ZIF-8 with exceptional thermal stability: Role of organic cosolvents in phase control and structure stabilization

Abstract

Thermal stability is one of the key factors in catalysis and high-temperature industrial applications. The thermal degradation of ZIF-8, particularly its nanocrystals, could result in a significant loss of surface area and crystallinity, limiting its large-scale applicability. In this study, ZIF-8 nanocrystals with improved thermal stability were synthesized at room temperature (RT) through a facile, one-pot method – an ultrasound-assisted, solvent-guided ZIF-8 growth. The effect of synthesis variables, including the type and concentration of solvent, ultrasonication, and synthesis time, in improving the material’s thermal stability was studied systematically. The as-prepared samples were analyzed by TGA/DSC, XRD, SEM/TEM, FTIR, and BET to monitor the weight loss with temperature, sample crystallinity, crystal morphology, chemical structure, and surface area. Notably, synthesis with 5 v/v% of n-hexane (ZIF-8new-ref) demonstrated the formation of highly-crystalline ZIF-8 nanocrystals with extraordinary thermal stability (with only ∼2.0–3.0 % weight loss up to 600 °C), large surface area (i.e., 975 m2 g−1), and an excellent particle size distribution in the range of 0.2–0.4 μm. More importantly, the short synthesis time (2 h) and sonication-assisted pretreatment of the precursor (for promoting solvent-ligand interaction) proved effective and played a crucial role in controlling ZIF-8’s thermal stability. The mechanism for the thermal stability enhancement was thus proposed. The highly thermally stable ZIF-8 nanocrystals were also synthesized with significantly reduced chemical usage, e.g., 1/11 of the normal ligand-to-metal ratio (70), achieving cost-effective and environmentally friendly synthesis.