THERMAL BEHAVIOUR OF SUCCINIC ACID , SODIUM SUCCINATE AND ITS COMPOUNDS WITH SOME BIVALENT TRANSITIONS METAL IONS IN DYNAMIC N 2 AND CO ATMOSPHERES

Thermal stability and thermal decomposition of succinic acid, sodium succinate and its compounds with Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) were investigated employing simultaneous thermogravimetry and differential thermal analysis (TG-DTA) in nitrogen and carbon dioxide atmospheres and TG-FTIR in nitrogen atmosphere. On heating, in both atmospheres the succinic acid melt and evaporate, while for the sodium succinate the thermal decomposition occurs with the formation of sodium carbonate. For the transition metal succinates the final residue up to 1180 oC in N atmosphere was a mixture of metal and metal oxide in no 2 simple stoichiometric relation, except for Zn compound, where the residue was a small quantity of carbonaceous residue. For the CO atmosphere the final residue up to 980 oC was: MnO, Fe O , CoO, ZnO and 2 3 4 mixtures of Ni, NiO and Cu, Cu O. 2


Introduction
Preparation and investigation on the thermal behaviour and thermal decomposition of several metal-ion succinates have been reported.These papers are concerned with the thermal decomposition of bivalent succinates in the solid state [1], the thermal, spectral and magnetic studies of succinic acid and compounds of some transition metal ions [2], thermal dehydration of manganese (II) dicarboxylate hydrates [3], synthesis, properties and thermal decomposition of Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) succinates [4], a study of the thermal decomposition of Cu(II) and Zn(II) malonate, maleate and succinate complex [5] and a comparative study on the thermal decomposition of some transition metal carboxylates [6].As continuation of our previous study on thermal behavior of succinic acid, sodium succinate and its compounds with some bivalent transition metal ions in air atmosphere [7], this paper deals with the thermal behavior and thermal decomposition of these compounds in dynamic N and CO atmospheres.2 2

Experimental
The succinic acid C H O , and its sodium salt obtained from Sigma.Solid state Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) compounds were prepared by mixing the aqueous suspension of the corresponding metal carbonates in excess with 50.0 mL of succinic acid -1 0.10 mol L solution, and heated slowly up to near ebullition, until total neutralization of the acid.The carbonate in excess was removed by filtration and the aqueous solution of the respective metal succinates were evaporated to near dryness in a water bath, dried in air and kept in a desiccator over anhydrous calcium chloride.To avoid the oxidation of Fe(II), all the solutions, as well as in the evaporation of the solution in water bath were purged with nitrogen gas.
In the solid-state compounds, metal ions, hydration water and succinate contents were determined from TG curves.The metal ions were also determined by complexometry with standard EDTA solution, after igniting the compounds to the respective oxides and their dissolution in hydrochloric acid solution [8].
Carbon and hydrogen contents were determined by microanalytical procedures, with an EA 1110 CHNS-O Elemental Analyser from CE Instruments.
Simultaneous TG-DTA curves were obtained with the thermal analysis system, model SDT 2960 from TA Instruments.The purge gases were nitrogen -1 and carbon dioxide flow of 100 mL min .A heating -1 rate of 20ºC min was adopted, with sample weighing about 7 mg and alumina crucible was used for recording the TG-DTA curves.
The measurements of the gaseous products were carried out using a Thermogravimetric Analyzer Mettler TG-DTA coupled to a FTIR spectrophotometer Nicolet is10, with gas cell and DTGS KBr detector.The furnace and the heated gas cell (250ºC) were coupled through a heated (T = 200ºC) 120 cm stainless steel line transfer with diameter of 2 mm, both purged with dry nitrogen (50 -1 mL min ).The FTIR spectra were recorded with 32 -1 scans per spectrum at a resolution of 4 cm .

Succinic acid
The TG-DTA curves and infrared spectrum of the released product during the heating of succinic acid in N and CO atmospheres are shown in Fig. 1. fusion and evaporation of succinic acid, respectively.The product evaporated was monitored and identified mostly on base of their FTIR reference available on Nicolet libraries, as shown in Fig. 1c.

Sodium Succinate hexahydrated
For the sodium succinate hexahydrated, the TG-DTA curves in N and CO atmospheres are shown in 2 2 Fig. 2a and 2b, respectively.These curves show mass losses in two steps and thermal events corresponding to these losses or due to physical phenomenon.The first mass loss between 30 -130ºC (N ) and 30 -150 ºC  The endothermic peak at 410 ºC (N , CO ), without mass loss in TG curve is due to fusion of the compound.In The analytical and thermoanalytical (TG) data for the synthesized compounds are shown in table 1.These results permitted to establish the stoichiometry of these compounds, which are in agreement with the general formula, MC H O .nHO, shown in Fig. 3.These curves show mass losses or gain in two up to six steps, corresponding to endothermic peaks due to dehydration or pyrolysis of the anhydrous compounds and exothermic peaks attributed to physical phenomenon.The thermal stability of the anhydrous compounds, in both atmospheres, as shown by the TG-DTA curves depend on the nature of the metal ion and they follow the order: For each compound a great similarity is observed concerning the TG-DTA profiles around 400ºC, in both atmospheres and above this temperature the thermal decomposition depend on the nature of the purge gas used and so the features of each of these compounds are discussed individually.
Manganese compound.The simultaneous TG-DTA curves in N and CO atmospheres are shown  to the exothermic peak at 490 ºC attributed to the decarboxylation with formation of sodium carbonate accompanied by carbonaceous residue.dehydration with loss of 2 H O (Calcd.17.41 %, TG = 17.70 % (N2), 17.35 % (CO )).The exothermic peak at 2 315 ºC observed in both atmospheres is attributed to the crystallization process, as shown in Fig. 4.
The thermal decomposition of the anhydrous compound in N atmospheres occurs in three steps 2 between 320 -410ºC, 410 -510ºC and 510 -1150ºC, with losses of 13.08% , 19.45 % and 17.50%, respectively, corresponding to the endothermic peaks at 400ºC, 455ºC and 1100ºC attributed to the thermal decomposition, with formation of a mixture of Mn and MnO in no simple stoichiometric relation.
For the CO atmospheres, the thermal 2 decomposition also occurs in three steps between 320 -410ºC, 410 -505ºC and 505 -830ºC, with losses of 12.34%, 20.68% and 15.67%, corresponding to the endothermic peaks at 400ºC, 480ºC and 790ºC with formation of MnO as final residue (Calcd.= 65.74 %, TG = 66.04 %).Iron Compound.The simultaneous TG-DTA curves in N and CO atmospheres are shown in Fig. (Calcd.= 9.49 %, TG = 9.24 % (N ), 9.63 % (CO )).The thermal decomposition of the anhydrous compound in N atmosphere, occurs in four steps 2 between 215 -300 ºC, 300 -480 ºC, 565 -635 ºC and 635 -900 ºC, corresponding to the endothermic peaks at 295 ºC and 605 ºC, with losses of 9.25 %, 26.78 %, 19.13 % and 6.06 %, respectively.Calculations based on the mass losses observed up to 900 ºC, are in agreement with the formation of Fe (Calcd.= 70.59%, TG = 70.46%).The mass gain (5.66 %) observed between 900 and 1180 ºC is attributed to the partial oxidation of Fe with formation a mixture of Fe and FeO.No thermal event is observed corresponding to the second and fourth mass losses, probably because endothermic and exothermic events must to occur simultaneously and the net heat or the small heat involved in these steps is insufficient to produce a thermal event.
For the CO atmosphere, the thermal decompo-2 sition occurs in three steps between 225 -300ºC, 300 -480ºC and 500 -800ºC, corresponding to the endothermic peaks at 295ºC, 355 ºC, 455ºC and 760ºC, with losses of 9.97 %, 25.78 % and 34.95 %, respectively.The mass losses up to 800 ºC, suggest the formation of a mixture of FeO and Fe O .The mass gain (1.12 %) 3 4 observed between 800 and 980 ºC is attributed to the oxidation of FeO with the formation of Fe O , as final 3 4 residue (Calcd.= 59.37 %, TG = 59.61 %).TG curve is attributed to the irreversible solid phase transition, which was confirmed by X-ray powder, with sample before and after the exothermic peak.
The thermal decomposition of the anhydrous compound in N occurs in two steps between 365 -2 520 ºC, corresponding to the endothermic peak at 465 ºC and 520 -1180 ºC, with losses of 41.40 % and 6.90 %, respectively.No thermal event corresponding to the second mass loss in observed in the DTA curve, probably because the small mass loss occurs so slowly that the heat involved in this step is not sufficient to produce a thermal event.Calculations based on the mass losses up to 510 ºC suggest the formation of Co O (Calcd.= 64.69%, TG = 64.32Fig. 3e and 3e , respectively.These curves show that the anhydrous compound is stable up to 305 ºC, in both atmospheres and above this temperature the thermal decomposition occurs in three steps.These curves also show that the first two steps occur through fast processes and overlapping one between 305 -375 ºC and 375 -420 ºC, corresponding to the endothermic peaks at 365 ºC and 400 ºC (N , CO ) with losses of 27.77 % and 31.80   and 3f , respectively.These curves show that the anhydrous compound is stable up to 430ºC and above this temperature the thermal decomposition occurs in two steps between 430 -535 ºC and 535 -> 1080ºC (N ) or

Evolved gas analysis
The gaseous products evolved during the thermal decomposition in dynamic nitrogen atmosphere of the sodium and transition metal ion compounds studied in this work were monitored by FTIR and identified on basis of their FTIR references available on Nicolet libraries.For the sodium succinate the gaseous products evolved during a thermal decomposition were identified as methane, acetone, CO, and  CO ; for the cobalt and nickel compounds were methane, propanoic acid, CO, and CO ; for the copper 2 compound were propenoic acid , CO, and CO and for 2 the manganese, iron, and zinc compounds were methane, CO and CO .The IR spectra of the gaseous 2 products evolved during the thermal decomposition of sodium, manganese, nickel and copper as representative of other compounds, are shown in Fig. 4.

Conclusion
The present study showed that the succinic 2 acid melt and evaporate, without decomposition.
In the sodium and all the synthesized compounds, the evolved products during the thermal decomposition in nitrogen atmosphere was CO and CO , besides methane and acetone for sodium 2 succinate, propanoic acid for cobalt and nickel compounds and propenoic acid for nickel compound.

2 2 The 2 corresponding
TG-DTA curves, Fig 1a and 1b show mass loss in single step between 165 and 260ºC (N , CO ) 2 to the endothermic peaks at 194 and 247 ºC (N ) or 194 and 250 ºC (CO ), attributed to the 2 2

Figure 2 .
Figure 2. Simultaneous TG-DTA curves of the sodium succinate in (a) N (3.726 mg) and (b) Co

2 2 % 2 based 2 % 2 Zinc
(N ) or 30.60 % and 30.79 % (CO ).Calculations 2 on the mass losses up to 420 ºC suggest the formation a mixture of Cu and CuO.The last step that occurs through a slow process between 420 -1000 ºC (N ) or 420 -750 ºC (CO ) with losses of 0.7 % and 3.15 2 , respectively, is attributed to the reduction of the mixture (Cu, CuO) to Cu, as final residue (Calcd.= 64.22%, TG = 64.48% (N ), 64.54 % (CO ).The endothermic 2 2 peak at 1090 ºC (N ) is due to the fusion of Cu.Compound.The simultaneous TG-DTA curves in N and CO atmospheres are shown in Fig. 3f

2 2 For the CO atmospheres the mass lost up to 2 885
ºC is in agreement with the formation of ZnO as final residue (Calcd.= 55.16 %, TG = 54.95%).For the N atmospheres in the last step occurs the reduction of 2 Zn(II) to Zn followed by ebullition (boiling point = 907 ºC).The final residue (7.95 %) up to 1080 ºC is attributed to the carbonaceous residue, which was confirmed after to analysis the crucible residue.The mass gain attributed to the oxidation reaction of Fe to FeO in N atmosphere or Co to CoO and 2 Ni to NiO in CO atmosphere occurs, probably because 2 the equipment is not hermetically sealed and/or a trace of oxygen in the N or CO which were used as purge gas.

Table 1 .
Analytical data for the MC 4 H 4 O 4 •nH 2 O compounds.