Sol–gel preparation of hierarchically porous magnesium aluminate (MgAl2O4) spinel monoliths for dye adsorption

Abstract Hierarchically porous spinel-type binary metal oxide MgAl 2 O 4 with cocontinuous skeletons and interconnected macropores has been synthesized by a facile sol–gel method accompanied by phase separation, in which poly (ethylene oxide) (PEO) works as phase separation inducer to form micrometer-scale phase-separated macrochannels at the same time of sol–gel transition. The obtained xerogels exhibit in the form of a monolith with well-defined macropores and cocontinuous dense skeletons by adjusting the amount of PEO. The xerogel and those heat-treated at or below 600 °C are amorphous, and heat-treatment at or above 700 °C allows the precipitation of complete crystalline MgAl 2 O 4 spinel with a macropore size of 0.7–0.9 μm, a porosity of 60–70%, and a BET surface area of 40–80 m 2  g −1 , without spoiling the macrostructure of monoliths. Three anionic dyes and two cationic dyes are selected as targeted adsorbates to investigate the selective adsorption ability of a hierarchically porous MgAl 2 O 4 spinel. The resultant MgAl 2 O 4 spinel shows a high affinity and adsorption rate toward anionic dyes Congo Red, Methyl Orange, and Methyl Blue. The maximum adsorption capacity of MgAl 2 O 4 spinel toward these three anionic dyes are 2721.6 mg g −1 for Congo Red, 2469.7 mg g −1 for Methyl Orange, and 6771.9 mg g −1 for Methyl Blue, respectively. The strong electrostatic interaction as well as hydrogen bond between the positively charged sites in hierarchically porous MgAl 2 O 4 spinel and amino groups or negatively charged sulfonate groups (-SO 3 - ) in the anionic dyes. The macropores provide delivery channels and the mesopores produce enough adsorption sites.