Article pubs.acs.org/EF
Upgrading of Bio-oil over Bifunctional Catalysts in Supercritical Monoalcohols Jixiang Zhang, Zhongyang Luo,* Qi Dang, Jun Wang, and Wen Chen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *
ABSTRACT: In this paper, bio-oil from fast pyrolysis of Pinus sylvestris L. was upgraded over supported noble metal catalysts in supercritical monoalcohols under a hydrogen atmosphere. Esterification, cracking (both alcoholysis and hydrolysis), hydrogenation, along with acetalization, isomerization, and other reactions were combined during the upgrading process. The product analysis showed that processing in ethanol over Pt/SO42−/ZrO2/SBA-15 had a good upgrading performance. The removal of acids and aldehydes and the decrease of ketones, phenols, sugars, and polycyclic aromatic hydrocarbons were achieved. Meanwhile, esters became dominant in upgraded oil. The effects of solvents, noble metal catalysts, and catalyst supports had been briefly discussed. Pretreatment tests suggested that the presented upgrading process can be applied to the whole bio-oil without fractionation. As a result, an effective solvent recovery and a post-water-removal process will be required for the application of upgraded oil.
1. INTRODUCTION Because lignocellulosic liquid biofuels are clean, renewable, carbon-neutral, and based on the world’s most abundant resources, they might replace gasoline and diesel fuels as the main global transportation fuel in the future. Lignocellulosic biomass can be converted into a dense liquid through pyrolysis or liquefaction. The main advantage of pyrolysis is that it can be applied to all kinds of biomass feedstocks with low capital investments.1 However, the main product, bio-oil, cannot be directly used as transportation fuel because of its undesirable properties, such as high oxygen content, high acidity, high viscosity, low heating value, thermal instability, etc. An upgrading process is required for its practical application. The hydrodeoxygenation (HDO) process is the most widely researched upgrading method. Complicated equipment (usually a dual reactor), high-pressure hydrogen, and metal catalysts combined with superior techniques required in the HDO process result in excess costs. Therefore, the catalytic cracking process, which is performed at atmosphere without hydrogen requirement, draws great interests. However, it does not seem promising because of the high coking and poor quality of upgraded oil.2 Supercritical fluid (SCF) presents a gas-like flow and transfer property (low viscosity and high diffusivity) as well as highly tunable solvent properties (solvent power and polarity). It forms a homogeneous reaction environment using SCF as a reaction medium. Therefore, SCF offers several advantages for both reaction and downstream processing in terms of product purification and/or catalyst recycle.3 Supercritical hydrocarbons have been used as effective hydrotreating reaction media for upgrading of heavy oil and bitumen.4,5 For biofuel production, methanol has been widely researched in supercritical transesterification of vegetable oil to biodiesel, which resulted in elimination of catalyst use, dramatically reduced processing time, and simplified feedstock pretreatment and product purification.6,7 Ethanol has also been tested in sub- and supercritical liquefaction © 2012 American Chemical Society
of lignocellulosic biomass. It has been reported that ethanol not only acted as a solvent but also reacted with lignocellulose and became part of the product.8,9 From the above, upgrading in supercritical methanol/ethanol can be a possible way to produce high-quality liquid fuels from bio-oil. Upgrading of bio-oil from pyrolysis of rice husk in supercritical ethanol has been carried out with and without hydrogen.10−12 Lou et al. separated bio-oil into low- and highboiling fractions through reduced pressure distillation (∼0.009 MPa) at 110 °C, and then, upgrading of both fractions in supercritical methanol was applied.13,14 In this paper, bio-oil from fast pyrolysis of Pinus sylvestris L. was upgraded over supported Pt and Pd catalysts in supercritical methanol/ethanol under a hydrogen atmosphere. HZSM-5, an acidic zeolite commonly used in the petroleum process, and SO42−/ZrO2/SBA-15, a modified acidic mesoporous material, were used as catalyst supports. The performance of the upgrading process was evaluated, and the effects of solvents, noble metal catalysts, and catalyst supports have been briefly discussed on the basis of product analysis. The necessity of pretreatment to remove water before upgrading has also been discussed.
2. EXPERIMENTAL SECTION 2.1. Materials. The bio-oil obtained from the pyrolysis of P. sylvestris L. was provided by the State Key Laboratory of Clean Energy Utilization, Zhejiang University (Hangzhou, China). The chemical materials, including high-purity nitrogen and hydrogen, anhydrous methanol and ethanol, sulfuric acid, zirconium nitrate pentahydrate [Zr(NO3)4·5H2O], cetyltrimethylammonium bromide (CTAB), chloroplatinic acid (H2PtCl6·6H2O), palladium(II) nitrate [Pd(NO3)2·2H2O], HZSM-5 (Si/Al = 25), and SBA-15, were all commercially available. All chemicals were used as received without further purification. Received: December 9, 2011 Revised: March 21, 2012 Published: March 22, 2012 2990
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2.2. Catalyst Preparation. SO42−/ZrO2/SBA-15 (designated as SZr) was prepared via a two-step wetness impregnation method.15 The 5 wt % Pt catalysts and 5 wt % Pd catalysts were prepared by incipient wetness impregnation of catalyst supports (SZr and HZSM-5) with aqueous solution of H2PtCl6·6H2O and Pd(NO3)2·2H2O, respectively, followed by drying at 110 °C overnight, calcination at 500 °C in air for 2 h, and reduction (500 °C for Pt catalysts and 280 °C for Pd catalysts) in flowing hydrogen for 3 h. 2.3. Catalyst Characterization. Nitrogen adsorption−desorption isotherms were measured by a Micromeritics TRISTAR 3020 system. The surface area of the catalyst was calculated according to the Brunauer−Emmett−Teller (BET) equation in the range of relative pressure (P/P0) between 0.0 and 0.2. The total pore volume was determined from the adsorption and desorption branches of the nitrogen isotherms at P/P0 = 0.97. The X-ray diffraction (XRD) analysis was conducted on an X’Pert PRO X-ray diffractometer using Cu Kα radiation over 2θ ranges from 10° to 70°. 2.4. Experimental Procedures. First, reduced pressure (∼0.009 MPa) distillation at 55 °C was applied to the bio-oil as pretreatment to remove water. Crude bio-oil (CB) was fractionated into distillate (DL) and distillation residue (DR). For the upgrading process, 5 g of DR was mixed with 50 g of solvent (methanol or ethanol) as feedstocks. Then, 55 g of feedstocks and 0.3 g of catalysts (Pt/SZr, Pd/SZr, Pt/ HZSM-5, and Pd/HZSM-5) were added in a 100 mL stainless-steel autoclave. For pretreatment tests: 10 g of DR/CB was mixed with 100 g of ethanol as feedstocks. Then, 110 g of feedstocks and 0.6 g of Pt/SZr catalysts were added in a 300 mL stainless-steel autoclave. After the autoclave was flushed 5 times with nitrogen and 10 times with hydrogen to remove the air inside, 2.0 MPa hydrogen was injected into the autoclave at room temperature. The autoclave was sealed and heated to 260 °C, and the reaction was held for 3 h at 260 °C with stirring at 500 rpm. The pressure of the system ranged from 7.5 to 11.5 MPa. A schematic diagram of the experiment is shown in Figure 1. Finally, the autoclave was cooled to room temperature, and
the solid mixture was carried out using a TGA/SDTA 851 thermogravimetric analyzer. The weight of solid products was calculated from the weight of catalysts added and the weight loss of the mixture after TG/DTA. The samples were heated over the range of 25−650 °C at a rate of 10.0 °C/min under oxygen, and the gas flow rate was 50 mL/min.
3. RESULTS AND DISCUSSION 3.1. Catalyst Properties. Figure 2 illustrates the nitrogen adsorption−desorption isotherms of the catalysts and catalyst
Figure 2. N2 absorption−desorption isotherms of the samples: SZr, Pt/SZr, Pd/SZr, HZSM-5, and Pd/HZSM-5.
supports. The isotherms of SZr illustrated a type-IV isotherm with a clear H1-type hysteresis loop in the P/P0 range of 0.6− 0.8, indicating that SZr had uniform mesoporous channels. Meanwhile, the isotherms of HZSM-5 illustrated a type-I isotherm with clear H4-type hysteresis loop in the P/P0 range of 0.4−1.0, indicating that HZSM-5 had slit-like pores but the pore size distribution was mainly in the micropore range. Supported Pd and Pt catalysts had similar isotherms as the catalyst supports, suggesting that the ordered structure was retained after the impregnation process. The textural properties of the catalysts and catalyst supports calculated from the nitrogen adsorption− desorption isotherms are presented in Table 1.
Figure 1. Schematic diagram of the upgrading experiment.
Table 1. Textural Properties of the Samples: SZr, Pt/SZr, Pd/SZr, HZSM-5, and Pd/HZSM-5
the gas product was discharged without analysis. The liquid and solid products (along with the used catalysts) were separated using vacuum filtration. 2.5. Product Analysis. The liquid products were analyzed by gas chromatography−mass spectrometry (GC−MS, Voyager) with a DBWAX column (30 m × 0.25 mm × 0.25 μm, Thermo Scientific). The injector temperature was 280 °C in split mode, and nitrogen was the carrier gas. The GC−MS operating conditions were as follows: the oven temperature was 40 °C for 3 min, heated at 4.0 °C/min to 180 °C, then heated at 10 °C/min to 280 °C, and held at this temperature for 10 min. Compounds were identified by means of the National Institute of Standards and Technology (NIST) library. In this paper, the relative content was determined by area normalization. Components with the degree of certainty of the matches below 40% were labeled with an asterisk. The solvent and water contents were determined with an external standard method and a Karl Fischer titrimetric method, respectively, and then the heating value of the solvent and water could be deduced from the bomb calorimeter measurement result. The used catalysts were mixed with the solid products after filtration. Themogravimetric analysis/differential thermal analysis (TG/DTA) of
properties BET surface area (m2/g) pore volume (m3/g)
SZr
Pt/SZr
Pd/SZr
HZSM-5
Pd/HZSM-5
436
385
329
357
312
0.749
0.667
0.474
0.182
0.156
The XRD patterns of the catalysts and catalyst supports are shown in Figure 3. Three diffraction peaks at 2θ = 39.7°, 46.2°, and 67.4°, which were clear in Pt/SZr and Pt/HZSM-5, were attributed to the tetragonal Pt crystallite. Three diffraction peaks at 2θ = 40.1°, 46.7°, and 68.1°, which were low and broad in Pd/SZr, were attributed to the tetragonal Pd crystallite. No peak of Pd crystallite was detected in Pd/HZSM-5. The disappearance of XRD reflection indicated good dispersion of Pd on the HZSM-5 surface. Four peaks attributed to tetragonal ZrO2 crystallite (2θ = 30.3°, 35.1°, 50.4°, and 60.3°) cannot be found in Pt/SZr but can be found in Pd/SZr and SZr, indicating that ZrO2 was presented in a highly dispersed form 2991
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in Pt/SZr because of the loading of Pt and also implying an interaction between Pt and ZrO2. 3.2. Upgrading Performance and Influencing Factors. CB was a dark brown liquid with a smoky odor. After reduced pressure distillation, DL was a colorless and clear liquid and DR was nearly black and had almost no fluidity at room temperature. The heating values of CB and DR were 20.8 and 22.3 MJ/kg, respectively. The organic composition and relative abundance of different fractions from reduced pressure distillation of bio-oil are listed in Table 2. The typical compositions of CB and DR were acids, aldehydes, alcohols, esters, ketones, phenols, sugars, polycyclic aromatic hydrocarbons (PAHs), and other oxygenated compounds. A large amount of organic acids results in a low pH value and high corrosiveness of bio-oil, while the thermal instability is mainly attributed to aldehydes and phenols. These undesired components should be removed or converted to stable and combustible components during the upgrading process. 3.2.1. Comparison of Distillation Residue and Upgraded Oil. Upgraded oil from distillation residue (U-DR) from 100 mL batch reaction over Pt/SZr in supercritical ethanol was compared to DR to evaluate the performance of the upgrading process.
Figure 3. XRD patterns of the samples: SZr, Pt/SZr, Pd/SZr, HZSM5, Pt/HZSM-5, and Pd/HZSM-5.
Table 2. Organic Composition and Relative Abundance of Upgrade Oil and Different Fractions from Reduced Pressure Distillation of Bio-oil relative abundance (%)
relative abundance (%) CBa
DLb
16.66 2.05 1.39
0.32 35.24 1.58 2.48 0.47
compound name
DRc
U-DRd
compound name
Acids D-(+)-glyceric
acid*
acetic acid formic acid* propanoic acid butanoic acid
Aldehydes acetaldehyde, hydroxy4.38 furfural 3.45 vanillin 2.40 benzaldehyde, 4-hydroxy-3,51.91 dimethoxy4-hydroxy-2-methoxycinnamaldehyde 1.43 Alcohols methyl alcohol 1.83 1,2-ethanediol 3.30 Esters formic acid, ethyl ester ethyl acetate butanoic acid, ethyl ester propanoic acid, 2-hydroxy-, ethyl ester acetic acid, methyl ester 1.31 butanoic acid, 2-hydroxy-, ethyl ester acetic acid, hydroxy-, ethyl ester 2-hydroxy-γ-butyrolactone* pentanoic acid, 4-oxo-, ethyl ester diethyl methylsuccinate* butanedioic acid, diethyl ester Ketones 2-butanone, 3-hydroxy2-propanone, 1-hydroxy13.91 2-cyclopenten-1-one 2-cyclopenten-1-one, 2-methyl-
0.79 7.20
Ketones 1-hydroxy-2-butanone 2(5H)-furanone 2.12 2-cyclopenten-1-one, 2-hydroxy-31.92 methyl5-hydroxymethyldihydrofuran-2-one* 1.39 Phenols phenol, 2-methoxy3.89 phenol, 2-methoxy-4-methyl3.05 phenol* 10.55 phenol, 4-methyl-* 5.54 phenol, 2-methoxy-3-(2-propenyl)-* 2.01 phenol, 2-methoxy-4-(1-propenyl)-* 1.36 phenol, 2,6-dimethoxy3.21 2,5-dimethoxybenzyl alcohol* 2.94 5-(diethylamino)-2-nitrosophenol* 1,2-benzenediol 1.71 Sugars 1,6-anhydro-β-D-glucopyranose* 3.73 PAHs naphthalene* 2.58 anthracene* Others methane, diethoxy2-ethoxytetrahydrofuran 1-propanol, 2-ethoxy-* propane, 1,1,3,3-tetraethoxy-* 2-furaldehyde diethyl acetal 1,4-dioxaspiro[4.5]decane-7methanol* heptaethylene glycol monododecyl ether* (3-ethylthio-5-isopropyl-4-methoxy2,4,6-cycloheptatrienylidene) malononitrile*
10.15
9.71 1.77 2.81 1.67
15.49 2.99
2.72 4.19 33.66 1.27 6.35
0.49
2.55 1.52 11.17 2.06 1.37 1.72 2.51
0.47 22.63 1.39 0.45
12.48
CBa
1.56 1.95
DLb
DRc
U-DRd
1.27 1.97 1.71
1.74 0.67 2.69 0.53
0.33
3.33 3.35 10.92 5.72 1.90
2.92 1.60 4.22 1.38
3.23 3.27 2.01
1.56 1.62
3.27
1.57
2.52 3.40
1.69
5.52 2.36 1.66 2.32 0.34 1.60 1.19
1.61
2.24
5.59
a
CB = crude bio-oil. bDL = distillate. cDR = distillation residue. dU-DR = upgraded oil from distillation residue from 100 mL batch reaction over Pt/SZr in supercritical ethanol. 2992
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Table 3. Analyses of Upgraded Products from the Distillation Residue from 100 mL Batch Reaction under Different Processing Conditions solvents
methanol
catalyst supports noble metal catalysts
liquid products (area %)
solid products (wt %)
ethanol
SZr acids aldehydes alcohols esters ketones phenols sugars PAHs others weight loss
HZSM-5
SZr
HZSM-5
Pt
Pd
Pt
Pd
Pt
Pd
Pt
Pd
0 0 1.78 33.13 2.20 30.96 2.80 0 29.13 72.7
0 0 1.82 19.44 5.11 43.72 0 0 29.92 67.9
0 3.64 1.33 23.39 3.95 15.94 2.56 0 49.19 96.4
0 2.15 1.88 33.52 6.76 27.27 2.68 1.90 23.84 91.9
0 0 2.72 63.76 3.50 13.30 1.57 1.69 13.46 66.0
0 0 1.93 52.79 3.52 11.15 0 0 30.61 62.6
0 2.56 1.82 60.37 0 21.54 0 0 13.70 75.1
0 4.45 1.41 53.41 3.72 13.49 1.90 0 21.62 45.7
parison for the total percentage peak area could not give accurate information, but we could still draw some conclusions. Methanol has higher activity according to the molecular structure, but ethanol with a longer alkyl chain could dissolve higher molecular-weight products, which led to less solid products. Furthermore, more kinds of esters were detected in the upgraded oil when processing in ethanol. Pd with more dissolved active hydrogen had a higher hydrogenation activity for a large molecular-weight compound with a complex space structure and, hence, inhibited the unstable polymers to form solid products. The removal of acids and aldehydes were achieved using SZr as the catalyst supports, while vanillin remained unconverted using HZSM-5. Meanwhile, a small amount of acetaldehyde may originate from the decomposition of furfural. The conversion of PAHs was not clear. No product from the decomposition of PAHs was detected in the upgraded products. However, processing over HZSM-5-supported catalysts resulted in less high-molecular-weight components with aromatic groups. More experiments based on model compound reactions may give approaches to explore the specific effects of the solvents, noble metal catalysts, and catalyst supports. 3.3. Pretreatment Tests. The water contents of CB, UDR, and upgraded oil from crude bio-oil (U-CB) were 31.3, 25.2, and 28.7%. Therefore, the upgraded oil was still high in water content. The solvent recovery and post-water-removal process will be necessary for the application of upgraded oil. Because the water content of DR was 1.68% (DR had almost no fluidity at room temperature), it can be concluded that the water content in the upgraded oil was mainly from the upgrading reactions during processes. Because a large amount of hydrophilic groups contained in both CB and upgraded oil resulted in the difficulty of water removal, we proposed that the upgrading process could be applied to the whole bio-oil without fractionation, followed by the solvent recovery and post-water-removal process. In this section, the necessity of pretreatment was discussed. DL and DR accounted for approximately 30 and 70 wt % CB, respectively. As seen from Table 2, a large amount of acids, alcohols, and ketones and a small fraction of aldehydes and phenols were distilled from CB into DL. Additionally, esterification, decomposition of sugars, and other reactions might occur during reduced pressure distillation, which contributed to the changes of the relative abundance as well. The major components of DL were acetic acid (35.24%), 2-propanone, 1-hydroxy- (22.63%), and methanol (15.49%). During the upgrading process, acetic acid was converted to the
The organic composition and relative abundance of U-DR are also listed in Table 2. Acids and aldehydes, which were abundant in DR (the total percentage of peak area was 10.15 and 15.96%, respectively), were not detected in U-DR. The amounts of ketones, phenols, and PAHs were also decreased dramatically (from 16.17, 33.73, and 5.92% to 3.50, 13.30, and 1.69%, respectively). The removal of acids and aldehydes and the decrease of phenols would decrease corrosiveness and increase stability of bio-oil. Meanwhile, esterification led to the formation of esters, which became dominant in U-DR (63.76%). Ethanol acted as both a reaction medium and reactant in the upgrading process. The amounts of sugars were also decreased (from 3.27 to 1.57%). The heating value of U-DR was elevated to 27.4 MJ/kg. These results showed that the upgrading process performed effectively. Supercritical ethanol medium, bifunctional catalyst, and highpressure hydrogen atmosphere facilitated the upgrading process. Esters were produced through esterification with corresponding acids. Some of the acids already existed in the feedstock; others might be the intermediate products from the conversion of certain oxygenated compounds during processes.13,16−18 Several oxygenated compounds with ether bonds were detected in U-DR, which were mainly formed through etherification and acetalization.10 Another achievement was that the unsaturated double bonds at the substituted group of phenols were not detected in U-DR. However, some components may also undergo polymerization to form solid products. Because bio-oil was a very complex mixture, the detailed reaction pathways in the upgrading process cannot be easily demonstrated. 3.2.2. Effect of Solvents, Noble Metal Catalysts, and Catalyst Supports. Analyses of upgraded products from the distillation residue from 100 mL batch reaction under different processing conditions are summarized in Table 3. The organic components were classified, and their peak areas were added together (detailed GC−MS results are shown in Tables S1 and S2 of the Supporting Information). The solid product content was calculated as the weight loss of the solid mixture after TG/DTA. Note that the detailed components in products were different; for example, ethyl esters were dominant when processing in ethanol, while methyl esters were in high content when using methanol as the solvent. The GC−MS technique could not give the quantitative analysis because the response factor is different for each compound. However, the percentage peak area is linear with the concentration. As a result, a com2993
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the DTA curve. The coke content from upgrading of CB was even less, which further indicated that the water content was not the most important factor of catalyst deactivation during the upgrading process. The heating value of U-DR and U-CB were 25.5 and 29.2 MJ/kg, respectively. The use of whole bio-oil resulted in a higher heating value of the upgraded oil. The recovered catalysts from 300 mL batch reaction were calcined at 550 °C in air for 2 h. The comparison of fresh and recovered Pt/SZr catalysts is presented in the Supporting Information. The peaks of the tetragonal Pt crystallite remained. However, the intensity was weakened, and the peaks were widened, indicating that the size of the Pt crystallite decreased after the reaction. The BET surface area and pore volume were almost the same. The uniform mesoporous channels of SBA-15 retained after the upgrading process, indicating that the SBA-15 catalyst support could withstand the supercritical monoalcohol environment. On the basis of product analysis and consideration of the difficulty in water removal, it was concluded that the presented upgrading process could be applied to the whole bio-oil without pretreatment of water removal. However, the effective solvent recovery and post-water-removal process with low energy consumption will be required for the application of upgraded oil.
Figure 4. Organic composition and relative abundance of U-CB and U-DR from 300 mL batch reaction over Pt/SZr in supercritical ethanol.
corresponding ester. The major products of 2-propanone, 1hydroxy-, were esters, ketones, alcohols, and furans according to model reaction results.13 Methanol took place in a variety of reactions and became part of the products. Most of the products from these three major components were desirable for bio-oil upgrading. Figure 4 illustrates the organic composition and relative abundance of U-DR and U-CB. No acids were detected, and the products had similar organic composition and relative abundance, which indicated that the water content had no evident negative impact on the upgrading process. It can be explained because alcohols were extremely excessive in the upgrading process, so that the influence of the water content was negligible. The experimental data of TG/DTA are presented in Figure 5. The solid product contents (weight loss of
4. CONCLUSION In this study, upgrading of bio-oil was carried out over supported noble metal catalysts in supercritical methanol/ ethanol under a hydrogen atmosphere. The product analysis showed that the upgrading process performed effectively over Pt/SO42−/ZrO2/SBA-15 in supercritical ethanol. The removal of acids and aldehydes and the decrease of ketones, phenols, sugars, and PAHs were achieved. Meanwhile, esters became dominant in upgraded oil. Because bio-oil is a very complex mixture, more experiments based on model compound reactions may give approaches to explore the reaction pathways and the effects of the solvents, noble metal catalysts, and catalyst supports. Pretreatment tests indicated that water had no evident negative impact on the upgrading process. In consideration of the difficulty in water removal, we suggested that the presented upgrading process can be applied to the whole bio-oil without pretreatment of water removal. However, the effective solvent recovery and post-water-removal process with low energy consumption will be required for the application of upgraded oil. More studies are underway to optimize the reaction conditions with less solvent consumption and higher energy efficiency.
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ASSOCIATED CONTENT
S Supporting Information *
Calculating method for heating value calculation (S1), detailed GC−MS results of U-DR from 100 mL batch reaction under different processing conditions (S2), comparison of fresh and recovered Pt/SZr catalyst analyses using XRD patterns and N2 absorption−desorption isotherms after 300 mL batch reaction and calcination (S3), elemental composition of CB, DR, U-CB, and U-DR (S4), and mass balance of 300 mL batch reaction (S5). This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 5. TG/DTA plots of the solid mixture from 300 mL batch reaction over Pt/SZr in supercritical ethanol.
the mixture) were about 60%. At this high level of the solid product content, the solid products could be considered as coke of thermal origin, formed by condensation−degradation of certain oxygenated compounds. According to the literature, this coke deposited on the meso- and macroporous structure of the catalyst matrix or even on the surface area of the catalysts.19 Consequently, its combustion is less limited, which is consistent with the exothermic peak at a relatively low temperature of about 225 °C in
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-0571-87952440. Fax: +86-0571-87951616. E-mail:
[email protected]. 2994
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from The National Science and Technology Supporting Plan through Contract 2011 BAD22B07 is greatly acknowledged.
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REFERENCES
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