Zirconium and Aluminum MOFs for Low-Pressure SO₂ Adsorption and Potential Separation: Elucidating the Effect of Small Pores and NH₂ Groups

Abstract

Finding new adsorbents for the desulfurization of flue gases is a challenging task but is of current interest, as even low SO₂ emissions impair the environment and health. Four Zr- and eight Al-MOFs (Zr-Fum, DUT-67(Zr), NU-1000, MOF-808, Al-Fum, MIL-53(Al), NH₂-MIL-53(Al), MIL-53(tdc)(Al), CAU-10-H, MIL-96(Al), MIL-100(Al), NH₂-MIL-101(Al)) were examined toward their SO₂ sorption capability. Pore sizes in the range of about 4-8 Å are optimal for SO₂ uptake in the low-pressure range (up to 0.1 bar). Pore widths that are only slightly larger than the kinetic diameter of 4.1 Å of the SO₂ molecules allow for multi-side-dispersive interactions, which translate into high affinity at low pressure. Frameworks NH₂-MIL-53(Al) and NH₂-MIL-101(Al) with an NH₂-group at the linker tend to show enhanced SO₂ affinity. Moreover, from single-gas adsorption isotherms, ideal adsorbed solution theory (IAST) selectivities toward binary SO₂/CO₂ gas mixtures were determined with selectivity values between 35 and 53 at a molar fraction of 0.01 SO₂ (10.000 ppm) and 1 bar for the frameworks Zr-Fum, MOF-808, NH₂-MIL-53(Al), and Al-Fum. Stability tests with exposure to dry SO₂ during ≤10 h and humid SO₂ during 5 h showed full retention of crystallinity and porosity for Zr-Fum and DUT-67(Zr). However, NU-1000, MOF-808, Al-Fum, MIL-53(tdc), CAU-10-H, and MIL-100(Al) exhibited ≥50-90% retained Brunauer-Emmett-Teller (BET)-surface area and pore volume; while NH₂-MIL-100(Al) and MIL-96(Al) demonstrated a major loss of porosity under dry SO₂ and MIL-53(Al) and NH₂-MIL-53(Al) under humid SO₂. SO₂ binding sites were revealed by density functional theory (DFT) simulation calculations with adsorption energies of -40 to -50 kJ·mol^-1 for Zr-Fum and Al-Fum and even above -50 kJ·mol^-1 for NH₂-MIL-53(Al), in agreement with the isosteric heat of adsorption near zero coverage (Δ<i>H</i>_ads⁰). The predominant, highest binding energy noncovalent binding modes in both Zr-Fum and Al-Fum feature μ-OH^δ+···^δ-OSO hydrogen bonding interactions. The small pores of Al-Fum allow the interaction of two μ-OH bridges from opposite pore walls with the same SO₂ molecule via OH^δ+···^δ-OSO^δ-···^δ+HO hydrogen bonds. For NH₂-MIL-53(Al), the DFT high-energy binding sites involve NH^δ+···^δ-OS together with the also present Al-μ-OH^δ+···^δ-OS hydrogen bonding interactions and C₆-π^δ-···^δ+SO₂, N^δ-···^δ+SO₂ interactions.