One-pot synthesis of Mg[sbnd]Al layered double hydroxide (LDH) using MgO and metakaolin (MK) as precursors

Abstract

Layered double hydroxides (LDH) are a class of basic inorganic layered compounds, which are widely used in the fields of adsorption and catalysis owing to their unique structure and properties. In previous studies, costly soluble Al salts or Al(OH)3 were used as the Al sources for the synthesis of MgAl LDH. The present work proposes a facile and one-pot method to synthesize MgAl LDH in a water bath with MgO and MK as precursors, which are both obtained from abundant natural resources. The effects of liquid/solid ratio (30:1 or 60:1), synthesis temperature (55–90 °C) and alkali concentration (3 or 6 mol/L) were investigated on the composition and characteristics of the synthetic products. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Thermogravimetric analysis (TGA), Scanning electron microscope (SEM) and Nuclear magnetic resonance (NMR). Findings showed that a highly alkaline solution (6 M NaOH) promoted the dissolution of MK, resulting in the release of trivalent Al for the formation of LDH. Increasing temperature also led to faster dissolution of MK and released more Al and Si, which resulted in the formation of both LDH and zeolite. More importantly, it was found that Mg2+ released during the hydrolysis of MgO combined with Al-containing substances directly to precipitate LDH, whereas its hydration product, i.e., brucite, remained stable in the alkaline condition and released little Mg2+ to participate in the formation of LDH. Zeolites may form depending on the Si/Al ratio of the solution which is governed by the various synthesis parameters. The findings not only shed lights on the reaction mechanism between the MgO and MK and the role of key synthetic conditions in the formation of LDH and zeolitic phases, but also demonstrated the feasibility of using widely-available, low-cost natural minerals to produce commercial adsorbents/catalysts.