Hydrothermal conversion of magnesium oxysulfate whiskers to magnesium hydroxide nanobelts

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Materials Chemistry and Physics 109 (2008) 381–385

Hydrothermal conversion of magnesium oxysulfate whiskers to magnesium hydroxide nanobelts Xiaotao Sun, Lan Xiang ∗ Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 17 July 2007; received in revised form 8 November 2007; accepted 1 December 2007

Abstract Mg(OH)2 nanobelts with a length of 1.5–3.0 ␮m, a width of 200–250 nm and a thickness of 40–50 nm were formed by treating magnesium oxysulfate (5Mg(OH)2 ·MgSO4 ·3H2 O, abbreviated as 513MOS) whiskers in 0.5 mol L−1 NaOH solution at 200 ◦ C for 2.0 h. The mechanical mixture model was employed to estimate the standard Gibbs free energy of 513MOS and the mechanism involved in the hydrothermal conversion from 513MOS whiskers to Mg(OH)2 nanobelts was discussed thereafter. © 2007 Elsevier B.V. All rights reserved. Keywords: Magnesium hydroxide nanobelt; Magnesium oxysulfate whisker; Hydrothermal conversion; Mechanical mixture model

1. Introduction One-dimensional (1D) magnesium-bearing materials have attracted much attention in recent years owing to their specific shapes, structures and mechanical properties distinctive from those of the conventional bulk materials [1–12]. As an effective reinforcing and flame retardant filler, 1D Mg(OH)2 has been paid much attention by virtue of its 1D shape, high decomposition temperature (340 ◦ C) and wide application in reinforcing thermoplastics or alloys. As an ecological and environmentfriendly flame retardant, 1D Mg(OH)2 can act simultaneously as a reinforcing agent, a flame retardant and a smoke suppressant additive with low or zero emissions of toxic or hazardous substances. Mg(OH)2 was usually produced from magnesium chloride contained in seawater or natural brines, using alkaline sources (e.g. calcium hydroxide, ammonia or sodium hydroxide) as the precipitation agents. Ways of synthesizing Mg(OH)2 with 1D shape have been sought via careful control of the crystallization parameters, such as solvents, supersaturation, temperature, the specific impurity level, etc. It was reported that 1D Mg(OH)2 with a diameter of 10–95 nm and a length of 100–4000 nm can

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be synthesized by careful mixing of MgCl2 , NH4 OH and NaOH solution at temperature lower than 10 ◦ C [13,14]. The Mg(OH)2 rods with a diameter of about 30 nm and a length of 3.0 ␮m were prepared by mixing 0.1 mol L−1 MgSO4 and ammonia solution at room temperature followed by the hydrothermal treatment of the slurry at 160 ◦ C for 16.0 h [15]. Mg(OH)2 nanoneedles were also synthesized using Mg(CH3 COO)2 and NaOH as the reactants and citrate as the additive. The bonding of citrate with OH− led to the preferential adsorption of citrate on (0 0 1) plane of Mg(OH)2 and inhibition of the growth along the [0 0 1] direction [16]. Mg(OH)2 nanorods with a diameter of about 20 nm and a length of about 200 nm were prepared by hydrothermal treatment of Mg in ethylenediamine aqueous solution. The selective interaction between the solvents and surface ions slowed the growth of specific planes [17]. Our former work showed that 513MOS whiskers can be formed under hydrothermal condition, using MgSO4 and NaOH as the reactants [2,3]. On the basis of the former work, a new hydrothermal conversion method was developed to synthesize Mg(OH)2 nanobelts, using 513MOS whiskers as the precursor and NaOH–water solution as the hydrothermal medium. The influence of the reaction time and the NaOH concentration on the hydrothermal conversion process was investigated. The mechanical mixture model [18] was used to estimate the standard Gibbs energies of 513MOS at different temperatures and to discuss the thermodynamic behaviors.

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Fig. 1. Influence of reaction time on the morphology of hydrothermal products. Time (h): (a) 2.0, (b) 6.0, temperature (◦ C): 200.

2. Experimental 2.1. Experimental procedure 2.1.1. Preparation of 513MOS whiskers [2,3] Commercial NaOH and MgSO4 ·7H2 O with an analytical grade were provided by Beijing Chemical Regent Factory of China. For preparing 513MOS whiskers, 20 mL of 2–4 mol L−1 NaOH was injected (2 mL min−1 ) into 25.0 mL of 1.5–2 mol L−1 MgSO4 . The slurry was then transferred to a Teflon-lined stainless steel autoclave with an inner volume of 80 cm3 . The autoclave was heated (5 ◦ C min−1 ) to 200 ◦ C and kept in isothermal condition for 2.0–6.0 h. After hydrothermal treatment, the autoclave was cooled down to room temperature naturally and the 513MOS product was filtrated, washed and dried at 105 ◦ C for 12.0 h. 2.1.2. Preparation of Mg(OH)2 nanobelts 3.0 g of the as-prepared 513MOS whiskers and 50 mL of 0– 2.0 mol L−1 NaOH solution were added and mixed in the Teflon-lined stainless steel autoclave. The autoclave was then heated (5 ◦ C min−1 ) to 200 ◦ C and kept under isothermal condition for 2.0 h. After hydrothermal treatment, the autoclave was cooled down to room temperature naturally and the hydrothermal product was filtrated, washed and dried at 105 ◦ C for 12.0 h.

2.2. Analysis The morphology of the samples was examined by the field emission scanning electron microscopy (FESEM, JSM7401F, JEOL, Japan). The composition and structure of the samples were identified by X-ray powder diffraction (XRD, ˚ and the D/max2500, Rigaku, Japan), using Cu K␣ radiation (λ = 1.54178 A) selected area electron diffraction (SAED).

phase at hydrothermal condition. MgSO4 , a part of 513MOS, was partially dissolved with the prolongation of the hydrothermal reaction time, producing Mg(OH)2 particles with irregular morphology. The corresponding reaction concerned with the hydrothermal conversion of 513MOS to Mg(OH)2 can be written as follow: 5Mg(OH)2 ·MgSO4 ·3H2 O(s) = 5Mg(OH)2(s) + Mg2+ + SO4 2− + 3H2 O

(1)

3.2. Conversion of 513MOS to Mg(OH)2 in NaOH solution Figs. 3 and 4 show the morphology and the XRD composition of the products formed by treating the 513MOS whiskers at 200 ◦ C for 2.0 h in 0–2.0 mol L−1 NaOH solution, respectively. Mg(OH)2 and 513MOS whiskers coexisted if the NaOH concentrations were lower than 0.2 mol L−1 . Mg(OH)2 nanobelts with a length of 1.5–3.0 ␮m, a width of 200–250 nm and a thickness of 40–50 nm, coexisted with minor amount of irregular Mg(OH)2 particles, were synthesized in 0.5 mol L−1 NaOH solution. The SAED pattern shown in Fig. 3c indicated the good crystallization of Mg(OH)2 nanobelts and its preferen-

3. Results and discussion 3.1. Stability of 513MOS whiskers Figs. 1 and 2 show the morphology and the XRD patterns of the hydrothermal products formed after treating the slurry containing MgSO4 and NaOH solutions at 200 ◦ C for 2.0 and 6.0 h. 513MOS whiskers with a length of 50–80 ␮m and a diameter of 0.5–1.5 ␮m were prepared after hydrothermal treatment of the slurry at 200 ◦ C for 2.0 h (Fig. 1a). Some of the 513MOS whiskers were resolved and converted to Mg(OH)2 when the reaction time was prolonged to 6.0 h, leading to the decrease of the length of the 513MOS whiskers to 30–50 ␮m and the occurrence of the Mg(OH)2 fine particles(Fig. 1b). The above results indicated that the 513MOS whisker was a metastable

Fig. 2. Influence of reaction time on the composition of the hydrothermal products. Time (h): (a) 2.0, (b) 6.0, temperature (◦ C): 200.

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Fig. 3. Influence of NaOH concentration on the morphology of hydrothermal products. NaOH concentration (mol L−1 ): (a) 0, (b) 0. 2, (c) 0.5, (d) 2.0, temperature (◦ C): 200.

tial orientation along [1 1 0] direction. More Mg(OH)2 plates were produced if the NaOH concentration increased up to 2.0 mol L−1 . The above work showed that the existence of appropriate amount of NaOH was favorable for the hydrothermal conversion of 513MOS whiskers to Mg(OH)2 nanobelts. In the case of low NaOH concentration, OH− was insufficient to react with Mg2+ produced by the dissolution of MgSO4 in 513MOS, leading to the incomplete conversion of 513MOS whiskers to Mg(OH)2 . The increase of NaOH concentration was favorable for the conversion of 513MOS to Mg(OH)2 , but

the 1D morphology was inclined to be destroyed due to the quick formation of Mg(OH)2 as well as the quick dissolution of MgSO4 in 513MOS whiskers if the NaOH concentration was too high. Figs. 5 and 6 show the morphology and the composition of the products formed by treating the 513MOS whiskers in 0.5 mol L−1 NaOH at 25 ◦ C, respectively. 513MOS whiskers remained no changes after 2.0 h of reaction. Mg(OH)2 phase, existed as both 1D morphology with a length of 1.0–2.0 ␮m and irregular particles, occurred after 120.0 h of reaction, revealing that 513MOS whiskers could also be converted to Mg(OH)2 at room temperature, but the conversion speed was lower than that at elevated temperature. Meanwhile, it was also difficult to maintain the 1D morphology at room temperature. 3.3. Thermodynamic discussion From the thermodynamic viewpoint, the hydrothermal conversion process of 513MOS to Mg(OH)2 in NaOH–water solution can be described as follows. Firstly 513MOS and NaOH were partially dissolved in solution (Eqs. (1) and (2)): NaOH(aq) = Na+ + OH−

Fig. 4. Influence of NaOH concentration on the composition of hydrothermal products NaOH concentration (mol L−1 ): (a) 0, (b) 0. 2, (c) 0.5, (d) 2.0, temperature (◦ C): 200.

(2)

Then Mg2+ reacted with OH− to form Mg(OH)2 precipitate: Mg2+ + 2OH− = Mg(OH)2(s)

(3)

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Fig. 5. Influence of reaction time on the morphology of products formed at 25 ◦ C. Time (h): (a) 2.0, (b) 120.0.

Fig. 7. G◦T of reactions (1)–(4). Fig. 6. Influence of reaction time on the composition of products formed at 25 ◦ C. Time (h): (a) 2.0, (b) 120.0.

The total reaction for the hydrothermal conversion of 513MOS to Mg(OH)2 : 5Mg(OH)2 ·MgSO4 ·3H2 O(s) + 2OH

−

= 6Mg(OH)2(s) + SO4 2− + 3H2 O

(4)

In order to calculate the standard Gibbs energy (G◦T ) of Eq. (4) at different temperatures, it’s essential to estimate the G◦T of 513MOS. The mechanical mixture model, which considered the complex compound as a combination of several simple compounds [18], was employed to estimate the G◦T of 513MOS

using Eq. (5). G◦T 513 MOS = 5G◦T [Mg(OH)2 ] + G◦T [MgSO4 ] + 3G◦T [H2 O]

(5)

The G◦T of the substances involved in the hydrothermal conversion of 513MOS to Mg(OH)2 , including MgSO4 , NaOH, Mg(OH)2 , H2 O, Mg2+ , Na+ , OH− and SO4 2− , were listed in Table 1 [19]. The G◦T values for reactions (1)–(4) were then calculated and shown in Fig. 7. The G◦T of reactions (1) and (2) increased while the G◦T of reaction (3) and (4) decreased with the increase of temperature from 25 to 200 ◦ C, revealing that the dissolution of MgSO4

Table 1 G◦ of substances at different temperatures (kJ mol−1 ) T (◦ C)

MgSO4 (s)

NaOH(aq)

Mg(OH)2 (s)

H2 O

Mg2+

Na+

OH−

SO4 2−

20 40 60 80 100 120 140 160 180 200

−1176.4 −1168.8 −1161.1 −1153.4 −1145.7 −1137.9 −1130.1 −1122.3 −1114.5 −1105.6

−374.9 −372.1 −369.3 −366.5 −363.7 −360.8 −358.0 −355.1 −352.3 −349.6

−835.2 −829.1 −822.9 −816.9 −810.8 −804.6 −798.5 −792.4 −786.4 −780.3

−237.9 −234.7 −231.5 −228.3 −225.2 −222.1 −218.9 −215.9 −212.9 −209.9

−454.3 −453.5 −452.6 −451.8 −450.9 −450.1 −449.3 −448.3 −447.3 −446.2

−261.5 −262.9 −264.5 −266.0 −267.6 −269.1 −270.6 −272.8 −273.4 −275.2

−158.5 −153.6 −148.4 −143.1 −137.6 −131.9 −126.1 −120.1 −113.9 −107.5

−747.1 −735.9 −724.3 −712.2 −699.8 −687.0 −673.8 −660.1 −646.0 −631.4

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was inhibited and the formation of Mg(OH)2 precipitate was promoted at higher temperatures. The negative values of the G◦T for reaction (4) (−88.8 to −90.2 kJ mol−1 ) revealed that it was thermodynamic possible for the conversion of 513MOS to Mg(OH)2 in the temperature range of 25–200 ◦ C, which was consistent with the experimental results shown in Figs. 1–6. The faster conversion rate of 513MOS to Mg(OH)2 at hydrothermal condition may be attributed to the acceleration of the reaction rate at elevated temperature. 4. Conclusions Magnesium hydroxide nanobelts with a length of 1.5–3.0 ␮m, a width of 200–250 nm and a thickness of 40–50 nm were formed by treating 513MOS whiskers in 0.5 mol L−1 NaOH–water solution at 200 ◦ C for 2.0 h. Both the thermodynamic calculation based on the mechanical mixture model and the experimental results indicated that it was thermodynamic possible for the conversion of 513MOS to Mg(OH)2 in the temperature range of 25–200 ◦ C. Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 50574051) and the Open Foundation of Tsinghua University.

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