The exploration of the reversed enantioselectivity of a chitosan functionalized cellulose acetate membranes in an electric field driven process

Stereospecific supramolecular polymerization of nanoclusters into ultra-long helical chains and enantiomer separation

During the construction of supramolecular polymers of smaller nanoparticles/nanoclusters bearing hierarchy and homochirality, the mechanism understanding via intuitive visualization and precise cross-scale chirality modulation is still challenging. For this goal, a cooperative self-assembly strategy is here proposed by using ionic complexes with uniform chemical composition comprising polyanionic nanocluster cores and surrounded chiral cationic organic components as monomers for supramolecular polymerization. The single helical polymer chains bearing a core-shell structure at utmost length over 20 μm are demonstrated showing comparable flexibility resembling covalent polymers. A nucleation-elongation growth mechanism that is not dealt with in nanoparticle systems is confirmed to be accompanied by strict chiral self-sorting. A permeable membrane prepared by simple suction of such supramolecular polymers displays high enantioselectivity (e.e. 98% after four runs) for separating histidine derivatives, which discloses a benefiting helical chain structure-induced functionalization for macroscopic supramolecular materials in highly efficient racemate separation.

A Homochiral Porous Organic Cage-Polymer Membrane for Enantioselective Resolution

Membrane-based enantioselective separation is a promising method for chiral resolution due to its low cost and high efficiency. However, scalable fabrication of chiral separation membranes displaying both high enantioselectivity and high flux of enantiomers is still a challenge. Here, the authors report the preparation of homochiral porous organic cage (Covalent cage 3 (CC3)-R)-based enantioselective thin-film-composite membranes using polyamide (PA) as the matrix, where fully organic and solvent-processable cage crystals have good compatibility with the polymer scaffold. The hierarchical CC3-R channels consist of chiral selective windows and inner cavities, leading to favorable chiral resolution and permeation of enantiomers; the CC3-R/PA composite membranes display an enantiomeric excess of 95.2% for R-(+)-limonene over S-(-)-limonene and a high flux of 99.9 mg h-1 m-2. This work sheds light on the use of homochiral porous organic cages for preparing enantioselective membranes and demonstrates a new route for the development of next-generation chiral separation membranes.

A Homochiral Poly(2-oxazoline)-based Membrane for Efficient Enantioselective Separation

Chiral separation membranes have shown great potential for the efficient separation of racemic mixtures into enantiopure components for many applications, such as in the food and pharmaceutical industries; however, scalable fabrication of membranes with both high enantioselectivity and flux remains a challenge. Herein, enantiopure S-poly(2,4-dimethyl-2-oxazoline) (S-PdMeOx) macromonomers were synthesized and used to prepare a new type of enantioselective membrane consisting of a chiral S-PdMeOx network scaffolded by graphene oxide (GO) nanosheets. The S-PdMeOx-based membrane showed a near-quantitative enantiomeric excess (ee) (98.3±1.7 %) of S-(-)-limonene over R-(+)-limonene and a flux of 0.32 mmol m^-2  h^-1 . This work demonstrates the potential of homochiral poly(2,4-disubstituted-2-oxazoline)s in chiral discrimination and provides a new route to the development of highly efficient enantioselective membranes using synthetic homochiral polymer networks.

Advances of enantioselective solid membranes

Rosy prospects of chiral membranes are proposed with novel and robust materials.

Green preparation of d-tryptophan imprinted self-supported membrane for ultrahigh enantioseparation of racemic tryptophan

Green and clean preparation of molecularly imprinted membrane for ultrahigh enantioseparation of racemic tryptophan.

Enantiomeric selective adsorption of amino acid by polysaccharide composite materials

A composite containing cellulose (CEL) and chitosan (CS) synthesized by a simple and recyclable method by using butylmethylimmidazolium chloride, an ionic liquid, was found to exhibit remarkable enantiomeric selectivity toward the adsorption of amino acids. The highest adsorption capacity and enantiomeric selectivity are exhibited by 100% CS. A racemic amino acid can be enantiomerically resolved by 100% CS in about 96-120 h. Interestingly, adsorption by 50:50 CEL/CS is more similar to that by 100% CS than to 100% CEL. Specifically, whereas 100% CEL has the lowest adsorption capacity and enantiomeric selectivity, 50:50 CEL/CS has sufficient enantiomeric selectivity to enable it to be used for chiral resolution. This is significant because in spite of its high enantiomeric selectivity 100% CS cannot practically be used because it has relatively poor mechanical properties and undergoes extensive swelling. Adding 50% CEL to CS substantially improves the mechanical properties and reduces its swelling while it retains sufficient enantiomeric selectivity to enable it to be used for routine chiral separations. The kinetic results indicate that the enantiomerically selective adsorption is due not to the initial surface adsorption but rather to the subsequent stage in which the adsorbate molecules diffuse into the pores within the particles of the composites and consequently are adsorbed by the interior of each particle. The strong intermolecular and intramolecular hydrogen bond network in CEL enables it to adopt a very dense structure that makes it difficult for adsorbate molecules to diffuse into its interior, thereby leading to low enantiomeric selectivity. Compared to hydroxy groups, amino groups cannot form strong hydrogen bonds. The hydrogen bond network in CS is not as extensive as in CEL, and its inner structure is relatively less dense than that of CEL. Adsorbate molecules can, therefore, diffuse from the outer surface to its inner structure relatively more easily than in CEL, thereby leading to higher enantiomeric selectivity for 100% CS.