Synthesis of Mg–Al Mixed Oxides with Markedly High Surface Areas from Layered Double Hydroxides with Organic Sulfonates

Mg–Al mixed oxides with record-high surface areas and basic site concentrations were synthesized from Mg–Al layered double hydroxides with interlayer isethionate (Ise) or 3-hydroxy-1-propanesulfonate (HPS). Anion exchange of interlayer CO₃^2– in synthetic hydrotalcites with the organic sulfonates induces disorders in layer stacking as characterized by powder X-ray diffraction and enables facile delamination in water. Thermal treatment of materials anion-exchanged by Ise (MgAl–Ise) and HPS (MgAl–HPS) in N₂ and H₂ resulted in the formation of Mg–Al mixed oxides with marked enhancement in Brunauer–Emmett–Teller (BET) surface area relative to those treated in air. Treatment in a flow of H₂ is particularly effective, doubling the surface area of mixed oxides derived from MgAl–Ise relative to those obtained in a flow of N₂. A higher degree of disorder in layer stacking in MgAl–HPS than MgAl–Ise resulted in the formation of Mg–Al mixed oxides with higher surface areas than those from MgAl–Ise. As a result, thermal activation of MgAl–HPS in a flow of H₂ yielded Mg–Al mixed oxides with the highest BET surface area (410 m² g^–1) and CO₂ uptake (1.6 mmol g^–1 at 25 °C and 100 kPa) in all samples. These values are significantly higher than those obtained from the initial hydrotalcites as well as those reported in the literature with similar Mg–Al ratios. Investigation of the thermal activation steps by thermogravimetric analysis and mass spectrometry indicates that the key factors to achieve high surface area and CO₂ uptake are to weaken interactions between layers by inducing stacking disorders and to facilitate the removal of interlayer sulfonates by preventing the formation of sulfates from them via thermal activation under a reducing environment.