Structural Characterisation of Complex Layered Double Hydroxides and TGA‐GC‐MS Study on Thermal Response and Carbonate Contamination in Nitrate‐ and Organic‐Exchanged Hydrotalcites

Abstract

Layered double hydroxides (LDHs) are versatile materials used for intercalating bioactive molecules in the fields of pharmaceuticals, nutraceuticals and cosmetics, with the purpose of protecting them from degradation, enhancing their water solubility to increase bioavailability and improving their pharmacokinetic properties and formulation stability. Moreover, LDHs are used in various technological applications to improve stability and processability. The crystal chemistry of hydrotalcite-like compounds was investigated by X-ray powder diffraction (XRPD), automated electron diffraction tomography (ADT) and thermogravimetric analysis (TGA)-GC-MS to shed light on the mechanisms involved in ion exchange and absorption of contaminants, mainly carbonate anions. For the first time, ADT allowed a structural model of LDH_NO3 to be obtained from experiment, shedding light on the conformation of nitrate inside LDH and on the loss of crystallinity due to the layer morphology. The ADT analysis of a hybrid LDH sample (LDH_EUS) clearly revealed an increase in defectivity in this material. XRPD demonstrated that the presence of carbonate can influence the intercalation of organic molecules into LDH, since CO3 -contaminated samples tend to adopt d spacings that are approximate multiples of the d spacing of LDH_CO3 . TGA-GC-MS allowed intercalated and surface- adsorbed organic molecules to be distinguished and quantified, the presence and amount of carbonate to be confirmed, especially at low concentrations (<2 wt %), and the different types and strengths of adsorption to be classified with respect to the temperature of elimination.

Controlling the Synthesis Conditions for Tuning the Properties of Hydrotalcite-Like Materials at the Nano Scale

“…Although for sample M11 (and partially for sample M12), the value calculated should also include the amount of water adsorbed on the external surface of the crystallites, which is easily removed up to 150 • C. Thus, the two broad DTA endothermic effects (clearly recorded for sample M12, and somewhat obscured by the first one for sample M11) should correspond to the removal of externally adsorbed water molecules, while the sharp, intense effect somewhat above 200 • C for all samples studied should correspond to the removal of more strongly held structure-bonded interlayer water molecules. Two endothermic effects (better defined for samples M2X and M3X) in the same temperature range as the second mass loss should correspond, as indicated above, to the removal of CO 2 (from interlayer carbonate species) and of H 2 O (through condensation of layer hydroxyl groups), as previously reported for this family of materials [49,51,52].…”

“…Thus, the two broad DTA endothermic effects (clearly recorded for sample M12, and somewhat obscured by the first one for sample M11) should correspond to the removal of externally adsorbed water molecules, while the sharp, intense effect somewhat above 200 °C for all samples studied should correspond to the removal of more strongly held structure-bonded interlayer water molecules. Two endothermic effects (better defined for samples M2X and M3X) in the same temperature range as the second mass loss should correspond, as indicated above, to the removal of CO2 (from interlayer carbonate species) and of H2O (through condensation of layer hydroxyl groups), as previously reported for this family of materials [49,51,52]. As already observed from the FTIR studies (sharper bands as the treatment becomes more severe), it is clear that the endothermic differential thermal analysis (DTA) effects become sharper for samples surface area equivalent to adsorption on micropores, from the t-plot (m 2 /g); e pore volume (mm 3 /g); f average pore diameter (nm).…”

DFT‐Based Simulation and Experimental Validation of the Topotactic Transformation of MgAl Layered Double Hydroxides

“…Because, for the first two steps of the decomposition of the intermediate Mono‐LDH, the Δ G values for water release are negative and that of the removal of an interlayer CO 2 are positive (Figure ), it can be deduced that water release occurs prior to the removal of CO 2 . However, it is noteworthy that the concomitant release of water and CO 2 in the experimental calcination of LDHs is still practicable, because of the existence of defects and inhomogeneity in heating. The gallery distance of the LDH material decreases to 4.42 Å after the decomposition of the interlayer anions, with the corresponding loss weight of 24.8 wt %.…”

Rational design of the solid-state synthesis of materials based on poly-aromatic molecular complexes

“…Moreover, it is confirmed that PCA could be useful in all kinetic investigations where Rietveld analysis is very complicated or unfeasible, because of disorder (like in layered structures) 40,41 or for the lack of a known crystal structure. Non-isothermal kinetic analysis was carried out since it is much faster.…”