Synthesis, characterisation and application of zeolite-like metal imidazole frameworks for n-butanol adsorption

The use of zeolite-like metal imidazole frameworks has been investigated for the adsorption of n-butanol and separation from aqueous media. The materials investigated (ZIF-8, MAF-6 and TIF-1 Zn) have highly microporous framework structures with high adsorption capacities for n-butanol. The adsorption capacity is shown to depend significantly on the structural characteristics of the material, governed by the size and nature of the imidazole ligand incorporated into the framework. The equilibrium adsorption capacities of ZIF-8, MAF-6 and TIF-1 Zn from 5 wt% n-butanol solutions were determined to be 0.364, 0.310 and 0.268 g g-1, respectively. Subsequently, the mechanism of adsorption of n-butanol by ZIF-8 was investigated in detail by advanced spectroscopic techniques (DRIFTS and SS NMR) as well as quantum mechanical molecular simulations. The maximum capacity of ZIF-8 for n-butanol was determined to be seven n-butanol molecules per β-cage of the sodalite structure. n-Butanol molecules were shown to interact through a combination of intermolecular hydrogen bonds and weaker π-hydrogen bonds to the framework. SSNMR experiments suggest that the n-butanol molecules are not fixed to one rigid adsorption site inside the cage, and instead possess a high degree of mobility within the framework. Extended simulations employing the DFTB method show that the n-butanol molecules adopt several stable configurations at room temperature, with the overall adsorption energy varying from -325 to -470 kJ mol-1. The kinetics of n-butanol desorption from the framework were shown to be significantly impacted by the size and morphology of the ZIF-8 particles. For ZIF-8 synthesised solvothermally at 100 °C with large (5 – 40 μm) truncated rhombic dodecahedral particles, n-butanol desorbed gradually from the material up to 350 °C. For ZIF-8 synthesised by room temperature chemical precipitation, with small (405 nm) rhombic dodecahedral particles, n-butanol was desorbed rapidly below 150 °C. ZIF-8 has been synthesised for the first time by membrane emulsification to provide precise control over the final ZIF-8 particle size and morphology. The method resulted in substantially larger particles than the conventional chemical precipitation method, yielding truncated cubic particles (1524 nm) after 1 h and truncated rhombic dodecahedral particles (835 nm) after 24 h. By mixing the reagents simultaneously at a reduced rate, the uniformity of the particles was improved, and smaller particles were obtained. After just 1 h, well-defined rhombic dodecahedral particles were obtained with a mean diameter of 209 nm, compared to 228 nm rhombic dodecahedral particles after 24 h. Due to the presence of surfactant which could not be removed from the ZIF-8 samples synthesised by this method, the equilibrium adsorption capacity was negatively impacted, with just 0.013 g g-1 adsorbed from a 5 wt% n-butanol solution.