*Article* **On the Reconstruction Peculiarities of Sol–Gel Derived Mg2**−**<sup>x</sup>Mx**/**Al1 (M** = **Ca, Sr, Ba) Layered Double Hydroxides**

**Ligita Valeikiene 1, Marina Roshchina 2, Inga Grigoraviciute-Puroniene 1, Vladimir Prozorovich 2, Aleksej Zarkov 1, Andrei Ivanets 2 and Aivaras Kareiva 1,\***


Received: 6 May 2020; Accepted: 30 May 2020; Published: 2 June 2020

**Abstract:** In this study, the reconstruction peculiarities of sol–gel derived Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) layered double hydroxides were investigated. The mixed metal oxides (MMO) were synthesized by two different routes. Firstly, the MMO were obtained directly by heating Mg(M)–Al–O precursor gels at 650 ◦C, 800 ◦C, and 950 ◦C. These MMO were 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). Secondly, in this study, the MMO were also obtained by heating reconstructed LDHs at the same temperatures. The synthesized materials were characterized using X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM). Nitrogen adsorption by the Brunauer, Emmett, and Teller (BET) and Barrett, Joyner, and Halenda (BJH) methods were used to determine the surface area and pore diameter of differently synthesized alkaline earth metal substituted MMO compounds. It was demonstrated for the first time that the microstructure of reconstructed MMO from sol–gel derived LDHs showed a "memory effect".

**Keywords:** layered double hydroxides; sol–gel processing; alkaline earth metals; mixed metal oxides; reconstruction effect; surface properties
