Mechanisms for the formation of monomers and oligomers during the pyrolysis of a softwood lignin

• Aprotic solvent inhibits the thermal polymerization of coniferyl alcohol.
• H-donor promotes the side-chain reduction of coniferyl alcohol.
• “Polymer effect” and primary radicals promote secondary reactions of lignin.
• Oligomers should also be focused as the initial products along with monomers.
• Aprotic solvent/H-donor improves the monomer/oligomer yields.

The formation of monomers and oligomers during the pyrolysis of softwood lignin has been studied with particular emphasis on the reactivity of coniferyl alcohol (CA), which is the anticipated product of the primary pyrolysis of lignin. Under the pyrolysis conditions (N2/250–350 °C/5 min), the lignin contained in Japanese cedar (Cryptomeria japonica) as well as the milled wood lignin (MWL) fraction gave the monomeric guaiacols, including CA, coniferyl aldehyde, dihydroconiferyl alcohol, isoeugenol and 4-vinylguaiacol. These products were similar to those obtained from the pyrolysis of CA, although the yields were much lower. The addition of an aprotic solvent, such as diphenoxybenzene (DPB), led to a substantial increase in the recovery of CA from its pyrolysis reaction, likely by inhibiting the formation of the quinine methide intermediates for the polymerization products, whereas the solvent effects were comparatively small for the lignin pyrolysis. Alternatively, the inclusion of a H-donor (1,2,3,10b-tetrahydrofluoranthene) to the DPB led to a substantial increase in the yields of the monomers and oligomers from lignin. Based on the present data, the pyrolytic formation of the monomers and oligomers from the softwood lignin is discussed at the molecular level.