Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol Academic Article uri icon

abstract

  • The nascent steps in the pyrolysis of the lignin components, salicylaldehyde (o-HOC6H4CHO) and catechol (o-HOC6H4OH), 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 µsec. 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, 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.

publication date

  • June 12, 2018