中低阶煤的选择性催化加氢解聚机理研究

Catalytic hydrogenolysis is a predominant approach used to depolymerize weak covalent bridging bonds aiming to obtain chemicals from middle- and low-rand coals under a mild condition. In this project, our main investigations are: (1) to prepare metallic model catalysts and supported metallic catalysts, which facilitate hydrogenolysis but depress hydrogenation, by controllable synthesis for hydrogenolysis reaction, and subsequently examine the effects of catalytic active sites, including their structure, morphology and dimension, and supports on hydrogenolysis reaction, to reveal the synergic effects resulting from various components of catalyst; (2) to evaluate the influence of intrinsic structure of reactants on the reactivity of hydrogenolysis using model compounds, which contain distinct weak covalent linkages (such as aliphatic bridges and orheteroatom-containing bridging bonds) and aromatic units with various substituents, and finally to illustrate the mechanisms related to catalytic hydrogenolysis as well as the generative behavior and transfer mechanism of active hydrogen species in above-mentioned processes by means of isotope labeling technique; (3) to catalytically hydrogenolyze organic macromolecules of middle- and low-rank coals, afterwards use multiple advanced instruments and clustering method to analyse hydrogenolysis products from organic macromolecules in middle- and low-rank coals, meanwhile incorporate the results from hydrogenolysis of model compounds for understanding the types of weak bridging bonds and structural units at a molecular level, and gain insights into the structure of organic macromolecules of middle- and low-rank coals. These investigations will provide the crucial scientific basis for process optimization of catalytic hydrogenolysis of middle- and low-rank coals and efficient use of their higher value-added products.

选择性催化加氢解聚是切断弱共价桥键旨在由中低阶煤直接获取化学品的重要手段。通过金属模型催化剂和负载型金属催化剂的加氢解聚反应，重点考察金属活性中心的结构、形貌和尺寸以及载体效应对加氢解聚反应的影响，揭示催化剂各组分间的协同作用，制备能够选择性加氢解聚弱共价桥键（如脂肪桥键和含杂原子桥键）且能抑制产物过度加氢的催化剂；选用一系列分别含不同类型弱共价桥键且芳环上取代官能团丰富的模型化合物作为反应探针分子，详细考察模型化合物的内禀结构与加氢解聚反应性之间的关系，借助同位素示踪技术揭示催化加氢解聚机理以及反应过程中活性氢的产生行为和迁移方式；对中低阶煤有机大分子进行催化加氢解聚，利用多种先进的分析手段和聚类分析方法详细解析产物的组成结构，结合模型化物解聚反应提供的相关信息，以期从分子水平上了解中低阶煤有机大分子的结构及存在形态，为优化中低阶煤加氢解聚工艺及解聚产物的高附加值利用提供重要的科学依据。

中低阶煤有机质的“结构单元”主要由含氧桥键和次甲基键相联，化学反应活性较高。中低阶煤有机质的上述性质为在温和条件下高效地“裁剪”中低阶煤弱共价键进而从煤直接获取芳香化学品提供了可能。迄今为止，有关中低阶煤有机质中弱共价键解聚方面的研究主要集中在如何有效地切断含氧桥键，而加氢解聚是能够有效“裁剪”煤有机大分子中含氧桥键的方法之一。本研究制备了一系列催化剂，选取多种含醚键化合物及木质素为探针分子，在温和条件下考察了载体的酸性和几何形貌以及活性组分对加氢解聚醚键的影响。采用蒸汽辅助法合成了无定形硅铝（ASA）-TiO2复合载体，并在此基础上制备了双功能催化剂Ru5/ASA-TiO2，将其应用于煤模型化合物二苯醚的加氢解聚反应。当ASA与TiO2的质量比为1：1时所制备的催化剂Ru5/1ASA-1TiO2表现出较高的解聚二苯醚的反应活性和苯选择性。在250 oC、氢压0.2 MPa条件下，二苯醚的转化率高于98%，苯收率为67.1%。结合NH3-TPD和pyridine-IR表征结果发现，弱酸和/或Lewis酸而非强Brӧnsted酸有利于提高二苯醚的加氢解聚转化率和苯收率。更重要的是，反应温度显著影响各类型酸的相对含量进而影响二苯醚加氢解聚产物的选择性，即低温有利于加氢反应，高温则促进脱氧和脱氢反应从而提高苯收率。制备了一系列非晶态Ru基双金属磷化物复合催化剂，并将其应用于α-O-4和4-O-5型醚键的加氢解聚。在温和的反应条件下，可以在不过度加氢的情况下实现较高的加氢解聚转化率和目标产物收率。最重要的是，源于Lewis酸的总酸量是促进醚键加氢解聚的主要因素，并且总酸量和加氢解聚产物的收率呈线性关系。基于二苯醚、3-苯氧基甲苯、苄基苯基醚和二苄醚的反应结果,α-O-4和4-O-5型醚键的加氢解聚可能分别由H-攻击α-C和芳碳取代原子所引发。制备了一系列大孔S-1负载Ru-Ni双金属磷化物催化剂（Ru-Ni12P5@S-1x），用于木质素及其模型化合物的C−O键的加氢解聚。未还原的Ru物种（Ruδ+）产生的路易斯酸位点和大孔载体的几何结构显著影响了加氢解聚转化率和产物选择性。特别注意的是，芳基醚和脂肪族醚键都能在Ru-Ni12P5@S-15上直接加氢解聚。