Synthesis and absorptive properties of metal-loaded nano-sized zinc zeolitic 2-methlimidazolate frameworks (ZIF-8) with applications in heterogeneous catalysis
Zeolitic Imidazole Framework-8 nanoparticles, a subclass of Metal Organic Frameworks, were successfully synthesized with particle size control by temperature, using isothermal benchtop reactions in methanol. As the synthesis temperature was increased from -15 °C to 60 °C, the average nanoparticle sizes decreased from 78 nm to 26 nm, with a simultaneous narrowing of the particle size distribution. Porosity analysis showed that the external surface area of the nanoparticles increases from 254 m² g-1 till 336 m² g-1 , as the particle size decreases from 78 nm till 26 nm. This increased external surface area led to a significant increase in the catalytic activity of the ZIF-8 nanoparticles in a Knoevenagel condensation test reaction. nZIF-8 was impregnated with silver and palladium salts, AgNO3 and Pd(acac)2, which after reduction with NaBH4 gave Ag@nZIF-8 and Pd@nZIF-8 respectively. In Ag@nZIF-8 the reduced silver metal nanoparticles (3.8 wt %) are situated on the external surface of nZIF-8, while in Pd@nZIF-8, the ~1 nm palladium metal cluster (3.1 wt %) were well dispersed throughout the porous structure of nZIF-8. While Aghigh@nZIF-8 showed a decrease in BET surface area, Pd@nZIF-8 showed an increase in BET surface area, with respect to the nZIF-8 starting material. nZIF-8 subjected to time resolved solvent assisted ligand exchange (SALE) with imidazole, gave a maximum conversion of 90 % after 72 hours. The ratio of 2-methylimidazole and imidazole linkers within the SALEM-2 products was determined by digestive 1H NMR for each timed exchange reaction. PXRD studies showed that the SOD topology of nZIF-8 starting material is retained till 24 h (or 58 %) of exchange, after which the structure begins to transform from SOD to ZNI topology. With the change to the non-porous ZNI topology, the porosity of the SALEM-2 compounds decreased. During post synthetic modification by lithiation of the C2 postion of the imidazole linkers a butyl chain was attached to SALEM-2 with a 0.6 % loading.