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.