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

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 A 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 A 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 (Delta H-ads(0)). The predominant, highest binding energy noncovalent binding modes in both Zr-Fum and Al-Fum feature mu-OH delta+center dot center dot center dot delta-OSO hydrogen bonding interactions. The small pores of Al-Fum allow the interaction of two mu-OH bridges from opposite pore walls with the same SO2 molecule via OH delta+center dot center dot center dot delta-OSO delta-center dot center dot center dot delta+HO hydrogen bonds. For NH2-MIL-53(Al), the DFT high-energy binding sites involve NH delta+center dot center dot center dot delta-OS together with the also present Al-mu-OH delta+center dot center dot center dot delta-OS hydrogen bonding interactions and C-6-pi(delta-)center dot center dot center dot delta+SO2, N delta-center dot center dot center dot delta+SO2 interactions.

Automated summary

Research on Zr- and Al-based metal-organic frameworks (MOFs) for SO2 adsorption showed that frameworks with pore sizes in the range of 4-8 angstroms exhibit optimal sorption capabilities at low pressures. MOFs with NH2 functional groups tend to have enhanced affinity for SO2. Stability tests and density functional theory (DFT) simulations provided insights into the adsorption mechanisms and binding energies of different MOFs towards SO2.