DTG and DSC

Journal of Alloys and Compounds 344 (2002) 195–198

L

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Dehydration studies of rare earth p-toluenesulfonate hydrates by TG / DTG and DSC a b, A.V. dos Santos , J.R. Matos * a

b

´ , CCT, UEPB, 58100 -001, Campina Grande, PB, Brazil Departamento de Quımica ˜ Paulo, Sao ˜ Paulo, 05508 -900, SP, Brazil ´ , Universidade de Sao Instituto de Quımica

Abstract The dehydration process of the compounds RE(C 7 H 7 SO 3 ) 3 ?xH 2 O (where x52 for RE5La–Ho, Tm, Yb and Y; x57 and 6 for RE5Er and Lu, respectively) has been investigated by thermogravimetry / derivative thermogravimetry (TG / DTG) and differential scanning calorimetry (DSC) techniques. The TG / DTG curves show that the dehydration process does not depend on the atmosphere used. For the compounds of La–Gd, dehydration occurs in a single step while for the compounds of heavier lanthanoid (Dy–Lu) and yttrium it occurs in two stages. The total dehydration DH for each compound was determined by DSC and the values vary between 67 and 381 kJ mol 21 along the series.  2002 Elsevier Science B.V. All rights reserved. Keywords: Dehydration; p-Toluenesulfonates; Rare earth; Differential scanning calorimetry (DSC); Thermogravimetry / derivative thermogravimetry (TG / DTG)

1. Introduction There are few data relating to the dehydration and thermal decomposition of several heavy and transition metallic salts of sulfonic acids, especially involving rare earth p-toluenesulfonates [1–6]. Even with advances in the synthesis, characterization and crystallographic investigation of some rare earth p-toluenesulfonates, there is no systematic study on the dehydration and thermal decomposition processes of this class of compounds. Moreover, detailed data of the dehydration process by thermogravimetry / derivative thermogravimetry (TG / DTG) and differential scanning calorimetry (DSC) after preparation in aqueous solution by the reaction between p-toluenesulfonic acid and rare earth basic carbonates are not available for these compounds. A detailed description of the above preparation method has been recently reported by dos Santos and Matos [7]. In the present paper we report the results of dehydration studies for all rare earth ptoluenesulfonates, using TG / DTG and DSC. In addition

*Corresponding author. Fax: 155-11-3818-3837. E-mail addresses: [email protected] (A.V. dos Santos), [email protected] (J.R. Matos).

and within the scope of this research, dehydration temperatures and DH were evaluated by DSC.

2. Experimental The compounds were prepared in aqueous solution by reaction between p-toluenesulfonic acid and metallic hydroxycarbonates in a molar ration of 3:1 at pH 5–6. Elemental analysis, complexometric methods, solid state IR absorption, conductivity measurements, X-ray powder diffraction patterns and thermal methods were the techniques used for characterization of the compounds [7]. TG / DTG curves were obtained on a Model TGA51 Thermogravimetric Analyzer (Shimadzu) in the temperature range 25–300 8C with a heating rate of 10 8C min 21 , under dynamic air, nitrogen and mixture of air1CO 2 (50 ml min 21 ) atmospheres and samples weighing |5 mg in Pt crucible. The mixture air1CO 2 was 50 / 50 (v / v). DSC curves were obtained on a Model DSC50 cell (Shimadzu) under dynamic nitrogen atmosphere (50 ml min 21 ), with samples (Table 2) in Al crucibles, with the same heating rate and temperature range used for TG. For DH measurements, the DSC system was calibrated with indium (m.p. 156.6 8C; DHfus 528.54 J g 21 ) and zinc (m.p. 419.6 8C).

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00339-0

A.V. dos Santos, J.R. Matos / Journal of Alloys and Compounds 344 (2002) 195–198

196

Table 1 Weight losses of dehydration process as well as the calculated values and the number of water molecules lost for RE(C 7 H 7 SO 3 ) 3 ?xH 2 O in each atmosphere

3. Results and discussion

RE

The dehydration process was studied in three dynamic atmospheres: air, nitrogen and air1CO 2 . The results indicated that the furnace atmosphere does not influence the dehydration process of the compounds (Table 1). The average values of Dm are in agreement with calculated values and confirm the proposed stoichiometry for this series of compounds. TG / DTG curves (Fig. 1) indicate that water loss occurs in a single step for the obtained compounds with the lighter rare earths (La–Gd), while for the compounds of the heavier lanthanoids (Tb–Yb) and Y the dehydration process occurs in two steps (Table 1). From TG / DTG curves, it can be seen that the dehydration process finishes completely at 160 8C and that the anhydrous species are thermally stable to |300 8C. In the cases of the compounds of Tb, Dy, Ho, Tm, Yb and Y an overlapping of reactions occurs; the first mass loss corresponds to the liberation of one water molecule and the second represents the additional loss of one more water

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y

x

2 2 2 2 2 2 2 2 2 2 7 2 2 6 2

%H 2 O

MExp

Calc.

Air atmosphere, exp.

N2 atmosphere, exp.

Air1CO 2 atmosphere, exp.

5.23 5.23 5.20 5.19 5.15 5.14 5.09 5.07 5.07 5.04 15.59 5.02 4.97 13.61 5.65

5.03 5.21 5.40 5.14 4.99 5.22 4.61 4.93 5.72 5.61 16.17 4.79 5.08 12.91 4.75

5.31 4.98 5.56 5.39 5.24 5.16 5.14 5.47 4.98 4.95 16.04 4.83 5.84 12.99 5.59

4.95 5.56 5.61 5.01 5.15 5.34 5.01 4.84 5.01 5.28 15.53 4.99 4.78 13.71 5.99

5.10 5.25 5.52 5.18 5.13 5.24 4.92 5.08 5.24 5.28 15.91 4.87 5.23 13.20 5.44

MExp , experimental values average; x, number of water molecules.

3.1. Thermogravimetry

Fig. 1. TG / DTG curves of RE(C 7 H 7 SO 3 ) 3 ?xH 2 O, in dynamic nitrogen atmosphere and heating rate of 10 8C / min.

A.V. dos Santos, J.R. Matos / Journal of Alloys and Compounds 344 (2002) 195–198

molecule. In the TG / DTG curves of the compounds of Er and Lu, it can be noted that the removal of water molecules occurs in two stages and corresponds to the loss of six and five water molecules, respectively, followed by the liberation of the last remaining water molecule. Calculations based on mass losses observed in TG / DTG curves are in concordance with the anhydrous species formation. The IR absorption spectra confirm the complete dehydration of the compounds, since the OH stretching that appears around 3400 cm 21 in RE(C 7 H 7 SO 3 ) 3 ?xH 2 O disappears in the spectra of the products isolated at 200 8C.

3.2. Differential scanning calorimetry The thermal events observed in DSC curves are in concordance with those observed in the TG / DTG curves. DSC curve profiles also allow separation of this series of compounds into two distinct groups. DSC curves (Fig. 2) show endothermic peaks characteristic of a dehydration process. For the series of La to Gd compounds only an endothermic peak can be observed indicating that the removal of the two water molecules occurs in a single step. In the case of the compounds of Tb–Er, Yb and Y, DSC curves show clearly two endothermic peaks indicating that water molecule removal occurs in two stages; this is not easily perceived from DTG curves. The temperature range in which water molecules are liberated point to crys-

197

tallization of water, consistent with IR data. There several stretching mode bands appear, which are continuous, wide and of weak / medium intensity, and related to water [8,9]. This is also in accordance with the fact that these compounds show evidence of crystalline structure (XRD) [7]. The average values of DH, measured in duplicate for each compound, corresponding to the endothermic processes are listed in Table 2. Good reproducibility of the results can be observed. The values of %Dr are smaller than 5%, except for the compounds of Tb, Dy, Lu and Y. This fact probably is due to the presence of weakly bonded water molecules, which can be liberated from room temperature. However, Wendlandt [10] reported that the accuracy and precision of DSC for determination of DH is 5–10% in most cases. DH values for compounds of La–Gd decreases regularly with the increase of the atomic number. For the compounds of Tb, Dy, Ho, Yb and Y, where dehydration events occur in two stages; enthalpy values corresponding to the second stage, in general, also decrease with atomic number increase. This is probably related to increase of repulsion forces due to the decrease in the RE 31 average ionic radii along the series. The compounds of Er and Lu behave in a different way when compared to other compounds, because both were obtained with a hydration degree larger than 2. Obviously, in both cases, DH values are larger than for dihydrates. Dehydration enthalpy values also suggest the presence of crys-

Fig. 2. DSC curves of RE(C 7 H 7 SO 3 ) 3 ?xH 2 O, in dynamic nitrogen atmosphere and heating rate of 10 8C / min.

A.V. dos Santos, J.R. Matos / Journal of Alloys and Compounds 344 (2002) 195–198

198

Table 2 DSC data of the dehydration process of RE(C 7 H 7 SO 3 ) 3 ?xH 2 O RE

x

DH of dehydration (kJ mol 21 )

m i (mg)

%Dr

Exp. 1

Exp. 2

Exp. 1

Exp. 2

MExp

110 99 91 89 87 83 78 15 67 9 64 46 46 354 29 141 42 234 13 53

109 97 93 88 86 84 77 18 68 10 62 45 45 349 30 142 41 252 16 53

109.5 98 92 88.5 86.5 83.5 77.5 16.5 67.5 9.5 63 45.5 45.5 351.5 29.5 141.5 41.5 243 14.5 53

La Ce Pr Nd Sm Eu Gd Tb

2 2 2 2 2 2 2 2

2.083 2.301 3.826 3.846 2.074 2.096 2.188 2.024

2.018 2.135 3.853 3.882 2.191 2.163 2.036 2.249

Dy

2

2.104

2.183

Ho

2

2.119

2.134

Er

7

2.068

2.122

Yb

2

2.105

2.214

Lu Y

6 2

2.060 2.258

2.163 2.068

60.9 62.0 62.1 61.1 61.1 61.2 61.3 616.7 61.5 610.0 61.6 62.2 62.2 61.4 63.3 60.7 62.4 67.1 618.7 0.0

MExp , experimental values’ average; m i , sample mass used for DSC measurement; %Dr , relative difference.

tallization of water since the values of DH are consistent with those found for the removal of this kind of water [9].

Acknowledgements The authors thank CNPq, CAPES and FAPESP for financial support and a scholarship.

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