Analytical pyrolysis of lignin: Products stemming from β-5 substructures

This paper addresses the behavior of β-5 substructures in analytical pyrolysis of lignin. Five phenolic 2-arylcoumarans containing a β-5 linkage, dehydrodiconiferyl alcohol (I) and II-V, and diazomethane-methylated I (Im), were pyrolyzed at 500 °C for 4 s. The products were analyzed by gas chromatography and gas chromatography/mass spectrometry. The major products of I-IV and Im were 4-methylguaiacol (4) and 1,2-dimethoxy-4-methylbenzene (4m), respectively, stemming from the A moiety of the 2-arylcoumarans. 2-Arylcoumarans I-IV having the ring B with a CHCHCH2OH or CHCHCHO side chain provided 4 in a larger yield than dehydrodiisoeugenol (V) having the ring B with a CHCHCH3 side chain; the yields of 4 were ∼6.6, 5.7-4.6, and 0.9 mol% from I, II-IV and V, respectively. Characterization of the dimeric products of I showed the α-aryl ether opened-dimers with the hydrogenated Cα-Cβ bond being the precursors of 4. Based on these results, guaiacyl and syringyl β-5 substructures were proposed as potential pyrolytic origins for 4 and 3,5-dimethoxy-4-methylphenol in analytical pyrolysis of soft- and hardwood lignins.