Thermo-oxidative-kinetic study of cinnamyl diesters

Thermochimica Acta 604 (2015) 94–105

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Thermo-oxidative-kinetic study of cinnamyl diesters Marta Worzakowska a, * , E. Torres-Garcia 1,b a b

Maria Curie-Skłodowska University, Faculty of Chemistry, Department of Polymer Chemistry, Gliniana 33 Street, 20-614 Lublin, Poland Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte # 152, 07730 México DF., Mexico

A R T I C L E I N F O

A B S T R A C T

Article history: Received 21 November 2014 Received in revised form 16 January 2015 Accepted 19 January 2015 Available online 23 January 2015

The thermo-oxidative behavior and kinetic study of cinnamyl diesters having different aliphatic chain lengths in the structure were studied by the TG/DSC/FTIR coupled method. The degradation processes on the studied esters occurred through several overlapping steps, between 150 and 450
C, linked to different gaseous products emitted and with changes of E(a) values as (a) increased. The initial stage of cinnamyl diester degradation (for a  0.15) was independent of the kind and length of the aliphatic chain, and the energetic barrier necessary to break the ester linkages was independent of the transformation degree (ca. 60–80 kJ mol1). A progressive increase in the E(a) values from ca. 80 kJ mol1 to 200–250 kJ mol1 (for 0.15  a  0.40) suggested the existence of competitive reactions which were due to chemical recombination processes in the gas phase, involving oxidation reactions of volatile organic fragments with oxygen. However, as a is changed between 0.40  a  0.75, the high data dispersion and the marked increase in experimental error in the E(a) values indicated a strong dependence of the kinetic parameters on conversion degree and complexity of chemical processes taking place. Finally, for 0.8  a the decrease in E(a) values suggested modifications of degradation mechanism and indicated that the easy oxygen accessibility toward the sample promotes the fast gasification reaction of the carbonaceous residues. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Cinnamyl diesters TG/FTIR Thermal stability Thermo-oxidative degradation Kinetic study

1. Introduction Organic esters are derivatives of organic acids which are characterized by the presence of at least one ester bond in their molecules. There is an important group of different structure compounds. Among esters, one can indicate natural or synthetic, saturated or unsaturated, aliphatic, aromatic or cycloaliphatic as well as monoesters, diesters or polyesters. Due to the variability in the structure, esters possess different physical and chemical properties and thus different, wide industrial applications [1–8]. The thermal stability, decomposition mechanism, the type of evolved gaseous products during pyrolysis or combustion and decomposition kinetics of various ester-structured compounds are important factors that allow a better understanding, control and optimization of both their reaction behavior and degradation mechanism, which allow a more detailed estimation of their usefulness as products or product components for industrial

* Corresponding author. Tel.: +48 81 524 22 51; fax: +48 81 524 22 51. E-mail addresses: [email protected] (M. Worzakowska), [email protected], [email protected] (E. Torres-Garcia). 1 Tel.: +52 55 9175 8430. http://dx.doi.org/10.1016/j.tca.2015.01.014 0040-6031/ ã 2015 Elsevier B.V. All rights reserved.

applications as well as their influence on environment pollution when manufactured at high temperatures. Generally, the cracking of organic compounds can proceed via free radical chain reaction or via the decomposition of one molecule into two or more molecules which is not accompanied by the formation of free radicals [9]. The pyrolysis process and pyrolysis kinetics of different organicstructured esters have been intensively and widely discussed and well documented [10–16]. It is commonly known that the pyrolysis of esters having b-hydrogen bonds causes the braking of C—O and C—H bonds and leads mainly to the formation of an acid and alkene fragments [17–20]. This reaction takes place through a concerted mechanism, where the formation of free radicals, responsible for the generation of multiple pyrolysis products, is not observed [17]. The carboxylic-ester-decomposition reactions, yielding an acid and olefin, are represented by an endothermic process which has very high activation energy (148–213 kJ mol1) depending on the ester structure [21–29]. However, the combustion process of organic esters is more complicated due to the influence of oxygen on the decomposition and thus, the production of different-structure free radicals which can further undergo, various subsequent reactions, causing the formation of various structure-intermediate-decomposition species. According to a literature survey, among the combustion process of esters,

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unimolecular initiations involving the breaking of C—C or C—H bonds, bimolecular initiations with oxygen-producing radicals, addition reactions of radicals to an oxygen molecule, isomerization, decomposition reactions, recombination, disproportionation processes of radicals, metatheses involving H-abstractions by radicals from the initial reactants, and other radical processes are highly expected [30]. There are numerous articles where the combustion process of organic esters using different experimental methods is discussed. Litwienko et al. [31] studied and described the kinetic parameters of the thermo-oxidation process of simple esters of free fatty acids by using the DSC method. They found that the activation energies regarding the oxidation of all the investigated esters were in the range of 102.5–127.3 kJ mol1 and, in general, were not dependent on the length of the carbon chain [31]. Also, the kinetic modeling of the oxidation of methyl esters, which are the main components of biodiesels in a jet-stirred reactor, has been studied [32–34]. In addition, the kinetics and combustion process of various vegetable oils and biomass, which can be converted into biofuels, have been studied by means of the TGA analysis [35–39]. Due to the importance of different structure organic esters in many industrial applications, the studies of their thermal stability, pyrolysis and combustion kinetics are still of high importance and intensely discussed. Regardless of the numerous papers on the thermal behavior, pyrolysis and combustion processes and kinetics, no kinetic data on the combustion process of diester derivatives of cinnamyl alcohol have been reported. Such esters may be an important component for industrial applications due to their properties and high thermal stability under inert conditions [40]. However, due to the fact that most commercial products are manufactured under air conditions, the thermal behavior and oxidation kinetic data of such esters are more important and useful for the producers and tool engineers. Due to the aforesaid, the objective of the present paper is to study the thermal behavior and combustion kinetics of different dicinnamyl-structured esters in detail. The influence of the ester structure on the thermal stability, nature of the occurring processes and type of emitted gaseous products under heating air conditions, and the estimation of the values of activation energies E(a) for each decomposition step and

their dependence on the reaction degree (a) have been evaluated and discussed. 2. Experimental 2.1. Materials Cinnamyl diesters were prepared through the esterification process of a stoichiometric ratio of cinnamyl alcohol (98%, Fluka) and acidic reagents such as succinic anhydride (99%, Merck), glutaric anhydride (95%, Fluka), adipic acid (99%, Merck) and sebacic acid (98%, Merck) in the presence of a catalyst. The accurate synthesis procedure, characterization and properties are given in Ref. [40] and their structure is presented in Scheme 1. 2.2. Methods Thermal degradation studies of cinnamyl diesters were carried out in a simultaneous TGA-DSC mode on a STA 449 Jupiter F1 Netzsch (Germany). The sample mass was ca. 10 mg. The analyses were done in open Al2O3 crucibles. The samples were heated from 40 up to 730
C with a heating rate of 10
C min1. As a gaseous atmosphere in the furnace was formed, synthetic air with a flow rate of 100 mL/min was applied. The analysis of the evolved gas formed during the degradation of the studied diesters was carried out in a FTIR spectrometer TGA 585 Brucker (Germany) coupled on-line to a STA instrument by Teflon transfer line. The gas cell and the transfer line of the FTIR spectrometer were heated up to 200
C in order to avoid condensation or adsorption of volatile decomposition products. The FTIR spectra were collected in the spectral range from 600 to 4000 cm1 with a resolution of 4 cm1. Each, single FTIR spectrum was recorded every 10
C. TGA/DTG experiments for kinetic studies were obtained at three different heating rates (2.5, 5 and 10
C min1) with the use of a STA apparatus and the same experimental conditions mentioned above. In the present work, the experimental TGA data were processed using the isoconversion principle in order to obtain the

O H C

CH

CH2

O

C

95

O (CH2)2

C

O

CH2

C H

O

CH2

C H

O

CH2

C H

O

CH2

C H

H C

dicinnamyl succinate O H C

CH

CH2

O

C

O (CH2)3

C

C H

dicinnamyl glutarate O H C

CH

CH2

O

C

O (CH2)4

C

H C

dicinnamyl adipate O H C

CH

CH2

O

C

O (CH2)8

C

dicinnamyl sebacate Scheme 1. The structure of studied cinnamyl diesters.

H C

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dependence of the activation energy E(a) as a function of the transformation degree (a). All isoconversional methods have their origin in the isoconversional principle which states that the reaction rate, at constant extent of conversion, depends only on the temperature [41]. If the Arrhenius equation is applicable, we can write:
dln ddta a EðaÞ (1) ¼ R dT 1 where the transformation degree, referred to as a, is a global parameter evaluated from the weight loss, i.e., a is the weight loss ratio as a function of time to the total weight loss, and the a subscript indicates the isoconversion values (ai)1 = (ai)2 = . . . (ai)n, for each experiment and temperature. This is the basis of the Friedman differential isoconversional method [42]. Usually, this equation is approximated to its linear form as:     da EðaÞ (2) ¼ ½const  ln RT dt a When ln (da/dt)a is plotted versus 1/T for each a value, a straight line should be obtained with a slope equal to Ea/R. This criterion allows an estimation of E(a) without the assumption of any reaction model, i.e., the model-free method. 3. Results and discussion 3.1. Thermal behavior The TG/DTG curves of the studied diesters for the experiments performed at a heating rate of 10
C/min under air conditions are presented in Fig. 1. In addition, the TG/DTG data were gathered in

110 90 dicinnamyl succinate

Mass (%)

70

dicinnamyl glutarate dicinnamyl adipate dicinnamyl sebacate

50 30

3.2. TGA-FTIR analysis

10 -10 40

140

240

340

440

540

640

Temperature (°C) 2 first decomposition step including two nonwell separated stages (i, ii)

second decomposition step (iii)

0 -2 DTG (%/min)

Table 1. The two main decomposition steps of cinnamyl diesters under oxidative conditions are visible from the TG/DTG curves. The first decomposition stage is described by an asymmetric, non-well separated peak which is composed by at least two main thermal events. Its shape reflects the simultaneous and complex decomposition path of the studied diesters under oxidative conditions. It occurred from temperatures above 150–160
C up to 430–450
C. In this stage, the significant mass loss, higher than 80% was indicated, Table 1. The second decomposition stage was visible from ca. 430–450
C to almost 615
C with the maximum mass loss temperature above 540
C. This decomposition stage was described by one, symmetrical DTG peak. The mass loss was from 14.25 to 19.40% depending on the ester structure. The original DSC curves are shown in Fig. 2. In addition, in order to analyze strictly the exo/endo effects happening during the first decomposition stage, the enlarged DSC curves are presented in Fig. 3. On the DSC curves, exo- and endothermic signals were clearly indicated. At temperatures where the first ester decomposition stage was observed on the TG/DTG curves, several overlapped thermal effects were detected, Fig. 3. The three, exothermic events with Tmax of 222, 275 and 365
C for dicinnamyl succinate and with Tmax of 232, 294 and 395
C for dicinnamyl glutarate are clearly observed. However, for dicinnamyl adipate and dicinnamyl sebacate, four exothermic events at Tmax of 250, 300, 365, 412
C and of 235, 295, 360, 380
C appeared, respectively. Generally, the exothermic signals are associated with the chemical reactions that happened in a gaseous phase between the primary decomposition products of the studied esters and oxygen [39]. In addition, it is worth noting that the endothermic signals can be detected between the exothermic ones. Their position is in accordance with the DTG signals. We believe that they are responsible of the mass loss linked to the decomposition and evaporation of intermediate products formed under oxidative conditions or products which are oxygen resistant. At temperatures above 430–450
C, only one, DSC exothermic signal was detected in all the samples. Its presence is related to the direct gasification of the char residue formed during the first decomposition stage. The obtained results are completely different from those recently published [40]. They indicate a different decomposition mechanism and thus different activation energies for the decomposition process of cinnamyl diesters in air with respect to an inert atmosphere.

557°C 548°C

-4

364°C

325°C

-6

320°C

366°C

305°C

348°C

542°C

548°C

-8 356°C

-10 295°C

325°C

-12 40

140

240

340

440

540

640

Temperature (°C) Fig. 1. TG/DTG curves of the studied esters in air atmosphere.

The 3D FTIR spectra and the extracted FTIR spectra of gaseous products evolved during the first decomposition stage of cinnamyl esters under oxidative conditions are presented in Figs. 4–7, respectively. The stacked FTIR spectra were gathered for temperatures where suitable mass losses from TGA curves were observed. They are almost in accordance with the temperature values given in Table 1. The FTIR spectra were recorded every 10
C, where the small variances between those observed from the TGA curves, Table 1 and those marked in Figs. 4–7 appeared. However, the extracted FTIR spectra for the first decomposition stage are representative and the main changes in the composition of the gaseous products emitted during the oxidation decomposition of the studied esters are precisely visible. At temperatures ranging from 40 to almost 150–160
C, no gaseous products are emitted by all the studied esters. This is in accordance with the TGA data, where the beginning of the thermal decomposition of esters above these temperatures was observed. However, when the temperature achieves the values where 1% of mass loss happens, the beginning of the emission of gaseous products is indicated. At T1% for all the studied esters, aldehyde fragments were the first detected decomposition products. This is confirmed by the

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Table 1 TG data for cinnamyl esters. Diester Dicinnamyl Dicinnamyl Dicinnamyl Dicinnamyl

T1% (
C)

T5% (
C)

T10% (
C)

T20% (
C)

T50% (
C)

T70% (
C)

T80% (
C)

First mass loss(%)a

Second mass loss(%)

170 176 158 175

227 237 233 248

250 262 261 276

272 289 291 307

303 334 343 361

335 364 366 392

365 408 420 428

85.75 81.23 80.60 81.78

14.25 18.77 19.40 18.22

First mass loss including two non-well separated decomposition stages.

second decomposition step (iii) dicinnamyl succ inate

Heat flow (mW/mg)

10 8

dicinnamyl glutarate

549°C

dicinnamyl adipate

550°C

dicinnamyl sebacate

559°C 543°C

6 first decomposition step including two non-well separated stages (i, ii)

4 2 0 -2 40

140

240

340

440

540

640

Temperature (°C) Fig. 2. Original DSC curves of the studied esters in air atmosphere.

presence of the absorption bands in the 1155–1280 cm1 regions (C—O stretching vibrations), the absorption signals at 1716– 1722 cm1 (C¼O stretching vibrations) and two bands at 2720 and 2806 cm1 (C—H stretching vibrations in aldehydes) [42–44], respectively. The emission of aldehyde fragments is indicated in a wide range of temperatures (from T1% to T80%). However, as the temperature increases, the changes in the FTIR peak intensity and the kind of gaseous products formed during the combustion of esters are observed. At T5%, the additional gaseous products are detected. For all the studied esters, the appearance of absorption bands in the 690–815 cm1 regions (out-of-plane deformation vibrations of CAr—H), the small bands at 1490 cm1 and at 1596 cm1 (stretching vibrations of ring C¼C) and the two absorption signals at 3034 and at 3074 cm1 (stretching vibrations of CAr—H and ¼C—H) are visible. The presence of these signals is the result of the beginning of the formation of aromatic fragments. The additional small bands at 885–960 cm1 (out-of-plane deformation vibrations of ¼C—H) and at 1635 cm1 (stretching vibrations of C¼C) together with the occurrence of the bands at 3034 and at 3074 cm1 (stretching vibrations of ¼C—H) indicate also on the creation of alkene fragments [18–20,44,45]. These observations are completely different from those obtained in our previous study [40]. During the pyrolysis of cinnamyl diesters, the detection of aromatic and alkene fragments at temperatures where 40% of mass loss occurred was really observed. As it was confirmed, the pyrolysis of cinnamyl diesters caused b-elimination reactions and the formation of an alkene (allene) and acids fragments as main, primary decomposition products, which is in agreement with other studies [18–20,40]. According to a literature survey, it is well known that allene is not a stable compound because of the occurrence of the extra strain, which is the result of one carbon atom forming two double bonds [46]. Due to this, it can undergo various reactions. As it was proved, under pyrolysis conditions, mainly the polymerization of formed primary alkene fragments was indicated. So, its depolymerization products were observed at relatively higher temperatures (T40%) [40].

Notwithstanding, the presence of oxygen influences the creation of the different gaseous products and thus different decomposition mechanisms. Meanwhile, we believe that the cleavage of ester bonds under an oxidizing environment can lead to the formation of unsaturated fragments as one of the primary decomposition products. In the present studies, reactions of primary formed unsaturated fragments with oxygen are expected instead of its polymerization since oxygen acts as an inhibitor of the polymerization processes [47–51]. According to literature data [52–57], alkene, alkyne and diene compounds can undergo free radical reactions with oxygen, which causes the formation of free radical species. The radicals and low mass intermediate compounds can be formed from unsaturated compounds in bimolecular steps and by thermal unimolecular decomposition. The free radical process is complex and parallel, including the creation of various order radicals, which are low stable fragments and undergo further, subsequent reactions. During this process, the formation of various intermediate and final products such as aldehydes, alkenes, aromatics, alkanes fragments, carbon monoxide, water, etc. is expected. Also in this case, the free radical reactions between 1,5 275°C (exo)

dicinnamyl succinate dicinnamyl glutarate

294°C (exo)

365°C (exo)

395°C (exo)

232°C (exo)

1

Heat flow (mW/mg)

12

222°C (exo)

0,5

250°C 240°C (endo) (endo)

0

310°C (endo)

328°C (endo)

368°C (endo)

-0,5

-1 40

140

240 Temperature (°C)

340

440

2,5 dicinnamyl adipate

2

Heat flow (mW/mg)

a

succinate glutarate adipate sebacate

dicinnamyl sebacate

1,5

235°C (exo)

295°C (exo) 250°C (exo)

360°C (exo)

365°C (exo)

380°C (exo)

412°C (exo)

300°C (exo)

1 325°C (endo)

0,5 255°C (endo)

0

270°C (endo)

338°C (endo)

-0,5 -1 -1,5 40

140

240 Temperature (°C)

340

440

Fig. 3. Enlarged DSC curves of the studied esters in air atmosphere (first decomposition step including two non-well separated stages).

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Fig. 4. 3D FTIR spectra (above) and stacked FTIR spectra (below) for dicinnamyl succinate.

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Fig. 5. 3D FTIR spectra (above) and stacked FTIR spectra (below) for dicinnamyl glutarate.

99

100

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Fig. 6. 3D FTIR spectra (above) and stacked FTIR spectra (below) for dicinnamyl adipate.

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Fig. 7. 3D FTIR spectra (above) and stacked FTIR spectra (below) for dicinnamyl sebacate.

101

102

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primary decomposition products and oxygen in gaseous phase are highly possible. The presence of the exothermic signals in the wide range of temperatures in the DSC curves, Fig. 3, can be assigned to free radical processes of primary formed decomposition products with oxygen. After breaking ester bonds during heating, oxygen reacts with formed unsaturated fragments and thus, the formation of intermediate products such as aldehydes are detected at T1% for once. However, the secondary reaction between unsaturated compounds and oxygen causes the creation of alkenes, aromatics, aliphatic fragments and CO [58,59], which are clearly observed above T10%,Figs. 4–7. As main secondary ester decomposition products under oxidative conditions, decarboxylated forms of dicarboxylic acids - aliphatic acid anhydrides for dicinnamyl succinate and dicinnamyl glutarate were detected. As it is commonly known, dicarboxylic acids are resistant to the oxidation process and thus their decarboxylation or dehydration processes at higher temperatures are mainly expected. This supposition is in accordance with the TG/FTIR results. In the case of dicinnamyl succinate at T5%, the beginning of the evaporation of an acid anhydride is well seen. It was confirmed by the presence of the absorption signals at 912 –1081 cm1 (C—O stretching vibrations), the bands in the 1810–1870 cm1 range (C¼O stretching vibrations), two vibration

signals at 1345 and 1448 cm1 (C—H deformation vibrations), the signals located at 2860 – 2970 cm1 (C—H stretching vibrations) [40,44,60,61] and the emission of CO2 visible at 670 cm1 and 2329–2365 cm1 [62–65], Fig. 4. Nevertheless, the evaporation of an acid anhydride (the characteristic bands for carbonyl groups at 1793–1824 cm1), which is one of the decomposition products of dicinnamyl glutarate, starts slowly at temperatures between T5% and T10%, Fig. 5. The maximum intensity for the evaporation of acid anhydrides was indicated at T50%. It is in accordance with the DTG curves (first decomposition peak), Fig. 1 and with the DSC curves (first endothermic signal), Fig. 3. However, at higher temperatures (ca. T20%) under oxidative conditions, parallel, partial decomposition of primary formed acid anhydrides may happen. It leads to the generation of CO2 [62–65], H2O (bands ranging from 3570 to 3900 cm1) [63,66–68] and condensation products such as aldehydes, Figs. 4, 5. On the other hand, the formation of products more stable than acid anhydrides such as cyclic ketones or carboxylic acids during the decomposition of dicinnamyl adipate and dicinnamyl sebacate is expected [40,44,62]. The presence of bands at 1020–1145 cm1 (C—O stretching vibrations) and the band at 1770 cm1 (C¼O stretching vibrations) for dicinnamyl adipate, Fig. 6, and the existence of the absorption bands at 1008–1178 cm1

Fig. 8. Apparent activation energy E(a) as a function of the transformation degree (a) determined by the Friedman (F) method for the decomposition processes of cinnamyl diesters in air atmosphere. Note: The error bars are also included for each case.

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(C—O stretching vibrations) and the broad signal with maximum at 1768 cm1 (C¼O stretching vibrations) for dicinnamyl sebacate, Fig. 7, indicated on the creation of cyclic ketones. However, the bands at 1145 cm1 and at 1178 cm1 and the appearance of the additional small signal at 1270 cm1 (stretching vibrations of C—O), the signal at 3580 cm1 (stretching vibrations of —OH) and the presence of the band at ca. 1770 cm1 may be also the result of the formation of acids. In addition, the emission of CO2 and H2O together with the formation of cyclic ketones and acids indicated the partial decarboxylation and dehydration processes of primary, formed acid anhydrides through oxidative decomposition of these two esters. The second decomposition stage, which is exothermic, happened at very similar temperatures for all the studied esters, Figs. 1 and 2. In this decomposition stage, the emission of only inorganic gaseous products such as CO, CO2 and H2O is observed, Figs. 4–7. Their presence is directly linked to the oxidation reactions of char residues formed during the first decomposition stage. 3.3. Kinetic analysis The results obtained at different heating rates (2.5, 5 and 10
C min1) using TGA and applying Eq. (2) are shown in Fig. 8. The analysis in terms of the activation energy shows the complex E(a) on a dependence and reveals the energetic behavior of the different thermal events during the degradation of cinnamyl diesters in oxidative atmosphere. In general, for all the samples, three possible zones, as a change between 0  a  1, were detected. Although the energetic profiles of E(a) in all the analyses have similar tendencies, the intervals and magnitudes as well as the dispersion and analysis errors for each sample are very different. This behavior shows the main physicochemical differences among these compounds and reveals the main processes during the thermo-oxidative degradation of cinnamyl diesters. Interestingly enough, during the first stage (I), for a  0.15, the activation energy values were independents of the transformation degree and very close to all cinnamyl diester samples (between 60–80 kJ mol1). This behavior suggests that the initial degradation mechanism is similar for all the studied cinnamyl diesters and that the breaking occurs on energetically equivalent bonds, proving that the reaction is not thermally auto-catalyzed despite its exothermic character. This stage, for temperatures lower than 330
C and E(a) values essentially independent of the transformation degree, is related to the initial degradation stage of the cinnamyl diesters through the breaking of ester linkages. Subsequently, a progressive increase in the E(a) values from ca. 80 to 200–250 kJ mol1, as a changes from 0.15 to 0.40 was detected. The existence of this zone reveals a mechanism change in the degradation process and suggests that the energetic barrier, necessary to overcome different interactions in this stage, comprises multiple and simultaneous reactions [69–71]. Therefore, the presence of exothermic events (DSC curves) in this interval is a clear indication of the chemical recombination processes in the gas phase, involving the oxidation reactions of volatile organic fragments with oxygen. This approach suggests that at least during the previous degradation stages, the highly evolved gaseous products apparently limit the oxygen accessibility towards the sample, which promotes that the reaction only occurs in gas phase. This fact explains why the reactions are not thermally auto-catalyzed, according to the physical property (mass) of choice in this work (TGA). Subsequently, a complex set of reactions, simultaneous and consecutive, may occur until full carbonization is reached. A second stage (II), for temperatures between 300–450
C, as a is changed between 0.40  a  0.75, shows as main characteristic a high dispersion of the data and a marked

103

increase in the experimental error in E(a) values. So, the experimental values of the activation energy might not be representative of some individual decomposition reaction steps in that interval. However, in this very interval, we can observe a zone (identified as ii-b) distinctive and characteristic in position and energetic magnitude for each of the studied cinnamyl diesters, which could be related to the nature and fragmentation form of the aliphatic chain in the structure. In broad terms, this means that the reaction mechanism, i.e., the step that always limits the reaction rate is not same, during this stage and that the kinetic parameters are highly dependent on the conversion degree. Finally, the common characteristic during the last degradation stage (III), for 0.8  a and temperatures higher than 450
C, was the marked decrease in activation energy values until reaching values between 60 and 70 kJ mol1 in all the cases, and particularly highlighting the value of ca. 24 kJ mol1 for dicinnamyl succinate. The existence of this zone reveals a mechanism change and suggests that once the partial pressure of the gaseous products is decreased, the easy oxygen accessibility toward the sample promotes the fast gasification reaction of the carbonaceous residues. Nevertheless, the increasing activation energy up to ca. 140–160 kJ mol1 values, for 0.9  a and temperatures higher than 500
C by the end of the reaction, suggests the presence of at least two char types. Such behavior could be associated with structural characteristics in the char residues, particularly with the presence of sp2 sites and mixed sp2-sp3 bond forms, which define the ordering degree and their chemical reactivity.

4. Conclusions The thermo-oxidative behavior and kinetic study of each stage, together with gaseous compounds emitted during the degradation process of different, aliphatic chain length cinnamyl diesters were investigated by simultaneous TG/DSC/FTIR analysis techniques. The results show that the degradation processes on the studied cinnamyl diesters occur through several overlapping steps, between 150 and 450
C. During the first decomposition stage (I), the emission of various gaseous products as a result of breaking ester bonds, and the further oxidation of volatile intermediate decomposition products, was observed from FTIR data. The values of activation energies for a  0.15 were similar for all the studied esters (60–80 kJ mol1), indicating a similar initial degradation mechanism through the breaking of ester linkages. Notwithstanding, as a was augmented up to 0.4, increasing E(a) values from ca. 80 kJ mol1 to 200–250 kJ mol1 suggest the existence of competitive reactions. For temperatures lower than 450
C, as a is changed between 0.40  a  0.75, (Stage II), the main characteristic is the high dispersion of the data and the marked increase in the experimental error in the E(a) values. This behavior is a clear evidence of the marked dependence of the kinetic parameters on the conversion degree and complexity of the present chemical processes. One of the key features in this stage is the presence of a distinctive and characteristic zone (ii-b), which has been related to the nature and length of the aliphatic chain in the cinnamyl diester structure. Finally, during the last stage (III), for temperatures higher than 450
C, analyses of the E(a) values showed a similar behavior for all the cinnamyl esters and it was assigned to direct gasification. In this stage, the significant decrease in E(a) values up to 60–70 kJ mol1 for a  0.8 and then increase in E(a) values ca. 140–160 kJ mol1 for a  0.9, suggested the formation of two char types during the previous decomposition stages, probably related to the presence of sp2 sites and mixed sp2–sp3 bonds, and that the kinetic parameters and rate limiting step are controlled by oxygen accessibility towards the residual solid.

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