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    外源酸催化对木材半纤维素热降解规律的影响

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

      摘要
    • 摘要:
        目的  高温热处理可使木材半纤维素部分降解，改善木材的尺寸稳定性。使用外源酸可降低木材半纤维素热降解温度，因此有必要明确其热降解规律。
        方法  采用碱法分离的方式提取出杨木和杉木的半纤维素，通过傅里叶红外光谱仪和热重分析法研究引入外源酸条件下（两种外源酸AlCl3和H3PO4，每种外源酸的浓度分别为0.1和0.3 mol/L）半纤维素的热降解规律。
        结果  碱法分离出的木材半纤维素，其红外谱图符合阔叶材和针叶材半纤维素的基本特征。经AlCl3和H3PO4处理后，相比未处理的半纤维素，羟基峰发生红移，且半纤维素的特征吸收峰强度显著降低。热重分析显示：两种外源酸预处理的木材半纤维素的起始降解温度（T5%）从200 ℃左右降低至95 ~ 150 ℃，且主要降解温度范围从200 ~ 300 ℃降低至100 ~ 150 ℃。在相同浓度下，AlCl3处理后半纤维素的T5%（150 ℃）大于H3PO4的（95 ℃）。当AlCl3浓度增大时，半纤维素热分解速率加快，但T5%无明显变化。随H3PO4浓度增大时，半纤维素热分解速率和T5%均减小。相比未处理的杨木半纤维素热解活化能（199.68 kJ/mol）和杉木半纤维素热解活化能（231.12 kJ/mol），AlCl3和H3PO4处理后的热解活化能显著降低。在相同浓度0.3 mol/L的条件下，AlCl3处理后杉木半纤维素的平均活化能（112.31 kJ/mol）要低于H3PO4的（125.82 kJ/mol）。
        结论  本研究采用的两种外源酸均可显著降低半纤维素的热降解温度。总体来讲，AlCl3催化效果优于H3PO4，杉木半纤维素对H3PO4较为敏感。可根据针、阔叶材半纤维素的差异性选择不同的外源酸性介质作为催化剂，加速热解反应速率，从而为外源酸在木材低温热处理的应用提供一定的理论支撑。

       

      Abstract:
        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 (T5%) 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 T5% 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 T5%. When the concentration of H3PO4 increased, the thermal decomposition rate of hemicellulose and T5% 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.

       

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