Characterization of the water-insoluble fraction from fast pyrolysis liquids (pyrolytic lignin) Part II.: GPC, carbonyl goups, and 13C-NMR

Fast pyrolysis has become an interesting technology for the conversion of solid biomass into a liquid (bio-oil) that can easily be stored and transported. The structural features of pyrolytic lignin is of paramount interest in order to explain reaction mechanisms during pyrolysis and aging processes. In addition, a well-characterized pyrolytic lignin can be of interest for commercial applications such as adhesives in wood based panel industry. This paper describes methods to elucidate the structural configuration of pyrolytic lignin. Using gel permeation chromatography (GPC) the average molecular weight of pyrolytic lignin was determined to be between 650 and 1300 g mol(-1). Ratios of hydroxyphenyl-, guaiacyl-, and syringyl- units (H, G, S-units) were obtained from pyrolysis gas chromatography-mass spectrometry (Pyroprobe-GC/MS) and were compared with milled wood lignin (MWL) data. Carbonyl groups were determined by oximation. Based on analytical data, a C-8-formula was established assuming a principal structure of phenyl ethane units. Combined with GPC results an average DP between 4 and 9 was calculated. Evaluation of C-13-NMR spectra shows some small signals indicating residual lignin moieties with ether structures. However, most of the ether bonds in the native lignin were split. As a consequence pyrolytic lignin seems to consist of oligomeric alkylated aromatic units linked by newly formed bonds, probably C-C bonds. (C) 2001 Elsevier Science B.V. All rights reserved.