Non-isothermal TGA, mechanism and thermodynamic study of the thermal degradation of mixed metal Zr/W-UiO-66-type MOF materials.

The thermal stability and decomposition behavior of Zr-based UiO-66 metal-organic frameworks (MOFs) are crucial for their use in practical thermocatalytic and high-temperature applications. This study systematically investigated the thermokinetic and thermodynamic behavior of pristine UiO-66 and tungsten-modified Zr/W-UiO-66 MOFs containing 25% and 60% W to elucidate the effect of W incorporation on their thermal decomposition pathways. The activation energy and pre-exponential factor were determined using model-free Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods, combined with an iterative procedure. Thermogravimetric analysis revealed distinct multistep decomposition pathways, with total mass losses of 50.5% for UiO-66, and 35.3% and 35.2% for Zr/W-UiO-66 (25% and 60%, respectively). The apparent activation energies ranged from 80.5 to 363.8 kJ mol-1 depending on the W content and decomposition stage. This demonstrates the strong influence of tungsten incorporation on the thermal decomposition energy landscape. Mechanistic analysis identified random nucleation and subsequent growth (G5), three-dimensional diffusion (D 8) and three-dimensional diffusion under spherical symmetry (D 3) as the most probable mechanisms for successive decomposition steps. The thermodynamic parameters (ΔS ≠, ΔH ≠ and ΔG ≠) obtained from the FWO and KAS approaches were highly consistent, with deviations of approximately 6%. Furthermore, PXRD, SEM-EDX, FTIR and N2 sorption analyses confirmed the structural and textural evolution of the materials, providing evidence of tungsten's role in the Zr/W-UiO-66 framework. Combined thermokinetic and structural analyses provide new insights into the relationships between W incorporation, decomposition mechanisms and the formation of ZrO2 and ZrO2@WO3-derived hybrid oxides. These findings establish a comprehensive framework for understanding and controlling the thermal transformation of W-modified UiO-66 MOFs toward functional oxide materials.