Modeling pyrolytic behavior of pre-oxidized lignin using four representative β-ether-type lignin-like model polymers

A better understanding of the pyrolytic characteristics of pre-oxidized lignin in which CαH-OH was oxidized to CαO group was obtained through four selected β-ether-type lignin-like model polymers containing the H- and G-type subunits. These models were depolymerized in closed ampoule reactor system and interrogated using in-situ FTIR. The results show that their degradation products differ significantly in species and yield due to CαH-OH, CαO, and aryl-OCH3. The yield of phenolic monomers from oxidized G-type polymer rose to 27.87 wt%, almost twice that of initial G-type polymer. Nevertheless, as for H-type polymer, oxidation of the CαH-OH to CαO group lead to decreases in phenolic monomers yield by ~half. The in-situ FTIR indicates that oxidation of the CαH-OH to CαO accelerates cleavage of interunit linkages and simplifies depolymerization pathways resulting in better selectivity of phenolic monomers, especially at low pyrolysis temperatures (<500 °C). Several important transformation pathways are proposed that clearly explain the pyrolytic behavior of pre-oxidized polymers.