Search

Chiral surfaces are an object of interest due to their application in catalysis, separation and sensors. Such surfaces can exhibit chirality on the molecular or supramolecular level. Previously, we have studied the thermodynamic features of adsorption on non-porous surfaces with supramolecular chirality. However, for their application in enantiomer separation, enantioselectivity and capacity should be greater. Porous adsorbents are promising for this purpose. In this work, the enantioselectivity of copper borophosphate LiCu2[BP2O8(OH)2] was studied. This zeotype material was obtained by hydrothermal synthesis without any source of chirality. Supramolecular chirality was achieved by spontaneous symmetry breaking via Viedma ripening. XRD and SEM were used to prove the synthesis accuracy. Enantioselectivity during adsorption was studied using the adsorption isotherms of α-pinene and limonene enantiomer analysis. The experimental isotherms were approximated by Langmuir, Dubinin–Radushkevich, Freundlich and Fowler–Guggenheim equations. The t-test was used to prove the reliability of the differences in enantiomer adsorption. The isosteric heats of adsorption were calculated from the adsorption isotherms. The data obtained have shown the ability of the LiCu2[BP2O8(OH)2] surface to recognize both α-pinene and limonene enantiomers. The molecules of α-pinenes adsorbed without pores, with the maximum enantioselectivity coefficient of 1.23 at 80 °C. Limonene molecules adsorbed in pore volume as single molecules or 1D chains. This phenomenon was proved by analysis of isosteric heats. The enantioselectivity of copper borophosphate to limonenes and α-pinenes was similar. The ability of pores with supramolecular chirality (and without molecular chirality) to recognize enantiomers was discovered in this paper for the first time. [ABSTRACT FROM AUTHOR]

The chiral recognition ability of microporous MOF [{Cu12I(trz)8}·4 Cl·8 H2O]n in adsorption processes was studied. The MOF sample was obtained from achiral molecules in the absence of an external source of chirality. It has, in its structure, a right-handed single-stranded helix nanotube with a size of 14 Å and a left-handed single-stranded helix nanotube measuring 4.9 Å. The aim of this study was to use substances capable of adsorbing only in one type of nanotubes. This could lead to chiral recognition by racemic crystals. To test the recognition ability, the enantiomers of limonene were used. The limonene dimensions were too large to fit in the left-handed nanotube but small enough to get adsorbed by the right-handed nanotube. To analyze the enantioselectivity of [{Cu12I(trz)8}·4 Cl·8 H2O]n, inverse gas chromatography was used. The specific retention volumes were calculated and the adsorption isotherms were plotted. In addition, the MOF sample was characterized by XRD, IR spectrometry, and porosimetry analyses. It has been shown that the MOF-based stationary phase was capable of chiral recognition. The maximum selectivity coefficient was 1.43. The reliability of differences in enantiomer adsorption isotherms was confirmed by t-test. The data obtained proved the chiral recognition ability. During adsorption data collection, a decrease in enantioselectivity was observed. It was caused by the irreversible (under gas chromatography conditions) adsorption of the limonene enantiomers in the MOF pores. This fact was confirmed by eluting n-heptane through a MOF column after a gas chromatographic experiment. Furthermore, irreversible nitrogen adsorption was observed during the porosimetry experiment. [ABSTRACT FROM AUTHOR]