Abstract:

The cellulose-water model system was established and optimized by using Materials Studio software, and the molecular dynamic simulation of the cellulose hydrothermal reaction process was conducted by using LAMMPS software. The evolution law of the hydrothermal products and the hydrothermal reaction mechanism of the cellulose were simulated and studied. The results show that temperature is an important factor affecting the distribution of hydrothermal products of cellulose. With the increase of the hydrothermal temperature, cellulose decomposition intensifies, the relative content of hydrothermal solid products in the system decreases sharply, the aromaticity and calorific value of the solid products increase, and the hydrophilicity decreases. The relative contents of heavy tar and light tar increased first and then decreased, and the reaction of secondary cracking of tar into small molecules intensified. With the increase of hydrothermal temperature, the production of CO₂ and H₂ in the system continued to increase, the production of formic acid showed a trend of increasing first and then decreasing, and the difference of formaldehyde was small. The simulation results also show that H₂O molecules participated in the hydrothermal reaction of cellulose mainly in the form of hydrated hydrogen ions (H₃O⁺). With the increase of the hydrothermal temperature, the ionization reaction of water molecules and the dehydration reaction of cellulose were enhanced. The analysis of hydrothermal mechanism of cellulose showed that the cellulose depolymerization paths included the decomposition into glucose and the dehydration into L-glucan. During the rearrangement isomerization process of the glucose monomer, the main intermediates such as furans and aldehydes were formed by dehydration and retrograde aldol condensation reactions from the three transition state structures of enediol. The research on the cellulose hydrothermal reaction mechanism can provide a theoretical basis for the biomass hydrothermal conversion.

Key words: biomass, hydrothermal treatment, cellulose, ReaxFF reactive force field, reaction path

摘要：

使用Materials Studio软件建立并优化纤维素-水模型体系，利用LAMMPS软件对其进行水热过程的分子动力学模拟，研究水热体系内纤维素水热产物的演变规律及其水热反应机理。结果表明：温度是影响纤维素水热产物分布的重要因素。随着水热温度的升高，纤维素分解加剧，体系内水热固相产物相对含量急剧下降，固相产物的芳香性和热值增高，亲水性降低；重质焦油、轻质焦油的相对含量均先增加后降低，焦油二次裂解为小分子反应加剧；随着水热温度的升高，体系内CO₂和H₂生成量均持续升高，甲酸生成量呈先升高后降低的趋势，甲醛生成量差异较小。模拟结果亦表明H₂O分子主要以水合氢离子（H₃O⁺）的形式参与纤维素水热反应，随着水热温度的升高，体系中水分子的电离反应与纤维素的脱水反应均增强。纤维素水热机理分析表明水热过程中纤维素的解聚包括分解为葡萄糖和脱水为左旋葡聚糖两条路径，而葡萄糖单体重排异构过程中会经历烯二醇3种过渡态结构，不同的结构通过脱水、逆羟醛缩合等反应分别生成呋喃类和醇醛类等主要中间产物。对纤维素水热反应机理的分析可为生物质水热转化提供理论指导。

关键词: 生物质, 水热处理, 纤维素, ReaxFF反应力场, 反应路径