Switching adsorbent molecular materials (SAMMs) for C8 separation and water capture

With the increasing demand for the separation and purification of industrial commodities, the energy consumption for their production has increased at a fast pace. Previous studies on metal-organic materials, materials based upon coordination chemistry with organic ligands, have shown that their modularity makes them amenable to crystal engineering. Furthermore, rigid 3D coordination networks can store and separate gases/vapours. Switching materials, i.e. materials that can undergo a sudden stimulus driven structural transformation induced by adsorption, are less studied but can offer higher working capacities than rigid variants. This thesis explores zerodimensional metal-organic switching materials, Werner complexes, to investigate their potential as energy-efficient materials for (i) separation of aromatic C8 isomers and (ii) water harvesting. The molecular nature of Werner complexes allows them to reversibly switch between a densely packed ‘closed phase’ and a porous ‘open phase’ with little structural strain/degradation. We introduce the term Switching Adsorbent Molecular Material (SAMM) to describe a Werner complex which exhibits a switching isotherm. The sorption properties of the Werner complex Ni(NCS)2(4-phenylpyridine)4 (SAMM3-Ni-NCS), were studied, including the collection of isotherms for OX, MX, PX and EB and multiple sorption cycles to investigate its recyclability. New Werner complex variants were prepared by applying crystal engineering principles to modify (i) anionic axial ligands, (ii) neutral N-donor equatorial ligands including pyridines and imidazoles and (iii) divalent metal ions. The sorption properties of these SAMMs were investigated using Dynamic Vapour Sorption (DVS). The C8 selectivity values of SAMMs were found to outperform rigid C8 sorbents. The variants studied in relation to water harvesting exhibit switching isotherms for water vapour: the first report of such behaviour in a Werner complex. It was found that the choice of the anionic axial ligand may play a role in determining whether a complex will exhibit switching induced by water vapour. The work described herein demonstrates the application of molecular physisorbents in the separation of C8 aromatic hydrocarbons, as well as the previously unexplored application of Werner complexes as switching water sorbents.