Study On The Preparation And Performance Of Metal-Organic Framework Toward Organic Separations

With the development of industry,chemical separation plays a vital role in the chemical production process.Compared with traditional separation technologies with high energy consumption（such as distillation,crystallization,etc）,solid adsorbent-based separation technology has attracted extensive attention due to its advantages of low cost and high separation efficiency.And the key is the development of advanced materials.Therefore,it is of great significance to develop functional materials with efficient separation performance.Metal-organic frameworks（MOFs）,as a new kind of nano-porous materials,have been widely used in many fields,due to its high specific surface area,highly tunable pore size,and structure designability.Herein,based on the structure characteristic of the guest molecules,MOF materials with high performance were synthesized for the separation of trace drug contamination from water and C8 aromatic hydrocarbon isomers.And the separation mechanism was explored at the molecular level by combining experiments with theory.The main research contents are as follows:（1）For the separation of trace pharmaceutical pollutants from water,a zirconium-based metal-organic framework（PCN-777）was synthesized as an adsorbent to remove cephalexin antibiotics from water.The experimental results show that PCN-777 exhibits a high adsorption capacity of 442.48mg·g^-1,which is higher than that of other types of adsorbents.In addition to the large specific surface area and pore size,the synergistic effect of electrostatic interaction and coordination interaction further enhances the interaction between the PCN-777 and cephalexin,thereby improving the adsorption capacity of cephalexin.This research work also shows that PCN-777 has a promising application prospect for the purification of wastewater containing trace antibiotics.（2）Due to the similar physicochemical properties of C8 aromatic isomers,it is a great challenge to separate them.In this work,an aluminum-based metal-organic framework（Al-MOF）was synthesized to study the ability to separate C8 aromatic isomers.Both single-component and multi-component adsorption experiments showed the sample exhibits preferential adsorption of o-xylene with the adsorption order of OX>PX>MX>EB.In particular,the separation selectivity of OX/MX can be up to 8.1,which is higher than the reported values using other MOFs in the liquid phase.Combining the results of calculations,it can be concluded that except for the stacking interaction between the guest molecules,the interaction between the polar groups in the framework and the guest plays a significant role in the separation process.Meanwhile,the Al-metal clusters with high electrostatic potential gradients also enhance the electrostatic interactions between the host and guest.Besides,with the help of DFT calculations,it can be concluded that the difference between the affinity of the framework and the guest molecules endows the MOF material with excellent selective adsorption capacity for o-xylene.（3）Based on the pre-synthesis modification method,using Ni（BDC）（TED）_0.5 as the fundamental framework structure,Ni（NDC）（TED）_0.5was synthesized by replacing 1,4-benzene dicarboxylic acid（H₂BDC）with1,4-naphthalene dicarboxylic acid（H₂NDC）.The introduction of the naphthalene ring not only successfully regulated the pore size（from 7.5?to6.0?）and pore microenvironment,but also introduced more benzene rings that can interact with positively charged/Lewis base sites.Ni（NDC）（TED）_0.5exhibits the high-efficiency separation ability of m-xylene from the C8aromatic mixture.The mechanism analysis showed that the main reasons for such separation performance are that the shape difference of C8 aromatic molecules and the unique pore structure synergy lead to the difference in the van der Waals and C-H...πinteractions between host and guest.In addition,the presence of the naphthalene ring group in the backbone may rotate,providing an optimal adsorption site for guest molecules,thereby improving the separation selectivity of m-xylene.Meanwhile,this work also provides new ideas for the separation of other systems.（4）Taking advantage of the flexibility of MOF materials,a locally flexible MOF material,Ni（ADC）（TED）_0.5,was synthesized.The coated capillary column can achieve baseline separation of the reverse boiling point sequence of C8 aromatic mixtures with the elution order of EB>PX>MX>OX.Static adsorption experiments and liquid-phase breakthrough experiments also confirmed the selective adsorption of EB over other isomers.Powder X-ray diffraction and Micro ED studies showed that it can adapt to specific guest molecules through disordered transformations of organic ligand units,thereby achieving the selective adsorption of EB.Density functional theory（DFT）and Hirshfeld surface calculations indicated that guest-induced splint-like confinement structure of ADC ligand makes the main contribution to such isomer separation performance.This work not only realizes the separation of C8 aromatic isomers using local flexibility but also provides a rational strategy for the separation of other isomers.