Molecular Products and Radicals from Pyrolysis of Lignin

Abstract

Thermal degradation of lignin under two reaction regimes (pyrolysis in N(2) and oxidative pyrolysis in 4% O(2) in N(2)) has been investigated in a tubular, isothermal, flow-reactor over the temperature range 200-900 °C at a residence time of 0.2 s. Two experimental protocols were adopted: (1) Partial pyrolysis in which the same lignin sample was continuously pyrolyzed at each temperature and (2) conventional pyrolysis, in which new lignin samples were pyrolyzed at each pyrolysis temperature. The results identified common relationships between the two modes of experiments, as well as some differences. The majority of products from partial pyrolysis peaked between 300 and 500 °C, whereas for conventional pyrolysis reaction products peaked between 400 and 500 °C. The principal products were syringol (2,6-dimethoxy phenol), guaiacol (2-methoxy phenol), phenol, and catechol. Of the classes of compounds analyzed, the phenolic compounds were the most abundant, contributing over 40% of the total compounds detected. Benzene, styrene, and p-xylene were formed in significant amounts throughout the entire temperature range. Interestingly, six ringed polycyclic aromatic hydrocarbons were formed during partial pyrolysis. Oxidative pyrolysis did not result in large differences from pyrolysis; the main products still were syringol, guaiacol, phenol, the only significant difference being the product distribution peaked between 200 and 400 °C. For the first time, low temperature matrix isolation electron paramagnetic resonance was successfully interfaced with the pyrolysis reactor to elucidate the structures of the labile reaction intermediates. The EPR results suggested the presence of methoxyl, phenoxy, and substituted phenoxy radicals as precursors for formation of major products; syringol, guaiacol, phenols, and substituted phenols.

Phenols from pyrolysis and co-pyrolysis of tobacco biomass components

“…The sampling of the reactor pyrolysate was done at the head of a GC column at À60°C using an in-line GC-MS. The experimental details for STDS are reported elsewhere (Rubey and Grant, 1988;Evans and Dellinger, 2005;Kibet et al, 2012). It is a well-established technique for analysis of a wide range of organic materials including amino acids (Marnett and Wilcox, 1988).…”

“…The fractional pyrolysis technique applied in this study is an experimental procedure in which the same sample is continuously pyrolyzed at each pyrolysis temperature (Kibet et al, 2012(Kibet et al, , 2013Fig. 1.…”

“…It is defined as a selective, in situ conversion of biopolymers to products (Kibet et al, 2013;Agblevor et al, 2010). The benefits of this method are discussed elsewhere (Kibet et al, 2012(Kibet et al, , 2013.…”

“…Phenolic compounds found in cigarette smoke considered toxic are those believed to originate from the thermal degradation of lignin; one of the major components of tobacco (Smith and Hansch, 2000). These phenols include 4-vinyl phenol, 4-vinyl guaiacol, syringol and vanillin (Selassie et al, 1999;Smith and Hansch, 2000;Kibet et al, 2012). Substituted phenols bearing electron-donating substituents would therefore be expected to be more toxic (Smith and Hansch, 2000;Kibet et al, 2013).…”

Formation of dibenzofuran, dibenzo-p-dioxin and their hydroxylated derivatives from catechol

“…CT originates from the fission of polyphenolic constituents in lignin. 4 Degradation of CT affords the formation of anisole, phenol, benzene and hydroxylated naphthalenes. 3 Dellinger's group has thoroughly investigated thermal decomposition of CT reporting that both pyrolytic and oxidative decomposition reactions of CT result in the formation of dibenzo-p-dioxin (DD) and dibenzofuran (DF).…”