【作者】 李贺；

【导师】 吴卫泽；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2024， 博士

【摘要】 由于能源资源危机及能源利用过程中引发的环境问题,能源结构亟待转型。低阶煤作为一种储量丰富、分布广泛的碳源,由于组成结构的限制,导致其利用率很低,低阶煤的清洁、高效、高值转化迫在眉睫。物质的结构是反应性与应用的基础。基于此,本文利用化学解聚手段在制备化学品的同时,对不同转化过程中低阶煤结构及演化规律进行探索,并对低阶煤结构模型提出新的见解,为其转化和工业应用提供理论支撑;生物质在替代传统化石能源方面具有重大潜力。木质素作为生物质中最稳定的组分,其分子结构与低阶煤之间存在重要关联,通过探讨木质素与低阶煤结构的异同点,试图为低阶煤的进化及木质素的“类煤化”利用提供新的观点。主要研究内容及结果如下:（1）低阶煤是由植物体演化而来,作为未来新兴能源的生物质与低阶煤之间肯定存在某种共性关系。通过对生物质中结构稳定的木质素组分与褐煤进行物理表征及分步解聚,发现二者结构上存在一定的相似性,但又有其各自的特点。木质素富氧,大多为酚类和芳香醚类,有丰富的C_al-O桥键和甲氧基、酚羟基取代的芳香结构,长链脂肪酸以游离态、弱键或芳环侧链形式存在。而褐煤富碳,桥键多为C_al-C_al,芳香结构以烷基取代芳碳为主,大量的长链脂肪酸以桥键或者芳环侧链的形式存在于褐煤中,褐煤中桥链长度、长链烷烃丰度均优于木质素,以上实验结果说明了植物体在成煤过程中发生了脱氧反应,木质素中芳环上甲氧基、酚羟基,以及大量的醚键结构脱除,结构稳定的长链烷烃以及稠环芳香结构保留、富集,并伴随结构单元间的交联反应。（2）相比于热解和液化,碱-氧氧化因反应温度低,转化率高,且适合低阶煤高氧含量的结构特性,在低阶煤转化制备化学品方面具有独特的优势。了解低阶煤结构特性与氧化反应性间的关联尤为重要。通过对低阶煤——小龙潭褐煤进行分步碱-氧氧化,发现随氧化反应进行,稠环芳烃结构富集于腐殖酸中,而长链烷烃结构富集于未反应的残渣内。基于剥离的特殊结构,本文构建了分子式为C₃₆₆₁H₃₄₃₅O₁₀₃₆N₈₁S₇的小龙潭褐煤结构模型,分子量为65301 amu,单环、双环、三环、四环芳香团簇的比例分别为43.7%,39.3%,14.7%和2.4%,其中包括15.9%的吡咯和2.4%的吡啶。而可控氧化解聚避免了稠环芳烃的开环和长链烷烃的过度氧化反应,进而深入了解了褐煤结构及氧化过程。（3）煤直接液化是一种通过加氢反应捕获自由基并获得高收率油品的转化方式,而液化煤种的选择也是影响液化效果的关键因素。什么结构的煤炭适于直接液化是一个关键问题。通过对典型液化所用低阶煤——淖毛湖煤的结构进行了研究,结合物理表征分析及氧化解聚,构建了分子尺寸为128755 amu的淖毛湖煤的结构模型,其中单、双、三、四芳环的比例为149:164:102:63。三、四环芳香团簇之间主要是由2-4个碳原子构成的C-C键连接,而单双环芳香团簇主要是通过醚、酯键等连接在三、四环芳香团簇结构上。淖毛湖煤中芳香碳和脂肪碳比例分别为62.60%和31.03%,平均芳环结构尺寸大约为2环。最终,确定了高H/C比,高醚键、酯键等弱键连接的含氧结构,丰富的1～4环构成的小尺寸芳香团簇结构以及C2-C4烷基桥键的结构特征有利于淖毛湖煤的液化行为。（4）热解是最广泛的利用方式,几乎所有的热转化过程均会涉及热解反应。研究低阶煤在热解过程中结构变化规律及特性,可以为低阶煤分级分质利用及煤化过程提供理论指导。本工作对低阶煤及不同温度下的热解半焦进行氧化解聚,结合物理表征,研究了低阶煤在热解过程中结构演化特征和路径,揭示了芳香团簇尺寸与热解温度的相关性。随热解温度增加到500℃,低阶煤的脂肪侧链和桥键断裂,小分子逸出,芳香结构富集增长,层间距(d₀₀₂)变化很小,而芳香片层尺寸（L_a）和堆叠高度（L_c）增加显著。芳香团簇平均尺寸由低阶煤的单、双环结构,逐渐演化为500℃半焦的三、四环。通过氧化解聚发现,热解温度升高,对应半焦的氧化时间延长,产物苯羧酸收率升高,说明热解后煤结构变得更加复杂且反应活性降低,并生成了大量与氧原子不直接相连的芳环结构,特别是苯二羧酸、苯三羧酸和苯四羧酸的前驱体结构。更多还原

【Abstract】 Owing to the crisis of energy resources and the environmental issues arising from the process of energy utilization,it is imperative to transform the energy structure.As a carbon source with abundant reserves and wide distribution,the utilization rate of low-rank coal is low due to its structural limitation,and the clean,efficient,and high-value conversion of low-rank coal is imminent.The structure of a substance determines its reactivity and application.Based on the above,this paper explores the structure and evolution laws of low-rank coal in different transformation processes through depolymerization methods while preparing chemicals,and provides new insights into the structural model of low-rank coal,which can further provide theoretical support for the conversion and industrial application of low-rank coal.As a primary source of low-rank coal,biomass has significant potential to replace traditional fossil energy resources.The molecular structure of lignin,the most stable component of biomass,is importantly related to that of low-rank coal.By exploring the structural similarities and differences between lignin and low-rank coal,we attempt to provide new insights into the evolution of low-rank coal and the"coal-like"utilization of lignin.The main studies and conclusions are as follows.（1）Low-rank coals evolved from plant bodies,and there must be some common relationship between biomass as a future emerging energy resource and low-rank coals in fossil energy resources.The physical characterizations and stepwise depolymerizations of structurally stable lignin fraction and lignite revealed some structural similarities between the two,but with their own individual characteristics.However,lignin is rich in oxygen,mostly phenols and aromatic ethers,with abundant C_al-O bridge bonds and aromatic structures substituted with methoxy and phenolic hydroxyl groups,and long-chain aliphatic acids in the form of free state,weak bonded or aromatic ring side chains.In contrast,lignite is rich in carbon,with mostly C_al-C_al bridge bonds and aromatic structures dominated by alkyl-substituted aromatic carbons,and a large number of long-chain aliphatic acids existed in the form of aromatic ring side chains.The bridge chain length and the abundance of long-chain alkanes in lignite were all better than those in lignin.The above experimental results indicated that the deoxygenation reactions occurred in the process of coal formation from the plant,and a large amount of methoxy and phenolic hydroxyl groups on the aromatic rings,as well as a large number of ether bond structures were removed from lignin,and the structurally stable long-chain alkane structures and fused-ring aromatic structures were retained and enriched,and accompanied by occurrence of cross-linking between structural units.（2）Compared with pyrolysis and liquefaction,alkali-oxygen oxidation has unique advantages in the conversion of low-rank coals to prepare chemicals because of its low reaction temperature and high conversion rate,and it is suitable for the structural properties of low-rank coals with high oxygen content.It is particularly important to understand the correlation between structural properties and oxidation reactivity of low-rank coals.By stepwise alkali-oxygen oxidation of low-rank coal,Xiaolongtan coal,it was found that as the oxidation reaction proceeded,the fused-ringed aromatic structures were enriched in humic acids,while the long-chain alkane structures were enriched in the unreacted residues.Based on special structures that were stripped out,a structural model of Xiaolongtan lignite with a molecular formula of C₃₆₆₁H₃₄₃₅O₁₀₃₆N₈₁S₇ and molecular weight of 65301 amu was constructed.There are 43.7%single-,39.3%double-,14.7%triple-,and 2.4%quadruple-ring aromatic clusters,amount which there 15.9%pyrrole-and 2.4%pyridines-based aromatic clusters.The controllable oxidation avoids the peroxidation reaction of aromatic clusters and the long-chain alkanes,thereby gaining a deeper understanding of the structure and oxidation process of lignite.（3）Direct coal liquefaction is a conversion method to capture free radicals and get a high yield of oils by hydrogenation reaction.However,the selection of types of coal during liquefaction is also a key factor in influencing the liquefaction result.And what structure of coal is suitable for direct coal liquefaction is a critical issue.The structure of Naomaohu coal,typical low-rank coal used for liquefaction,has been studied,and a structural model of Naomaohu coal with a molecular size of 128755 amu has been constructed by combining physical characterization analyses and oxidation depolymerizations,in which the ratio of single-,double-,triple-and quadruple-aromatic rings is 149:164:102:63.The triple-and quadruple-aromatic clusters are mainly connected by C-C bonds consisting of 2-4 carbon atoms,while the single-and double-aromatic clusters are mainly connected to the triple-and quadruple-aromatic clusters by ether and ester bonds.The proportions of aromatic and aliphatic carbon in Naomaohu coal were 62.60%and 31.03%,respectively,and the average size of aromatic ring structures was about 2 rings.Finally,it was determined that the high H/C ratio,a large amount of oxygen-containing structures（ether and ester bonds,and other weak bonds）,the small-size aromatic clusters structures composed of abundant 1～4 rings,and the C2-C4 alkyl bridge bonds are favorable to the liquefaction behavior of Naomaohu coal.（4）Pyrolysis is the most widely utilized method,involving almost all thermal conversion processes.The study of the evolution characteristics and pathways of low-rank coal during pyrolysis can provide theoretical guidance for low-rank coal grading and fractionation utilization and the coalification process.In this work,the oxidative depolymerization of low-rank coal and pyrolyzed coal at different temperatures,combined with physical characterization,was used to deduce the structural evolution characteristics and pathways during pyrolysis,revealing the correlation between aromatic cluster size and pyrolysis temperature.As the pyrolysis temperature proceeded to 500℃,the aliphatic side chains and bridges scission as well as the aromatic structures were enriched and grew up in the low-rank coal matrix.The layer spacing(d₀₀₂)varied very little,while the aromatic lamellae size（L_a）and stacking height（L_c）increased significantly.The average size of aromatic clusters gradually evolved from single-and double-ring aromatic structures of low-rank coals to triple-and quadruple-ring aromatic clusters by pyrolysis at 500℃.By oxidative depolymerization,it was found that the reaction time of pyrolyzed char samples was prolonged and the yield of the product of benzene carboxylic acids was increased by increasing the pyrolysis temperature.It indicated that the coal structure became more complex and less reactive after pyrolysis,and a large number of aromatic ring structures that were not directly connected with oxygen atoms were generated,especially the precursor structures of benzene carboxylic acids containing 2～4-COOH.更多还原