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Layered double hydroxides (LDHs), also called hydrotalcite-like compounds or anionic clays, consist of positively charged metal hydroxide sheets, interlayer anions and water. They can be represented by a general formula, , where M2+ and M3+ stand for divalent and trivalent metal cations respectively, and An- is the interlayer anions; y is the amount of interlayer H2O. Owing to their adjustable composition and structure, LDHs have many unique chemical properties and are used as fire-retardants [2-4], catalysts [5, 6], adsorbents [7, 8], electrochemical materials [9-11], and others [12, 13].
LDHs are mainly prepared by co-precipitation [14, 15], ion exchange , hydrothermal method [17, 18], and the calcination regeneration method . Among them, coprecipitation is the mostly used method because of its convenience and low cost. The most common precipitant is sodium hydroxide, which leads to formation of sodium salt that is cheap and difficult to sell; therefore, they may be released to the environment and result in pollution. Nevertheless, using NH3·H2O instead may solve this problem, because the byproduct ammonium salts can be used as fertilizers.
In this article, we report a much greener method to synthesize a calcium-doped [Mg3Al(OH)8]Cl. The LDH can be used as a fire-retardant or thermal stabilizer for PVC processing. By using cheap Mg(OH)2, Ca(OH)2, and AlCl3·6H2O as the metal sources, NH3·H2O as the precipitant, the synthetic reaction completes readily if certain amount of NH4Cl exists. This process produces less amount of by-product NH4Cl; and it is clean, economic and environmental friendly by means of reuse of the mother solution, and the distilled water from crystallization of NH4Cl.
MATERIALS AND EXPERIMENTAL METHODS
The reagents used in this work were all of analytical grade; they were purchased locally and used without further purification.
Decarbonated distilled water was used for all solutions. The samples were prepared by the co-precipitation method. The preparation was conducted in four steps:
(1) AlCl3·6H2O, Mg(OH)2 and Ca(OH)2 were mixed with the molar ratio of 1:2.8:0.2 and dispersed in distilled water in a flask submerged in an oil bath. The suspension was then stirred continuously for 24 h at room temperature;
(2) A certain amount of NH4Cl was added into the suspension, with control of the molar ratios of AlCl3·6H2O and NH4Cl;
(3) NH3·H2O of 2 equivalent of AlCl3·6H2O was added into the suspension, and the temperature of the oil bath was respectively maintained at 15 ºC, 60 ºC, 95 ºC and the temperature at which water is refluxing. The suspension was stirred continuously for a period of time, and then it was directly filtered, washed and dried in an oven.
(4) By distilling some of the water from the filtrate, part of NH4Cl was crystallized out. The mother solution will be used in the second batch of the previous procedure.
Powder X-ray diffraction (XRD) patterns of the samples were performed using a Bruker D8-ADVANCE X-ray diffractmeter at 40 kV and 30 mA, Cu Kα (λ = 0.15406 nm) radiation. Analyses of metals were carried out with a J-A1100 ICP spectrometer. The Fourier transformed infrared spectra (FTIR) of the LDH materials were obtained by using a Nicolet 750 spectrometer. The thermo-gravimetric analysis (TG) of the sample was conducted on a TA Instruments SDT-Q600 in static air with a rate of 10°C/min.
RESULTS AND DISCUSSION
The Principle of the Reaction
The Scheme 1 shows the flow chart of preparation of the LDH, in which the reactants are Mg(OH)2, Ca(OH)2, AlCl3·6H2O, and NH3·H2O, NH4Cl as a catalyst. The reaction is as follows, and only two products are produced:
During the preparation, all those compounds are mixed in sufficient water which makes the mechanic stirring smooth. After the formed slurry is stirred at a temperature for a few hours, it is centrifuged or filtered to separate the solid and the solution directly. The solid is washed with water for several times until all the NH4Cl is removed. Each filtrate is stored in a separate container, which are marked as the mother solution, the first washing liquid, the second washing liquid, etc. By distilling the mother solution under reduced pressure, the crystalline NH4Cl can be obtained, meanwhile, the obtained distilled water is used for the last washing of the LDH. The saturated NH4Cl solution supplied by the first washing liquid is used in the next batch of preparation.
Because of the low solubility of Mg(OH)2, which prefers to stay isolated from the reaction liquid, it is important to find a way to make it more reactive. It is known that Mg2+ can be released from Mg(OH)2 in a solution of ammonium salt because of the following equilibrium:
The equilibrium constant K of the reaction can be calculated by Eq. (3), in which NH4OH is used instead of NH3·H2O just for explicitness (it is known that there is no compound as stated by NH4OH, consequently, NH3·H2O is suggested to be used instead of NH4OH):
Furthermore, the solubility product constant of Mg(OH)2 and dissociation constant of are from the CRC Handbook of Chemistry and Physics, Version 2010. Suppose the Mg2+ and NH3·H2O are solely produced by dissolution of Mg(OH)2 in solution, the concentration of NH3·H2O is twice of that of Mg2+, which depend on the concentration of . Consequently, the molar concentration of is 0.47 mol L−1 if that of Mg2+ is 0.1 mol L−1; and it is 0.01 mol L−1 if that of Mg2+ is 0.01 mol L−1. Thus, the molar ratios of Mg2+ and NH4Cl should be 1:4.7 and 1:1 if the concentrations of Mg2+ is 0.1 and 0.01 mol L−1. Therefore, to make the Mg(OH)2 reactive, the amount of NH4Cl added into the suspension should be at least the same amount of Mg(OH)2.
The Influence of NH4Cl on the Co-Precipitation Reaction
To verify the influence of NH4Cl on the reaction, three experiments were done according to the above theoretical considerations, in which NH4Cl is separately added to the suspension of Mg(OH)2 and AlCl3 in water to make the molar ratios of AlCl3·6H2O and NH4Cl be 1:0, 1:1 and 1:3, and the suspension formed were stirred for 24 h at 15°C, and then filtered. The XRD patterns of the dried solid are shown in Figure 1. It can be seen that the solids still contain Mg(OH)2, but there is clearly LDH formed when the molar ratio of Mg(OH)2 to NH4Cl is 1:3. Therefore, high concentration of NH4Cl has a positive effect on the Eq. (4):
In Eq. (4), the LDH is arbitrarily represented by [Mg3Al(OH)8]Cl, but it should be known that the composition may be different. Here, NH4Cl does not appear in the reaction, it therefore accelerates the reaction just like a catalyst.
Synthesis of LDH by Using NH3·H2O as Precipitant
According to Eq. (4), some MgCl2 will be a by-product if there is no more base added. In this research we chose NH3·H2O as the base. Figure 2 shows the XRD patterns of the product when NH3·H2O of 2 equivalent of AlCl3·6H2O is added into the suspension and stirred at 15°C for 24 h. It can be seen that addition of NH3·H2O forms more LDH. This is because NH3·H2O not only satisfies the requirement of hydroxyls, but also generates more in the reaction (Eq. (1)), which accelerates the reaction as shown above. Consequently, the reaction is self-catalyzed.
Although the addition of NH3·H2O makes the characteristic diffraction peaks of layered double hydroxides at 11.3°, 23.2°, and 34.9° much stronger, the peaks from magnesium hydroxide persist. We believe it is caused by low reaction speed; therefore, we carried out the following experiments at elevated temperatures.
Effects of Temperature and Time on the Synthesis of LDHs
The following experiments are carried out with a proportion of AlCl3·6H2O : Mg(OH)2 : Ca(OH)2 : NH4Cl : NH3·H2O as 1:2.8:0.2:3:2. Figure 3 shows the powder X-ray diffraction patterns of the products prepared at 15°C, 60°C, 95°C and the refluxing temperature of water, respectively. As it is shown in Figures 3a and 3b, attempts at 15°C and 60°C cannot synthesize pure phase LDH in short time because Mg(OH)2 persists even when the reaction time are extended to 24 and 14 h. When the temperature is raised to 95°C, the diffraction peaks of Mg(OH)2 gradually reduce in intensity as time prolongs, and they disappear eventually after 10 h; when the temperature is raised to 102°C, only 4 h are taken for the diffraction peaks of Mg(OH)2 completely vanished.
The XRD patterns of the pure sample in Figure 3c after 12 h and Figure 3d after 4 h can be indexed with a hexagonal cell. The lattice parameters a, b, and c are 3.07, 3.07, and 23.38, which are calculated according to the different reflection peaks. The corresponding d-spacing is 0.78 nm, which is consistent with the reported result .
It can be observed in Figure 3d that the crystallinity of the LDH increases as the time prolongs, because crystals grow more rapidly at higher temperature . For the best of usage, good crystallinity may not be needed; therefore, we chose the 4 h at refluxing temperature as the optimal condition for the reaction.
The Solid–Liquid Ratio of the Reactants
We understand that water is an important resource for lives on the earth, we do not want use too much of it although it is cheap. Two experiments are compared with solid-liquid ratios as 1:1.5 and 1:5 to minimize the use of water, and the results are shown in Figure 4. Obviously, both ratios make pure LDH. It has been found that the solid–liquid ratio must be lower than 1:1.5 in order to smooth the mechanical stirring.
The Reuse of the NH4Cl in the Mother Solution
The mother solution of the first batch of preparation is reused for the next batch of preparation, as it contains mainly NH4Cl and tiny amount of Mg2+, Ca2+, and Al3+ chlorides. Consequently, no more NH4Cl from outside source will be used throughout next batches of preparation. Figure 5 shows three batches of such preparations, and it shows that all succeed as expected.
Obviously, more and more NH4Cl will be produced as the batch number increases; some NH4Cl has to be crystallized out of the reaction system. To investigate how much of NH4Cl should be removed by crystallization, four parallel experiments were carried out, in which the content of NH4Cl from 58 mL of the mother solution of previous preparation is about 23.9 g. After some NH4Cl is crystallized by evaporation of water, the mother solution is used for the next preparation, maintaining the optimized conditions for the preparation.
It has been found that when the 58 mL of the mother solution was distilled under reduced pressure to 50, 47, 44, and 42 mL, 4, 6, 7, and 8 g of NH4Cl was crystallized, respectively. The remaining mother solutions were diluted to 78 mL with the first washing liquid of the product, and were used to synthesize next batch of LDH by providing NH4Cl. The XRD patterns of the four new products are shown in Figure 6. It is shown that there is Mg(OH)2 in the LDH if too much of NH4Cl is removed. Consequently, the original composition of the suspension, which is that the molar ratio of AlCl3·6H2O:Mg(OH)2:Ca(OH)2:NH4Cl:NH3·H2O should be maintained at 1:2.8:0.2:3:2 when decision of how much of NH4Cl is removed.
The Structural Information of As-Prepared LDH
The infrared spectrum of the sample prepared under optimized synthetic conditions is shown in Figure 7. A strong, broad band at around 3450 cm−1 is assigned to the hydroxyl groups stretching mode from layer and the interlayer water molecules . The band at 1625 cm−1 is due to δOH bending vibration . The sharp absorption band at around 1380 cm−1 is attributed to a small amount of carbonate produced by CO2 dissolved in the solution . The appearance of strong bands at 612 cm−1 and 416 cm−1 can be mostly attributed to lattice vibrations of metal-oxygen bonds vibrations.
To make sure that the LDH contained the three metals (Mg, Ca, Al), ICP analysis was carried out, and the result was shown in Table 1.
Table 1. Chemical composition of the sample
It can be seen that all the three metals were coprecipitated into the LDH, but the content of Ca2+ is very low. The reason is that Ca(OH)2 is much more soluble than Mg(OH)2 and Al(OH)3. This has been well-known for LDHs containing Ca2+. If one wants to synthesize [Ca2Al(OH)6]NO3, the molar ratio of Ca2+ to Al3+ must be increased to 2.3:1 at least .
The TG-DTA curves for the LDH are shown in Figure 8. The DTA curve displayed two apparent endothermic peaks at 113.2°C and 374°C. The first weight loss of 10.5% can be resulted from loss of interlayer water. A further weight loss of 27.5% is due to the dehydroxylation of the layers to form oxides . According to the chemical formula of the LDH, [Mg2.985Ca0.015Al(OH)8]Cl·2H2O, the loss of the 2 crystal waters results in a weight loss of 11.7%, and loss of 4 waters from dehydroxylation results in a weight loss of 23.4%. The total weight loss is 35.1%, which is in accordance with the experimental value (38.0%) since HCl may be produced during the thermal decomposition . The thermal decomposition analysis of sample indicates that it absorbs a large amount of heat during decomposition, so this material can be used for polymer thermal stabilizer, or a flame-retardant.
Calcium-doped [Mg3Al(OH)8]Cl has been successfully prepared from AlCl3·6H2O, Mg(OH)2, Ca(OH)2, NH4Cl and NH3·H2O in water under refluxing for 4 hours. This is the first time that Mg(OH)2 and Ca(OH)2 are used as raw materials for the preparation of LDHs to reduce the amount of by-product, NH4Cl.
This process produces only a solid product, the LDH, and a by-product NH4Cl in the mother solution. Because the mother solution is reused in the process, it does not release anything to the environment. By using Mg(OH)2 instead of MgCl2 as the raw material, only 1/4 of NH4Cl is formed, and the atom utilization is much higher. Therefore, it is a green chemical process.
The existence of enough amount of is a must; otherwise, pure LDH cannot be obtained in short time. However, it must be crystallized from the mother solution in case it is accumulated too much. By evaporating water from the hot mother solution under reduced pressure, required amount of NH4Cl can be crystallized and used as a fertilizer; also, the distilled water can be used to wash the LDH.
The conditions for the preparation are optimized, such as the molar ratios of reactants (AlCl3·6H2O:Mg(OH)2:Ca(OH)2:NH4Cl:NH3·H2O was 1:2.8:0.2:3:2), temperature, time, and the solid–liquid ratio. In summary, this is an environmentally attractive synthetic route.
The authors would like to thank The Office of Education, Jiangsu Province (JH07-003) for financial supports.