An in situ investigation of the thermal decomposition of metal-organic framework NH2-MIL-125 (Ti)

Titanium based metal-organic frameworks (MOFs) are interesting self-sacrificial precursors to derive semiconducting porous nanocomposites for highly efficient heterogeneous catalysis. However, there is a lack of systematic and in-depth mechanistic understanding of the pyrolytic conversion of MOF precursors into the desired functional composite materials. In this work, TGA-MS and in situ STEM/EDX combined with other characterization techniques were employed to investigate the evolution of the structural, physicochemical, textural and morphological properties of NH2-MIL-125(Ti) pyrolysis at different temperatures in an inert gaseous atmosphere. In situ thermal analysis of NH2-MIL-125(Ti) reveals the presence of 3 rather defined stages of thermal transformation in the following order: phase-pure, highly porous and crystalline MOF -> intermediate amorphous phase without accessible porosity -> recrystallized porous phase. The three stages occur from mom temperature till 300 degrees C, between 350 and 550 degrees C and above similar to 550 degrees C respectively. It is found that the framework of NH2-MIL-125(Ti) starts to collapse around 350 degrees C, accompanied with the cleavage of coordination and covalent bonds between organic linkers [O2C-C6H3(NH2)-CO2](6) and the Ti oxo-cluster Ti8O8(OH)(4). The organic linker continues fragmentation at 450 degrees C causing the shrinkage of particle sizes. The dominant pore size of 0.7 nm for NH2-MIL-125(Ti) gradually expands to 1.4 nm at 800 degrees C along with the formation of mesopores. The derived disc-like particles exhibit an approximately 35% volume shrinkage compared to the pristine MOF precursor. Highly crystalline N and/or C self-doped TiO2 nanoparticles are homogeneously distributed in the porous carbon matrix. The original 3D tetragonal disc-like morphology of the NH2-MIL-125(Ti) remains preserved in derived N and/or C doped TiO2/C composites. This study will provide an in-depth understanding of the thermal conversion behavior of MOFs to rationally select and design the derived composites for the relevant applications.

Automated summary

This study investigates the thermal conversion behavior of NH2-MIL-125(Ti) MOFs, revealing three distinct stages of transformation at different temperatures and providing insights into the mechanistic understanding of the process.