Syntheses and Functional Studies of Cationic Metal-organic Frameworks (MOFs) toward Light Hydrocarbons Separation and Environmental Pollutants Remediation

Abstract

Metal-organic frameworks (MOFs) which are crystalline solids constructed from organic ligands and held together by metal nodes in a periodic manner, have rapidly emerged as a distinguished class of porous materials. The features enabling tunable architectures and modulation of chemical/physical properties on demand at molecular level has led to exploration of MOFs for various functions such as gas storage, separation, sensing, ion-conduction, catalysis etc. Very recently, MOFs are seeking attention for the challenging task of selective and efficient separation of industrially relevant light hydrocarbons. In addition, chemical water pollution by different toxic and hazardous wastes like dye molecules, metal ions, oxo-anions, radioactive wastes, pharmaceutical wastes etc. has called for huge concern globally. Typically, carboxylate-based linkers have been employed for the synthesis of MOFs, but neutral N-donor linkers which offer suitable alternatives to construct functional MOFs have been less explored. Such N-donor linkers afford the formation of cationic frameworks and offer extra framework functionality in the form of uncoordinated anions inside the porous cavity. Generally, cationic MOFs are less explored in studies of aquatic pollutants remediation while they still remain untapped for separating light hydrocarbons as they are unstable in aqueous medium and do not yield rigid porous frameworks. In this regard we sought to understand in detail the design principles influencing formation of stable N-donor linker based cationic MOFs by utilizing a higher dentate N-donor linker. The design strategies were focused to yield compounds which were specifically targeted for separation of light hydrocarbons and remediation of hazardous pollutants. In particular, we examined at the ability of cationic MOFs to capture toxic pollutants like heavy metal oxo-anions and radioactive species (via surrogate anions) via ion-exchange pathways, as well as compounds with various functional uncoordinated anions for the otherwise difficult separation of industrially important C2H2 molecules over other hydrocarbons.