Urea method for the synthesis of hydrotalcites

Jointly published by React.Kinet.Catal.Lett.Akadémiai Kiadó, Budapest Vol. 83, No. 2, 275-282and Kluwer Academic Publishers, Dordrecht (2004)

0133-1736/2004/US$ 20.00.© Akadémiai Kiadó, Budapest.

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RKCL4455

UREA METHOD FOR THE SYNTHESIS OF HYDROTALCITES

Piaoping Yang, Jianfeng Yu, Zhenlü Wang, Qingsheng Liuand Tonghao Wu*

Chemistry College, Jilin University, Changchun, 130023, P R China

Received November 11, 2003In revised form April 29, 2004

Accepted May 3, 2004

Abstract

Mg-Al, Zn-Al and Ni-Al hydrotalcite-like compounds with high crystallinity weresynthesized by using the decomposing property of urea at high temperature. Thecrystallinity and the regularity of the samples were much preferable to thosesynthesized by other methods.

Keywords: Hydrotalcite-likes, urea method, pH value of control

INTRODUCTION

The chemical composition of hydrotalcite-like compounds (HTlcs) can berepresented by the general formula [MII

1-xM�

x(OH)2]x+[An-

x/n]·H2O, where MII

and MIII are di- and trivalent metal cations, respectively, which occupyoctahedral positions in hydroxide layers. Nn- is an exchangeable anion, such asCO3

2-, NO3-, Cl-, etc. Recently, HTlcs have attracted much attention due to wide

applications in the field of catalysis [1-3]. So far three methods have been used to synthesize the anionic clay: titrationco-precipitation method, anionic exchange method and thermal decomposition-reconstitution method [1,4,5]. Titration co-precipitation method includes thelow and high supersaturation methods. The common drawback of the methodsis that the instantaneous pH value is certainly different in different parts of the_________________________*Corresponding author. E-mail: [email protected]

276 PIAOPING YANG et al.: UREA METHOD

slurry no matter how fast is the stirring speed. Although it is easier tocrystallize in low supersaturation conditions, which usually gives rise toprecipitation, than in high supersaturation conditions, it is still difficult toobtain hydrotalcites with high crystallinity [1]. To our knowledge there is little information available on using the ureamethod to synthesize hydrotalcite-like compounds. Urea can form ahomogeneous solution with metal nitrates at low temperature owing to its highstability. When the temperature of the solution is raised to 90°C, urea begins todecompose slowly and the pH of the solution increases subsequently. Thisprocess can sustain the pH in every part of the solution at a homogeneous level.Therefore, highly crystalline Mg-Al, Zn-Al and Ni-Al hydrotalcite-likecompounds were synthesized by this method in this work.

EXPERIMENTAL

Sample preparation

0.15 mol MII(NO3)2·6H2O (MII = Mg, Zn, Ni), 0.075 mol Al(NO3)3·9H2Oand some urea were dissolved in 250 mL distilled water under stirring. Theurea/NO3

- ratio in the solution was in the range of 1~4. When the temperaturewas higher than 90°C, urea began to dissolve and gas evolved from thesolution. The pH of the solution began to rise simultaneously. The precipitatebegan to appear when the pH reached 7 after 1 h, the solution turned into aslurry after further 10 min. The slurry was heated at 95°C for 10 h understirring, and then was aged statically at the same temperature for another 20 h.Finally, the resulting precipitate was washed several times with distilled waterand dried at 90°C. For comparison HTlcs were prepared by titration co-precipitation methodsaccording to the literature [1].

Catalyst characterization

X-ray diffraction (XRD) patterns were collected on a RigakuD/MaxIIIB(λ=0.1541 Å) diffractometer with �� radiation. FT-IR spectra were recordedon a Jas Co. FTIR-610 instrument using KBr pellet technique. The chemicalcomposition of the materials was determined by CE-EA-1110. SEM pictureswere obtained on Philips XL30S.

PIAOPING YANG et al.: UREA METHOD 277

RESULT AND DISCUSSION

XRD analysis

Figure 1 shows the XRD patterns of Mg-Al, Zn-Al and Ni-Al HTlcssynthesized by different methods. All the samples synthesized by titration co-precipitation methods show some general features of hydrotalcite (Figs 1 (A)band c, (B)b and (C)b). However, the relatively low diffraction intensities, broadpeaks and asymmetric characteristics of the samples indicate that disorder maybe present in the stacking of the layers and poor crystallinity is evolved in thesematerials. However, the diffraction peaks of HTlcs prepared by urea method arenarrow and sharp, and show excellent symmetry. These phenomena suggest thatthe higher crystallinity and regularity exist in the structure of the layeredmaterials synthesized by urea method.

10 20 30 40 50 60

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

(A)

2θ

a

b

c

10 20 30 40 50 60

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

2θ

a

b

(B)

10 20 30 40 50 60

(C)

2θ

a

b

Fig. 1. XRD patterns of Mg-Al(A), Zn-Al(B) and Ni-Al(C) HTlcs synthesized bydifferent methods. a: Urea method, n(urea)/n(NO3

-)=2, b: Low supersaturation, c:High supersaturation


For an identical hydrotalcite, the hydration degree and the type of aniondirectly affect the thickness of one layer, which can be deduced by the changeof 003 and 006 reflections in the XRD spectra [6,7]. Analyzing the position ofthe characteristic reflections of the HTlcs synthesized by different methods, wecan find that the positions of 003 and 006 reflections of the samples��

These results indicate that the interlayer thickness of the HTlcs synthesized byurea method may be lower than that synthesized by titration methods. TG-DTAresults have already proved that the water content in the samples synthesized byurea method is similar to that in other samples, the influence of hydrationdegree can be ruled out. Therefore, the difference of the interlayer region isonly due to the factor of the gallery anions. When nitrate was used in thesynthesis by urea method, a large amount of NH3 was produced from thedecomposition of urea at high temperature, which will react with NO3

-. As aresult, a large amount of CO3

2- entered the interlayer as main gallery anion.This was proved by the results of elemental analysis (shown in Table 1).

Table 1

Results of elemental analysis for the HTlc samples

Element Mg-Al-urea

method

Mg-Al-lowsupersaturation

Mg-Al-highsupersaturation

Zn-Al-urea

method

Zn-Al-lowsupersaturation

C 2.37 0.52 0.56 2.02 0.18H 3.16 3.52 3.52 1.40 1.92O 28.84 34.49 32.67 21.86 27.03N 0.45 4.58 4.21 0.20 3.66

The XRD spectra of Mg-Al hydrotalcite with different urea/Al ratiosprepared by the urea method are shown in Fig. 2. The XRD pattern of Al(OH)3

synthesized by urea method is presented for comparison. The amorphousAl(OH)3 demonstrates the absence of characteristic reflections of hydrotalcite.Although the layered compounds with characteristic and symmetric reflectionsof hydrotalcite can be obtained when the urea/NO3

- ratio is equal to 1, the drift��

crystalline hydrotalcite is obtained at urea/NO3- = 2. Further increase of the

urea/NO3- ratio does not show obvious effects on the crystallinity of the

samples.


10 20 30 40 50 60

0

1000

2000

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4000

5000

6000

7000

8000

9000

10000

×2

d

c

ba

2θ

Fig. 2. XRD patterns of Mg-Al hydrotalcite with different urea/NO3- ratios

synthesized by urea method. a: Al(OH)3, b: n(urea)/n(NO3-) = 1, c: n(urea)/n(NO3

-) =2, d: n(urea)/n(NO3

-) = 4

IR and SEM analysis

IR analysis is not a diagnostic tool to study the structure of HTlcs, but it canbe useful to identify the presence of foreign anions in the interlayer. Inaddition, information about the type of bonds formed by the anions and theirorientations can also be obtained [1]. Figure 3 shows the IR spectra of the samples prepared by different methods.Broad absorption bands in the 3400~3600 cm-1 region can be attributed to OHstretching vibrations of hydroxy groups, water molecules in the interlayer andphysically adsorbed water. The band at 1650 cm-1 is mainly due to the HOHbending vibration of physically adsorbed water [8]. In Fig. 3a, the shoulder at3000~3100 cm-1 is due to hydrogen bonding of H2O to CO3

2- ions in theinterlayer space [4,8,9]. Usually, three absorption bands in the IR spectra of hydrotalcite-CO3

2- areobserved at 1350~1380 cm-1

��3), 850~880 cm-1 ��2) and 670~690 cm-1 ��4),corresponding to D3h planar symmetry of the carbonate anion. The appearanceof a shoulder at 1400 cm-1 and a double absorption band at 1350~1400 cm-1 isdue to the loss of carbonate symmetry from D3h to C2v and the disorder in theinterlayer [9,10]. It can be seen that the samples prepared by low supersaturation and highsupersaturation methods show similar IR spectra [Fig. 2 (A)b and c, (B)b]. Butin the case of the HTlcs prepared by urea method, a shoulder at 1400 cm-1, the


split of the vibration at about 1380 cm-1 and a new peak at approximate 790 cm-1

can be observed, especially in the spectrum of Mg-Al hydrotalcite [Fig. 2 (A)a].These results indicate that the gallery anions of the HTlcs synthesized by ureamethods are much different from those prepared by titration methods.

Fig. 3. IR spectra of Mg-Al(A), Zn-Al(B) HTlcs synthesized by differentmethods. a: Urea method, n(urea)/n(NO3

-) = 3, b: Low supersaturation, c: Highsupersaturation

The morphology of samples prepared by different methods was investigatedby SEM (Fig. 4). The Mg-Al, Zn-Al samples synthesized by urea method showmuch higher crystallinity than those synthesized by co-precipitation method.


The average crystallite size is about 5 ��

the samples prepared by co-precipitation is in the range of 10-30 ��

results revealed by SEM are in agreement with above XRD analysis.

Fig. 4. SEM pictures of Mg-Al and Zn-Al HTlcs synthesized by differentmethods.(A) Mg-Al, high supersaturation, (B) Mg-Al, low supersaturation, (C) Mg-Al,urea method, (D) Zn-Al, low supersaturation, (E) Zn-Al, urea method


In summary, the urea method is an efficient pH control method for thesynthesis of HTlcs. The crystallinity and the regular degree of the samplesprepared by this method are clearly preferable to those synthesized by othermethods.

REFERENCES

1. F. Cavani, F. Trifirò, A. Vaccari: Catal. Today, 11, 173 (1991).2. A. Schutz, P. Bileon: J. Solid State Chem., 68, 360 (1987).3. R. Vicente, A.U. Maria: Coordination Chem. Rev., 181, 61 (1999).4. S. Miyata: Clays Clay Miner., 23, 369 (1975).5. K. Chibwe, W. Jones: J. Chem. Soc. Chem. Commun., 9, 26 (1989).6. S. Miyata: Clays Clay Miner., 31, 305 (1983).7. J. Pérez, G. Mol, J.A. Moulijn: Vibrational Spectroscopy, 27, 75 (2001).8. M.K. Titulaer, J.B.H. Jansen, J.W. Geus: Clays Clay Miner., 42, 249 (1994).9. D.L. Bish, G.W. Brindley: Am. Min., 62, 458 (1977).

10. Sankarasubbier Narayanan, K. Krishna: Appl. Catal. A, 198, 1 (2000).

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Urea method for the synthesis of hydrotalcites