Preparation Of Hierarchically Porous UiO-66@SiO₂ Core-Shell Stationary Phase And Its Application In Liquid Chromatography

High performance liquid chromatography（HPLC）is one of important technologies for separation and analysis of complicated samples.The development of new stationary phase with good selectivity,high separation efficiency and high speed is always the key to promote the improvement of chromatographic technology.Metal-organic frameworks（MOFs）are demonstrated to be promising chromatographic packing materials for HPLC,because of the adjustable pore sizes,large specific surface area and abundant action sites.Among them,the Ui O-66 MOFs have the characteristics of simple preparation,excellent stability,acid and alkali resistance,which is a potential ideal chromatographic filler.However,Ui O-66-based stationary phases exhibit some disadvantages including low chromatographic efficiency,high column pressure and poor chromatographic peaks.Therefore,to solve these problems,in this work,we firstly prepared UiO-66@SiO₂ core-shell microspheres through in situ self-assembly method.And then the MOFs shell was etched by H₂O₂ to obtain hierarchically porous UiO-66@SiO₂ stationary phases.The chromatographic retention behaviors of various types of small molecules on UiO-66@SiO₂ packed column were carefully studied.Moreover,the effect of pore size on the separation performances were also investigated.This thesis is mainly divided into three parts:Chapter 1:In this chapter,a brief overview of the structures,characteristics and types of MOFs,application of MOFs and its composite materials in chromatography,the perparation of hierarchically porous MOFs,the research progress of Ui O-66 materials as chromatographic stationary phase,the relationship between MOFs and compounds in separation,as well as the performance evaluation method of chromatographic stationary phase was summerized.Chapter 2:In this chapter,2μm silica microspheres were employed as the substrate,and 3-aminopropyl trimethoxysilane（APTES）was used to modify the surface of silica to form aminosilica（SiO₂-NH₂）.Then the aminosilica was mixed together with metal ions of Zr⁴⁺and organic ligands（terephthalic acid or 2-amino terephthalic acid）in one-pot to prepare microporous UiO-66@SiO₂ and Ui O-66-NH₂@SiO₂ core-shell microspheres through in situ self-assembly process.The two core-shell microspheres were further characterized via FT-IR,SEM,XRD and BET.The retention behaviors of small solutes on the two stationary phases were investigated,and the separation performance discrepancies of both hydrophobic and hydrophilic small molecules were explored with varying the mobile phase compositions as well as MOFs types.The results showed that the two microporous stationary phase presented a hydrophilic/hydrophobic mixed mode.And for UiO-66@SiO₂ column,the separations of alkylbenzenes and xylene compounds were better than other kinds of solutes under reverse phase mode.In contrast,for the Ui O-66-NH₂@SiO₂ column,its separation performace for hydrophobic compounds was not as good as UiO-66@SiO₂ column,whilst better separation for hydrophilic compounds was obtained.Nevertheless,the pore sizes of MOFs in both the two MOFs@SiO₂ core-shell microspheres are in the range of micropore（<2 nm）,which greatly hindered the molecular diffusion and mass transfer.As a result,these two columns still have some disadvantages such as low resolution,low column efficiency and long analysis time.Therefore,it is necessary to further develop hierarchically porous UiO-66@SiO₂ chromatographic stationary phase for efficient separation of small molecules.Chapter 3:In order to resolve the problems of microporous UiO-66@SiO₂ column mentioned in the chapter 2,we successfully preared hierarchically porous UiO-66@SiO₂microspheres through etching method with hydrogen peroxides as an etching agent.Moreover,the hierarchical pore size of UiO-66@SiO₂was adjusted by varying the amount of hydrogen peroxides added during the etching process.The hierarchically porous core-shell microspheres were characterized via FT-IR,SEM,XRD and BET.Under reversed-phase mode,the separations of six groups of small molecules on hierarchically porous UiO-66@SiO₂ column were investigated and their chromatographic properties were evaluated.The results showed that the hierarchically porous UiO-66@SiO₂ column prepared by etching with 4 m L hydrogen peroxides displayed the best separation performances for all the six group of solutes.Compared with microporous UiO-66@SiO₂ column,hierarchically porous UiO-66@SiO₂ column showed better separation resolution and higher column efficiency.Baseline separations for six groups of hydrophobic solutes were achieved and the theoretical plate number of toluene was 8300 plate/m.Therefore,the hierarchically porous UiO-66@SiO₂ core-shell microspheres was demonstrated to effectively overcome the shortcomings of microporous one with smaller pore size and lower mass transfer,which obiviously improved the separation of small hydrophobic solutes with higher efficiency and faster speed.This strategy will provide a new pathway for developing new MOFs-based stationary phase.