## 2 Materials and Methods  
The following chemicals were used for the synthesis: MgCl\({}_{2}\)6H\({}_{2}\)O (\(>\)99%, Penta s r.o., Prague, Czech Republic) and MgO (\(>\)98%, Penta s r.o., Prague, Czech Republic). Deionized water (16.8 M\(\Omega\)) was used for all syntheses. The stoichiometric ratio 5 MgO to MgCl\({}_{2}\)6H\({}_{2}\)O was used for the synthesis of MOC phase with composition 5 Mg(OH)\({}_{2}\)-MgCl\({}_{2}\)\(\cdot\)8H\({}_{2}\)O (also known as Phase 5, MOC 5-1-8, and Mg\({}_{3}\)(OH)\({}_{5}\)Cl\(\cdot\)4H\({}_{2}\)O). According to the well-established notation, our sample was named MOC 5-1-8. To prepare the desired phase, we dissolved 15.32 g of MgCl\({}_{2}\)\(\cdot\)6H\({}_{2}\)O in 9.48 g of deionized water. Next, an amount of 15.18 g of magnesium oxide was added to the solution, and then the suspension was intensively stirred for five minutes. The suspension was then divided into two parts, where one was applied to the X-ray powder diffraction (XRD) holder for the analysis of kinetics of formation, and the other one was poured into a plastic beaker for other analyses. The following equation (Equation (1)) summarizes the formation of MOC 5-1-8:  
\[5\text{MgO }+\text{ MgCl}_{2}\cdot\text{6H}_{2}\text{O }+\text{ 3H}_{2}\text{O }\to 5\text{Mg(OH)}_{2}\cdot\text{MgCl}_{2}\cdot 8\text{H}_{2}\text{O} \tag{1}\]  
Scanning electron microscopy (SEM) was used to study the surface morphology (Tescan MAIA3, TSCAN Brno, s.r.o., Brno, Czech Republic). The elemental composition and mapping were characterized using an energy-dispersive spectroscopy (EDS) analyzer (X-Max150, Oxford Instruments, High Wycombe, UK) with a 20-mm\({}^{2}\) SDD (silicon drift detector) detector and AZtecEnergy software (Oxford instruments, HighWycombe, UK). The sample was set on a carbon conductive tape in order to unsure the conductivity of the experiments. For both SEM and SEM-EDS analysis, the electron beam was set to 10 kV with 10-mm working distance. Simultaneous thermal analysis (STA) was performed using the Setsys Evolution apparatus from Setaram in a temperature range up to 1300 \({}^{\circ}\)C. The measurements were performed in a dynamic helium atmosphere with a flow rate of 50 mL/min and a heating rate of 10 \({}^{\circ}\)C/min. The mass spectrometer OmniStar\({}^{\text{TM}}\) from Pffeifer Vacuum (Pffeifer Vacuum GmbH, Aflar, Germany) was used in order to analyze gases that evolved during the heating. X-ray powder diffraction (XRD) was carried out using a Bruker D2 Phaser, a powder diffractometer with Bragg-Brentano geometry, applying CuK\({}_{\alpha}\) radiation (\(\lambda\) = 0.15418 nm, U = 30 kV, I = 10 mA) and rotation (five rounds per minute). The step size was set to of 0.02025\({}^{\circ}\) (28) and the overall data were acquired in the angular range of 5\({}^{\circ}\)-80\({}^{\circ}\). The kinetics of formation of MOC 5-1-8 was determined by taking and analyzing a test sample every 12 h for seven days. X\({}^{\prime}\)Pert HighScore Plus software (PANalytical, Almelo, Netherlands) was applied to evaluate the obtained data including quantitative phase analysis using Rietveld refinement. X-ray fluorescence (XRF) was used to determine the chemical composition of the sample with an Axios sequential WD-XRF spectrometer (PANalytical, Almelo, Netherlands) equipped with a Rh anode end-window X-ray tube fitted with a 50-\(\upmu\)m beryllium window. To collect the measured data, the software SuperO was used. The sample was pressed (with no binding agents) on a pellet of H\({}_{3}\)BO\({}_{3}\), where the total thickness was \(\sim\)5 mm, and the diameter was 40 mm. High-resolution transmission electron microscopy (HR-TEM) was performed using an EFTEM (energy-filtering transmission electron microscopy) Jeol 2200 FS microscope (Jeol, Tokyo, Japan). A 200-keV acceleration voltage was used for measurement.