Synthesis, characterization and thermal behavior of some trivalent lanthanide 4-amino-benzenesulfonate salts Fabrı́cio Rossi Marques Matias1 • Vivian Martins F. da Silva1 • Ronaldo Spezia Nunes2 • José Marques Luiz2 Received: 16 December 2016 / Accepted: 14 July 2017 / Published online: 27 July 2017 � Akadémiai Kiadó, Budapest, Hungary 2017 Abstract The lanthanide 4-amino-benzenesulfonate salts (or sulfanilates) were obtained by the reaction of lanthanide carbonates (La3?, Ce3?, Pr3?, Nd3?, Sm3?, Eu3? and Gd3?) and 4-amino-benzenesulfonic (or sulfanilic) acid in aqueous solution. Simultaneous thermogravimetry, differ- ential thermal analysis and differential scanning calorimetry, elemental analysis, X-ray powder diffractom- etry and infrared spectroscopy (FTIR) were used to char- acterize and to study the thermal behavior of these compounds. The general formula could be established as LnL3�nH2O, where Ln represents trivalent lanthanide ions (La–Gd), L is the 4-amino-benzenesulfonate anion (NH2 C6H4SO3 -) and n = 8 (La–Gd) and n = 7 only for Ce. The thermal behavior indicates that dehydration occurs in a single step up to 443 K, characterized by an endothermic peak, and the enthalpy of dehydration was evaluated by DSC. Thermal decomposition takes place above 523 K producing a stable intermediate Ln2O2SO4 characterized by X- ray diffraction, which in the sequence decomposed to the respective oxides. For the La3?, Nd3? and Pr3? sul- fanilates, the residual oxide formation occurs only after 1573 K. Keywords Lanthanides � Sulfanilates � 4-Amino- benzenesulfonates � Thermal behavior Introduction The molecular formula of sulfanilic acid (pKa = 3.232) or 4-amino-benzenesulfonic acid may be represented by (C6 H7NO3S) or (NH2C6H4SO3H). It is an organic synthetic compound obtained from aniline and sulfuric acid. It is a zwitterion, in other words, a dipolar ion having opposite charges on different atoms. In the complex compound formation, the sulfonic group (–SO3 -) can act as a mon- odentate, bidentate or tridentate ligand for different metallic ions. In some cases, both the amine (–NH2) and sulfonic groups participate in the metal coordination [1]. Rare earths are a group of metallic elements which contain, in addition to the lanthanides (58Ce to 71Lu), the elements 21Sc, 39Y and 57La [2]. In coordination chemistry, the lanthanide ions (Ln3?) are classified as hard acid by Pearson’s theory. Among the electron donor species, namely bases, the preference to form a bond follows the O[N[ S[F order, with ionic nature interactions. The sulfanilate ion (NH2C6H4SO3 -) has suitable chemical characteristics to act as a ligand with lanthanide ions. In the literature, some studies report the synthesis and the structure determination of some sulfanilates with alkali metal, ammonium and transition metals [3–11]. Gunder- man [7] described the synthesis of the two complexes, bis(4-amino-benzenesulfonate)diaquocopper(II) dihydrate, [Cu(C6H6NO3S)2(H2O)2]�2H2O and bis(4-amino-benzene- sulfonate)diaquomanganese(II) [Mn(C6H6NO3S)2(H2O)2]. Vinciguerra et al. [8] studied the magnetic moment, infrared and optical spectra of ortho-, meta- and para- amino-benzenesulfonates of copper (II). Shakeri and Haussühl [9] studied the crystal structure of the cobalt sulfanilate tetrahydrate [Co(NH2C6H4SO3)2(H2O)4]. Zhao et al. [12] studied the structure of 4-amino-ben- zenesulfonates of some lanthanide metals (La, Nd, Sm, Eu, & José Marques Luiz jmluiz@feg.unesp.br 1 Departamento Engenharia de Materiais e Tecnologia, Faculdade de Engenharia, Universidade Estadual Paulista (UNESP), Guaratinguetá, SP, Brazil 2 Departamento de Fı́sica e Quı́mica, Faculdade de Engenharia, Universidade Estadual Paulista (UNESP), Guaratinguetá, SP, Brazil 123 J Therm Anal Calorim (2017) 130:2185–2190 DOI 10.1007/s10973-017-6581-0 http://crossmark.crossref.org/dialog/?doi=10.1007/s10973-017-6581-0&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1007/s10973-017-6581-0&domain=pdf Tb, Dy and Er) by means of FTIR and X-ray diffraction. In most of these studies, the main goal was the structure determination of the synthesized compound, and a few of them have reported the thermal behavior of the compounds derivatives from sulfonic acid. Experimental Synthesis The 4-amino-benzenesulfonic acid, NH2C6H4SO3H, with 99.6% was obtained from Sigma. Carbonates of La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III) and Gd(III) were prepared by adding slowly, with continuous stirring, the saturated sodium hydrogen carbonate solution to the cor- responding metal chlorides or nitrate solutions until total precipitation of the metal ions. The precipitates were washed with distilled water until the elimination of chlo- ride or nitrate ions (qualitative test with AgNO3/HNO3 solution for chloride ions or diphenylamine in sulfuric acid solution for nitrate ions) and maintained in aqueous suspension. Solid-state Ln3? sulfanilates were prepared by mixing the aqueous solution of 4-amino-benzenesulfonic acid with the corresponding metallic carbonate suspension, in slight excess. The aqueous suspension was heated slowly up to near boiling until the total neutralization of the acid occurs. The carbonates in excess were filtered, and the aqueous solutions of the respective metallic sulfanilates were slowly evaporated. The compounds were dried in air and kept in a desiccator over anhydrous calcium chloride. Experimental equipment and conditions In solid-state compounds, hydration water, ligand and metal contents were determined from the TG curves. All TG, DTA and DSC curves were recorded using a SDT– Q600 simultaneous TG–DTA–DSC thermal analysis con- trolled by Thermal Advantage (4.2.1) software, both from TA Instruments, under dynamic dry air atmosphere (gas flow of 100 mL min-1), heating rate of 10 K min-1 in the range of 303–1573 K, with sample masses of ca. 5 mg in a sample holder of alumina. The dehydration enthalpy was evaluated using SDT Q600 from TA Instruments under dynamic dry air atmosphere (100 mL min-1), heating rate of 10 K min-1 in the range of 303–473 K and sample masses of ca. 4 mg in a platinum sample holder. Elemental analysis for C, H, N and S was performed using a Leco CHNS Analyzer. X-ray powder patterns were obtained using a BRUKER system D8 Advance diffrac- tometer, employing CuKa radiations (k = 1.54184 Å) and setting of 40 kV and 20 mA. Each run was performed with 2h values between 10 and 70 at a step size of 0.010 and a count time of 0.5 s per step. The attenuated total reflectance (ATR) spectra for 4-amino-benzenesulfonic acid as well as for its trivalent lanthanide compounds were run on a Spectrum system 100 ATR FTIR spectrophotometer (Perkin-Elmer), using an ATR accessory with Ge windows. The FTIR spectra were recorded with 16 scans per spectrum at a resolution of 4 cm-1. Results and discussion The FTIR spectra are shown in Fig. 1. The main absorption bands observed are those associated with the stretches (m O–H, m N–H and m SO3) and deformations (d H–S–H and d H–C–H), and all absorptions are in agreement with Zhao et al. [12]. Significant differences are noted between the spectra of the sulfanilic acid and its compounds sug- gesting the complex formation. The absorption bands found between 3700 and 3000 cm-1 are related with the presence of water molecules of hydration, which are not observed in the IR spectrum of sulfanilic acid. The absorption bands of m N–H were not observed, associated 4000 3500 3000 2500 2000 1500 1000 500 Sm Nd Pr Ce Sulfanilic acid Gd Eu La 20% Wavenumber/cm–1 Tr an sm itt an ce /% Fig. 1 FTIR spectra of the sulfanilic acid and its trivalent lanthanide compounds 2186 F. R. M. Matias et al. 123 with the –NH2 group in the sulfanilic acid spectrum, while the spectra of lanthanide sulfanilates presented a broad band attributed to absorption of H2O and –NH2 (1650–1631 cm-1) overlapped. The elemental analysis results are presented in Table 1 and are in agreement with the proposed general formula. The X-ray powder patterns presented in Fig. 2 showed that all compounds were obtained with good crystallinity degree and with evidence for the formation of an isomor- phic series. Zhao et al. [12] reported that in these com- pounds, the metallic ion is coordinated by two oxygen atoms of two sulfonate groups and seven oxygen atoms from water molecule. The simultaneous TG–DTA curves of the compounds are shown in Fig. 3. These curves exhibit mass losses in three main steps and thermal events corresponding to these losses (endothermic and exothermic peaks). The thermal analytical data from TG curves are shown in Table 2. The first mass loss in the range of 323–433 K is attributed to dehydration, which occurs in a single step with formation of anhydrous compounds (stable up to 533 K). The anhydrous compounds decompose above 533 K, and the TG curves profile suggests that the oxida- tion occurs at least with three overlapping events, pro- ducing Ln2O2SO4 as a stable intermediate. After this event, the stable intermediates decomposed to the respective oxides as a final residue (La2O3, Pr6O11, Nd2O3, Sm2O3, Eu2O3 and Gd2O3). For the cerium compound, the decomposition of organic matter occurs directly to the CeO2 without the formation of the stable intermediate. These results indicate that the stability of the compounds decreases with the decreasing ionic radius of the lanthanide ions. La(NH2C6H4SO3)3�8H2O The TG–DTA curves are shown in Fig. 3a. The dehydra- tion process occurs between 330 and 449 K, with mass loss of 18.8% (^8 H2O) associated with an endothermic peak at 389 K in DTA curve. In DSC curve, this event was evaluated and an enthalpy of 539 kJ mol-1 was calculated (Fig. 4). The anhydrous compound is stable up to 529 K and after this temperature the thermal decomposition of organic material occurs until 1083 K, with the formation of La2O2SO4 as stable intermediate (Fig. 3a) and X-ray diffraction patterns (Fig. 5). In these steps were observed Table 1 Elemental analysis of the solid Ln3? sulfanilates Compound % C % H % N % S Calc. EA Calc. EA Calc. EA Calc. EA La(NH2C6H4SO3)3�8H2O 27.17 28.07 4.30 4.06 5.26 5.65 12.03 12.36 Ce(NH2C6H4SO3)3�7H2O 27.62 28.34 4.13 4.08 5.37 5.59 12.29 12.83 Pr(NH2C6H4SO3)3�8H2O 26.97 27.59 4.28 4.11 5.24 5.40 12.00 12.36 Nd(NH2C6H4SO3)3�8H2O 26.86 27.29 4.26 4.09 5.22 5.46 11.95 12.39 Sm(NH2C6H4SO3)3�8H2O 26.66 27.50 4.23 4.14 5.18 5.46 11.86 12.42 Eu(NH2C6H4SO3)3 8H2O 26.60 26.99 4.23 4.08 5.17 5.47 11.84 12.21 Gd(NH2C6H4SO3)3�8H2O 26.43 26.98 4.20 4.08 5.14 5.45 11.76 12.24 10 20 30 40 50 60 1000 cps Nd Gd Eu Sm Pr Ce In te ns ity /c ps La 2θ /° Fig. 2 X-ray diffraction pattern of the trivalent lanthanide compounds Synthesis, characterization and thermal behavior of some trivalent lanthanide… 2187 123 two exothermic peaks at 723 and 843 K. The La2O2SO4 is stable up to 1483 K �C when its decomposition takes place to produce La2O3. At 1573 K, the final residue is a mixture of La2O2SO4 and La2O3. Ce(NH2C6H4SO3)3�7H2O The TG–DTA curves are shown in Fig. 3b. The dehydra- tion occurs between 325 and 441 K, with mass loss of 16.8% (^7 H2O) characterized by an endothermic peak in DTA curve at 390 K, as can be seen in Fig. 3b. From DSC data, this event was evaluated with dehydration enthalpy of 495 kJ mol-1 (Fig. 4) and the anhydrous compound remains stable up to 809 K. The thermal decomposition of organic material occurs between 536 and 1013 K, with the formation of the respective oxide CeO2 (Fig. 5). Two exothermic peaks are observed in DTA and DSC curves for 20 % (b) 5.0 K mg–1 (c) Temperature/K (d) M as s/ % (a) (e) (f) 400 600 800 1000 1200 1400 Exo up (g) T/ K m g–1 ∆ Fig. 3 TG–DTA curves: (a) LaL3 8H2O (m = 4.304 mg), (b) CeL3 7H2O (m = 3.426 mg), (c) PrL3 8H2O (m = 5.343 mg), (d) NdL3 8H2O (m = 4.299 mg), (e) SmL3 8H2O (m = 5.342 mg), (f) EuL3 8H2O (m = 4.207 mg) and (g) GdL3 8H2O (m = 4.005 mg) Table 2 Thermal analytical data from TG, DTA and DSC curves Sample M.M./g mol-1 % H2O DH(dehydr) % metal % Ln2O2SO4 % residue Calc. TG Calc. TG kJ mol-1 Calc. TG Calc. TG Calc. TG LaL3�8H2O 799.53 806.71 18.02 18.75 539 17.37 17.22 25.38 23.38 20.38 19.56* CeL3�7H2O 782.73 789.35 16.11 16.81 495 17.90 17.75 – – 21.99 22.50 PrL3�8H2O 801.53 805.99 17.98 18.43 536 17.58 17.48 25.57 25.58 21.23 16.97* NdL3�8H2O 804.86 814.37 17.90 18.86 524 17.92 17.71 25.88 23.78 20.90 18.84* SmL3�8H2O 810.99 816.19 17.77 18.29 431 18.55 18.43 26.43 25.16 21.46 21.33 EuL3�8H2O 812.59 816.51 17.73 18.14 420 18.71 18.06 26.58 23.39 21.66 21.55 GdL3�8H2O 817.88 810.34 17.61 16.85 412 19.26 19.41 27.06 25.44 22.16 22.37 M.M. molar mass * Incomplete decomposition up to 1573 K 340 350 360 370 380 390 400 410 La Ce Pr H ea t f lo w /W g –1 Nd Sm Eu Temperature/K Gd Fig. 4 DSC curves of all compounds showing the dehydration step 2188 F. R. M. Matias et al. 123 this step, the first one at 714 K and another at 817 K. There was no observed stable intermediate for this compound. Pr(NH2C6H4SO3)3�8H2O The TG–DTA curves are shown in Fig. 3c. The dehydra- tion process occurs in the range of 313–447 K with mass loss of 18.4%, attributed to dehydration (^8 H2O) asso- ciated with an endothermic peak in DTA curve at 390 K. In the DSC analysis, the enthalpy was evaluated around 536 kJ mol-1 (Fig. 4). The anhydrous compound is stable up to 517 K and after this temperature, the thermal decomposition of the sulfanilate ligand takes place between 517 and 991 K, producing a stable intermediate Pr2O2SO4 (Fig. 5), associated with exothermic peaks at 721 and 846 K in both DTA and DSC curves. The last event is the partial thermal decomposition of Pr2O2SO4 to Pr6O11 above 1553 K. At 1573 K, the final residue is a mixture of Pr2O2SO4 and Pr6O11. Nd(NH2C6H4SO3)3�8H2O The TG–DTA curves are shown in Fig. 3d. As can be noted, the initial mass loss of 18.6% is in agreement with the release of eight water molecules and this event was associated with an endothermic peak in DTA and DSC curves (397 K). The dehydration enthalpy found for this compound was 524 kJ mol-1 (Fig. 4) in the DSC analysis. The anhydrous compound is stable up to 529 K and the thermal decomposition of organic material takes place between 529 and 1033 K, with exothermic peaks in DTA and DSC curves at 723 and 843 K, respectively. The Nd2O2SO4 (Fig. 5) was obtained as a stable intermediate between 1033 and 1473 K, and after this temperature, its decomposition takes place producing a mixture of Nd2O2 SO4 and Nd2O3 as final residue at 1573 K. Sm(NH2C6H4SO3)3�8H2O The TG–DTA curves are shown in Fig. 3e. The initial mass loss of 18.3% (8 H2O) is characterized by an endothermic peak (Tpeak = 391 K) in the DTA curve and an enthalpy of 431 kJ mol-1 in the DSC curve (Fig. 4). The anhydrous compound is stable between 440 and 515 K, and the thermal decomposition of organic contents occurs in the range of 515 and 991 K, where Sm2O2SO4 is produced (Fig. 5). In this last step, two exothermic peaks at 721 and 849 K are observed. The stable intermediate starts its decomposition only in higher temperature than 1553 K producing the samarium oxide as a final residue. Eu(NH2C6H4SO3)3�8H2O The TG–DTA curves are shown in Fig. 3f. The initial mass loss of 18.1% (8 H2O) is characterized by an endothermic peak at 387 K, and the dehydration enthalpy of 420 kJ mol-1 was evaluated by means of DSC analysis (Fig. 4). The anhydrous compound is stable up to 573 K where the second event of mass loss takes place until 1111 K with two exothermic peaks at 716 and 814 K, producing the Eu2O2SO4 (Fig. 5) as a stable intermediate. The Eu2O2SO4 is stable until 1314 K, and after this tem- perature the europium oxide is obtained as a final residue from 1453 K. Gd(NH2C6H4SO3)3�8H2O The TG–DTA curves are shown in Fig. 3g. For this sam- ple, the dehydration process occurs with an initial mass loss of 16.9% corresponding to eight water molecules released and an endothermic peak at 383 K in DTA curve. The dehydration enthalpy was evaluated in 412 kJ mol-1 in the DSC analysis (Fig. 4). The anhydrous compound is 10 20 30 40 50 60 70 Gd2O2SO4 Eu 2 O 2 SO 4 Sm2O2SO4 Nd2O2SO4 Pr2O2SO4 CeO 2 La2O2SO4 2 /° N or m al iz ed in te ns ity ( I / I m ax ) θ Fig. 5 X-ray diffraction pattern of the samples after heating at 1273 K for 2 h in a muffle furnace Synthesis, characterization and thermal behavior of some trivalent lanthanide… 2189 123 stable up to 526 K, and above this temperature the decomposition of organic material occurs up to 1037 K, showing two exothermic peaks associated with this event at 728 and 821 K in DTA curve. The decomposition of organic material produces the Gd2O2SO4 (Fig. 5) that is stable until 1311 K where the gadolinium oxide is formed as a final residue at 1423 K. These results indicate that the thermal stability of the compound decreases for the lanthanide ions with lower ionic radius. From the TG–DTA data, it was possible to establish a general formula to the compound as LnL3�nH2O, where Ln represents trivalent lanthanide ions (La, Ce, Pr, Nd, Sm, Eu and Gd), L is 4-amino-benzenesulfonate anion (NH2C6 H4SO3 -) and n = 7 (Ce) and 8 (La, Pr, Nd, Sm, Eu and Gd). The DTA curves show that organic material decom- position (NH2C6H4SO3 -) occurred in consecutive steps with, at least, two or three exothermic peaks, related to release of CO2, H2O and SO3 gases. When heated at 1273 K for 2 h in a muffle furnace, the compounds formed the respective dioxysulfates (Ln2O2 SO4). The formation of a stable intermediate, Ln2O2SO4, during the thermal decomposition of these compounds based on the TG–DTA curves was confirmed by compar- ison with the diffraction pattern for La2O2SO4 (JCPDF No. 01-085-1534), as can be seen in Fig. 5. All dioxysulfates obtained are isomorphic. For cerium, the formation of CeO2 was also confirmed. Conclusions The infrared spectroscopic data indicate that the trivalent coordination of lanthanide ions occurs through the sulfonic group and the bands observed were associated with the stretches (m O–H, m N–H and m SO3) and deformations (d H–S–H and d H–C–H). The X-ray powder diffraction suggests that the compounds are crystalline and the for- mation of an isomorphic series. TG–DTA analysis pro- vided previously unreported information concerning the thermal behavior and thermal decomposition of these compounds, and based on the TG curves, a general formula could be established for the synthesized compounds: LnL3.nH2O, where Ln = (La, Ce, Pr, Nd, Sm, Eu and Gd); L = (NH2C6H4SO3 -) and n = 7 for Ce3? and 8 for (La3?, Pr3?, Nd3?, Sm3?, Eu3? and Gd3?). The dehydration process occurs in a single step up to 453 K and the enthalpy of dehydration decreases from La3? to Gd3? as a consequence of the decrease in the ionic radius of the lanthanide ion. The thermal decomposition of all com- pounds, except cerium, presented in its thermal pathway a stable intermediate Ln2O2SO4, which subsequently decompose to their respective oxides. For the La3?, Pr3? and Nd3? sulfanilates, the formation of residual oxide occurs above 1573 K and in the present work it was obtained as a mixture. The formation of Ln2O2SO4 was verified by X-ray diffraction after heating at 1273 K for 2 h. Acknowledgements This research was supported by resources sup- plied by the Faculdade de Engenharia de Guaratinguetá (UNESP). The authors thank Prof. Dr. Ivonete Ávila (DEN-FEG-UNESP) and Luiz Carlos Rios for TG–DTA measurements, Prof. Dr. Jivaldo O. 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Matias et al. 123 Synthesis, characterization and thermal behavior of some trivalent lanthanide 4-amino-benzenesulfonate salts Abstract Introduction Experimental Synthesis Experimental equipment and conditions Results and discussion La(NH2C6H4SO3)3middot8H2O Ce(NH2C6H4SO3)3middot7H2O Pr(NH2C6H4SO3)3middot8H2O Nd(NH2C6H4SO3)3middot8H2O Sm(NH2C6H4SO3)3middot8H2O Eu(NH2C6H4SO3)3middot8H2O Gd(NH2C6H4SO3)3middot8H2O Conclusions Acknowledgements References