**1. Introduction**

Layered double hydroxides ([M<sup>2</sup>+1−<sup>x</sup>M3<sup>+</sup>x(OH)2]x<sup>+</sup>(Ay−)x/y·zH2O, where M2<sup>+</sup> and M3<sup>+</sup> are divalent and trivalent metal cations, respectively, and Ay− is a intercalated anion, LDHs) are widely used in catalysis, in ion-exchange processes, as catalyst support precursors, adsorbents, anticorrosion inhibitors, anion exchangers, flame retardants, polymer stabilizers, and in pharmaceutical applications, optics, in separation science and photochemistry [1–7]. The most common preparation technique of LDHs is the co-precipitation method starting from the soluble salts of the metals [8–10]. The second synthetic technique also widely used for the preparation of LDHs is anion exchange [11–13]. Recently, for the preparation of Mg3Al1 LDHs, we developed the indirect sol–gel synthesis route [14–17]. In this synthetic approach, the synthesized Mg–Al–O precursor gels were converted to the mixed metal oxides (MMO) by heating the gels at 650 ◦C. The LDHs were fabricated by the reconstruction of MMO in deionized water at 80 ◦C. The proposed sol–gel synthesis route for LDHs showed some benefits over the co-precipitation and anion-exchange methods such as simplicity, high homogeneity, and good crystallinity of the end synthesis products, effectiveness, cost efficiency, and suitability for the synthesis of different LDH compositions.

Recently, this newly developed sol–gel synthesis method has been successfully applied for the synthesis of the transition metal substituted layered double Mg3-x Mx/Al1 (M = Mn, Co, Ni, Cu, Zn) [18]. Calcined at temperatures higher than 600–650 ◦C, the M/Mg/Al LDHs form MxMg1−<sup>x</sup>Al2O4 solid solutions having the spinel structure and various cations distributions [19–23]. It was reported that these spinel structure compounds obtained at high temperatures cannot be reconstructed to the LDHs [24–29].

The investigation of mixed oxides derived from calcined LDHs prepared by direct and indirect methods is an interesting topic, since the reformation conditions could have an e ffect not only on the composition of a solid but also on the morphology of oxides and consequently on the properties. Usually, the calcined LDHs materials or mixed metal oxides have high surface areas. During the calcination, the dehydroxylation of LDHs with di fferent chemical composition gave rise to the crystal deformation and interstratified structure of metal oxides, resulting in the development of mesopores and enhancement of specific surface area and enhanced sorption capacity [30–34]. Interestingly, the obtained MMO sometimes can preserve the morphology of the LDH precursor and show also the efficient recycling of the spent adsorbent [35,36]. It has been found that the nature of the partially introduced cation into the M2<sup>+</sup> position influences the conditions of thermal decomposition of LDHs and also the structural and morphological features of the formed mixed metal oxides [37]. The obtained data can be used to synthesize the oxide supports with desired adsorption and other physical properties. In this study, the alkaline earth metal substituted Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) layered double hydroxides were synthesized by an indirect sol–gel method. The aim of this study was to decompose the sol–gel-derived LDHs at di fferent temperatures and investigate the possible reconstruction of obtain mixed metal oxides to LDHs. The surface area and porosity as important characteristics of these alkaline earth metal substituted MMO materials were investigated in this study as well.
