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Infra-red spectra from 2OWtOOOcm~ of lignin precipitated from black liquor produced from different pulping processes of bagasse, e.g. soda, kraft, sulfite,.
Polvner

c PII:

1998

SO14l-3910(97)00072-4

Degradation

und Sluhility

60 ( 1998) 241

25

I

Elsevier Science Lmited. All rights reserved Printed m Northern Ireland Ol41-3910/98~%19.00

Infra-red spectroscopic study of lignins Abd-Alla M. A. Nada, Mohamed El-Sakhawy Cellulose

(Received

& Paper

Department,

28 October

National

1996; revised

Research

I I February

Centre.

& Samir M. Kamel Dokki.

1997; accepted

Cairo,

10 March

Egypt

1997)

Infra-red spectra from 2OWtOOOcm~ of lignin precipitated from black liquor produced from different pulping processes of bagasse, e.g. soda, kraft, sulfite, peroxyacid and butanol, have been characterized. Peroxyacid lignins are more degraded than other lignins. However, peroxyacid lignin has a higher intensity band at 1720cm-’ than other types of lignins. At the same time, the aromatic ring of lignin produced from peroxyacid pulping of bagasse undergoes severe degradation. Syringyl type of lignin is predominant in all isolated lignins. Peroxyacid and butanol lignins have lower quantities of syringyl lignin shown by the lower ratio of relative absorbance of band intensity at 1500 cm-’ to the band at 1600cm-’ than other lignins. Kraft lignin has a broad weak band at about 630cm-’ that is probably due to a C-S bond. A sharp band at 655cm-‘, which is due to SOsH, is characteristic of lignosulfonate, which is precipitated from black liquor produced from sulfite pulping process of bagasse. Generally, degradation of different lignins during pulping of bagasse with different processes has the following sequence: peroxyformic > peroxyacetic > butanol-water > butanol-alkaIi > kraft > sulfite > soda. ,6 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

isolated from waste black liquor from bagasse pulping with soda, kraft, sulfite, peroxyformic, peroxyacetic, butanol-water and butanol-alkali processes.

All lignins are built from poly phenylpropane units. They are obtained as a by-product from black liquor, which is produced in pulping of lignocellulosic materials to produce cellulosic or modified pulp. The main reactive groups of lignin that are presented in the side-chain are p-substituted phenolic hydroxyl groups. The characterization of lignin has been the subject of much interesti Lignin character is much influenced by the starting lignocellulosic materials. Some of the methoxyl groups of native lignin are hydrolyzed, causing an increase in the phenolic hydroxyl groups of lignin precipitated from black liquor.5,6 The type of pulping process and the different pulping variables, e.g. time, temperature, liquor ratio and pH, have an important role in the properties and chemical constituents of isolated lignin.7,8 Infra-red spectroscopy can be used as a tool to follow the structure of separated lignin, and to give information about the actual chemical groups altered, removed from and/or added to lignin during the pulping process. In this paper, infra-red spectra are used to follow the chemical constituents of different lignins

2 EXPERIMENTAL Bagasse was cooked with different pulping processes. All pulping processes were carried out in electrically heated rotating autoclave, except pulping of bagasse with peroxyacids, which was carried out in polyethylene bags. Conditions of the different pulping processes were as follows: 1. Soda pulping of bagasse: 15% sodium hydroxide (based on weight of bagasse). 2. Kraft pulping: (11.25% sodium hydroxide + 3.75% sodium sulfide) (based on bagasse). 3. Sulfite pulping: (18% sodium sulfite+4% sodium carbonate) (based on bagasse). 4. Butanol-water pulping: [butanol: water (50:50)] 5. Butanol-alkali pulping: [butanol: water (50:50) + 12% sodium hydroxide (based on bagasse)]. All of these pulping processes were carried out with a 5:l liquor ratio (5 solution: bagasse) at 170°C for 2 h. 241

A. M. A. Nada et al.

248

6. Peroxyformic pulping: lOOmI H202 (30%) + 200 ml formic acid + 100 g bagasse, pulping at 90°C for 2 h. 7. Peroxyacetic pulping: 100 ml H202 (30%) + 200 ml acetic acid + 100 g bagasse, pulping at 90°C for 2 h. Lignin was precipitated from soda and kraft black liquor by using 5% sulfuric acid at pH 223. Lignin was precipitated from sulfite black liquor by using methyl alcohol to isolate lignosulfonate. Lignin was precipitated from butanol black liquor by using benzene. Lignin was precipitated from peroxyacid black liquor by using water. Infra-red spectra of these lignins were obtained using a Jacco FT/IR 300E spectrophotometer. The samples were measured as KBr discs.

3 RESULTS

AND DISCUSSION

Infra-red spectra of different lignins isolated from the black liquor by-product of bagasse pulping with different methods are shown in Fig. 1. The band positions and corresponding assignments are given in Table 1. From Table 1, it is seen that the band at 3430 cm-* seems to be characteristic of OH groups of lignin; this band is strong and sharp, whereas the OH vibration of water is very broad. Also, the sample was prepared under vacuum to eliminate the effect of adsorbed water on KBr and sample spectra. The band at 1370cm-’ is due to the bending vibration of the phenolic OH group. Bands at 1140 and 1035 cm-’

Table 1. Assignment of infra-red absorption of lignin Maximum band position (cm-‘) 344&3430 2940-2930 2689-2880 172771690 161&1690 I505 1458 1425 1420 1370- 1250 I260 116CL1140 1044 844 655 630

Band origin

OH stretching (H-bonded) CH stretching of methyl or methylene group CH vibration of methyl group of methoxyl C = 0 stretching Aromatic skeletal vibration Aromatic skeletal vibration CH stretching of methyl or methylene group CH vibration of methyl group Bending vibration of OH band Syringyl ring breathing with CO stretching Guaiacyl ring breathing with CO OH stretching of secondary alcohol OH stretching of primary alcohol Aromatic CH out of plane bending CS vibration of sulfonic group A broad band of CS bond

are characteristic of secondary and primary OH groups, respectively.’ Moreover, it is clear that a shoulder at 28502860 cm-’ is assignable to vibration of OCH3 groups. Bands at 1460 and 1415 cm-’ include a considerable contribution from CH bonds of OCHs groups. Two bands at 1600 and 1500cm- ’ are characteristic of aromatic rings and are due to vibrations of the aromatic skeleton. Also, a band at 836cm-’ which is due to aromatic C-H ‘out of plane’ vibration in p-hydroxy phenylpropane unitI is detected. The band at 1700-l 720cm-’ arises, in lignin, from isolated beta-keto structures. The ratio of absorbance maxima of individual bands to the band at 1600 cm-’ has been recorded in Table 2. The 1600 cm-’ band has been chosen as an internal standards here, as it is present as a strong band in all different lignins. From Table 2, it is clear that the band at 3430cm-‘, which is attributed to OH groups of lignin compounds, has a lower ratio of absorbance in the case of peroxyformic and peroxyacetic lignins, compared with other kinds of lignins (soda, kraft, ligno-sulfonate and butanol lignins). This is attributed to the high oxidation and degradation power of peroxy acids during pulping of bagasse. However, the relative absorbance of this band (3430cm-‘) in the case of peroxyformic acid lignin is lower than in the case of peroxyacetic acid lignin. This can be attributed to the higher oxidation power as well as higher penetration of peroxyformic acid through bagasse during pulping. Comparing the relative absorbance, the bands of primary and secondary OH groups at 1044 and 1120cm~‘, respectively, of butanol-water and butanol-alkali lignins are lower than for soda, kraft lignins and lignosulfonate and higher than for peroxyformic and peroxyacetic lignins. This can be explained by the fact that the precipitated lignin from black liquors produced from bagasse pulping with butanol-water and butanol-alkali undergoes oxidation and degradation during pulping with a lower degree than peroxyacid pulping of bagasse. The relative absorbance of these two bands, 1044 and 1120cm~‘, of the lignosulfonate have higher values than for the soda and kraft lignins. This is attributed to the higher solubility of lignin during the sulfite pulping process in pulping solution in the form of lignosulfonate, which is easily soluble in the produced black liquor. Thus, this lignosulfonate undergoes less degradation than the other types of lignin.

249

Infia-red spectroscopic study of lignins Table 2. Relative absorbance of band of different groups (band intensity of different groups/band 16OOcm-‘) in lignin Bands of different

Relative

groups

absorbance

3430 1370 1165 1043 Mean value of OH groups

2.40 0.35 0.85 0.96 1.29

2.40 0.50 0.80 0.88 1.19

I .86 0.26 1.15 1.17 1.11

1.64 0.22 0.43 0.47 0.69

1.70 0.22 0.50 0.49 0.73

I .80 0.26 0.50 0.60 0.80

1.83 0.30 0.55 0.63 0.82

2890 1460 1420 Mean value of OCH groups

1.45 0.65 0.30 0.80

1.60 0.44 0.38 0.81

1.70 0.60 0.50 0.93

1.45 0.84 0.48 0.92

1.62 0.86 0.39 0.96

1.53 0.84 0.46 0.94

1.66 0.89 0.39 0.98

1600

1.oo 0.80 0.64 0.81

1.00 0.94 0.61 0.85

1.00 0.74 0.41 0.72

1.oo 0.82 0.42 0.75

1.00 0.72 0.60 0.77

1.00 0.80 0.50 0.77

1.oo 0.95

0.62 0.88

1.52 0.96

1.40 0.96

1.30 0.96

I .25 0.96

822 Mean value of aromatic

ring

1720 Mean value of previous

groups

0.72 0.96

655 633

Peroxy-acetic

lignins

Kraft

1.oo 0.95 0.89 0.95

Peroxy-formic

groups in different

Soda

1500

Lignodulfonate

of different

intensity of aromatic ring at

Butanol-water

Butanol-alkali

0.65 0.20

In general, the mean values of the relative absorbances of the different bands of OH groups in different lignins has the following sequence: soda > kraft > lignosulfonate > butanol-alkali > butanol-water > peroxyacetic > peroxyformic. The mean value of the relative absorbance of methoxyl group bands at 2920-2890cm-‘, 1480 and 1420 cm-’ of peroxyacid and butanol lignins is higher than in the case of soda, kraft and lignosulfonate. This can be attributed to the oxidation of hydroxyl groups to carboxyl groups. Moreover, the ratio of the mean value of relative absorbances of hydroxyl group bands to the mean value of relative absorbances of methoxyl group bands is lower in the case of peroxyacid lignins than the butanol lignins. This indicates that lignin produced from peroxyacid pulping of bagasse undergoes degradation more than lignin produced from butanol pulping. Thus, peroxyacid pulping causes an oxidation of lignin and/or formylation of OH groups or ring cleavage to formation of lactones.’ ’ In the case of soda, kraft lignins and lignosulfonate, it is clear that the mean values of relative absorbance of OH group bands are higher than that of 0CH3 group bands. This can be attributed to the fact that, during pulping of bagasse with soda, kraft and sulfite processes, methoxyl groups of lignin undergo partial hydrolysis to phenolic OH groups. This causes an increase of the mean values of relative absorbance of hydroxyl groups. This can be confirmed by the high relative absorbance of phenolic OH group

band at 1370 cm-’ of soda, kraft lignin and lignosulfonate. Also, it can be seen that the relative absorbance of phenolic OH group band at 1370cm-’ of kraft lignin is higher than all other lignins. However, kraft lignin has a lower relative absorbance of primary and secondary hydroxyl groups bands at 1044 and 1120 cm-‘, respectively, than soda lignin and lignosulfonate.12,13 This is due to the oxidation of these bands to carboxylic groups. This can be confirmed by the higher relative absorbance of the carboxylic group band at 1720 cm-’ of kraft lignin than soda lignin and lignosulfonate. For this reason, the mean value of relative absorbance of hydroxyl group bands of kraft lignin is lower than that for soda lignin. The mean value of relative absorbance of the aromatic ring bands at 1600, 1500 and 822 cm-’ compared with the relative absorbance of carboxylic group at 1720cm-’ indicates the decomposition of aromatic ring structures.i4 In the case of soda lignin and lignosulfonate, the relative absorbance of carboxylic group band at 1720 cm- ’ is lower than the mean value of relative absorbance of aromatic ring bands at 1600, 1500 and 822 cm-‘. This means that the aromatic rings of soda lignin and lignosulfonate undergo less degradation than other lignins. In the case of kraft lignin, it exhibits a carboxylic band at 1720 cm-‘, higher than soda lignin and lower than peroxyacid and butanol lignin. From Table 2, it is clear that all different lignins have a lower relative absorbance of aromatic ring

A. M. A. Nuda et al.

attributed to the ring cleavage of lignin aromatic ring during peroxyacid pulping of bagasse to form lactones. From Fig. 1 and Table 2, it is observed that the precipitated lignin from black liquor produced from kraft pulping of bagasse has a broad, weak band at about 630 cm-‘. However, this band is absent in soda and other lignins. This band arises from CS bonds. I3 However, there is one sharp band at 655 cm-’ with a relative absorbance of 0.6, and one broad band at 560-520cm-’ that are characteristic of lignosulfonate. This lignosulfonate was precipitated from black liquor produced from pulping of bagasse with sulfite process. Thus, the mean value of relative absorbance of methoxyl groups, aromatic rings, hydroxyl groups and carboxylic groups bands is lower than other lignins due to the presence of sulfonic in the lignosulfonate that is produced from pulping of bagasse with the sulfite process.

4 CONCLUSIONS

3

I

I

3200

2400

I 1800

I 1400

I 1000

I

I

600

200

Wavenumber (cm-‘) Fig. 1. Infra-red spectra of different lignins: 1, soda; 2, kraft; 6. butanol4, peroxyacetic; 5, peroxyformic: 3, lignosulfonate; water and 7, butanol-alkali lignin.

vibration at 1500cm-’ than the relative absorbance of aromatic ring vibration at 1600cm-‘. This indicates that the syringyl type of lignin has been dissolved in preference to the guaiacyl type lignin. lo Therefore, the syringyl type of lignin is more concentrated in soda lignin than other lignins, as shown by the higher ratio of the relative absorbance of the band at 1500cm-’ to the band at 1600cm-‘. Also, it is seen from Table 2 that the peroxyacid lignins have the lowest ratio of relative absorbance band at 1500-1600cm-’ due to the high degradation of the aromatic ring and consequently guaiacyl type of lignin increases. This can be confirmed by the very weak signal intensity band at 1268 cm- ’ of guaiacyl content of peroxyacid lignin than other lignins.t5 This can be

1. Peroxyacid lignins have lower relative absorbances of different bands of OH groups than other types of lignin, due to the high oxidative effect of peroxyacid during pulping process. 2. Relative absorbances of the carboxyl group bands in peroxyacids and butanol lignins are higher than other lignins. 3 Kraft lignin has a higher relative absorbance of phenolic OH band and lignosulfonate has higher relative absorbances of primary and secondary OH group bands than other types of lignins. 4. The ratio of the mean value of relative absorbance of OCH3 group bands to OH groups bands of peroxyacid lignin is higher than other lignins. 5. OCH3 groups of lignin undergo partial hydrolysis during soda and kraft pulping processes of bagasse more than other pulping processes. 6. The syringyl type of lignin is more concentrated in soda lignin than other types of lignin. 7. A sharp band was seen at 655cm ‘, which was characteristic of lignosulfonate. From the previous discussion, it is clear that kraft lignin is more reactive than other lignins due to its

Infia-red

spectroscopic

higher content of phenolic OH and low OCH3 groups. Lignosulfonate is reactive due to the presence of sulfonic groups that enhance its solubility in water.

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