Comprehensive Investigation of the Thermal Degradation

Article pubs.acs.org/EF

Comprehensive Investigation of the Thermal Degradation Characteristics of Biodiesel and Its Feedstock Oil through TGA−FTIR Hui Li, Shengli Niu,* Chunmei Lu, and Yongzheng Wang School of Energy and Power Engineering, Shandong University, Jingshi Road, No. 17923, Jinan, Shandong 250061, PR China ABSTRACT: With the rapid growth of biodiesel production in recent years, it is essential to evaluate the thermal degradation characteristics of biodiesel and its feedstock oil, for they are concerned with plenty of scientific applications. This study investigates thermal degradation of biodiesel and its feedstock oil through thermogravimetric analysis in conjunction with Fourier transform infrared spectroscopy (TGA−FTIR). The experiments are conducted under inert conditions from 298 to 873 K, and are operated at temperature heating rates of 5, 10, 15, and 20 K min−1. Based on TGA results, the activation energy and enthalpy are calculated via the model free approach and the reaction order is determined by the Avrami theory. Besides the preexponential factor, the Gibbs free energy and entropy are calculated at the initial weight loss and the maximum weight loss temperature. Finally, the evolved products during the thermal degradation of biodiesel and its feedstock oil are detected by FTIR in real time.

1. INTRODUCTION The progressive depletion of fossil fuel, together with global warming and environmental pollution, has urged the development of renewable energy. Triacylglycerides, namely, the feedstock oil usually derived from vegetable oil or animal fat, are green and sustainable energy resource. However, the direct use of feedstock oil is not satisfactory because of their high viscosity and poor ignition properties. Transesterification of feedstock oil with short-chain alcohol (methanol or ethanol) under the catalytic effect of bases, acids, or enzymes leads to the formation of less viscous fatty alkyl methyl esters (FAMEs), which are also called biodiesel. Biodiesel, a promising renewable substitute for fossil diesel, is gaining particular attention for its abundant availability, eco friendliness, renewability, and similar characteristics with fossil diesel.1,2 Thermal degradation characteristics provide efficient tools for measuring properties such as the activation energy, enthalpy, heat capacity, and boiling point.3−5 Thermogravimetric analysis (TGA) is a common method to quantify thermal degradation of biological materials for the in-depth analysis of weight loss and determination of kinetic parameters.5 Chien et al.6 studied the thermal degradation kinetics of soybean biodiesel by using TGA. The activation energy and pre-exponential constant were 68.75 kJ mol−1 and 2.6 × 107 min−1, respectively. Similarly, the thermal characteristic of the poultry fat biodiesel was evaluated through TGA/ DTG/DTA/DSC and the result revealed that it was an endothermic reaction with only one weight loss step under nitrogen atmosphere.7 With the rapid growth of biodiesel production in recent years, it is necessary to investigate its thermal degradation characteristics for industrial application. Meanwhile, the feedstock oil also needs to be evaluated for comparison. For instance, the quality of biodiesel and its feedstock oil is deteriorated during long-term storage. The thermal degradation characteristics could determine the decomposition temperature to optimize the storage conditions.8 Santos et al.9 evaluated the volatilization kinetic parameters of Brazilian sunflower oil and © XXXX American Chemical Society

its biodiesel. They found that the activation energy for biodiesel was lower than that of feedstock oil. Yet, with the increasing of biodiesel application, it is imperative to quantify the thermodynamics of thermal degradation as well as the kinetics. Also, the evolved products from a sample are indispensable to obtain a comprehensive understanding of thermal degradation, and Fourier transform infrared spectroscopy (FTIR) is a useful tool to get the type and quantity of evolved products.10,11 Consequently, the technology of thermogravimetric analysis in conjunction with Fourier transform infrared spectrometry (TGA−FTIR) could be employed to evaluate the thermal degradation characteristics for a sample in real time.10 The purpose of this fundamental work is to comprehensively investigate and compare the thermal degradation characteristics for biodiesel and its feedstock oil non-isothermally under nitrogen atmosphere. Based on the iso-conversional method, the activation energy and enthalpy are calculated through the model free approaches of the Friedman method, the Kissinger− Akahira−Sunose method, and the Flynn−Wall−Ozawa method, respectively. Meanwhile, the reaction order is determined by Avrami theory. Besides the pre-exponential factor, Gibbs free energy and entropy are also calculated at the initial weight loss and the maximum weight loss rate temperature. Finally, the evolved products during the thermal degradation for biodiesel and its feedstock oil from TGA are detected by FTIR in real time.

2. MATERIALS AND METHODS 2.1. Materials. The feedstock oil used in this study is peanut oil (denoted as PO), which is the major edible oil in China and makes a great contribution to waste cooking oil, and is purchased from the local market. The fatty acids composition is analyzed with a gas chromatograph (Shimadzu Co., Ltd., Japan) equipped with a DBINNOWAX capillary column (30 m × 320 μm × 0.5 μm) and a flame Received: May 11, 2015 Revised: July 21, 2015

A

DOI: 10.1021/acs.energyfuels.5b01054 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels ⎛ dα ⎞ E ln⎜β · ⎟ = ln(A · f (α)) − a ⎝ dT ⎠ RT

ionization detector (GC−FID). Column temperature is initially set at 333 K for 1 min and then increased to 533 at 10 K min−1, where it is held constant for 15 min. Injector and detector temperature are 533 and 573 K, respectively. Nitrogen (99.999% purity) is employed as the carrier gas at 1.4 mL min−1. The peanut oil biodiesel (denoted as POB) is obtained from PO through transesterification as described by previous study,12 and the fatty acids compositions of POB and PO are shown in Table 1.

Besides the differential method, eq 1 also can be calculated by integral, where the Kissinger−Akahira−Sunose method (KAS) and the Flynn−Wall−Ozawa method (FWO) are widely demonstrated to be proper.4,13,14 And the integration method of eq 1 is expressed as

∫0

Table 1. Fatty Acids Composition of Peanut Oil Biodiesel (POB) and Peanut Oil (PO) POB

PO

Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Daturic acid (C17:0) Heptadecenoic acid (C17:1) Stearic acid (C18:0) Oleinic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic acid (C20:0) Arachidonic acid (C20:1) Behenic acid (C22:0) Tetracosanoic acid (C24:0)

0.64 21.00 0.52 0.08 1.39 2.53 17.00 54.90 0.92 0.30 0.13 0.20 0.39

0.63 19.46 0.58 0.07 0.08 2.10 16.40 59.92 0.07 0.18 0.29 0.13 0.09

dα = g (α) = Aβ −1 f (α)

g (α) =

∫0

T

⎛ E ⎞ ⎟dT exp⎜ − ⎝ RT ⎠

(3)

⎛ E ⎞ A RT 2 · · exp⎜ − a ⎟ ⎝ RT ⎠ β E

(4)

After taking the natural logarithm, eq 4 is changed into eq 5,

⎛ AE ⎞ ⎛ β⎞ E ln⎜ 2 ⎟ = ln⎜ ⎟− ⎝T ⎠ ⎝ Rg (α) ⎠ RT

(5)

As for the FWO method, the Doyle’s approximation, which is given as log(P(X)) = −2.315 − 0.4567X, is used for temperature integration as shown in eq 6,

g (α) =

⎛ E ⎞ A 0.00484· exp⎜ −1.052 a ⎟ ⎝ β RT ⎠

(6)

By taking the natural logarithm, eq 6 is transformed into eq 7,

⎛ AEa ⎞ E ln(β) = ln⎜ ⎟ − 5.331 − 1.052 a RT ⎝ Rg (α) ⎠

2.2. FTIR analysis of POB and PO. Characteristic functional groups presented in POB and PO are analyzed with a Vertex70 FTIR analyzer (Bruker, Co., Ltd., Germany) with a single reflection horizontal ATR accessory (Pike Technologies, Co., Ltd., USA). Each ATR spectrum is the average of 16 scans by using air as reference with 4 cm−1 spectral resolutions ranging from 4500 to 600 cm−1. 2.3. TGA−FTIR experiments. A TGA/SDTA 851e (Mettler− Toledo, Co., Ltd., Switzerland) coupled with a Vertex70 FTIR analyzer (Bruker, Co., Ltd., Germany) is used to study the thermal degradation process of POB and PO. For TGA, the sample is weighted to 10 ± 0.1 mg and conducted from 298 to 873 K at the linear temperature heating rates of 5, 10, 15, and 20 K min−1, respectively. Meanwhile, TGA cell is flushed with 50 mL min−1 nitrogen (99.999% purity) to maintain inert atmosphere for thermal degradation. Released products from TGA are online monitored with a FTIR gas cell which is heated to 473 K to prevent condensation of evolved products. 2.4. Kinetic and thermodynamic calculation theory. The activation energies (Ea) of POB and PO are calculated through the model free approaches, also called the iso-conversional method. They are derived with the assumption that the reaction rate depends on temperature and conversion degree only. This method has been successfully used for investigating the kinetics of different substances due to no reaction model being estimated. The thermal degradation rate under a linear temperature heating rate is generally expressed by eq 1,

⎛ E ⎞ dα dα = k(T )·f (α) → β · = A · exp⎜ − a ⎟ ·f (α) ⎝ RT ⎠ dt dT

α

For the KAS method, the Coats−Redfern approximation of P(X) = x2·e−x is applied for the temperature integration as exhibited in eq 4,

Composition (wt %) Fatty acids

(2)

(7)

Based on the same conversion at the different heating rates, the regression lines of ln(β·dα/dT) versus 1/T (eq 2) for the Friedman method, ln(β/T2) versus 1/T (eq 5 for the KAS method, and ln(β) versus 1/T eq 7 for the FWO method have been fitted for Ea calculation from the slope. Besides Ea, the reaction order (n) is also an important index to illustrate the thermal degradation characteristics, and the Avrami theory,14,15 which is described as eq 8, is used to evaluate the values of n in this study.

α = 1 − exp

−k(T ) βn

(8)

By taking rearrangement and double natural logarithm, eq 8 is transformed into ln(− ln(1 − α)) = ln A −

Ea − n·ln(β) RT

(9)

Thus, plots of ln(−ln(1 − α)) versus ln β are established at the same temperature, and the slope of the regression line is the reaction order. The enthalpy is the state function and reflects the heat energy absorbed or released of a chemical reaction, which is calculated by eq 10,16,17

ΔH = Ea − RT

(10)

Further, the pre-exponential factor (A), the Gibbs free energy (ΔG), and the entropy (ΔS) of POB and PO are calculated from eq 11 to eq 13, respectively.16,18

(1)

where α refers to the reaction conversion, α = (M0 − Mt)/(M0 − Mf) (M0 refers to the initial weight; Mt refers to the sample weight at time t; Mf refers to the final weight), f(α) refers to the reaction mechanism model, k(T) refers to the reaction rate constant described by the Arrhennius equation, namely, k(T) = A·exp(−Ea/RT) (A refers to the pre-exponential factor; Ea refers to the activation energy; R refers to the gas constant), T is temperature, and β refers to the heating rate (β = dT/dt = constant). The Friedman method4 is a differential method which is obtained by taking the natural logarithm on both sides of eq 1 as shown in eq 2,

⎛E ⎞ A = β · Ea · exp⎜ a ⎟/RT 2 ⎝ RT ⎠

(11)

⎛ K ·T ⎞ ΔG = Ea + RT · ln⎜ B ⎟ ⎝ h·A ⎠

(12)

ΔS = B

ΔH − ΔG T

(13) DOI: 10.1021/acs.energyfuels.5b01054 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels where KB is the Boltzmann constant and h is the Planck constant.

3. RESULTS AND DISCUSSION 3.1. Functional groups of POB and PO. FTIR spectra for the functional groups of POB and PO are depicted in Figure 1.

Figure 1. FTIR spectra of peanut oil biodiesel (POB) and peanut oil (PO).

The similarities in the FTIR spectra of POB and PO at 3009 cm−1, 2923 cm−1, 2853 cm−1, 1742 cm−1, 1460 cm−1, 1362 cm−1, 1166 cm−1, and 722 cm−1 suggest the presence of identical chemical groups in their chemical composition, and these are attributed to C−H axial deformation in olefinic double bonds, C−H symmetric axial deformation in CH3, C−H symmetric axial deformation in CH2, CO axial deformation in the ester groups, C−H angular deformation in CH3, C−O− C asymmetric vibration in the ester group, C−O stretching vibration in the ester groups, and C−H out-of-plane bending vibration in CH2, respectively.10,11 However, it should be noted that the unique band at 1435.6 cm−1 attributed to the CH3 asymmetric deformation in O−CH3 is only detected in POB but not in PO, and this is the distinctive difference between FAMEs and triacylglycerides.11 3.2. TGA−DTG analysis of POB and PO. TGA and derivative thermogravimetric (DTG) curves for POB and PO at the heating rates of 5, 10, 15, and 20 K min−1 are shown in Figure 2. POB and PO exhibit similar trends throughout the thermal degradation process where only one distinct weight loss stage is obviously detected. Santos et al.9 evaluated the volatilization of sunflower oil and its biodiesel under nitrogen atmosphere, and the TGA curves also showed a single welldefined step from 303 to 873 K at heating rates of 5, 10, and 20 K min−1. Similarly, Todaka et al.19 compared the thermal decomposition of three biodiesels under air and nitrogen atmosphere, and only one weight loss step was observed. Therefore, the TGA results in the current study are comparable to the published literature. To be specific, the thermal characteristic parameters of POB and PO are presented in Table 2. The initial weight loss temperature (Tinitial) is used to indicate the resistance to thermal degradation. POB is thermally stable up to 398.32 at 5 K min−1, and then shifted to 447.15 at 20 K min−1. However, Tinitial for PO starts from 603.65 at 5 K min−1 to 674.82 at 20 K min−1. Dantas et al.20 found that corn oil biodiesel and its feedstock oil were thermally stable up to 418 and 609 K under nitrogen atmosphere, which was similar to the results in this study.

Figure 2. Thermogravimetric analysis (TGA) (a) and derivative thermogravimetric (DTG) (b) curves for peanut oil biodiesel (POB) and peanut oil (PO) at heating rate of 5, 10, 15, 20 K min−1 in nitrogen atmosphere.

Table 2. Characteristic Parameters of Thermal Degradation for Peanut Oil Biodiesel (POB) and Peanut Oil (PO) at Different Heating Rates β Sample (K min−1) POB

PO

5 10 15 20 5 10 15 20

T initial (K)

DTGm × 103 (% K −1)

Tm (K)

Final carbon residue (%)

398.32 423.32 434.65 447.15 603.65 633.50 651.90 674.82

1.53 2.56 3.12 3.47 1.06 2.05 2.82 3.73

500.98 527.65 538.15 541.15 686.65 710.65 717.15 727.15

2.53 3.19 3.65 3.57 3.67 5.17 5.54 5.48

With temperature increasing, small molecule and weak chemical bonds are gradually decomposed and TGA curves descend. The major weight loss occurs from 398.32 to 629.15 K for POB and from 603.65 to 768.48 K for PO, respectively. This result demonstrates that PO is more thermally stable than POB, and this is explained by the fact that PO has components with a higher molar mass, resulting in a product with higher viscosity and intermolecular forces. Further, the observed thermal characteristic temperatures for POB are closer to the volatilization of conventional diesel temperatures. Hence, POB is validated as a potential alternative fuel.9 Finally, the final carbon residues for POB and PO are 2.53%, 3.19%, 3.65%, 3.57% and 3.67%, 5.17%, 5.54%, 5.48%, respectively. As pointed out by Gai et al.,15 the residual weight percentage C


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Figure 3. Regression lines for peanut oil biodiesel (POB) based on the Friedman method (a), the Kissinger−Akahira−Sunose (KAS) method (c), and the Flynn−Wall−Ozawa (FWO) method (e) and for peanut oil (PO) based on the Friedman method (b), the Kissinger−Akahira−Sunose (KAS) method (d), and the Flynn−Wall−Ozawa (FWO) method (f) at 5, 10, 15, and 20 K min−1.

with PO. Therefore, POB is used at large scale as alternative fuel, for it can be easily burned out. In addition, it can be seen from Figure 2 and Table 2 that the entire thermal degradation process is shifted to a higher temperature region with the heating rate increasing from 5 K min−1 to 20 K min−1. A similar relationship between the thermal characteristics and heating rate was also reported in the previous study.21 Heating rate is a crucial factor to affect the thermal degradation due to the heat transfer limitation at the higher temperature heating rate. This is explained by the fact that a higher heating rate decreases the heat distribution in molecules; therefore, the thermal degradation starts at a higher temperature.22 3.3. Kinetic and thermodynamic calculation of POB and PO. In this study, TGA experiments are conducted at the four heating rates of 5, 10, 15, and 20 K min−1. Meanwhile, eight conversion values from 10% to 80% with an increment of

after thermal decomposition is mainly determined by the total content of ash and fixed carbon. In this study, the final residue of POB is lower than that of PO, which is attributed to the elimination of the glycerol backbone (C−C−C), resulting in less ash content for POB. The DTG curves in Figure 2(b) exhibit a single peak for both POB and PO, which confirms one weight loss stage of the thermal degradation. Specifically, the maximum derivation weight loss (DTGm) and the corresponding temperature (Tm) for POB increase with the increment of heating rates, which are heightened from 1.53 × 10−3 % K−1 at 500.98 K to 3.47 × 10−3 % K−1 at 541.15 K. Meanwhile, DTGm and Tm for PO are heightened from.1.06 × 10−3 % K−1 at 686.65 K to 3.73 × 10−3 % K−1 at 727.15 K. Taking the above results into consideration reveals that POB could be thermally degraded at lower temperature compared D


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Table 3. Correlation Coefficient (R), Activation Energy (Ea), and Standard Deviation (SD) for Peanut Oil Biodiesel (POB) and Peanut Oil (PO) Calculated from the Friedman Method, the Kissinger−Akahira−Sunose (KAS) Method, and the Flynn−Wall− Ozawa (FWO) Method Friedman method

KAS method

FWO method

Sample

α (%)

R

Ea (kJ/mol)

SD

R

Ea (kJ/mol)

SD

R

Ea (kJ/mol)

SD

POB

10% 20% 30% 40% 50% 60% 70% 80% 10% 20% 30% 40% 50% 60% 70% 80%

0.99 0.97 0.98 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.97 0.99 0.99 0.99

48.32 50.31 51.49 52.83 53.81 54.55 55.07 55.11 92.71 97.25 111.51 123.44 130.98 138.33 150.7 153.75

0.09 0.17 0.13 0.08 0.08 0.07 0.07 0.06 0.07 0.06 0.08 0.09 0.23 0.03 0.08 0.08

0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

43.22 45.24 46.28 47.07 48.24 49.03 49.55 50.28 86.24 99.85 108.83 116.5 125.36 137.39 144.78 153.83

0.03 0.08 0.10 0.10 0.09 0.09 0.09 0.09 0.10 0.08 0.07 0.05 0.05 0.05 0.04 0.04

0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

48.54 50.74 51.9 52.78 54.01 54.86 55.45 56.22 92.54 105.71 114.38 121.79 130.32 141.84 148.99 157.73

0.04 0.08 0.10 0.10 0.09 0.09 0.09 0.09 0.10 0.08 0.07 0.05 0.05 0.05 0.04 0.04

PO

the presumed reaction model.24,25 As pointed out by White et al.,25 the first-order model was formulaically employed in a kinetic calculation, but their feasibilities under specific conditions were not rigorously verified. Therefore, to obtain the accurate reaction order of thermal degradation for POB and PO in this study, eight temperatures basically distributed in the conversion range from 10% to 80% are selected at heating rates of 5, 10, 15, and 20 K min−1. By adopting the Avrami theory, the correlation coefficient (R), the calculated reaction order (n), and the standard deviation (SD) are shown in Table 4.

10%, which mainly covers the thermal degradation for POB and PO, are selected for Ea calculation. The parallel fitted lines depicted in Figure 3 indicate Ea values at different conversions follow a single mechanism or unification of multiple reaction mechanisms and are in accordance with TGA−DTG results. From Table 3, it is found that the calculated data for POB and PO show the satisfying fitness of the Friedman method, the KAS method, and the FWO method to experimental data for the high value of the correlation coefficient (R) and the low value of the standard deviation (SD). At the same time, the Ea values of POB and PO present a similar trend with conversion for each method. The activation energy is the minimum energy required to break the chemical bonds and it determines the reactivity and sensitivity of a reaction rate.14 Within the whole process, Ea for POB gradually get increased from 43.22 kJ mol−1 to 56.22 kJ mol−1 while for PO are heightened from 86.24 kJ mol−1 to 157.73 kJ mol−1. The broad carbon number distribution and various covalent bonds existed in POB and PO lead to the extent of Ea. It further reveals that the thermal degradation for POB and PO is a complex reaction involving parallel, competitive and consecutive reactions. The average Ea of POB calculated by the Friedman method, KAS method and FWO method are 52.68 kJ mol−1, 47.36 kJ mol−1 and 53.06 kJ mol−1, respectively. As for PO, the average Ea are 124.83 kJ mol−1, 121.59 kJ mol−1 and 126.66 kJ mol−1. Typically, a reaction with lower activation energy requires less energy to crack down the chemical bonds.10,22 As PO possesses higher molecular weight with stronger viscosity and intermolecular forces, thus it requires more energy to be degraded. After transesterification with methanol, the molecular weight of PO is almost decreased to 1/3,23 where the viscosity and intermolecular forces are correspondingly decreased. As a result, POB requires less energy to be thermally degraded. It further confirms that POB is more suitable to be used as an alternative diesel than PO for it could be more easily atomized, decomposed or burned out. In a word, Ea of thermal degradation mainly depends on molecular weight. To determine the kinetic parameters, most studies applied the model fitting method and took zero-order or first-order as

Table 4. Correlation Coefficient (R), Reaction Order (n), and Standard Deviation (SD) for Peanut Oil Biodiesel (POB) and Peanut Oil (PO) Calculated with the Avrami Theory POB

PO

T (K)

R

n

SD

T (K)

R

n

SD

490 495 498 500 503 508 512 517 Average

0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

1.55 1.58 1.61 1.62 1.65 1.69 1.72 1.82 1.65

0.12 0.13 0.14 0.14 0.15 0.16 0.17 0.20

691 693 697 701 703 706 710 713

0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

1.23 1.19 1.13 1.04 1.00 0.96 0.94 0.89 1.05

0.04 0.04 0.06 0.09 0.11 0.12 0.12 0.11

From the high value of R and the low value of SD listed in Table 4, it is seen that the Avrami theory is fitted well to estimate the n for POB and PO thermal degradation. n for both POB and PO vary slightly with increasing temperature but they exhibit reverse trend, where n for POB increases gradually from 1.55 to 1.82, as for PO, it decreases from 1.23 to 0.89. This result demonstrates that thermal degradation for POB and PO is a complicated physiochemical process and it might be hard to determine a single value of n during the whole process. On average, n is 1.65 for POB and 1.05 for PO, which are in accordance with the reported results.26 E


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Energy & Fuels Positive ΔH in Figure 4 indicates the thermal degradation for POB and PO is an endothermic reaction. Because of the high

disorder degree at Tinitial due to the highest ΔS absolute value of 172.38 J/(mol K). Yet, at Tm, POB needs more energy to decrease the disorder degree of evolved products for the higher ΔS absolute value of 143.75 J/(mol K). 16 ΔG is a comprehensive index to evaluate the heat flow and disorder change, and larger ΔG indicates lower favorability of a reaction.10,27 The favorability order of thermal degradation for POB is 109.45−137.25 kJ/mol and that for PO is 167.82− 185.84 kJ/mol, which is consistent with Ea. 3.4. Evolved products for POB and PO analyzed by FTIR. Generally, thermal degradation of FAMEs is the chain breakage of hydrocarbons with evolved volatile compounds under anaerobic conditions at high temperature. To specify the types of evolved products, the gas evolution for POB and PO at Tm from 298 to 873 K at 10 K min−1 is analyzed by FTIR (Figure 5). POB and PO resulted in evolved products that have

Figure 4. ΔH for peanut oil biodiesel (POB) (a) and peanut oil (PO) (b) based on the Friedman method, Kissinger−Akahira−Sunose (KAS) method, and Flynn−Wall−Ozawa (FWO) method.

Figure 5. Functional groups of evolved products for peanut oil biodiesel (POB) and peanut oil (PO) at the maximum derivation weight loss temperature (Tm).

molecular weight of PO, ΔH for PO is higher than that of POB. In detail, ΔH for POB calculated by the Friedman method, the KAS method, and the FWO method is heightened from 43.22 kJ mol−1 to 55.11 kJ mol−1. As for PO, ΔH is increased from 80.71 kJ mol−1 to 147.68 kJ mol−1. It is clearly seen that the ΔH extent for POB is narrower than that of PO, revealing the thermal degradation for PO is more complex than that of POB. For POB, the average ΔH values calculated by the Friedman method, the KAS method, and the FWO method are 48.41 kJ mol−1, 47.36 kJ mol−1, and 48.78 kJ mol−1, respectively. The corresponding average ΔH values for PO calculated from these three methods are separately 119.01 kJ mol−1, 115.77 kJ mol−1, and 120.83 kJ mol−1. Thermal degradation occurs at Tinitial, and the chemical bonds begin to decompose with increasing temperature. At Tinitial and Tm, the negative ΔS and positive ΔG presented in Table 5 validate the nonspontaneous process of thermal degradation for POB and PO. Considering that thermal degradation is irreversible, more energy is required for PO to decrease the

FTIR profiles similar to those of the alkane group (νH−C−H 2931 cm−1, 2859 cm−1; δC−H 1458 cm−1), the alkene group (νC−H 3014 cm−1 in CC−H), the aldehyde group (νC−H 3093 cm−1; δCO 1746; δCO 669 cm−1), the ether group (νC−O−C 1152 cm−1), and CO2 (νCO 2360 cm−1, 2342 cm−1).10,28−30 It can be seen from Figure 6(a)−(d) that the functional groups of alkanes, alkenes, ethers, and aldehydes for evolved products of POB and PO show a single peak pattern and reach the maximum around 550 K for POB and 667 K for PO, respectively. The alkane groups for POB and PO are first detected at 472 and 640 K (Figure 6(a)), and with temperature increasing, the alkene group is formed from the cracking of alkenyl. Ether and aldehyde groups are formed successively from deoxygenation of ester. These results reveal that C−C bond cleavage of the hydrocarbon chain occurs first and deoxygenation of the short chain molecule follows subsequently.17,31 Further, taking the Lambert−Beer Law into

Table 5. Thermodynamic Parameters of Peanut Oil Biodiesel (POB) and Peanut Oil (PO) Calculated at the Initial Weight Loss Temperature (Tinitial) and Maximum Derivation Weight Loss Temperature (Tm) Sample

Tinitial

log A (s−1)

ΔS (J/mol K)

ΔG (kJ/mol)

SD

Tm

log A (s−1)

ΔS (J/mol K)

ΔG (kJ/mol)

SD

POB PO

423.32 633.50

4.42 4.55

−171.32 −172.38

109.45 167.82

0.05 0.07

527.65 710.65

5.97 9.50

−143.75 −78.55

137.25 185.84

0.10 0.15

F


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Figure 6. Absorbance of functional groups for evolved products changed with temperature for peanut oil biodiesel (POB) and peanut oil (PO): (a) alkane group, (b) alkene group, (c) ether group, (d) aldehyde group, (e) CO2, (f) ketone group, (g) H2O.

Conversely, CO2 for PO exhibits double peaks which are detected at 685 K for the breaking of C−O connected to glycerol bone and at 781 K for the cracking of carbonyl (C O) in the ester group, respectively. The unique bands around 1716.8 cm−1 and 3567−3735 cm−1 only detected in the evolved products for PO are ascribed to a

account, the absorbance of gas is in linear correlation with concentration; thus, the dominant group of evolved products for both POB and PO is the alkane group.32 In general, CO2 is produced via decarboxylation,31 and as shown in Figure 6(e), the CO2 structure for POB is a unimodal pattern at 791 K, which is attributed to ester decarboxylation. G


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ketone group (Figure 6(f)) and H 2 O (Figure 6(g)), respectively. It is postulated that the ketone group is formed during thermal degradation for both PO and POB. However, the ketone group could be isomerized into aldehyde groups.33 According to results exhibited in section 3.3, the average reaction order for POB is higher than that of PO; namely, under the same reaction rate, more POB and intermediate products, such as the ketone group, are involved in thermal degradation. Therefore, the ketone group evolved from POB is isomerized into an aldehyde group completely, which is not observed in the FTIR spectrum (Figure 5). Tudorachi and Mustata34 evaluated the evolved gas of thermal degradation for vegetable oil in nitrogen atmosphere through TGA−FTIR/MS and found that H2O was one of the gaseous products for corn oil and castor oil. On the contrary, no obvious H2O is detected in the evolved products for POB (Figure 5). The possible reason is that the oxygen-containing group in POB, namely the ester group (RCOOR) is only decomposed into CO− and C−O− and no hydroxyl (OH) is formed.

4. CONCLUSIONS In this study, both POB and PO exhibit only one weight loss stage during the whole thermal degradation process and the thermal characteristics of Tinitial and Tm for POB are lower than those of PO. Ea calculated by three different methods for POB and PO increased from 43.22 to 56.22 kJ mol−1 and from 86.24 to 157.73 kJ mol−1, respectively. Also, ΔH shows a similar variation trend with Ea. These results prove that POB is more suitable to be used as an alternative diesel at large scale than PO, for it could be easily atomized and burned out. The average n determined by Avrami theory is 1.65 for POB and 1.05 for PO. At Tinitial and Tm, negative ΔS and positive ΔG validate the thermal degradation is a nonspontaneous process. Further, the thermal degradation favorability order is ΔG of 109.45−137.25 kJ/mol for POB and 167.82−185.84 kJ/mol for PO. The common functional groups of the evolved products detected by FTIR are alkanes, alkenes, aldehydes, ethers, and CO2 for both POB and PO. Particularly, the CO2 structure for POB is an unimodal pattern, whereas CO2 for PO exhibits double peaks which are detected at 685 and 781 K. Ketone groups and H2O are only observed in the evolved products of PO but not detected in that of POB.

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AUTHOR INFORMATION

Corresponding Author

*Tel:+86−531−88392414. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by National Natural Science Foundation of China (51206098), Young Scholars Program of Shandong University (2015WLJH33), Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province, China (BS2012NJ005), and China Postdoctoral Science Foundation Special Funding (2013T60667).

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Comprehensive Investigation of the Thermal Degradation

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