Thermal behavior of nicotinate of some bivalent transition metal ions in dry CO 2 and N 2 atmospheres

Synthesis, characterization and thermal decomposition of bivalent transition metal nicotinates M(C6H4NO2)2 . nH2O (M = Mn (II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)), as well as the thermal decomposition of sodium nicotinate, were investigated employing simultaneous thermogravimetry and differential thermal analysis (TG-DTA), simultaneous thermogravimetry and differential scanning calorimetry (TGDSC) coupled to infrared spectroscopy (FTIR) and complexometry. In both atmospheres, the thermal decomposition of sodium nicotinate up to 500 oC, occurs with the formation of sodium carbonate and carbonaceous residue and up to 800 oC the mass loss is still being observed. In CO2 atmosphere the thermal decomposition of these compounds occurs in three consecutive steps, with the formation of the respective metal or metal oxides: MnO, FeO, CoO, Ni°, Cu° and ZnO. In N2 atmosphere, the thermal decomposition also occurs, in three consecutive steps and only iron and cobalt compounds, with the formation of Fe3O4 and CoO, respectively, while the other compounds the mass loss is still being observed up to 1000 oC.

The literature shows that the papers involving nicotinic acid and bivalent manganese, cobalt, nickel, copper and zinc reported the spectroscopic, thermogravimetric, magnetic studies and thermochemical behavior of solid nicotinic hydrazide [1,2], thermal decomposition of copper (II) nicotinate, isonicotinate and synthesis and characterization of copper (II) complexes with nicotinate in different coordination style [3,4], a new 2-D chiral coordination polymer of [Zn(nicotinate)2] n [5], simultaneous thermal analysis of a cobalt (II) complex with nicotinate [6], hydrothermal synthesis, crystal structures and properties of two 3-D network nickel nicotinate coordination polymers and hydrothermal synthesis, structural determination and thermal properties of 2-D cobalt-and nickel-based coordination polymers incorporating pendant-arm 3-pyridinecarboxylate ligands [7,8], synthesis, structures and properties of 3d/5d-4f metal complexes with novel polycationic chains [9], a pionner study on the anti-ulcer activies of copper nicotinate complex [CuCl(HNA) 2 ] in experimental gastric ulcer induced by aspirin-pyloris ligation model (shay model) [10], Growth and characterization of a novel polymer of manganese (II) nicotinate single crystal [11] and thermal behavior of nicotinic acid, sodium nicotinate and its compounds with some bivalent transition metal ions [12].
In this paper, solid-state compounds of some bivalent transition metal ions (i.e.Mn, Fe, Co, Ni, Cu and Zn) with nicotinate were prepared.These compounds were investigated by means of complexometric, simultaneous thermogravimetry and differential thermal analysis (TG-DTA) in CO 2 and N 2 atmospheres and simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC) coupled to infrared spectroscopy (FTIR), since the use of the coupled techniques makes possible a correct interpretation for the mechanism of a thermally induced reaction, involving the formation of gaseous species evolved during the thermal decomposition [13].This work is primarily a continuation and extension of a previously reported study [12].

Experimental
The nicotinic acid (C 6 H 4 NO 2 ) with 99.5% purity was obtained from Sigma and it was used as received.Aqueous solution of sodium nicotinate 0.1 mol L -1 was prepared by neutralization of an aqueous solution of nicotinic acid with sodium hydroxide solution 0.1 mol L -1 .Aqueous solutions of bivalent metal ions 0.1 mol L -1 were prepared by dissolving the corresponding chloride (Mn(II), Co(II), Ni(II)) or sulphate (Fe(II), Cu(II), Zn(II)).
The solid-state compounds were prepared by adding slowly with continuous stirring 100 mL sodium nicotinate solution 0.1 mol L -1 to 50 mL of the respective metal ions solutions 0.1 mol L -1 heated up to near ebullition, following the same procedure already described [12].In the solid-state metal ions, hydration water and nicotinate contents were determined from TG curves obtained in CO 2 atmosphere.The metal ions were also determined by complexometric with standard EDTA solution after igniting the compounds to the respective oxides and their dissolution in hydrochloric acid solution [14,15].Carbon, hydrogen and nitrogen contents were determined by calculations based on the mass losses of the TG curves obtained in CO 2 atmosphere, since the water and ligand lost in the thermal decomposition occurred with the formation of the respective metal or oxides with stoichiometry known, as final residue.
Simultaneous TG-DTA curves were recorded on a model SDT 2960 Thermal analysis system from TA Instruments.The purge gases were CO 2 and N 2 flow of 50 and 100 mL min-1, respectively.A heating rate of 10 ºC min-1 was adopted, with samples weighing about 7 mg.Alumina crucibles were used for recording the TG-DTA curves.
The measurements of the gaseous products were carried out by using a TG-DSC Mettler Toledo coupled to FTIR spectrophotometer Nicolet with gas cell and DTGS KBr detector.
The furnace and the heat gas cell (250 ºC) were coupled through a heated (200 ºC) 120 cm stainless steel line transfer with 3 mm diameter both purged with dry N 2 (50 mL min -1 ).The FTIR spectra were recorded with 16 scans per spectrum at a resolution of 4 cm -1 .

Results and Discussion
The TG-DTA curves of sodium nicotinate in CO 2 and N 2 atmospheres are shown in Fig. 2 a, a*.In both atmospheres, the TG curve shows that the thermal decomposition occurs in two consecutive steps between 410 and 780 ºC, in spite of the mass loss is still being observed.The endothermic peak at 410 ºC (CO 2 , N 2 ) without mass loss in the TG curve is attributed to the fusion of the compound, in agreement with the ref.[12].These curves also show a great similarity in the TG-DTA profiles in both atmospheres, although the mass loss in each step of the TG curve is characteristic of each atmosphere.
For CO 2 atmosphere, the first step between 410 and 500 ºC, with loss of 51.84%, corresponding to a small endothermic peak at 447 ºC is attributed to the thermal decomposition with the formation of a mixture of sodium carbonate and carbonaceous residue.The second step between 600 and 780 ºC, with loss of 9.56% without thermal event is attributed to the partial pyrolysis of the carbonaceous residue.
For N 2 atmosphere, the mass losses occur between 410-500 ºC and 500-780 ºC, without thermal event with losses of 41.96% and 17.71%, respectively.The first mass loss is also attributed to the thermal decomposition with the formation of a mixture of sodium carbonate and carbonaceous residue and the second one to the partial pyrolysis of the carbonaceous residue.
For the synthesized compounds the analytical and thermoanalytical (TG) data are shown in Table 1.These results permitted to stablish the stoichiometry of these compounds, which are in agreement with the general formula M(L) 2 .nH 2 O, where M represents bivalent manganese, iron, cobalt, nickel, copper, zinc, L is nicotinate and n = 4.5 (Ni), 4 (Co), 3.5 (Fe), 2 (Mn), 0.4 (Cu) and 0.23 (Zn).
The simultaneous TG-DTA curves of the synthesized compounds in CO 2 and N 2 atmospheres are shown in Fig. 2 in CO 2 (a-f) and N 2 (a* -f*), respectively.In both atmospheres, these curves show mass losses in steps characteristic of each compound and thermal events corresponding to these losses or due physical phenomenon.These curves also show a great similarity in the TG-DTA profiles as much in CO 2 as in N 2 atmosphere up to 475 ºC (Mn, Zn), 450 ºC (Fe, Co, Ni) and 300 ºC (Cu).This similarity suggests that the thermal decomposition mechanism for each compound must be the same.Thus, in both atmospheres the thermal stability of the hydrated compounds (I), as well as of the anhydrous ones, as shown by TG-DTA curves depends on the nature of the metal ions, and they follow in both atmospheres the order: The TG-DTA curves also show that the thermal behavior of the compounds is heavily dependent on the nature of the metal ion and so the features of each of these compounds are discussed individually.

Manganese compound
The simultaneous TG-DTA curves in CO 2 and N 2 atmospheres are shown in Fig. 3 a, a*.In both atmospheres these curves show mass losses in three steps, between 150 -780 ºC (CO 2 ) and 150 -> 1000 ºC (N 2 ) and thermal events corresponding to these losses or due to physical phenomenon.The first mass loss between 150 and 210 ºC in both atmospheres is attributed to dehydration with loss of 2 H2O (Calcd.= 10.75%,TG = 11.02%(CO 2 ), 11.01%(N 2 ).The anhydrous compound is stable up to 390 ºC, and above this temperature the thermal decomposition occurs in two consecutive steps.
For CO 2 atmosphere, calculations based on the mass loss up to 780 ºC, are in agreement with the formation of MnO, as residue (Calcd.= 78.84%,TG = 78.80%).The small mass gain between 780 and 1000 ºC is attributed to the oxidation of MnO, with the formation of Mn3O4, as residue (Calcd.= 1.59%,TG = 1.52%).
For N 2 atmosphere, the mas loss is still being observed up to 1000 ºC.The sharp exothermic peak at 255 ºC (CO 2 ) or 253 ºC (N 2 ), without mass loss in the TG curve is attributed to irreversible transition phase, as already observed in the thermal decomposition of this compound in air atmosphere [12].
For CO 2 atmosphere, calculations based on the mass loss 840 ºC suggest the formation of FeO, as residue (Calcd.= 80.21%, TG = 79.70%) and based on the residue color (black) and magnetic property.
For N2 atmosphere, calculations based on the mass loss up to 690 ºC, also suggests the formation of a mixture of Fe° and FeO in no simple stoichiometric relation (Calcd.= Fe° = 84.62%,FeO = 80.21%, TG = 82.52%).The mass gain (4.14%), between 690 at 1000 ºC is FeO to Fe3O4.The formation of FeO and Fe3O4 was also based on the residue color (black) and magnetic property.
For N 2 atmosphere, the mass loss up to 840 ºC, is in agreement with the formation of Co° as residue (Calcd.= 84.43%,TG = 84.73%).The mass gain between 840 and 980 ºC is attributed to the oxidation of Co to CoO (Calcd.= 4.22%, TG = 4.19%).The exothermic peak at 255 ºC (CO 2 ) or 249 ºC (N 2 ), without mass loss in the TG curve is attributed to the irreversible transition phase, in agreement with the result of ref. [12].
The anhydrous compound is stable up to 240 ºC in both atmospheres and above this temperature up to 980 ºC (CO 2 ) and > 1000 ºC (N 2 ), the mass loss occurs in two consecutive step, being the first step a fast process followed by slow one.For CO 2 atmosphere the mass loss up to 980 ºC is in agreement with the formation of Cu° as final residue (Calcd.= 79.82%,TG = 79.67%),while for N 2 atmosphere the mass loss is still being observed up to 1000 ºC.

Zinc compound
The simultaneous TG-DTA curves in CO 2 and N2 atmospheres are shown in Fig. 3 f, f*.These curves show mass losses in three (CO 2 ) or two (N 2 ) steps between 70 -840 ºC and 330 -> 1000 ºC, respectively.For CO2 atmosphere, the first mass loss that occurs slow between 70 and 250 ºC, without thermal event is attributed to the loss of adsorbed water, which correspond to 0.23 H20 (Calcd.= 1.32%,TG = 1.31%).In both atmospheres, the anhydrous compound is stable up to 340 ºC, and above this temperature the mass losses occur in two consecutive steps up to 840 ºC (CO 2 ) and 980 ºC (N 2 ).
For CO 2 atmosphere the mass loss up to 840 ºC is in agreement with the formation of ZnO, as final residue (Calcd.= 74.07%,TG = 74.09%).For N2 atmosphere, the total mass loss of 88.14% suggests that during the thermal decomposition must occur the formation a mixture of Zn° and ZnO in no simple stoichiometric relation and evaporation of Zn° up to 980 ºC (mp = 420 ºC, bp = 907 ºC).
The X-ray diffraction powder patterns of the final residue, was not obtained due to the small mass of the final product of thermal decomposition.For the compounds where the mass gain was observed in the last step of the TG curve, the oxidation reaction occurs, probably because the equipment is not hermetically sealed and/or presence of oxygen in the N 2 and CO 2 which were used as purge gas.
In the thermal decomposition of these compounds, where the TG curve shows mass loss and no thermal event corresponding to this loss is observed in the DTA curve, undoubtedly is because the mass loss occurs slowly and the heat involved is not sufficient to produce a thermal event.
The temperature ranges, mass losses and the peak temperatures observed in each step of the TG-DTA curves in CO 2 and N 2 atmospheres are shown in Table 2.The gaseous products evolved during the thermal decomposition of the compounds studied in this work, in N2 atmosphere were monitored by FTIR, and in all the compounds water, carbon monoxide, carbon dioxide and pyridine were detected.The IR spectra of the gaseous products evolved during the thermal decomposition of cobalt compound, as representative of all the compounds are shown Fig. 4.

Conclusion
The thermal decomposition of sodium nicotinate in CO 2 and N 2 atmospheres, up to 780 ºC occurs with the formation of mixture of sodium carbonate and carbonaceous residue, although the mass loss is still being observed up to this temperature.
From TG curve in CO2 atmosphere and complexometry results, as well as the carbon, hydrogen and nitrogen contents determined from TG curve, a general formula could be established for the synthesized compounds.
The TG-DTA curves also provided previously unreported information about the thermal stability and the thermal decomposition of these compounds in CO 2 and N 2 atmospheres.
The monitoring of the evolved gases during the thermal decomposition of the compounds studied in this work occurs with release of H 2 O, CO, CO 2 and pyridine.

Figure 4 .
Figure 4. IR spectra of the gaseous products evolved during the thermal decomposition of cobalt compound: Co(L) 2 4H 2 O, as representative all the nicotinates compounds studied in this work.

Table 1 .
Analytical and thermoanalytical (TG) data for the M(C 6 H 4 NO 2 ) 2 •nH 2 O compounds in CO 2 atmosphere.

Table 2 .
Temperatures Ranges Θ, mass losses (m) and peak temperatures (P) observed for each steps of TG-DTA curves of the M(L) 2 •nH 2 O compounds where M = bivalent transition metal ions, L = nicotinate