Structure of Alkali Lignins Fractionated from Ricinus communis and Bagasse. 3. IR Spectra

Abstract

Abstract The lignin fractions isolated by one- and multistage soda and sulfate cookings showed almost identical IR spectra, indicating the similarity of the lignin skeletal structure throughout the plant. However, the absorbances reveal some differences. Similarity of the spectra includes: 1) chelation and bonding of the hydroxyl groups. 2) Stretching vibration of C-H bonds in methyl, methoxyl, and methylene groups. 3) Stretching vibration of C[tbnd]N. 4) Carbonyl unconjugated β-ketone, conjugated acids, or esters at 1725 cm−1. There is no change in the intensity of absorption at this band from that at 1515 cm−1 with the cooking stage. 5) Aromatic skeletal vibration at 1610 and 1515 cm−1, affected by ring substituents at 1425 cm−1. 6) The band at 1465 cm−1 showed a higher intensity for soda and soluble kraft lignins than for insoluble kraft ones. 7) The band at 1370 cm−1, assigned to phenolic OH bending, is affected by the methoxyl group. 8) The absence of condensed guaiacyl and the presence of syringyl a...