Rational design of high performance metal organic framework stationary phase for gas chromatography

Metal organic frameworks ( MOFs) are assembled from metal ions or clusters and organic ligands. The high tunability of these components offers a solid structural foundation for achieving efficient gas chromatography ( GC) separation. This review demonstrates that the de. sign of high performance MOFs with suitable stationarity should consider both the thermody. namic interactions provided by these MOFs and the kinetic diffusion of analytes. Thermodynam. ic parameters are basic indicators for describing the interactions between various analytes and the stationary phase. Thermodynamic parameters such as retention factors, McReynolds con. stants, enthalpy changes, and entropy changes can reflect the relative intensity of thermody. namic interactions. For example, a larger enthalpy change indicates a stronger thermodynamic interaction between the analytes and stationary phase, whereas a smaller enthalpy change indi. cates a weaker interaction. In addition, the degree of entropy change reflects the relative de. grees of freedom of analytes in the stationary phase. A larger entropy change indicates that the analytes have fewer degrees of freedom in the stationary phase. The higher the degree of re. striction, the closer the adsorption of the analytes and, thus, the longer the retention time. Thermodynamic interactions, such as metal affinity, p.p interactions, polarity, and chiral sites, can be rationally introduced into MOF structures by pre. or post. modifications depending on the target analytes. These tailored thermodynamic interactions create a favorable environ. ment with subtle differences for efficient analyte separation. For example, MOF stationarity may require large conjugation centers to provide specific p.p interactions to separate benzenes. Chiral groups may be required in the MOF structure to provide sufficient interactions to separate chiral isomers. The kinetic diffusion rate of the analytes is another critical factor that affects the separation performance of MOFs. The diffusion coefficients of analytes in the stationary phase ( D s) can be used to evaluate their diffusion rates. The chromatographic dynamics equation illustrates that the chromatographic peak of analytes tends to be sharper and more symmetrical when the D s is large, whereas a wider trailing peak may appear when the D s is small. The Van Deemter equation also proves that a low D s may lead to a high theoretical plate height and low column efficiency, whereas a high D s may lead to a low theoretical plate height and increased column efficiency. Analyte diffusion can be significantly influenced by the pore size, shape, particle size, and packing mode of MOFs. For instance, an excessively small pore size results in increased mass transfer resistance, which affects the diffusion of analytes in the stationary phase, probably leading to serious peak trailing. Thus, a suitable pore size is required to en. hance the kinetic diffusion of analytes and improve the separation performance of MOFs. Theo. retically, the design of a high performance MOF stationary phase requires the creation of routes for the rapid diffusion of analytes. However, the separation ability of an MOF is determined by not only the kinetic diffusion rate of the analytes but also the thermodynamic interactions it pro. vides. An excessively fast diffusion rate may lead to insufficient interactions between the ana. lytes and MOFs, compromising their ability to effectively separate different analytes. The ther. modynamic interactions and kinetic diffusion of analytes are synergistic and mutually essential. Therefore, this review concludes with research on the influence of both the thermodynamic in. teractions and kinetic diffusion of analytes on the performance of MOF stationary phases. Based on the findings of this review, we propose that high performance MOF stationary phases can be achieved by balancing the thermodynamic interactions and kinetic diffusion of analytes in these phases through the rational design of the MOF structure. We believe that this review provides useful guidelines for the design of high performance MOF stationary phases.