**4. Conclusions**

Water stability of NH2-MIL-125(Ti) was studied by analyzing quantitatively the amount of ligand released to the liquid phase along the exposition time in water. In many studies, the water stability of MOFs has been mainly investigated using the X-ray di ffraction, N2 adsorption–desorption and electron microscopy of filtered powders, comparing the properties before and after exposition. This study shows that these characterization techniques are not enough to assess that stability. We demonstrated, for the first time, the quantitative lixiviation of this MOF, which achieved values up to 25% after 24 h of contact, with water still being far from equilibrium after that time. The present study also shows that the post-modification of NH2-MIL-125(Ti) by thermal treatments stabilizes its structure, making it less susceptible to partial dissolution. Temperature and time showed to be the determining variables, while the atmosphere (air or vacuum) showed no significant e ffect. Thermal treatment under these two atmospheres within 150–250 ◦C did not alter the structure and porous texture of the MOF, except for some slight increase in the specific surface area accompanied by a small reduction in crystallinity. Increasing the treatment temperature up to 300 ◦C caused the collapse of the crystalline MOF structure accompanied by the loss of the pore network. Certain reduction on the surface area were observed by extending the thermal treatment but maintaining its structure.

The results obtained provide a new understanding of the effects of thermal treatments on the water stability of NH2-MIL-125(Ti), which was improved as the temperature of the treatment increased up to 250 ◦C. Some significant differences were found on the effects of the atmosphere used in the thermal treatment. Under air, greater stabilization of the MOF was achieved due to the removal of excess linker molecules, with lower leaching percentages and the attainment of a quasi-equilibrium state in a relatively low contact time in water. The best results were obtained upon thermal treatment in air at 250 ◦C for 48 h. After that treatment, less than 7% ligand leaching occurred upon 24 h of contact with water. These findings have significant implications regarding the potential use of MOFs in water-related applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/6/603/s1, Figure S1: UV–vis diffuse absorbance spectra of NH2-MIL-125(Ti) treated under vacuum at different temperatures for 16 h. Original NH2-MIL-125(Ti) and MIL-125(Ti) spectra are included as reference, Figure S2: UV–vis diffuse absorbance spectra of NH2-MIL-125(Ti) heated in air at different temperatures for 16 h (**a**) and at 250 ◦C for 48 and 72 h (**b**). Original NH2-MIL-125(Ti) and MIL-125(Ti) spectra are included as reference, Figure S3: UV–vis diffuse absorbance spectra of M-250-48 before and after contact with water for 24 h. Original NH2-MIL-125(Ti) and MIL-125(Ti) spectra are included as reference, Figure S4: (**a**) Chemical structure of Ti-oxo clusters linked to NH2-BDC ligands, from the Cambridge Structural Database (CSD). Chemical structure of NH2-MIL-125(Ti) viewed from a-axis (**b**) and c-axis (**c**), from cif data of Crystallography Open Database.

**Author Contributions:** Conceptualization, J.B., C.B. and J.J.R.; methodology, A.G.-A. and V.M.-R.; writing— original draft preparation, A.G.-A. and C.B.; writing—review and editing, A.G.-A., V.M.-R., J.B., C.B. and J.J.R.; supervision, J.B., C.B., J.J.R.; funding acquisition, J.B., C.B. and J.J.R. All authors have read and agree to the published version of the manuscript.

**Funding:** This research was funded by Spanish MINECO (project CTQ2016-78576-R).

**Acknowledgments:** V. Muelas-Ramos thanks to Spanish MCIU for BES-2017-082613 grant.

**Conflicts of Interest:** The authors declare no conflict of interest.
