Pyrolytic reactivities of deuterated β-ether-type lignin model dimers

• Deuterium isotope effects at Cα and OH groups in lignin dimers were identified.
• Deuteration at OHs confirmed the CαCβ pathway via phenoxy radicals.
• Deuteration at Cα confirmed the CαO pathway via Cα radicals.
• The CαCβ pathway via phenoxy radicals is sensitive to the environment.
• The CαCβ pathway via phenoxy radicals acts as a radical sensitizer.

To achieve a better understanding of the radical chain reactions in lignin pyrolysis, the reactivities of three phenolic and nonphenolic deuterated lignin model dimers, i.e., 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propan-1-d-ol, 1-(4-hydroxy-d-3-methoxyphenyl)-1-hydroxy-d-2-(2-methoxyphenoxy)-propane, and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1-propan-1-d-ol, were studied in closed ampoule reactors under N2 at 250–400 °C, the temperature at which the major primary pyrolysis reactions of lignin normally occur, including cleavage of the β-ether linkages. Deuteration at Cα and hydroxyl groups decreased the β-ether cleavage reactivities of the dimers, which indicated that hydrogen abstractions from Cα and hydroxyl groups play direct roles in pyrolytic β-ether cleavage reactions. The influences of deuteration on the yields of the cleavage products indicated the reaction pathways. Significant changes in the reactivity of a mixture of pyrolysis dimers supported the involvement of radical chain reactions. The phenolic Cα-deuterated dimer had complex effects on the reactivity, probably because of the secondary isotope effects of Cα-d on the conformation.