Research progress of stationary phase of gas chromatography based on chiral organic frameworks

Abstract

Enantiomers typically show different pharmacological, toxicological, and physiological properties. Thus, the preparation of enantiopure compounds is of great significance for human health and sustainable development. Compared with asymmetric catalysis, enantiomeric separation is simpler, faster, and more efficient; as such, it has become the preferred method for obtaining pure enantiomers. At present, enantiomeric separation methods mainly include chromatography, nanochannel membrane separation, selective adsorption, and recrystallization. In particular, gas chromatography (GC) plays an important role in enantioseparation because of its high sensitivity, excellent reproducibility, and outstanding processing capacity for various enantiomers. The stationary phase is key to the separation efficiency of GC, and more efficient, stable, and cost-effective materials that could serve as stationary phases are constantly being explored. Organic frameworks, such as covalent organic frameworks (COFs), metal-organic frameworks (MOFs), porous organic cages (POCs), metal-organic cages (MOCs), and hydrogen-bonded organic frameworks (HOFs), possess large specific surface areas, high porosities, tunable pore sizes, and easy functionalization, rendering them promising candidates for the separation of mixed analytes. Research has shown that the use of organic frameworks as stationary phases for GC results in excellent column efficiency and high resolution for various analytes, including n-alkanes, n-alcohols, polycyclic aromatic hydrocarbons, positional isomers, and organic fluorides. Furthermore, organic frameworks can be prepared as chiral stationary phases for GC by the intelligent introduction of a chiral moiety, thereby enabling the efficient separation of enantiomers. Synthetic strategies for chiral organic frameworks are primarily categorized as post-synthesis or bottom-up approaches. In general, the post-synthesis strategy can introduce various chiral sites to the framework; however, the distribution of chiral sites may not be uniform, and the ordered framework may be destroyed during the post-synthesis process. The bottom-up strategy allows for the uniform and precise distribution of chiral sites in the framework, but the synthesis of chiral monomers and the constraint between asymmetry and crystallinity limit its development. Chiral induction has been proposed as an alternative strategy for synthesizing chiral organic frameworks. The use of this strategy has led to the successful preparation of organic frameworks with abundant chiral sites and excellent crystallinity. Dynamic coating and in situ growth are the main approaches used to transform the as-prepared chiral organic frameworks into stationary phases. Notably, the in situ growth approach can yield chiral COF/MOF-coated capillary columns that provide high resolution for the separation of enantiomers with excellent repeatability and reproducibility. Nevertheless, owing to the slightly complex pretreatment process and the difficulty of synthesizing chiral organic frameworks, the in situ growth approach has not yet been widely applied. Owing to their excellent solvent processing performance, POCs, MOCs, and HOFs can be easily coated on the inner walls of columns to form membranes via dynamic or static coating. A series of enantiomers have been successfully separated and analyzed by immobilizing chiral COFs, MOFs, POCs, MOCs, and HOFs on GC capillary columns, demonstrating the great potential of chiral organic frameworks for enantiomeric separation. In general, the mechanisms by which chiral organic frameworks recognize enantiomers could be mainly categorized as van der Waals interactions, hydrogen bonding, π-π interactions, and size-exclusion effects. While molecular simulations can offer some insights into these recognition mechanisms, clarifying these mechanisms based on effective characterization remains challenging. In summary, organic frameworks show outstanding advantages for enantiomer separation. Given breakthroughs in synthetic strategies for chiral organic frameworks and the in-depth study of chiral recognition mechanisms, chiral organic frameworks may be expected to become an important aspect in the field of chiral materials, further realizing the large-scale analysis and production of chiral analytes. A total of 64 references, most of which are from the American Chemical Society, Springer Nature, Wiley Online Library, and Elsevier databases, are cited in this review.