Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol

The nascent steps in the pyrolysis of the lignin components salicylaldehyde ( o-HOC₆H₄CHO) and catechol ( o-HOC₆H₄OH) were studied in a set of heated microreactors. The microreactors are small (roughly 1 mm ID × 3 cm long); transit times through the reactors are about 100 μs. Temperatures in the microreactors 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, photoelectron photoion concidence mass spectroscopy, and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: o-HOC₆H₄CHO (+ M) → H₂ + CO + C₅H₄═C═O (fulveneketene). At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of HC≡C-C≡C-CH₃, HC≡C-CH₂-C≡CH, and HC≡C-CH═C═CH₂. These alkynes decompose to a mixture of radicals (HC≡C-C≡C-CH₂ and HC≡C-CH-C≡CH) and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: o-HOC₆H₄CHO + H → C₆H₅OH + CO + H. Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: o-HOC₆H₄OH (+ M) → H₂O + C₅H₄═C═O. These findings have implications for the pyrolysis of lignin itself.