Article pubs.acs.org/IECR
Reaction-Free Lignin Whitening via a Self-Assembly of Acetylated Lignin Yong Qian,† Yonghong Deng,*,† Hao Li,† and Xueqing Qiu*,† †
School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China 510640 ABSTRACT: Acetylated lignin (ACL), obtained from the acetylation of alkali lignin (AL), has been found to self-assemble into ordered colloidal spheres in a THF/H2O media. This colloid formation causes the color of ACL to change from dark brown to light yellow. ACL molecules form colloidal spheres at critical water content (CWC) of 48 vol % for an initial concentration of ACL in THF of 0.4 mg/mL. The colloidization process is completed at a water content of 77.75 vol %. The colloidal spheres formed from the gradual hydrophobic aggregation of ACL were approximately 80 nm in size. The concentration of Fe3+ before and after colloid formation did not change significantly, which indicates that the whitening of ACL colloids is not influenced by metal ions, but is affected by the rearrangement of chromophores during self-assembly and the light reflectance of uniform particles. Self-assembly of ACL into colloidal spheres not only whitens the dark color of lignin but also provides a novel method to prepare ordered lignin-based nanomaterials.
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INTRODUCTION In recent years, the amount of unexploited fossil energy sources has depleted rapidly, increasing the need for alternative and sustainable resources. Lignin is the second most abundant renewable biomass resource, but large quantities of lignin are obtained as a byproduct of the pulp and paper industry.1−4 Lignin is nearly colorless in wood, but alkali lignin (AL) that is separated from pulping black liquor has a strong black color. This black color is due to a variety of chromophores that are introduced into the structure during the pulping processes and isolating procedures. AL is mainly treated by hydrophilic modification to improve its solubility in water for industrial applications such as concrete water reducers, water-coal-slurry dispersants, and dyestuff dispersants.5−7 AL and its derivatives give negative effects in many application areas due to the intensively darkened color. For example, if a lignin-based biopolymer is used as a dyestuff dispersant, it will cause staining problems and distort the authentic color of the dye in the fabric fibers. The chromophore formed and the mechanism for its formation during the pulping process are not completely understood but result in the dark black color. Five possible chromophores have been proposed: (1) carbon−carbon double bonds conjugated with the aromatic ring; (2) quinonemethides and quinones; (3) chalcone structures; (4) free radicals; and (5) metal complexes with catechol structures. The first two chromophores contribute most of the color in the lignin, while the latter three only contribute to a minor extent.8 To improve high-value utilization of lignin resources, it is necessary to reduce the dark color of lignin-based biopolymers. The two-step processes for reducing the color of sulfonated alkali lignin and lignosulfonate has been reported.9,10 These processes include the blocking of phenolic hydroxyl groups in the lignin with known blocking agents, followed by oxidation with air, molecular oxygen, hydrogen peroxide, or chlorine dioxide. Compared with the unmodified sulfonated alkali lignin and lignosulfonate, the light-colored lignin dispersants prepared © 2014 American Chemical Society
by the two-step process exhibit a lower staining and a lower dye reduction on the fabric fibers.10 Acetylated lignin (ACL) naturally exists in hardwoods such as kenaf.11 However, it is usually regarded as nonexistent after being hydrolyzed and removed during the isolation and degradative procedures.12 In most conditions, AL is acetylated so it can be dissolved in solvents for NMR analysis.13 Surprisingly, recent findings show that ACL, a random branched biopolymer, can also form nanocolloidal spheres in selected solvents.14 This self-re-organization can be utilized to modify lignin without chemically reacting it. In this work, AL was acetylated to block the free phenolic hydroxyl groups to reduce the color of lignin. The color was further whitened through the self-assembly of ACL into colloidal nanoparticles. The formation of the ACL colloids and its whitening mechanism was investigated using static light scattering (SLS), dynamic light scattering (DLS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and atomic absorption spectroscopy (AAS). Overall, this work provides a new method to whiten lignin without the use of reactive mediators.
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EXPERIMENTAL SECTION
Materials. Wheat alkali lignin (AL) was obtained from Quanlin Paper Mill Co. Ltd. (Shandong, China). Prior to acetylating, the AL sample was processed by acidification and filtration, and was washed a minimum of three times. Acetyl bromide was purchased from SigmaAldrich (Shanghai, China). Deionized water (resistivity ⩾18 MΩ/cm) was obtained from a Millipore water purification system and was used for the experiments conducted in this work. Other reagents and solvents were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) without further purification. Received: Revised: Accepted: Published: 10024
March 10, 2014 May 8, 2014 May 24, 2014 May 25, 2014 dx.doi.org/10.1021/ie5010338 | Ind. Eng. Chem. Res. 2014, 53, 10024−10028
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Acetylation of AL. A purified lignin sample was acetylated according to the procedure reported by Guerra.15 After acetylation, most of the hydrophilic hydroxyl groups were modified into hydrophobic ester groups, and the obtained acetylated lignin could be effectively dissolved in THF. The structural changes of AL before and after acetylation were characterized by FTIR. As shown in Figure 1, a stretching vibrational band of the hydroxyl groups between 3200
The absorption of ACL in the investigated concentration range was very weak at an incident laser wavelength of 632.8 nm; therefore, the effect of absorption on light scattering was neglected. The details of light scattering theory can be found in our previous work.18 All the samples were measured at a temperature of 25 °C after being filtered with 0.45 μm PVDF Millipore syringe filters, with the goal of removing possible interference from dust.
Figure 1. FTIR spectra of AL and ACL.
RESULTS AND DISCUSSION Whitening Treatment from AL to ACL Colloids. We have found that the dark black color of AL can be whitened to dark brown by acetylation treatment, and the dark brown color of the acetylated lignin can be further whitened to light yellow when it is dissolved in a THF/H2O mixture, followed by drying. The free phenolic hydroxyl groups contribute greatly to the dark color of lignin.9,10 Therefore, it is possible to whiten the dark color of AL by an acetylation treatment to block the free phenolic hydroxyl groups. Potentiometric titration measurements indicate that 94% of the phenolic hydroxyl groups in AL can be modified into ester groups of ACL.14 This results in the dark color of the AL significantly fading due to the blocking of the phenolic hydroxyl groups. The dark black color of AL fades to a dark brown color after the acetylation reaction, which is illustrated in Figure 2. In addition to this, acetylation of
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and 3400 cm−1 has a significant decrease after acetylation. Characteristic ester groups (CO and CO) corresponding to the stretching vibrational absorptions are seen to appear at 1675−1820 cm−1 and 1100−1300 cm−1. These results indicate that most of the hydroxyl groups were modified during the acetylation of alkali lignin via acetyl bromide. Characterization. Fourier transform infrared (FTIR) spectra were recorded in the range 800−4000 cm−1 on a Vector 333 FT-IR spectrometer (Bruker, Germany). Content of phenolic hydroxyl groups in the AL before and after acetylation were measured by nonaqueous potentiometric titration using the automatic potentiometric titrator (809 Titrando, Metrohm Corp. Switzerland).16 During nonaqueous potentiometric titration, the titration is carried out in DMF, using tetra-n-butyl ammonium hydroxide (TnBAH) as the titrant, and p-hydroxybenzoic acid as an internal standard. The electrodes are immersed in the solution, which is then titrated with TnBAH under a nitrogen atmosphere. SEM images of the colloidal spheres were obtained by using a Nova NanoSEM 430 electron microscope with an accelerating voltage of 10 kV. The SEM sample was prepared by dropping colloidal dispersions onto the silica slice and then dried at 25 °C for 24 h. Gold was sprayed prior to characterization. AFM images were observed using AFM (Park XE-100, Park SYSTEMS Co., Korea) in tapping mode. The AFM samples were prepared by immersing the quartz slide into diluted colloidal dispersions with a specific angle, pumping out the dispersions as slow as possible and drying the slide coated with ACL colloids under room temperature dispersions for 24 h. The absorbance of Fe3+ in different solutions was determined by atomic absorption spectroscopy (AA-6800, Shimadzu, Japan). A hollow cathode discharge lamp operated at 10 mA was used as the radiation source. The operational conditions were as follows: purge gas flow, argon, 250 mL/min and stop gas flow at atomization stage; wavelength, 283 nm; slit width, 0.7 L. The absorbance values are the average values of five measurements. Light scattering experiments were performed on the light scattering instrument (ALV/CGS-3, ALV GmbH, Germany) equipped with a multi−t digital time correlator (ALV-7004) and a solid-state He−Ne laser (JDS-Uniphase, output power = 22 mW, 632.8 nm). A high performance laser-line bandpass filter (Edmund, NT47-494) was placed between the sample solution and the photomultiplier to avoid an overestimation of the scattered intensity caused by fluorescence.17
Figure 2. Photos of (a) AL raw material, (b) particles obtained from ACL in THF, (c) freshparticles obtained from ACL in THF/H2O mixture, and (d) particles obtained from ACL in THF/H2O mixture followed by storing for three months.
AL also increased its hydrophobicity allowing for dissolution in THF. It was surprising to find that light yellow particles (Figure 2C) were obtained when excessive water was added into an ACL/THF solution followed by spray drying. When the ACL particles were stored for three months, their color darkened to some degree as shown in Figure 2D. This darkening may be related to the “unblocked” phenolic hydroxyl groups in lignin, which are further oxidized with oxygen in the air.9 The major topics studied in this report are as follows: (1) What happens when ACL is dissolved into a THF/H2O mixture? (2) How can the dark brown color of ACL be whitened into light yellow without further reaction? Formation of Colloidal Spheres by Self-Assembly. To explore what happens when water is added to the ACL/THF solution, light scattering was used to monitor colloidal formation. ACL was first dissolved in THF to form a 0.4 mg/mL ACL solution. No change in scattered light intensity was observed until the H2O content was increased to the critical value of 48 vol %, as seen in Figure 3. Beyond this critical volume, the ACL has formed colloids in the solutions resulting in a dramatic increase in scattering intensity. Several studies have shown that amphiphilic polymers tend to form colloids at critical water content (CWC) in selective solvents.19,20 10025
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Figure 3. Scattered light intensity as a function of added water content in the ACL/THF solutions. The initial concentration of ACL in THF was 0.4 mg/mL.
Figure 4. Fraction of the associated ACL molecules as a function of the water increment beyond CWC. Arrow indicates the water content where 99.5% of lignin molecules are involved in the colloidal dispersion of the ACL.
The CWC depends on both the initial ACL concentration in THF and the molecular weight.19 For ACL with a specific molecular weight, there is an inverse linear relationship between the CWC and the logarithm of the initial concentration (C0) of the ACL.14 CWC = − 12.93 log C0 + 55.66
(1)
The colloidal fraction of ACL as a function of the water content can be estimated using a method reported in our previous work.21 Equation 1 can also be rearranged for C0: C0 = exp[2.303(55.66 − CWC)/12.93]
(2)
When the water content is beyond CWC, more and more ACL molecules associate to form colloidal spheres, and the concentration of the unassociated ACL molecules (Cunass) decreases. Cunass in colloidal dispersions obeys a relationship similar to eq 2. Therefore, a similar equation can be obtained Cunass = exp[2.303(55.66 − Cw )/12.93]
Figure 5. Rh distributions of ACL molecules in THF and colloidal spheres in water based on the CONTIN algorithm. The concentration of ACL in THF was 0.4 mg/mL, and the DLS measurement was performed at 60°.
(3)
where Cw represents the water content (vol %) in the solution. The colloidal fraction can be defined as the ratio of associated chains to total chains, (C0 − Cunass)/C0. By combining eq 2 and 3, the following relationship can be obtained: (C0 − Cunass)/C0 = 1 − exp[2.303(CWC − Cw )/12.93] = 1 − exp[−2.303ΔH 2O/12.93]
(4)
where ΔH2O represents the water increment beyond CWC. By using eq 4, the change of (C0 − Cunass)/C0 as a function of ΔH2O can be calculated. A plot of (C0 − Cunass)/C0 against ΔH2O is shown in Figure 4. When ΔH2O is 29.75 vol % (i.e., water content is 77.75 vol % when initial concentration of ACL in THF is 0.4 g/L), the percentage of ACL molecules involved in the colloids is calculated to be 99.5%. Therefore, all “isolated” single ACL molecules in the solution form colloids when the water content is more than 77.75 vol %. After reaching a water content above 77.75 vol %, an excess amount of water was added into the dispersion, followed by dialyzing against water to remove THF. The size of the ACL molecules and ACL colloids was determined by DLS. Figure 5 shows the average size of the ACL molecules was 1.5 nm, while the average size of the colloids was approximately 90 nm. Figure 6a shows a typical SEM image of the ACL dispersion. As
Figure 6. SEM and AFM images of the colloidal spheres obtained from the ACL dispersion. The initial concentration of ACL in THF was 0.4 mg/mL.
shown in Figure 6b, uniform colloidal spheres can be obtained from the ACL colloidal spheres by careful arrangement. The average radius of the spheres is estimated to be about 80 nm, obtained from the statistics of 100 contiguous spheres in the AFM image. The size of ACL colloidal spheres obtained through AFM was slightly less than the DLS result, which is reasonable since the AFM image was obtained under dry conditions. 10026
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Table 1. Absorbance of Fe3+ in AL, ACL, ACL Colloid Solutions, and the Solvent of the ACL Colloids samplesa Fe
3+
absorbance
AL solution
ACL solution
ACL colloid solution
solvent of the ACL colloids
0.8113 ± 0.0547
0.8057 ± 0.0141
0.8087 ± 0.0134
0.1412 ± 0.0171
a
The concentrations of AL in THF, ACL in THF, ACL colloid solutions in THF/H2O mixture are all 1 g/L. The solvent of the ACL colloids is the THF/H2O mixture after removing the ACL colloids by centrifuging.
for forming ACL spherical colloids is through π−π interactions. As proposed by Gierer,25 quinonoids such as stilbenequinone, benzoquinone, and methylene quinone are possible chromophoric structures in lignin. Conjugated quinonoid structures easily form π−π interactions. As a result, large amount of chromophoric groups are packaged inside of ACL spherical colloids suppressing their color. The result is colloids appearing as a light yellow color.
Lignin Whitening and Lignin Colloidal Spheres. Lignin colloid spheres are formed from ACL through gradual hydrophobic association of ACL in a THF/H2O mixture induced by a continuous increase in the water content. Although ACL is whitened when ACL colloidal nanoparticles are formed, the whitening mechanism is still unclear. There are three possible reasons: (1) loss of colored metal complexes; (2) light reflectance of uniform particles; (3) the self-assembled structure of ACL colloids. All three possibilities were studied to determine what the likely cause of the whitening is. During the self-assembly process of ACL, there is no chemical reaction to reduce the color, but a loss of transition metal ions could contribute to the color reduction. Transition metal ions, such as iron, can be introduced into lignin and form colored metal complexes during pulping processes and isolating procedures.8,22,23 Peart and Ni have studied several transition metal ions, such as Fe3+, Fe2+, Cu2+, Mn2+, and Al3+, and found that the Fe3+ has the strongest effect on the absorption spectra of lignin.23 If there were a significant loss of Fe3+ in lignin during the self-assembly process, the loss of Fe3+ would be an important reason for lignin-whitening. To determine if this was the reason for the whitening, we monitored the concentration of Fe3+ during the self-assembly process. The Fe3+ concentration in different solutions was determined by AAS, as show in Table 1. When ACL colloids were formed by the addition of water, the absorbance of Fe3+ in the ACL colloidal solution remained nearly unchanged. Very little Fe3+ was found in the remaining solution after the ACL colloids were centrifuged out indicating that most of Fe3+ remains in the ACL colloids and the removal of Fe3+ does not occur during colloidal formation. Therefore, removal of Fe3+ is not the cause of the lignin whitening. Light scattering from particles of about 100 nm produces red or yellow in air, while scattering from micrometer-scale particles is nonselective and produces white (Mie scattering). Similarly, ACL colloid spheres with a radius of 80 nm scatter yellow light while scattering from ACL colloidal sphere aggregates produces white. Combination of the two scatterings makes the color of dry ACL colloid spheres appear light yellow. Therefore, the light reflectance of uniform particles should be added as a factor for reducing lignin color but is not the major factor. ACL solid particles are composed of ACL molecular aggregates, which are typically irregular in size and shape. Large amounts of chromophoric groups are distributed on the outside of ACL aggregates, which cause ACL particles to possess a dark brown color. Observing the structure and formation of ACL colloids, they are spherical formed by hydrophobic interactions.14 The hydrophobic effect is related to van der Waals and π−π interactions. The van der Waals interaction is too weak to drive ACL molecules to form colloids in a THF/H2O solution because there is not a large enough carbon chain concentration in the ACL. However, the basic unit of lignin is phenylpropane, making lignin rich in π−π bonds. The π−π interactions between the aromatic nucleuses and other conjugated structures play an important role in lignin intramolecular aggregation.24 As a result, the main driving force
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CONCLUSIONS We found the dark brown color of ACL can be whitened into a light yellow when it self-assembles into ordered colloidal spheres. ACL colloids are formed through hydrophobic association with a cores consisting of a more hydrophobic ACL fraction, and shells consisting of a less hydrophobic ACL fraction. Due to the rearrangement of chromophores into the inner part of the colloid by self-assembly and the light reflectance of uniform particles, ACL spherical colloids whiten into light yellow. This understanding provides significant insight into both the whitening of lignin without reaction and the converting the aggregates of lignin-based polymers from a disordered to an ordered state.
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86-20-87114722. Email:
[email protected]. *Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Basic Research Program of China (973 Program) (2012CB215302), the Chinese Scholarship Council (CSC) and the National Natural Science Foundation of China (21374032, 21176096).
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