Free-Radical Analysis on Thermochemical Transformation of Lignin to Phenolic Compounds

The chemical characteristics of lignin isolated from industrial black liquor were identified by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, and two-dimensional (2D) heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR), concerning its average molecular weight, distribution of typical interunit linkages, and functional groups. The frequency of β–O–4 linkage was determined to be 17–28/100 C9 units by 2D NMR, while the content of unit [guaiacol (G), syringol (S), and p-hydroxyphenyl (H)] presents a ratio of 7:2:1.5 for G/S/H. The mass-average molecular weight of lignin was characterized to be 2238 g/mol by GPC analysis. The low polymerization degree of the units in lignin leads to the substantial extent of interunit linkage cleavage at low temperatures. The guaiacol-, syringol-, and phenol-type compounds from fast pyrolysis of lignin in a pyroprobe at 500 °C were notably identified by gas chromatography–mass spectrometry (GC–MS) and presented a ratio of the peak area as 7:2:1. More fragments were observed at higher temperatures from pyrolysis (Py)–GC–MS analysis, because of the commencement of demethoxylation and cracking of side chains. The scheme concerning the cleavage of characterized interunit linkages in lignin was proposed to produce the free radicals. The side chains on the free radicals were preferably to crack on β-site bonds to produce a number of methyl phenolic compounds. The methoxyl group was intensively cracked with the increased temperature because of its high bond dissociation energy (BDE), giving rise to the notable increase of cresol-, phenol-, and catechol-type compounds under high temperatures.