Synthesis of layered double hydroxide anionic clays intercalated by carboxylate anions

Materials Chemistry and Physics 85 (2004) 207–214

Synthesis of layered double hydroxide anionic clays intercalated by carboxylate anions Jie Zhang, Fazhi Zhang, Lingling Ren, David G. Evans, Xue Duan∗ Education Ministry Key Laboratory of Science and Technology of Controllable Chemical Reactions, Beijing University of Chemical Technology, Box 98, 15 Beisahuan Dong Lu, Beijing 100029, PR China Received 31 October 2003; received in revised form 25 November 2003; accepted 19 January 2004

Abstract Layered double hydroxides (LDHs) containing carboxylate anions are often contaminated with carbonate or other anions arising from atmospheric carbon dioxide or precursor salts, respectively. If the structure of the layered host is to be exploited in order to control features such as the stereochemistry of reactions of intercalated guest species or the kinetics of their ion-exchange, or if the structure of the host–guest assembly is to be compared with molecular dynamics simulations it is essential to prepare materials containing the target anion only. In this paper, we show that LDHs containing citrate, oxalate, tartrate and malate ions free from other anions may be obtained by a simple procedure involving dissolution of a magnesium–aluminium, nickel–aluminium or zinc–aluminium carbonate-containing LDH precursor by addition of the appropriate carboxylic acid followed by precipitation of the product by addition of the mixture to a basic solution. In contrast to other synthetic procedures for LDHs, the reaction does not need to be carried out under nitrogen in order to prevent co-intercalation of carbonate ions. © 2004 Elsevier B.V. All rights reserved. Keywords: Layered double hydroxides; Hydrotalcite-like compounds; Intercalation; Carboxylate

1. Introduction Layered double hydroxides (LDHs) are a class of synthetic anionic clays whose structure can be described as containing brucite (Mg(OH)2 )-like layers in which some of the divalent cations have been replaced by trivalent ions giving positively charged sheets [1]. This charge is balanced by intercalation of anions in the hydrated interlayer regions. LDHs can be represented by the general formula II MIII (OH) ]x+ (An− ) [M1−x 2 x/n · yH2 O. The identities of the x di- and trivalent cations (MII and MIII , respectively) and the interlayer anion (An− ) together with the value of the stoichiometric coefficient (x) may be varied over a wide range, giving rise to a large class of isostructural materials. Anions can range from simple inorganic species [1], through polyoxometallates [2] to organic anions [3] to polymers [4]. LDHs containing interlayer carboxylate anions have attracted considerable attention in recent years because these materials have a number of interesting properties and potential applications. For example, Jones and coworkers [5] ∗ Corresponding author. Tel.: +86-10-6442-5395; fax: +86-10-6442-5385. E-mail address: [email protected] (X. Duan).

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.01.020

have shown that guest anions such as terephthalate adopt different orientations with respect to the LDH layers as a function of the charge density on the layers and the degree of hydration of the interlayer region. Intercalation of carboxylic acid compounds such as ibuprofen [6] and Vitamin A acid (retinoic acid) [7] in LDH hosts has been reported and it has been suggested that these materials may find application in the storage, transport and controlled release of these and other pharmaceutically active compounds. Mann and coworkers [8] have reported that intercalation of aspartic acid may be followed by in situ thermal polymerization in the interlayer region and the stereoselectivity of in situ photochemical dimerization of cinnamate [9] and 4 -chloro-4-stilbenecarboxylate [10] in LDHs has been shown to be dependent on the arrangement in the interlayer region. In order to develop these potentially interesting applications of carboxylate-intercalated LDHs, the ability to tune key properties of the materials, such as the rate of drug release or the stereochemistry of polymerization, is required. These properties will depend on the supramolecular host–guest and guest–guest interactions. There has also been recent interest in molecular dynamics simulations of LDHs containing carboxylate anions [5,11]. In order to have the

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maximum control over the structure of the interlayer region, as well as be able to compare the results of experimentally determined structures with computer models, it is desirable that there should be only one type of interlayer guest anion present in the interlayer region, i.e. the carboxylate anion of interest. Unfortunately, this objective is often not easy to achieve in practice. Synthetic procedures for the intercalation of carboxylic acids into LDHs have been critically reviewed by Carlino [12]. In coprecipitation reactions, even when the reaction is carried out under nitrogen, co-intercalation of carbonate ions arising from atmospheric carbon dioxide is sometimes observed. Even where carbonate contamination is prevented, the materials are often contaminated by precursor anions from the mixed salt solution [8,13]. Alternatively, interlayer chloride or nitrate ions in LDH precursors can be replaced by carboxylate anions in an ion-exchange reaction, but exchange is often not complete [14]. A final method involves the calcination of LDH-carbonates at intermediate temperatures (450–500 ◦ C) to give mixed metal oxides which on subsequent reaction with an aqueous solution of a carboxylate salt under nitrogen can reform the LDH layers with intercalated carboxylate anions. In this case, the calcination conditions have a crucial effect on the purity and crystallinity of the resulting LDH-carboxylate and must be optimized in each case [12]. In addition, the strongly basic nature of the intermediate solids can lead to ready uptake of carbon dioxide. Furthermore, in some cases, no reconstruction of the LDH phase is observed [15]. In this paper, we report a general method for the preparation of carboxylate-pillared LDHs which does not suffer from any of the problems discussed above such as competitive intercalation of carbonate or other anions and is easy to carry out. The method involves dissolution of an LDH-carbonate precursor by addition of an organic acid followed by re-precipitation of the LDH-carboxylate at variable pH. Unlike the usual coprecipitation reactions, the

re-precipitation does not need to be carried out under nitrogen in order to prevent contamination by co-intercalated carbonate. We [16] have recently developed a new process for the synthesis of the carbonate-pillared LDH precursors with a narrow range of particle sizes, which facilitates production of the LDH-carbonate precursor required for the reactions reported here in large quantities. 2. Experimental 2.1. Preparation of LDH precursors An Mg/Al-LDH (Mg/Al = 2.11) containing carbonate as the interlayer anion was synthesized by a modified coprecipitation method as described elsewhere [16]. Samples of Ni/Al-LDH (Ni/Al = 1.92) and Zn/Al-LDH (Zn/Al = 2.17) carbonate-LDHs were prepared in a similar fashion. 2.2. Preparation of citrate, oxalate, tartrate and malate-containing LDHs Citrate-LDH was prepared by adding citric acid (8.82 g, 0.042 mol) to a suspension of Mg/Al-LDH-carbonate (4.0 g, ca. 0.008 mol) in 100 ml distilled water at 50 ◦ C. The suspension slowly dissolved with effervescence and a clear solution was obtained. This solution was added dropwise to an alkaline solution (NaOH (4.0 g, 0.10 mol) dissolved in 100 ml water) maintaining the pH above 9, followed by refluxing for 2 h. The resulting solid was recovered by filtration, washed, and dried at 70 ◦ C for 16 h. Oxalate-LDH was prepared by using oxalic acid (7.62 g, 0.060 mol) in place of the citric acid. Tartrate-LDH was prepared by using tartaric acid (9.90 g, 0.066 mol) in place of the citric acid. Malate-LDH was prepared by using malic acid (8.04 g, 0.060 mol) in place of the citric acid. Reactions of Ni/Al-LDH and Zn/Al-LDH carbonate-LDHs with citric acid were carried out in a similar fashion.

Fig. 1. Powder XRD pattern of the Mg/Al-LDH-carbonate precursor. The 003 reflection at low angle corresponds to the basal spacing.

J. Zhang et al. / Materials Chemistry and Physics 85 (2004) 207–214

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Fig. 2. Powder XRD patterns for the carboxylate-intercalated Mg/Al-LDHs. The basal reflection shifts to lower angle when carbonate is replaced by the larger carboxylate anions: (a) citrate; (b) oxalate; (c) tartrate; (d) malate.

Table 1 Comparison of basal spacings for materials prepared in this work with previously reported values Basal spacing/nm

This work

Literature

Mg/Al–CO3 -LDH Mg/Al–citrate-LDH Mg/Al–oxalate-LDH Mg/Al–tartrate-LDH Mg/Al–malate-LDH Ni/Al–CO3 -LDH Zn/Al–CO3 -LDH Ni/Al–citrate-LDH Zn/Al–citrate-LDH

0.761 1.18 0.960 1.20 1.22 0.750 0.753 1.17 1.23

0.760 [1] 1.20 [18], 1.22 [14] 0.96 [20] 0.960, 1.23 [15] None 0.789 [21] 0.755 [22] None 1.21 [14]

FT-IR spectra were recorded on a Bruker Vector 22 instrument. The sample was finely ground for 1 min, combined with oven dried spectroscopic grade KBr and pressed into a disc. The spectrum of each sample was recorded in triplicate by accumulating 20 scans at 2 cm−1 resolution between 400 and 4000 cm−1 . Thermogravimetric analysis and differential thermal analysis (TG–DTA) were carried out on PCT-1A thermal analysis system produced locally in flowing air at a heating rate of 10 ◦ C min−1 . Samples were dried at 100 ◦ C for 24 h prior to analysis. 3. Results and discussion

2.3. Characterization Powder XRD patterns of the samples were recorded using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, Cu K␣ (λ = 0.15406 nm) radiation. The samples as unoriented powders were step-scanned in steps of 0.02◦ (2θ) in the range from 3 to 70◦ using a count time of 4 s per step. Elemental analyses were performed by ICP emission spectroscopy using an Ultima instrument on solutions prepared by dissolving the samples in dilute HNO3 .

The XRD pattern for the magnesium–aluminium carbonate-LDH precursor showed the characteristic reflections corresponding to a well-crystallized layered phase with an interlayer spacing of 0.76 nm (Fig. 1) similar to that reported in [17]. When citric, tartaric, oxalate or malic acids were added to a suspension of the magnesium–aluminium carbonate-LDH in warm water it dissolved readily with effervescence to give colorless solutions. Addition of these solutions to aqueous sodium hydroxide solution afforded gelatinous precipitates, which after aging, separation, washing and drying gave white solid products. The powder

Table 2 Chemical compositions of the Mg/Al-LDH-carbonate precursor and the carboxylate-intercalated LDHs

Citrate Oxalate Tartrate Malate

Found (calculated)

Mg (%)

Al (%)

C (%)

H (%)

Mg/Al

Mg0.67 Al0.32 (OH)2.0 (CO3 )0.15 ·0.38H2 O Mg0.68 Al0.34 (OH)2.0 (C6 H5 O7 )0.12 ·0.68H2 O Mg0.68 Al0.32 (OH)2.0 (C2 O4 )0.16 ·0.66H2 O Mg0.66 Al0.34 (OH)2.0 (C4 H4 O6 )0.17 ·0.36H2 O Mg0.64 Al0.36 (OH)2.0 (C4 H4 O5 )0.18 ·0.58H2 O

21.35 17.03 19.02 17.51 16.07

11.38 9.62 9.93 9.89 10.33

2.38 9.79 4.72 9.24 9.34

3.65 4.06 3.76 3.80 3.72

2.1 2.0 2.1 1.9 1.8

(21.22) (17.29) (19.20) (17.56) (16.20)

(11.40) (9.66) (10.15) (10.05) (10.27)

(2.38) (9.19) (4.57) (9.06) (9.20)

(3.64) (4.16) (3.90) (3.78) (4.15)

(2.1) (2.0) (2.1) (1.9) (1.8)

J. Zhang et al. / Materials Chemistry and Physics 85 (2004) 207–214

Transmittance(a.u.)

Transmittance(a.u.)

210

1581

1399

1361

4000

3500

3000

2500

2000

1500

1000

500

-1

(a)

3500

3000

2500

2000

1500

4000

1408

3000

2500

2000

1500

1000

500

-1

4000

3500

3000

1390

2500

2000

1500

1000

500

-1

(d)

Wavenumber(cm )

Wavenumber(cm )

Transmittance(a.u.)

3500

500

Wavenumber(cm )

1613

(c)

1000

-1

Transmittance(a.u.)

Transmittance(a.u.)

Wavenumber(cm )

16 36

4000

(b)

1399 16 25

4000

(e)

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber(cm )

Fig. 3. Infrared spectra of the Mg/Al-LDH-carbonate precursor and the carboxylate-intercalated LDHs. The characteristic carbonate absorption at 1361 cm−1 is absent in the spectra of the carboxylate derivatives: (a) carbonate; (b) citrate; (c) oxalate; (d) tartrate; (e) malate.

J. Zhang et al. / Materials Chemistry and Physics 85 (2004) 207–214

XRD patterns of the four materials (Fig. 2) are typical of layered materials, showing the basal peak and higher order reflections at low angle. Intercalation of the carboxylate anions is clearly seen in each case by virtue of the increase in basal spacing (Table 1) compared with that in the carbonate precursor. The values of basal spacing for citrate, oxalate and tartrate-pillared LDHs are closely comparable

211

to those reported in the literature for LDH materials intercalated with the same carboxylate anions produced by other methods (Table 1), although as discussed above these latter materials may be contaminated with carbonate or other anions. Given that the thickness of the brucite-like layer of LDH is 0.48 nm, the gallery height in the citrate- and oxalate-pillared materials is 0.70 and 0.48 nm, respectively,

Fig. 4. TG and DTA curves for the Mg/Al-LDH-carbonate precursor and the carboxylate-intercalated LDHs: (a) carbonate; (b) citrate; (c) oxalate; (d) tartrate; (e) malate.

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compared with 0.28 nm in the carbonate-containing LDH [16]. These data suggest a monolayer arrangement for the intercalated carboxylic acids oriented perpendicular to the LDH layers. As shown in Table 1, Forano and coworkers have reported [15] two values for the basal spacing for tartrate-pillared LDHs depending on the drying conditions. They have rationalized the contraction in basal spacing observed on drying at higher temperatures in terms of a reorientation of the tartrate ion from initially being perpendicular to the layers to becoming parallel to the layers. In our case, the interlayer spacing (1.20 nm) and gallery height (0.72 nm) for the tartrate-pillared material corresponds to the perpendicular orientation. The intercalation of malate ions in LDHs has not previously been reported to our knowledge, but on the basis of its molecular structure the basal spacing can be expected to be very similar to that of tartrate-pillared LDHs. As shown in Table 1, the interlayer spacing (1.22 nm) and gallery height (0.78 nm) are consistent with the perpendicular orientation of the guest ion. Elemental analysis results and calculated chemical compositions for the carboxylate-pillared LDHs are shown in Table 2. The observed Mg2+ /Al3+ ratios for citrate- and oxalate-pillared LDHs are very similar to that in the precursor LDH-carbonate (Section 2.1). The analytical data suggest that the oxalate, tartrate and malate ions exist as dianions, consistent with the literature for other synthetic routes. The analytical data for the citrate intercalate can best be interpreted in terms of a trianionic species being present in the interlayer. Previous reports [14,18] of citrate-intercalated LDHs do not include any analytical data so it is not clear whether the citrate is also present in a trianionic form in these materials. Since our method avoids the introduction of foreign anions, the only possibility of contamination is from carbonate arising from atmospheric carbon dioxide. As mentioned above, this is a common problem in LDH synthesis, even when the reaction is carried out under a nitrogen atmosphere.

For the carboxylate-intercalated LDHs reported here, the analytical data suggest no incorporation of carbonate anions in each case even though the reactions were carried out in air. The absence of carbonate was confirmed by the absence of effervescence when the samples were dissolved in dilute mineral acids. The absence of carbonate may be a consequence of the relatively high concentration of the organic anion in the reaction mixture. An alternative explanation is that the magnesium and aluminium ions are complexed by the carboxylate anions at low pH and as the pH is increased by addition of base, the complexes break down allowing the cations to enter the layers. The liberated organic ions enter the interlayer region in a pseudo-concerted process. The presence of a mixture of complex ions has been demonstrated by multinuclear NMR for aluminium–citrate solutions, supporting this interpretation [19]. The absence of interlayer carbonate was further confirmed by examination of the FT-IR spectra shown in Fig. 3. The spectrum of the LDH-carbonate precursor has an intense absorption at 1361 cm−1 which is not present in the spectra of any of the carboxylate intercalates. These spectra are dominated by the asymmetric and symmetric RCO2 − stretches of the organic guests at around 1600 and 1400 cm−1 , respectively [15]. Weaker bands arising from their alkyl C–H stretches are observed in the range 3000–2800 cm−1 and a broad intense absorption centered around 3500 cm−1 is associated with the hydrogen bonded network of layer hydroxyl groups and interlayer water molecules. Thermogravimetric analysis results for the carbonate-LDH showed an initial distinct reduction in mass between 65 and 235 ◦ C (about 15%, w/w) arising from loss of surface adsorbed and interlayer water. A major mass loss was observed starting at 270 ◦ C and continuing up to 500 ◦ C, resulting from simultaneous dehydroxylation of the inorganic layers and decomposition of intercalated carbonate anions (about 24%, w/w) (Fig. 3(a)). The corresponding DTA trace for the carbonate-LDH showed two endotherms centered at

Fig. 5. Powder XRD patterns for citrate-intercalated LDHs: (a) Ni/Al; (b) Zn/Al.

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Table 3 Chemical compositions of the Zn/Al and Zn/Al-LDH-carbonate precursors and their citrate intercalates Found (calculated)

MII (%)

MIII (%)

C (%)

H (%)

MII /MIII

MII = Ni, MIII = Al Ni0.66 Al0.34 (OH)2.0 (CO3 )0.17 ·0.68H2 O Ni0.64 Al0.36 (OH)2.0 (C6 H5 O7 )0.12 ·0.66H2 O

37.40 (37.22) 31.56 (31.91)

8.91 (8.82) 8.03 (8.38)

1.94 (1.96) 7.68 (7.48)

3.20 (3.23) 3.19 (3.37)

1.9 (1.9) 1.8 (1.8)

MII = Zn, MIII = Al Zn0.69 Al0.32 (OH)2.0 (CO3 )0.16 ·0.62H2 O Zn0.68 Al0.32 (OH)2.0 (C6 H5 O7 )0.11 ·0.58H2 O

41.66 (41.68) 37.26 (37.44)

7.91 (7.98) 7.18 (7.31)

1.80 (1.77) 6.95 (6.72)

3.04 (2.99) 3.11 (3.13)

2.2 (2.2) 2.1 (2.1)

229 and 400 ◦ C associated with these processes (Fig. 4(a)). In contrast, the carboxylate-intercalated LDHs underwent a mass loss between ambient temperature and 250 ◦ C (about 20%, w/w) (Fig. 4(b)–(e)), resulting from loss of surface adsorbed and interlayer water. The onset of a rapid and ma-

jor mass loss ensued between 320 and 500 ◦ C (about 30% loss, w/w) as dehydroxylation and decomposition of the organics occurred in an exothermic process (Fig. 4(b)–(e)). The corresponding DTA traces (Fig. 4(b)–(e)) showed exotherms in the temperature range 340–420 ◦ C associated

Fig. 6. Infrared spectra of citrate-intercalated LDHs: (a) Ni/Al; (b) Zn/Al.

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with combustion of the organic components. These results are consistent with those reported in the literature for these compounds prepared by other routes. In order to demonstrate the general applicability of the method, the synthesis was repeated using nickel–aluminium carbonate (Ni2+ /Al3+ = 1.92) and zinc–aluminium carbonate-LDH precursors (Zn2+ /Al3+ = 2.17) with citric acid as the carboxylic acid. As shown in Fig. 5, the powder XRD patterns are similar to that of the magnesium–aluminium citrate-LDH (see Fig. 2(a)). The analytical data (see Table 3) indicate the presence of the citrate trianion as for the magnesium analogue. The FT-IR spectra of the nickel–aluminium and zinc–aluminium citrate-LDHs illustrated in Fig. 6 shows the same asymmetric and symmetric RCO2 − stretches of the citrate, at around 1600 and 1400 cm−1 , respectively, as observed for the magnesium–aluminium analogue (see Fig. 3(b)). The bands below 1000 cm−1 are M–O vibration modes and the pattern is similar for the three LDH citrates, although the bands show shifts in position associated with the different atomic weights and bond strengths to oxygen of magnesium, nickel and zinc.

4. Conclusions LDHs containing citrate, oxalate, tartrate and malate ions free from other anions may be obtained by a simple procedure involving dissolution of a suspension of a carbonate-containing LDH precursor by addition of the appropriate carboxylic acid followed by precipitation of the product by addition of the mixture to aqueous sodium hydroxide solution. The reaction does not need to be carried out under nitrogen in order to prevent co-intercalation of carbonate ions. The absence of any co-intercalated carbonate ions has been confirmed by elemental analysis, IR spectroscopy and lack of evolution of carbon dioxide on reaction with dilute acid.

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