FT-IR It is well known that the del

FT-IR
It is well known that the delignification method used for the isolation of lignin plays an important role in the final structure of lignin.
It was considered important to investigate whether structural changes in the lignin would result from various conditions. FT-IR has been shown to be a useful tool in monitoring structural changes in polymers. The FT-IR spectrum and corresponding assignment of lignin are shown in Figure 3 and Table 5, respectively.
The results showed that the FT-IR spectra of all lignin samples were very similar, indicating that there was a similar core structure of lignin.
The band at 3390 cm-1 seemed to be a characteristic of the phenolic-OH groups of lignin, while the band at 2920 cm-1 was attributed to C-H stretching of the -CH3 or -CH2 groups. A shoulder at 2850 cm-1 was assignable to vibration of the -OCH3 groups. The band at 1690 cm-1 corresponded to conjugated carbonyl groups, whereas two bands at 1605 and 1515 cm-1 were characteristic of aromatic rings and were due to vibrations of the aromatic skeleton.
Bands at 1460 and 1425 cm-1 included a considerable contribution from the C-H bonds of the -OCH3 groups.
The Kraft process cleaves β-O-4 and α– O-4 linkages, leaving a lot of non-etherified phenolic-OH groups in the lignin that are visible in the spectrum at 1365 cm-1 (Tejado et al., 2007).
Syringyl units in lignin molecules were clearly seen at 1330 cm-1, while guaiacyl units appeared as a shoulder at 1270 cm-1.
This is in accordance with the results obtained by Ibarra et al. (2005), which showed that the lignin in eucalypt wood and pulps is mainly composed of syringyl units with small amount of guaiacyl units (Ibarra et al., 2005).
Bands at 1155 and 1042 cm-1 were characteristic of secondary and primary -OH groups, respectively. A band at 832 cm-1 was due to aromatic CH out of plane vibration in a p- hydroxy phenylpropane unit of the syringyl units. Due to the scission of ether bonds by the nucleophilic sulfide (S2-) or bisulfide (HS-) ions, a weak band at 650 cm-1 arose from C-S bonds, which is a characteristic of Kraft lignin.
All FT-IR results were in agreement with those of numerous previous works (Nada et al., 1998; El Mansouri and Salvado, 2007; Tejado et al., 2007; Garcia et al., 2009; Yuan et al., 2009).