Recycling Spent Synthesis Liquor for Enhanced Production of ZIF-67: Characterization and Morphological Insights

This study reports the preparation and characterization of zeolitic imidazolate framework-67 (ZIF-67) through the recycling of spent synthesis liquor containing unreacted 2-methylimidazole. The recycling enhances the yield of ZIF-67 while maintaining the dodecahedral rhombic morphology of the particles. In the first synthesis, a molar ratio of 1:26 (Co²:2-methylimidazole) results in particles with an average diameter of 230 nm and a surface area of 1374 m² g⁻¹. The first recycling step produces ZIF-67 particles that doubl in diameter and surface area, reaching 520 nm and 1690 m² g⁻¹, respectively. In the second recycling step, the particles further increase to 1040 nm in diameter and 1806 m² g⁻¹ in surface area. This increase in diameter is attributed to changes in the metal-to-ligand ratio, which affects the nucleation and growth rates. Increased surface area is linked to a reduction in the average micropore diameter, which decreases from 1.42 nm (first synthesis) to 1.37 nm (second recycling step). There is a 6 m² g⁻¹ increase in surface area for every 0.001 cm³ g⁻¹ increase in the volume of micropores. This indicates that the spent liquor can be utilized in consecutive batches to produce ZIF-67, minimizing reagent waste.