Thermal behavior of β-1 subunits in lignin: Pyrolysis of 1,2-diarylpropane-1,3-diol-type lignin model compounds

1,2-Diarylpropane-1,3-diol-type lignin model compounds, 1,2-bis(4-hydroxy-3-methoxyphenyl)propane-1,3-diol (1) and 1-(3,4- diethoxyphenyl)-2-(4-methoxyphenyl)propane-1,3-diol (2), were pyrolyzed at 500°C for 4 s to clarify the thermal behavior of β-1 subunits in lignin. Products were monitored by gas chromatography/mass spectrometry. The cleavage of the Cα-Cβ bond to produce benzaldehydes such as 4-hydroxy-3-methoxybenzaldehyde (9) and phenylethanals as the counterparts such as 4-hydroxy-3-methoxyphenylethanal (10) occurred in pyrolyses of both 1 and 2. In pyrolysis of 1, an oxetane pathway leading to the formation of Z/E-stilbenes without the γ-CH₂OH group such as Z/E-4,4′-dihydroxy-3, 3′-dimethoxystilbene (3) was predominant. In pyrolysis of 2, the oxetane pathway was minor, while pathways producing a dimer with a =CγH ₂ group by loss of water and a dimer with an α-carbonyl group were predominant. Pyrolysis of Japanese cedar wood provided 3 and 10 in ∼0.8% and 0.6% yields, respectively, based on the Klason lignin content, while pyrolysis of a guaiacyl bulk dehydrogenation polymer gave them in a very small amount.