Synthesis, Characterization, and Thermal Decomposition Kinetics of Manganese Complex of Methionine Hydroxy Analogue

The manganese complex of methionine hydroxy analogue was synthesized with methionine hydroxy analogue and manganese chloride as main raw materials. The composition and structure of the complex were characterized by elemental analyses, infrared spectra, and X-ray powder diffraction. The formula of the complex was Mn(C 5 H 9 O 3 S) 2 . The experimental results indicated that the manganese ion was, respectively, coordinated by the carboxylic and hydroxyl oxygen atoms from the methionine hydroxy analogue ligand.The crystal structure of the complex belonged tomonoclinic systemwith the lattice parameters of a= 1.2775 nm, b= 1.5764 nm, c = 1.5764 nm, and β = 94.06.The thermal decomposition process of the complex was studied by thermogravimetry and differential thermal analysis. The decomposition of the complex has taken place above 200C. The residue was mainly manganese sulfide, and the experimental and calculated percentagemass loss was also given.The parameters of thermal decomposition kinetics for the complex, such as activation energy, reaction order, and preexponential factor, were calculated by usingKissinger, Flynn-WallOzawa, and Freeman-Carroll methods, and the kinetic equations of the thermal decomposition were obtained.


Introduction
DL-2-Hydroxy-(4-methylthio) butanoic acid, which is known as methionine hydroxy analogue (abbreviated as H 2 MHA), is equivalent to methionine in biological value.In the digestive tract, the complexes of H 2 MHA are absorbed by the amino acid pathway, and the absorption speed is fast and the absorption rate is high [1].The complexes of methionine hydroxy analogue can be used as feed additives, which can provide the essential trace elements and methionine sources for animal growth, and these complexes have the advantages of stable chemical properties, nontoxicity, little stimulation to animals, small side effects, and so on [2].Many forms of metal complexes with amino acids and hydrolysed proteins, less commonly, with other organic ligands molecules are commercially available.The functions of the complexes of H 2 MHA are similar to that of the complexes of natural amino acid and trace elements, while they are easier to meet the needs of the market than the complexes of natural amino acid and trace elements [3,4].So far, the complexes of H 2 MHA have been rarely reported [5][6][7], especially in the thermal decomposition mechanism and the thermal decomposition kinetics of the complexes of H 2 MHA.Here we report the synthesis, characterization, and thermal decomposition mechanism of a manganese complex of H 2 MHA, and the thermal decomposition kinetic parameters of the complex are studied by using three different methods.These results can provide reliable scientific basis for the further research and the development of new products.

Materials and Physical
Measurements.All chemicals purchased were of analytical reagent grade and used without further purification.Methionine hydroxy analogue (industrial product with water content of 12%) was provided by Xingjia Bio-Engineering Co., Ltd., Changsha, and manganese chloride tetrahydrate was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai.
The elemental analyses for C, H, S, and O in the complex were measured on a Vario EL CUBE elemental analyzer, and the content of manganese was determined by EDTA complexometric titration with eriochrome black T as indicator.The IR spectrum was obtained with KBr pellets on a Nicolet 5700 FTIR spectrophotometer in the range of 4000-400 cm −1 .The powder X-ray diffraction (XRD) pattern of the complex was recorded by a D/max-II X-ray diffractometer with Cu  1 radiation and Ni filter, voltage of 35 kV, current of 60 mA, and scanning speed of 8 ∘ min −1 at room temperature, and the XRD data were collected in the diffraction angle range of 3−80 ∘ .The thermal analysis data of the complex were obtained by using a SDT Q600 thermogravimetry analyzer in the air atmosphere from room temperature to 600 ∘ C, and the test sample mass was about 3.5 mg. 2 .88% of methionine hydroxy analogue (20 mmol, 3.41 g) and manganese chloride tetrahydrate (10 mmol, 1.98 g) were weighed and mixed in a beaker and heated to dissolve completely.The solution of sodium hydroxide (20 mmol, 0.80 g) was added to the above solution by drops under stirring.The pH of the solution was 6-7 after adding, and no precipitation was generated.Continue to heat with stirring until the solution was concentrated to about 10 mL, then the pink precipitation was generated, and the pH of the solution was about 6.After the solution cooling, the resultant was separated from the reaction mixture by filtration and washed by a small amount of anhydrous ethanol.The resultant was placed in a silica gel desiccator for 1 week.The product was pink powder (2.74 g) and the yield was about 77.6%.

Results and Discussion
3.1.Composition and Property.The results of elemental analyses for the complex are shown in Table 1.The experimental results coincide with the theoretical calculation, and the molecular formula of the complex is Mn(C 5 H 9 O 3 S) 2 (  = 353.30);namely, its composition is Mn(HMHA) 2 .The solid complex is stable in the air, easily dissolved in water, and not easy to absorb moisture.

X-Ray Powder Diffraction
Analysis.The XRD pattern of the complex is shown in Figure 1.The background of the XRD pattern is small and the diffractive intensity is strong, and the result indicates that the complex has a fine crystalline state.The index calculation of the XRD data is based on the computer programs of least squares method [8,9], and the calculated results are shown in Table 2.As Table 2 shows, all the diffraction data can be very well indexed by a set of cell parameters according to monoclinic system, the calculated spacing values  cal are consistent with the experimental spacing values  exp , and the maximum relative error is less than 0.3%.The result indicates that the complex is a single phase compound and the crystal structure of the complex belongs to the monoclinic system with the lattice parameters of  = 1.2775 nm,  = 1.5764 nm,  = 1.5764 nm, and  = 94.06 ∘ .

IR Spectroscopy Analysis.
The IR spectrum of the complex is shown in Figure 2. The peaks at 2914, 1438, and 1313 cm −1 are attributed to the characteristic absorption peaks of the CH 3 S-group.The moderate intensity broad peak at 3434 cm −1 is attributed to the stretching vibration absorption peak of the -OH group between 3650 and 3200 cm −1 , and the characteristic stretching vibration absorption peak originated from the C-O bond of the secondary alcohols can be found at 1071 cm −1 , so we can infer that the -OH group participates in the coordination with metal ion.The absorption peaks at 1633 and 1578 cm −1 and 1383 cm −1 are attributed, respectively, to the antisymmetric and symmetric stretching vibration of the COO − group.The difference values between ] as (COO − ) and ] s (COO − ) are 250 and 195 cm −1 , which is a sign that the carboxyl oxygen atoms are coordinated with manganese ion by the bridging coordination mode [10].As shown in the spectrum, there are two strong absorption peaks near 1600 cm −1 , which are in accord with the absorption peaks at 1637 and 1580 cm −1 of the zinc complex of H 2 MHA without water molecule [6].The result illustrates that there is not any water molecule in the manganese complex of H 2 MHA, and it is in line with the thermal decomposition process.In the low-frequency region, the absorption peaks at 572, 500, and 445 cm −1 in the complex are assigned to the stretching vibration of the Mn-O bond.In conclusion, the six oxygen atoms from two HMHA − anionic ligands are directly coordinated to the manganese ion, which should exhibit hexacoordination as occurs for the corresponding Mn 2+ glycolate complex [11].

Thermal Decomposition Mechanism.
Studying thermal decomposition process of complexes is helpful in the understanding of the coordination structure of the complexes  [12,13].The TG-DTA curves of the complex of H 2 MHA from room temperature to ∘ C with a heating rate of 10 ∘ C min −1 are shown in Figure 3, and the TG and the DTG curves of the complex at the heating rates of 5, 10, 15, and 20 ∘ C min −1 are presented in Figure 4. We can come to the conclusion from Figures 3 and 4 that the thermal decomposition process of the complex consists of three stages.With the increasing of the heating rate, the DTG curves move to the high temperature and the decomposition peak temperature   of every stage is also moving in the direction of high temperature [14].This reveals that the change of the heating rate has a slight influence on the temperature range of each mass loss stage.In the first stage, the percentage of mass loss at different heating rates is basically unchanged.But the final residual mass of the thermal decomposition for the complex is decreased with the increasing of the temperature.The percentage of the residual mass at the heating rate of 15 ∘ C min −1 is 24.60%, and it is in good agreement with the theoretical calculated value of the residue of manganese sulfide in 24.62%.Figure 3 shows that the complex is stable in the air and is decomposed over 200 ∘ C, and its thermal stability is very good.The final residue is black and stable powder after the complex undergoes three consecutive mass loss stages.Figure 3 shows that there are one endothermic peak and three exothermic peaks in the DTA curve.First, there is a very weak endothermic peak at 129 ∘ C, while there   is no mass loss in the TG curve, and it is a melting and endothermic process of the complex of H 2 MHA.At the first mass loss stage, there is a sharp exothermic peak at 251 ∘ C in the DTA curve, and it is ascribed to the oxidative decomposition of the ligand and the loss of one H 2 MHA molecule CH 3 SCH 2 CH 2 CH(OH)COOH.The experimental percentage mass loss of 43.26% in the TG curve is close to the calculated result of 42.51%.At this time, the title complex is transformed into the complex [Mn(MHA)] of the chelate ratio of 1 : 1.At the second and the third mass loss stages, there are one very weak exothermic peak at 352 ∘ C and one strong exothermic peak at 458 ∘ C in the DTA curve, and it is considered to be the oxidation and decomposition of the deprotonated MHA 2− ligand.The percentage of the residual mass in the TG curve is 28.58%, and there is some difference in the theoretical residual mass of 24.62% of manganese sulfide.It can be explained that partial manganese sulfide is oxidized to produce manganese sulfate when the heating rate is low.

Thermal Decomposition Kinetics Analysis.
In order to obtain the thermal decomposition kinetic parameters of the title complex, two different dynamic analysis methods, namely, the Kissinger method and the Flynn-Wall-Ozawa (F-W-O) method, which are utilized to calculate the activation energy , the preexponential factor , and the correlation coefficient  at the first and the third mass loss stages of the complex, and the related results are shown in Figures 5   and 6 and Table 3.The basic equations of the Kissinger method [15][16][17] and the F-W-O method [18][19][20][21] are as follows: where  is the heating rate (K min −1 ),  is the preexponential factor (s −1 ),  is the gas molar constant (8.314J K −1 mol −1 ),  is the activation energy (kJ mol −1 ),  is the conversion rate (%),   is the thermal spectrum peak temperature of different heating rates (K), and () is the integral mechanism function.Both of the two methods require several kinetic curves to perform the analysis and thus are sometimes called multicurve methods [22].The Kissinger method is a model-free method, but it is not isoconversional method because it assumes constant activation energy with the progress of conversion instead of calculating activation energy at different constant extents of conversion [23].According to Kissinger, the maximum reaction rate occurs with an increase in the reaction temperature [16].As shown in Figures 5(a) and 6(a), the plot of ln(/ 2  ) versus 1/  obtained from thermal curves recorded at different heating rates is a straight line whose slope can be used to calculate the activation energy  and the intercept can be used to calculate the preexponential factor .In the F-W-O method, when the percentage mass loss of the complex in different heating rates  is 20% at the thermal spectrum peak temperature   , the conversion rate  is approximately equal.The activation energy can be obtained from the plot of lg  versus 1/  for a series of experiments at different heating rates, the data of which are linearly fitted by least square method, and the results are exhibited in Figures 5(b) and 6(b).From Table 3, the activation energy of the pyrolysis of the complex obtained by the Kissinger method is extremely close to the one by the F-W-O method.
There are various possibilities to express the conversion function for the solid-state reactions.Most of the previous authors used the conversion function, namely, () = (1−)  .The equation which can be obtained by the Freeman-Carroll method [24][25][26] is as follows: The average value of the activation energy obtained from the above two methods is used as the activation energy of the thermal decomposition processes.The preexponential factor  can be obtained by Kissinger and Freeman-Carroll methods.As shown in Figure 7, according to the thermal decomposition data with a heating rate of 5 ∘ C min −1 , the plot of ln(d/d)+/ versus ln(1−) is obtained in the first and  the third mass loss stages.The reaction order can be found out through the slope of the line, and the preexponential factor  can be gotten by the intercept.The results are displayed in Table 4.The kinetic equation for the thermal decomposition of the title complex can be obtained by the nonisothermal kinetic equation [17]: The thermal decomposition kinetic parameters of , , and  are substituted into (3), and the thermal decomposition kinetics equations of the title complex in the first and the third mass loss stages can be obtained as follows: (4)

Conclusion
The manganese complex of H 2 MHA was synthesized with DL-2-hydroxy-(4-methylthio) butanoic acid and manganese chloride as main raw materials, and the complex was characterized by elemental analyses, FTIR, XRD, and TG-DTA-DTG.The index results of the XRD data show that the crystal structure of the complex belongs to monoclinic system with the cell parameters of  = 1.2775 nm,  = 1.5764 nm,  = 1.5764 nm, and  = 94.06 ∘ .The Mn(II) ion is hexacoordinated by the carboxylic and hydroxyl oxygen atoms from the H 2 MHA ligand.The thermal decomposition processes of the complex in the air illustrate that the complex is stable at room temperature and is decomposed over 200 ∘ C, and the final residue is mainly manganese sulfide.The thermal decomposition kinetics equations and parameters of the complex in the first and the third mass loss stages are obtained by nonisothermal kinetic theory and dynamic methods.The results are as follows: the activation energy  is 236.33 and 178.20 kJ mol −1 , the reaction order  is 0.91 and 0.91, and the preexponential factor ln() is 57.173 and 29.828, respectively.

Figure 4 :
Figure 4: TG (a) and DTG (b) curves of the complex Mn(HMHA) 2 at different heating rates.

Figure 5 :Figure 6 :
Figure 5: Linear relationship at different heating rates and   for the first stage.

Figure 7 :
Figure 7: Curves of ln(d/d) + / versus ln(1 − ) at the first (a) and the third (b) stages of the thermal decomposition.

Table 2 :
Experimental data and calculated results for X-ray powder diffraction pattern of the complex Mn(HMHA) 2 .

Table 3 :
Calculated results for the complex kinetic parameters of thermal decomposition using Kissinger and F-W-O methods.

Table 4 :
Calculated results for the complex kinetic parameters using Kissinger and Freeman-Carroll methods.