Post-synthetic modification and chemical modulation of the ZIF-8 MOF using 3-mercaptopropionic acid (MPA): a multi-technique study on thermodynamic and kinetic aspects

Abstract

We hereby report on the thermodynamic and kinetic aspects of ZIF-8 MOF synthesis when performed using methanolic solutions of precursors and 3-mercaptopropionic acid (MPA) acting as a chemical modifying agent. MPA is of great interest as it allows us to gain control over the surface chemistry, shape, and size of colloidal porous nanounits formed. In particular, small and wide-angle X-ray scattering (SAXS/WAXS), and electron microscopy were employed for determination of suitable MPA : MeImH modification molar ratios for two different approaches: post-synthesis modification (PSM) and coordination modulation (CM). Using the CM approach, in situ time-resolved SAXS/WAXS experiments and dynamic light scattering (DLS) experiments were carried out for a detailed characterization of the MPA influence on the early stages of nucleation and growth processes. Complementary X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption experiments showed that MPA remains mainly surface confined and adds mesoporosity to the synthesized MOF, thus it can be used for positioning thiol moieties without compromising structural integrity. Moreover, isothermal titration calorimetry (ITC) experiments carried out using the PSM strategy allowed us to determine an enthalpy value of ∼7 kJ mol⁻¹ associated with the coordination between Zn²⁺ and MPA for the first time to our knowledge.