纤维素热裂解过程中活性纤维素的生成和演变机理

在辐射加热闪速热裂解实验台上获取了一种可溶性的黄色固体中间产物, 在两个不同辐射热流下其产量随着反应的进行均呈现升高-稳定-再升高的变化趋势. 高效液相色谱分析发现, 其主要成分包括低聚糖、葡萄糖、左旋葡聚糖、丙酮醛等. 将其中的纤维二糖、纤维三糖等低聚糖, 以及葡萄糖等单糖的混合物归为活性纤维素. 在高热流下, 活性纤维素的相对产量呈现先升后降的趋势, 在可溶性产物中的含量可高达68%(w, 质量分数), 且聚合度较高的低聚糖占主导地位. 在低热流下活性纤维素产量相对较低, 最高达到约57%(w), 更多地由聚合度较低的糖类组成. 在高温下活性纤维素极不稳定, 易降解生成聚合度更低的糖类, 甚至焦炭、挥发份和气体产物. 最后提出了一个改进的机理模型, 描述了活性纤维素的生成和演变的反应路径.

Mechanism of Formation and Consequent Evolution of Active Cellulose during Cellulose Pyrolysis

An intermediate product that was yellow, soluble, and solid was obtained in a high-radiation flash pyrolysis reactor. Under two different radiant heat fluxes, the yields tended to both increase initially until achieving a steady state, and then increase again with the progress of reaction. The compositional analysis of the yellow product was performed on high performance liquid chromatography (HPLC). It was indicated that the product mainly consisted of oligosaccharides, glucose, levoglucosan, methylglyoxal and so on. The compounds including oligosaccharides such as cellobiose and cellotriose, and monosaccharides such as glucose were regarded as active cellulose. Under the higher heat flux, the relative yield of the active cellulose increased initially, followed by a decreasing trend, and achieved a maximummass fraction of 68% (w) in the soluble yellowproduct. The oligosaccharides with higher degree of polymerization (DP) were the primary components. Under the lower heat flux the yield of active cellulose was relatively lower, achieving a maximum of about 57% (w), and more saccharides with lower DP were contained. It was suggested that active cellulose was quite unstable at high temperature, and easily decomposed into saccharides with lower DP, even char, volatiles, and gaseous products. Finally an improved mechanism was proposed to describe the reaction route of formation and consequent evolution of active cellulose during cellulose pyrolysis.