Comparison of Mg–Al layered double hydroxides intercalated with OH⁻ and CO₃²⁻ for the removal of HCl, SO₂, and NO₂

Two different Mg–Al layered double hydroxides (LDHs), OH⋅Mg–Al LDH and CO₃⋅Mg–Al LDH, are prepared and utilized for the efficient removal of acidic gases. The former contains OH⁻ anions intercalated between the Mg–Al layers, whereas the latter contains CO₃²⁻. The synthesized Mg–Al LDHs were characterized via X-ray diffraction (XRD) and tested for their ability to remove three important acidic gases, HCl, NO₂, and SO₂ via chemical reaction with the intercalated OH⁻ and CO₃²⁻ anions (for HCl and NO₂) and surface adsorption (in case of SO₂). Both OH⋅Mg–Al LDH and CO₃⋅Mg–Al LDH showed appreciable gas removal abilities with respect to HCl and NO₂. However, in case of OH⋅Mg–Al LDH, a higher gas removal efficiency was observed due to the facile reaction between the acidic gases and the intercalated OH⁻ anions between the LDH layers. Briefly, the HCl removal amount was 0.53 mmol for OH⋅Mg–Al LDH and 0.49 mmol for CO₃⋅Mg–Al LDH in 90 min. The NO₂ removal amount was 0.09 mmol for OH⋅Mg–Al LDH and 0.07 mmol for CO₃⋅Mg–Al LDH in 90 min. The SO₂ removal amounts by the LDHs were comparable since the gas was adsorbed on the surface of the LDHs. The SO₂ removal amount was 0.04 mmol for OH⋅Mg–Al LDH and 0.04 mmol for CO₃⋅Mg–Al LDH in 90 min. The XRD results indicated that the LDHs retained their structures after gas removal, despite attenuation of the XRD peak intensities. Thus, two easily synthesized LDHs with efficient acidic gas removal characteristics have been developed via judicious utilization of the reactions that dictate gas removal by LDHs.