Revealing the pyrolysis mechanism of Fe-based MOFs for robust CO2 hydrogenation

Understanding the complex pyrolysis mechanisms of metal-organic frameworks (MOFs) is pivotal for the rational design of high-performance catalysts. However, a critical knowledge gap remains in tracking the precise structural and chemical evolution during this process. In this work, taking MIL-53(Fe) as a representative example, we deciphered its pyrolysis process and applied the pyrolysis-derived materials to CO 2 hydrogenation. A four-stage pyrolysis pathway was identified: dehydration (50 – 250 °C), ligand decomposition (250 – 400 °C), major carbonization (400 – 500 °C), and structural reorganization/reduction (500 – 600 °C). These transformations are accompanied by a stepwise valence reduction of iron (Fe 3+ → Fe 3 O 4 → Fe 0 /Fe 3 O 4 ). Crucially, 3D electron tomography revealed a dramatic morphological reconstruction: the initial rhombic bipyramid collapses into a porous, flocculated aggregate, rather than retaining its shape. Nitrogen adsorption analysis confirms this reconstruction, revealing a sharp recovery of specific surface area from ∼8 m 2 g −1 at 400 °C to 273 m 2 g −1 at 600 °C, attributed to the formation of a hierarchically porous carbon matrix with both micropore-related and mesopore-related features. This is coupled with the transformation of the ordered organic linker into a disordered, nanocrystalline carbon matrix. The final pyrolysis product exhibits excellent catalytic activity for CO 2 hydrogenation, achieving 18.77% CO 2 conversion and >95% CO selectivity. Furthermore, it demonstrated outstanding stability with no decay over 100 h. These findings provide critical mechanistic insights into the structural evolution of MOF-derived catalysts, offering experimental guidance for the rational design of robust CO 2 hydrogenation catalysts. • A four-stage pyrolysis pathway of MIL-53(Fe) was experimentally resolved. • 3D tomography reveals collapse of rhombic crystals into porous aggregates. • Pyrolysis drives Fe 3+ → Fe 3 O 4 → Fe 0 /Fe 3 O 4 and carbon reconstruction. • Porosity recovers sharply at 600 °C, yielding a hierarchically porous Fe/C solid. • The 600 °C-derived catalyst shows high CO selectivity and 100 h stability.