Controlled synthesis and size distribution of zeolitic imidazolate framework crystals via the reaction-diffusion framework

Abstract

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal-organic frameworks (MOFs), have been recently employed in various fields such as gas separation, catalysis, water purification and drug delivery. Their high importance is due to their chemical and thermal stability in addition to the flexibility of their design. Several routes of synthesis were employed to produce these materials. In this thesis, we investigate a new synthesis method where ZIF crystals are produced using the reaction-diffusion framework (RDF) in a hydrogel medium at room temperature. The method is based on the diffusion of an outer solution of the organic linker or mixed linkers into an agar gel containing the inner metal ions Zn (II) where a precipitation reaction takes place leading to the formation of the ZIF crystals. Remarkably, the nonlinear coupling between reaction and diffusion with the nucleation and growth of the crystals results for many ZIF systems in the formation of Liesegang bands with direct spacing. This banding can be further exploited to control the crystal size distribution of the ZIFs along the tubular reactor. The ZIF-8 and ZIF-7 particles are then collected and fully characterized using powder x-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller (BET) analysis. Moreover, the scaling laws of these specific Liesegang system are determined, and their effect on the crystal size distribution is studied. The effect of different parameters such as the concentrations of the reactants, the gel thickness, and the temperature is investigated. Furthermore, cobalt-doped ZIF-8 (CoxZn1-x(HmIm)2, HmIm = 2-methylimidazole) is successfully synthesized using the same framework starting with an initial ionic mixture of Zn (II) and Co (II) in the gel at a fixed composition and HmIm as the outer linker. This system also exhibits periodic precipitation concomitant with variable cobalt-doping percentages among the Liesegang bands.