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Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol

  • Author(s): Ormond, TK
  • Baraban, JH
  • Porterfield, JP
  • Scheer, AM
  • Hemberger, P
  • Troy, TP
  • Ahmed, M
  • Nimlos, MR
  • Robichaud, DJ
  • Daily, JW
  • Ellison, GB
  • et al.
Abstract

© 2018 American Chemical Society. 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.

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