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Crystallization of synthetic hydrotalcite under hydrothermal conditions
Applied Clay Science 28 (2005) 101 – 109 www.elsevier.com/locate/clay
Crystallization of synthetic hydrotalcite under hydrothermal conditions Frantisˇek Kovandaa,*, David Kolousˇeka, Zuzana Cı´lova´b, Va´clav Hulı´nsky´b a
Department of Solid State Chemistry, Institute of Chemical Technology, Technicka´ 5, 166 28 Prague, Czech Republic b Department of Glass and Ceramics, Institute of Chemical Technology, Prague, Czech Republic Received 13 November 2003; received in revised form 20 December 2003; accepted 28 January 2004 Available online 2 July 2004
Abstract The synthetic Mg–Al hydrotalcite with Mg/Al molar ratio of 2 was hydrothermally treated in autoclaves under autogenous water vapour pressure at 120–200 8C for 2–18 h. The obtained samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements. A well-crystallized hydrotalcite-like phase was present in the coprecipitated product. No other crystalline phases were detected in the powder XRD patterns of both coprecipitated and hydrothermally treated samples. The integral intensity and full width in half maximum (FWHM) of the (003) and (006) hydrotalcite diffraction lines were evaluated in order to compare the crystallinity of samples hydrothermally treated under various conditions. The hydrothermal treatment increased the hydrotalcite content in the samples and improved significantly the hydrotalcite crystallinity. In general, the higher temperature and longer time of hydrothermal treatment, the higher hydrotalcite enrichment, as well as an increasing growth of crystallites, were observed. After the first 2–4 h, the time dependence was less evident, whereas temperature seemed to be a crucial parameter affecting the hydrothermal crystallization. The temperature of 120 8C was too low to increase significantly the sample crystallinity during several hours. According to XRD results, the marked crystallinity improvement was observed at 160 8C and higher temperatures. The hydrothermal treatment resulted in a marked decrease of surface area and a growth of hydrotalcite crystals. SEM micrographs of coprecipitated samples showed the aggregates composed of small thin crystals of ca. 0.1 Am in diameter, which were gradually transformed into thin plates with hexagonal morphology and particle size of several tenths of micrometers. Both a decrease of surface area and the crystallinity improvement of hydrothermally treated samples can be explained by the increase of hydrotalcite crystal size. D 2004 Elsevier B.V. All rights reserved. Keywords: Mg–Al hydrotalcite; Layered double hydroxide; Hydrothermal treatment; Crystallinity improvement
1. Introduction * Corresponding author. Fax: +420 2 2431 1082. E-mail address: [email protected] (F. Kovanda). 0169-1317/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2004.01.009
Hydrotalcite, a magnesium–aluminum hydroxycarbonate, is a naturally occurring mineral of chemical
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composition Mg6Al2(OH)16CO3d 4H2O exhibiting a layered crystal structure, which is comprised of positively charged hydroxide layers and interlayers composed of carbonate anions and water molecules. The ordering of hydroxide layers is similar to that of brucite, Mg(OH)2, where each Mg2+ cation is octahedrally surrounded by six OH anions and the different octahedra [Mg(OH)6]4 share edges to form infinite sheets. In the hydrotalcite, the Mg2+/Al3+ isomorphous substitution in octahedral sites of the hydroxide sheet results in a net positive charge, which has to be neutralized by interlayer anionic species. Hydrotalcite has been taken as a reference name for many isomorphous compounds, containing various MII and MIII metal cations in hydroxide layers and various interlayer anions. These materials, known as hydrotalcite-like compounds, layered double hydroxides, anionic clays, etc., have a great variety of uses in many applications. They are used in heterogeneous catalysis as catalysts, catalyst precursors and support materials, as neutralizing agents and halogen scavengers in polymer processing or in pharmacy as antacids. They are also known as good adsorbents and anion exchangers (Cavani et al., 1991; Trifiro and Vaccari, 1996; Rives, 2001). Recently they have been used in the preparation of nanocomposite materials (Leroux and Besse, 2001). The most common method applied to preparation of hydrotalcite-like compounds is coprecipitation, which is based on the reaction of a solution containing MII and MIII metal cations in adequate proportions with an alkaline solution. The products obtained by coprecipitation at low supersaturation are usually more crystalline in comparison with those prepared at high supersaturation conditions. However, the product crystallinity may be affected by various experimental parameters such as reaction pH and temperature, concentration of used solutions, flow rate during addition of reactants, hydrodynamic conditions in the reactor and/or postsynthesis operations (e.g. an ageing of obtained precipitate). A large number of studies on physical– chemical properties and applications of hydrotalcitelike compounds was reported, but only a few reports discussed the control of particle size, which is an important parameter in industrial applications. In many cases, an optimization of experimental conditions does not lead to a well-crystallized
hydrotalcite-like phase. An improvement of the crystallinity may be achieved by hydrothermal treatment in the presence of water vapour at temperatures which do not exceed the decomposition temperature of the hydrotalcite-like compound. The hydrothermal crystallization is usually carried out at temperatures up to ca. 200 8C under autogenous pressure for a time ranging from hours to days. The effect of a hydrothermal treatment on the crystallite size and strain of synthetic Mg–Al hydrotalcite-like compounds with various Mg/Al molar ratios was studied by Miyata (1980). Maximum crystallite size was achieved by 24 h of hydrothermal treatment between 180 and 200 8C, with molar ratio Al/(Al+Mg) ranging from 0.337 to 0.429. Some aspects of the hydrothermal treatment of hydrotalcite-like compounds at high temperatures have been discussed in the comprehensive article of Cavani et al. (1991). The hydrothermal treatment was used in order to improve the crystallinity of various layered double hydroxides such as Ni–Al and Ni–Cr (Clause et al., 1991), Ni–Al–Cr and Ni– Al–Fe (Kooli et al., 1995), Ni–Fe (del Arco et al., 1999), Mg–Cr (Prakash et al., 2000), Mg–V (Rives and Labajos, 1993; Labajos et al., 1996), or Zn–Al intercalated with polyoxovanadate (Barriga et al., 1998). The crystallinity of Mg–MIII compounds (MIII=Al, Cr or Fe) was also enhanced by the microwave irradiation (Kannan and Jasra, 2000). Hickey et al. (2000) studied the influence of hydrothermal treatment on the crystallinity of Mg–Al hydrotalcite. It was shown that the ageing at increased temperature and pressure improved crystallinity and the water treatment resulted in a more crystalline material in comparison with the treatment in the mother liquid. The crystal size increased with ageing. The growth occurred on the edges, resulting in the formation of hexagonal plate shaped hydrotalcite crystals. The effect of synthetic conditions and a hydrothermal ageing on the particle size was reported by Oh et al. (2002). The aim of the present work is to study the effect of hydrothermal treatment on crystallinity, crystal size and surface area of synthetic hydrotalcite depending on temperature and time. The synthetic hydrotalcite with Mg/Al molar ratio of 2 was aged under hydrothermal conditions at various temper-
F. Kovanda et al. / Applied Clay Science 28 (2005) 101–109
atures in the range 120–200 8C for 2–18 h. An influence of hydrothermal crystallization was evaluated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements.
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maintain all surface details at high resolution. The very low accelerating voltage of about 2 kV was applied to avoid the surface charging.
3. Results and discussion 2. Experimental 2.1. Preparation of samples Hydrotalcite was prepared by coprecipitation. An aqueous solution (450 ml) of MgSO4d 7H2O and Al2(SO4)3d 18H2O with Mg/Al molar ratio equal to 2 and total metal ion concentration of 1.2 mol l 1 was added dropwise into 500 ml of 1 M Na2CO3 solution under vigorous stirring. During the synthesis, the temperature was maintained at 75 8C and pH at about 10 by the simultaneous addition of 10 M NaOH solution. The addition of both solutions took about 1 h. The resulting suspension was then maintained at 75 8C, with stirring, for 1 h. The product was filtered off, washed with hot distilled water, resuspended in 1000 ml of hot distilled water, again filtered off and thoroughly washed until free of SO42 . The washed filtration cake was resuspended in distilled water to obtain a suspension containing about 10 wt.% of solid. Suspension portions of 55 ml volume were placed into 100 ml Teflon lined stainless steel bombs and hydrothermally treated at temperatures from 120 to 200 8C for 2–18 h. After hydrothermal ageing, all samples were filtered off and dried at 130 8C. 2.2. Characterization of the products Powder X-ray diffraction patterns were recorded using a Seifert XRD 3000P instrument with Co Ka radiation (k=0.179 nm, graphite monochromator, goniometer with Bragg–Brentano geometry) in 2h range 11–758, step size 0.058. The integral intensity and full width in half maximum (FWHM) of the (003) and (006) diffraction lines were evaluated in order to compare the crystallinity of samples hydrothermally treated under various conditions. Two regions between 11.08 and 14.58 2h, and 24.58 and 29.58 2h corresponding to the location of (003) and (006) reflections, respectively, were chosen for detailed XRD measuring. Every sample was measured three times while the sample holder was rotated in face to primary XRD beam. Surface area measurements were carried out by nitrogen adsorption at 77 K and evaluated by one point BET method. Scanning electron micrographs (SEM) of the samples were taken with a scanning electron microscope Hitachi S-4700 at a magnification of about 50,000 times. No conductive layer was applied for coating of observed samples in order to
A relatively well-crystallized hydrotalcite-like phase was observed in the dried products before hydrothermal treatment. No other crystalline phases were detected in the powder XRD patterns of the coprecipitated samples. The hydrothermal treatment resulted in an increasing intensity of diffraction lines with increased temperature and time, particularly at higher temperatures. No other crystalline phases were formed during a hydrothermal treatment (Fig. 1). All samples, i.e. both coprecipitated products and hydrothermally treated ones, exhibited a broadening of (01l) reflections. The OH sheets in brucite-like layers may exhibit two stacking sequences, rhombohedral and hexagonal, respectively. Hydrotalcite crystallizes with a rhombohedral 3R stacking sequence with the unit cell parameters a=3.054 2 and c=3cV=22.81 2, where cV is the thickness of one layer consisting of a brucite-like sheet and one interlayer. The polytype form, manasseite, crystallizes with a hexagonal 2H stacking sequence, the parameters of unit cell being a=3.10 2 and c=2cV=15.6 2. The hydrotalcite-like compounds synthesized at usual conditions are the three-layer polytype (i.e. rhombohedral), while the two-layer polytype (i.e. hexagonal) may be the form obtained at high temperatures (Cavani et al., 1991). The broadening of (01l) reflections may be related to stacking faults as a result of the intergrowth of the rhombohedral and hexagonal polytypes (Bellotto et al., 1996). The hydrothermal process had no influence on improvement of stacking faults of the synthesized hydrotalcite. The evaluated integral intensities and FWHM values of hydrotalcite samples are compared in Figs. 2 and 3, respectively. Coprecipitated samples (i.e. hydrothermally non-treated samples signed as d0 hT) differed a little from each other. The integral intensities of both (003) and (006) reflections exhibited close values and all coprecipitated samples had approximately the same hydrotalcite content. A variability of the FWHM values reflected a slightly different crystallinity of the synthesized samples.
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Fig. 1. Powder X-ray diffraction patterns of hydrotalcite samples depending on temperature and time of hydrothermal treatment.
Based on comparison of integral intensity values, the hydrothermal treatment considerably increased the hydrotalcite content in the samples, which was dependent both on temperature and time of the ageing procedure (Fig. 2). Temperature seems to be a crucial parameter affecting the hydrothermal crystallization. At 160 8C and higher temperatures, a marked increase of hydrotalcite content was observed in all hydro-
thermally treated samples. The influence of time was less apparent but it can be generally considered that with the longer time, the higher hydrotalcite content in sample was obtained. A change in the ratio of integral intensities of (003) and (006) diffraction lines was observed during hydrothermal treatment. The ratio of 1.4–1.7 was evaluated for the coprecipitated samples. In the first
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Fig. 2. Integral intensity of hydrotalcite (003) and (006) diffraction lines depending on temperature and time of hydrothermal treatment.
few hours of hydrothermal crystallization, a rapid increase of the integral intensity ratio was detected (Fig. 4). The highest ratios of 2.8–3.1 were found during hydrothermal treatment at 200 8C, the maximum values in the interval from 1.7 to 2.7 were observed at lower temperatures between 120 and 180 8C. The reason of this effect is not very clear. Perhaps the recrystallization processes taking place during hydrothermal treatment led to the formation of other polytypes and their intergrowth resulted in a slightly different phase composition of products obtained. Based on evaluated FWHM values, the hydrotalcite crystallinity was significantly improved by hydrothermal treatment, particularly dependent on processing temperature. In general, the higher temperature in the studied interval 120–200 8C was applied, the lower FWHM values were observed
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Fig. 3. Full width in half maximum (FWHM) values of hydrotalcite (003) and (006) diffraction lines depending on temperature and time of hydrothermal treatment.
Fig. 4. Integral intensity of (003) to that of (006) hydrotalcite diffraction lines ratio depending on temperature and time of hydrothermal treatment.
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(Fig. 3). The FWHM values decreased markedly in a relatively short time (2–4 h) and then the time dependence was less evident. Using the FWHM values, the average crystallite size in [001] direction was calculated according to Scherrer’s formula: crystallite size=Kk/(B cos h), where B corresponds to the structural broadening (the difference in the integral profile width between a standard and an unknown sample), K is the shape factor (value of K=0.9 was used), k is the wavelength of Co Ka radiation and h is the diffraction angle. The enlargement in [001] direction is demonstrated in Fig. 5. For dried precipitates, the average crystallite size of about 100 2 was evaluated. During the hydrothermal treatment, the crystallite size increased with increasing temperature and time, achieving almost 250 2 after 18 h ageing at 200 8C. The maximum crystallite sizes were much lower than that (from 526 to 1653 2) reported by Miyata (1980). The different experimental conditions used could influence the obtained results. In this study, 55 ml of suspension containing ca. 10 wt.% of hydrotalcite was treated in 100 ml autoclave, whereas 100 g of hydrotalcite suspended in 700 ml of water was treated in a 1000 ml autoclave during experiments reported by Miyata (1980). The physical–chemical properties of the hydrotalcite used might also be slightly different. The product prepared by coprecipitation of sulphate solutions was used in this work, but Miyata (1980) used a product prepared from chloride solutions.
Fig. 5. Crystallite size in [001] direction depending on temperature and time of hydrothermal treatment.
Fig. 6. Surface area of hydrotalcite samples depending on temperature and time of hydrothermal treatment.
According to XRD results, the higher the temperature and time of hydrothermal treatment applied, the higher the hydrotalcite enrichment, as well as an increasing growth of crystallites, were observed. These processes may be explained by dissolving an amorphous part present in the coprecipitated samples and by growing of crystallites formed during hydrotalcite synthesis. The growth of crystallites was connected with increasing hydrotalcite content in the hydrothermally treated samples. The surface area of samples decreased with time and especially with temperature of hydrothermal treatment (Fig. 6). The dried precipitates exhibited a surface area of about 80–90 m2 g 1. At lower processing temperatures (120 and 140 8C, respectively), a slight increase of surface area was observed after 2 h treatment. The possible explanation of this rather surprising effect may be that the starting recrystallization processes affected the coprecipitated sample and the very small crystallites not detectable by XRD and/or an incompletely crystallized solid formed during autoclave cooling was present in the obtained product. When longer processing times were applied, a slow decrease of surface area with time was observed and values of about 20–30 m2 g 1 lower in comparison with initial ones were achieved after 18 h. The hydrothermal treatment at 160 8C and higher temperatures resulted in a rapid decrease of surface area in the first 2–4 h; the subsequent surface area decrease with time was less evident. The surface area lower than 20 m2 g 1 was
F. Kovanda et al. / Applied Clay Science 28 (2005) 101–109
Fig. 7. SEM images of hydrotalcite samples hydrothermally treated at 160 and 180 8C for various times.
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evaluated for samples treated at 180 and 200 8C after 6–8 h. SEM images showed a gradual crystallization of hydrotalcite during hydrothermal treatment accompanied by the considerable increase of crystal size (Fig. 7). The aggregates composed of small thin crystals of ca 0.1 Am in diameter were observed in coprecipitated samples. Thin plates with hexagonal morphology and particle size of several tenths of micrometers were gradually formed during hydrothermal treatment, depending on temperature and time applied. The SEM observations were in good agreement with XRD results and surface area measurements. An increase of the sample crystallinity may reflect only a structure ordering without substantial crystal growth, but in most cases, it is accompanied by an increase of crystallite size. A dissolution and recrystallization of the smallest crystallites and amorphous parts during hydrothermal treatment may be considered. The obtained results confirmed an increase in both crystallinity and crystallite size upon hydrothermal ageing.
4. Conclusions The Mg–Al hydrotalcite was hydrothermally treated under autogenous pressure at various temperatures in the range 120–200 8C for 2–18 h. Based on XRD results, the hydrothermal treatment increased the hydrotalcite content in the samples and improved significantly the hydrotalcite crystallinity, particularly dependent on processing temperature. The crystallite size in [001] direction was enlarged from ca. 100 2, evaluated for coprecipitated samples, to about 250 2, found in the sample crystallizing at 200 8C for 18 h. In general, the higher the temperature and longer time of hydrothermal treatment were applied, the higher the hydrotalcite enrichment, as well as an increasing growth of crystallites, were observed. After the first 2–4 h, the time dependence was less evident, whereas temperature seems to be a crucial parameter affecting the hydrothermal crystallization. The hydrothermal treatment resulted in a marked decrease of surface area and growth of hydrotalcite crystals. The aggregates composed of small thin crystals were transformed into thin plates with hexagonal morphology and particle size of several tenths of micrometers.
Both the decrease of surface area and the crystallinity improvement can be explained by the increase of hydrotalcite crystal size. Acknowledgements This work was supported by the Czech Ministry of Education, Youth and Sports (research project No. CEZ:MSM 223/10/0002) and by the Grant Agency of Czech Republic (projects No. 106/02/0523 and 103/ 03/0506). References Barriga, C., Jones, W., Malet, P., Rives, V., Ulibarri, M.A., 1998. Synthesis and characterization of polyoxovanadate-pillared Zn– Al layered double hydroxides: an X-ray absorption and diffraction study. Inorg. Chem. 37, 1812 – 1820. Bellotto, M., Rebours, B., Clause, O., Lynch, J., Bazin, D., Elkaim, E., 1996. A reexamination of hydrotalcite crystal chemistry. J. Phys. Chem. 100, 8527 – 8534. Cavani, F., Trifiro, F., Vaccari, A., 1991. Hydrotalcite-type anionic clays: preparation, properties and application. Catal. Today 11, 173 – 301. Clause, O., Gazzano, M., Trifiro, F., Vaccari, A., Zatorski, L., 1991. Preparation and thermal reactivity of nickel/chromium and nickel/aluminium hydrotalcite-type precursors. Appl. Catal. 73, 217 – 236. del Arco, M., Malet, P., Trujillano, R., Rives, V., 1999. Synthesis and characterization of hydrotalcites containing Ni(II) and Fe(III) and their calcination products. Chem. Mater. 11, 624 – 633. Hickey, L., Kloprogge, J.T., Frost, R.L., 2000. The effects of various hydrothermal treatments on magnesium–aluminium hydrotalcites. J. Mater. Sci. 35, 4347 – 4355. Kannan, S., Jasra, R.V., 2000. Microwave assisted rapid crystallization of Mg–M(III) hydrotalcite where M(III)=Al, Fe or Cr. J. Mater. Chem. 10, 2311 – 2314. Kooli, F., Kosuge, K., Tsunashima, A., 1995. New Ni–Al–Cr and Ni–Al–Fe carbonate hydrotalcite-like compounds: synthesis and characterization. J. Solid State Chem. 118, 285 – 291. Labajos, F.M., Rives, V., Malet, P., Centeno, M.A., Ulibarri, M.A., 1996. Synthesis and characterization of hydrotalcite-like compounds containing V3+ in the layers and of their calcination products. Inorg. Chem. 35, 1154 – 1160. Leroux, F., Besse, J.-P., 2001. Polymer interleaved layered double hydroxide: a new emerging class of nanocomposites. Chem. Mater. 13, 3507 – 3515. Miyata, S., 1980. Physico-chemical properties of synthetic hydrotalcites in relation to composition. Clays Clay Miner. 28, 50 – 56. Oh, J.-M., Hwang, S.-H., Choy, J.-H., 2002. The effect of synthetic conditions on tailoring the size of hydrotalcite particles. Solid State Ionics 151, 285 – 291.
F. Kovanda et al. / Applied Clay Science 28 (2005) 101–109 Prakash, A.S., Vishnu Kamath, P., Hegde, M.S., 2000. Synthesis and characterization of the layered double hydroxides of Mg with Cr. Mater. Res. Bull. 35, 2189 – 2197. Rives, V. (Ed.), 2001. Layered Double Hydroxides: Present and Future. Nova Science Publishers, New York, pp. 251 – 411. Rives, V., Labajos, F.M., 1993. A new hydrotalcite-like compound containing V3+ ions in the layers. Inorg. Chem. 32, 5000 – 5001.
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Trifiro, F., Vaccari, A., 1996. Hydrotalcite-like anionic clays (layered double hydroxides). In: Atwood, J.L., Davies, J.E.D., MacNicol, D.D., Vfgtle, F., Lehn, J.-M., Albert, G., Bein, T. (Eds.), Comprehensive Supramolecular Chemistry, vol. 7. Pergamon, Oxford, pp. 251 – 291.
Crystallization of synthetic hydrotalcite under hydrothermal conditions Frantisˇek Kovandaa,*, David Kolousˇeka, Zuzana Cı´lova´b, Va´clav Hulı´nsky´b a
Department of Solid State Chemistry, Institute of Chemical Technology, Technicka´ 5, 166 28 Prague, Czech Republic b Department of Glass and Ceramics, Institute of Chemical Technology, Prague, Czech Republic Received 13 November 2003; received in revised form 20 December 2003; accepted 28 January 2004 Available online 2 July 2004
Abstract The synthetic Mg–Al hydrotalcite with Mg/Al molar ratio of 2 was hydrothermally treated in autoclaves under autogenous water vapour pressure at 120–200 8C for 2–18 h. The obtained samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements. A well-crystallized hydrotalcite-like phase was present in the coprecipitated product. No other crystalline phases were detected in the powder XRD patterns of both coprecipitated and hydrothermally treated samples. The integral intensity and full width in half maximum (FWHM) of the (003) and (006) hydrotalcite diffraction lines were evaluated in order to compare the crystallinity of samples hydrothermally treated under various conditions. The hydrothermal treatment increased the hydrotalcite content in the samples and improved significantly the hydrotalcite crystallinity. In general, the higher temperature and longer time of hydrothermal treatment, the higher hydrotalcite enrichment, as well as an increasing growth of crystallites, were observed. After the first 2–4 h, the time dependence was less evident, whereas temperature seemed to be a crucial parameter affecting the hydrothermal crystallization. The temperature of 120 8C was too low to increase significantly the sample crystallinity during several hours. According to XRD results, the marked crystallinity improvement was observed at 160 8C and higher temperatures. The hydrothermal treatment resulted in a marked decrease of surface area and a growth of hydrotalcite crystals. SEM micrographs of coprecipitated samples showed the aggregates composed of small thin crystals of ca. 0.1 Am in diameter, which were gradually transformed into thin plates with hexagonal morphology and particle size of several tenths of micrometers. Both a decrease of surface area and the crystallinity improvement of hydrothermally treated samples can be explained by the increase of hydrotalcite crystal size. D 2004 Elsevier B.V. All rights reserved. Keywords: Mg–Al hydrotalcite; Layered double hydroxide; Hydrothermal treatment; Crystallinity improvement
1. Introduction * Corresponding author. Fax: +420 2 2431 1082. E-mail address: [email protected] (F. Kovanda). 0169-1317/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2004.01.009
Hydrotalcite, a magnesium–aluminum hydroxycarbonate, is a naturally occurring mineral of chemical
102
F. Kovanda et al. / Applied Clay Science 28 (2005) 101–109
composition Mg6Al2(OH)16CO3d 4H2O exhibiting a layered crystal structure, which is comprised of positively charged hydroxide layers and interlayers composed of carbonate anions and water molecules. The ordering of hydroxide layers is similar to that of brucite, Mg(OH)2, where each Mg2+ cation is octahedrally surrounded by six OH anions and the different octahedra [Mg(OH)6]4 share edges to form infinite sheets. In the hydrotalcite, the Mg2+/Al3+ isomorphous substitution in octahedral sites of the hydroxide sheet results in a net positive charge, which has to be neutralized by interlayer anionic species. Hydrotalcite has been taken as a reference name for many isomorphous compounds, containing various MII and MIII metal cations in hydroxide layers and various interlayer anions. These materials, known as hydrotalcite-like compounds, layered double hydroxides, anionic clays, etc., have a great variety of uses in many applications. They are used in heterogeneous catalysis as catalysts, catalyst precursors and support materials, as neutralizing agents and halogen scavengers in polymer processing or in pharmacy as antacids. They are also known as good adsorbents and anion exchangers (Cavani et al., 1991; Trifiro and Vaccari, 1996; Rives, 2001). Recently they have been used in the preparation of nanocomposite materials (Leroux and Besse, 2001). The most common method applied to preparation of hydrotalcite-like compounds is coprecipitation, which is based on the reaction of a solution containing MII and MIII metal cations in adequate proportions with an alkaline solution. The products obtained by coprecipitation at low supersaturation are usually more crystalline in comparison with those prepared at high supersaturation conditions. However, the product crystallinity may be affected by various experimental parameters such as reaction pH and temperature, concentration of used solutions, flow rate during addition of reactants, hydrodynamic conditions in the reactor and/or postsynthesis operations (e.g. an ageing of obtained precipitate). A large number of studies on physical– chemical properties and applications of hydrotalcitelike compounds was reported, but only a few reports discussed the control of particle size, which is an important parameter in industrial applications. In many cases, an optimization of experimental conditions does not lead to a well-crystallized
hydrotalcite-like phase. An improvement of the crystallinity may be achieved by hydrothermal treatment in the presence of water vapour at temperatures which do not exceed the decomposition temperature of the hydrotalcite-like compound. The hydrothermal crystallization is usually carried out at temperatures up to ca. 200 8C under autogenous pressure for a time ranging from hours to days. The effect of a hydrothermal treatment on the crystallite size and strain of synthetic Mg–Al hydrotalcite-like compounds with various Mg/Al molar ratios was studied by Miyata (1980). Maximum crystallite size was achieved by 24 h of hydrothermal treatment between 180 and 200 8C, with molar ratio Al/(Al+Mg) ranging from 0.337 to 0.429. Some aspects of the hydrothermal treatment of hydrotalcite-like compounds at high temperatures have been discussed in the comprehensive article of Cavani et al. (1991). The hydrothermal treatment was used in order to improve the crystallinity of various layered double hydroxides such as Ni–Al and Ni–Cr (Clause et al., 1991), Ni–Al–Cr and Ni– Al–Fe (Kooli et al., 1995), Ni–Fe (del Arco et al., 1999), Mg–Cr (Prakash et al., 2000), Mg–V (Rives and Labajos, 1993; Labajos et al., 1996), or Zn–Al intercalated with polyoxovanadate (Barriga et al., 1998). The crystallinity of Mg–MIII compounds (MIII=Al, Cr or Fe) was also enhanced by the microwave irradiation (Kannan and Jasra, 2000). Hickey et al. (2000) studied the influence of hydrothermal treatment on the crystallinity of Mg–Al hydrotalcite. It was shown that the ageing at increased temperature and pressure improved crystallinity and the water treatment resulted in a more crystalline material in comparison with the treatment in the mother liquid. The crystal size increased with ageing. The growth occurred on the edges, resulting in the formation of hexagonal plate shaped hydrotalcite crystals. The effect of synthetic conditions and a hydrothermal ageing on the particle size was reported by Oh et al. (2002). The aim of the present work is to study the effect of hydrothermal treatment on crystallinity, crystal size and surface area of synthetic hydrotalcite depending on temperature and time. The synthetic hydrotalcite with Mg/Al molar ratio of 2 was aged under hydrothermal conditions at various temper-
F. Kovanda et al. / Applied Clay Science 28 (2005) 101–109
atures in the range 120–200 8C for 2–18 h. An influence of hydrothermal crystallization was evaluated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements.
103
maintain all surface details at high resolution. The very low accelerating voltage of about 2 kV was applied to avoid the surface charging.
3. Results and discussion 2. Experimental 2.1. Preparation of samples Hydrotalcite was prepared by coprecipitation. An aqueous solution (450 ml) of MgSO4d 7H2O and Al2(SO4)3d 18H2O with Mg/Al molar ratio equal to 2 and total metal ion concentration of 1.2 mol l 1 was added dropwise into 500 ml of 1 M Na2CO3 solution under vigorous stirring. During the synthesis, the temperature was maintained at 75 8C and pH at about 10 by the simultaneous addition of 10 M NaOH solution. The addition of both solutions took about 1 h. The resulting suspension was then maintained at 75 8C, with stirring, for 1 h. The product was filtered off, washed with hot distilled water, resuspended in 1000 ml of hot distilled water, again filtered off and thoroughly washed until free of SO42 . The washed filtration cake was resuspended in distilled water to obtain a suspension containing about 10 wt.% of solid. Suspension portions of 55 ml volume were placed into 100 ml Teflon lined stainless steel bombs and hydrothermally treated at temperatures from 120 to 200 8C for 2–18 h. After hydrothermal ageing, all samples were filtered off and dried at 130 8C. 2.2. Characterization of the products Powder X-ray diffraction patterns were recorded using a Seifert XRD 3000P instrument with Co Ka radiation (k=0.179 nm, graphite monochromator, goniometer with Bragg–Brentano geometry) in 2h range 11–758, step size 0.058. The integral intensity and full width in half maximum (FWHM) of the (003) and (006) diffraction lines were evaluated in order to compare the crystallinity of samples hydrothermally treated under various conditions. Two regions between 11.08 and 14.58 2h, and 24.58 and 29.58 2h corresponding to the location of (003) and (006) reflections, respectively, were chosen for detailed XRD measuring. Every sample was measured three times while the sample holder was rotated in face to primary XRD beam. Surface area measurements were carried out by nitrogen adsorption at 77 K and evaluated by one point BET method. Scanning electron micrographs (SEM) of the samples were taken with a scanning electron microscope Hitachi S-4700 at a magnification of about 50,000 times. No conductive layer was applied for coating of observed samples in order to
A relatively well-crystallized hydrotalcite-like phase was observed in the dried products before hydrothermal treatment. No other crystalline phases were detected in the powder XRD patterns of the coprecipitated samples. The hydrothermal treatment resulted in an increasing intensity of diffraction lines with increased temperature and time, particularly at higher temperatures. No other crystalline phases were formed during a hydrothermal treatment (Fig. 1). All samples, i.e. both coprecipitated products and hydrothermally treated ones, exhibited a broadening of (01l) reflections. The OH sheets in brucite-like layers may exhibit two stacking sequences, rhombohedral and hexagonal, respectively. Hydrotalcite crystallizes with a rhombohedral 3R stacking sequence with the unit cell parameters a=3.054 2 and c=3cV=22.81 2, where cV is the thickness of one layer consisting of a brucite-like sheet and one interlayer. The polytype form, manasseite, crystallizes with a hexagonal 2H stacking sequence, the parameters of unit cell being a=3.10 2 and c=2cV=15.6 2. The hydrotalcite-like compounds synthesized at usual conditions are the three-layer polytype (i.e. rhombohedral), while the two-layer polytype (i.e. hexagonal) may be the form obtained at high temperatures (Cavani et al., 1991). The broadening of (01l) reflections may be related to stacking faults as a result of the intergrowth of the rhombohedral and hexagonal polytypes (Bellotto et al., 1996). The hydrothermal process had no influence on improvement of stacking faults of the synthesized hydrotalcite. The evaluated integral intensities and FWHM values of hydrotalcite samples are compared in Figs. 2 and 3, respectively. Coprecipitated samples (i.e. hydrothermally non-treated samples signed as d0 hT) differed a little from each other. The integral intensities of both (003) and (006) reflections exhibited close values and all coprecipitated samples had approximately the same hydrotalcite content. A variability of the FWHM values reflected a slightly different crystallinity of the synthesized samples.
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Fig. 1. Powder X-ray diffraction patterns of hydrotalcite samples depending on temperature and time of hydrothermal treatment.
Based on comparison of integral intensity values, the hydrothermal treatment considerably increased the hydrotalcite content in the samples, which was dependent both on temperature and time of the ageing procedure (Fig. 2). Temperature seems to be a crucial parameter affecting the hydrothermal crystallization. At 160 8C and higher temperatures, a marked increase of hydrotalcite content was observed in all hydro-
thermally treated samples. The influence of time was less apparent but it can be generally considered that with the longer time, the higher hydrotalcite content in sample was obtained. A change in the ratio of integral intensities of (003) and (006) diffraction lines was observed during hydrothermal treatment. The ratio of 1.4–1.7 was evaluated for the coprecipitated samples. In the first
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Fig. 2. Integral intensity of hydrotalcite (003) and (006) diffraction lines depending on temperature and time of hydrothermal treatment.
few hours of hydrothermal crystallization, a rapid increase of the integral intensity ratio was detected (Fig. 4). The highest ratios of 2.8–3.1 were found during hydrothermal treatment at 200 8C, the maximum values in the interval from 1.7 to 2.7 were observed at lower temperatures between 120 and 180 8C. The reason of this effect is not very clear. Perhaps the recrystallization processes taking place during hydrothermal treatment led to the formation of other polytypes and their intergrowth resulted in a slightly different phase composition of products obtained. Based on evaluated FWHM values, the hydrotalcite crystallinity was significantly improved by hydrothermal treatment, particularly dependent on processing temperature. In general, the higher temperature in the studied interval 120–200 8C was applied, the lower FWHM values were observed
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Fig. 3. Full width in half maximum (FWHM) values of hydrotalcite (003) and (006) diffraction lines depending on temperature and time of hydrothermal treatment.
Fig. 4. Integral intensity of (003) to that of (006) hydrotalcite diffraction lines ratio depending on temperature and time of hydrothermal treatment.
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(Fig. 3). The FWHM values decreased markedly in a relatively short time (2–4 h) and then the time dependence was less evident. Using the FWHM values, the average crystallite size in [001] direction was calculated according to Scherrer’s formula: crystallite size=Kk/(B cos h), where B corresponds to the structural broadening (the difference in the integral profile width between a standard and an unknown sample), K is the shape factor (value of K=0.9 was used), k is the wavelength of Co Ka radiation and h is the diffraction angle. The enlargement in [001] direction is demonstrated in Fig. 5. For dried precipitates, the average crystallite size of about 100 2 was evaluated. During the hydrothermal treatment, the crystallite size increased with increasing temperature and time, achieving almost 250 2 after 18 h ageing at 200 8C. The maximum crystallite sizes were much lower than that (from 526 to 1653 2) reported by Miyata (1980). The different experimental conditions used could influence the obtained results. In this study, 55 ml of suspension containing ca. 10 wt.% of hydrotalcite was treated in 100 ml autoclave, whereas 100 g of hydrotalcite suspended in 700 ml of water was treated in a 1000 ml autoclave during experiments reported by Miyata (1980). The physical–chemical properties of the hydrotalcite used might also be slightly different. The product prepared by coprecipitation of sulphate solutions was used in this work, but Miyata (1980) used a product prepared from chloride solutions.
Fig. 5. Crystallite size in [001] direction depending on temperature and time of hydrothermal treatment.
Fig. 6. Surface area of hydrotalcite samples depending on temperature and time of hydrothermal treatment.
According to XRD results, the higher the temperature and time of hydrothermal treatment applied, the higher the hydrotalcite enrichment, as well as an increasing growth of crystallites, were observed. These processes may be explained by dissolving an amorphous part present in the coprecipitated samples and by growing of crystallites formed during hydrotalcite synthesis. The growth of crystallites was connected with increasing hydrotalcite content in the hydrothermally treated samples. The surface area of samples decreased with time and especially with temperature of hydrothermal treatment (Fig. 6). The dried precipitates exhibited a surface area of about 80–90 m2 g 1. At lower processing temperatures (120 and 140 8C, respectively), a slight increase of surface area was observed after 2 h treatment. The possible explanation of this rather surprising effect may be that the starting recrystallization processes affected the coprecipitated sample and the very small crystallites not detectable by XRD and/or an incompletely crystallized solid formed during autoclave cooling was present in the obtained product. When longer processing times were applied, a slow decrease of surface area with time was observed and values of about 20–30 m2 g 1 lower in comparison with initial ones were achieved after 18 h. The hydrothermal treatment at 160 8C and higher temperatures resulted in a rapid decrease of surface area in the first 2–4 h; the subsequent surface area decrease with time was less evident. The surface area lower than 20 m2 g 1 was
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Fig. 7. SEM images of hydrotalcite samples hydrothermally treated at 160 and 180 8C for various times.
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evaluated for samples treated at 180 and 200 8C after 6–8 h. SEM images showed a gradual crystallization of hydrotalcite during hydrothermal treatment accompanied by the considerable increase of crystal size (Fig. 7). The aggregates composed of small thin crystals of ca 0.1 Am in diameter were observed in coprecipitated samples. Thin plates with hexagonal morphology and particle size of several tenths of micrometers were gradually formed during hydrothermal treatment, depending on temperature and time applied. The SEM observations were in good agreement with XRD results and surface area measurements. An increase of the sample crystallinity may reflect only a structure ordering without substantial crystal growth, but in most cases, it is accompanied by an increase of crystallite size. A dissolution and recrystallization of the smallest crystallites and amorphous parts during hydrothermal treatment may be considered. The obtained results confirmed an increase in both crystallinity and crystallite size upon hydrothermal ageing.
4. Conclusions The Mg–Al hydrotalcite was hydrothermally treated under autogenous pressure at various temperatures in the range 120–200 8C for 2–18 h. Based on XRD results, the hydrothermal treatment increased the hydrotalcite content in the samples and improved significantly the hydrotalcite crystallinity, particularly dependent on processing temperature. The crystallite size in [001] direction was enlarged from ca. 100 2, evaluated for coprecipitated samples, to about 250 2, found in the sample crystallizing at 200 8C for 18 h. In general, the higher the temperature and longer time of hydrothermal treatment were applied, the higher the hydrotalcite enrichment, as well as an increasing growth of crystallites, were observed. After the first 2–4 h, the time dependence was less evident, whereas temperature seems to be a crucial parameter affecting the hydrothermal crystallization. The hydrothermal treatment resulted in a marked decrease of surface area and growth of hydrotalcite crystals. The aggregates composed of small thin crystals were transformed into thin plates with hexagonal morphology and particle size of several tenths of micrometers.
Both the decrease of surface area and the crystallinity improvement can be explained by the increase of hydrotalcite crystal size. Acknowledgements This work was supported by the Czech Ministry of Education, Youth and Sports (research project No. CEZ:MSM 223/10/0002) and by the Grant Agency of Czech Republic (projects No. 106/02/0523 and 103/ 03/0506). References Barriga, C., Jones, W., Malet, P., Rives, V., Ulibarri, M.A., 1998. Synthesis and characterization of polyoxovanadate-pillared Zn– Al layered double hydroxides: an X-ray absorption and diffraction study. Inorg. Chem. 37, 1812 – 1820. Bellotto, M., Rebours, B., Clause, O., Lynch, J., Bazin, D., Elkaim, E., 1996. A reexamination of hydrotalcite crystal chemistry. J. Phys. Chem. 100, 8527 – 8534. Cavani, F., Trifiro, F., Vaccari, A., 1991. Hydrotalcite-type anionic clays: preparation, properties and application. Catal. Today 11, 173 – 301. Clause, O., Gazzano, M., Trifiro, F., Vaccari, A., Zatorski, L., 1991. Preparation and thermal reactivity of nickel/chromium and nickel/aluminium hydrotalcite-type precursors. Appl. Catal. 73, 217 – 236. del Arco, M., Malet, P., Trujillano, R., Rives, V., 1999. Synthesis and characterization of hydrotalcites containing Ni(II) and Fe(III) and their calcination products. Chem. Mater. 11, 624 – 633. Hickey, L., Kloprogge, J.T., Frost, R.L., 2000. The effects of various hydrothermal treatments on magnesium–aluminium hydrotalcites. J. Mater. Sci. 35, 4347 – 4355. Kannan, S., Jasra, R.V., 2000. Microwave assisted rapid crystallization of Mg–M(III) hydrotalcite where M(III)=Al, Fe or Cr. J. Mater. Chem. 10, 2311 – 2314. Kooli, F., Kosuge, K., Tsunashima, A., 1995. New Ni–Al–Cr and Ni–Al–Fe carbonate hydrotalcite-like compounds: synthesis and characterization. J. Solid State Chem. 118, 285 – 291. Labajos, F.M., Rives, V., Malet, P., Centeno, M.A., Ulibarri, M.A., 1996. Synthesis and characterization of hydrotalcite-like compounds containing V3+ in the layers and of their calcination products. Inorg. Chem. 35, 1154 – 1160. Leroux, F., Besse, J.-P., 2001. Polymer interleaved layered double hydroxide: a new emerging class of nanocomposites. Chem. Mater. 13, 3507 – 3515. Miyata, S., 1980. Physico-chemical properties of synthetic hydrotalcites in relation to composition. Clays Clay Miner. 28, 50 – 56. Oh, J.-M., Hwang, S.-H., Choy, J.-H., 2002. The effect of synthetic conditions on tailoring the size of hydrotalcite particles. Solid State Ionics 151, 285 – 291.
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