Design and Syntheses of Neutral N-donor Linker Based Metal-Organic Frameworks (MOFs) Towards Environmental Applications

Abstract

The main objective of the work in this thesis is to develop neutral N-donor linker based metal-organic frameworks (MOFs) towards their application for the recognition and capture of environmental pollutants. Metal-organic frameworks (MOFs) have emerged as a distinguished subset in the domain of porous materials owing to several advantages which are inaccessible in conventional porous solids. The facile access to tune pore character, modulate physical/chemical characteristics on demand and examine host-guest chemistry on the basis of structure-property correlation has rendered MOFs as a unique class of polymeric crystalline solid material. The features of MOFs have been tapped for suitability in several applications such as gas storage, separation of industrially important mixtures, catalysis, sensing, ion-conduction etc.; but hitherto these compounds have not been explored profoundly for their utility as solid sorbents/sensory probes of environmental pollutants. Broadly, MOFs are classified into neutral and ionic on the basis of the residual charge on the framework backbone. Different donor groups have been evaluated for the preparation of MOFs such as carboxylates, sulfonates, phosphonates, N-heterocyclic aromatic rings etc. Although relatively unexplored, neutral N-donor based linkers offer several advantages alike other donor groups and in some cases can render unique features to the resulting frameworks. Typically N-donor ligand based MOFs have been found to vulnerable to dissociation under humid conditions and hence very few applications have been evaluated using standalone neutral N-donor linker based MOFs. Thus the factors governing the stability of such frameworks were sought to be investigated, as hydrolytic stability is a pre-requisite for any material seeking environmental applications. Further the insights gained from these studies were applied to synthesize MOFs as ion-exchange materials for the capture of heavy metal contaminants (Angew. Chem. Int. Ed. 2016, 55, 7811-7815) and highly absorbing organic dyes (iScience 2018, 3, 21-30), or as solid adsorbents for the capture of air pollutants such as greenhouse gas CO2.