Mixed Metal-Organic Framework Stationary Phases for Liquid Chromatography

In modern separation science and technology, the stationary phase structures have drawn significant research attention owing to their influence on the separation performance of liquid chromatography (LC). Numerous LC separation modes and their respective stationary phases have been developed to meet the growing demands for improved separation performance and control. Lately, mixed-mode LC enabling the simultaneous operation of multiple retention modes has emerged as a powerful separation phase design strategy, replacing the conventional design strategies. Although it has facilitated improved phase separation, it still encounters numerous challenges in recognizing and separating complex molecules in practical scenarios. To this end, developing versatile LC stationary phases is highly desirable in overcoming these challenges.

Metal-organic frameworks (MOFs) offer versatile porous platforms for facilitating the structural design of desirable molecular adsorption media. These nanoporous materials are formed through the coordination and self-assembly of organic linkers and metal ions. For improved separation performance and retention control, highly designed features of MOF-based stationary phases have been used. Nevertheless, most MOF-based molecular separations reported so far focus on using single-component and single-phase stationary phases, which still have some limitations. The design of LC stationary phases using mixture of different MOFs, both in the form of physical mixture of different MOF particles and solid solution MOFs where different linkers are homogeneously incorporated in a single crystal structure, is yet to be reported despite its promising advantages.

Herein, Mr. Kaoru Kioka, Mr. Nagi Mizutani, Professor Nobuhiko Hosono and Professor Takashi Uemura from The University of Tokyo proposed a new design approach based on MOFs as tunable LC stationary phases. The retention performance of the polyethylene glycol for the mixed-MOF stationary phases with filled packed columns was investigated to illustrate the rational tunability of the retention behavior, where the physical mixture of different MOF particles and mixed-linker solid-solution MOFs were utilized as LC stationary phase. The work is currently published in the journal, ACS Nano.

In their approach, a total of three sub-nanoporous MOFs having isostructural pillared-layer structure were employed: 1,4-naphthalenedicarboxylate (ndc), 1,4-benzenedicarboxylate (bdc) and 9,10-anthracenedicarboxylate (adc). The isostructural frameworks of these MOFs had sub-nanosized pores, and their dicarboxylate ligand layers could be replaced by isostructural dicarboxylates during solvent synthesis. This systematic design of the constituent ligands allowed tunability of the MOF pore sizes by varying the mixing composition of the dicarboxylate linkers.

The authors showed that the mixed-particle MOF columns exhibited better control of the polyethylene glycol retention behavior in a predictable fashion. This was achieved by simply tunning the MOF mixing ratio. On the other hand, a multicomponent effect where the mixed-linker solid-solution MOF columns with specific linker incorporation ratios and appropriate ligand combinations exhibited stronger polyethylene glycol affinity in LC than those synthesized using pure-phase parent MOFs, was observed.

The peculiar retention mechanism of the mixed-linker solid solution MOF column was attributed to a fine balance between the adsorption kinetics and adsorption enthalpy, which increased the interactions between the specific substrate and the MOF stationary phase. Specifically, the mixed linker solid-solution MOFs comprising of ndc/bdc binary ligands exhibited the strongest retention performance higher than those of ndc- and bdc-MOF stationary phases. Overall, the mixed approach allowed the use of a variety of MOFs and desired combination of the solid-solution/particle mixture to achieve versatile design of LC stationary phases.

In summary, the efficiency and practicability of mixed-linker and mixed-solution metal-organic framework as tunable stationary phases for improving the separation performance and control of liquid chromatography was successfully demonstrated. The retention performance of the solid-solution column was significantly dependent on the combined influence of the two main counterbalancing factors: adsorption kinetics and adsorption enthalpy. In a joint statement to Advances in Engineering, the authors explained that the findings provide a blueprint for precise modulation and tunning of MOF columns and will pave the way for advanced separation technology.