*3.2. Characterization*

Thermogravimetric analysis was carried out using a Q600 TA instruments (New Castle, DE, USA) in an alumina crucible. All measurements were carried out within the temperature range 25–1200 ◦C at a heating rate of 10 ◦C·min−<sup>1</sup> under a continuous air flow of 20 mL·min−1. X-ray diffractograms (XRD) were registered in the 2–45◦ 2θ range with a rate of 1.5 ◦·min−<sup>1</sup> and using a Cu Kα radiation. A Bruker D8 diffractometer (Bremen, Germany) was used, equipped with a Sol-X energy dispersive

detector. Crystallinity percentage was calculated after di ffractogram deconvolution with OriginPro by the following expression [53,54]:

$$\text{CrystalIntity} \left( \% \right) = 100 \times \frac{\text{Area of crystalline peaks}}{\text{Area of all \"RDD pattern}} \tag{2}$$

The average crystal size was determined by using the Scherrer's equation from the reflection peak (121) of NH2-MIL-125(Ti), not overlapped with any other peak. UV-vis di ffuse reflectance spectroscopy was carried out on Shimadzu UV2600i (Kyoto, Japan) equipment, recording the spectra in the 250–700 nm range. Scanning electron microscopy images (SEM) were taken with a Hitachi S4800 microscope (Krefeld, Germany) and transmission electron microscopy images (TEM) with a TECNAI G2 20 Twin equipment (Hillsborough, NC, USA). N2 adsorption–desorption isotherms at −196 ◦C were carried out with a TriStar 123 equipment (Micromeritics II 3020, Norcross, GA, USA). Solids were outgassed before testing under vacuum at 120 ◦C for 18 h. Specific surface area (SBET) was calculated by the Brunauer–Emmet–Teller method [55], while micropore surface area (SMP) was obtained using the *t-plot* method [56] and the external or non-microporous surface area (SEXT) by di fference between SBET and SMP. The total pore volume (VT) corresponded to the amount of nitrogen adsorbed at a relative pressure of 0.99, while the micropore volume (VMP) was obtained from the *t-plot* method.

#### *3.3. Stability Testing in Water*

The water stability of the NH2-MIL-125(Ti) derived solids was tested through the leaching of the NH2-BDC in deionized water (Type II). The solid was suspended in water (250 mg·L−1) and stirred for 24 h at 25 ◦C and 170 rpm in a water bath orbital shaker (JULABO SW22, Seelbach, Germany). At di fferent interval times, aliquots of 2 mL were collected and filtered with Polytetrafluoroethylene (PTFE) syringe filters (Whatman 0.45 μm). The filtrates were analyzed to determine the concentration of the ligand by high performance liquid chromatography (HPLC) (Shimadzu Prominence-I LC-2030C, Kyoto, Japan), equipped with a diode array detector (SPDM30 A) and a C18 column (Eclipse Plus, 5 μm). A gradient elution method was used for the analyses, changing the mobile phase (aqueous acetic acid solution 0.1%, Sigma Aldrich, Madrid, Spain, ≥99%) by elution with acetonitrile (HPLC grade, Scharlab, Barcelona, Spain) and using a constant flow of 0.7 mL·min−1. The wavelength of the detector was fixed at 358 nm for the detection of the NH2-BDC that appeared at a 6.3 min retention time. Solid samples were recovered from the aqueous medium by centrifugation after stability test and stored for later characterization.
