Synthesis of magnesium hydroxide and its calcinates by a precipitation method with the use of magnesium sulfate and poly(ethylene glycols)
Graphical abstract
The influence of PEG modifiers on unique physicochemical properties of magnesium hydroxide obtained by precipitation was presented. Magnesium hydroxide obtained from magnesium sulphate and sodium hydroxide was subjected to thermal decomposition at 450 °C. The synthesised samples of magnesium hydroxide and magnesium oxide have the crystalline structures of brucite and periclase, respectively. Dielectric study of MgO modified with PEG exhibits insulating properties.
Highlights
► Application of poly(ethylene glycols) in synthesis of Mg(OH)2 and MgO. ► Mechanism of the adsorption modes of PEG on the Mg(OH)2 nanoplates surfaces. ► The influence of modifiers on Mg(OH)2 and MgO dispersive properties. ► The effect of PEGs on the MgO dielectric properties.
Introduction
Magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2) have been extensively studied. Magnesium and its alloys possess unique properties such as low weight and non-toxicity [1]. These properties are advantageous for various devices in the fields of electronics, aerospace, automotive applications and biomedicine [2], [3], [4]. Magnesium oxide, a very important wide-band gap insulator, has attracted much attention due to its applications in catalysis [5], toxic waste remediation [6], as additives in refractory, paint, and superconductor products [7], [8], [9], and in steel manufacturing because of its high corrosion-resistant behaviour [10]. Magnesium hydroxide is commonly used as a flame-retardant filler in composite materials due to its ability to undergo endothermic dehydration in fire conditions [11], [12]. It has also been used as an acidic waste neutralizer in environmental protection, in paper manufacturing, as a fertiliser additive [13], [14], [15], and as the most important precursor for the synthesis of magnesium oxide [16].
Recently, much attention has been paid to the synthesis of MgO and Mg(OH)2 nanostructures. Mg(OH)2 nanostructures with versatile morphological structures can be prepared by several methods, such as electrodeposition [17], sol–gel technique [18], precipitation [19], [20], [21], hydrothermal [22], solvothermal [23], preparation using a bubbling setup [24], and microwave assisted synthesis [25]. Several reports have demonstrated that these structures can be converted into each other (i.e. MgO↔Mg(OH)2) by either hydration [26] or dehydration [27] procedures. Generally, the final properties of nanocrystals strongly depend on their shape, agglomeration state and preparation process.
In this study magnesium hydroxide was obtained by precipitation with addition of non-ionic compounds representing a group of poly(ethylene glycols) with different molecular mass. The properties of the modified magnesium hydroxide were compared with those of the unmodified product. Magnesium oxide was obtained by calcination at 450 °C. The literature contains only a few reports about modification of the synthesis of magnesium hydroxide and oxide with PEG. Wei Wang [28] has provided evidence of the influence of PEG 400 on the formation of nanosized MgO, and Peipei Wang [29] has reported a method of synthesis of magnesium hydroxide with different morphology by precipitation with the addition of PEG 1200 as modifier. In the same study the conditions of the process, such as the magnesium salt and ammonium concentration and temperatures of the reaction, were analysed, and an interesting mechanism of crystal growth was proposed. The influence of temperature on the type of modifier adsorption on the crystal walls as well as on the development of different morphologies (nanoneedles, nanodiscs) was demonstrated.
In this paper we describe a simple method to synthesise Mg(OH)2 and MgO nanoplates via chemical precipitation using sodium hydroxide as precipitating agent and magnesium sulfate as a precursor in the presence of the poly(ethylene glycols). The use of poly(ethylene glycols) with different molecular mass and different concentrations, and detailed discussion of the influence of this parameter on products properties, undoubtedly represents the originality of this work compared with the previous literature reports. Another point of interest is the proposed mechanism of the adsorption modes of PEG. The morphology of magnesium hydroxide was characterised using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Also the products' dispersive properties (non-invasive backscattering technique —NIBS), wettability profiles, Fourier transform infrared spectroscopy (FT-IR) data, and thermogravimetry and differential thermal analysis (TG/DTA), were determined. Calcined magnesium hydroxides were subjected to structural identification by the wide angle X-ray scattering method (WAXS). Fundamental adsorption parameters such as surface area, pore volume and mean pore diameter (based on the Brunauer–Emmett–Teller and Barrett–Joyner–Halenda methods) were found for magnesium hydroxide and oxides. As MgO is a potential insulating material, the dielectric properties of modified magnesium oxide were also investigated.
Section snippets
Materials
To obtain magnesium hydroxide and then magnesium oxide, the substrates used were hydrated magnesium sulfate (analytical grade) and sodium hydroxide (also of analytical grade) in the form of solutions, produced by POCh SA. Non-ionic compounds from the group of poly(ethylene glycols), PEG 200, PEG 8000 and PEG 20 000 purchased from Sigma-Aldrich, were used as modifiers.
Magnesium hydroxide and MgO synthesis
Precipitation of magnesium hydroxide was performed in a reactor of capacity 500 cm3 equipped with a high-speed propeller stirrer,
Dispersive properties
Table 2 presents data on the dispersive properties of magnesium hydroxide samples precipitated with the use of magnesium sulfate at 80 °C, with an excess of salt (MgSO4/NaOH→1.5:1). Samples 2–9 were subjected to in situ modification by non-ionic compounds: PEG 200, PEG 8000, PEG 20 000 used at concentrations 1 wt.%, 2 wt.% or 5 wt.%. To emphasise the effects of this modification, for the sake of comparison, Table 2 also displays the data for the unmodified sample.
As a result of addition of a
Conclusions
Magnesium hydroxide with plate morphology was obtained by precipitation with the use of magnesium salt and ammonium hydroxide as precipitating agent. The samples were modified with PEGs of different weights and in different concentrations. Introduction of a 2 wt.% poly(ethylene glycol) (PEG 200) solution into the reaction system leads to a product of hydrophobic character consisting of particles with diameters of 28–79 nm. This modified sample of Mg(OH)2 shows a relatively large BET surface area
Acknowledgements
This work was supported by Poznan University of Technology research grant no. 32-125/2012−DS.
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