Effect of Molecular Weight on the Reactivity and Dispersibility of

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Effect of molecular weight on the reactivity and dispersibility of sulfomethylated alkali lignin modified by horseradish peroxidase Zixian Ding, Xueqing Qiu, Zhiqiang Fang, and Dongjie Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02826 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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ACS Sustainable Chemistry & Engineering

Effect of molecular weight on the reactivity and dispersibility of sulfomethylated alkali lignin modified by horseradish peroxidase Zixian Dinga ，Xueqing Qiua ,b*, Zhiqiang Fangb , Dongjie Yanga∗ a

School of Chemistry and Chemical Engineering, bState Key Lab of

Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China *Corresponding author: Tel: +86-20-8711-4722. Fax: +86-20-8711-4721. E-mail Address: [email protected] (Prof. X. Qiu); [email protected] (Prof. D. Yang).

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Abstract: Understanding the relationship between the molecular weight (Mw) and reactivity or dispersibility of sulfomethylated alkali lignin (SAL) modified by horseradish peroxidase (HRP) is of significance for its practical applications such as concrete additives, feed additives, and phenolic resins. In this study, SAL with different Mw was polymerized with HRP to obtain HRP polymerized SAL (HRP-SAL), with the goal of unveiling the effects of the Mw of SAL on the reactivity and dispersibility of HRP-SAL. The results showed that the SAL with a lower Mw exhibited the highest reactivity and a significant increase in the weight-average molecular weight (Mw) by 15.4-fold. At the same time, HRP-SAL polymerized from the SAL with the middle Mw exhibited the largest final Mw and significantly enhances the dispersibility of TiO2. Keywords: molecular weight; sulfomethylated alkali lignin; horseradish peroxidase; reactivity; dispersibility

Introduction Sulfomethylated alkali lignin (SAL) is a modified natural material with excellent water solubility and surface activity.

1

Previous reports have shown that the surface

activity of sulfonated lignin mainly depended on its sulfonic group content and Mw. 2–4 However, obtaining water-soluble SAL with a large Mw is still a challenge. Polycondensation is the main chemical method used in industry to improve the Mw of lignin, but this approach involves toxic organic reagents, and the Mw of lignin obtained by polycondensation is relatively low. Additionally, there is a competitive reaction between the sulfomethylation and polycondensation, because the C5 position


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of the phenylpropane unit in lignin is a reaction site for both sulfomethylation and polycondensation, 5,6 thus impeding the preparation of SAL with both a high degree of sulfonation and large Mw. Recently, environmentally friendly HRP modification has been proposed to prepare SAL with a high degree of sulfonation and large Mw, which endowed lignosulfonate with excellent adsorption properties.

1,7

Generally, SAL with a lower

Mw presents better reactivity because it contains more active functional groups,

8

while SAL with a large Mw has a highly condensed structure and less active functional groups,

9

thus leading to its low reactivity. However, alkali lignin obtained from a

pulp mill has a broad and uneven Mw distribution, which is far from achieving the optimal reactivity for polymerization and desired dispersibility. Therefore, it is of significance to understand the effect of SAL’s Mw on the reactivity of sulfonated lignin incubated with HRP. In this study, an ultrafiltration method is employed to fractionate AL into three fractions with different Mw. SAL and HRP-SAL were both obtained according to our previous study under the optimal conditions1,7. The structural characteristics and dispersion stabilities of different fractions of the modified AL were emphatically investigated, with the goal of understanding the effect of the Mw on the reactivity of SAL in HRP catalytic polymerization. This study provides a way to explore the mechanism of HRP when applied as catalyst in the polymerization of lignin.

Experimental Materials



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AL was isolated from pine pulping black liquor during the sulfuric acid treatment. It was supplied and purified by the Jilin Paper Co. Ltd. (Jilin, China) and used without further purification. 2,2'-Azinobis-(3-ethylbenzthiazoline-6-sulphonate)(ABTS), hydroiodic acid (57%), Folin

Ciocalteu

phenol

reagent,

methyl

iodide,

PDAC

[poly

(diallyldimethylammonium chloride), Mw of 200,000 ~ 350,000)] and vanillin were supplied by Sigma Aldrich (Shanghai, China). All other chemicals were of analytical grade. HRP was supplied by the Xueman Biotech Co. Ltd. (Shanghai, China) and was preserved at -20℃. The activity of HRP was determined in accordance with the method reported by Childs et al.

10

The enzyme activity of HRP detected in this study

was 12708 U·g-1. Ultrafiltration of AL A 5% concentration of AL was treated by fractionation-membrane separation. The membranes were made of polysulfone and manufactured by Separa Tech (Wuxi, China). The molecular weight cut-offs (MWCOs) of the membranes used in the experiments are 2500, 10000 and 50000. In addition, three cut off fractions of 2500– 10000 (AL-1), 10000–50000 (AL-2), and greater than 50000 (AL-3) were obtained and used as raw materials. The fraction of AL less than 5000 was cast out for containing small molecular weight impurities including sugars and inorganic salts. Preparation of sulfomethylated alkali lignin (SAL) The AL sample was first dissolved in sodium hydroxide solutions to the mass


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fraction of 30% in a reaction vessel (Carousel 6, Radleys Corp., England) and heated to 70℃. Subsequently, 15% (based on the mass of AL)

of formaldehyde aqueous

solution was added and stirred at 600 rpm for 1h.Then, it was heated to 95 ℃, and after that, 30% (based on the mass of AL) of anhydrous sodium sulfite was added and stirred for 3 h. At last, the product was adjusted to a pH of 6 and filtered by a Büchner funnel. SAL was obtained from the filtrate by freeze-drying, and the SAL samples acquired from AL1~AL3 were marked as SAL-1~SAL-3. Preparation of HRP-SAL by HRP modification A 20g·L-1 SAL solution was prepared with a buffer solution of NaH2PO4-Na2HPO4, and 6 g·L-1 HRP was added to the solution. Then the polymerization reaction was initiated by adding a 2% volume fraction of H2O2 while stirring at 480 rpm at 30 °C for 2 h. After that, all samples were immediately heated in boiling water for 8 min to inactivate the HRP, followed by a precipitation of the sample through adjusting the pH to 1. After that, the samples were filtered, and HRP-SAL was obtained from the filter residues by freeze-drying. HRP-SAL1, HRP-SAL2 and HRP-SAL3 were prepared by HRP modification using SAL1, SAL2 and SAL3 as materials, respectively. Measurement of molecular weight distribution An aqueous GPC (Gel Permeation Chromatography, UltrahydrageTM250 and UltrahydrageTM120 columns/1515 Isocratic HPLP pump/2487 Dual λ Absorbance Detector, Waters Corp., USA) was used to monitor change of molecular weight before and after HRP incubation. Sodium polystyrene sulfonate in the range from 2000 to 100000 g·mol-1 was used as calibration standards. A 0.10 mol·L-1NaNO3 solution (pH



=10) was used as the eluent at a flow rate of 0.50 mL·min-1. All samples were ion-exchanged and filtered by a 0.22 µm filter prior to detection. Functional group content measurements All samples were purified by ion-exchange before detection to cast out organic acids, salts and impurities. The sulfonic group content 12

11

and carboxyl group content

of lignin samples were measured by an automatic potentiometric titrator (905

Titrando, Metrohm Corp., Switzerland). The phenolic hydroxyl content in the lignin samples was detected by the FC method

13

with vanillin used as the standard. The

methoxyl content in the lignin samples was determined by head-space gas chromatography (HS-GC) 14 with methyl iodide used as the standard. Spectroscopic analysis The UV-Vis spectra of all lignin samples were recorded by a UV-2550 instrument (Shimadzu Corp., Japan), and the absorbances between 190 and 700 nm were measured with a scanning interval of 0.5nm. The IR spectra of all lignin samples were recorded by a Nexus spectrometer (Thermo Nicolet Corp., USA) over a frequency range from 400cm-1 to 4000 cm-1 and the resolving power is 4 cm-1. Tablets were prepared by grinding a certain amount of sample with potassium bromide in a mortar box and pressing them with a tablet machine. FTIR was first calibrated by potassium bromide for background signal scanning prior to the measurement15,16. The 1H-NMR spectra of lignin samples were determined by a DRX-400 instrument (400 MHz 1H frequency, Bruker Corp., Germany). Thirty milligrams of lignin


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samples was dissolved in 0.5 mL of DMSO-d6 to obtain the 1H-NMR spectroscopy data. The spectra were acquired at 30℃. Measurement of the dispersion stability Taking SAL or HRP-SAL as the dispersant, and TiO2 as disperser, the dispersion stabilities of modified products were studied. The mass concentration of TiO2 particles in the dispersion system was 3%, and the mixing amount of the lignin samples was 0.5 wt % based on mass of TiO2. The dispersion stability of the dispersion system was measured by a Turbscan Lab Expert dispersion analyzer (Formulaction, Corp., FR) with a scanning time of 1h every and a measurement taken 2 minutes.

Results and discussion Molecular weights and main functional content of the SAL fractions To determine the influence of the Mw on the reactivity of polymerization, the molecular weights and content of the main functional groups of SAL were first measured, and the results were shown in Figure 1. Table 1 displays the Mw and polydispersity (Mw/Mn). The Mw of SAL increased from 1200 to 2600 Da, and the polydispersities were all approximately 4.0 with increasing Mw values of the SAL. The content of carboxyl group and sulfonic group content decreased while that of methoxyl group increased. All three fractions have similar phenolic group contents.



a

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b

Figure 1. a) Molecular weight distribution and b) main functional group content of different SAL fractions. Table 1 Mw and polydispersity values of different SAL fractions Sample

Mw/Da

polydispersity

SAL1

1200

3.77

SAL2

1600

4.26

SAL3

2600

3.81

Molecular weights and main functional content of HRP-SAL fractions The molecular weights and content of the main functional groups of HRP-SAL were also measured after polymerization (Figure 2). The Mw and polydispersity were calculated likewise, and moreover, the Mw ratio of HRP-SAL (MwH) to the corresponding SAL (MwS) was used to compare the reactivities of different fractions (Table 2).


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a

b

Figure 2. a) Molecular weight distribution and b) main functional group content of different HRP-SAL fractions. Table 2 Mw and polydispersity values of various HRP-SAL fractions Sample

Mw/Da

polydispersity

MwH/ MwS

SAL1

18500

6.86

15.41

SAL2

23400

6.91

14.63

SAL3

20700

6.73

7.96

Compared to the corresponding SAL, the Mw values of HRP-SAL1, HRP-SAL2 and HRP-SAL3 increased rapidly to 15.41, 14.63, and 7.96 times those of SAL, respectively. This was due to the low steric hindrance of SAL with a low Mw which made it much easier to take part in radical polymerization under the catalysis of HRP. In addition, the polydispersity increased from approximately 4 to approximately 6.8, indicating that some small molecules didn’t participate in the polymerization. Despite the highest degree of polymerization of HRP-SAL1, its Mw is lower than that of HRP-SAL2 (As shown in Figure 2b). This results was attributed to the high Mw of SAL2 (1600 Da), which was 33% higher than that of SAL1. As shown in Figure 2b, similar to SAL, the content of carboxyl groups



decreased, while the content of methoxyl group and phenolic group increased with increasing Mw values of HRP-SAL. However, the content of sulfonic group decreased with the increase of the Mw. Compared with SAL, the carboxyl group content of the corresponding HRP-SAL significantly increased by 70 ~ 80%, while the phenolic hydroxyl and methoxyl group content respectively decreased by 37 ~ 49% and 40 ~ 46%. With the increasing Mw of SAL, the MwH/ MwS of HRP-SAL decreased, which indicated that SAL with a low Mw had a high polymerization activity, and the content of phenolic hydroxyl groups decreased simultaneously which further confirmed that phenolic hydroxyl groups played a key role in HRP modification in the published literature. 1 In conclusion, a preliminary presumption was given as follows: a phenolic hydroxyl group was oxidized into a phenoxy radical by HRP to initiate the polymerization reaction. 10 The ortho-position, and para-position of the phenolic group and Cβ of SAL 11 were activated due to the delocalization of phenoxy radicals. Then, SAL polymerized and even further reacted with Na2SO3 12–14,17 through transmitting free radicals to improve the degree of sulfonation. In addition, the increase in number of carboxyl groups and decrease in number of methoxy groups were both due to the strong oxidation characteristics of HRP. The strongly oxidative HRP converted the hydroxyl and aldehyde groups of SAL into carboxyl groups, while the methoxyl groups were oxidized to CH3OH and released, which turned methoxy benzene structures into benzoquinones.

18

At the same time the presence of the phenolic

hydroxyl groups acting as electron donor can help reduce the redox potential of the


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aromatic nucleus to promote the removal of the methoxyl group. 18 With the increasing Mw of SAL, two decisive factors that caused the decrease in HRP polymerization activity should be taken into consideration. First, the low molecular weight SAL with better water solubility possessed more sulfonic groups and carboxyl group which made it easier to combine with the active center of HRP and further produced more free radicals, which were conducive to the polymerization of radicals. Second, the phenolic groups played a vital role in the activation of HRP polymerization, while the methoxyl groups were the major factor which limited the polymerization reactivity of SAL. Additionally, SAL with a low Mw has the highest content of phenolic group and the lowest content of methoxyl groups, and thus, it possessed the highest reactivity. The structures of SAL and HRP-SAL

a



b Figure3. a) IR spectra of different SAL and HRP-SAL fractions. b) 1H-NMR spectra of different SAL and HRP-SAL fractions The IR spectra of different SAL and HRP-SAL fractions were all measured, as shown in Figure. 3a. The broad peak near 3423 cm-1 was attributed to O-H stretching vibrations, the multi-band between 1650 ~1450 cm-1 was assigned to aromatic vibrations, and the band near 1325 cm-1 was due to the C-O stretching vibrations in the syringyl ring along with the in-plane deformation vibration of phenol hydroxyl groups

19,20

. The three different fractions of SAL and HRP-SAL all had the above

typical absorption peaks of lignin after sulfomethylation and HRP modification, indicating that the basic structure of lignin remained in SAL and HRP-SAL. By comparison of the different fractions of SAL, with the increase in the Mw, the absorption peak positions and intensities were rather similar. The results showed that the Mw had a limited impact on the structure and types of functional groups of SAL.


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The Mw only caused a difference in the functional group content of SAL, which cannot be reflected in the IR spectrum. For the different fractions of HRP-SAL, similar results to those with SAL were obtained. In addition, compared with SAL, the characteristic absorption peak of the syringyl ring of the corresponding HRP-SAL near 1325 cm-1 decreased, and the stretching vibrations of the methoxy group at 2844 cm-1 disappeared, which proved that some demethylations occurred during HRP catalytic polymerization. Bands near 1708 cm-1 were attributed to the non-conjugated carbonyl absorption peak. Bands near 1463 cm-1 were assigned to the C-H deformation bands of asymmetric methyl and methylene. The vibrational absorption peak at 1115 cm-1 belonged to typical C-H in-plane bending vibrations of the aromatic ring. SAL had strong absorption in the above absorption band and the absorption band of the aromatic ring skeleton. However, the corresponding absorption peak intensity of HRP-SAL was reduced, and a new strong conjugated carbonyl absorption peak near 1655 cm-1 appeared. This was because HRP catalytic polymerization converted the benzene ring structure of lignin to a benzoquinone structure, thus leading to the formation of a large number of new conjugated carbonyl groups. All SAL and HRP-SAL fractions had strong absorption at 1040 cm-1 corresponding to S=O stretching. The 1H-NMR spectra of different fractions of SAL and HRP-SAL was shown in Figure. 3b. The integrations of different spectra regions were normalized to the DMSO-d6 cross-signal (2.56~2.44 ppm) with the semi quantitative method as shown in Table 3. For different SAL fractions, the main proton signals changed as follows:



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the signals at approximately 7.80 ~ 7.25 ppm, 7.25 ~ 6.80 ppm and 6.80 ~ 6.25 ppm were assigned to the protons of hydroxy-phenyl (H), guaiacyl (G) and syringyl (S) unit, respectively. With the increase in the Mw, the H and G proton intensities of SAL decreased from 1.25 and 1.35 to 1.06 and 1.06, while the S proton intensity increased from 0.74 to 1.14 accordingly. The signals at approximately 6.25 ~ 5.75 ppm, 5.75 ~ 5.24 ppm, 4.90 ~ 4.43 ppm and 4.30 ~ 4.00 ppm were due to the proton signals of β-O-4’, β-1’, β-5’and β-β’structures, respectively. The signal intensities of the above chemical shifts in different SAL fractions were very close, suggesting that the Mw of SAL had a small effect on the content of β-O-4’, β-1’, β-5’and β-β’ structures. The strong signal between 4.00 ppm and 3.20 ppm was attributed to the protons in the methoxyl group. With the increase in the Mw, the proton intensity increased from 6.05 to 7.36 which showed that SAL with a high Mw contained more methoxyl groups.

Table 3 The chemical shifts in the 1H-NMR spectra of different SAL and HRP-SAL fractions δH/ppm

Assignment

11.50-8.0 0

H of carboxyl and aldehyde group Aromatic protons in hydroxy-phenyl units Aromatic protons in guaiacyl units Aromatic protons in syringyl units Hα of β-O-4’ and β-1’ structures Hα of β-5’ structures H of xylan residue Hα and Hβ of β-O-4’ structures

7.80-7.25 7.25-6.80 6.80-6.25 6.25-5.75 5.75-5.24 5.20-4.90 4.90-4.43

21

Intensity SAL1

SAL2

SAL3

HRP-SAL1

HRP-SAL2

HRP-SAL3

1.17

1.08

1.05

1.79

1.50

1.47

1.30

1.25

1.06

1.16

1.12

0.99

1.35

1.42

1.06

0.71

0.77

0.85

0.74

0.76

1.14

0.64

0.58

0.82

0.52

0.43

0.55

0.57

0.54

0.51

0.73 0.60

0.65 0.55

0.52 0.47

0.76 0.61

0.69 0.49

0.56 0.46

1.33

1.22

1.32

1.55

1.39

1.21


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4.30-4.00 4.00-3.20 3.20-3.00 2.50 2.40-2.22 2.22-1.60 1.60-0.38

Hγ of β-β’ structures H of methoxyl group Hβ of β-β’ structures DMSO H of aromatic acetates H of aliphatic acetates Aliphatic protons

1.30 6.05 0.43 1.00 0.33 1.12 2.26

1.37 7.23 0.37 1.00 0.32 0.96 2.48

1.21 7.36 0.70 1.00 0.41 1.23 2.29

1.67 4.26 0.49 1.00 0.26 1.19 3.97

1.66 4.99 0.55 1.00 0.30 1.17 3.87

1.42 5.05 0.60 1.00 0.36 1.29 3.53

For different HRP-SAL fractions, proton signals mainly changed as shown below: all three proton intensities of different HRP-SAL fractions had the same varying trend as that of SAL. Namely, with the increase in the Mw, the H and G proton intensities of SAL decreased from 1.79 and 1.16 to 1.47 and 0.99, while the S proton intensity increased from 0.71 to 0.85 accordingly. Moreover, SAL with a lower Mw possessed higher reactivity and consumed more G protons which led to the decrease in G proton intensity. In addition, the proton signal intensities of β-O-4’, β-1’, β-5’and β-β’ structures decreased from 0.57, 0.76, 1.55 and 1.67 to 0.51, 0.56, 1.21 and 1.42 with the increasing Mw of HRP-SAL. The results indicated that the increase in the Mw was detrimental to the formation of new bonds and thus reduced the reactivity in HRP polymerization. For SAL and the corresponding HRP-SAL, proton signals mainly changed during HRP modification as follows: the aromatic ring proton intensities of H, G and S structure decreased significantly, which may be due to that phenolic hydroxyl groups participated in free radical polymerization reaction and the hydrogen protons of the aromatic ring were substituted or the benzene ring structure was oxidized to a benzoquinone structure. The proton signals of β-O-4’, β-1’, β-5’and β-β’ structures increased, especially, the β-O-4’ and β-β’structures did show a significant increase



which suggested that the HRP catalytic polymerization reaction formed β-O-4’, β-1’, β-5’and β-β’ structures, mainly including β-O-4’ and β-β’ types. The methoxyl proton intensity decreased significantly indicating that HRP modification had the effect of removing the methoxyl group which was consistent with the results of the IR spectra. Multiple absorption signals between 1.60 and 0.38 ppm were assigned to aliphatic proton. Compared with SAL, these characteristic absorption peaks of HRP-SAL significantly enhanced with an increment between 1.24 and 1.71 ppm. This was because the significant increase in the Mw of HRP-SAL led to the enhancement of the shielding effect,

22

and further caused the increase in high field and low shift

hydrocarbon protons. Dispersion stability It is generally known that TiO2 is one of the most important mineral oxides widely used in the areas of ceramics, paints, papermaking, etc.23 Its wide application required good dispersion stability. However, TiO2 without the addition of dispersant could easily form aggregates due to its large surface area and surface properties. In this paper, the influence of different SAL and HRP-SAL fractions on the dispersion stability of TiO2 within 1 h was investigated. The variation of the particle size of TiO2 was shown in Figure. 4.


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Figure.4. Influences of different SAL or HRP-SAL fractions on the particle size of the TiO2 system A control experiment without SAL or HRP-SAL acting as a dispersant was also established, which not shown in Figure.4. The grain sizes of TiO2 in the control run increased rapidly from 16.8 µm to 25.5 µm with the increase in time within 1 h. As shown in Figure.4, the growth of the TiO2 particle size was much slower after the addition of SAL or HRP-SAL. The minimum size of TiO2 was below 4µm, while its maximum size was approximately 6µm. Moreover, the change in the particle size was not remarkable. The results showed that the addition of SAL or HRP-SAL could effectively prevent the aggregation of TiO2 which was beneficial to the preparation of a stable and dispersed TiO2 solution. The particle size of TiO2 with different SAL fractions as the dispersant followed the order of SAL 2

Effect of Molecular Weight on the Reactivity and Dispersibility of

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