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

Metal organic frameworks (MOFs) are porous crystalline materials composed of metal ions or clusters of metal ions coordinated with organic linkers (such as terephthalic acid, 1,3,5-benzenetricarboxylic acid, or imidazoles). MOFs exhibit tunable structures, low density, ultrahigh surface area and have various potential applications in catalysis, hydrogen storage, and adsorption/separation of liquid or gaseous mixtures. Among the MOFs structures, the zeolitic imidazolate frameworks (ZIFs) have recently attracted considerable attention. In these materials, metal atoms such as Zn2+ are linked through N atoms by the ditopic 2-methylimidazolate ligand to form neutral frameworks. ZIF-8 has a sodalite zeolite-type topology with cages of 11.6 A and pores of 3.4 A in diameter. ZIF-8 are characterized by high thermal stability (550 oC in N2), large surface area (BET: 1630 m2/g) and high resistance to various solvents. Concerning ZIF-8 chemical properties, these materials can successfully be used for hydrogen, carbon dioxide and iodide storage, Knoevenagel condensations, cycloadditions, oxidations, trans-esterification, and Friedel-Crafts alkylations. Variations of synthetic parameters (solvent, concentration, temperature, time, molar ratio of reactants) are commonly used to manipulate the morphology and size of ZIF-8 crystals. In this work, we demonstrate that the reactivity of the Zn(+2) salt in the growth solution can also markedly affect the size and the morphology of ZIF-8 particles. Small ZIF-8 nanocrystals 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. 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 was found to generate microsized crystals. These significant changes in particle size induced distinctive changes in adsorption properties as demonstrated by BET measurements but also in the catalytic performances of ZIF-8 crystals in a Knoevenagel condensation used as model. The small sized crystals produced from Zn(NO3)2 exhibit the highest surface area and the best catalytic activity.