Urea Aqueous

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Activation of Enzymatic Hydrolysis Lignin by NaOH/Urea Aqueous Solution for Enhancing Its Sulfomethylation Reactivity Binpeng Zhang,† Dongjie Yang,*,†,‡ Huan Wang,† Yong Qian,† JinHao Huang,† Lixuan Yu,† and Xueqing Qiu*,†,‡ †

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by AUSTRALIAN NATL UNIV on 12/14/18. For personal use only.

School of Chemistry and Chemical Engineering, Guangdong Provincial Engineering Research Center for Green Fine Chemicals, South China University of Technology, 381Wushan Road, Tianhe District, Guangzhou, 510641, China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 381Wushan Road, Tianhe District, Guangzhou, 510641, China S Supporting Information *

ABSTRACT: Enzymatic hydrolysis lignin (EHL), from straw bioethanol process, was activated using NaOH/urea aqueous solution under low temperature condition to enhance its sulfonation degree and dispersion property. The sulfonation degree of EHL increased by 62.60% after the activation of NaOH/urea aqueous solution at −10 °C for 12 h, which is resulted from the breakage of the hydrogen bonds and ether bonds in lignin−carbohydrate complexes (LCCs) and the removal of residual carbohydrate. The functional group content, 2D-HSQC, and AFM analysis indicated that the increase of phenolic groups and the weakened aggregation degree of EHL after the activation are in favor of the increase of activity sites and decrease of steric hindrance, which is beneficial to the enhancement of sulfomethylation reactivity. Owing to the enhancement of reactivity, the dispersion properties of sulfomethylated EHL on graphite suspension were significantly enhanced. KEYWORDS: Enzymatic hydrolysis lignin, NaOH/urea, Sulfomethylation reactivity, Activation, Dispersion performance

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INTRODUCTION In order to alleviate the serious energy crisis, the cellulosic bioethanol industry has developed rapidly in recent years. Enzymatic hydrolysis lignin (EHL), a byproduct in this process, accounts for more and more proportion of industrial lignin.1 But a large amount of EHL is discarded into the environment as a castoff or burned for energy. The value-added applications of EHL not only greatly cut down the cost of bioethanol but also ease the burden on the environment.2 Due to being waterinsoluble at neutral pH, EHL is hardly applied in industrial fields.3 Thus, in order to increase the utilization of EHL, we must modify it for improving its water-solubility. Sulfomethylation, similar to the Mannich reaction, is often used to improve the water-solubility of lignin through introducing sulfonic groups to benzene rings.4 Sulfomethylated lignin was widely applied in industry as a cement additive,5 pesticide,6 dyestuff,7 and coalwater slurry8 dispersant. Lots of research has shown that sulfomethylated lignin with more sulfonic groups had better dispersion performance.4,9 However, due to the mild condition of the enzymatic hydrolysis process, there was still a lot of residual cellulose and other carbohydrates in EHL after separation from enzymatic hydrolysis residues, which resulted in the agglomeration and © XXXX American Chemical Society

decrease of active functional groups. So the sulfomethylation of EHL was extremely restricted, which would affect greatly the performance of sulfomethylated EHL as dispersant. Therefore, EHL must be activated to enhance its reactivity before modification. Chemical activation methods, such as hydroxymethylation,10 phenolation,11 demethylation,12 oxidation,13 and so on, have been used in the industry to enhance the reactivity of lignin. However, the above activation methods have some inherent disadvantages such as the addition of toxic organic solvent, tedious procedures, and so on, which not only would result in environmental pollution but also increase the production cost. Thus, it is necessary to look for some green and low-cost methods for enhancing the reactivity of EHL. In recent years, a nontoxic and low-cost NaOH/urea aqueous system has been attracting much research interest because of its ability to dissolve cellulose at low temperature which was confirmed by Zhang et al.14 The hydrogen bonds between cellulose were destroyed by NaOH hydrate and urea hydrate in aqueous solution at low temperature, and then the cellulose Received: September 18, 2018 Revised: November 23, 2018 Published: November 30, 2018 A

DOI: 10.1021/acssuschemeng.8b04781 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

and poly(diallyldimethylammonium chloride) (PDAC) were supplied by Sigma-Aldrich (Shanghai, China). All other reagents used in the experiment were of analytical grade. Graphite powder was obtained by Aladdin Crop with purity of 99%. The granularity of graphite was about 800 mesh determined by Mastersizer 2000 Particle Size Analyzer (Malvern Co. Ltd., England). Activation of EHL with NaOH/Urea Aqueous System. At first, the mixture with NaOH/urea/EHL of 8/12/100 by mass was dispersed into deionized water with continuous stirring for 30 min to get an EHL solution with concentrations 20 wt %. Then the mixture was preserved under low temperature condition for 12 h. Subsequently, the mixing solution was constantly stirred for 3 h after adjusting the pH of solution to ∼3 using 20% sulfuric acid solution. The samples were obtained through filtration, washing several times using deionized water until the filtrate was neutral, and freeze-drying. The samples after activation under low temperature condition were named as EHL-(T) (T is the corresponding treatment temperature). Sulfomethylation of Lignin. A certain amount of EHL was dissolved in 10% sodium hydroxide solution in a three-necked flask to obtain 20% lignin solution. Then 15% formaldehyde solution (wHCHO/ wlignin) was added and heated to 75 °C. The reaction was maintained at 300 rpm for 1 h. 30% Na2SO3 (wNa2SO3/wlignin) was added after increasing the temperature of samples to 95 °C and reacted another 3 h at 300 rpm. The mixed samples were cooled to room temperature and filtered to remove insoluble solid. The filtrate was freeze-dried to obtain sulfomethylated EHL (SEHL or SEHL-(T)). Glucose and Cellulose Contents Measurement. EHL and EHL-(T) were hydrolyzed with H2SO4 in two steps according to the method reported before.20 The pH of filtrate after hydrolysis was adjusted to 5−6 using anhydrous calcium carbonate. Then 2.0 mL aliquots of samples were taken for glucose analysis after centrifugation. Glucose in hydrolysis products was determined using the SBA-40E biosensor with a H2O2 electrode sensor (Institute of Biology of the Shandong Academy of Sciences, China). Then, the cellulose content was calculated based on the content of glucose. Determination of Functional Group Content. The content of sulfonic group in EHL was measured with the automatic potential titration method according to the method reported before.4,21 Headspace gas chromatography (HS-GC) was used to measure the methoxyl content of EHL based on the method described previously.22,23 All samples were determined through a DK-3001A headspace sampler (Beijing Zhongxing Huili Science & Technology Co., Ltd., Beijing, China) and Model GC9800 capillary gas chromatographer (Kechuang Co., Shanghai, China). The content of the phenolic group was measured according to the method reported before.24 The absorbance at 760 nm was measured using an ultraviolet visible spectrophotometer. The vanillin was chosen as the standard. Gel Permeation Chromatography (GPC). Aqueous GPC was used to measure the molecular weight distribution of EHL with the method described in our previous report.21 Polystyrenesulfonates, with molecular weights from 500 to 10000 g/mol, were selected as standard substance. The NaNO3 solution (0.10 mol/L) at pH 11.5 was selected as mobile phase. The samples obtained for measurement were first dissolved in double distilled water and then filtered through a 0.22 μm syringe filter. 50 μL samples were injected into a sampler for measurement. Spectroscopy Analyses. The changes of functional groups were also confirmed by Thermo Nicolet 380 Fourier transform infrared spectrometer (Thermo Nicolet Corp., U.S.A.). A thin KBr disk was prepared through pressing a mixture of 200 mg KBr (for spectroscopy) and 2 mg of dried samples under a pressure of 12 MPa for 2 min. Two-dimensional heteronuclear single quantum coherence (2DHSQC) NMR has been applied in quantification of lignin structures. 2D-HSQC NMR spectra of EHL were recorded through 60 mg of samples dissolved in 0.5 mL of DMSO-d6 by the Bruker AVANCE III HD 600 instrument (Bruker Corp., Germany). The central solvent (DMSO) peak was used as an internal reference. The spectral widths were 5000 and 20000 Hz for the 1H and 13C dimensions, respectively.25,26

molecules surrounded by hydrate were dissolved. Nowadays, the NaOH/urea aqueous system has been applied in the pretreatment of biomass for increasing utilization. Wang et al.15 reported that the NaOH/urea aqueous system was used to pretreated wheat straw at low temperature. They demonstrated that hemicellulose and lignin were dissolved and the hydrogen bond between cellulose was destroyed after the pretreatment under low temperature, which led to the deconstruction of the plant cell wall. After pretreatment, the lignin, xylan, and cellulose were easily obtained respectively from wheat straw. Mohsenzadeh et al.16 confirmed that NaOH/urea (7/12 wt %) aqueous solution could be used to pretreat softwood spruce and hardwood birch for improving enzymatic efficiency. After pretreatment, the structure of wood was significantly modified and then the yield of ethanol and biogas was raised. In addition, this study also proved that the treatment temperature would greatly influence the composition of wood. In our previous works, urea was added to lignocellulose substrate for significantly improving the enzymatic hydrolysis.17 Hydration layers could be formed on the lignin surface through the hydrogen bonds between urea hydrate and lignin when lignocellulose was added in urea aqueous solution. Then the adsorption capacity of cellulase on lignin could decrease significantly due to the steric hindrance effect. So the enzymatic efficiency of cellulase was improved because of the increase of adsorption capacity on cellulose. Our previous study also demonstrated that alkali lignin could be dissolved in urea aqueous solution.18 The urea disrupted the inter- and intramolecular hydrogen bonds and then formed new hydrogen bonds with alkali lignin. In addition, the O-π structures were formed between urea and alkali lignin, which could weaken π−π stackings in alkali lignin. Thus, lignin molecules were dissolved. Furthermore, researchers have proved that the cleavage of ether would occur in alkali lignin under alkaline condition, which will result in the increase of active sites.19 The aformentioned works have confirmed the NaOH/ urea aqueous system could disrupt the hydrogen bonds and ether bonds in lignocellulose and weaken the aggregation degree of lignin. Therefore, we proposed that the NaOH/urea aqueous system can be used to remove the residual carbohydrate in EHL and weaken the aggregation degree of EHL, which resulted in the enhancement of reactivity. In this study, the influence of the NaOH/urea activation on the sulfomethylation reactivity of enzymatic hydrolysis lignin (EHL) was studied. First, EHL was treated with NaOH/urea aqueous solution at low temperature to obtain activated EHL. Subsequently, sulfomethylated EHL was introduced to investigate the sulfomethylation reactivity by its sulfonic group content. In order to reveal the enhancement mechanism, the structural characterization of EHL was studied before and after activation. In addition, the graphite suspension was applied to research the dispersion properties of sulfomethylated EHL.

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EXPERIMENTAL SECTION

Materials. Enzymatic hydrolysis lignin (EHL) was extracted from the enzymatic hydrolysis residue of corn straw supplied by Tianguan Bioethanol Co. Ltd. (Henan, China). The extraction process was as follows: enzymatic hydrolysis residue was first added in pure water, and then the pH of the mixture was adjusted to ∼12 using sodium hydroxide. The mixture was filtered to remove the insoluble residue after stirring for 1 h. The pH of the filtrate solution was adjusted to ∼3 with sulfuric acid solution (20%). The mixed liquor was stirred for 3 h, and the precipitate was enzymatic hydrolysis lignin (EHL). 2,2′-Azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS), Folin Ciocalteau’s phenol reagent (2 N), hydroiodic acid (57%), vanillin, methyl iodide, B


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Figure 1. Diagram of NaOH/urea aqueous system activation and sulfomethylation of EHL. Preparation and Characterization of Self-Assembly Film. Self-assembled multilayer films were prepared with the alternating adsorption of EHL and PDAC as described in our previous report.27 EHL or EHL-(T) was first added in double distilled water to get the polyanionic solution (1 g/L). Then the pH of the solution was adjusted to 11.0 using diluted NaOH solution. 0.1 mM of PDAC solution was selected as the polycation. The morphological images of the films were observed using an atomic force microscope (AFM) (XE-100, Park Systems, Korea) with the tapping mode. Root-mean-square height was calculated for determining the surface roughness of the prepared films. Dispersion Properties. The dispersion performance of EHL on graphite suspension was determined using Turbiscan Lab Expert (Formulaction, France). Three wt % graphite powders were added in 0.1 wt % lignin samples to obtained graphite suspension. Then the suspension was measured after stirring at 1000 rpm for 20 min. This method could be used to detect particle size changes and phase separation with the time increase and the aggregation extent of graphite powders was observed.

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indicated sulfomethylation reactivity of EHL was significantly enhanced after activation. Moreover, low treatment temperature was beneficial for the activation process, which was similar to the results described before.16 Purity and Molecular Weight Distribution of EHL. There is always some unhydrolyzed cellulose in EHL because of the mild enzymatic hydrolysis process. The purity is the important factor that influences the reactivity of EHL. The cellulose and lignin contents of samples were explored at different treatment temperatures (Figure S1). The purity of EHL increased and the content of cellulose decreased after activation of NaOH/urea aqueous solution compared with unactivated EHL. Moreover, the purity of EHL increased and the content of cellulose decreased continuously with the treatment temperature decrease. When the treatment temperature was −10 °C, the purity of EHL was increased to 92.59% from 75.01% and the content of cellulose was decreased to 1.56% from 3.59%. The molecular weight of lignin also has a significant effect on its application. Our early study demonstrated lignosulfonates with higher molecular weight had better dispersion property.28 So the EHL with higher molecular weight would also have a better dispersion performance after sulfomethylation. The weight-average (Mw) and numberaverage molecular weight (Mn) and the polydispersity (Mw/ Mn) of EHL are listed in Table S1. After the activation of NaOH/urea aqueous solution, Mw of EHL all decreased. The hydrogen bonds and ether bonds were broken between LCCs, which resulted in the removal of residual cellulose and other carbohydrate. At the same time, the EHL was depolymerized partly. So Mw of EHL decreased after activation. However, Mw increased obviously with the decrease of treatment temperature. Low temperature was favorable to the dissolution of cellulose, but it was not favorable to the depolymerization of lignin. Thus, low treatment temperature was beneficial to getting high molecular weight and high-purity EHL. Functional Group Content of EHL. In order to reveal the reaction mechanism of activation, the phenolic group content and methoxyl group content of EHL were determined before and after activation. As Figure 2 shows, the content of the phenolic group increased, while the content of the methoxyl group decreased with decreasing treatment temperature. When the treatment temperature was −10 °C, the phenolic group content increased from 1.21 mmol/g to 1.45 mmol/g and the methoxyl group content decreased from 1.33 mmol/g to 1.05 mmol/g compared

RESULTS AND DISCUSSION

Influence of NaOH/Urea Aqueous System Activation on Sulfomethylation Reactivity of EHL. The process of NaOH/urea aqueous system activation and sulfomethylation of EHL is shown in Figure 1. EHL was first activated at certain temperatures with NaOH/urea aqueous solution, and then sulfonic groups were introduced by sulfomethylation. The study mainly focused on the effect of treatment temperature on the reactivity of EHL at a given NaOH/urea/EHL ratio (8%/12%/ 100%) and treatment time (12 h). The sulfonation degree of sulfomethylated EHL was used to evaluate its sulfomethylation reactivity. As shown in Table 1, the sulfonic group contents of SEHL-(T) all obviously increased compared with unactivated SEHL. In addition, the sulfonic group contents of SEHL-(T) increased as the treatment temperature decreased. When the treatment temperature was −10 °C, the sulfonic group content reached 2.13 mmol/g and the sulfomethylation reactivity was 62.60% higher than that of EHL unactivated. The results Table 1. Influence of Treatment Temperature on Sulfonic Group Content of SEHL treatment temp (°C)

SEHL products

sulfonic group content (mmol/g)

\ 5 0 −10

SEHL SEHL-(5) SEHL-(0) SEHL-(−10)

1.31 1.54 1.71 2.13 C


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while, the adsorption peak near 1322 cm−1, attributed to syringyl ring breathing with C−O stretch vibration, also decreased.21 It was suggested that the amount of syringyl units decreased after the activation, which may result in the decreasing of methoxyl groups in EHL. The peak near 1250 cm−1 for the C−O stretching vibration of the phenolic hydroxyl groups was enhanced. Moreover, the C−O stretching vibration of aliphatic C−OH and methylol C−OH near 1030 cm−1 was also increased after activation.32 These all confirmed that the ether linkages between LCCs were broken, and then more hydroxyl was informed. 2D-HSQC Analysis. In order to get an in-depth comprehensive structural characterization of EHL before and after activation with NaOH/urea aqueous solution, EHL and EHL-(−10) were subjected to 2D-HSQC analysis (Figure 4). The substructures of lignin such as syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units were identified through the cross peaks in 2D-HSQC spectra.33 In the aromatic region, the main C−H correlations from different lignin units were used to estimate their relative abundances as shown in Table 2. A great number of cross-signals, resulting from different units of EHL, were observed from the aromatic region (δC/δH 95−160/5.5− 8.5 ppm) in 2D-HSQC spectra. For example, S units showed strong signal for C2,6-H2,6 at δC/δH 104/6.60, G units exhibited strong signal for C5−H5 at δC/δH 115/6.66, and H units showed strong signal for C2,6-H2,6 at δC/δH 128/7.12.26,34 The aromatic region signals of EHL after activation were enhanced obviously, especially the signals from G and H unites, compared with the original EHL, which confirmed that the purity of EHL increased significantly. In addition, the S/G ratio of EHL could be estimated by relevant region. The S/G ratio decreased from 1.4 to 0.27 after activation. According to the previous reporter, the sulfomethylation reactivity of G and H units were higher than S units owing to more ortho position of phenolic hydroxyl.21 Thus, the reactivity of EHL was improved with activation of NaOH/urea aqueous solution. This result was consistent with the decrease of methoxyl group content after activation determined above. Influence of NaOH/Urea Aqueous System Activation on Aggregation of EHL. The EHL/PDAC and EHL-(T)/ PDAC layer-by-layer self-assembly films were obtained through alternative absorption of EHL or EHL-(T) with PDAC on quartz glass. Atomic force microscopy (AFM) was used to investigate the morphology of multilayer with EHL or EHL-(T) as the outmost layer. Thus the aggregation behavior of EHL before and after activation was investigated with AFM. As shown in Figure 5, before the activation, the root-mean-square roughness (RMS) of the EHL/PDAC bilayer was 12.59 nm and large aggregate of EHL was seen from the image of AFM. After the activation, the RMS of EHL-(5)/PDAC, EHL-(0)/ PDAC, and EHL-(−10)/PDAC were 3.53, 2.06, and 1.83 nm, respectively. The peak of EHL macromolecule became narrower and smaller after the activation treatment, which indicated that the aggregation degree of EHL was weakened. The results showed that the aggregation behavior of EHL in aqueous solution was reduced effectively after the activation of NaOH/ urea aqueous solution. Enhancing Mechanism of NaOH/urea Aqueous System on Sulfomethylation Reactivity. The hydroxymethylation, which belongs to electrophilic addition reaction, is the key step of sulfomethylation of EHL. Hydroxymethylation is easier to occur on the carbon atom rich in electron cloud. Our previous report has indicated that sulfomethylation of lignin

Figure 2. Influence of treatment temperature on functional group content of EHL.

with unactivated EHL. In the process of activation, the cleavage of hydrogen bands and ether bonds between LCCs resulted in the stripping of cellulose and other carbohydrate from EHL, which improved the purity of EHL. Thus, the content of phenolic groups increased after activation. Moreover, EHL was oxidized easily with alkali catalyst. Base-catalyzed oxidation of lignin was mainly because of the rupture of the β-O-4 ether bond.29,30 There were higher content of ether bond and lower content of C−C bond in the syringyl unit compared with the guaiacyl unit and hydroxyl-phenyl unit. So the syringyl unit, with a higher content of methoxyl, was easier to be stripped and removed with cellulose after the activation of EHL. Thus, the content of methoxyl group decreased after the activation of NaOH/urea aqueous solution. IR Analysis. In order to more deeply explore the effect of activation with NaOH/urea aqueous solution on EHL, IR spectroscopy was used to study the structural changes of EHL before and after activation. EHL-(−10) was chosen as representative sample to study as shown in Figure 3.

Figure 3. IR spectra of EHL before and after the activation of NaOH/ urea aqueous system.

The corresponding assignments and references are summarized in Table S2. The broad adsorption peak around 3400 cm−1 corresponded to the stretching vibration of the hydroxyl groups (O−H). The peak near 2930 cm−1 was due to the C−H stretching vibration of the −CH2 and −CH3. The multiband from 1450 to 1620 cm−1 was observed, indicating aromatic skeletal vibrations in EHL. It is noteworthy that the peak at 2850 and 1450 cm−1 assigned to the C−H stretching vibration of the −OCH3 group decreased obviously after activation.31 MeanD


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Figure 4. 2D-HSQC spectra of EHL before and after NaOH/urea aqueous system activation.

Table 2. 13C−1H Correlation Signals Assignment and Relative Proportion of Monomers in 2D-HSQC Spectra of EHL

Figure 5. AFM images of EHL/PDAC and EHL-(T)/PDAC self-assembly film.

occurred on the carbon of the ortho position of Ph−OH.35 The content of Ph−OH increasing along with the content of -OCH3

decreasing could lead to the increase of active sites, which would enhance its sulfomethylation reactivity. The results measured E


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Figure 6. Charge density of EHL model before and after activation.

because of lower C−C bonds content. Low temperature was favorable for forming hydration layers on cellulose and lignin, which stabilized the EHL solution through preventing agglomeration of the obtained EHL mixture. The syringyl unit, with higher content of methoxyl and low reactivity, was easier to be removed with residual cellulose at lower temperature. Therefore, the content of the phenolic group increased and the content of methoxyl group decreased with the decrease of treatment temperature, which resulted in the enhancement of sulfomethylation reactivity. However, low temperature was unfavorable for the oxidation of lignin. So the Mw increased with the decrease of treatment temperature. The content of phenolic and methoxy groups was essential to the sulfomethylation reactivity of EHL. For further exploring the relationship between groups content and sulfomethylation reactivity of EHL, the relativity between them was analyzed (Figure 7). With different treatment temperature, there was a

above have confirmed that there was more phenolic group content and less methoxyl group content in EHL after activation. Moreover, 2D-HSQC showed there was more content of guaiacyl and less content of syringyl. Thus the sulfomethylation of EHL was enhanced after the activation of NaOH/urea aqueous solution. The steric hindrance and charge density were also the important factors affecting the reactivity of EHL. The AFM analysis showed that the aggregation degree of EHL was weakened efficiently after the activation. The NaOH/urea aqueous system could disrupt hydrogen bonds and ether bands between LCCs and remove the residual carbohydrate (mainly including cellulose). Lignin-cellulose complexes (EHL0) linked with a glycosidic bond were chosen as the molecular model of EHL before activation, and lignin (EHL1) was chosen as the molecular model of EHL after activation. Then EHL model componds were optimized based on energy minimization by means of AM1 in the semiempirical molecular orbital (MOPAC) method using the Chem3D Pro 14.0 software. The charge density of model complexes was shown in Figure 6. The hydroxymethylation of lignin, a key intermediate reaction of sulfomethylation, occurred on a carbon atom of the benzene ring. Moreover, hydroxymethylation was considered to take place on the C5 position of the benzene ring (ortho position of phenolic hydroxyl) and the position of C5 was shown in Figure 6. So the charge density of C5 was calculated and decreased after activation as shown in Table 3. It indicated that C5 positions Table 3. Charge Density on C5 and Steric Energy of EHL Model Charge density EHL model

C5

Steric energy (kcal/mol)

EHL0 EHL1

−0.09 −0.12

68.91 9.73

Figure 7. Relativity between phenolic group, methoxyl group content in EHL, and sulfonic group content in SEHL.

donate electrons more easily after activation, resulting in the occurrence of electrophilic addition reaction. In addition, the steric energy of model complexes decreased from 68.91 to 9.73 kcal/mol after activation. Steric energy was decreased by 1 order of magnitude, which indicated steric hindrance was greatly reduced after the activation of NaOH/urea aqueous solution. The sulfomethylation reactivity of EHL increased with the decrease of treatment temperature. Hydrogen bands and ether bonds between LCCs were disrupted in the NaOH/urea aqueous system, which resulted in the stripping of cellulose and other carbohydrate from EHL. Moreover, EHL was depolymerized partly and the syringyl unit of EHL was easily stripped

linear relation between the sulfonation degree of sulfomethylated EHL and the methoxyl group content of EHL and the correlation coefficient R2 was 0.999. Meanwhile the sulfonation degree of sulfomethylated EHL and the phenolic group content of EHL presented an exponential correlation relationship. The results indicated that the decrease of methoxyl group content and the increase of phenolic group content enhanced the sulfomethylation reactivity. EHL was often connected with carbohydrate (mainly including cellulose) through a large number of ether bonds and hydrogen bonds, which resulted in the aggregation of EHL. F


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Figure 8. Activation of EHL in NaOH/urea aqueous system under low temperature.

The mechanism of the activation by NaOH/urea aqueous solution is shown in Figure 8. As seen in Figure 8a, when EHL was immersed in the low-temperature NaOH/urea aqueous solution, the lignin-carbohydrate complexes were surrounded by NaOH hydrates, urea hydrates, and free water. Then NaOH hydrates and urea hydrates destroyed the hydrogen bonds and ether bonds of lignin-carbohydrate complexes as shown in Figure 8b, which resulted in the stripping of carbohydrate and weakened aggregation degree of EHL. The β-O-4 ether bond was easily disrupted in EHL. So the syringyl unit, lower content of C−C bond, was easier stripped from EHL. At last, lignin and carbohydrate in mixed solution were attached by NaOH hydrates and urea hydrates like an overcoat, which resulted in the stability of the solution corresponding to Figure 8c. Activated EHL was collected through adjusting the pH of mixed solution to ∼3, while cellulose and a small part of lignin (mainly including syringyl lignin) were removed together. Dispersion Properties. It is well-known that graphite is widely used in various fields because of its self-lubricating property, corrosion resistance, high temperature resistance, and so on.36,37 Due to the hydrophobicity properties, graphite particles often tend to form an agglomerate, which restricts its applications in industry.38 The dispersion stabilization of graphite is essential for its end-use. In this study, the influence of activation with NaOH/urea aqueous solution on dispersion properties of suifomethylated EHL on the graphite suspension was studied. The smaller the particle was, the more stable and less aggregation extent the suspension was. As displayed in Figure 9, the dispersion performance of SEHL-(−10), SEHL(−0), and SEHL-(5) were obviously better than SEHL because of the higher sulfonic group. Furthermore, the dispersion properties of SEHL-(−10) was best among them. After adding SEHL-(−10), it was adsorbed on the surface of graphite particles. High content of sulfonic group was favorable to

Figure 9. Influence of SEHL with NaOH/urea aqueous solution activation on the particle size of graphite slurry.

enhance the electrostatic repulsion and steric hindrance between graphite particles. Thus, the dispersion performance of sulfomethylated EHL was improved significantly through the activation of the NaOH/urea aqueous system.

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CONCLUSIONS This study demonstrated that activation with NaOH/urea aqueous system under low temperature could improve the sulfomethylation reactivity of EHL significantly. NaOH hydrates and urea hydrates could destroy the hydrogen bonds and ether bonds between lignin-carbohydrate complexes and then lead to a major increase in the purity of EHL, while also dramatically reducing the aggregation degree of EHL. When the treatment temperature was −10 °C, the phenolic hydroxyl group content increased from 1.21 mmol/g to 1.45 mmol/g and the G


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methoxyl group content decreased from 1.33 mmol/g to 1.05 mmol/g compared with unactivated EHL. Sulfomethylation reactivity of EHL increased 62.60%. The dispersion effect of sulfomethylated EHL-(T) on graphite suspension was improved significantly. Therefore, NaOH/urea aqueous system activation is a promising method for improving the reactivity of EHL. This work represented an activation way to adjust the microstructure of EHL and increase the active sites for enhancing its reactivity. The activation strategy provided a theoretical basis for the highly effective modification of EHL. It also has an important significance for high value-added application of EHL in the field of dispersant.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04781.

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Influence of treatment temperature on the purity of EHL, molecular weight of EHL under different treatment temperatures, IR band assignments of EHL before and after NaOH/urea aqueous system activation. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D. Yang). Tel.: 86-2087114722. Fax: +86-20-87114721. *E-mail: [email protected] (X. Qiu). Tel.: 86-20-87114722. Fax: +86-20-87114721. ORCID

Dongjie Yang: 0000-0002-6987-0800 Yong Qian: 0000-0001-9164-5119 Xueqing Qiu: 0000-0001-8765-7061 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of National Natural Science Foundation of China (21690083, 21436004, 21878114), Natural Science Foundation of Guangdong (2017A030308012), and Guangdong Province Science Foundation (2017B090903003).

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