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Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides
Applied Clay Science 22 (2003) 295 – 301 www.elsevier.com/locate/clay
Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides Tsveta Stanimirova *, Georgi Kirov Department of Mineralogy, Sofia University, 15 Tzar Osvoboditel, Sofia 1000, Bulgaria Received 8 March 2002; accepted 7 April 2003
Abstract Changes in cation composition and M2 +/M3 + ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations. Heating Mg – Al – CO3 hydrotalcites with Mg/Al = 2, 3 and 3.7, at 600 jC for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations. The presence of Mg2 +, Zn2 +, Ni2 +, Co2 +, Cu2 + salts or of low soluble hydromagnesite increased the M2 +/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn2 +, Ni2 +, Co2 + and Cu2 + cations in the newly formed hydrotalcite was detected, while Mg2 + remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH)3, the M2 +/Al ratio approximated the minimal possible value of 2. The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value. The results obtained support the conception of the dissolution – crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Hydrotalcite; Layered double hydroxides; Cation modification; Regeneration
1. Introduction Layered double hydroxides (LDH) are described m by the general formula [M12 + x M3x +(OH)2]x +Ax/m 3+ nH2O, where M (Al, Fe, Cr, Ga) partially substitutes M2 + (Mg, Ni, Zn, Cu, Fe, etc.) in the brucitelike layers. Surplus positive charge is balanced by exchangeable anions Am (CO32 , SO42 , NO3 , Cl , * Corresponding author. E-mail address: [email protected] (T. Stanimirova).
Br , OH , etc.) in the interlayer space, where water is also present (Cavani et al., 1991). Thermal treatment causes structural changes and yields several metaphases (Stanimirova et al., 1999). The wide possibilities for thermal modification and for isomorphic substitutions both in the cationic and anionic parts make it possible that several materials are produced for various applications (Cavani et al., 1991; Vaccari, 1999). Several methods are used for the synthesis of LDH: precipitation from metal salts solutions with
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Table 1 Batch compositions, layer thickness (c0) and M2 +/Al ratio of the starting and recrystallized layered double hydroxides No.
Starting samples
a
1 2a 3 4 5 6 7 a b
Batch compositions
Mg/Al
c0 (nm)
a (nm)
2.0 + 3.7 2.0 + 3.0 3.0 3.0 2.0 2.0 2.0
0.758, 0.790 0.758, 0.778 0.778 0.778 0.758 0.758 0.758
0.3044, 0.3070 0.3044, 0.3060 0.3060 0.3060 0.3044 0.3044 0.3044
Recrystallized samples
(2HT-P + 3.7HT-P) + H2O (2HT-P + 3HT-P) + H2O 3HT-P + 0.5 M Na3Al(OH)6 3HT-P + Al(OH)3 + H2O 2HT-P + Mg4(CO3)3(OH)23H2O + H2O 2HT-P + 0.5 M Mg(NO3)2 (CO23 exch.)b 2HT-P + 0.5 M Mg(NO3)2
c0 (nm)
a (nm)
M2 +/Al
0.779 0.769 0.760 0.758 0.792 0.792 0.816
0.3062 0.3051 0.3045 0.3044 0.3072 0.3071 0.3071
3.05 2.50 2.10 2.04 3.8 3.8 n.d.
A 1:1 mixture before heating. Sample was exchanged after crystallization in 0.2 M Na2CO3 solution.
bases; reactions of low-soluble M2 + and/or M3 + compounds with solutions and reaction of mixed (Mg, Al) oxides with water or aqueous solutions of suitable anions (Miyata, 1980; Pausch et al., 1986; Misra and Perrotta, 1992; Martin et al., 1998). The last method is widely used to obtain hydrotalcites with various inorganic or organic anions, as well as to extract anions from solutions (Chibwe and Jones, 1989; Cavani et al., 1991; Newman and Jones, 1998). The mixed oxides are prepared by thermal decomposition of carbonate or nitrate form of hydrotalcites. Between 400 and 800 jC, the mixture becomes finegrained and porous and assumes a periclase-like structure (NaCl-type) (Miyata, 1980). The ability of the periclase-like metaphase (HT-P according to our notation, Stanimirova et al., 1999) to restore the layered hydroxide structure after wetting is described in the literature as ‘‘memory effect’’ (Cavani et al.,
1991). The processes of transformation of hydrotalcite into HT-P and vice versa usually are considered topotactic (Sato et al., 1988) and structural schemes of these transformations are suggested (Marchi and Apesteguia, 1998). We have demonstrated in a recent publication (Stanimirova et al., 2001) that: (1) during regeneration of the mixed oxide (produced either from natural macrocrystals or synthetic microcrystals), microcrystalline HT with entirely different morphology were formed; (2) regenerated samples were always of 3R1 polytype, unaffected by the precursor structure. Those results indicate that the regeneration proceeds by dissolution of the Mg, Al-oxide solid solutions and subsequent crystallization. A dissolution – crystallization mechanism was also supposed by Millange et al. (2000). The dissolution – crystallization mechanism explains the ease with which different anions are incor-
Table 2 Batch compositions, chemical composition, lattice parameter a, layer thickness c0 and Mg/M2 +/Al ratio of recrystallized layered double hydroxides Batch compositions
Modified samples CO32
As synthesised
2HT 2HT-P + 2 M Ni(NO3)2 2HT-P + 1 M Ni(NO3)2 2HT-P + 1 M CoSO4 2HT-P + 0.5 M ZnCl2
exch.
Mg (wt.%)
Al (wt.%)
M2 + (wt.%)
c0 (nm)
a (nm)
c0 (nm)
20.68 6.35 9.07 10.37 13.84
11.51 5.56 7.20 7.68 8.79
– 30.58 21.92 20.61 15.54
0.758 0.816 0.828 0.866 0.798
0.3044 0.3038 0.3042 0.3076 0.3060
– 0.790 0.776 0.775 0.785
Mg/M2 +/Al
2.00:0.00:1 1.27:2.53:1 1.40:1.40:1 1.51:1.23:1 1.75:0.73:1
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porated. This mechanism has the important consequence that the cations of the brucite-like layer can also be changed. In the present work, this modification is verified.
2. Material and methods Initial hydrotalcite samples were prepared by coprecipitation of 1 M nitrate solutions (Mg/Al = 2:1
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(2HT), 3:1 (3HT) and 3.7:1 (3.7HT)) with solutions of 0.8 M NaOH and 0.2 M Na2CO3 at pH c 10.5 and 60 jC. The suspensions aged for 2 days at 90 jC in polypropylene bottles. The precipitates were washed until pH = 8 of washing water and air-dried. The mixed oxides 2HT-P, 3HT-P and 3.7HT-P were obtained heating the 2HT, 3HT and 3.7HT at 600 jC for 2 h and then air-cooled. Hydrotalcite recrystallization was performed at room temperature through mixing of the HT-P sam-
Fig. 1. X-ray powder diffraction pattern of: (a) 3HT, (b) 3HT-P regenerated in an Al(OH)3 dispersion, (c) 2HT, (d) 2HT-P regenerated in Mg(NO3)2 solution, (e) 2HT-P regenerated in Mg(NO3)2 solution, CO23 exchanged.
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Fig. 2. Layer thickness (c0) vs. Mg/Al ratio: (D) hydrotalcites, prepared by precipitation: open symbols—our data, shaded symbols— after Miyata (1980); ( ) hydrotalcite, recrystallized from HT-P. Arrows connect starting and recrystallized compositions by the added reagents.
.
ples with: (1) 0.5, 1.0 or 2.0 M solutions of Mg(NO3)2, Ni(NO3)2, CoSO4, ZnCl2 and Na3Al (OH)6 (4 ml/1 g HT-P); (2) water suspensions of less soluble compounds as commercial magnesia alba (Mg4(CO3)3(OH)23H2O) or fresh-precipitated, washed until pH f7 and air-dried Al(OH)3 or Fe(OH)3 with low-soluble salts and water; (3) a mixed oxide of another Mg/Al ratio. Batch compositions are shown in Tables 1 and 2. After crystallization, the solution was pipetted; the precipitates were washed several times with water and air-dried. In order to compare results, parts of the obtained samples were exchanged with carbonate ions (0.2 M Na2CO3 solution for 24 h at 20 jC), washed and airdried. The phase composition of the initial, intermediate and end products was studied by X-ray powder diffractometry (TUR M62, Co Ka radiation, energy 30 kV, 25 mA, Si as internal standard). The Mg/Al ratio of the recrystallized samples was derived from the lattice parameters a and c (Miyata, 1980; Pausch et al., 1986). Ni, Mg, Co, Zn and Al contents were determined by atomic absorption spectroscopy (AAS).
Fig. 3. Computer fitting of the 006 reflection profile of: mixture of 2HT and 3.7HT (perfect homogenized)—before heating (a) and after regeneration (b); mixture of 2HT and 3HT (insufficient homogenized)—before heating (c) and after regeneration (d).
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3. Results and discussion The regenerated samples consist of HT-like phases with a few admixtures in some cases (Fig. 1). Their anionic composition is related to the composition of the regenerating solution and has a strong influence on the layer thickness c0. All samples exchange quantitatively carbonate ions. The powder diffraction patterns of the regenerated samples show some broadening of the basal reflections and no turbostratic disorder effects (high angle tail of the h0l-lins), which were observed for the starting samples. The layer thickness c0 (c0 = d003 for 3R polytipe) rather than lattice parameter c was used accounting for the possible mixed-layered character of the samples.
299
Fig. 2 is a plot of c0 vs. the Mg/Al ratio of our and literature data (Miyata, 1980) of carbonate samples, prepared by co-precipitation. The layer thickness c0 of the carbonate samples derived from mixed oxides are lying on a trend line. The data in Table 1 and in Figs. 1 and 2 show that crystallization in the presence of Mg2 + or Al3 + cations in the solution leads to drastic changes in the brucite-like layer composition. Addition of Mg2 + in the recrystallization solution increases the Mg/Al ratio to 3.8 as well as c0 and a (c0 due to the layer charge reduction and a due to the larger ion radius of Mg2 +). On the contrary, Al3 + addition to 3HT-P produced a sample with the smallest possible ratio Mg/Al = 2 and smaller c0 and a values. The regenerating process is very slow (it is completed for 24 –48
Fig. 4. X-ray powder diffraction pattern of 2HT and recrystallized LDH.
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h at room temperature), so the cation composition can be modified both by easily soluble salts and by very low soluble reagents such as Mg-alba, Al(OH)3, Mg(OH)2, etc. In the experiments, Mg2 + and Al3 + ions are added in significant excess in order to modify the Mg/Al ratio of the initial samples to the extremely possible values. The lower limit (2:1) is determined by Pauling’s rules and Mg/Al = 3.8 seems to be the upper limit at these crystallization conditions. If only limited amounts of Mg2 + or Al3 + ions are added, any composition within the these limits can be obtained from a given initial composition. Intermediate compositions can also be produced applying appropriate proportions of mixed oxides with different Mg/Al ratios. Fig. 3 shows the 006 reflection of mixtures of 2HT and 3.7HT (a) and 2HT and 3HT (c) before heating and after regeneration of the resulting mixed oxides in water (b, d). In all investigated mixtures, the Mg/Al ratio was equal to the weighted average of both ratios of the initial samples. The width of the diffraction lines was affected by the degree of homogenization of the mixtures. The 006 reflection could be plausibly fitted by the sum of the reflections of mainly HT of an average composition similar to the initial samples (Fig. 3b,d). This indicates that dissolved ions
from HT-P migrates at only very small distances before crystallizing in the new HT phase. Table 2 summarizes data of hydrotalcite samples regenerated in solutions of cations other than Mg2 + or Al3 +. Fig. 4 shows the X-ray powder diffraction patterns and Fig. 5 is a plot M3 +/(M2 + + M3 +) vs. Mg/(M2 + + Mg) for the starting and newly formed samples. The presence of the cations in the regenerating solution caused substantial changes in the composition of the octahedral layer. This is clearly expressed by the changes of the a parameter and the powder diffraction patterns. Similar observation were reported by Tichit et al. (2001) in the case of samples obtained by precipitation. The incorporation of Ni2 + and Co2 + ions in the samples is visually evident. The sediment becomes greenish-blue and pink and the solution become less intensive colored down to colorless. Like in the case of Mg2 + and Al3 +, the M2 +/M3 + ratio also changed when heavy divalent cations were added (Table 2 and Fig. 5). The maximum ratio Ni2 +/ Al was again 3.8 with a significant Ni2 + surplus in the solution. A part of the heavy cations were incorporated in the octahedral layer on Mg2 + positions. In all cases, the Mg content was less than in the initial sample and Mg2 + ions were detected in the regenerating solutions. Probably, some divalent cations are preferred to Mg2 +. A similar preference was detected when multicomponent HT-like compositions were directly precipitated from solutions. The preference was related to the different precipitation pH of the pure hydroxides (f 8 of Ni and Co and >9 of Mg) (Tichit et al., 2001). Thus, that multi-component HT-like compositions may be obtained by HT-P crystallization in more complex solutions. In the experiments with Cu2 + and Fe3 +, some changes in the diffraction patterns indicated that these cations were incorporated in the newly formed hydrotalcites. However, some impurities were detected, visually and by X-ray so that a quantitative estimation of the degree of substitution was impossible.
4. Conclusions Fig. 5. Composition diagram of the starting 2HT and recrystallized samples.
The distinct changes of the composition of the octahedral layer is explained by a dissolution –
T. Stanimirova, G. Kirov / Applied Clay Science 22 (2003) 295–301
crystallization mechanism. An average composition of the newly formed LDH from mixtures of HT-P with different Mg/Al ratios (Fig. 3) is not expected, if the regeneration proceeds by a topotactic mechanism. On the basis of the dissolution – crystallization mechanism, it is understandable that different anions of the regeneration solution are implanted in the growing crystals. The process is, therefore, not an ‘‘ion exchange’’ as claimed for the topotactic regeneration. As we noted before (Stanimirova et al., 2001), ‘‘the only observed indication for the ‘memory effect’ is the preservation of the Mg/Al ratio of the initial sample in the regenerated samples’’. The present data indicate that this argument of the ‘‘memory effect’’ is only realized in a special case, when no suitable octahedral cations are present in the regeneration solution.
References Cavani, F., Trifiro, F., Vaccari, A., 1991. Hydrotalcite-type anionic clays: preparation, properties and applications. Catal. Today 11, 173 – 301. Chibwe, K., Jones, W., 1989. Intercalation of organic and inorganic anions unto layered double hydroxides. J. Chem. Soc., Chem. Commun., 926 – 927. Marchi, A.J., Apesteguia, C.R., 1998. Impregnation-induced mem-
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ory effect of thermally activated layered double hydroxides. Appl. Clay Sci. 13, 35 – 48. Martin, E.S., Stinson, J.M., Cedro III, V., Horn Jr., W.E., 1998. Two-powder synthesis of hydrotalcite and hydrotalcite-lice compounds with polyvalent inorganic anions. US Pat. No. 5,730,951. Millange, F., Walton, R.I., O’Hare, D., 2000. Time-resolved in situ X-ray diffraction study of the liquid-phase reconstruction of Mg – Al – carbonate hydrotalcite-like compounds. J. Mater. Chem. 10, 1713 – 1720. Misra, C., Perrotta, A.J., 1992. Composition and properties of synthetic hydrotalcites. Clays Clay Miner. 40, 145 – 150. Miyata, S., 1980. Physico-chemical properties of synthetic hydrotalcites in relation to composition. Clays Clay Miner. 28, 50 – 56. Newman, S.P., Jones, W., 1998. Synthesis, characterization and application of layered double hydroxides containing organic guests. New J. Chem., 105 – 115. Pausch, I., Lohse, H.-H., Schurmann, K., Allmann, R., 1986. Synthesis of disordered and Al-rich hydrotalcite-like compounds. Clays Clay Miner. 34, 507 – 510. Sato, T., Fujita, H., Endo, T., Shimada, M., Tsunashima, A., 1988. Synthesis of hydrotalcite-like compounds and their physicochemical properties. React. Solids 5, 219 – 228. Stanimirova, Ts., Petrova, N., Vergilov, I., Kirov, G., 1999. Thermal decomposition products of hydrotalcite-like compounds: low-temperature metaphases. J. Mater. Sci. 34, 4153 – 4161. Stanimirova, Ts., Kirov, G., Dinolova, E., 2001. Mechanism of hydrotalcite regeneration. J. Mater. Sci. Lett. 20, 453 – 455. Tichit, D., Riber, S., Coq, B., 2001. Characterization of calcined and reduced multi-component Co – Ni – Mg – Al-layered double hydroxides. Eur. J. Inorg. Chem., 539 – 546. Vaccari, A., 1999. Clays and catalysis: a promising future. Appl. Clay Sci. 14, 161 – 198.
Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides Tsveta Stanimirova *, Georgi Kirov Department of Mineralogy, Sofia University, 15 Tzar Osvoboditel, Sofia 1000, Bulgaria Received 8 March 2002; accepted 7 April 2003
Abstract Changes in cation composition and M2 +/M3 + ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations. Heating Mg – Al – CO3 hydrotalcites with Mg/Al = 2, 3 and 3.7, at 600 jC for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations. The presence of Mg2 +, Zn2 +, Ni2 +, Co2 +, Cu2 + salts or of low soluble hydromagnesite increased the M2 +/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn2 +, Ni2 +, Co2 + and Cu2 + cations in the newly formed hydrotalcite was detected, while Mg2 + remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH)3, the M2 +/Al ratio approximated the minimal possible value of 2. The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value. The results obtained support the conception of the dissolution – crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Hydrotalcite; Layered double hydroxides; Cation modification; Regeneration
1. Introduction Layered double hydroxides (LDH) are described m by the general formula [M12 + x M3x +(OH)2]x +Ax/m 3+ nH2O, where M (Al, Fe, Cr, Ga) partially substitutes M2 + (Mg, Ni, Zn, Cu, Fe, etc.) in the brucitelike layers. Surplus positive charge is balanced by exchangeable anions Am (CO32 , SO42 , NO3 , Cl , * Corresponding author. E-mail address: [email protected] (T. Stanimirova).
Br , OH , etc.) in the interlayer space, where water is also present (Cavani et al., 1991). Thermal treatment causes structural changes and yields several metaphases (Stanimirova et al., 1999). The wide possibilities for thermal modification and for isomorphic substitutions both in the cationic and anionic parts make it possible that several materials are produced for various applications (Cavani et al., 1991; Vaccari, 1999). Several methods are used for the synthesis of LDH: precipitation from metal salts solutions with
0169-1317/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-1317(03)00122-4
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Table 1 Batch compositions, layer thickness (c0) and M2 +/Al ratio of the starting and recrystallized layered double hydroxides No.
Starting samples
a
1 2a 3 4 5 6 7 a b
Batch compositions
Mg/Al
c0 (nm)
a (nm)
2.0 + 3.7 2.0 + 3.0 3.0 3.0 2.0 2.0 2.0
0.758, 0.790 0.758, 0.778 0.778 0.778 0.758 0.758 0.758
0.3044, 0.3070 0.3044, 0.3060 0.3060 0.3060 0.3044 0.3044 0.3044
Recrystallized samples
(2HT-P + 3.7HT-P) + H2O (2HT-P + 3HT-P) + H2O 3HT-P + 0.5 M Na3Al(OH)6 3HT-P + Al(OH)3 + H2O 2HT-P + Mg4(CO3)3(OH)23H2O + H2O 2HT-P + 0.5 M Mg(NO3)2 (CO23 exch.)b 2HT-P + 0.5 M Mg(NO3)2
c0 (nm)
a (nm)
M2 +/Al
0.779 0.769 0.760 0.758 0.792 0.792 0.816
0.3062 0.3051 0.3045 0.3044 0.3072 0.3071 0.3071
3.05 2.50 2.10 2.04 3.8 3.8 n.d.
A 1:1 mixture before heating. Sample was exchanged after crystallization in 0.2 M Na2CO3 solution.
bases; reactions of low-soluble M2 + and/or M3 + compounds with solutions and reaction of mixed (Mg, Al) oxides with water or aqueous solutions of suitable anions (Miyata, 1980; Pausch et al., 1986; Misra and Perrotta, 1992; Martin et al., 1998). The last method is widely used to obtain hydrotalcites with various inorganic or organic anions, as well as to extract anions from solutions (Chibwe and Jones, 1989; Cavani et al., 1991; Newman and Jones, 1998). The mixed oxides are prepared by thermal decomposition of carbonate or nitrate form of hydrotalcites. Between 400 and 800 jC, the mixture becomes finegrained and porous and assumes a periclase-like structure (NaCl-type) (Miyata, 1980). The ability of the periclase-like metaphase (HT-P according to our notation, Stanimirova et al., 1999) to restore the layered hydroxide structure after wetting is described in the literature as ‘‘memory effect’’ (Cavani et al.,
1991). The processes of transformation of hydrotalcite into HT-P and vice versa usually are considered topotactic (Sato et al., 1988) and structural schemes of these transformations are suggested (Marchi and Apesteguia, 1998). We have demonstrated in a recent publication (Stanimirova et al., 2001) that: (1) during regeneration of the mixed oxide (produced either from natural macrocrystals or synthetic microcrystals), microcrystalline HT with entirely different morphology were formed; (2) regenerated samples were always of 3R1 polytype, unaffected by the precursor structure. Those results indicate that the regeneration proceeds by dissolution of the Mg, Al-oxide solid solutions and subsequent crystallization. A dissolution – crystallization mechanism was also supposed by Millange et al. (2000). The dissolution – crystallization mechanism explains the ease with which different anions are incor-
Table 2 Batch compositions, chemical composition, lattice parameter a, layer thickness c0 and Mg/M2 +/Al ratio of recrystallized layered double hydroxides Batch compositions
Modified samples CO32
As synthesised
2HT 2HT-P + 2 M Ni(NO3)2 2HT-P + 1 M Ni(NO3)2 2HT-P + 1 M CoSO4 2HT-P + 0.5 M ZnCl2
exch.
Mg (wt.%)
Al (wt.%)
M2 + (wt.%)
c0 (nm)
a (nm)
c0 (nm)
20.68 6.35 9.07 10.37 13.84
11.51 5.56 7.20 7.68 8.79
– 30.58 21.92 20.61 15.54
0.758 0.816 0.828 0.866 0.798
0.3044 0.3038 0.3042 0.3076 0.3060
– 0.790 0.776 0.775 0.785
Mg/M2 +/Al
2.00:0.00:1 1.27:2.53:1 1.40:1.40:1 1.51:1.23:1 1.75:0.73:1
T. Stanimirova, G. Kirov / Applied Clay Science 22 (2003) 295–301
porated. This mechanism has the important consequence that the cations of the brucite-like layer can also be changed. In the present work, this modification is verified.
2. Material and methods Initial hydrotalcite samples were prepared by coprecipitation of 1 M nitrate solutions (Mg/Al = 2:1
297
(2HT), 3:1 (3HT) and 3.7:1 (3.7HT)) with solutions of 0.8 M NaOH and 0.2 M Na2CO3 at pH c 10.5 and 60 jC. The suspensions aged for 2 days at 90 jC in polypropylene bottles. The precipitates were washed until pH = 8 of washing water and air-dried. The mixed oxides 2HT-P, 3HT-P and 3.7HT-P were obtained heating the 2HT, 3HT and 3.7HT at 600 jC for 2 h and then air-cooled. Hydrotalcite recrystallization was performed at room temperature through mixing of the HT-P sam-
Fig. 1. X-ray powder diffraction pattern of: (a) 3HT, (b) 3HT-P regenerated in an Al(OH)3 dispersion, (c) 2HT, (d) 2HT-P regenerated in Mg(NO3)2 solution, (e) 2HT-P regenerated in Mg(NO3)2 solution, CO23 exchanged.
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Fig. 2. Layer thickness (c0) vs. Mg/Al ratio: (D) hydrotalcites, prepared by precipitation: open symbols—our data, shaded symbols— after Miyata (1980); ( ) hydrotalcite, recrystallized from HT-P. Arrows connect starting and recrystallized compositions by the added reagents.
.
ples with: (1) 0.5, 1.0 or 2.0 M solutions of Mg(NO3)2, Ni(NO3)2, CoSO4, ZnCl2 and Na3Al (OH)6 (4 ml/1 g HT-P); (2) water suspensions of less soluble compounds as commercial magnesia alba (Mg4(CO3)3(OH)23H2O) or fresh-precipitated, washed until pH f7 and air-dried Al(OH)3 or Fe(OH)3 with low-soluble salts and water; (3) a mixed oxide of another Mg/Al ratio. Batch compositions are shown in Tables 1 and 2. After crystallization, the solution was pipetted; the precipitates were washed several times with water and air-dried. In order to compare results, parts of the obtained samples were exchanged with carbonate ions (0.2 M Na2CO3 solution for 24 h at 20 jC), washed and airdried. The phase composition of the initial, intermediate and end products was studied by X-ray powder diffractometry (TUR M62, Co Ka radiation, energy 30 kV, 25 mA, Si as internal standard). The Mg/Al ratio of the recrystallized samples was derived from the lattice parameters a and c (Miyata, 1980; Pausch et al., 1986). Ni, Mg, Co, Zn and Al contents were determined by atomic absorption spectroscopy (AAS).
Fig. 3. Computer fitting of the 006 reflection profile of: mixture of 2HT and 3.7HT (perfect homogenized)—before heating (a) and after regeneration (b); mixture of 2HT and 3HT (insufficient homogenized)—before heating (c) and after regeneration (d).
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3. Results and discussion The regenerated samples consist of HT-like phases with a few admixtures in some cases (Fig. 1). Their anionic composition is related to the composition of the regenerating solution and has a strong influence on the layer thickness c0. All samples exchange quantitatively carbonate ions. The powder diffraction patterns of the regenerated samples show some broadening of the basal reflections and no turbostratic disorder effects (high angle tail of the h0l-lins), which were observed for the starting samples. The layer thickness c0 (c0 = d003 for 3R polytipe) rather than lattice parameter c was used accounting for the possible mixed-layered character of the samples.
299
Fig. 2 is a plot of c0 vs. the Mg/Al ratio of our and literature data (Miyata, 1980) of carbonate samples, prepared by co-precipitation. The layer thickness c0 of the carbonate samples derived from mixed oxides are lying on a trend line. The data in Table 1 and in Figs. 1 and 2 show that crystallization in the presence of Mg2 + or Al3 + cations in the solution leads to drastic changes in the brucite-like layer composition. Addition of Mg2 + in the recrystallization solution increases the Mg/Al ratio to 3.8 as well as c0 and a (c0 due to the layer charge reduction and a due to the larger ion radius of Mg2 +). On the contrary, Al3 + addition to 3HT-P produced a sample with the smallest possible ratio Mg/Al = 2 and smaller c0 and a values. The regenerating process is very slow (it is completed for 24 –48
Fig. 4. X-ray powder diffraction pattern of 2HT and recrystallized LDH.
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h at room temperature), so the cation composition can be modified both by easily soluble salts and by very low soluble reagents such as Mg-alba, Al(OH)3, Mg(OH)2, etc. In the experiments, Mg2 + and Al3 + ions are added in significant excess in order to modify the Mg/Al ratio of the initial samples to the extremely possible values. The lower limit (2:1) is determined by Pauling’s rules and Mg/Al = 3.8 seems to be the upper limit at these crystallization conditions. If only limited amounts of Mg2 + or Al3 + ions are added, any composition within the these limits can be obtained from a given initial composition. Intermediate compositions can also be produced applying appropriate proportions of mixed oxides with different Mg/Al ratios. Fig. 3 shows the 006 reflection of mixtures of 2HT and 3.7HT (a) and 2HT and 3HT (c) before heating and after regeneration of the resulting mixed oxides in water (b, d). In all investigated mixtures, the Mg/Al ratio was equal to the weighted average of both ratios of the initial samples. The width of the diffraction lines was affected by the degree of homogenization of the mixtures. The 006 reflection could be plausibly fitted by the sum of the reflections of mainly HT of an average composition similar to the initial samples (Fig. 3b,d). This indicates that dissolved ions
from HT-P migrates at only very small distances before crystallizing in the new HT phase. Table 2 summarizes data of hydrotalcite samples regenerated in solutions of cations other than Mg2 + or Al3 +. Fig. 4 shows the X-ray powder diffraction patterns and Fig. 5 is a plot M3 +/(M2 + + M3 +) vs. Mg/(M2 + + Mg) for the starting and newly formed samples. The presence of the cations in the regenerating solution caused substantial changes in the composition of the octahedral layer. This is clearly expressed by the changes of the a parameter and the powder diffraction patterns. Similar observation were reported by Tichit et al. (2001) in the case of samples obtained by precipitation. The incorporation of Ni2 + and Co2 + ions in the samples is visually evident. The sediment becomes greenish-blue and pink and the solution become less intensive colored down to colorless. Like in the case of Mg2 + and Al3 +, the M2 +/M3 + ratio also changed when heavy divalent cations were added (Table 2 and Fig. 5). The maximum ratio Ni2 +/ Al was again 3.8 with a significant Ni2 + surplus in the solution. A part of the heavy cations were incorporated in the octahedral layer on Mg2 + positions. In all cases, the Mg content was less than in the initial sample and Mg2 + ions were detected in the regenerating solutions. Probably, some divalent cations are preferred to Mg2 +. A similar preference was detected when multicomponent HT-like compositions were directly precipitated from solutions. The preference was related to the different precipitation pH of the pure hydroxides (f 8 of Ni and Co and >9 of Mg) (Tichit et al., 2001). Thus, that multi-component HT-like compositions may be obtained by HT-P crystallization in more complex solutions. In the experiments with Cu2 + and Fe3 +, some changes in the diffraction patterns indicated that these cations were incorporated in the newly formed hydrotalcites. However, some impurities were detected, visually and by X-ray so that a quantitative estimation of the degree of substitution was impossible.
4. Conclusions Fig. 5. Composition diagram of the starting 2HT and recrystallized samples.
The distinct changes of the composition of the octahedral layer is explained by a dissolution –
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crystallization mechanism. An average composition of the newly formed LDH from mixtures of HT-P with different Mg/Al ratios (Fig. 3) is not expected, if the regeneration proceeds by a topotactic mechanism. On the basis of the dissolution – crystallization mechanism, it is understandable that different anions of the regeneration solution are implanted in the growing crystals. The process is, therefore, not an ‘‘ion exchange’’ as claimed for the topotactic regeneration. As we noted before (Stanimirova et al., 2001), ‘‘the only observed indication for the ‘memory effect’ is the preservation of the Mg/Al ratio of the initial sample in the regenerated samples’’. The present data indicate that this argument of the ‘‘memory effect’’ is only realized in a special case, when no suitable octahedral cations are present in the regeneration solution.
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