半纤维素模型化合物热解机理的理论研究

Theoretical study on thermal degradation mechanism of hemicellulose model compound

针对半纤维素模型化合物4-O-甲基葡萄糖醛酸的热解,提出了六种可能的反应路径,对各种反应路径中的反应物、产物、中间体和过渡态的结构进行了几何结构全优化,计算了各步反应的标准动力学参数。结果表明,4-O-甲基葡萄糖醛酸热解时,首先通过分子内的氢原子转移发生开环反应而形成链状中间体,然后中间体进一步分解,主要产物是甲醇、乙醇醛、2-羟基-3-甲氧基丁醛酸、乙二醛和2-羟基丁醛酸等;主要的热解竞争产物是甲酸、CO_2、CO、4-羟基-3-丁烯酮和甲基乙烯醚等。在半纤维素的热解中,CO_2是通过不饱和反应物或中间体脱羧基反应而形成,乙酸则是通过脱O-乙酰基反应而形成。

Six possible reaction pathways for the thermal degradation of 4-O-methyl-glucuronic acid as a hemicellulose model compound were proposed; the reactants, products, intermediates and trasistion states involved in these reaction pathways were structurally optimized and the related standard kinetic parameters were calculated. The results show that for the thermal degradation of the hemicellulose model compound, 4-O-methyl-glucuronic acid is first converted to catenulate intermediate through a ring-opening reaction with a intramolecular hydrogen transfer, the intermediate is then decomposed, with methanol, glycolaldehyde, 2-hydroxy-3-methoxy-butyl aldehyde acid, glyoxal, 2-hydroxy-butyl aldehyde acid and so on as the major products; the competitive degradation products are formic acid, CO2 , CO, 4-hydroxy-3-vinylmethylketone, methoxyethene and so on. During the thermal degradation of hemicellulose, CO2 is likely formed through decarboxylation of unsaturated reactants or intermediates, whereas acetic acid is probably produced through the elimination of O-acetyl.