The impact of thermal activation conditions on physicochemical properties of nanosheet-derived Mg-Al mixed oxides

Abstract

Mg-Al mixed oxides with highly accessible basic sites were synthesized from Mg-Al layered double hydroxides bearing interlayer isethionates (Ise). Anion-exchange of interlayer carbonates in synthetic hydrotalcites with Ise induces stacking disorders in layered structure, and subsequent delamination in water followed by drying causes further disordering as characterized by X-ray diffraction technique. However, thermal activation of these materials in air formed Mg-Al mixed oxides with low surface area and low CO2 adsorption capacity. In contrast, thermal activation in N2 or N2 followed by air formed materials exhibiting marked increases in CO2 adsorption capacity up to ∼800% relative to those obtained by thermal activation in air. Thermogravimetric and elemental analyses and IR spectroscopy characterization indicate that thermal activation in N2 enables removal of sulfur species formed from Ise at ∼400 °C, and limits the formation of sulfate species that bridge nanosheets and cause irreversible stacking of them. The proper thermal activation procedure in addition to disordered structure of aggregates of Mg-Al double hydroxide nanosheets enabled the formation of Mg-Al mixed oxides with higher surface area and larger micropore volume than those obtained from the initial hydrotalcites. These results highlight the critical importance of thermal activation steps for such materials, and demonstrate a prospective route to synthesize new porous materials based on Mg-Al mixed oxides.