Hydrogen Adsorption Behavior of Highly Stable Assorted Functionalized UiO-66-MOFs at Low Pressures: A Combined Experimental and DFT Study

Three distinct functionalized UiO-66-MOFs, namely, UiO-66-SO₃H@Si (2-sulfoterephthalic acid monosodium salt), UiO-66-PDCA (2,5-pyridinedicarboxylic acid), and UiO-66-AO@Si (amidoxime), with Zr⁴⁺ metal, have been synthesized. In the present study, hydrogen adsorption behavior of synthesized UiO-66-MOFs has been demonstrated at low pressures (0.67 to 2.1 bar) and fixed temperature (77 K). The functionalized UiO-66-MOFs exhibited variations in surface area, porosity, thermal, chemical stability, and hydrogen adsorption efficiency. Besides this, the synthesized UiO-66-SO₃H@Si and UiO-66-AO@Si MOFs showed outstanding chemical stability in a nitric acid medium up to 8 and 6 M, respectively. In comparison to the other two MOFs, UiO-66-AO@Si demonstrated the highest hydrogen adsorption, with a saturation value of 0.35 wt % at 2.1 bar pressure and 77 K, making amidoxime-based MOFs attractive contenders for potential H₂ storage materials. The increasing order of hydrogen adsorption by three UiO-66-MOFs are as follows: 0.12 wt % (UiO-66-SO₃H@Si) < 0.25 wt % (UiO-66-PDCA) < 0.35 wt % (UiO-66-AO@Si) at 2.1 bar and 77 K. However, these three MOFs showed the same trend in hydrogen adsorption over low to high pressure range, i.e., as pressure increases, hydrogen adsorption capacity also increases. Hydrogen storage in these functionalized UiO-66-MOFs is accomplished through solely physical adsorption. The high surface area and porosity of UiO-66-AO@Si are important factors in achieving the high hydrogen adsorption capacity. Theoretical evidence for hydrogen adsorption by these UiO-66-MOFs were validated by DFT studies. The computed complexation energies of UiO-66-MOFs and H₂ using DFT are in excellent agreement with the experimental results.