峨山褐煤的分子结构和分子模拟

作为重要的化石能源，褐煤资源潜力巨大、分布广泛但综合利用率低。研究褐煤的分子结构模型，有助于预测褐煤在热解、液化和气化过程中的化学反应机理及反应路径，进而提高褐煤的综合应用水平。以云南峨山褐煤为研究对象，利用傅里叶变换红外光谱、13C核磁共振波谱及X射线光电子能谱等分析测试方法，获取了峨山褐煤的含碳、含氧及含氮结构参数。在此基础上，借助Gaussian 09计算平台，采用量子化学建模的方法构建并优化了峨山褐煤的分子结构模型。研究结果表明：峨山褐煤的芳碳率为39.20%，芳香碳结构主要为苯和萘，且芳香桥头碳与周边碳的比值χ_b为0.07；脂碳率为49.51%，脂肪碳结构主要为亚甲基，季碳和氧接脂碳；氧原子主要存在于羟基、醚氧、羰基和羧基结构中；含氮结构则以吡啶为主。基于元素分析、13C 核磁共振波谱分析，又经过热重实验消除褐煤中残余水分的影响后，计算出峨山褐煤的分子式为C₁₅₃H₁₃₇O₃₅N₂。依据分子式及分析结果计算出峨山褐煤的结构单元含量并构建出其初始结构模型，采用半经验法PM 3基组及密度泛函理论M06-2X/3-21G基组对初始分子构型进行优化。优化后的分子模型具有明显的三维立体特征，芳香环之间较为分散且在空间中排列不规则，芳香簇主要通过亚甲基、醚氧基、羰基、酯基和脂肪环连接，含氧官能团主要分布在分子边缘，脂肪族侧链较多。对优化后的分子模型进行振动频率计算进而获得了分子模型的模拟红外光谱，其与实验红外谱图吻合度良好，证明了峨山褐煤分子结构模型的准确性、合理性。分子结构模型的构建有利于直观地了解峨山褐煤的分子结构特征，从而有助于从微观分子角度研究峨山褐煤的宏观性质。同时，峨山褐煤分子结构模型可为其在热解、液化和气化等领域研究中提供理论指导。

Molecular Structure and Molecular Simulation of Eshan Lignite

As a critical fossil energy, lignite has a huge resource, wide distribution, and a low comprehensive utilization rate. Investigations regarding the molecular structure model of lignite are beneficial for pre-judging the chemical reaction mechanism and reaction path of lignite in pyrolysis, liquefaction and gasification, thereby improving its comprehensive utilization. Eshan lignite was studied by Fourier transform infrared spectroscopy, 13C Nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy in this paper. Moreover, the structural unit parameters of carbon, oxygen and nitrogen of Eshan lignite were obtained. According to these parameters, the molecular structure model of Eshan lignite was established and optimized by using the quantum chemical modeling method in the Gaussian 09 computing platform. The results indicate that the content of aromatic carbon and aliphatic carbon is 39.20% and 49.51%, respectively. In detail, the aromatic carbon structure mainly includes benzene and naphthalene, and the ratio of aromatic bridgehead carbon to surrounding aromatic carbon is 0.07. The aliphatic carbon structure mainly contains methylene, methyl and oxy-aliphatic carbon. Furthermore, the oxygen atoms mainly exist in hydroxyl, ether oxygen, carboxyl and carbonyl. Moreover, the nitrogen structure mainly involves pyridine. Based on the results of ultimate analysis and 13C nuclear magnetic resonance spectroscopy analysis, the molecular formula of Eshan lignite was calculated as C₁₅₃H₁₃₇O₃₅N₂ after eliminating the influence of water by thermogravimetric experiment. The initial structural model of Eshan lignite was constructed via the connecting structural unit. The PM 3 basis set of semi-empirical method and density functional theory M06-2X/3-21G basis set were used to optimize the initial molecular configuration. The optimized model has obvious three-dimensional characteristics. Among these, the aromatic rings arrange irregularly in space, and the distance between every aromatic ring is far. The aromatic carbon structures are mainly connected by methylene, ether oxygen, carbonyl ester and aliphatic ring. The oxygen functional groups mainly distributed at the edge of molecular and aliphatic structures possess many side chains. The simulated infrared spectrum of the molecular model was obtained by analyzing the vibration frequency of the optimized molecular model, and it agrees with the experimental infrared spectrum well, representing the accuracy and rationality of the molecular structure model of Eshan lignite. This molecular structure model is conducive to understanding the physicochemical properties of Eshan lignite more intuitively and revealing its macroscopic properties. Meanwhile, the molecular structure model can provide a theory basis for further research on lignite pyrolysis, liquefaction and gasification.