木质纤维素的结构自相似性及酶水解分形动力学研究

The high cost of fuel ethanol conversion from lignocelluloses is mainly attributed to its biorecalcitrance. The lignocellulose structure is complex with self-similarity and fractal characteristics, so the fractal theory can be used for nonlinear quantitative describing the variation of structure in the whole process. In this project, acid or alkaline pretreatment, refining and enzymatic hydrolysis were conducted on the raw materials, the microstructure and pore structure of different solid samples were characterized and analyzed. The fractal dimension were calculated by image analysis method and low temperature nitrogen adsorption method based on FHH and Yu Boming theory. Based on the above data, the relationship between fractal dimension and the chemical composition and structural properties was revealed, then the fractal structure model was deduced, which is expected for predicting the lignocellulose structure. As to the enzymatic hydrolysis kinetics, a new fractal kinetics model was deduced aiming at the imperfection of the existing ones. The reaction rate constant k and fractal exponent h were simulated and calculated, and the influence law by pretreatment temperature, time, pH, enzyme dosage, inhibitor, et al. was revealed, then the enzymatic hydrolysis mechanism of fractal kinetics was clarified. Based on all the above, the fractal dimension of substrates and the fractal exponent of enzymatic hydrolysis kinetics were related, and the structure-function relationship was fitted and illustrated. The research method and results can help simplify the process, and provide guidance and reference for the determination of process parameters in the fuel ethanol conversion from lignocelluloses.

木质纤维素转化燃料乙醇成本较高，主要原因之一在于其抗降解性结构。由于生物结构复杂且具自相似性特点，符合分形特征，可应用分形理论对其整个过程的结构变化进行非线性定量描述。本项目对原料进行酸/碱预处理、磨浆、酶水解，分析表征不同阶段固形物的微观结构与孔结构（低温氮吸附法），分别采用图像分析法与基于FHH、郁伯铭理论计算分形维数，揭示木质纤维素分维与化学组成及结构特征参数的关系，建立结构分形模型，实现对木质纤维素结构的预测。针对现有分形动力学方程存在的缺陷进行改进，利用模型模拟计算反应速率常数k与动力学分形指数h，揭示预处理温度、时间、pH值、酶用量、抑制物等因素对二者的影响规律，阐明酶水解分形动力学机制。在上述基础上，将底物的结构分形维数与酶水解动力学分形指数联系起来，明确二者之间的构效关系。本项目的研究方法与结果有助于简化研究过程，对乙醇转化过程工艺条件的选取具有指导与参考意义。

木质纤维素具有抗降解性结构，且具有结构自相似性特点，符合分形特征，可采用分形的数学方法来进行研究。本项目以相思木和辐射松为原料进行预处理和酶水解，分析固形物的微观结构与孔结构（低温氮吸附法），并采用图像分析法与孔结构分析法计算分形维数，揭示分维与化学组成及结构特征参数的关系，建立结构分形模型，并对现有分形动力学方程进行改进，揭示预处理温度、时间、pH值、酶用量、抑制物等因素对二者的影响规律，阐明酶水解分形动力学机制。研究结果表明，相思木经乙酸预处理后，试样分维值上升，且分维随预处理温度的升高而降低。分形维数与纤维素、木质素含量之间呈现负相关，而与半纤维素含量、结晶度以及木素酚型/非酚型比之间呈现正相关。结构分形模型显示，木质素对试样的结构分维起主要作用，其它因素的影响均不太显著。酶水解动力学的研究结果表明，反应速率常数k随预处理温度的升高而增大，分形指数h无规律性变化。乙酸用量、纤维素酶用量、酶解体系中添加葡萄糖及Tween-80均对k和h有不同程度的影响。基于图像处理法所构建的结构分形模型，方程中木素的系数k2绝对值最大，即木素缩合/非缩合比对分维的影响最显著，其次为半纤维素，故木质素结构对试样的结构分维起主要作用，其它参数的影响次之。基于孔结构FHH分维的结构模型显示，比较针叶木单独建模与针叶木、阔叶木综合建模，方程中系数的正负不一致，各因素的变化趋势不同，针叶木建模中木质素酚型/非酚型比k1对结构分维的影响最显著，而综合建模后纤维素含量对结构分维的影响最显著，这与建模原料或预处理方式的不同有关。本项目所采用的研究方法与结果有助于简化研究过程，可以为木质纤维素转化利用过程中结构的研究提供一定指导与参考。