超临界流体活化配位不饱和MOFs材料的热力学基础与过程调控

Metal-organic frameworks (MOFs) have shown uniquely high performance in a wide range of applications such as gas storage, gas/liquid separations, and heterogeneous catalysis. High internal surface area is one of the foremost attributes of MOFs. The removal of guest solvent molecules from the pores (MOF activation) without diminishing porosity is an extremely important step during MOF preparation. The current activation method for MOFs containing coordinately unsaturated metal sites (CUS) is highly time-consuming, volatile solvent- and energy-intensive. To solve this problem, we propose to remove the guest molecules binding to the CUS rapidly and efficiently by supercritical CO2 mixed with cosolvent. This new activation method for MOFs containing CUS would be faster, less volatile solvent demanding and energy saving. The activated MOFs would show better performance. In this proposal, we will first experimentally measure the single and competitive adsorption equilibria of guest molecules and cosolvents on several series of MOFs containing different CUS and pore structures in supercritical CO2, then investigate the activation results of MOFs with CUS by supercritical fluids, and finally experimentally measure the dynamic desorption curves of guest molecules by supercritical fluids and cosolvents by supercritical CO2. The experimental results will be combined with molecular simulation and mathematical analysis to understand how to select the cosolvent and its concentration in supercritical CO2, the transport mechanism of desorption process by supercritical fluids and how to control the activation process based on different MOF structures. The relationship of the microscopic structures of MOFs and their macroscopic adsorption properties will be also studied. The outcomes of this project will provide fundamental principles for the activation of MOFs by supercritical fluids, and also have significant impact on the preparation of MOFs on large scale and their application in the petrochemical industry.

金属有机骨架材料(MOFs)在气体储存、吸附分离和多相催化等领域已展现出优异的性能。MOFs的活化对材料性能影响很大，是制备中一至关重要环节，但含配位不饱和金属(CUS)MOFs现有活化方法过程冗长、能耗高且往往CUS活性位点暴露不完全而影响材料性能。本申请提出在超临界CO2中添加极性共溶剂，以夹带剂浓度梯度变化的渐进方式快速完全脱除与CUS结合的客体分子制备高性能MOFs材料。通过实验结合量化计算和数值模拟，研究超临界CO2条件下客体分子和极性共溶剂与不同结构系列MOFs的作用方式和强度、超临界流体脱除客体分子和极性共溶剂的动态过程及传递特性，探索超临界流体活化时极性共溶剂种类和用量的选择依据、活化条件和过程调控与MOFs结构之间的依赖关系，并考察MOFs活化方法与其微观结构和宏观吸附性能的关系。本申请的研究成果将为MOFs的超临界流体活化方法奠定基础，推动MOFs的规模化制备及其应用。

针对MOFs材料现有活化方法时间长、溶剂消耗量大且往往活化不完全而影响材料性能，本项目研究了超临界流体技术用于MOFs材料活化的热力学和动力学规律，以及活化完全的MOFs材料对溶液中有机物的选择性吸附性能进行了探索。. 以配位饱和（MOF-5、IRMOF-3、MOF-177和UiO-66）、配位不饱和（MOF-74、HKUST-1、MIL-101（Cr）和UMCM-150）以及骨架结构稳定性差（FJI-1和DUT-9）这三类典型MOFs为对象，合成以后无需使用低沸点有机溶剂置换，用超临界流体技术直接进行活化，得到以下规律：（1）配位饱和MOFs，液态和超临界CO2的效果是基本一致的，然而对于配位不饱和MOFs，超临界CO2能脱除部分与配位不饱和金属位点结合的溶剂分子，活化效果更佳；（2）对配位饱和MOFs，超临界CO2无需添加夹带剂可在1个小时内完全活化，对配位不饱和MOFs，需添加乙醇或丙酮作为夹带剂的超临界CO2才能完全活化；（3）对骨架结构稳定性差的MOFs，超临界流体在减压膨胀过程时吸热降低体系的温度，如进入液态CO2区域将使骨架结构塌陷，活化过程中应保持50℃以上的较高温度并控制减压膨胀速率。. MOFs材料的液相吸附性能以燃料油中硫化物/氮化物和水中有机污染物的去除为目标，研究了含多种官能团的系列MIL-101(Cr)的吸附性能，系统阐述了MOFs材料基于氢键相互作用对液相体系中目标物质的选择性吸附性能。实验结果表明，MIL-101(Cr)中引入氨基对喹啉和吲哚的吸附量可分别提高25%和10%，MIL-101(Cr)-NH2对燃油中喹啉和吲哚的吸附量高达50和70 mg-N/g，高于绝大部分吸附剂。通过红外光谱表征，阐述了MOFs材料中氨基官能团与有机氮化物具有较强氢键相互作用，引入氨基功能基团，可有效提高其选择性吸附脱氮性能，对新型脱氮MOFs的设计有指导意义。项目成果为MOFs材料的清洁化、规模化制备以及功能吸附MOFs材料的制备提供了理论和技术基础。