Flash vacuum pyrolysis of methoxy-substituted lignin model compounds

The Bash vacuum pyrolysis (FVP) of methoxy-substituted beta-O-4 lignin model compounds has been studied at 500 degrees C to provide mechanistic insight into the primary reaction pathways that occur under conditions of fast pyrolysis. FVP of PhCH2CH2OPh (PPE) a model of the dominance beta-O-4 Linkage in lignin, proceeds by C-O and C-C cleavage, in a 37:1 ratio, to produce styrene plus phenol as the dominant products and, minor amounts of toluene, bibenzyl, and benzaldehyde, From the deuterium isotope effect in the FVP of PhCD2CH2OPh, it was shown that C-O cleavage occurs by homolysis and by 1,2-elimination in a ratio of 1.4:1, respectively. Methoxy substituents enhance the homolysis of the beta-O-4 linkage, relative to PPE, in o-CH3O-C6H4OCH2CH2Ph (o-CN3O-PPE) and (o-CH3O)(2)-C6H3OCH2CH2Ph ((o-CH3O)(2)-PPE) by a factor of 7.4 and 21, respectively. The methoxy-substituted phenoxy radicals undergo a complex series of reactions, which are dominated by 1,5-, 1,6-, and 1,4-intramolecular hydrogen abstraction, rearrangement, and beta-scission reactions. In the FVP of o-CH3O-PPE, the dominant product, salicylaldehyde, forms from the methoxyphenoxy radical by a 1,5-hydrogen shift. to form 2-hydroxyphenoxymethyl radical, 1,2-phenyl shift, and beta-scission of a hydrogen atom. The 2-hydroxyphenoxymethyl radical can also cleave to form formaldehyde and phenol in which the ratio of 1,2-phenyl shift to beta-scission is ca, 4:1. In the FVP of o-CH3O-PPE and (o-CH3O)(2)-PPE, products (ca. 20 mol %) are also formed by C-O homolysis of the methoxy group. The resulting phenoxy radicals undergo 1,5- and 1,6-hydrogen shifts in a ratio of ca. 2:1 to the aliphatic or benzylic carbon, respectively, of the phenethyl chain. In the: FVP of (o-CH3O)(2)-PPE, o-cresol was the dominant product. It was formed by decomposition of 2-hydroxy-3-hydroxymethylbenzaldehyde and 2-hydroxybenzyl alcohol, which are formed from a complex series of reactions from the 2,6-dimethoxyphenoxy radical. The key step in this reaction sequence was the rapid 1,5-hydrogen shift from 2-hydroxy-3-methoxybenzyloxy radical to 2-hydroxymethy-6-methoxyphenoxy radical before beta-scission of a hydrogen atom to give the substituted benzaldehyde. The 2-hydroxybenzyl alcohols rapidly decompose under the reaction conditions to o-benzoquinone methide and pick up hydrogen front the reactor walls to form o-cresol.