基于金属有机骨架材料固定相的气相色谱分离应用

金属有机骨架材料(MOFs)是一类由有机配体和金属离子(或金属簇)自组装形成的新型多功能材料。MOFs具有孔隙度高、比表面积大、孔径可调、化学和热稳定性高等特点,被广泛应用于吸附、分离、催化等多个领域。近年来,MOFs作为新型气相色谱固定相用于分离异构体受到了广泛关注。与传统无机多孔材料相比,MOFs在结构和功能上展现出高度的可调性,通过合理地选择配体和金属中心,可以设计合成具有不同孔道大小和孔道环境的MOFs,从而分别从热力学和动力学角度优化色谱分离效果,有效提高分离选择性。该文结合MOFs的结构,讨论了MOFs气相色谱固定相分离不同类型分析物的分离机理。分离机理主要包括MOFs孔道的分子筛效应或形状选择性,MOFs不饱和的金属位点与分析物中不同的官能团之间产生的相互作用,分析物与MOFs孔道之间产生的不同范德华力、π-π相互作用和氢键相互作用。此外,MOFs的手性分离可能主要依赖于外消旋体与手性MOFs中手性活性位点之间的相互作用。该文也对不同分析目标物进行了归类,综述了多种MOFs气相色谱固定相对烷烃、二甲苯异构体和乙基甲苯、外消旋体、含氧有机物、环境有机污染物的气相色谱分离效果。最后,该文还对MOFs在该领域的应用进行了总结与展望,旨在为MOFs气相色谱高效分离的研究提供参考。

Application of gas chromatography separation based on metal-organic framework material as stationary phase

Metal-organic frameworks(MOFs)are a new class of porous materials,which are synthesized using organic ligands and inorganic metal ions or metal clusters.MOFs possess tunable structures through the self-assembly of a large number of organic linkers and metal nodes,which is beyond the scope of conventional porous materials.In addition,MOFs have excellent properties,including the lowest density(as low as 0.13 g/cm),highest specific surface area(as high as 10400 m~2/g),and largest pore aperture(as large as 9.8 nm)among all porous materials reported till date.Because of their high porosity,large surface area,tunable apertures,as well as high chemical and thermal stabilities,MOFs have been widely applied in the fields of adsorption,separation,and catalysis.In addition,MOFs have been successfully applied as stationary phases for isomer separation in gas chromatography(GC).Since the use of the first MOF(MOF-508)packed column for the separation of alkane isomers in GC,several other MOFs(e.g.,MIL-47,MOF-5,and ZIF-8)have been employed for the GC separation of isomers.However,packed-column-type separation not only requires gram-scale quantities of MOFs,thereby increasing the analysis cost,but also results in poor separation efficiency.The first MOF(MIL-101)capillary column designed toward cost reduction allowed for the baseline separation of xylene and ethylbenzene isomers within 100 s under constant-temperature conditions.Since then,the capillary-type column has been widely utilized in the MOF-based stationary phase for GC separation.Alkanes,xylene isomers and ethyl toluene,oxy-organics and organic pollutants are not only important chemicals in industry but also harmful environmental pollutants.Thus,the separation of these analytes is of practical importance environmental monitoring and industrial quality control.However,it is difficult to realize the efficient separation and detection of these isomers or racemates because of their similar boiling points and molecular sizes.In the past decades,GC was utilized as a rapid and efficient technique for the separation of the abovementioned analytes.The stationary phase used in GC plays a dominant role in the separation processes.This review summarizes the MOF-based GC separation of the abovementioned targets based on the different classification of analytes,including alkanes,xylenes,racemates,oxy-organics and persistent organic pollutants.The separation mechanisms of different analytes are also discussed according to the structural benefits of MOFs.The separation mechanisms mainly involve van der Waals forces between the MOFs and analytes,interactions between the unsaturated metal sites and different functional groups of the analytes,molecular sieve effect or shape selectivity,and hydrogen-bond orπ-πinteractions.In addition,the chiral recognition abilities of MOFs possibly depend on the interactions between the chiral active sites in chiral MOFs and racemates.Furthermore,efficient GC separation is influenced by thermodynamic and kinetic factors.The thermodynamic factor is mainly the difference between the partition coefficients of the separated components,which also reflects the properties of the analytes as well as the interactions between the stationary phase and the analytes.The kinetic factor also affects the column efficiency and chromatographic peak shape.Compared with traditional inorganic porous materials,MOFs with tunable structures are more favorable for optimizing the separation of isomers from both thermodynamic and kinetic standpoints.Therefore,this review summarizes the separation mechanism when using MOFs as stationary phases for isomer separation via thermodynamic and kinetic analyses.We hope the review would aid the state-of-art design of MOF stationary phases for high efficient isomer separations in GC.