Two-dimensional chiral metal-organic-framework nanosheets based on Co-BDC-NH<sub>2</sub> used as stationary phases for gas chromatography

Two-dimensional metal-organic-framework (2D-MOF) materials have emerged as a new class of functional 2D material. Compared to bulk crystals, 2D-MOFs are easily derivatized, highly porous, and have sufficient active sites. While 2D-MOFs are of considerable research interest, they are also efficient candidates for multiple applications in a variety of fields owing to their numerous advantages. The ability to separate and analyze chiral compounds is greatly significant for progressing human society, and chromatographic separation is widely used in this regard owing to its high resolution and sensitivity. Few reports on the use of 2D-MOFs in chromatographic-separation applications currently exist, and those use gas chromatography to analyze and separate enantiomers are even rarer. Unsurprisingly, the development of novel stationary phases has become a popular topic in the chiral-chromatography field. In this study, 2D-MOF nanosheets (Co-BDC-NH2) were synthesized using a surfactant-assisted solvothermal method. The nanosheets were subsequently characterized by scanning electron microscopy and X-ray diffractometry. MOFs can be post-synthetically modified without affecting their frameworks, and such modifications can lead to the construction of chiral MOFs. Accordingly, Co-BDC-NH2 was post-synthetically modified with glycyl-L-aspartic acid and glycyl-L-glutamic acid as chiral ligands to afford two chiral 2D-MOF nanosheets, namely Co-BDC-NH2-glycyl-L-aspartic acid and Co-BDC-NH2-glycyl-L-glutamic acid. These chiral 2D-MOF nanosheets were characterized by Fourier-transform infrared spectroscopy, circular dichroism, and thermogravimetric analysis. The two 2D-MOF nanosheet materials were used as chiral stationary phases in gas chromatography by coating them onto prepared capillary columns using a dynamic coating method; this process leaves a homogeneous coating layer on the inner wall of the column. Scanning electron microscopy confirmed that the two chiral columns had been successfully prepared. The columns were finally tested through gas-chromatography-separation experiments. Theoretical plates can be used to evaluate column efficiency. The Co-BDC-NH2-glycyl-L-aspartic acid and Co-BDC-NH2-glycyl-L-glutamic acid columns were determined to have 3538 and 3108 N/m theoretical plates, respectively, which implies that these columns are highly efficient. The McReynolds constant can be used to determine the polarity of the stationary phase in a chromatographic column; the two capillary columns exhibited McReynolds constants of 181 and 208, consistent with materials of medium polarity. The two chromatographic columns exhibited good abilities to resolve positional isomers and racemates (especially amino-acid derivatives), with seven racemates identified using the Co-BDC-NH2-glycyl-L-aspartic acid column, and eight identified using the Co-BDC-NH2-glycyl-L-glutamic acid column. In addition, the former column also separated mixed n-alkanes, mixed n-alcohols, and Grob mixtures. In this study, we augmented 2D-MOF nanosheets with chiral functional groups and confirmed that they are effective stationary phases for use in gas-chromatography applications. The study also expands the applicability of 2D-MOF nanosheets to chiral separation.