Development of functionalized and hydrolytically stable Cu(II)-Diimide based Metal Organic Polyhedra (MOP) and their application in light hydrocarbon separation

Abstract

The development of advanced functional porous materials capable of selective hydrocarbon separation is a pressing need. In this backdrop, metal-organic polyhedra (MOPs) that combine solution processability with water resistance and separation ability represent a particularly important target for industrial applications. Herein, two novel MOPs, designated BiPhe-Ala and BiPhe-IL, were successfully synthesized using di-imide-based ligands derived from alanine and isoleucine, respectively. The materials were prepared under solvothermal conditions and characterized with the help of single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, and Fourier-transform infrared spectroscopy. Shift of the appended alkyl chain from a methyl group (alanine) to a more branched isobutyl group (isoleucine) resulted in a rapid increase in hydrophobicity, as confirmed by water contact angle measurements and water adsorption studies. The isoleucine-derived MOP exhibited enhanced water stability, retaining structural integrity after eight days of aqueous exposure, along with superior solution processability in tetrahydrofuran. Both MOPs demonstrated promising performance in vapor adsorption and hydrocarbon separation, particularly for C2H2/CO2 mixtures, with BiPhe-IL showing higher selectivity and uptake ratios. Isosteric heat of adsorption calculations indicated stronger interactions with acetylene for the more hydrophobic cage. Furthermore, both materials were successfully synthesized at room temperature on a bulk scale using a facile method with 2,6-lutidine as a base, yielding products with properties comparable to their solvothermally prepared counterparts. Cumulatively, these findings highlight the role of alkyl chain engineering in modulating the hydrophobicity, stability, and separation performance of MOPs, and suggest a versatile strategy for developing functional cage materials using amino acid-derived ligands for targeted applications.