Chiral Separation via Molecular Sieving: A Computational Screening of Suitable Functionalizations for Nanoporous Graphene

Unconventional approaches for chiral resolution

Chirality is a fundamental and ubiquitous property of nature involved in multiple fields of science. In particular, the possible resolution of the enantiomeric forms of a molecule is crucial in the pharmaceutical, food, and agrochemical industries. The search for efficient, broad-spectrum, and yet simple methods for obtaining enantiomerically pure substances is a current challenge. Enantioselective resolution methods rely on an asymmetric environment that allows the two antipodes of a chiral molecule to be distinguished. In addition to enantiomeric separation techniques, such as chromatography and electrophoresis, new promising approaches involving out-of-the-scheme synergistic effects between chiral selectors (CS) and external stimuli are emerging. This Trends article discusses different enantioselective mechanisms triggered by unconventional physicochemical stimuli for the design of avant-garde approaches that could offer novel perspectives in the field of chiral resolution.

Enantioseparation Membranes: Research Status, Challenges, and Trends

The purity of enantiomers plays a critical role in human health and safety. Enantioseparation is an effective way and necessary process to obtain pure chiral compounds. Enantiomer membrane separation is a new chiral resolution technique, which has the potential for industrialization. This paper mainly summarizes the research status of enantioseparation membranes including membrane materials, preparation methods, factors affecting membrane properties, and separation mechanisms. In addition, the key problems and challenges to be solved in the research of enantioseparation membranes are analyzed. Last but not least, the future development trend of the chiral membrane is expected.

Vapor Adsorption Measurements with Two-Dimensional Membranes

Two-dimensional (2D) membranes display extraordinary mass transfer properties, in particular for the permeation of gaseous substances. Their ultimate thickness not only ensures the shortest diffusion pathways, but also makes the membrane surface play a significant role in accommodating and guiding the permeating molecules. As saturated vapors of water and organic solvents are often observed to pass 2D membranes faster than inert gases, condensation is believed to be responsible for surface-mediated transport. Here, we present a spectroscopic experiment to probe adsorption of condensable species on 2D membranes under realistic conditions. Polarization-modulation infrared reflection absorption spectroscopy (PM IRAS) is coupled with a reaction chamber and a vacuum system to control the vaporous environments. The measurements are demonstrated to yield quantitative information on the amount of adsorbates onto supported 2D layers. As a case study, the azeotropic mixture of water and propanol is revealed to maintain its molar composition upon interaction with carbon nanomembranes.

Enantioseparation processes and mechanisms in functionalized graphene membranes: Facilitated or retarded transport?

Up to date, functionalized graphene-based membranes have exhibited a promising potential in the enantioseparation. However, since precisely controlling the interlayer distance of two-dimensional materials is a great challenge in practical experiments, the transport mechanism of chiral guests in such membranes, together with various critical parameters that play a controlling role in the transport behaviors of the preferentially binding enantiomer in narrow channels, remains to be explored. The molecular dynamics (MD) simulation, especially using the steered MD (SMD) method, might be an alternative way to investigate the enantioseparation processes and mechanisms of layered membranes with different interlayer distances. In this work, D-alanine modified graphene sheets with different interlayer distances were built as membrane models, whereas D- and L-phenylalanine were selected as chiral probes. The effect of the interlayer distance and the applied external force on the enantioseparation performance was examined. Results show that such two parameters exert a significant influence on the enantioseparation performance: (a) Increasing the interlayer distance would result in a conversion from the retarded to the facilitated mechanism at a proper external force (medium); (b) both the large and small driving forces would only lead to the appearance of the retarded transport for the preferential enantiomer, unlike the moderate force; (c) the interaction energy of L-phenylalanine with D-isomer selector decreases with the rising interlayer distances studied in this work, regardless of what the external force is. Our findings can provide guidance on the practical applications in the membrane-based chiral separation.

Chiral Separation via Molecular Sieving: A Computational Screening of Suitable Functionalizations for Nanoporous Graphene

In a recent study [Angew. Chem. Int. Ed., 2014, 53, 9957–9960] a new concept of chiral separation has been suggested, which is based on functionalized, nanoporous sheets of graphene. In this follow‐up article we discuss the underlying principle in greater detail and make suggestions for suitable pore functionalizations with respect to a selection of chiral prototype molecules. Considering drug molecules as future targets for a chiral separation via membranes, the necessary pore sizes represent a big challenge for standard methods of computational chemistry. Therefore, we test two common force fields (GAFF, CGenFF) as well as a semiempirical tight‐binding approach recently developed by the Grimme group (GFN‐xTB) against the computationally much more expensive density functional theory. We identify the GFN‐xTB method as the most suitable approach for future simulations of functionalized pores for the given purpose, as it is able to produce reaction pathways in very good agreement with density functional theory, even in cases where force fields tend to an extreme overestimation of barrier heights.