Pyrolytic formation of monomers from hardwood lignin as studied from the reactivities of the primary products

Abstract

The pyrolytic reactivity of sinapyl alcohol (SA), which is the expected primary product during the early stage pyrolysis of syringyl (S)-type lignin, was studied under nitrogen pyrolysis at 200–350 °C for 5 min and compared with the reactivity of coniferyl alcohol (CA) from the guaiacyl (G)-type lignin. The pyrolytic pathways of SA were similar to those reported for CA, with some differences in their reactivities. The major products were the condensation products, along with smaller amounts of a variety of side-chain conversion products (cis-SA, sinapyl aldehyde, dihydrosinapyl alcohol, 4-propenylsyringol and 4-vinylsyringol). The recoveries of SA through its evaporation were lower than those of CA, most likely because of its low evaporation efficiency against the condensation reactivity. From the influences of the addition of an aprotic solvent (1, 3-diphenoxybenzene) and a H-donor (1, 2, 3, 10b-tetrahydrofluoranthene), SA was found to be more susceptible to the free radical reactions than CA at 350 °C. This promoted secondary reactions of SA. The use of the H-donor suppressed the radical coupling reactions effectively, which led to an increase in yield of monomeric syringols. With these features in mind, monomer formation from Japanese beech (Fagus crenata, a hardwood) wood and milled wood lignin (MWL) was studied, as compared with those from Japanese cedar (Cryptomeria japonica, a softwood). Both S- and G-type monomers were obtained, which were similar to those from the SA and CA pyrolysis, respectively. The results were mostly explainable with the reactivities of SA and CA as the lignin pyrolysis intermediates, except for the differences observed between beech MWL and wood; only lignin in the beech wood produced much greater amounts of monomers than other samples at 300 and 350 °C, even without addition of any H-donors for stabilization of the radical intermediates. Other wood constituents in beech, probably hemicellulose, were proposed to serve as a source of H-donors during pyrolysis.