**2. Experimental**

Aluminium nitrate nonahydrate (Al(NO3)3·9H2O, 98.5%, Chempur, Plymouth, MI, USA), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), 99,0% Chempur, Plymouth, MI, USA), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, 99%, Chempur, Plymouth, MI, USA), strontium nitrate (Sr(NO3)2, 99.0%, Chempur, Plymouth, MI, USA) and barium nitrate (Ba(NO3)2, 99.0%, Chempur, Plymouth, MI, USA) were used as metal sources in the preparation of Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) layered double hydroxides. In the sol–gel processing, citric acid monohydrate (C6H8O7·H2O, 99.5%, Chempur, Plymouth, MI, USA) and 1,2-ethanediol (C2H6O2, 99.8%, Chempur, Plymouth, MI, USA) were used as complexing agents. Ammonia solution (NH3, 25%, Chempur, Plymouth, MI, USA) was used to change pH of the solution.

For the synthesis of Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba; x is a molar part of substituent metal) LDHs, the stoichiometric amounts of starting materials were dissolved in distilled water under continuous stirring. Citric acid was added to the above solution, and the obtained mixture was stirred for an additional 1 h at 80 ◦C. Then, 2 mL of 1,2-ethanediol was added to the resulting solution. The transparent gels were obtained by the complete evaporation of the solvent under continuous stirring at 150 ◦C. The synthesized precursor gels were dried at 105 ◦C for 24 h. The mixed metal oxides (MMO) were obtained by heating the gels at 650 ◦C, 800 ◦C, and 950 ◦C for 4 h. The Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) LDHs were obtained by reconstruction of the MMO in water at 50 ◦C for 6 h under stirring and by changing the pH of the solution to 10 with ammonia.

X-ray di ffraction (XRD) analysis was performed using a MiniFlex II di ffractometer (Rigaku, The Woodlands, TX, USA) (Cu K α radiation) in the 2θ range from 10◦ to 70◦ (step of 0.02◦) with the exposition time of 2 min per step. The morphological features of MMO samples were estimated using a scanning electron microscope (SEM) Hitachi SU-70, Tokyo, Japan. Nitrogen adsorption by the Brunauer, Emmett, and Teller (BET) and Barret method was used to determine the surface area and pore diameter of the materials (Tristar II, Norcross, GA, USA). The pore-size distribution was

evaluated by the Barrett–Joyner–Halenda (BJH) procedure. Prior to analysis, the calcined samples were outgassed at 523 K for 5 h.

#### **3. Results and Discussion**

To study the reconstruction peculiarities of sol–gel derived Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) layered double hydroxides (LDHs), the precursor gels were firstly annealed at 650 ◦C, 800 ◦C, and 950 ◦C for 4 h. The XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg2/Al1 LDHs precursor gels at different temperatures are presented in Figure 1. The XRD pattern of the sample heated at 650 ◦C had two XRD peaks, which show the formation of a mixed metal oxide (MMO) phase with an MgO-like structure (JCPDS No. 96-100-0054) [14]. Thermal treatment of the precursor gels at 800 ◦C resulted in the formation of two phases, namely MMO and a low-crystallinity spinel phase with the composition of MgAl2O4 (JCPDS No. 96-154-0776). After heating at 950 ◦C, evidently, the highly crystalline MgAl2O4 phase has formed along with the MgO phase.

**Figure 1.** XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg2/Al1 precursor gels at 650 ◦C, 800 ◦C, and 950 ◦C.

The XRD patterns of the Mg/Al LDH synthesized by the indirect sol–gel method (reconstruction of sol–gel derived MMO) show the formation of layered double hydroxides independent of the annealing temperature of the precursor gels (see Figure 2).

Three basal reflections typical of an LDH structure were observed: at 2θ of about 10◦ (003), 23◦ (006), and 35◦ (009) [14,15]. Besides, the spinel phase obtained at 800 ◦C and 950 ◦C remain almost unchanged during the reconstruction process. These results are in good agreemen<sup>t</sup> with those previously published elsewhere [38].

The XRD patterns of synthesis products with the same substitutional level of Ca, Sr, and Ba obtained at 800 ◦C and 950 ◦C are almost identical and revealed in all cases with the formation of crystalline magnesium oxide, magnesium spinel phase MgAl2O4 and an appropriate spinel of alkaline earth metal (CaAl2O4, SrAl2O4 and BaAl2O4). Again, during the partial reconstruction process, the phase purity of sol–gel derived Mg2−<sup>x</sup>Mx/Al1 LDHs evidently is dependent on the nature of introduced metal. As was expected, the spinel phases obtained at 800 ◦C and 950 ◦C remained almost unchanged during the partial reconstruction process. Moreover, during the reconstruction process, a negligible amount of metal carbonates (CaCO3, SrCO3, and BaCO3) have formed as well. The XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr, Ba) precursor gels at different temperatures and reconstructed LDHs are presented in Figures 3–5, respectively. The XRD analysis results confirmed that the phase purity of alkaline earth substituted LDHs obtained by an indirect sol–gel synthesis approach is highly dependent on both the annealing temperature of the precursor gels and that of the alkaline earth metal.

**Figure 2.** XRD patterns of sol–gel derived Mg2/Al1 layered double hydroxides (LDHs, reconstructed from MMO). The annealing temperature of the precursor gels was 650 ◦C, 800 ◦C, and 950 ◦C.

**Figure 3.** XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg1.95Ca0.05/Al1 precursor gels at 800 ◦C and 950 ◦C (**top**) and reconstructed Mg1.95Ca0.05/Al1 LDHs (**bottom**).

**Figure 4.** XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg1.95Sr0.05/Al1 precursor gels at 800 ◦C and 950 ◦C (**top**) and reconstructed Mg1.95Sr0.05/Al1 LDHs (**bottom**).

The obtained mixed metal LDH samples were repeatedly heated at different temperatures to obtain MMO and compare the phase composition, morphology, and surface properties with obtained ones after initial annealing. The XRD patterns of non-substituted and Ca, Sr, and Ba containing MMO obtained after the heating of LDHs are shown in Figures 6 and 7, respectively. Evidently, the XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg2/Al1 precursor gels (see Figure 1) and obtained by heating the Mg2/Al1 LDHs (Figure 6) are very similar, confirming the same phase composition. However, the reflections of just obtained MMO are more intense in comparison with ones presented in the repeatedly obtained MMO from LDHs. Obviously, the second time obtained Ca and Sr substituted MMO samples contain much more side phases (see Figures 3, 4 and 7). However, this is not the case for the Ba-substituted MMO samples. Both synthesis products obtained from precursor gels and by heating LDHs were composed of several crystalline phases.

**Figure 5.** XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg1.95Ba0.05/Al1 precursor gels at 800 ◦C and 950 ◦C (**top**) and reconstructed Mg1.95Ba0.05/Al1 LDHs (**bottom**).

**Figure 6.** XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg2/Al1 LDHs at different temperatures.

**Figure 7.** XRD patterns of mixed metal oxides (MMO) obtained by heating the Mg1.95Ca0.05/Al1 LDHs (**bottom**), Mg1.95Sr0.05/Al1 LDHs (**middle**), and Mg1.95Ba0.05/Al1 LDHs (**top**) at different temperatures.

The interplanar spacings and lattice parameters of sol–gel derived Mg2−<sup>x</sup>Mx/Al1 (M = Ca, Sr and Ba) LDHs with standard deviations in parentheses are presented in Table 1.


**Table 1.** The interplanar spacings and lattice parameters of sol–gel derived Mg1.95 M0.05/Al1 (M = Ca, Sr and Ba) LDHs.

Surprisingly, the calculated values of parameter *a* are not increasing monotonically with the increasing ionic radius of metal in Mg1.95 M0.05/Al1. On the other hand, the amount of substituent is rather small, and the obtained LDHs were not fully monophasic.

Figures 8–11 show the morphological features of non-substituted and alkaline earth metal-substituted LDHs and MMO obtained by heating the precursor gels or Mg2/Al1 LDHs. The SEM micrographs of Mg–Al MMO obtained by heating Mg–Al–O precursor gel (Figure 8) confirm that the surface of synthesized compounds is composed of large monolithic particles at about 15–20 μm in size independent of the annealing temperature (800 ◦C and 950 ◦C). The surface of these monoliths is randomly covered with smaller needle-like particles, and some pores also could be detected. The SEM micrographs of reconstructed from MMO Mg2/Al1 LDH samples showed di fferent morphological features. The formation of round particles (3–15 μm) could be observed, and these particles are composed of nanosized plate-like crystallites. The most interesting observation is that the surface morphology of MMO samples obtained by heating Mg2/Al1 LDH specimens show "memory e ffect". In this case, the surface morphology of MMO is almost identical to the morphology of primary Mg2/Al1 LDHs. On the other hand, the morphological features of di fferently obtained MMO (MMO obtained by heating Mg–Al–O precursor gel and MMO obtained by heating Mg2/Al1 LDHs) di ffer considerably (see Figure 8).

The SEM micrographs of MMO obtained by heating the Mg1.95Ca0.05/Al1 precursor gels, sol–gel derivedMg1.95Ca0.05/Al1 LDHs, andMMO obtained by heating theMg1.95Ca0.05/Al1 LDHs are presented in Figure 9. The surface of Ca containing MMO obtained by heating the Mg1.95Ca0.05/Al1 precursor gels is composed of large monolithic particles (≥20 μm). Apparently, the di fferent morphological features could be determined for the reconstructed Mg2−<sup>x</sup>Cax/Al1 LDH samples. The plate-like crystals with sizes of 5–15 μm composed of nanosized plate-like crystallites have formed. An almost identical microstructure was observed for the MMO specimens obtained after heating Mg2−<sup>x</sup>Cax/Al1 LDH samples. SEM micrographs of strontium containing MMO and related Mg2−<sup>x</sup>Srx/Al1 LDHs are presented in Figure 10. The surface microstructure of Sr-containing MMO obtained by heating the Mg1.95Sr0.05/Al1 precursor gels is very similar to the Ca-containing ones. However, on the surface of plate-like crystals of reconstructed Mg2−<sup>x</sup>Srx/Al1 LDH samples, additionally spherical particles (approximately 1 μm) were determined. These spherical particles as a "memory e ffect" remain on the surface of already heat-treated Sr containing LDHs. Again, the microstructure of investigated samples was not dependent on the annealing temperature. Interestingly, the barium containing Mg2−<sup>x</sup>Bax/Al1 LDH samples showed the formation of smaller LDH particles (2–5 μm) (Figure 11). The formation of plate-like crystals of MMO with the size of 7.5–12.5 μm was observed by heating these LDHs at elevated temperatures.

**Figure 8.** SEM micrographs of MMO obtained by heating the Mg2/Al1 precursor gels (**top**), sol–gel derived Mg2/Al1 LDHs (**middle**) and MMO obtained by heating the Mg2Al1 LDHs (**bottom**). Annealing temperatures: 800 ◦C (**A**) and 950 ◦C (**B**).

**Figure 9.** SEM micrographs of MMO obtained by heating the Mg1.95Ca0.05/Al1 precursor gels (**top**), sol–gel derived Mg1.95Ca0.05/Al1 LDHs (**middle**) and MMO obtained by heating the Mg1.95Ca0.05/Al1 LDHs (**bottom**). Annealing temperatures: 800 ◦C (**A**) and 950 ◦C (**B**).

**Figure 10.** SEM micrographs of MMO obtained by heating the Mg1.95Sr0.05/Al1 precursor gels (**top**), sol–gel derived Mg1.95Sr0.05/Al1 LDHs (**middle**), and MMO obtained by heating the Mg1.95Sr0.05/Al1 LDHs (**bottom**). Annealing temperatures: 800 ◦C (**A**) and 950 ◦C (**B**).

The results received by the BET method on Mg–Al MMO obtained by heating Mg–Al–O precursor gel and Mg2/Al1 LDHs are presented in Figure 12. Interestingly, these results of MMO obtained from Mg2/Al1 LDHs are comparable with those determined for the Mg3/Al1 LDH samples [18]. These samples exhibit type IV isotherms independent of the annealing temperature. At higher pressure values, the H1 hystereses are seen. This type of hysteresis is characteristic for the mesoporous (pore size in the range of 2–50 nm) materials. However, in the case of the MMO obtained by heating Mg–Al–O precursor gel, the steep increase at relatively low pressures let us predict the type of H4 isotherms, especially for the MMO samples obtained at lower temperature (800 ◦C). The surface area of these MMO samples evidently depends on the synthesis temperature.

**Figure 11.** SEM micrographs of MMO obtained by heating the Mg1.95Ba0.05/Al1 precursor gels (**top**), sol–gel derived Mg1.95Ba0.05/Al1 LDHs (**middle**) and MMO obtained by heating the Mg1.95Ba0.05/Al1 LDHs (**bottom**). Annealing temperatures: 800 ◦C (**A**) and 950 ◦C (**B**).

Thus, the isotherms and hystereses are dependent on both synthesis pathway and annealing temperature. The nitrogen adsorption–desorption results obtained for the mixed metal oxides containing Ca, Sr, and Ba (Figure 13) demonstrated that the N2 adsorption–desorption isotherms show very similar trends.

**Figure 12.** Nitrogen adsorption–desorption isotherms of mixed metal oxides (MMO) obtained by heating the Mg2/Al1 precursor gels (**top**) and obtained by heating the Mg2Al1 LDHs (**bottom**) at 800 ◦C and 950 ◦C.

However, in the case of barium-substituted MMO samples synthesized at 800 ◦C, the determined N2 adsorption–desorption isotherms exhibited same type of isotherms independent of the synthesis method.

The results of the BET analysis of MMO samples are summarized in Table 2.

**Figure 13.** Nitrogen adsorption–desorption isotherms of mixed metal oxides (MMO) obtained by heating the Mg1.95Ca0.05/Al1 precursor gels (**top, left**), by heating the Mg1.95Ca0.05/Al1 LDHs (**top, right**), by heating the Mg1.95Sr0.05/Al1 precursor gels (**middle, left**), by heating the Mg1.95Sr0.05/Al1 LDHs (**middle, right**), by heating the Mg1.95Ba0.05/Al1 precursor gels (**bottom, left**), and by heating the Mg1.95Ba0.05/Al1 LDHs (**bottom, right**) at 800 ◦C and 950 ◦C.

Figure 14 shows the pore size distributions obtained by the BJH method for the MMO specimens obtained by heating Mg–Al–O precursor gels and Mg2/Al1 LDHs. Both samples demonstrate narrow pore size distributions (PSD) almost at the mesoporous level, but very close to micropores domain.


**Table 2.** Brunauer, Emmett and Teller (BET) surface area of sol–gel derived Mg1.95M0.05/Al1 (M = Ca, Sr and Ba) MMO.

**Figure 14.** The pore size distribution of mixed metal oxides (MMO) obtained by heating the Mg2/Al1 precursor gels (**top**) and obtained by heating the Mg2/Al1 LDHs (**bottom**) at 800 ◦C and 950 ◦C.

Surprisingly, the PSD width does not depend neither on the synthetic procedure nor on the annealing temperature. The determined average pore diameter in the mesopore region is approximately 3.0–5.5 nm. The PSD results obtained for the mixed metal oxides containing Ca, Sr, and Ba are shown in Figure 15.

**Figure 15.** The pore size distribution of mixed metal oxides (MMO) obtained by heating the Mg1.95Ca0.05/Al1 precursor gels (**top, left**), by heating the Mg1.95Ca0.05/Al1 LDHs (**top, right**), by heating the Mg1.95Sr0.05/Al1 precursor gels (**middle, left**), by heating the Mg1.95Sr0.05/Al1 LDHs (**middle, right**), by heating the Mg1.95Ba0.05/Al1 precursor gels (**bottom, left**), and by heating the Mg1.95Ba0.05/Al1 LDHs (**bottom, right**) at 800 ◦C and 950 ◦C.

As seen, various surface properties could be detected for the MMO samples synthesized by two different methods. The pore size distribution of directly obtained MMO by heating Mg–Al–O precursor gels depends on the heating temperature and less on the nature of alkaline earth metal. The determined average pore diameter in the mesopore region is approximately 2.5–8 nm, 2.5–7 nm, and 2.5–9 nm

for the Ca-MMO, Sr-MMO, and Ba-MMO samples, respectively, synthesized at 800 ◦C. The pore size distribution is visible wider for the MMO synthesized at 950 ◦C (approximately 3–10.15 nm, 3–12.5 nm, and 3.5–10.1 nm for the Ca-MMO, Sr-MMO, and Ba-MMO samples, respectively). As seen from Figure 15, the pore size distributions obtained by the BJH method for the MMO specimens synthesized from the reconstructed Mg2/Al1 LDHs depends on both synthesis temperature and nature of 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 has very broad pore size distribution. In general, the gain in the volume of mesopores is clearly visible for the MMO samples synthesized at lower temperature. However, the pore diameter, wall thickness, and pore size distribution depend on the used synthesis method, heating temperature, and nature of alkali earth metal in the MMO host matrix, indicating that these MMO could have the potential for the application as catalysts, catalyst supports, and adsorbents.
