NMR a critical tool to study the production of carbon fiber from lignin

The structural changes occurring to hardwood Alcell (TM) lignin as a result of fiber devolatilization/extrusion, oxidative thermo-stabilization and carbonization are investigated in this study by solid-state and solution nuclear magnetic resonance (NMR) spectroscopy techniques. Solution based H-1-C-13 correlation NMR of the un-spun Alcel (TM) lignin powder and extruded lignin fiber detected modest changes occurring due to fiber devolatilization/extrusion in the type and proportion of aliphatic side-chain carbons or monolignol inter-unit linkages. Molecular weight analysis by gel permeation chromatography (GPC), along with an additional P-31 NMR method used to indicate changes in terminal hydroxyl functionality, suggest fiber devolatilization/extrusion causes both chain scission and condensation reactions. H-1 CRAMPS (combined rotation and multiple-pulse spectroscopy) and C-13 cross-polarization/magic angle spinning (CP/MAS) spectra of extruded and stabilized lignin fibers indicate stabilization severely reduces the proportion of methoxy groups present, while also increasing the relative proportion of carbonyl and carboxyl-related structures, typically associated with cross-linking chemistries. C-13 direct-polarization/magic angle spinning (DP/MAS) analysis of stabilized and carbonized fibers shows an increased relative amount of carbon-carbon bonds on aryl structures and a relative decrease of aryl ethers. DP/MAS dipolar dephasing experiments suggest that a majority of non-protonated carbons convert from carbonyl to aryl and condensed aryl structures during carbonization. (C) 2012 Elsevier Ltd. All rights reserved.