**4. Conclusions**

In this study, the reconstruction peculiarities of sol–gel derived Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) layered double hydroxides (LDHs) were investigated. For the synthesis of Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) LDHs, the indirect sol–gel synthesis method has been used. Citric acid and 1,2-ethanediol were used as complexing agents in sol–gel processing [39]. The mixed metal oxides (MMO) were synthesized by two different routes in this work. Firstly, the MMO were obtained directly by heating Mg(M)–Al–O precursor gels at 650 ◦C, 800 ◦C, and 950 ◦C. The XRD pattern of the MMO sample obtained by heating Mg–Al–O precursor gels at 650 ◦C showed the formation of monophasic MMO. However, with increasing annealing temperature up to 800 ◦C or 950 ◦C and upon the substitution of Mg by Ca, Sr, and Ba, highly crystalline spinel (MgAl2O4, CaAl2O4, SrAl2O4 and BaAl2O4) phases have also formed. All MMO samples were successfully reconstructed to the Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) layered double hydroxides (LDHs) in water at 50 ◦C for 6 h (pH 10). However, the spinel phases were not reconstructed and remained as impurity phases. Moreover, during the reconstruction process, a negligible amount of metal carbonates (CaCO3, SrCO3, and BaCO3) have formed as well. Secondly, the MMO were also obtained by heating the reconstructed LDHs at the same temperatures and the phase composition, morphology, and surface properties of MMO were compared with obtained ones after initial annealing. It was demonstrated that the second time obtained Ca and Sr-substituted MMO samples contained more side phases. However, this was not the case for the Ba-substituted MMO samples, since both synthesis products obtained from precursor gels and by heating LDHs were composed of several crystalline phases. It was demonstrated for the first time that the microstructure of reconstructed MMO from sol–gel derived LDHs showed a "memory effect", i.e., the microstructural features of MMO were almost identical as was determined for LDHs. Besides, the microstructure of investigated samples was not dependent on the annealing temperature and substitution. The synthesized Mg(M)–Al MMO samples exhibited type IV isotherms independent of the annealing temperature. At higher pressure values, the H1 hystereses were detected, which are characteristic for the mesoporous (pore size in the range of 2–50 nm) materials. It was found that the pore size distributions obtained by the BJH method for the MMO specimens synthesized from the reconstructed Mg2/Al1 LDHs depended on both the synthesis temperature and nature of the substituent. The most narrow pore size distribution was determined for Sr-containing MMO (2.5–3.5 nm for the sample heat-treated at 800 ◦C). On the other hand, the sample with 5% mol of Ba and prepared at 950 ◦C had very broad pore size distribution. The pore diameter, wall thickness, and pore size distribution was found to be dependent on used synthesis method, heating temperature, and nature of alkali earth metal in the MMO host matrix.

**Author Contributions:** Formal Analysis, L.V., A.Z., A.I., and A.K.; Investigation, L.V., M.R., I.G.-P., A.Z., and V.P.; Resources, A.Z., A.I., and A.K.; Data Curation, L.V.; Writing—Original Draft Preparation, L.V., I.G.-P., and A.K.; Writing—Review and Editing, A.K.; Visualization, A.Z. and V.P.; Supervision, A.I. and A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a Research gran<sup>t</sup> N◦CAMAT (No. S-LB-19-2) from the Research Council of Lithuania and Belarusian Republican Found for Fundamental Research (No. 19LITG-007).

**Conflicts of Interest:** The authors declare that they have no conflict of interest.
