Thermal stability and decomposition pathways of zeolitic imidazolate frameworks for future catalytic thermochemical applications

Abstract

Metal-organic frameworks (MOFs) have recently gained attention as catalysts in catalytic pyrolysis (CP); however, their application is constrained by thermal instability at high operating temperatures, which calls for work to find the most suitable class for this purpose. In this context, this work introduces zeolitic imidazolate frameworks (ZIFs) as thermally stable and commercially available MOF candidates for CP applications by investigating their decomposition behaviour, kinetics, and structure stability. Thermogravimetric analysis (TGA) was carried out under various heating rates, coupled with TG-FTIR and GC/MS to observe evolved vapors. The kinetic and thermodynamic characteristics were simulated using linear and nonlinear models, and a trained artificial neural network (ANN) was used to predict decomposition regions. The results showed that ZIF-67 and ZIF-8 exhibited high thermal stability up to 580 °C and 600 °C, respectively, followed by rapid degradation releasing 1H-imidazole, 2-methyl- compound. ZIF-8 exhibited higher activation energy (450–461 kJ/mol) compared to 340–352 kJ/mol (ZIF-67), with both materials following endothermic decomposition approach. The ANN model successfully captured the decomposition region (R2 ≥ 0.999). Finally, the structure analysis showed that porosity loss rises with faster heating rates, with ZIF-67 proving more stable than ZIF-8, especially at higher rates. SEM-EDX results also confirmed that cobalt nodes are more stable than zinc. These results demonstrate that ZIFs have high thermal and porosity stability at low heating rates, while at higher heating rates, ZIF-67 exhibits exceptional stability, making it a promising catalyst for the CP of feedstocks with decomposition temperatures below 580 °C (such as biomass and plastic waste).