Structural characterization of the lignin of Cocos nucifera coir

Abstract

The chemical structure of the lignin isolated from coconut coir has been comprehensively characterized in this work by analytical pyrolysis (Py-GC/MS), two-dimensional nuclear magnetic resonance (2D-NMR) and derivatization followed by reductive cleavage (DFRC). The analyses demonstrated that coconut coir lignin is a p-hydroxyphenyl-guaiacyl-syringyl (H-G-S) lignin, with a predominance of G-lignin units (S/G 0.24), and with a considerable amount of associated p-hydroxybenzoates (13%). 2D-NMR indicated that the main substructures present in this lignin include β–O–4´ alkyl-aryl ethers, followed by phenylcoumarans (β–5´) and resinols (β–β´). In addition, 2D-NMR indicated that coconut coir lignin is partially acylated at the γ-carbon of the side-chain. DFRC results showed that acetates are one of the acylating group and preferentially acylate the γ-OH in syringyl rather than in guaiacyl units.