Role of crystal nucleation and growth conditions on the soft-templated preparation of HKUST-1 MOFs with hierarchical porosity involving gemini surfactants

Abstract

Utilization of porous materials such as metal-organic frameworks (MOFs) will get enhanced if a connectivity between the micro- and meso-pore structures within such solids can be achieved. Soft-templated synthesis has been a widely used method where surfactant based molecular self-assemblies act as templates to introduce mesopores in the intrinsically microporous MOFs and thereby generate hierarchical porosity (HP). In this work, we present our investigations on the formation of HP HKUST-1 MOFs by using two gemini surfactants, G16 and G14 as soft-templates. Crystal nucleation and growth conditions in the soft-templated synthesis play a significant role in the success of HP formation. Here, we have employed variations in the composition of water:ethanol (W:E) solvent mixtures mainly to understand and to arrive at the favourable templation conditions. Depending on the relative ratio of the W:E compositions and the gemini surfactants, we have employed additional parameters such as temperature, reaction time and reactant compositions that crucially influence the nucleation and growth factors to achieve HP. Gas sorption, DLS, zeta potential, thermogravimetry, SEM and TEM investigations indicate the formation of unique types of HP structures including the ones which have formed via bilayer and vesicular based soft-templation mechanism in our samples. Presence of the mesopores in our HP samples have been demonstrated by correlating the higher values of both the hierarchy factor (HF) and dye-sorption with a bulky Rhodamine molecule. A direct relation found between the HF values and the dye-uptake in our HP samples indicate that HF can be used as a quantitative marker of HP.