Structural modifications of flax and sisal lignin during the pulping and bleaching processes

We have studied the structural modifications of the lignin of flax and sisal during the pulping (soda/AQ) and bleaching (TCF and ECF) processes. The residual lignins were isolated by acidolysis and subsequently characterized by Py-GC/MS and 2D-NMR. Flax residual lignins have a predominance of G-lignin and low amounts of S-lignin units in the unbleached pulp, which are also present in similar abundances in the residual lignin after TCF bleaching. After ECF bleaching, lignin was still present, with the presence of G- and H-lignin units, the latest being prominent now, and the complete absence of S-moieties. In the case of sisal, the residual lignin isolated from unbleached pulp was enriched in S-lignin units, although the S/G ratio decreases to 2 indicating a preferential removal of the S-lignin during cooking. In the TCF bleached pulp, the residual lignin also presented a predominance of S-units, similar to that found in the unbleached pulp, indicating that TCF bleaching did not affect in great extent the lignin composition. However, the analysis of the ECF bleached pulp indicated the complete removal of lignin markers. The NMR data gave also additional information on the lignin linkages. The main linkages observed in flax residual lignins were β-O-4' alkyl-aryl ether, β-β' resinol and β-5' phenylcoumaran, while in sisal residual lignins the main linkages were β-O-4' and β-β' resinol, with only small amounts of β-5' phenylcoumarans. The distributions of the different linkages also changed during cooking and bleaching.

This study has been supported by the Spanish MEC project AGL2005-01748, the EU contracts NMP2-CT-2006-26456 (BIORENEW) and KBBE-2009-244362 (LIGNODECO), and the CSIC project 2010-40E-075. JR thanks the Spanish CSIC for an I3P fellowship, and GM thanks the Spanish Ministry of Education for a FPI fellowship.

Peer reviewed