Effect of pressure on porous materials

Abstract

Research to design and synthesise new porous materials is a rapidly growing field with thousands of new systems proposed every year due to their potential use in a multitude of application in a wide range of fields. Pressure is a powerful tool for the characterisation of structure-property relationships in these materials, the understanding of which is key to unlocking their full potential. In this thesis we investigate a range of porous materials at a range of pressures. Over time the chemical architecture and complexity of porous materials has increased. Although some systems display remarkable stability to high-pressures, which we generally think of as being above 1 GPa (10,000 bar), in general, the compressibility of porous materials have increased substantially over the last 10 years, rendering most unstable at GPa pressures. Here we present new methods for investigating porous materials at much more moderate pressures (100’s of bar), alongside more traditional high-pressure methods (diamond anvil cell techniques), finishing with gas sorption studies in a molecular based porous material.

Here, the design and development of a new moderate pressure sapphire capillary cell for the small molecule beamline I19 at the Diamond Light Source is described. This cell allowed access to pressures of more than 1000 bar regularly with a maximum operating pressure of 1500 bar with very precise pressure control (< 10 bar) on both increasing and decreasing pressure. This cell closes the gap between ambient pressure and the lowest pressures attainable using a diamond anvil cell (DAC), which is generally above 0.2 GPa (2000 bar). Along with the development of the sapphire capillary pressure cell, the compression to 1000 bar of the small organic sample molecule Hexamethylenetetramine (hexamine, C6H12N4) and its deuterated form (C6D12N4) was determined, demonstrating the precision possible using this cell.

Solvent uptake into porous materials can induce large structural changes at 100’s of bar. In the case of the Sc-based Metal-organic framework (MOF), Sc2BDC3 (BDC = 1,4-benzenedicarboxylate), we used the sapphire capillary pressure cell to study changes in the framework structure on the uptake of n-pentane and isopentane. This work shows how the shape and smaller size of n-pentane facilitated the swelling of the framework that could be used to explain the increase in stability of the MOF to applied pressure.

The effect of pressure on the previously unreported Cu-framework bis[1-(4- pyridyl)butane-1,3-dione]copper(II) (CuPyr-I) was investigated using high-pressure single-crystal diffraction techniques (DAC). CuPyr-I was found to exhibit high-pressure and low-temperature phase transitions, a pressure induced Jahn- Teller switch (which was hydrostatic medium dependent), piezochromism, and negative linear compressibility. Although each of these phenomena has been reported numerous times in a range of materials, this is to the best of our knowledge the first example to have been observed within the same material.

The final two chapters investigate the exceptional thermal, chemical, and mechanical stability of a porous molecular crystal system (PMC) prepared by the co-crystallisation of a cobalt phthalocyanine derivative and a fullerene (C 60 or C70). The stabilising fullerene is captured in the cavity between two phthalocyanines in a ball and socket arrangement. These PMCs retain their porous structure: on the evacuation of solvent of crystalisation; on heating to over 500 K; on prolonged immersion in boiling aqueous acid, base, and water; and at extreme pressures of up to 5.85 GPa, the first reported high-pressure study of a PMC. the reactive cobalt cation is accessible via the massive interconnected voids, (8 nm3), as demonstrated by the adsorption and binding of CO and O2 to the empty metal site using in situ crystallographic methods available at beamline I19, Diamond Light Source.