Ni@MFI封装催化剂定向转化酚制芳烃氢解选择性调控机理研究

Catalytic upgrading of phenolics to aromatics via hydrogeneolysis is an important direction for the high value utilization of medium and low-temperature coal tar. However, most of metal particles, with ununiform sizes were unevenly distributed on the external surface of current catalysts, which resulted in the inferior hydrogenolysis activity, selectivity of aromatics and stability. With phenol/m-cresol hydrogenolysis to benzene as the model reaction, we design Ni@MFI model catalysts with different Ni properties and polarity, through encapsulation of Ni nanoparticles into MFI zeolites. The influence of interactions between metal and MFI zeolites on catalysts' properties is revealed. Selective adsorption of hydroxyl groups over model catalysts is quantitatively characterized by in situ FT-IR and 17O MAS NMR technologies. The kinetic investigations disclose the crucial factors influencing selective adsorption of hydroxyl groups, and the mechanism of phenol/m-cresol hydro-conversion over Ni@MFI encapsulated catalysts via the end-up adsorption conformation in confined space. Corresponding, the structure-activity relationship between properties of Ni@MFI encapsulated catalysts and hydrogenolysis performance of phenol/m-cresol can be established. This research will provide new insights into the design of encapsulated catalyst and selectivity control methology for phenolic hydrogenolysis to drive the upgrading phenolics in medium and low-temperature coal tar to value-added aromatics.

中低温煤焦油中的酚类化合物定向催化制备芳烃，是其高值化利用的重要方向。但现有催化剂，金属多分布于外表面、粒径不均一，难以兼顾氢解活性和芳烃选择性，且稳定性差。基于苯酚/间甲酚氢解模型反应，本项目拟设计MFI分子筛封装具有良好加氢活性的Ni纳米颗粒，构建金属性质和极性可调控的Ni@MFI模型催化剂，揭示封装过程中金属与分子筛相互作用对催化剂性质的影响机制。通过原位FT-IR和17O MAS NMR技术，研究苯酚/间甲酚在模型催化剂上的吸附，建立酚羟基选择性吸附的量化表达方法。通过研究反应动力学特征，阐明影响酚羟基选择性吸附的关键因素、及苯酚/间甲酚在Ni@MFI催化剂限域空间和端点吸附模式下临氢转化的反应机理，建立催化剂性质与苯酚/间甲酚氢解性能间构效关系。本项目的开展为酚类化合物氢解制芳烃催化剂设计和选择性调控提供机理认识，支撑中低温煤焦油高值化利用技术开发。

中低温煤焦油中酚类物质定向催化制备芳烃是高值化利用的重要方向。传统浸渍法所得催化剂，金属颗粒多分布于载体外表面、粒径不均一，难兼顾氢解活性和芳烃选择性，且稳定性差。基于此，本项目旨在开发一种氢解活性高、芳烃选择性优和催化稳定性好的酚油氢解催化剂；从调控酚吸附形态入手，理性设计和可控制备系列酚油氢解封装催化剂。通过调控封装方法、金属Ni前驱体及封装主体MFI硅铝比，成功制备出系列具有不同形貌的Ni@MFI封装催化剂，揭示了封装方法致使Ni纳米颗粒在MFI晶体表面或内部的分布规律以及晶体自身孔结构性质的影响规律。优选以TPAOH为结构导向剂、乙二胺为Ni前驱体配体及正硅酸乙酯为硅源，采用原位低温分段合成法所制备的椭球型纯硅Ni@MFI封装催化剂，可实现金属Ni纳米颗粒完全均匀被封装于MFI晶体内部，其结构类似于火龙果，所封装Ni纳米颗粒可类比于火龙果中的黑色籽，被均匀镶嵌在MFI主体（火龙果）内部。基于封装金属Ni纳米颗粒限域催化与MFI孔道择形的协同作用，通过酚模型物（苯酚和间甲酚）临氢反应的动力学实验，成功实现了MFI孔道限制酚苯环的平躺吸附而表现出较高的氢解活性、芳烃选择性和催化稳定性，但扩孔后的MFI则利于酚苯环平躺吸附进而促进苯环加氢反应和降低酚羟基氢解反应。在反应温度300 ℃时，优选的Ni@MFI在间甲酚体积空速6 h-1和略高于常压条件下酚羟基氢解转化率和选择性均超过95.0%，芳烃收率约74.2%。尽管酚油馏分中硫/氮化合物含量极少，低于1.0wt%，但却是导致Ni@MFI封装催化剂失活的根本原因，尤其是硫化合物。本项目不仅开发了分子筛封装非贵金属的新方法，亦为中低温煤焦油氢解制芳烃技术提供数据支撑。