Synthesis of linear and cyclic organic sulfonic acid-modified Cu-Al layered double hydroxides and their adsorption properties

Over the past few years, many substances have been investigated for their potential application as adsorbent materials. The combination of a layered double hydroxide (LDH) and CH/π interactions enables selective adsorption through the differences in the electron density of the aromatic ring. Thus, in this study, two organic anions, namely the linear 1-octane sulfonate (OS-) and cyclic sulfonated-β-cyclodextrin (SCD13-), were intercalated into the interlayers of a Cu-Al LDH by coprecipitation (Cu/Al ratio, 3.0; chemical ratio, 2.0). The SCD13- anion was selected because β-cyclodextrin comprises hydrophobic cavities that allow the formation of inclusion compounds. X-ray diffraction (XRD), total organic carbon (TOC) analysis, and inductively coupled plasma-atomic emission spectrometry (ICP-AES) revealed that the as-synthesized OS-Cu-Al LDH and SCD-Cu-Al LDH were both oriented perpendicular to the LDH layer. Subsequent absorption experiments with m-aminophenol (AP) and m-nitrophenol (NP), which have similar structures but different aromatic ring electron densities, confirmed a Freundlich-type adsorption mechanism and revealed that adsorption occurred on a non-uniform surface for both LDHs. The results also revealed excellent NP and AP adsorptions for the linear OS•Cu-Al LDH and cyclic SCD•Cu-Al LDH, respectively. This was attributed to the strong hydrophobic interactions in the former and strong CH/π interactions in the latter. This study shows the potential of using linear and cyclic organic sulfonic acid-modified LDHs for the selective recovery of the desired aromatic organic compounds.