Complete dissolution of ball-milled Masson pine using an aqueous sodium hydroxide solvent

Abstract

An inorganic sodium hydroxide aqueous solution was developed for the dissolution of ball-milled softwood. Extractive-free Masson pine powder was prepared from the wood meal by planetary ball milling for various lengths of time. The effects of ball milling on the dissolution and regeneration of Masson pine powder were investigated. After 4 h of ball milling, the wood powder was completely dissolved in 6 wt% NaOH aqueous solution under vigorous magnetic stirring for 10 to 20 min at room temperature. The alkaline nitrobenzene oxidation products yield of 4 h ball-milled wood was very close to that of coarse wood meal without ball milling. Regeneration of the lignin fraction was easier than cellulose during the acid neutralization process, and the dialysis regeneration yield of total biomass was 91.1%. Therefore, the complete dissolution of the Masson pine in sodium hydroxide solution is of great potential for enhancing lignin isolation with a very little structural change.

Full Article

Complete Dissolution of Ball-Milled Masson Pine Using an Aqueous Sodium Hydroxide Solvent

Yangsu Zhu, Linfeng Yang, Wenjuan Wu, Zhiguo Wang, and Yongcan Jin *

An inorganic sodium hydroxide aqueous solution was developed for the dissolution of ball-milled softwood. Extractive-free Masson pine powder was prepared from the wood meal by planetary ball milling for various lengths of time. The effects of ball milling on the dissolution and regeneration of Masson pine powder were investigated. After 4 h of ball milling, the wood powder was completely dissolved in 6 wt% NaOH aqueous solution under vigorous magnetic stirring for 10 to 20 min at room temperature. The alkaline nitrobenzene oxidation products yield of 4 h ball-milled wood was very close to that of coarse wood meal without ball milling. Regeneration of the lignin fraction was easier than cellulose during the acid neutralization process, and the dialysis regeneration yield of total biomass was 91.1%. Therefore, the complete dissolution of the Masson pine in sodium hydroxide solution is of great potential for enhancing lignin isolation with a very little structural change.

Keywords: Masson pine; Sodium hydroxide aqueous solution; Ball milling; Dissolution; Regeneration

Contact information: Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing 210037, China; *Corresponding author: jinyongcan@njfu.edu.cn

INTRODUCTION

The main components of lignocellulosic biomasses are cellulose, hemicellulose, and lignin. Cellulose, as a linear polysaccharide, has highly developed hydrogen bond networks and a partially crystalline structure (Yang et al. 2011). The lignin, derived primarily from p-hydroxycinnamyl alcohols, consists of a three-dimensional network of coupled monomers of a varied 4-hydroxyphenylpropanoid type (Fasching et al. 2008). Unlike most natural polymers, lignin binds with polysaccharides to form a tight chemical structure through molecular associations and covalent bonds (Chang et al. 1975; Lee et al. 1981; Ikeda et al. 2002). Therefore, it is difficult to dissolve native lignocelluloses completely in traditional solutions.

Complete dissolution of lignocellulosic biomass is of great importance for both efficient isolation and optimum use of the cell wall components. There are many approaches for dissolving lignocelluloses in organic or inorganic solvent systems. These solvent systems, such as N-methylmorpholine-N-oxide (NMMO) (Fink et al. 2001), lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) (McCormick et al. 1985), and ionic liquid (Kosan et al. 2008), have been successfully applied to dissolve cellulose. However, most of these solvent systems cannot completely dissolve lignin in lignocellulosic materials. For example, Sjoholm et al. (1997) and Yanagisawa et al. (2005) reported that softwood kraft pulps were not completely dissolved in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc). Lu and Ralph (2003) succeeded in dissolving ball-milled wood in dimethyl sulfoxide and N-methylimidazole (DMSO/NMI) at room temperature for several hours. Wang et al. (2009) and Gu et al. (2015) reported that wood or straw samples milled in a planetary ball mill for 2 h could be completely dissolved in lithium chloride/dimethyl sulfoxide (LiCl/DMSO). Moderate ball milling is often necessary for the complete dissolution of lignocellulosic materials without delignification (Lee et al. 1981; Fujimoto et al. 2005; Holtman et al. 2007; Wang et al. 2009). However, the solvent systems suffer from serious environmental problems, difficult recycling, and high cost. Thus, an environmentally friendly solvent for complete dissolution of lignin-containing lignocellulosic materials is needed.

Recently, an aqueous sodium hydroxide solvent system with urea or thiourea was successfully developed for cotton linter dissolution at low temperature (-12 °C) (Cai et al. 2006, 2007). This solvent system is promising with respect to both environmental and economic issues (Jin et al.2007; Qi et al. 2008). However, only low degree of polymerization of cellulose can be dissolved in this system (Qi et al. 2008). Native cellulose in lignocelluloses is difficult to dissolve in this solvent system and must be pretreated to reduce the polymer chain length (Trygg and Fardim 2011). Therefore, solvents that dissolve lignin-containing fiber materials, such as wood meal and unbleached pulps should be developed (Shi et al. 2014).

Masson pine (Pinus massoniana) is the most widespread species in the forests in southern China (Zhou et al. 2001). It has great potential as a raw material for a complete dissolution study. Ball milling treatment is a cost-effective and environmentally friendly physical modification method (Liu et al. 2011). The main purpose of this work is to develop a novel method to dissolve the ball-milled Masson pine power completely at room temperature in aqueous NaOH solution without significant structural change. The method application would widely expand the utilization of lignocelluloses as a new source for both efficient isolation and use of the cell wall components.

EXPERIMENTAL

Materials

Masson pine (Pinus massoniana) was collected from Sanming, Fujian Province, China, in October 2011. Air-dried chips were ground using a Wiley mill. The particles between the sizes of 40 and 80 mesh were collected and extracted with benzene-ethanol (2:1, v/v) in a Soxhlet apparatus for 12 h. The extractive-free wood meals were further dried at 40 °C overnight prior to ball milling. The ball milling was carried out with a planetary ball mill (Fritsch GMBH, Pulverisette 7 premium line, Idar-Oberstein, Germany) at 600 rpm for 0.5 h to 8 h at room temperature. The two grinding bowls were made of silicon nitride (80 mL), and each bowl contained 25 zirconium dioxide grinding balls (1 cm diameter). In each milling run, 4 g oven-dried (o.d.), extractive-free Masson pine was charged for each bowl. Each 5-min milling run was followed by a 10-min break to avoid over-heating.

N,O-bis(trimethylsilyl)acetamide (BSA) was used as silylation derivatization reagent in the analysis of nitrobenzene oxidation products and was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). All other chemicals were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd., Nanjing, China.

Dissolution of Milled Wood in Sodium Hydroxide Aqueous Solution

A series of sodium hydroxide aqueous solutions with a concentration range from 2 wt% to 18 wt% was prepared. The pine powder (1% by weight) subjected to various ball milling treatments were added to NaOH aqueous solution under vigorous magnetic stirring at room temperature.

Pine powder samples subjected to 0.5 h to 4 h of ball milling were suspended into 6 wt% NaOH aqueous solution with a concentration of 5 wt% at room temperature. After 10 to 20 min of vigorous magnetic stirring, the insoluble matter was separated and then washed with deionized water several times by centrifugation until the pH of the supernatant was 7. The washed insoluble matter was freeze-dried and weighed to calculate the dissolution ratio in NaOH aqueous solution.

Regeneration of NaOH Aqueous Solution Dissolved Masson Pine

Masson pine treated with 4 h of ball milling was completely dissolved in 6 wt% NaOH aqueous solution. Two processes, acid neutralization and dialysis regeneration, were used to regenerate the dissolved pine. For acid neutralization, the solution was adjusted to pH 0, 1, 4, or 7 using 3 M HCl and then washed three times with the corresponding hydrochloric acid solution or deionized water. For dialysis regeneration, pine dissolved in NaOH solution was poured into a seamless cellulose tube with a diameter of 28.6 mm and MWCO of 14 kDa (UC 36-32-100, EIDIA Co., Ltd., Tokyo, Japan). The sample was dialyzed in ultra-purified water at room temperature until the pH of the dialysate was 7. The regenerated solids were freeze-dried and weighed to calculate the regeneration yield. The structural components (lignin and cellulose) in dried regenerated samples were analyzed.

Analytical Methods

Klason lignin and cellulose contents were determined in the solid samples (Masson pine Wiley, milled Masson pine, and regenerated Masson pine) according to Sluiter et al. (2008) and Tan and Lee (2012), respectively. The yield of extractable lignin from the milled wood was determined as described in Fujimoto et al. (2005) and calculated as the weight percentage based on the Klason lignin content of the milled wood. The absorption coefficient of softwood lignin (15.4 L/(g·cm)) was used to calculate the extractable lignin.

Alkaline nitrobenzene oxidation (NBO) analyses were applied to the wood samples according to Chen et al. (1992). Nitrobenzene oxidation products were silylated with BSA at 100 °C for 10 min and then analyzed by gas chromatography under the following conditions: gas chromatograph, GC-14B with flame ionization detector (FID) (Shimadzu Co., Kyoto, Japan); column, InertCap 1 (fused-silica capillary column, 30 m, 0.25 mm i.d.) (GL Science Inc., Tokyo, Japan); column program, 5 min at 150 °C, raised by 5 °C/min to 210 °C for 0 min and by 20 °C/min to 280 °C for 6 min; injection temperature, 280 °C; detector temperature, 280 °C. Because very little p-hydroxybenzaldehyde and p-hydroxybenzoic acid were detected, the vanillin (4-hydroxy-3-methoxybenzaldehyde) and vanillic acid (4-hydroxy-3-methoxybenzoic acid) from uncondensed guaiacyl structures in lignin were the major products of nitrobenzene oxidation.

RESULTS AND DISCUSSION

Dissolution of Ball-Milled Wood in NaOH Aqueous Solution

In this work, 18 wt% NaOH aqueous solution was selected to test the solubility of the wood powder due to its strong capacity of swelling and dissolving at room temperature. The samples treated with ball milling for various amounts of time were suspended in 18 wt% NaOH aqueous solution under vigorous magnetic stirring for 10 to 20 min at room temperature. A low solid content of 1 wt% was adopted for the observation of dissolution (Fig. 1). Wood meals with short ball milling time were not completely soluble in 18 wt% NaOH. The solubility increased as the number of ball milling hours increased. The solution became clear when the ball milling time was 4 h, which indicated that the wood sample was completely dissolved.

Fig. 1. The solubility of Masson pine in 18 wt% NaOH aqueous solution with various ball-milling hours

A high concentration of NaOH favors the swelling and dissolution of cellulose in lignocellulosic biomass. However, strong alkali may result in severe degradation of lignin. To minimize structural changes in lignin, the possibility of dissolving 4 h ball-milled wood in alkali aqueous solution with the NaOH concentration lower than 18 wt% was investigated. As shown in Fig. 2, only the solution of 1 wt% wood in 4 wt% NaOH aqueous solution was not very clear. Thus, 4 h ball-milled wood was completely dissolved in alkali aqueous solution with a concentration of NaOH beyond 6 wt%.

Fig. 2. The solubility of 4 h ball-milled Masson pine in various NaOH aqueous solutions

Effects of Ball Milling Hours on the Extractable Lignin and Alkaline Nitrobenzene Oxidation Products Yield

Violent milling of wood meal causes structural changes in lignin (Lee et al. 1981; Fujimoto et al.2005; Holtman et al. 2007). Therefore, it is of great importance for dissolution of lignocelluloses to shorten the ball milling time without reducing final lignin yield. However, the effect of milling on the structural changes of lignin depends not only on milling time, but also on the other conditions, such as ball mill model, ball material, and operation frequency. The yields of extractable lignin under specific conditions are more useful than milling time to evaluate the milling intensity (Fujimoto et al. 2005).

The yields of extractable lignin and nitrobenzene oxidation products of Masson pine with various ball milling hours are given in Table 1. The extractable lignin yield increased as the ball milling time increased from 0 to 8 h. A longer ball milling time results in more cleavage of β-aryl ether bonds in lignin (Lee et al. 1981; Gu et al. 2015). However, Wang et al. (2009) reported that the side-chain regions of lignin changed little when the extractable lignin yield was lower than 20 wt%. In this work, the extractable lignin yield of 4 h ball-milled Masson pine was 17.4 wt%. Therefore, although the β-aryl ether structure of lignin changes depending on the milling time, the degree of change was negligible at 4 h of ball milling time.

Table 1. Extractable Lignin Yield and Nitrobenzene Oxidation Products in Masson Pine Powder with Various Ball Milling Times

Nitrobenzene oxidation has been used extensively for studying the structural changes of aromatic moieties in lignin (Felisa et al. 2006). The NBO products yield of Masson pine without ball milling was 2.0 mol/g-lignin. The NBO products yield changed little when the ball milling time was lower than 4 h, and it was as high as 1.8 mmol/g-lignin when the milling time extended to 8 h. This result showed that the aromatic structures in lignin were not obviously changed under the milling conditions.

The Dissolution of Structural Components of Ball-Milled Masson Pine in NaOH Solution

As mentioned earlier, 4 h ball-milled pine was completely dissolved in 6 wt% NaOH aqueous solution at room temperature. As the ball milling time significantly affects the dissolution of the lignocellulosic materials (Lee et al. 1981; Holtman et al. 2007; Wang et al. 2009), the dissolution rate of the structural components of Masson pine with various ball milling times in 6 wt% NaOH aqueous solution was investigated (Fig. 3). With the ball milling hours from 0 to 4 h, the dissolution ratio of total biomass in pine increased.

The ball-milled Masson pine was completely dissolved when the milling time reached 4 h. The dissolution ratio of cellulose was higher than that of lignin in the pine sample when the wood meal was milled from 0.5 to 3 h. This result indicated that cellulose in Masson pine showed better solubility than lignin in aqueous NaOH solvent. For pine without ball milling, the solubility of cellulose and lignin was almost the same. The crystal structure of cellulose was destroyed by ball milling even with a short milling time (Wang et al. 2009), which was important for the swelling and dissolution of cellulose. The difference between the dissolution rate of cellulose and lignin in Masson pine increased with the ball milling time from 1 to 2 h, and then the dissolution rate of cellulose become slower than that of lignin.

The cause of the difference between the dissolution rate of cellulose and lignin might be the different structural units, linked types, and distribution of cellulose and lignin in the cell wall components.

Fig. 3. The content of cellulose and lignin in undissolved residues (a) and their dissolution ratio of Masson pine with various ball milling hours in 6% (w/w) NaOH aqueous solution

Regeneration of Ball-Milled Masson Pine Dissolved in NaOH Aqueous Solution

In this work, the regeneration of ball-milled Masson pine dissolved in NaOH aqueous solution was carried out by adding HCl into the solution (Yan and Gao 2008). The pH of this solution was adjusted to 0, 1, 4, and 7 using 3 M HCl, and the regenerated lignin and cellulose were analyzed to understand the effect of pH on the regeneration of different structural components in the ball-milled pine sample.

Fig. 4. The regeneration yield of 4 h ball-milled Masson pine at different pH after dissolving in 6% (w/w) NaOH aqueous solution

As shown in Fig. 4, the regenerated solid yield increased gradually as the pH was decreased from 7 to 0. The highest acid regeneration yield reached 83 wt% at pH 0, such that 17 wt% of the components in Masson pine was lost. Ball milling reduces the molecular weight of lignin and the crystallinity of cellulose (Tan et al. 2015). The small fractions produced in the ball milling process are difficult to recover even with regeneration under strongly acidic conditions. The regeneration ratio of lignin was higher than that of cellulose in Masson pine when the pH ranged from 7 to 0. For example, when regenerated in water at pH 7 and 0, the lignin recovery was 64.7 wt% and 95.0 wt%, respectively, but cellulose recovery was only 44.5 wt% and 77.7 wt%, respectively. The hydrophilic property of cellulose may explain its lower regeneration yield.

The dialysis regeneration of 4 h ball-milled Masson pine dissolved in 6 wt% NaOH aqueous solution was also investigated. The regenerated solid yield of dialysis in water was 91.1 wt%, which was noticeably higher than that of the acid neutralization process (51.8 wt% at pH 7 and 83.4 wt% at pH 0). Compared to acid neutralization, more of the small fractions dissolved in aqueous alkaline solution were regenerated by dialysis.

CONCLUSIONS

1. Extractive-free Masson pine with a relatively short ball milling time (4 h at 600 rpm) was completely dissolved in 6 wt% NaOH aqueous solution in 10 to 20 min under vigorous stirring at room temperature.
2. The extractable lignin yield of Masson pine increased with the ball milling hours, and it was 17.4 wt% at the ball milling time of 4 h. In different ball milling conditions, the effects of extractable lignin yields on the alkaline nitrobenzene oxidation products yields of Masson pine were small. The cellulose component in Masson pine was easier to dissolve in the NaOH aqueous solution than lignin. The lignin fraction was easier to regenerate by acid neutralization. Compared with acid neutralization, more regenerated solids were obtained by dialysis.

ACKNOWLEDGMENTS

The authors are grateful for the financial support of National Natural Science Foundation of China (Grant No. 31370571), Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20133204110006), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Article submitted: March 5, 2016; Peer review completed: May 1, 2016; Revised version received: May 10, 2016; Accepted: May 11, 2016; Published: May 20, 2016.

DOI: 10.15376/biores.11.3.6017-6025