Inverse Gas Chromatography Demonstrates the Crystallinity-Dependent Physicochemical Properties of Two-Dimensional Covalent Organic Framework Stationary Phases

Two-dimensional polymers (2DPs), in the form of layered 2D covalent organic frameworks (COFs), are promising candidates for adsorbent-based separations because their pore sizes, shapes, functionalities, and interlayer stacking arrangements can be tuned by modifying their building blocks. Recently, high-quality single crystals of two 2D COFs exhibited distinct and improved separation characteristics in the gas chromatography (GC) separation of benzene and cyclohexane relative to polycrystalline samples of the same materials. These surprising findings motivate the present study, in which inverse pulse gas chromatography (IGC) was used to characterize the dispersive and specific adsorption properties of the surfaces of single-crystalline and polycrystalline TAPPy-PDA COFs for the separation of linear n-alkanes as well as a series of standard polar probes. Major differences in separation behavior were again observed that provide insight into how analytes interact with the single-crystalline and polycrystalline 2D COFs. A polarity study based on McReynolds constants revealed the nonpolar nature of the single-crystalline TAPPy-PDA COF, whereas the polycrystalline TAPPy-PDA COF surface was found to have a slightly polar character. Three common approaches to calculating the specific interaction parameter, Isp, were tested to examine their validity in the context of 2D COFs, revealing that the single-crystalline TAPPy-PDA COF possessed an electron donor character that we attribute to the imine nitrogen atoms inside the well-defined pore channels. In contrast, the polycrystalline TAPPy-PDA COF showed a relative electron acceptor character, which may be more heavily influenced by interactions between the analytes and dangling bonds or functionalities at grain boundaries. These findings provide a quantitative comparison of 2D COF materials quality by determining the acid–base interactions (represented by the electron donor–acceptor properties), polarity, and other physiochemical parameters. Furthermore, these results indicate the importance of establishing high materials quality for 2D COF samples prior to establishing rigorous structure–property relationships for separation performance.