(24c) Pyrolysis Chemistry of Lignin Model Compounds; ?-O-4 and ?-O-4 Linkages
Pyrolysis chemistry of lignin model compounds; β-O-4 and α-O-4 linkages
Fast pyrolysis is considered a promising thermochemical technology for converting solid lignocellulosic biomass into a liquid bio-oil. Unlike the relatively homogeneous structure of cellulose, lignin is an amorphous polymer of irregularly bonded hydroxyl- and methoxy- substituted phenylpropane units. These phenylpropane units are linked primarily by β-O-4 and α-O-4 bonds which occupy 45-60% and 6-15% of the total linkages, respectively, in most lignins. While significant effort has been spent to understand the pyrolysis chemistry of cellulose with key success, the complex structure of lignin has been a bottleneck in understanding lignin pyrolysis. To overcome this issue, researchers have used lignin model compounds. However, most experimental setups obscure primary reactions in fast pyrolysis, since long vapor residence times can lead to secondary reaction. Herein, we focused on the primary reactions with a micropyrolyzer-GC-MS/FID system allowing short vapor residence time of 15-20 milliseconds and high heating rate of 1000 °C/s. Phenolic dimers connected by the most common linkages in lignin, β-O-4 and α-O-4, were selected and pyrolyzed at 500 °C. The simplest β-O-4 compound is 2-phenoxyphenylethanol (PPE: PhCHOHCH2OPh). To characterize the effects of different substituents, –CH2OH, -OH, –OCH3 groups, on the pyrolysis chemistry, three other compounds with the same base structure of PPE were used. For exploring pyrolysis chemistry of α-O-4 compounds, benzylphenyl ether (BPE: PhCH2OPh) and BPE with –CH3 group at Cα were pyrolyzed. Performing mole balances from GC-FID and initial pathway screening using the Gibbs free energies of the reactants and products provides insight into the primary reaction pathways during pyrolysis thereby leading to improved understanding of lignin pyrolysis chemistry.