Raman spectra of lignin model compounds

Abstract

To fully exploit the value of Raman spectroscopy for analyzing lignins and lignin containing materials, a detailed understanding of lignins’ Raman spectra needs to be achieved. Although advances made thus far have led to significant growth in application of Raman techniques, further developments are needed to improve upon the existing knowledge. Considering that lignin’s heterogeneous structure consists not only of inter-phenylpropane-unit linkages of carbon-to carbon and carbon-to- oxygen, but also of side chains with various substituent and functional groups, a study of a large number of representative models is essential. In the present work, 40 lignin models were investigated in the neat state (near-IR FT-Raman and FT-IR), in solution (near-IR FT-Raman), and on cellulose (near-IR FT-Raman). These models represent a large number of substituent/functional groups and sub-structures in lignin - aliphatic and phenolic OH, C=O, CHO, COOH, CH3, OCH3, a,b C=C, furan, and interunit C-O- C and C-C linkages. Raman band positions associated with various groups were identified and Raman frequencies and intensities were compared between the models. For example, for the phenyl group, some of the noteworthy findings were as follows. The intensities of the 1600 (aromatic ring stretch) and 3070 (aromatic C-H stretch) cm-1 bands varied significantly between the models. The position of the 1600 cm-1-Raman-mode was found to be only minimally sensitive to substituents because in about 70 % of the models, including those containing more than 1 phenyl group, the vibrational frequency was confined between 1594 and 1603 cm-1. Similar information was obtained on other functional groups as well. It is hoped that the findings from this study will greatly aid in developing a better understanding of the Raman spectra of lignins and lignocellulosic materials.

Citation

Agarwal, Umesh P.; Reiner, Richard S.; Pandey, Ashok K.; Ralph, Sally A.; Hirth, Kolby C.; Atalla, Rajai H. 2005. Raman spectra of lignin model compounds. Proceedings of the 59th APPITA Annual Conference and Exhibition incorporating the 13th ISWFPC (International Symposium on Wood, Fibre, and Pulping Chemistry), held in Auckland, New Zealand (May 16-19, 2005). Carlton, Victoria, Australia]: APPITA, c2005. Pages 1-8.