New insight into the phase transformation of sepiolite upon alkali activation: Impact on composition, structure, texture, and catalytic/sorptive properties

Abstract

Wet NaOH activation of natural sepiolite was studied for the purpose of endowing the material with enhanced basicity for potential catalytic/sorptive applications. A series of synthesized solids were characterized with X-ray powder diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM), X-ray fluorescence (XRF), atomic absorption spectroscopy (AAS), N2 adsorption/desorption, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Surface basicity was determined by titration with benzoic acid. The study led to the major reinterpretation of the existing knowledge on the sepiolite phase transformation to its sodium form (loughlinite). Activation with NaOH involved significant desilication of sepiolite, while the loss of Mg from the solid was prevented due to the intracrystalline preciptation of amorphous Mg(OH)2. Formation of Mg(OH)2 affected the material composition, hindered the access of nitrogen molecules to the micropore system, and enhanced the product basicity. For the first time the Si sites at the centre of structural ribbons were identified as part of the silicate framework most susceptible to leaching upon alkaline treatment. Although the NaOH treated product had the structure of loughlinite, it was in fact a composite of desilicated loughlinite and amorphous Mg(OH)2. The materials were tested as catalysts in aldol self condensation of acetone and oxidation of cyclohexanone to ε-caprolactone, and as CO2 sorbents.