Structural Modifications of Sugarcane Bagasse Lignins during Wet

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Structural Modifications of Sugarcane Bagasse Lignins during Wet-storage and Soda-oxygen Pulping Fengxia Yue, Wu Lan, Songnan Hu, Ke-Li Chen, and Fachuang Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00726 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 26, 2016

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ACS Sustainable Chemistry & Engineering

Structural Modifications of Sugarcane Bagasse Lignins during Wet-storage and Soda-oxygen Pulping Fengxia Yue,1,2,3 Wu Lan,3 Songnan Hu,1 Ke-Li Chen,1,* and Fachuang Lu2,3,* 1

Faculty of Chemical Engineering, Kunming University of Science and Technology

(Chenggong Campus), 727 Jingmingnan Road, Kunming, 650500, China; 2

State Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, 381 Wushan Rd, Tianhe District, Guangzhou, 510640, China 3

Department of Biochemistry, Department of Biological System Engineering, and The

DOE Great Lakes Bioenergy Research Center, The Wisconsin Energy Institute, University of Wisconsin-Madison, 1552 University Ave., Madison, WI 53726, US *Corresponding authors: Prof. Ke-Li Chen (K.-L. Chen) Faculty of Chemical Engineering, Kunming University of Science and Technology (Chenggong Campus), 727 Jingmingnan Road, Kunming, 650500, China; E-mail: [email protected]; Tel: +86-871-659-20329; +86-871-659-20329.

Prof. Fachuang Lu (F. Lu) State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 381 Wushan Rd, Tianhe District, Guangzhou, 510640, China E-mail: [email protected]; Tel: +86 2087113953: +1-608-890-2403.

ACS Paragon Plus Environment


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Structural Modifications of Sugarcane Bagasse Lignins during Wet-storage and Sodaoxygen Pulping Fengxia Yue,1,2,3 Wu Lan,3 Songnan Hu,1 Ke-Li Chen1,*, and Fachuang Lu2,3,* 1

Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, 650500, China; 2

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China

3

Department of Biochemistry, Department of Biological System Engineering, and The DOE Great Lakes Bioenergy Research Center, The Wisconsin Energy Institute, University of Wisconsin, Madison, WI 53726, USA

ABSTRACT

Wet-storage is the most common way to maintain sugarcane bagasse in paper-making industry, although there were few studies on the structural alteration of lignins caused by wet storage system. The lignin preparations isolated from wet-stored bagasse in a laboratory simulated wet storage system and from the corresponding pulps and spent liquors of soda-oxygen pulping were characterized by various analytical techniques, including elemental analysis (EA), gel permeation chromatography (GPC), and heteronuclear single-quantum coherence (HSQC) NMR spectroscopy. The characteristics of these lignins were compared with those of lignin preparations isolated from fresh sugarcane bagasse samples. 11% decrease of p-coumarate (p-CA) in the lignins from


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wet-stored raw materials were observed. p-Coumarate and tricin were completely removed during pulping process. Results from this study suggested that syringyl units were more easily degraded and dissolved under low temperature soda-oxygen pulping conditions. Wet-storage for certain period of time (14 days) did not modify lignin structure or degrade cellulose significantly. KEYWORDS: wet-storage; dioxane lignin (DL); soda-oxygen lignin; elemental analysis (EA); gel permeation chromatography (GPC); heteronuclear single-quantum coherence (HSQC) NMR spectroscopy

INTRODUCTION

Lignins are the phenylpropanoid polymers produced by combinatorial radical coupling of three types of 4-hydroxycinnamyl alcohols differing in their degrees of methoxylation. Lignins are the second most abundant and important natural biopolymers accounting for approximately 30% of the organic carbon in the biosphere.1 As highly abundant natural aromatic polymers, lignins have drawn significant attentions from both academic and industrial sectors in fields related to their biosynthesis and chemical/mechanical properties.2 However, the understanding of the chemical compositions and structures of lignins is the most challenging task for scientists because of their complexity and heterogeneity.3-5 The mysteriously undefined lignin structure, to some extent, hinders the paces towards high-value utilizations of lignins. Lignin structural studies therefore play an important role not only in understanding the nature of this enigmatic class of polymers, but for optimizing the value of lignins through their potential applications.6,7



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Nowadays, wood shortage, environmental pollution, and high-energy consumption remain major obstacles hindering the development of pulp and paper industry. Energysaving and environmentally friendly pulping processes are in desperate need, especially for non-woody materials. In China, the use of non-wood fibers plays an important role in paper-making industry for which sugarcane bagasse (referred as bagasse from here on after) fiber is one of the most important renewable raw materials. Studies on bagasse pulping also attract tremendous attentions in other countries.8-10 Normally, a certain period of storage is necessary for bagasse-based pulping mills because they operate year-round whereas the bagasse raw material is available seasonally only. Considering dust control, wet weather influence, and protect against external sources of ignition, wet-form storage is superior to the dry-form, most paper mills now stock bagasse in a wet form.11 Then the wet-stored materials are subsequently entered pulping process after being stored for a certain period of time. However, the quality of stored bagasse can be influenced by the residue sugar, which is reflected by the high content of organic contaminant in the waste effluent discharged from the stockpiles. Moreover, the microbial environment around the stockpiles also affect the quality of bagasse. Therefore, the wet storage of bagasse acts somehow like a biological process that could modify the stocked raw material compositionally and structurally. However, there have been few detailed investigations into impacts of a wet storage system on lignin structure and resultant pulps, although the reported studies were mainly focusing on the white-rot fungus under culture conditions.12,13 Pulping of bagasse has been carried out mostly using the soda, sulfate and neutral sulfite methods.14 However, such industrial processes for bagasse pulp production face


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several problems such as high cooking temperature, low pulp yields, and troublesome environmental pollution. As a pulping technique, soda-oxygen pulping has some advantages over the conventional soda pulping processes due to the presence of oxygen that helps delignification of fibrous raw materials and reduces the chromophore groups in the resultant pulps. The delignification in this process is a result of the synergistic action of alkali and oxygen on the components of the raw materials. The cooking temperature used for soda-oxygen pulping of bagasse has been usually not lower than 125 °C. However, our recent work demonstrated that quality bagasse pulp with satisfied properties can be produced from newly crushed bagasse under soda-oxygen pulping conditions with cooking temperature of 100 °C.15 As a continuous effort to study soda-oxygen pulping at low temperature, we investigated the influence of wet storage on the lignin structures of wet-stored bagasse as well as the quality of pulps produced by the soda-oxygen pulping process performed at 100 °C. A laboratory simulated wet-storage system was built up. The wet-stored bagasse raw material was pulped under soda-oxygen pulping conditions and the resultant pulps were evaluated for pulp yields, lignin content (Kappa number), viscosity, and brightness. Meanwhile, compositional analysis was performed on the bagasse raw material and wetstocked bagasse (for 14 days and 48 days) to determine any changes caused by such a treatment. Then the isolated bagasse lignin preparations from fresh bagasse raw material (before storage), wet-stored bagasse (for 14 days and 48 days), pulps produced by sodaoxygen pulping, and spent pulping liquors were characterized by elemental analysis, gel permeation chromatography and NMR spectroscopy.

EXPERIMENTAL SECTION



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Materials. Sugarcane bagasse used in this study was obtained from Yunnan Xinping Nan’en Sugar and Paper Co., Ltd (Xinping, Yunnan Province, China). After being extracted for sugarcane juice, the crushed bagasse was screened, de-pithed, and air-dried. The original sugarcane bagasse used in this study was labeled as B. All the chemicals were analytical grade and used as supplied. Wet-storage process. Outdoor lab simulated wet storage of bagasse was carried out in a polyethylene cubic container. 81.22 kg of bagasse (absolutely dry) was loaded and saturated with alkaline water of pH value 8.5 (approximately 300 L) before stocking. The bagasse was sprayed by alkalescent water (pH value 8.5+0.1) every day during wet storage, the sprayed liquid volume was implemented by proper adjustments according to the discharged effluent volume the day before. The pH value of alkaline water was adjusted by adding NaOH. Sampling periods of stored bagasse were 7, 14, 21, 30, 40, and 48 days, respectively. The storage process was terminated at the 48th day. The samples obtained from the same storage period were mainly from two parts, the upper layer and the lower layer, considering that the height of the container (1 meter) may lead to difference of storage circumstances. In contrast, the 48-day sample was all mixed together because after 48-day stored the wet-stored bagasse from the upper layer and the lower layer showed similar color. Moreover, the height of the stored bagasse was reduced to less than 50 cm after a series of samplings, so the influence caused by height in the later stage was not as obvious as that in earlier storage period. After sampling, all the stored bagasse samples were washed 3 times by water and air-dried before used. Samples used in this study were obtained from 14th day upper layer and 48th day, which were labeled as WS14 and WS48, respectively.


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Chemical Composition Analyses. A portion of each non-treated/treated bagasse was ground and the 40–60 mesh fraction was used for chemical analysis. The analysis of typical composition, including cellulose (Kurschner–Hoffner method), holocellulose (Chlorination method), lignin (TAPPI T 222 om-88), 1% NaOH extractives (TAPPI T 212), and pentosan (TAPPI T 223 cm-01) was carried out by referenced standard methods. The analyses were conducted with three parallels and the relative standard deviation was below 5%. Pulping process and analysis of pulp properties. Soda-oxygen pulping of bagasse at 100 °C was carried out at following conditions: cooking time 180 min, alkali charge 23%, initial pressure of oxygen 0.6 MPa, MgSO4 charge 0.5%, and de-pithed bagasse consistency 12%, etc. The resultant pulp was thoroughly washed with water and screened. Properties of pulp such as Kappa number (ISO 302:1981, Reapproved 1991), viscosity (ISO 5351-1:1981, Reapproved 1991), and brightness (ISO 3688:1999, ISO 2470: 1999), were measured by the referenced standard methods. The analyses were conducted with three parallels and the relative standard deviation was below 5%. Isolation of mild acidolysis dioxane lignins. Dioxane lignins (DL) of raw materials and pulps were isolated according to the published procedure.16 1) DLs from raw materials. Each extractive-free coarsely grounded sample (40–60 mesh, 30 g dry weight) was suspended in 1000 mL of 0.1 M HCl in dioxane/distilled water solvent (82/18, v/v). The suspension was refluxed under gentle N2 bubbling and magnetic stirring for 2 h. The reaction mixture was cooled and vacuum-filtered through a porcelain Büchner funnel (Whatman filter paper). The residue was washed three times with 200 mL of dioxane/water solvent (82/18, v/v) and then with distilled water (~1.5 L) to a neutral pH



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value. The combined filtrates were evaporated under reduced pressure at 40 °C to remove dioxane. The total volume of water (containing HCl) was controlled not to below 500 mL in case the acidity of the solution increased. The solution was leaving in the refrigerator overnight and then the precipitated lignin was collected by filtration through a fine porous glass filter, washed with distilled water to a neutral pH and again suspended in distilled water. Such operations were repeated twice. After freeze-drying, the isolated lignin was extracted with pentane in a Soxhlet extractor for 8 h to remove any remaining trace of low molecular weight material. The isolated yields of DLs were 30-35 wt% of Klason lignin, recorded as B-1, WS14-1, and WS48-1. 2) DLs from pulps. Each of the bagasse pulp sample obtained by soda-oxygen pulping at 100 °C was thoroughly washed with distilled water and air-dried. Extractive-free pulp sample (200 g dry weight) was suspended in 6000 mL of 0.1 M HCl in dioxane/distilled water mixture (82/18, v/v) following the abovementioned procedures for raw material. The isolated yields of DLs were ~25% of residual lignin (based on Kappa number), recorded as B-2, WS14-2, and WS48-2. Isolation of lignins from black liquors. 300 mL of each black liquor obtained from related non-treated/treated bagasse by soda-oxygen pulping at 100 °C was centrifuged to remove the insoluble residue. The supernatant was collected and acidified to pH 2.0 by addition of HCl (12%) while kept stirring. The precipitate was recovered by centrifugation and kept in the freezer, overnight. Then, the precipitate was washed with distilled water to a neutral pH and vacuum-filtered through a porcelain Büchner funnel (Whatman filter paper), freeze-dried. The obtained powder was then dissolved in dioxane/water mixture (9/1, v/v), centrifuged and the supernatant was concentrated under


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reduced pressure at 40 °C. Then, the crude product was re-dissolved in acetic acid/water (9/1, v/v) and purified by centrifugation. The supernatant was then precipitated out by dropwisely adding into 10 volumes of cold water. After the precipitate was separated by centrifugation, washed 3 times with distilled water and freeze-dried. Crude isolated lignin was then re-dissolved in CH2Cl2/ethanol (2:1) and centrifuged. The supernatant part was then precipitated out by addition into 10 volumes of ether, following the separation by centrifugation. The crude product was then washed with ether twice and petroleum ether twice, respectively. The soda-oxygen lignins were finally obtained after vacuum drying, recorded as B-3, WS14-3, and WS48-3. Characterization of lignins Elemental analysis (EA) and methoxyl content. Elemental analyses (C, H, and N) were determined using vario EL III (Elementar, Germany) elemental analyzer. The percentage of oxygen was calculated by subtracting the C, H, and N contents from 100%. The methoxyl (MeO) groups of lignin were estimated using 1H NMR (CDCl3) analysis after acetylation according to published procedure.17-19 Briefly, The 1H NMR spectra of acetylated lignins show that syringyl proton signals occur between 6.28 ppm and 6.80 ppm, while the guaiacyl proton signals occur between 6.80 ppm and 8.00 ppm. The theoretical ratios between aromatic and methoxyl protons of guaiacyl and syringyl are 1.00 and 0.33, respectively, which could be measured from the integration ratio (x) = Haromatic/Hmethoxyl. By plotting this ratio against the percent content of methoxyl group (MeO) obtained by the classical hydroiodic acid method for the same lignins, the data were submitted to a statistical linear regression analysis and the following equation was finally obtained:



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%OMe =28.28436-19.750047x (1) Thus, the content of MeO group was calculated from the integration ratio x of the protons of aromatic groups (6.4-7.1 ppm) to the protons of methoxyl group (3.5-4.1 ppm) (x = Haromatic/Hmethoxyl) as reported.18 Gel permeation chromatography (GPC). The molecular weights of the lignin samples were determined by gel permeation chromatography (GPC, Agilent 1100) on PLgel Mixed-c 7.5 × 300mm, 5mm; PLgel 10E3 7.5 × 300mm, 5mm columns. Each acetylated sample (3 mg) sample was dissolved in 1 mL of tetrahydrofuran, and a 20 µL sample in solution was injected. The column was operated at ambient temperature and eluted with tetrahydrofuran at a flow rate of 1 mL/min. Monodisperse polystyrene was used as the standard for the molecular weight of lignin. Heteronuclear single quantum coherence (HSQC) NMR spectroscopy. NMR spectra were acquired on a Bruker Biospin (Billerica, MA, USA) AVANCE 500 (500 MHz) spectrometer fitted with a cryogenically-cooled 5-mm TCI gradient probe with inverse geometry (proton coils closest to the sample) and spectral processing used Bruker’s Topspin 3.1 (Mac) software. Standard Bruker implementations of one- and twodimensional (gradient-selected COSY, HSQC and HMBC) NMR experiments were used for routine structural assignments of acetylated lignin samples. The conditions used for all acetylated lignin samples were around 60 mg in 0.5 mL NMR solvent (CDCl3) with the central solvent peaks (δH/δC 7.26/77.23) used as internal reference.

RESULTS AND DISCUSSION


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Chemical composition and pulp properties. The chemical compositions and properties of pulps from non-stored and wet-stored sugarcane bagasse are listed in Table 1. No obvious chemical composition change of wet-stored bagasse can be observed after being stored for 14 days, although 1% NaOH extractive was slightly decreased and the other compositions were up a little, probably because the residual sugar and extractives soluble in weak basic water were washed away. The resultant pulp from bagasse wetstored for 14 days showed comparable or even better properties, i.e. higher yield. However, the longer storage time resulted in inferior compositions, i.e. lower content of holocellulose and cellulose; higher content of Klason lignin and 1% NaOH extractive. The bagasse sample, WS48, being wet-stored for 48 days, was the worst one. This probably was caused by the some microbial degradations on polysaccharides or lignin. Pulp produced from WS48 bagasse also showed inferior properties, i.e. higher Kappa number; lower pulp yield and brightness. Therefore the best timing for wet storage of bagasse should be about 14 days. Isolation of DLs and soda-oxygen lignins. Mild acidolysis hydrolysis of extractivefree raw materials as well as pulps is a convenient and rapid way to prepare lignins for structural analysis, although there were concerns about structural alteration of lignins because acidic conditions used to liberate the lignins from the raw materials or pulps.16 However, this lignin preparation is still validate and has been used in structural characterization for comparison,20-25 which was the exact purpose using DLs in current study. One of the feature or advantages of DLs is their high isolation yields, which is important for isolating lignins from low lignin-containing materials, for example, chemical pulps that was used here. The isolated yields of DLs from raw materials were



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30-35% (on Klason lignin basis), while the isolated yields of DLs were 20~25% of residual lignin (on Klason lignin basis). The yields of soda-oxygen lignins from acid precipitation of black liquors were around 50% (of total lignin dissolved in black liquor). Characterizations of DLs and soda-oxygen lignins Elemental analysis. The elemental compositions and methoxyl contents of these isolated lignins are listed in Table 2, together with the approximate C9 formulae derived from them.26 Apparently, the methoxyl content of DLs from raw materials are comparable to those from pulps, whereas the methoxyl content of soda-oxygen lignins are, to some extent, higher. One possible reason is that the syringyl units are much easier to be cleaved than guaiacyl units during low temperature soda-oxygen pulping. On the other hand, these data also suggests acid precipitated soda-oxygen lignins from black liquors (B-3, WS14-3, and WS48-3) contain more syringyl units. The higher oxygen content as revealed by C9 formula for soda-oxygen lignins could be explained by the oxidative reactions occurred during pulping process. Nitrogen content of lignins reflects the contamination of protein residues, implying the strong associations between proteins and lignin, and the protein residues are difficult to be removed by the extraction or washing processes.27,28 It is interesting to note that, the nitrogen contents of lignin samples from non-stored bagasse are much similar to those from wet-stored bagasse, which indicates that the abovementioned strong associations between proteins and lignin, , besides those already existed, might not have been established yet during the wetstorage process. For the notable nitrogen contents in the soda-oxygen lignins, the possible reason is lignins that associated or absorbed with proteins prone to be removed and dissolved in black liquors during soda-oxygen pulping.


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Molecular weight. The molecular weight distribution of isolated lignin samples was measured by gel permeation chromatography (GPC) (Figure 1). Weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity (Mw/Mn) are shown in Table 3. As can be seen from Figure 1, the molecular weight of DLs from raw materials and pulps, even soda-oxygen lignin, is not homogeneous, which distributes into three main dispersion ranges. The dispersion of DLs from raw materials and those from their corresponding pulps are similar, the polydispersity is ranging from 1.74 to 2.01 (Table 3). Slightly increment of lower molecular weight distribution of WS14-1 can be seen from Figure 1, which is in accordance with the slightly lower molecular weight as shown in Table 3. So different storage periods cause no significant degradations of the lignin macromolecules as the molecular weight of DL from nonstored bagasse was similar to those from wet-stored bagasse (Table 3). The molecular weights of DLs from pulps were somehow lower than those from raw materials suggesting that the residual lignins in pulps were degraded or depolymerized by the sodaoxygen pulping process. The polydispersity of soda-oxygen lignin samples (B-3, WS14-3, and WS48-3) were from 2.56 to 2.98, apparently bigger than those of DLs (Figure 1 and Table 3) as the results of increased portions of low molecular weight and high molecular in soda-oxygen lignins. This indicated that the degradation and depolymerization of lignin macromolecules takes place during low temperature soda-oxygen pulping process meanwhile the alkali solution used for pulping dissolves significant amount of high molecular weight lignin portion from raw material.29-33 On the hand the molecular weight of DLs from raw material and pulps was limited due to acid degradation of lignin caused by the extraction conditions. So overall the average molecular weights of soda-oxygen



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lignins (B-3, WS14-3, and WS48-3), are much higher than those DLs from raw material and pulps. The main reason for such results could be the loss of the low molecular portion (soluble part) of lignins during the acid-precipitating step. Another reason that cannot be ruled out was the possible condensation reactions occurring in this alkaline pulping process.16 HSQC spectra analysis. Additional structural information about these lignin samples was obtained from 2DHSQC NMR. The HSQC spectra of the lignins from bagasse raw material, the corresponding pulp, and black liquor for both non-stored and wet-stored (14 days) samples are shown in Figure 2. The percentage of the substructures, such as β-aryl ether (A, β–O–4), phenylcoumaran (B, β–5), resinol (C, β–β), tetrahydrofuran (C', β–β), tricin (T), p-coumarate (p-CA), ferulate (FA), guaiacyl (G), syringyl (S or S'), and cinnamaldehyde and benzaldehyde end-groups (X2 and X3) were calculated by integrating the contour volumes of the corresponding correlations in the HSQC spectra, (explanation of calculation) and the results are listed in Table 4. According to the spectra and the calculated results, the structures of the lignins obtained from raw material, pulp, and black liquor were slightly different from those obtained from bagasse wet-stored (for 14 and 48 days). Lignins from the wet-stored raw materials (WS14-1 and WS 48-1) showed a lower level of p-CA than that from the fresh bagasse (B-1), indicating that a hydrolysis of the ester occurred during wet storage where mild alkali solution was used. 31,34-36

However, the percentages of p-CA in the lignin samples from all kind of raw

materials are around 50% (Table 4), which are much higher than expected. The content of p-CA was, to some extent, overestimated because the calculation was based on the lignin


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units (S+G=100) while some of the G units were missing from the HSQC spectra, and also because the correlations of p-CA overlaps with some other unknown peaks. Furthermore, the integrals of these endgroups significantly overestimate the actual levels on a lignin basis in HSQC experiments.37 The content of tricin in WS14-1 and WS48-1 slightly decreased, suggesting that some tricin moiety associated with lignin was soluble or not stable even under mild base condition. It is reasonable to expect that soda-oxygen pulping may modify the structure of lignin to some extent. The p-CA was completely removed from the pulps (B-2, WS14-2, and WS48-2) mostly due to alkali hydrolysis.31,34 Meanwhile no p-coumaric acid was observed in the lignins (B-3, WS14-3, and WS48-3) recovered from black liquor due to its water solubility. The S/G ratios of lignins recovered from black liquors show a higher than the residual lignins, indicating that the syringyl units, which are predominantly forming β–O–4 alkyl-aryl ether, are degraded and dissolved preferably into the black liquor during soda-oxygen treatment. Another possible reason for the higher S/G ratios of lignins recovered from black liquors is that syringyl lignins were less soluble in acidified water than guaiacyl ones that may be better linked to Ferulic acid. Therefore the syringyl lignins were recovered preferably. Normally, the alkyl aryl ether (β–O–4) structure is the dominant linkage in lignin polymers. It is crucial for the cleavage of this linkage for depolymerization and removal of lignins from the plant cell walls. However, it is interesting to note that the β–O–4 (substructure A) inter-linkage was the dominant structure in the lignins isolated from bagasse, and even from the pulps after soda-oxygen treatment and from black liquors, which present almost the same amount (around 90%). One possible reason is the uncleaved β-aryl ether remains the dominant linkage in residual lignins from pulp as well as



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acid precipitated lignins from black liquor, although some β-aryl ethers were cleaved during alkaline pulping. On the other hand, it also indicates that low temperature sodaoxygen reaction cannot completely cleave aryl ether bond; that is, only part of the aryl ether bonds was degraded during the soda-oxygen treatment. The piece of lignin chain which was not covalent bonded to the cell wall was dissolved into the base solution, while the lignin polymer chain which was still linked onto the cell wall remained in the pulp. This also explains why the molecular weight of the lignin isolated from untreated bagasse, pulp, and black liquor were not much different from each other. Furthermore, these results are consistent with the A/(S+G) ratios (Table 4), the percentages of β-O-4 ether (based on all aromatic structural units), which provides another (or better) measures for structural changes lignins caused by microbes or pulping. For the linkages between side chains of lignin units, the percentage of β–5 (B) substructure declined, whereas the amount of β–β (sum of C and C') increased tremendously in the residual lignins of pulps. In the case of black liquor lignins, the percentages of these inter-linkages were similar to those of the lignin from raw material. Furthermore, the cinnamaldehyde end-group was degraded (converted) to the benzaldehyde end-group found in lignins from pulp and black liquor. This result indicated that the side chain of the end-groups was readily degraded via the Cα–Cβ double bond cleavage under the soda-oxygen pulping condition.

CONCLUSIONS

In summary, no obvious structural change was observed for the isolated lignins obtained from bagasse treated by a lab simulated wet storage system. A slight decrease in


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p-CA content was found in the wet-stored raw materials. The inconspicuous structural change of lignins caused by different wet-storage cycles here suggested that mild conditions, such as weak alkaline water (pH around 8.5) used for wet-storage process, were applied. Moreover, microbe degradations on lignins have not led to lignin structure change yet. However, a long period of wet-storage was undesirable for bagasse due to the degradation of cellulose and polysaccharides. The results obtained in this work indicated that there is an appropriate of time period for wet-storage, in which microbe actions are mainly on the residual sugar and the polysaccharides rather than on lignin or cellulose polymers. Therefore, short time (around 14 days in this study) wet storage of bagasse benefits the pulping process, and then paper production, whereas long term storage (over 48 days) led to a poor delignification of the raw materials during pulping process, and then poor quality of the resultant pulps.

AUTHOR INFORMATION

Corresponding Authors *(K.-L. Chen): E-mail: [email protected]; Tel: +86-871-659-20329; +86-871659-20329. *(F. Lu): E-mail: [email protected]; Tel: +86 2087113953: +1-608-890-2403. Notes The authors declare no competing financial interest. Acknowledgment



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This study was financially supported by the National Natural Science Foundation of China (20567001, 21276119, 51363013), and the Provincial Natural Science Foundation of Yunan (2004B0013M), China.

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Table 1. Chemical composition and pulp properties of original and wet-stored bagasse Chemical compositions Pulp sample B WS14 WS48

Pulp Properties

Holocellulose (%)

Cellulose (%)

Klason Lignin (%)

Pentosans (%)

77.06 77.67 75.40

48.80 49.23 48.87

20.19 20.52 22.50

24.34 25.25 24.58

1% NaOH Extractive (%) 28.49 26.51 28.86

Screened Yield (%)

Rejects (%)

Kappa Number

Viscosity (dm3/kg)

Brightness (% ISO)

62.48 65.11 62.15

0.65 0.92 1.00

14.6 14.8 16.5

802 808 811

62.3 58.2 56.1

Table 2. Elemental and methoxyl analysis of isolated lignin samples together with calculated C9 Formula Lignin sample B-1

0.06

56.51

5.53

38.02

20.09

C9H7.95O3.83(OMe)1.43

WS14-1

0.07

58.97

5.66

35.45

20.26

C9H7.81O3.30(OMe)1.38

WS48-1

0.08

59.38

5.72

34.98

18.83

C9H8.08O3.27(OMe)1.26

B-2

0.16

58.38

6.10

35.69

20.29

C9H8.84O3.37(OMe)1.40

WS14-2

0.18

58.13

6.32

35.73

20.42

C9H9.33O3.39(OMe)1.41

WS48-2

0.15

58.28

6.35

35.51

20.32

C9H9.39O3.35(OMe)1.40

B-3

0.87

55.84

5.80

39.23

20.54

C9H8.59O4.04(OMe)1.49

WS14-3

0.60

56.87

5.75

37.98

20.80

C9H8.26O3.77(OMe)1.48

WS48-3

0.61

51.44

5.25

43.92

19.99

C9H8.20O5.19(OMe)1.59

N%

C%

H%

O%

MeO%

C9 Formula*

Note: 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.26



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Table 3. Weight-average molecular weight (Mw, g/mol), number-average molecular weight (Mn, g/mol), and polydispersity (Mw/Mn) of acetylated lignin samples Main peaks molecular weight Mw1 Mw2 Mw3

Lignin sample

Mn

Mw

Mw/Mn

B-1 WS14-1 WS48-1 B-2 WS14-2 WS48-2

5600 5000 5200 4300 4700 4600

9900 9000 9600 7500 8300 9300

1.74 1.82 1.86 1.75 1.78 2.01

20000 20000 20000 13500 15000 18000

10500 10000 10500 6750 8000 9800

3750 3800 3750 2900 3200 3400

B-3

6500

16700

2.56

--

--

--

WS14-3

5500

15900

2.89

--

--

--

WS48-3

5700

16900

2.98

--

--

--

Table 4. Percentages of the substructures in the lignin samples on the basis of contour integration of the HSQC spectra B-1 WS14-1 WS48-1 B-2 WS14-2 WS48-2 B-3 WS14-3 WS48-3 p-CA 55% 51% 49% ------T 2% 1% 1% ------S'

Structural Modifications of Sugarcane Bagasse Lignins during Wet

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