Reversible Low-Temperature Metal Node Distortion during Atomic Layer Deposition of Al₂O₃ and TiO₂ on UiO-66-NH₂ Metal-Organic Framework Crystal Surfaces

Metal-organic frameworks (MOFs) are chemically functionalized micro- and mesoporous materials with high surface areas and are attractive for multiple applications including filtration, gas storage, and catalysis. Postsynthetic modification (PSM), via solution or vapor-based techniques, is a way to impart additional complexity and functionality into these materials. There is a desire to shift toward vapor-phase methods in order to ensure more controlled modification and more efficient reagent and solvent removal from the modified MOF material. In this work we explore how the metal precursors titanium tetrachloride (TiCl₄) and trimethylaluminum (TMA), commonly used in atomic layer deposition, react with UiO-66-NH₂ MOF. Using in situ quartz crystal microbalance (QCM) and Fourier transform infrared spectroscopy (FTIR) at 150 and 250 °C, we find that the ALD precursors react with μ₃-OH hydroxyl and μ₃-O bridging oxygen groups on Zr₆ nodes, as well as oxygen from carboxylate linker groups. The reactions occur predominantly at the crystal surface at μ₃-OH hydroxyl sites, with TiCl₄ exhibiting greater diffusion into the MOF subsurface. FTIR analysis suggests that, at 150 °C, both TiCl₄ and TMA reversibly dehydroxylate the hydroxylated UiO-66-NH₂, which is accompanied by distortion of the zirconium metal clusters. Finally, we show that TiCl₄ is able to react with the dehydroxylated UiO-66-NH₂ structure, suggesting that TiCl₄ is also able to react directly with the bridging oxygens in the metal clusters or carboxylate groups on the organic ligand. A better understanding of chemical and thermally driven MOF dehydroxylation reactions can be important for improved postsynthetic modification of MOFs.