Structural and Electronic Insights into Titanium-Modified UiO-66: Probing Cation Substitution via Multi-Technique Characterization and High-Field (1.2 GHz) NMR

Metal-organic frameworks (MOFs) based on the UiO-66 structure have garnered significant attention due to their stability, tunability, and potential applications in catalysis and electronic materials. This study investigates the partial substitution of zirconium (Zr) by titanium (Ti) in the UiO-66 framework using two synthetic approaches: postsynthetic modification (PSM) and one-pot synthesis (OP). Structural and electronic modifications were characterized through X-ray diffraction (XRD), solid-state nuclear magnetic resonance (ssNMR), scanning and transmission electron microscopy (SEM/TEM), infrared (FTIR) and Raman spectroscopies, UV–vis diffuse reflectance spectroscopy, and nitrogen sorption analysis. The results confirm that Ti is incorporated within the inorganic cluster rather than forming separated titanium phases, as evidenced by XRD and spectroscopic techniques. The lattice parameter decreased from 20.7696(8) Å in UiO-66(Zr) to 20.758(2) Å in UiO-66(Zr/Ti)-PSM and 20.651(1) Å in UiO-66(Zr/Ti)-OP, related to the smaller ionic radius of Ti4+ relative to that of Zr4+. Specific surface area (SSA) measurements revealed an increase for UiO-66(Zr/Ti)-PSM (1342 m2/g) compared to UiO-66(Zr) (1262 m2/g), which is indicative of the cation exchange. On the other hand, UiO-66(Zr/Ti)-OP exhibited a lower SSA (828 m2/g), possibly due to defect formation, which was later confirmed by ssNMR. Band gap analysis indicated that Ti incorporation reduced the optical bandgap from 4.07 eV (UiO-66(Zr)) to 3.87 eV (PSM) and 3.75 eV (OP), suggesting the formation of linker-to-metal charge transfer (LMCT) states. 91Zr and 47/49Ti ssNMR experiments confirmed the local perturbations in the metal cluster, further supporting the successful incorporation of Ti into the UiO-66 framework. This study provides comprehensive evidence of titanium substitution within UiO-66(Zr) without phase separation, offering insights into how controlled metal exchange can modify the electronic properties and enhance the functional potential of MOFs. These findings pave the way for future applications in catalysis, photocatalysis, and electronic materials.