Identification of a polyfuran network as the initial carbonization intermediate in cellulose pyrolysis: A comparative analysis with cellulosic hydrochars

Pyrolysis of cellulose is accompanied with different complex and superimposing transformations resulting in a broad mixture of isolable products with strong variations depending on the applied reaction conditions. The operative chemistry represents a challenge for analytical chemists and process engineers alike. Especially, the reactions leading to char formation cannot be described as sufficiently understood. In ongoing efforts to shed light on the major transformations during charring of cellulose, the occurrence of a thermostable condensed phase (TSCP) previously postulated as an important carbonization intermediate formed below 300 °C was revisited. It was attempted to isolate pure TSCP intermediates without cellulose contamination by applying dehydration catalysts known from cellulose based carbon fiber production and extensive isothermal treatments. It was shown that the weight loss levels off during isothermal treatment for 6 h in the temperature range of 200–250 °C, resulting in the formation of a common intermediate with an almost identical composition – irrespective of the employed temperature or catalyst. Moreover, isothermal treatment of pure cellulose at 270–280 °C for up to 12 hours also resulted in the formation of an intermediate which had a similar composition as the material prepared with added dehydration catalysts. To test the hypothesis of a proposed polyfuranic nature of the TSCP intermediate the prepared samples were compared with hydrochars obtained from hydrothermal treatment of cellulose as reference material for a polyfuranic humin. Similarities and differences are discussed and implications for the overall carbonization mechanism are summarized.