Synthesis of Highly Crystalline NH<sub>2</sub>‑MIL-125 (Ti) with S‑Shaped Water Isotherms for Adsorption Heat Transformation

NH<sub>2</sub>-MIL-125 has been investigated as a water adsorbent because of its high hydrothermal stability and S-shaped water adsorption isotherm. Herein, we report the synthesis of NH<sub>2</sub>-MIL-125 with high surface area and water capacity for an adsorption heat transformation (AHT) system. NH<sub>2</sub>-MIL-125 derived from Ti­(BuO)<sub>4</sub> shows higher surface area and water uptake than those of Ti­(<i>i</i>PrO)<sub>4</sub>-derived samples regardless of the synthesis method. In a sense of crystallinity, a solvothermal method with static conditions generated more distinct crystalline properties than the one synthesized by a reflux reaction as confirmed from powder X-ray diffraction analysis, UV–vis absorbance spectra, and scanning electron microscopy images. Considering it as an adsorbent for an AHT system, the Ti­(BuO)<sub>4</sub>-derived sample synthesized by a solvothermal method shows an ideal S-shaped isotherm with a steep rise in water uptake at lower relative pressure (0.550 g/g at <i>P</i>/<i>P</i><sub>0</sub> = 0.30), which is attributed to narrow triangle apertures and hydrophilic functional groups. This material shows the dynamic water adsorption/desorption cycle without any noticeable weight change.