THERMAL DECOMPOSITION OF THE LIGNIN MODEL COMPOUNDS: SALICYLALDEHYDE AND CATECHOL

The nascent steps in the pyrolysis of the lignin components, salicylaldehyde (o-\chem{HOC_6H_4CHO}) and catechol (o-\chem{HOC_6H_4OH}), have been studied in a set of heated micro-reactors. The micro-reactors are small (roughly 1 mm ID x 3 cm long); transit times through the reactors are about 100 $\mu$sec. Temperatures in the micro-reactors can be as high as 1600 K and pressures are typically a few hundred Torr. The products of pyrolysis are identified by a combination of photoionization mass spectrometry and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: $o$-\chem{HOC_6H_4CHO} (+ M) $\rightarrow$ \ce{H_2 + CO + C_5H_4=C=O}. At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of \ce{HC#C-C#C-CH_3}, \ce{HC#C-CH_2-C#CH}, and \ce{HC#C-CH=C=CH_2}. These alkynes decompose to a mixture of radicals (\ce{HC#C-C#C-CH_2} and \ce{HC#C-CH-C#CH} and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: $o$-\ce{HOC_6H_4CHO + H} $\rightarrow$ \ce{C_6H_5OH + CO + H}. Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: $o$-\ce{HOC_6H_4OH} (+ M) $\rightarrow$ \ce{H_2O + C_5H_4=C=O}. These findings have implications for the pyrolysis of lignin itself