. These
visible absorption bands correspond to the d–d transitions in
After UV irradiation for 5 h, 14.9% of methane was
consumed by Ga-ETS-10-0.2 (Table S1, entry 1), correspond-
ꢀ
1
ꢀ1
ing to a methane conversion rate of 29.8 mmolh g . For
3
+
2+
Fe and Cu cations, respectively. In contrast, the SPV
spectra are associated with separation of photogenerated
charges (electrons and holes) and d–d transitions in tran-
sition-metal ions do not contribute to the SPV signals. All the
ETS-10 based materials before and after ion exchange gave
comparison, under similar conditions, the methane conver-
+
sion rates for the Zn -modified ZSM-5 zeolite and SiO -
2
Al O -TiO were reported previously to be 9.8 and
2
3
2
ꢀ
1
ꢀ1
1.33 mmolh g , respectively. With increasing reaction time,
the produced ethane can react further with Ga-ETS-10-0.2 to
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3
+
form other hydrocarbons so that the content of ethane in the
product decreases whereas those of higher hydrocarbons
increase. To confirm this observation, pure ethane instead of
methane was used as the reactant. Using 0.2 g of Ga-ETS-10-
of Ga ions in Ga-Y-0.2 is comparable with that in Ga-ETS-
10-0.2. Evidently, the presence of photoactive TiO units in
2
the host framework is crucial for the photodriven methane
conversion reaction, and the possibility that the excellent
activity of Ga-ETS-10-0.2 for photodriven methane conver-
sion arises from one single factor (the semiconducting titanate
0
.2 and 200 mmol of ethane, a conversion of 11.0% and
a butane selectivity of 57% were achieved after UV
irradiation for 5 h (Table S1, entry 2). This result demon-
3
+
wires or the extraframework Ga cations) can be ruled out.
Presumably, the combination of the two factors in the
photodriven reaction results in the efficient methane CꢀH
3
+
strates that the Ga -exchanged ETS-10 material can also
activate the CꢀH bond in ethane under UV irradiation.
Furthermore, no hydrocarbons larger than butane (> C )
bond activation.
4
were observed in the reaction product in this case because of
the restriction of the pore size of the ETS-10 host.
In the ideal structure of ETS-10, the titanate wires are
surrounded by SiO tetrahedra, and in principle they are not
4
Using a series of UV-cut-out filters, we determined that
the minimum light energy required to drive the methane-
activation reaction over Ga-ETS-10-0.2 corresponds to
a wavelength of about 350 nm. To evaluate quantitatively
the performance of Ga-ETS-10-0.2 upon UV irradiation
within the wavelength range 300–400 nm, a UV-D35 filter
accessible to extraframework species. Nevertheless, there
exist structural defects in ETS-10 where titanium sites are
exposed, and guest species may get access to these sites.
Raman spectroscopy is very sensitive to the state of the
titanate wires in ETS-10 (Figure S7); the pristine ETS-10
material without ion-exchange treatment gives an intense
ꢀ
1
(
Figure S4) was carefully mounted in the system to block
completely wavelengths shorter than 300 nm and longer than
00 nm from the high-pressure Hg lamp. Using 0.2 g of Ga-
ETS-10-0.2 and 200 mmol of methane gave a conversion of
2.8% after irradiation for 24 h (Table S1, entry 3).
Raman band at 727 cm , corresponding to the stretching
[
19]
vibrations of the titanate wires.
However, after ion
4
exchange, this band decreases in intensity and its maximum
shifts toward higher frequency, suggesting that the length and
[20]
1
the coordination of the titanate wires varied significantly.
3
+
3+
2+
3+
Besides Ga -modified ETS-10, Al -, Zn -, and Fe -
The ion-exchange treatment brings structural defects along
the titanate wires, resulting in much more accessible Ti ions,
modified ETS-10 were also effective in promoting the
methane activation reaction (Figure 3a and Table S1,
which are coordination-saturated by hydroxy groups (Tiꢀ
+
+
[13,14]
entries 4–6). In contrast, the activities of (Na ,K )- and
OH).
As shown in the infrared (IR) spectra (Figure S8),
2
+
Cu -modified ETS-10 were distinctly low (Table S1, entries 7
the pristine ETS-10 without ion exchange has only one
3
+
ꢀ1
ꢀ1
and 8). In the case of Ga -modified ETS-10, the higher the
absorption band at 3734 cm between 4000 and 3000 cm ,
3
+
content of Ga cations, the higher the conversion of methane
Table S1, entries 9–11). These observations demonstrate that
corresponding to the stretching vibration of surface SiꢀOH,
(
whereas the metal-exchanged ETS-10 samples have addi-
tional absorptions that are attributed to the stretching
the modification metal ions play an important role in the
photodriven methane-activation reaction.
[
20]
vibration of TiꢀOH groups.
The dependence of the methane consumption upon the
initial irradiation time (0–60 s) is presented in Figure S5. The
result demonstrates that the Ga-ETS-10 material shows
activity for methane conversion immediately upon exposure
to the UV irradiation, and the rate of methane consumption
remains constant. No induction period was observed. How-
ever, the oxidation state of the trivalent gallium center in the
Ga-ETS-10 material remains unchanged during the methane
photoactivation process, as shown by X-ray photoelectron
spectroscopy. After UV irradiation for 5 h in the presence of
methane, the XPS spectrum for the Ga-ETS-10 material was
essentially the same as that for the parent Ga-ETS-10; neither
of the binding energy peaks with maxima at 19.1 (typical for
The as-prepared Ga-ETS-10-0.2 sample shows no electron
paramagnetic resonance (EPR) signals either before or after
UV irradiation, indicating that no unpaired electrons are
present. However, upon UV irradiation in the presence of
methane, the initially EPR-silent sample exhibits a distinct
3
+
EPR signal that is characteristic of Ti cation with an axial
[
21]
g tensor (g = 1.962, g = 1.937), and this signal increases in
?
k
intensity with irradiation time (Figure 4a). Presumably, when
Ga-ETS-10-0.2 is exposed to UV irradiation in the presence
of methane, the photogenerated charge carriers (electrons
and holes) from the semiconducting titanate wires migrate
rapidly to the surface (either internal or external) of the
[
13]
zeolite material and are trapped by the TiꢀOH moieties.
+
0 [16]
Ga ) and 17.7 eV (typical for Ga ) was detected, and the
only gallium signal (21.2 eV) in the XPS spectrum (see
The holes are captured by the OH groups, producing hydroxyl
radicals (COH), whereas the photogenerated electrons are
3
+
4+
3+
Figure S1) corresponds to Ga species. These results indicate
trapped at the Ti sites to form Ti ions. The COH radical
3
+
[10]
that the Ga cations in Ga-ETS-10 are not reduced to
become additional active centers during the photodriven
methane activation reaction at room temperature.
abstracts a hydrogen atom from methane to form water,
leaving CCH3 radicals, which then combine to release as
ethane. The disappearance of TiꢀOH groups and the for-
To evaluate the effect of the zeolite host on the photo-
activation of the methane CꢀH bond, two aluminosilicate
mation of water during the methane activation reaction were
shown by in situ IR spectroscopy (Figure S8).
[6]
[18]
zeolites ZSM-5 and Y were used as host materials in place
However, as mentioned earlier, only in the presence of
3
+
3+
3+
2+
of ETS-10 for the preparation of Ga -containing samples by
some particular extraframework metal ions (Ga , Al , Zn ,
3
+
3+
ion exchange (Figure S6). The photoactivities of Ga -
exchanged ZSM-5 and Y samples were very poor for methane
conversion (Table S1, entries 12 and 13), although the content
and Fe ) is the photoactivity of the ETS-10 host for the
methane conversion reaction significantly enhanced, whereas
+
+
2+
the Na , K , and Cu cations are not able to promote the
4
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Chemie
modified Y and ZSM-5), the photoactivity of Ga-ETS-10-0.2
for methane conversion is enhanced by at least a factor of 27
under identical reaction conditions.
Experimental Section
+
+
3+
3+
2+
3+
2+
Preparation of (Na ,K )-, Ga -, Al -, Zn -, Fe -, and Cu
-
modified ETS-10 samples: The details for the preparation of the
metal-exchanged ETS-10 materials are described in the Supporting
Information.
Photodriven methane activation reaction: All tests were per-
formed under a dry Ar atmosphere or in a vacuum using Schlenk
glassware, glove-box, or vacuum-line techniques. All the reactants
were dried by passing the corresponding gases through a column of
MgSO4 and CuSO4 prior to catalytic testing. The activity for the
photodriven methane activation reaction was evaluated in an airtight
Figure 4. a) Room-temperature X-band EPR spectra of Ga-ETS-10-0.2
in vacuum (curve 1), under UV irradiation in the presence of methane
(
curve 2), and followed by exposure to oxygen (curve 3); b) reaction
mechanism involved in the photodriven methane activation reaction.
3
quartz reactor (20 cm ) at room temperature. Solid material (0.2 g)
3
+
3+
2+
3+
photodriven reaction. Previously, Ga , Al , Zn , and Fe
was spread evenly on the wall of a closed quartz reactor, which was
then evacuated at 2508C for 2 h to remove water and other molecules
adsorbed in the zeolitic material. Afterwards, the reactor was cooled
slowly to room temperature under vacuum (P < 0.01 Pa), followed by
reaction with pure methane (200 mmol) under UV irradiation from
a 150 W high-pressure Hg lamp for 5 h. The light intensity of the high-
have been proved to be capable of adsorbing methane
dissociatively and forming metal–methyl intermediates at
[
8]
+
+
2+
elevated temperatures, and Na , K , and Cu ions do not
[
22]
have this function. Even at room temperature, the polar-
3
+
ization of alkane CꢀH bonds by Ga cations is very strong.
This is evidenced by the observation that the CꢀH symmetric
stretching mode of saturated alkane (IR symmetry forbidden
for the free molecule) becomes IR active and shifts much
ꢀ
2
pressure Hg lamp was measured to be approximately 100 mWcm . If
a UV-D35 filter (300 nm < ltransmittance < 400 nm) was used, the light
intensity measured at wavelengths between 300 and 400 nm was
ꢀ2
around 2.5 mWcm . The hydrocarbon products were thermally
ꢀ1
toward lower frequencies (> 100 cm ) upon interaction with
desorbed by heating the reactor gradually up to 2508C and continuing
3
+ [23,24]
heating for 60 min under evacuation, collected with a liquid-N trap,
cations such as Ga .
As the adsorbed methane is
2
and analyzed by gas chromatography (GC) with a flame-ionization
detector (FID). The amounts of carbon oxides or hydrogen (if any)
were measured directly by GC with a thermal conductivity detector
polarized by the interaction between the individual metal
3
+
3+
2+
3+
ion (Ga , Al , Zn , and Fe in our case) and the methyl
n+
dꢀ
d+ [25]
group (·M ···C ꢀH ),
the CꢀH bond is significantly
(TCD).
weakened, promoting the hydrogen abstraction from meth-
General characterization: Powder X-ray diffraction (XRD)
ane by the COH radicals in the vicinity. Thus, the photodriven
patterns were recorded on a Rigaku D/Max 2550 X-ray diffractom-
eter with CuKa radiation (l = 1.5418 ꢀ). EPR spectra were obtained
on a JES-FA 200 EPR spectrometer; scanning frequency: 9.45 GHz;
central field: 340 mT; scanning width: 500 mT; scanning power:
cleavage of the H CꢀH bond is in fact a synergistic process of
3
photogenerated COH radicals and extraframework metal
cations (Figure 4b).
0.998 mW; scanning temperature: 258C. The stable radical 2,2-
3
+
Thermal treatment of the Ti -containing Ga-ETS-10-0.2
sample in the presence of oxygen and water at about 2508C
for 2 h leads to recovery of the TiꢀOH groups in the material
diphenyl-1-picrylhydrazyl (DPPH) was used as a standard for the
calculation of g values. X-ray photoelectron spectroscopy (XPS) was
performed on an ESCALAB 250 X-ray photoelectron spectrometer
with a monochromated X-ray source (AlKa hn = 1486.6 eV). The
(
Figure 4a and Figure S8), and, as a result, the photoactivity
of the sample is regained. During the treatment, the photo-
energy scale of the spectrometer was calibrated using Au4f7/2, Cu2p3/2,
3
+
and Ag3d5/2 peak positions. The standard deviation for the binding
energy (BE) values was 0.1 eV. For in situ infrared spectral measure-
generated Ti ion transfers its electron to oxygen to form
ꢀ
superoxide O2 (Figure 4a), which further interacts with
ꢀ2
ments, a thin self-supporting wafer (10–15 mgcm ) of the sample was
[13,26]
water to produce hydroxyl radicals
at an elevated
evacuated in the preheating zone of a home-made IR cell by heating
the sample to 2508C for 2 h. Upon cooling to room temperature, the
sample was lowered from the preheating zone into the sample
compartment for the IR measurement. The sample in the IR cell was
then exposed to methane under UV irradiation, and the IR spectrum
was recorded again for comparison after reaction. All the FTIR
spectra were recorded on a Bruker IFS 66v/S FTIR spectrometer
equipped with a deuterated triglycine sulfate (DTGS) detector. 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 ones through Kubelka–Munk transformation. The surface
photovoltage spectroscopy (SPS) measurement system consisted of
a source of monochromatic light, a lock-in amplifier (SR830-DSP)
with a light chopper (SR540), a photovoltaic cell, and a computer. A
temperature (2508C in our case). The performance of Ga-
ETS-10-0.2 for the photodriven conversion of methane was
tested for five cycles (Figure S9), and the results indicate that
the material may be used repeatedly without noticeable
deactivation after the simple thermal treatment in moist air.
After the repeated tests, the crystal structure of the sample
remained intact, as judged by powder X-ray diffraction
(
Figure S10).
The key to the excellent performance of Ga-ETS-10-0.2
for the photodriven methane activation reaction is the
presence of binary active species, that is, the extraframework
metal cations and the TiꢀOH groups on titanate wires, which
interact with methane in a synergistic manner under UV
irradiation, leading to easy cleavage of the methane CꢀH
500-W xenon lamp and a grating monochromator (Omni-l 500) were
bond. In comparison with the materials bearing only one
combined to provide the monochromatic light. A low chopping
frequency of 23 Hz was used to obtain stable and intensive signals.
+
+
2+
3+
active species (Na , K , and Cu modified ETS-10 or Ga -
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Angewandte
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The photovoltaic cell was a sandwich-like structure consisting of
ITO–sample–ITO (ITO = indium tin oxide). Raman spectra were
obtained with a Renishaw inVia confocal Raman spectrometer, and
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32 nm radiation from a solid-state laser was used as the exciting
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photometer equipped with a rhodium tube as the source of radiation.
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