Synthesis of nanoscale zeolitic imidazolate framework-8 (ZIF-8) using reverse micro-emulsion for Congo red adsorption

To enhance the removal of dyes from water, the adsorption capacity of an adsorbent can usually be increased via reducing its particle size and enlarging the specific area. To this end, a reverse micro-emulsion system was developed in this study using an ionic liquid ([C14MIm]Cl) to prepare nanoscale zeolitic imidazolate framework-8 (ZIF-8) material for Congo red (CR) anionic dye separation from water. The size and morphology of nano ZIF-8 synthesized under different conditions, including reaction time, the order of reactants addition, the molar ratio of water and [C14MIm]Cl and the concentration of reactants, were analyzed by dynamic light scattering (DLS) particle size analyzer and transmission electron microscope (TEM). The optimal synthesis conditions including reaction time of 60 min, the ratio of water and ionic liquid (ω0) of 60, and the concentrations of B1 and B2 solutions of 0.07 M and 0.28 M, were determined using the response surface method (RSM). Studies have shown that the reverse micro-emulsion system can effectively control the crystal size of ZIF-8 nanoparticles, with a uniform particle size distribution from 15 to 35 nm, smaller than that of the general ZIF-8 material prepared in the aqueous phase (100–200 nm). The specific surface area of the nano ZIF-8 particles synthesized under optimal synthesis conditions was 1477 m2/g. The synthesized nano ZIF-8 material removed Congo red (CR) dye from water efficiently by adsorption. Congo red adsorption on nano ZIF-8 was observed as a spontaneous endothermic adsorption process. The adsorption conformed to the pseudo-second-order kinetic model and the Langmuir isotherm model. The maximum predicted adsorption amount was 1339.8 mg/g at a temperature of 323 K. Analysis of adsorption mechanism shows that the adsorption of CR on nano ZIF-8 materials is mainly due to electrostatic interaction and physical attraction including hydrogen bonds and π-π bond interactions. This study further expands the application of reverse micro-emulsion method in nanoscale framework materials preparation.