Effects of exogenous acid catalysis on the thermal degradation law of wood hemicellulose

  Objective  High-temperature heat treatment can partially degrade wood hemicellulose and improve wood dimensional stability. The use of exogenous acid can reduce the thermal degradation temperature of wood hemicellulose, so it is necessary to clarify its thermal degradation law.

  Method  In this study, hemicelluloses from poplar and Chinese fir were separated by alkali method. Thermal degradation of hemicellulose (The two exogenous acids were AlCl₃ and H₃PO₄, and the concentration of each acid was 0.1mol/L and 0.3mol/L, respectively) under different acidic conditions were investigated by Fourier Infrared Spectrometer (FTIR) and thermogravimetric (TG) analysis.

  Result  The FTIR spectra of wood hemicelluloses isolated by alkali method were consistent with the basic characteristic of hemicellulose of softwood and hardwood. After being treated with AlCl₃ and H₃PO₄, the hydroxyl peak showed red shift, and the characteristic absorption peak intensity of hemicellulose decreased significantly compared with the untreated hemicellulose. TG analysis showed that the initial degradation temperature (T_5%) of wood hemicellulose ranged from 200 ℃ to 95–150 ℃ after being pretreated by two kinds of exogenous acids, and the main degradation temperature ranged from 200–300 ℃ to 100–150 ℃. At the same concentration, the T_5% value of hemicellulose (150 ℃) after AlCl₃ treatment was greater than that of H₃PO₄ (95 ℃). When the concentration of AlCl₃ increased, the thermal decomposition rate of hemicellulose accelerated, but it had almost no effect on T_5%. When the concentration of H₃PO₄ increased, the thermal decomposition rate of hemicellulose and T_5% both decreased. Compared with untreated poplar wood hemicellulose (199.68 kJ/mol) and Chinese fir hemicellulose (231.12 kJ/mol), the pyrolysis activation energy was significantly reduced after AlCl₃ and H₃PO₄ treatment. When the concentration was 0.3M, the average activation energy of Chinese fir hemicellulose (112.31 kJ/mol) after AlCl₃ treatment was lower than that of H₃PO₄ (125.82 kJ/mol).

  Conclusion  The two exogenous acids used in this study can significantly reduce the degradation temperature of hemicellulose. In general, the catalytic effect of AlCl₃ was better than H₃PO₄, and the Chinese fir hemicellulose was more sensitive to the catalytic effect of H₃PO₄. Therefore, different exogenous acidic media can be selected as catalysts according to the difference of softwood and hardwood of hemicellulose to accelerate the pyrolysis reaction rate, thereby providing theoretical support for the application of exogenous acid in low-temperature heat treatment of wood.