Controlling ZIF-8 nano- and microcrystal formation and reactivity through zinc salt variations

Abstract

We report here a simple method for controlling the crystal size and morphology of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals in methanol solution. ZIF-8 crystals were prepared by mixing 2-methylimidazole (Hmim) with various zinc salts for 1 h and using a Hmim/Zn(+2) salt molar ratio of 8/1. All products prepared were assigned to a sodalite-type structure and both particle size and morphology were found to be dependent on the reactivity of the Zn(+2) salt. Small ZIF-8 crystals with diameters varying between ca. 50 and 200 nm were obtained with reactive zinc salts like Zn(acac)₂, Zn(NO₃)₂, ZnSO₄ or Zn(ClO₄)₂ as demonstrated by SEM, TEM and DLS analyses. The use of ZnCl₂, Zn(OAc)₂ or ZnI₂ afforded crystals with sizes varying between ca. 350 and 650 nm. Finally, the low reactive ZnBr₂ salt was found to generate microsized crystals. Taking ZIF-8 crystals prepared from Zn(NO₃)₂, Zn(OAc)₂ and ZnBr₂ as representatives and through thermogravimetric analysis and BET measurements, we also demonstrated that changes in particle size induced changes in stability and adsorption properties. The small sized ZIF-8 crystals produced from Zn(NO₃)₂ were found to exhibit the highest surface area (1700 m² g⁻¹) and the best catalytic activity in Knoevenagel and Friedländer reactions.