Microwave-Assisted Catalyst-Free Oxidative Conversion of a Lignin Model Compound to Value-Added Products Using TEMPO


As oxidative conversion of vanillyl alcohol (VAL) to vanillin (VN) represents an important process for valorization of lignin compounds, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is employed for the first time as an oxidant to oxidize VAL in the absence of catalysts using microwave irradiation (MWI). In comparison with H2O2, persulfate and peroxymonosulfate, TEMPO leads to the significantly higher selectivity of vanillin (SVN) and yield of VN. In addition, MWI is also more effective than conventional hydrothermal process for converting VAL to VN with a much higher SVN. A longer reaction time is highly favorable for VAL conversion to VN by TEMPO using MWI with a much higher SVN and yield of VN. Besides, the SVN and yield of VN from VAL oxidation by TEMPO using MWI could be also significantly improved at the elevated temperature with a higher TEMPO dosage. The optimal condition for VAL conversion to VN by TEMPO using MWI is the usage of 1.5 g of TEMPO for 7.5 min of MWI at 160 °C and the highest yield of VN is 17% with SVN of 99%, which is higher than many reported values of catalyst-involved processes. TEMPO can be also re-used to oxidize VAL to generate VN with quite stable yield of VN and SVN. These results indicate that TEMPO, even without catalysts, can successfully and effectively convert VAL to the value-added phenolic products, such as VN. The findings of this study can be useful for further investigating TEMPO-mediated oxidation of lignin compounds.

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This work is supported by the Ministry of Science and Technology (MOST 105-2628-E-005-001-MY3), Taiwan, and financially supported by the “Innovation and Development Center of Sustainable Agriculture” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE), Taiwan. The authors also thank Mr. Thomas Wi-Afedzi for his assistance on proofreading of the manuscript and English editing.

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Correspondence to Jechan Lee or Kun-Yi Andrew Lin.

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Lin, J., Lee, J. & Lin, K.A. Microwave-Assisted Catalyst-Free Oxidative Conversion of a Lignin Model Compound to Value-Added Products Using TEMPO. Waste Biomass Valor 11, 3621–3628 (2020). https://doi.org/10.1007/s12649-019-00646-5

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  • Vanillyl alcohol
  • Vanillin
  • Microwave
  • Hydrothermal