Mechanistic Insights on Solution-Based Green Synthesis of Phase-Pure Ca-Based Layered Double Hydroxides from Ca(OH)₂

Ca-based layered double hydroxides (LDHs) are ideal candidates for applications that require the selective adsorption of anions and acidic molecules due to their positively charged layers and high density of exposed basic adsorption sites. As such, they are rapidly gaining attention in the fields of drug delivery, alloy corrosion inhibition, industrial waste recycling, environmental remediation, and CO₂ capture. Traditional LDH synthesis methods such as coprecipitation, urea hydrolysis, and sol-gel require use of non-environmentally friendly precursors and/or additional post-synthetic treatments to achieve highly crystalline and phase-pure Ca-LDHs. More recently, green synthesis of crystalline Ca-LDH has been achieved by means of Ca(OH)₂ hydrolysis without the need for strong alkalis (e.g., NaOH); however, attainment of phase-pure Ca-LDHs using this approach remains elusive due to a lack of understanding of the Ca-LDH formation mechanism. In this work, we develop a comprehensive solution speciation model for the synthesis of Ca₂Fe(OH)₆Cl·2H₂O via Ca(OH)₂ hydrolysis and discuss the effect of initial Ca/Fe(III) solution molar ratio (SMR_Ca/Fe) on sample purity and crystallinity from both a theoretical and experimental viewpoint. Our calculations show a SMR_Ca/Fe ≈ 3 is needed to (i) achieve full conversion of the solid precursors and (ii) avoid the coprecipitation of less soluble impurities. This value stems from the chemical equilibria of species in solution and the need to be above a specific pH level to drive the precipitation of the Ca-LDH. Characterization of the samples using thermogravimetric analysis (TGA) coupled with gas Fourier transform infrared spectroscopy (gas-FTIR), high-resolution X-ray diffraction (HR-XRD), and pair distribution function analysis (PDF) supports the theoretical findings, showing evidence of high purity samples synthesized with SMR_Ca/Fe ≈ 3. Based on these results, we introduce a new simplified method for the green synthesis of crystalline Ca-LDHs of different chemistries via in situ hydrolysis of Ca(OH)₂, consisting of a one-pot synthesis at ambient temperature without the need for strong alkalis (i.e., NaOH).