Catalytic cleavage of C-O linkages in benzyl phenyl ether assisted by microwave heating

Article abstract of DOI:10.1039/c5ra04974a

The microwave-assisted cleavage of C-O linkages in benzyl phenyl ether was investigated at 200 °C in the presence of ZrP or SBA-15 doped with or without metal catalyst (Pd/C or Ru/C). The total yield of phenol and benzyl alcohol from benzyl phenyl ether depolymerization with ZrP was 26.91%, while the yield of phenol reached 26.11% for that with Pd/C. Hybrid catalysts performed effectively for promoting the formation of phenol. A conversion of 85.70% was achieved with ZrP-Pd/C catalyst, and the selectivity to phenol reached 47.32%. The highest yield of phenol was obtained at 1 h, and repolymerization was remarkable at longer reaction time. Based on the reaction using phenol and benzyl alcohol as the reactants, benzyl phenyl ether might be decomposed into phenol and benzyl alcohol following with the repolymerization producing thermally stable polymers in the presence of Pd.

Full text of DOI:10.1039/c5ra04974a

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Catalytic cleavage of C–O linkages in benzyl phenyl
ether assisted by microwave heating†
Cite this: RSC Adv., 2015, 5, 43972
Jun Hu, Dekui Shen, Shiliang Wu, Huiyan Zhang and Rui Xiao*
ꢀ
The microwave-assisted cleavage of C–O linkages in benzyl phenyl ether was investigated at 200 C in the
presence of ZrP or SBA-15 doped with or without metal catalyst (Pd/C or Ru/C). The total yield of phenol
and benzyl alcohol from benzyl phenyl ether depolymerization with ZrP was 26.91%, while the yield of
phenol reached 26.11% for that with Pd/C. Hybrid catalysts performed effectively for promoting the
formation of phenol. A conversion of 85.70% was achieved with ZrP–Pd/C catalyst, and the selectivity to
phenol reached 47.32%. The highest yield of phenol was obtained at 1 h, and repolymerization was
remarkable at longer reaction time. Based on the reaction using phenol and benzyl alcohol as the
reactants, benzyl phenyl ether might be decomposed into phenol and benzyl alcohol following with the
repolymerization producing thermally stable polymers in the presence of Pd.
Received 20th March 2015
Accepted 11th May 2015
DOI: 10.1039/c5ra04974a
www.rsc.org/advances
benzyl phenyl ether with Pd catalyst supported on cesium-
1
. Introduction
exchanged heteropolyacid, reporting a conversion of 78% aer
ꢀ
Biomass is the only renewable resource for the production of 1 h at 200 C, with phenol, benzene and toluene as the main
1
23
liquid fuels and chemicals. As one of the three main compo- products. These literatures conrmed that catalyst is impor-
nents of biomass, lignin is a potential feedstock to be converted tant for promoting the conversion.
into aromatics and phenolic compounds due to its abundant
methoxylated phenyl-propane units connected with C–O and assisted cleavage of C–O linkages in benzyl phenyl ether
The target of this work is to investigate the microwave-
2–4
C–C bonds. Lignin can be converted by various thermal– catalyzed by ZrP or SBA-15 doped with or without metal cata-
chemical ways and hydrothermal is a promising way to produce lyst (Pd/C or Ru/C). As a solid catalyst, ZrP performs well for
5
–9
24,25
liquid products. Extensive studies have been carried out on ether bonds cleavage in conversion of xylose and glucose.
the hydrothermal degradation of various lignins, and most of SBA-15 is widely used as catalyst support due to its large
5,10–13
26
these studies were conducted under electric heating.
surface area and uniform tubular mesopores. Metal catalysts
These days, microwave heating have drawn increasingly of Pd/C and Ru/C were reported as good catalysts for lignin and
2
7,28
attention in terms of biomass and lignin conversion, due to its biomass conversion.
cost-effective heating compared with electric heating.
The reactions of phenol and benzyl
alcohol which are the possible intermediate products were
14–18
Despite these work, the degradation mechanism of lignin is not carried out to further study the conversion pathway of benzyl
clear yet. One main reason is that the complex structure of real phenyl ether.
2
lignin usually lead to a wide range of product distribution. In
this work, benzyl phenyl ether is selected as the model
2
. Materials and methods
compound to give insight into the cleavage mechanism of a-O-4
19–22
2.1. Materials
bond which is one of the main ether bonds in real lignin.
Catalyst plays a vital role in the conversion of lignin. Roberts The solvents used in the study were purchased from Nanjing
et al. reported that the conversion of benzyl phenyl ether Reagent Co. and used as received. 5 wt% Ru/C catalyst and 5 wt%
ꢀ
19
without catalyst took 4 h to reach 91% at 270 C. When cata- Pd/C catalyst were purchased from Aladdin. SBA-15 was
lysts were used, efficient cleavage of C–O bonds can be fullled purchased from Nanjing XFNANO Materials Tech Co., Ltd.
under mild conditions. He et al. reported that the conversion of Benzyl phenyl ether (>98%, GC assay) was purchased from Tokyo
benzyl phenyl ether with Ni/HZSM-5 reached about 97% aer Chemical Industry Co., Ltd. (TCI). NH H PO and ZrOCl $8H O
4
2
4
2
2
ꢀ
20,21
1
00 min at 250 C.
Park et al. investigated the degradation of were purchased from Sinopharm chemical reagent Co., Ltd.
2
.2. Preparation and characterization of catalysts
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education,
Southeast University, Nanjing 210096, PR China. E-mail: ruixiao@seu.edu.cn
The zirconium phosphate catalyst (ZrP) was prepared according
to the published work. In brief, NH
was added to ZrOCl $8H O (1 mol L , 100 mL) at the P/Zr mole
29
ꢁ1
H
2
PO
4
(1 mol L , 200 mL)
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra04974a
4
ꢁ
1
1
2 2
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m
i
ratio of 2. The solution was stirred for 30 min. The precipitate
Y
i
ð%Þ ¼ m
ꢂ 100%
(1)
was ltered and washed with deionized water until the pH of 5,
bpe_0
ꢀ
ꢀ
dried at 100 C overnight, and then calcined at 400 C for 4 h
in air.
i
where m is the mass of product i, mbpe_0 is the initial mass of
benzyl phenyl ether.
3
Ammonia temperature-programmed desorption (NH -TPD)
The conversion (C) was determined by the weight difference
of benzyl phenyl ether before and aer reaction, and was
calculated as:
was carried out on a quartz U-tube apparatus. The 40–60 mesh
ꢀ
sample (0.1 g) was rstly degassed in a He stream at 400 C for
ꢀ
3
0 min. Then the temperature was cooled down to 100 C and
m
bpe_0 ꢁ mbpe
5
% NH
3
/N
2
was used for adsorption under a ow of 20 mL
Cð%Þ ¼
ꢂ 100%
(2)
ꢁ
1
m
bpe_0
min for 0.5 h. Aer that, pure He was switched to remove any
physically adsorbed NH for 0.5 h under the same temperature. where mbpe is the mass of benzyl phenyl ether aer reaction.
3
ꢀ
ꢀ
Desorption of NH was performed from 100 C to 700 C at a
i
The selectivity (S ) was calculated as:
3
ꢁ1
ꢀ
ꢀ
rate of 10 C min and the temperature was held at 700 C for
another 1 h. The desorbed NH was measured using a gas
chromatograph (GC-SP6890) with thermal conductivity
detector (TCD). XRD was performed on a D/max rC X-ray
Y
i
S
i
ð%Þ ¼ ꢂ 100%
(3)
3
C
a
ꢀ
ꢀ
diffractometer using Cu Ka radiation from 5 to 80 with a
ꢀ
3
. Results and discussion
step of 0.02 per s.
3.1. Characterization of the catalysts
2.3. Depolymerization of lignin
Fig. 1 shows the NH -TPD desorption proles of ZrP and
3
The hydrothermal degradation of benzyl phenyl ether was SBA-15. One strong broad desorption peak could be observed
ꢀ
conducted in a microwave reaction system (MDS-6, Sineo, for ZrP from 570 to 660 C, indicating the existence of strong
China). The 70 mL vessel was made of Teon, with a maximum acid site. SBA-15 was reported with no acid sites. However, one
ꢀ
ꢀ
working condition of 4 MPa and 220 C. In a typical run, 0.2 g small broad desorption peak could be found around 550 C for
benzyl phenyl ether and 0.1 g ZrP or SBA-15 doped with or SBA-15 in the present work. The weak peak may be attributed to
without 0.1 g Pd/C or Ru/C were loaded into the reactor vessel
with 15 mL deionized water. Then the reactor was heated by
ꢀ
microwave irradiation to 200 C in three steps (three minutes to
ꢀ
ꢀ
ꢀ
1
60 C, three minutes to 180 C and three minutes to 200 C).
ꢀ
Aer keeping at 200 C for a desired time, the reactor was cooled
to room temperature by forced-air. Each reaction condition was
repeated at least twice.
Aer reaching the room temperature, the reactor was
opened, and all liquid and solid products were collected. The
reactor was rinsed with ethyl acetate, then the ethyl acetate was
added into the collected products. The liquid product was
separated from the solid catalysts by ltration. The obtained
liquid product was then extracted with ethyl acetate three times,
and the ethyl acetate extracted liquid product was calibrated to
100 mL by adding ethyl acetate for further GC and GC MS
analysis.
2.4. Characterization of the products
The liquid products were identied by GC-MS (Agilent 7890A-
5
975C). The condition of GC MS was set to be: the injector
ꢀ
temperature was kept at 300 C; the chromatographic separa-
tion was performed with a HP-5 MS capillary column; the oven
temperature was programmed from 40 C (3 min) to 180
ꢀ
ꢀ
C
ꢀ
ꢁ1
ꢀ
(2 min) with 5 C min heating rate, and then to 280 C (2 min)
ꢀ
ꢁ1
with 10 C min heating rate; the mass spectra were operated
in EI mode at 70 eV. The mass spectra were obtained from m/z
5
0 to 650. The chromatographic peaks were identied according
30,31
to the NIST MS library and previously published work.
The
yield of product i (mass yield, Y ) was quantied by GC (Shi-
i
madzu 2014) with an AT.SE 54 capillary column and a FID
detector, and calculated as:
3
Fig. 1 NH -TPD desorption profiles of ZrP and SBA-15.
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Fig. 3 Product yields of benzyl phenyl ether without catalyst. Reaction
conditions: 0.2 g benzyl phenyl ether, 15 mL H O, 1 h.
2
ꢀ
ꢀ
ꢀ
obtained at 150 C and 180 C, respectively. At 200 C, the yields
of phenol and benzyl alcohol reached 4.07% and 2.74%, respec-
tively. Degradation at higher temperature cannot be investigated
in this system because of the working condition limitation.
3.3. Effect of catalyst species
ꢀ
Benzyl phenyl ether can be partially degraded at 200 C without
catalyst, but the product yields were low. Therefore, ZrP, SBA-15,
Pd/C and Ru/C were screened to promote the conversion. The
conversions of benzyl phenyl ether with different catalysts are
given in Table 1. The conversion with the treatment of ZrP was
Fig. 2 X-ray diffractogram patterns of ZrP, SBA-15(a) and Pd/C, 59.93%. While the conversion with SBA-15 was only 45.27%.
Ru/C(b).
The presence of metal catalysts resulted in higher conversions.
Conversions with Pd/C and Ru/C were 85.51% and 72.66%
respectively (Table 1).
3
2
the existence of silanols. The absolute peak area of the
NH -TPD desorption prole is considered to be linear with the all catalysts compared to that without catalyst. The main
amount of its acid sites. Calculated based on the peak area, products identied by GC MS were phenol, benzyl alcohol and
As listed in Table 1, higher product yields were obtained with
3
25
ꢁ1
ZrP possessed the acid sites of 0.2175 mmol g
.
toluene. Roberts et al. studied the products of benzyl phenyl
The X-ray diffractogram (XRD) patterns of ZrP, SBA-15, Pd/C ether under electric heating with LDI-TOF/MS, and reported
1
9
and Ru/C are shown in Fig. 2. For ZrP catalyst, two broad peaks that dimers and trimers could also be found in the products.
ꢀ
ꢀ
in 2q range of 15–40 and 40–70 indicated its amorphous The highest yields of phenol (13.29%) and benzyl alcohol
24,25
nature, which was consistent with the published work.
(13.62%) were obtained with ZrP. Only 8.59% yield of phenol
SBA-15 also exhibited amorphous structure with one strong and 5.33% yield of benzyl alcohol were produced over SBA-15.
ꢀ
ꢀ
broad peak in the range of 2q from 20 to 40 . For the two metal For the metal catalysts, 8.07% yield of phenol and 2.68% yield
catalysts supported on activated carbon, one board peak around of benzyl alcohol were obtained with the application of Ru/C.
ꢀ
2
4
could be observed and this peak corresponded to the Although no benzyl alcohol was produced over Pd/C, the yield
ꢀ
amorphous carbon. The Ru crystallite peak at 2q ¼ 44 and Pd of phenol reached the highest value of 26.11% (Table 1).
ꢀ
crystallite peak at 2q ¼ 39.34 were very weak, indicating that
both Ru and Pd particles were well dispersed on the support 3.4. Effect of hybrid catalysts
33,34
(
particle size less than 5 nm).
ZrP or SBA-15 was mixed with Ru/C or Pd/C to investigate the
performance of hybrid catalysts, and the conversions with
different hybrid catalysts are given in Table 2. Low conversion of
3
.2. Effect of temperature
Fig. 3 is the product yields of benzyl phenyl ether at different 47.87% and 50.02% were obtained over SBA-15–Ru/C and
temperatures without catalyst. The main products were phenol ZrP–Ru/C, respectively. The application of ZrP–Pd/C resulted in
and benzyl alcohol. Only 0.10% and 0.38% yield of phenol were the highest conversion of 85.7%.
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a
Table 1 Product yields and selectivities of benzyl phenyl ether with different catalysts
Toluene
Phenol
Benzyl alcohol
Entry
Catalyst
Conversion (%)
Yield (%)
Selectivity (%)
Yield (%)
Selectivity (%)
Yield (%)
Selectivity (%)
1
2
3
4
ZrP
59.93
45.27
85.51
72.66
0.00
0.00
2.75
0.00
0.00
0.00
3.18
0.00
13.29
8.59
26.11
8.07
22.18
18.98
30.18
11.10
13.62
5.33
0.00
2.68
22.73
11.77
0.00
SBA-15
Pd/C
Ru/C
3.69
a
ꢀ
2
Reaction conditions: 0.2 g benzyl phenyl ether, 0.1 g catalyst, 15 mL H O, 200 C.
The yields of phenol and benzyl alcohol over SBA-15–Ru/C the conversion increased, reaching 85.70% at 1 h, and 99.23%
were 7.8% and 0.95%. And the yields of these two products at 4 h.
over ZrP–Ru/C were 14.26% and 3.78%, respectively. The low
The products of benzyl phenyl ether degradation were
yields with Ru/C as the metal catalyst indicates that the phenol and toluene. At 0 h, 6.30% phenol was obtained. The
promotion of Ru/C is not as strong as Pd/C. The yield of phenol highest yield of phenol was obtained at 1 h as 41.55%. Aer
with the treatment of SBA-15–Pd/C was 28.55%. 2.11% yield of that, the yield decreased slightly to 41.38% at 2 h and 39.97% at
toluene was identied as well, while no benzyl alcohol was 4 h. Toluene reached the highest yield of 1.22% at 2 h. The
identied. When ZrP and Pd/C were used as the hybrid catalyst, results indicates that most of the benzyl phenyl ether has been
4
4
0.55% yield of phenol was obtained with the selectivity of consumed in 1 h with the treatment of ZrP–Pd/C. Longer reac-
7.95%. The yield of phenol over ZrP–Pd/C was higher than that tion time to 4 h resulted in the decreasing of product yields,
using ZrP (13.29%) or Pd/C (26.11%).
although the conversion is higher. This should be the result of
The hydrothermal stability of catalyst in aqueous phase repolymerization reaction.
condition is an important aspect. The X-ray diffractogram (XRD)
patterns of spent ZrP and spent Pd/C are shown in Fig. S1.† The 3.6. Chemical pathways for the formation of primary
pattern of spent ZrP showed no difference compared to that of products
fresh ZrP, showing that ZrP was stable in terms of the crystalline
According to He et al.'s report, the mole yields of phenol and
structure in aqueous condition. The pattern of Pd/C also showed
benzyl alcohol formed from the depolymerization of benzyl
a typical amorphous structure of activated carbon without sharp
phenyl ether catalyzed by Ni/HZSM-5 under electric heating
peak of Pd, indicating that the well dispersion of Pd particles
21
exhibited a stoichiometry balance. Therefore, they proposed a
hydrolysis reaction mechanism for the decomposition of benzyl
phenyl ether. However, no obvious stoichiometry balance was
found for phenol and benzyl alcohol in the present work. The
mole yield of phenol was generally higher than that of benzyl
alcohol. With Pd/C as the catalyst, benzyl alcohol was not iden-
tied in the liquid products. And the total yield of products
identied by GC was lower than the calculated conversion. One
possibility is that the products formed by hydrolysis reaction
reacted further to form some secondary products which could
remained in the aqueous condition. NH
3
-TPD desorption
revealed that the acid sites of ZrP decreased slightly to
ꢁ
1
0
.1575 mmol g aer the reaction. Liao et al. observed that
catalytic performance of ZrP decreased slightly for conversion of
24
cellulose aer the rst use. They claimed that this phenomenon
was attributed to P leaching under the hydrothermal reaction
condition, and P leaching only took place in the rst run.
3.5. Effect of reaction time
The effect of reaction time was investigated with ZrP–Pd/C not be identied by GC. To test the possibility, phenol and benzyl
ꢀ
catalyst at 200 C. The conversion of benzyl phenyl ether and alcohol which might be the possible primary products were used
the product yields at 0–4 h were shown in Fig. 4. At 0 h, 41.06% as the reactants to investigate if some further reaction existed in
conversion was observed. With the reaction time increasing, our reaction system. Product yields and selectivities of phenol
a
Table 2 Product yields and selectivities of benzyl phenyl ether with hybrid catalysts
Toluene
Phenol
Benzyl alcohol
Yield (%)
Entry
Catalyst
Conversion (%)
Yield (%)
Selectivity (%)
Yield (%)
Selectivity (%)
Selectivity (%)
1
2
3
4
SBA–15-Ru/C
SBA–15-Pd/C
ZrP–Ru/C
47.87
78.76
50.02
85.70
0.10
2.11
0.35
0.96
0.21
2.68
0.70
1.12
7.80
28.55
14.26
40.55
16.30
36.24
28.52
47.32
0.95
0.00
3.78
0.00
1.98
0.00
7.56
0.00
ZrP–Pd/C
a
ꢀ
Reaction conditions: 0.2 g benzyl phenyl ether, 0.1 g SBA-15 or ZrP catalyst and 0.1 g metal catalyst, 15 mL H
2
O, 200 C.
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unreacted amount of phenol was high as well (91.28–91.91%).
The results indicate that phenol is very stable even with the
catalyst present.
When benzyl alcohol was used as the reactant, 95.56%
benzyl alcohol remained aer 1 h with ZrP as the catalyst, and
4.50% phenol was formed. This shows that benzyl alcohol is
also relatively stable over ZrP. Treated with Pd/C or ZrP–Pd/C as
the catalyst, only 1.77% and 0.67% phenol was produced,
respectively. However, it is interesting that no unreacted benzyl
alcohol or any other products was identied, showing that
benzyl alcohol has all been consumed.
When phenol and benzyl alcohol were used as the reactants
and ZrP used as the catalyst, the amount of phenol increased
slightly, and the amount of benzyl alcohol decreased slightly to
96.29%. This is consistent with the result using phenol and
benzyl alcohol as the reactant respectively. With Pd/C or
ZrP–Pd/C as the catalyst, the amount of phenol increased
slightly, while the amount of benzyl alcohol decreased greatly to
12.24% and 4.17% respectively, which is also consistent with
the reaction using benzyl alcohol as the reactant. Therefore, it
could be proposed that catalyzed by Pd/C, benzyl alcohol
formed by the depolymerization of benzyl phenyl ether would
undergo some further reaction forming products which could
not be identied by GC, such as oligomers. If such repolyme-
rization could be inhibited, the total yield of monomeric
aromatics would be improved greatly.
3.7. Summary of the depolymerization mechanism of BPE
Various reaction mechanism has been proposed for the depoly-
merization of benzyl phenyl ether. One proposed mechanism was
that in supercritical water, the cleavage of a-O-4 bond was pro-
ceeded via parallel hydrolysis and pyrolysis pathways to form
Fig. 4 Product yield and selectivity of benzyl phenyl ether at different
reaction time. Reaction conditions: 0.2 g benzyl phenyl ether, 0.1 g ZrP
ꢀ
and 0.1 g Pd/C, 15 mL H
2
O, 200 C.
35
phenol and toluene. Siskin et al. proposed that the radical
and/or benzyl alcohol under microwave heating are shown in
Table 3.
With phenol as the reactant and ZrP as the catalyst, 97.20%
phenol was recovered, and no other products were detected by
GC. When Pd/C or ZrP–Pd/C were used as the catalyst, the
ꢀ
pathway dominated the reaction above 350 C in the aqueous
ꢀ
phase, while at the temperature lower than 250 C the ionic
36
chemistry was the main pathway. He et al. also proposed that the
C–O cleavage of benzyl phenyl ether proceeded along an ionic
ꢀ
21
pathway rather than a thermal/free-radical route at 250 C.
Because of the presence of H , only a slight amount of alkylation
2
products were found in their work. Roberts et al. proposed that in
the presence of alkali carbonate, the metal ion led to the forma-
tion of a cation-benzyl phenyl ether adduct, in which the ether
Table 3 Product yields and selectivity of phenol and/or benzyl alcohol
under microwave heating
19
Benzyl alcohol bond was polarized and prone to be heterolytically cleaved.
Reactant
Catalyst
Phenol (%)
(%)
Based on the previous results, possible catalyzed reaction
pathway of benzyl phenyl ether was shown in Fig. 5. It could be
proposed that in our case treated with ZrP, benzyl phenyl ether
will rstly be hydrolyzed forming phenol and benzyl alcohol.
Slight amount of benzyl alcohol will react further, resulting in
the relatively lower yield of benzyl alcohol than phenol.
a
a
Phenol
Phenol
Phenol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol + phenol
Benzyl alcohol + phenol
Benzyl alcohol + phenol
ZrP
Pd/C
ZrP–Pd/C
ZrP
Pd/C
ZrP–Pd/C
ZrP
Pd/C
97.2
0.00a
a
91.28a
0.00
a
91.91b
0.00b
4.50b
1.77
95.56b
0.00
b
b
0.67a
0.00b
For the circumstance of Pd/C, benzyl phenyl ether will be
depolymerized rst forming phenol and benzyl alcohol. One
possible pathway is that ether bond in benzyl phenyl ether will
be polarized by forming the metal-benzyl phenyl ether, and thus
101.83a
96.29b
109.53a
12.24b
ZrP–Pd/C
112.31
4.17
a
b
Calculated based on the mass amount of initial phenol. Calculated
based on the mass amount of initial benzyl alcohol. Reaction
19
was easily to be heterolytically cleaved. The benzyl alcohol will
conditions: 0.2 g benzyl phenyl ether, 0.1 g ZrP and 0.1 g Pd/C, 15 mL undergone further alkylation reaction forming undetected
ꢀ
H
2
O, 200 C.
oligomer products.
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9
J. Hu, D. K. Shen, S. L. Wu, H. Y. Zhang and R. Xiao, Energy
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1
1
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2
Fig. 5 Possible reaction pathway for the cleavage of benzyl phenyl
ether in presence of ZrP and Pd/C.
14 A. Toledano, L. Serrano, J. Labidi, A. Pineda, A. M. Balu and
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In the presence of ZrP–Pd/C mixture catalysts, both catalysts 15 A. Toledano, L. Serrano, A. Pineda, A. A. Romero, R. Luque
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2
2
2
2
2
2
2
2
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2
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. Conclusion
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The yield of phenol increased effectively in the presence of ZrP or
SBA-15 doped with metal catalyst (Pd/C or Ru/C), and 40.55%
phenol was produced over ZrP–Pd/C. As the main primary prod-
ucts, phenol was stable, while benzyl alcohol reacted easily
forming oligomers over Pd. It is proposed that under the catalysis
of ZrP–Pd/C hybrid catalyst, benzyl phenyl ether will rstly be
decomposed into phenol and benzyl alcohol, and benzyl alcohol
will then undergo repolymerization forming polymers.
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Acknowledgements
The authors greatly acknowledge the funding support from the
projects supported by National Basic Research Program of
China (973 Program) (Grant no. 2012CB215306 and
2011CB201505) and National Natural Science Foundation of
China (Grant no. 51476034, 51476035), the Major Research Plan
of the National Natural Science Foundation of China (Grant no.
91334205), and the Scientic Research Foundation of the
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