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Structural Characterization of Pre-hydrolysis Liquor Lignin and Its Comparison with Other Technical Lignins G. Yang1,2*, M. Sarwar Jahan1,3* and Y. Ni1 1
Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3; 2Key Laboratory of Pulp & Paper Science and Technology (Shandong Polytechnic University), Ministry of Education, Jinan, Shandong 250353, P.R. China; 3 Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dhaka-1205, Bangladesh Abstract: The kraft-based dissolving pulp production process can be a model for the forest biorefinery concept. The objective of this study is to characterize the dissolved lignin present in the pre-hydrolysis liquor (PHL) to facilitate its subsequent use following the forest biorefinery concept. In this work, lignin was isolated from PHL by acidification using dilute H2 SO4, followed by purification through dissolution in a dioxane solution (9:1) and re-precipitation with diethyl ether. The characteristics of PHL lignin were compared with those of the dioxane lignin, acetic acid (AA) lignin and ethanosolv (EL) lignin isolated from the same mixed hardwood (maple, poplar and birch wood chips, in a ratio of 7:2:1), which is the raw material used for dissolving pulp production. The obtained lignin samples were characterized by UV, FTIR, 1H-NMR spectroscopy, molecular weight determination, elemental and methoxyl analyses. The results showed that the absorptivity of dioxane lignin at 276 nm was 10.0 l g-1cm-1, while that of PHL lignin was 17.2 l g-1cm-1. The presence of condensed structures in the PHL lignin was also observed in the FTIR spectrum (strong bands at 870 and 890 cm1), which was also present in the AA and EL lignins. The lignin isolated from PHL had a lower molecular weight and methoxyl group per C9 unit, in comparison with other lignin samples. 1H-NMR analysis indicated a significant increase in the phenolic hydroxyl content in the PHL lignin, caused by cleavage of aryl-ether bonds during the prehydrolysis.
Keywords: PHL lignin, dissolving pulp, kraft pulping, characteristics, phenolic hydroxyl group, molecular weight, methoxyl group. 1. INTRODUCTION The development of biorefinery concept is aiming to find substitutes to the petroleum-based products and energy sources in the past years. The process profitability is also dependent on the conditions/ processes employed in separating the three main components: cellulose, hemicellulose, and lignin [1, 2]. In the production of dissolving pulp based on the kraft process, wood chips are pretreated with steam or hot water to remove hemicelluloses in a prehydrolysis stage [2]. In this process, a significant amounts of organics, such as hemicelluloses, lignin and acetic acid are dissolved in the prehydrolysis liquor (PHL). The recovery and potential use of these dissolved organics would fit well into the forest biorefinery concept for kraft-based dissolved pulp production [3, 4]. Lignin is a part of the dissolved organics in the PHL [5, 6] and its utilization as a raw material for many value-added products, e.g. phenols, biofuel, and plastics, can generate additional revenues to the pulp and paper industry [7]. The objective of this paper was to characterize the lignin present in the PHL so that its subsequent utilization in the production of value- added products will be facilitated. During the pre-hydrolysis, lignin depolymerization occurs, which may result in the cleavage of the lignin-carbohydrates bonds [8]. Also, re-polymerization/condensation can occur slowly because of the increased content of C–C condensed structures and the decrease in the content of -O-4 structures, as evidenced in one study on the characterization of lignin isolated after steam explosion of aspen wood [9, 10]. The main lignin -O-4 aryl ether *Address correspondence to these authors at the Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3; Tel: 1-506-453-4547; Fax 1-506-451-6860; E-mails:
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linkages are cleaved under acidic condition, with the release of the free phenolic hydroxyl groups [11, 12]. Early studies on methanol extracted lignin from steam-exploded aspen (Populus tremuloides) also indicated the cleavage of -O-4 aryl ether linkages, causing a decrease in the molecular weight and an increase in the phenolic content [13]. Results from beech-wood lignin also showed that molecular weight decreased with increasing temperature while condensed lignin structures began to be formed above 215 oC [14]. In the present paper, lignin was isolated from the pre-hydrolysis liquor (PHL) of a kraft-based dissolving pulp production process located in Eastern Canada. For comparison, dioxane lignin, acetic acid (AA) and ethanosolv (EL) processes lignin were isolated from the same mixed hardwood, as used in the kraft-based dissolving pulp production process. The isolated lignins were characterized by UV, FTIR and 1H-NMR spectroscopy, molecular weight and elemental analyses. 2. EXPERIMENTAL 2.1. Raw Materials Maple, poplar and birch wood chips, which were collected from a mill in Eastern Canada with a weight ratio of 7:2:1, was ground to 40/60 mesh in a Wiley mill, and ground meal was extracted with acetone for 24 h, followed by vacuum-dried over P2O5. The prehydrolysis liquor (PHL) was collected from the same mill. 2.2. Isolation of Lignin from Prehydrolysis Liquor (PHL) The collected PHL was acidified to pH 2, by using a dilute H2SO4 solution, and lignin was precipitated and separated by centrifugation at 5000 rpm. The precipitated lignin sample was thoroughly washed with distilled water by repeated centrifugation to neutrality. © 2013 Bentham Science Publishers
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2.3. Isolation of Lignin from Wood Meal
min-1. The samples were dissolved in THF and 10 μL were injected. Calibration was done using polystyrene standards in the molecular weight range of 96-2,560,000.
The same procedure applied in an early study [15] was followed. The acetone-extractive free wood meals were refluxed with an acidic dioxane (9:1) solution. HCl concentration in the dioxane solution was 0.2 N. The dioxane solution-to-wood meal ratio was 8. The wood meal was refluxed with a dioxane solution for about 1 hr, in N2 atm. Upon completion, the wood meal dioxane mixture was filtered on a Buchner funnel and the residue was washed with a dioxane solution (9:1), after which it was neutralized by adding solid Na2CO3 and filtered. The filtrate was concentrated in a vacuum evaporator at 50 oC, and a concentrated dioxane solution was added drop wise so that lignin could be precipitated. The lignin precipitate was washed and vacuum-dried over P2O5. 2.4. Ethanosolv Lignin (EL) Extraction 50g (od) of Maple, poplar and birch wood chips in the ratio of 7:2:1 was treated with aqueous ethanol (35/65 v/v) with 1.2 w/w% sulfuric acid as a catalyst at 190 °C for 60 min, following the method established by Pan et al. [16]. The solid to liquid ratio was 1:8. The pre-treatments were carried out in a 1.0 L glass-lined Parr reactor equipped with a 4842 temperature controller (Parr Instrument Company). The pre-treated wood chips were washed with warm (60 °C) ethanol/water (8:1). The pre-treated liquor and washes were combined, and 3 volumes of water were added so that Ethanosolv Lignin (EL) could be precipitated, which was separated by centrifugation, and then air dried. 2.5. Acetic acid (AA) Lignin Extraction The same mixed hardwood chips were treated with 90% aceticacid solutions that contained a small amount (0.2%v/v) of HCl. Experiments were performed at boiling temperature using a liquor/wood ratio of 4g/g for 2 hours [12]. Subsequently, they were cooled to room temperature. The spent liquor obtained by filtration was concentrated through a rotary vacuum evaporator at 60 C, and approximately 100 ml of the concentrated sample was obtained. Thereafter, 900 ml of deionized water was added to precipitate the dissolved lignin. After 24 hours, lignin was obtained by filtration followed by vacuum drying. 2.6. Lignin Purification The crude lignin isolated from wood meal and PHL was dissolved in dioxane (9:1), and again precipitated in ether, under constant stirring with a magnetic bar. The precipitated lignin was dried in vacuum over P2O5. 2.7. Acetylation The lignin samples obtained above (100 mg) were added separately to 2 mL of dry pyridine-acetic anhydride (1:1) and kept for 72 h. The solutions were added to a 10-fold volume of cold water, and the acetylated lignins were recovered as precipitates, subsequently purified by successive washing with water and dried under vacuum over P2O5. 2.8. Determination of Lignin Molecular Weight The weight average (Mw) and number average molecular (Mn) weight of the acetylated lignin was determined by GPC, on a Sodex KF-802.5 column with UV detection (280 nm) and differential refractive index (RI) detection. The analyses were carried out at 30 ° C using tetrahydrofuran (THF) as the eluent, at a flow rate of 1 mL
2.9. Elemental Analysis C, H, O and N analyses were carried out using a LECO CHNO analyzer. The methoxyl content in different lignin was determined in accordance with a Japanese International Standard Method (JIS P8013). In this method, methoxyl group react with HI acid at 135 °C and form methyl iodide, which is further react with bromine, and finally form HIO3 and subsequently titrated by 0.1 N Na2SO3. 2.10. Spectroscopy Ultraviolet: 7-8 mg dioxane lignin was dissolved in 100 mL dioxane (9:1), followed by two times dilution, and the spectra were recorded using a GENESYS 10S spectrophotometer. FTIR: IR spectra were recorded with a Shimadzu FTIR spectrometer model 8201PC. The dried samples were embedded in KBr pellets at concentrations of about 1mg/100 mg KBr. The spectra were recorded in the absorption band mode in the 4000-400 cm–1 range. 1 H NMR: The 1H NMR spectra were recorded for solutions of 10 mg acetylated lignin contained in 0.5 mL CDCl3, using the solvent as an internal standard. For quantification of protons, the signals of aromatic and aliphatic regions of the spectrum were integrated with respect to a spectrum-wide baseline drawn at the level of the background noise, and the results were referred to the signal for methoxyl protons, whose average number per C9 unit was established as described above. For example, from the empirical formula of PHL lignin (Table 1), the total OCH3 protons per C9 unit, calculated to be 1.04 x 3 = 3.12 equivalent to the integral for methoxyl area. 2.11. Thermal Gravimetrical Analysis (TGA) Thermal gravimetrical analysis (TGA) of about 5 mg of airdried lignins was performed using a TAQ500 Thermogravimetry (TG) analyzer at a heating rate of 10 oC/min under helium atmosphere. 3. RESULTS AND DISCUSSION 3.1. Lignin Recovery from PHL In opposition to black liquor, a typical industrially produced PHL has a pH of 3-4 [5]. Therefore, the method applied for recovering the organic materials of PHL for the purpose of producing byproducts is challenging and is recently the subject of intensive research [3, 5, 21, 22]. It has been well documented that the lignin removal/precipitation via acidification was significantly affected by the structure/functional groups and molecular weight of the dissolved lignin in the PHL [17-20]. There are contradictory results available in the literature on the efficiency of acidification on the lignin removal of the PHL. In one research, the acidification of industrially produced PHL (via steam hydrolysis) to pH 2 caused 50% lignin and 16% other carbohydrate removals [21]. However, the acidification (pH 2) of another industrially produced PHL (via steam hydrolysis) resulted in less than 10% lignin removal with marginal carbohydrate removals [22]. Another method proposed to recover the lignin of industrially produced PHL (via steam hydrolysis) is shown in (Fig. 1) [21]. As shown, stream 1 is for the possibility of using polymers (e.g., poly ethylene oxide (PEO), poly aluminum chloride (PAC)) to form
Structural Characterization of Pre-hydrolysis Liquor Lignin and Its Comparison with Other Technical Lignins
Current Organic Chemistry, 2013, Vol. 17, No. 15 3
Fig. (1). Proposed process flow diagram of lignin isolation from PHL [21].
complexes with lignin, which facilities the lignin flocculation and precipitation. The precipitates of PEO/PAC/lignin may be used as wet-end additives (retention aids) for papermaking. Stream 2 depicts the possibility for precipitating lignin via solvent extraction (e.g., ethyl acetate (EC) or ether), and the precipitated lignin can be further utilized in different processes for the production of various value-added products. The solvent should then be recovered from the PHL. In the case of polymer treatment of the PHL, about 20% of lignin was removed in one study via having 200 mg/l PEO (MW of 600k) concentration in the PHL solution (pH 1.5) [22]. The complex removed can be utilized as fuel source. In the case of solvent extraction, 64% of lignin was removed by adding the ethyl acetate to PHL (4/1 v/v). Although the efficiency of solvent extraction seems reasonable, its high recovery cost impairs the practical application of solvent extraction for removing lignin of the PHL. Preliminary results of PHL lignin showed that it can be substituted for phenolic resin [22]. The amount of lignin found in PHL was about 1%, which represented about 20% of the total solid content of PHL [5]. The lignin content of PHL depends on the condition of prehydrolysis process [5, 24]. Yoon et al. obtained 5.11% lignin (based on wood) in loblolly pine prehydrolyzed for 90 min at 190 °C [25]. The lignin content in the hot-water extract of sugar maple was 3.27 % (based on wood) [26]. Leschinsky et al. also reported that lignin was degraded and dissolved during the prehydrolysis process [27], and Liu et al. observed that lignin density increased on the external surface of wood chips upon pre-hydrolysis [28]. It is expected that lignin dissolved in the PHL would have a similar structure to native lignin (i.e. less chemical/structural changes in the hydrolysis process). Thus, hydrolysis lignin would be similar to organosolv lignin, but different from kraft or sulfite lignin. Masura observed that the Klason lignin content of prehydrolysate beech wood decreased in the first hour of prehydrolysis at 160-170 °C [28]. The amount of extractable lignin in auto-
hydrolyis was very high at a high temperature (e.g. 195-215 oC) [8], which was due to the cleavage of aryl-ether bonds and lignin depolymerization. 3.2. Elemental Analysis Based on the elemental analysis of the lignin samples listed in (Table 1), a possible empirical composition of PHL lignin was C9H9.14O3.68(OCH3)1.04 while that of dioxane lignin from the same mixed hardwood was C9H10.52O3.36(OCH3)1.20. This indicates that the lignin in PHL has a lower methoxyl content than the dioxane lignin, which resembles the native lignin and implies that demethylation occurred during pre-hydrolysis. Similar results were observed in aspen steam exploded lignin [13]. Leschinsky et al. also observed that the methoxyl content per C9 of the lignin precipitated from the prehydrolysate was lower than that in the MWL of Eucalyptus globules [27]. For comparison, empirical formula of ethanosolv (EL) lignin and acetic acid (AA) lignin was also determined and the analysis showed that the methoxyl content in EL and AA lignin were even lower than the PHL lignin. A lower methoxyl content in the EL lignin was also observed by others [31]. Zhao and Liu [31] also found that the OCH3 content in AA lignin are lower. The lower methoxyl contents in these lignin samples indicate that the lignins have more ortho positions of their phenyl rings unblocked by methoxyl groups and are therefore suitable for manufacturing lignin phenol–formaldehyde resins. 3.3. UV Spectroscopy The UV–Vis absorption measurements were performed using dioxane–water as solvent at the wavelength of 200–350 nm. All lignin samples exhibit the typical UV spectra of lignin, with two maxima originating from non-conjugated and conjugated phenolic groups (p-coumaric and ferulic acids) [32-34]. All lignin samples had a well-defined maximum at 276 nm, while the PHL lignin had a
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Table 1. Elemental Analyses, Methoxyl Contents and C 9 Formulas of the Lignin Samples
Lignin Sample
C
H
O
OCH3
C9 formula*
PHL lignin
57.8
5.77
35.5
15.2
C9H9.14O3.68(OCH3)1.04
Dioxane lignin
58.0
6.59
34.6
17.8
C9H10.52O3.36(OCH 3)1.20
AA lignin
56.24
5.29
38.47
14.4
C9H8.81O4.13(OCH3)0.99
EL lignin
60.58
5.85
33.57
15.3
C9H8.64O3.17(OCH3)0.97
*Empirical formula of CxHyOz(OCH3)n , were calculated as follows: n = (%OCH3)/31.04; x = (%C)/12 – n; y = (%H) - 3n; z = (%O)/16 – n
Wave Length at max. nm
150 Dioxane AA
100
PHL
50
EL
0 -1
3430 cm
4000 3500 3000 2500 2000 1500 1000 500
Table 2. UV Characteristics of Lignin Samples
Lignin
3430cm1 was assigned to the stretching of O–H groups, inclusive of R–OH and Ar–OH.
Absorbance (%)
wider peak at 276 nm, which may be due to the non-condensed phenolic groups in lignin. As is well known, the guaiacyl structures, especially those from softwood, exhibit maxima in the 280 region. However, the syringyl units (with an extra methoxyl group in the 5th position) shift the maxima to a lower wavelength [35], the results indicated that the lignin samples here were rich in syringyl units, as a mixed hardwood was the raw material. The absorptivity of dioxane lignin was 10.0 l g-1cm -1, while that of acetic acid, ethanosolv and PHL lignin was 18.2, 19.5 and 17.2 l g-1cm-1, respectively, as presented in (Table 2). The absorptivity of beech lignin obtained from steam explosion was 15.60 l g1cm1 at 277 nm [36]. The typical shoulder at 310-320 nm was absent in these lignin samples, indicating the absence of ester or ether bonds between hydroxycinnamic acids, such as p-coumaric acid and ferrulic acid of lignin. Some juvenile hardwood lignin and non-wood lignin show the presence of these bands [15, 37, 38].
-1 Wave number, cm
Absorptivity at max (L g-1 cm-1)
Dioxane lignin
276
10.0
PHL lignin
276
17.2
AA lignin
276
18.2
EL lignin
276
19.5
Fig. (2). FTIR spectra of PHL, AA, EL and dioxane lignins.
3.4. FTIR Spectroscopy The FTIR spectra of PHL, AA, EL and dioxane lignin samples were recorded, and shown in (Fig. 2). All lignin samples show strong band at 1720 cm-1 which is attributed to unconjugated C=O stretching. The relative ratio of band 1720 to 1518 cm -1 was the highest for EL lignin. The band at 1330 cm-1 is due to the presence of the phenolic OH group, whereas the band at 1030 cm-1 is assigned to a primary alcohol [39], which is weaker in PHL lignin than in AA, EL and dioxane lignin. The bands at 1330, 1220 and 1120 cm-1 correspond to the syringyl unit, whereas the small shoulders at 1275, 1153 and 1037 cm-1 are assigned to the guaiacyl unit of lignin molecules [40, 41], and again is much weaker in PHL lignin. The band at 1504 cm1 was shifted to 1515 cm-1 on AA, EL and PHL lignin, which is probably caused by the decreased amount of -O-4 linkages [8]. These results suggest that the prehydrolysis process cleaves the O-4 linkages of lignin. The 1594/1510, 1463/1510 and 1418/1510 ratios are lower in PHL lignin than in EL, AA and dioxane lignin, indicating that the syringyl-to-guaiacyl ratio decreased during the pre-hydrolysis, and suggesting that more syringyl structures were removed in the process. Similar results were observed in the lignin sample from the water pre-hydrolysis of Eucalyptus globules wood [8]. The presence of condensed structures in PHL lignin is indicated by the stronger bands at 870 and 890 cm1, which were also found in the AA and EL lignins [42]. The band at approximately
Fig. (3). Chromatogram of PHL, AA, EL and dioxane lignins.
3.5. Molecular Weight The molecular weight distribution of acetylated PHL, AA, EL and dioxane lignin samples was measured by GPC (Fig. 3), and weight average molecular weight (Mw), number average molecular weight (Mn) and polydispersity (Mw/Mn) are shown in (Table 3). AA lignin has a bimodal curve, which represents two lignin fractions with different amounts. The Mw of the lignin isolated from PHL was remarkably lower than that of the dioxane and AA lignin (about 3,000 vs. 6,500 and 6200), suggesting that a substantial lig-
Structural Characterization of Pre-hydrolysis Liquor Lignin and Its Comparison with Other Technical Lignins
nin degradation occurred during the prehydrolysis process. The molecular weight of PHL lignin depends on prehydrolysis intensity. Bentivenga et al. showed [36] that an extensive lignin degradation was occurred during the steam-explosion of beech wood at 188 °C for 3 min. Mitsuhiko showed that the molecular weights (Mw) of exploded lignin decreased with increasing steam pressure and time [43]. The lower polydispersity of AA, EL and PHL lignins compared to dioxane lignin indicates the higher fraction of low molecular weight. The lignin fractions with a low molecular weight are suitable for condensates with phenol formaldehyde because they are more reactive than those with a high molecular weight [44]. Table 3. Average Molecular Weight and Polydispersity of Dioxane, AA, EL and PHL Lignins
Sample
Polydispersity
Mw
Mn
Dioxane lignin
6521
1638
4.0
PHL lignin
2,975
794
3.7
AA lignin
6215
3515
1.8
2641
1790
1.5
(Mw/Mn)
3.6. 1H NMR The integrated NMR spectra obtained for acetylated PHL, AA, EL and dioxane lignins are shown in (Table 4), together with the signal assignments according to an early reference of Lundquist [45]. The significant increase in the phenolic hydroxyl content of PHL lignin indicates an extensive cleavage of the aryl-ether bonds during the prehydrolysis process. The aliphatic hydroxyl content in PHL lignin is much lower than that of dioxane, AA and EL lignins, which may be due to the elimination of the primary and secondary aliphatic hydroxyls, which is a result of the aryl-ether cleavage mechanism. These results are in agreement with those from the prehydrolysis of eucalyptus globules [8]. Li and Lundquist reported that the hydrolysis under slightly acidic conditions can bring about the loss of aliphatic OH groups through homolytic cleavage of the C-O bond [46]. According to the 1H NMR results (Table 4), the aromatic proton was much lower in AA and EL lignins than in the dioxane and PHL lignins, indicating that the former would have more condensation than the latter. The methoxyl content in PHL, AA and EL lignins was lower than that in dioxane lignin, supporting the conclusion that the demethylation of aromatic methoxyl groups in the
lignin structure occurred during prehydrolysis. Similar re-
Current Organic Chemistry, 2013, Vol. 17, No. 15 5
sults were obtained for lignin obtained from the steam explosion of lignocellusic biomass [47]. 3.7. Thermo Gravimetric Analysis The TGA results illustrated in (Fig. 4) show that the weight loss of all lignins occurred in two stages. In the first stage (30-150 °C), weight loss was only less than 10% in all lignins and mainly attributed to the moisture loss. In the second stage (200-500 °C), pyrolysis of lignin mainly happened. In this stage, interunit linkages break up and monomeric phenols evaporate and cause the mass loss at the given temperatures. Above 400 °C the disintegration of the aromatic rings of lignin molecules starts [48]. The PHL lignin was more thermally stable than the dioxane lignin, AA and EL lignins over the 200 - 450 °C temperature range. Dioxane lignin had nearly 40% of weight loss at temperatures below 335 °C, the PHL lignin had a weight loss of about 30% at 550 °C. The degradation temperature of AA and EL lignins was 365 and 354 °C, respectively. The degradation of dioxane lignin started at a lower temperature than that of PHL lignin did, suggesting that the degradation rate of dioxane lignin was faster than that of PHL lignin under the conditions studied. A higher thermal stability of PHL lignin indicated its suitability in polymer processing.
Fig. (4). Thermal degradation of PHL, AA, EL and dioxane lignin.
4. CONCLUSIONS The present study demonstrated that the prehydrolysis step of kraft-based dissolving pulp production removes part of the lignin and alters its structure. Lignin demethylation is supported by a
Table 4. Assignments of Signals and Protons per C 9 Structural unit in the 1H NMR Spectra of Acetylated Lignins
Range (ppm)
7.25-6.80 6.80-6.25 6.25-5.75 5.75-5.24 5.20-4.90 4.90-4.30 4.30-4.00 4.00-3.48 2.50-2.22 2.22-1.60
Main Assignments
PHL lignin
Dioxane lignin
Aromatic proton in syringyl units
0.58
0.64
H of -O-4 and -5 structures
0.52
0.75
H of -5 structure
0.26
0.41
H of xylan residue
0.29
0.31
H & H of -O-4 structures
0.20
0.19
H of - structures
1.16
1.09
H of xylan residue
3.12
3.60
H of methoxyl groups
1.16
0.51
H of aromatic acetates
1.88
2.55
AA lignin
El lignin
0.28
0.29
0.17
0.28
0.11
0.17
0.12
0.11
0.92
0.94
2.97
2.91
1.12
1.13
1.53
1.74
Aromatic proton in guaiacyl units
H of aliphatic acetates
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lower methoxyl content of the prehydrolysis liquor (PHL) lignin than the wood lignin. In addition, PHL lignin has a low molecular weight compared to dioxane and AA lignins. The main interunit O-4 aryl ether linkages are cleaved, resulting in an increased free phenolic hydroxyl group content, which may facilitate further utilization of PHL lignin in other products, such as phenolic resin. The PHL lignin is more thermally stable than the dioxane, AA and EL lignins. The characteristics of PHL lignin permit the conclusion that PHL lignin would be a suitable raw material for producing biobased polymers such as polyurethanes, phenolic resin etc.
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CONFLICT OF INTEREST
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The authors confirm that this article content has no conflicts of interest.
[19]
[20]
[21]
[22]
[24]
ACKNOWLEDGEMENTS The authors are grateful for the financial support from the National Science Foundation of China (Grant No.31070525), and the Prior Special Study of 973 Program (Grant No.2011CB211705). This project was also funded by an NSERC CRD grant.
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Received: ??????????, 2013
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Revised: ??????????, 2013
Accepted: ??????????, 2013
