Stable Oligomer Formation from Lignin by Pyrolysis of Softwood in an Aprotic Solvent with a Hydrogen Donor

Pyrolysis of Japanese cedar wood in diphenoxybenzene (an aprotic solvent) with a hydrogen donor was investigated between 270–380 °C. Under these conditions, re‐condensation via radical and quinone methide intermediates was efficiently suppressed and a thermally stable oligomer was obtained. The oligomer was stable even after the treatment time was extended. Yields of lignin‐derived products at 270 °C were limited to approximately 20 wt %, but increased to >80 wt % (lignin basis) at the higher temperatures. The oligomer yield increased directly with the extent of the cellulose degradation at 350 °C. Based on the NMR analysis results, the ether bonds in lignin were largely cleaved, but condensed linkages such as β‐aryl and β‐β and 5‐5’ types remained. The γ‐hydroxypropyl group was identified as a typical side chain, formed by hydrogenation of the double bond of a coniferyl alcohol‐type structure.