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Study on the thermal degradation kinetics and pyrolysis characteristics of chitosan-Zn complex
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Study on the thermal degradation kinetics and pyrolysis characteristics of chitosan-Zn complex Chunyan Ou a , Song Chen b , Yonghai Liu a , Jiangjuan Shao a , Sidong Li b,∗ , Tingming Fu c , Wenling Fan a , Hui Zheng a , Qin Lu a , Xiaolin Bi a a
School of Pharmacy, Nanjing University of Chinese Medicine, Nanjing 210023, China School of Chemistry and Environment, Guangdong Ocean University, Zhanjiang 524088, China c Jiangsu Collaborative Innovation Center of Chinese Medicinal Resources Industrialization, Nanjing 210023, China b
a r t i c l e
i n f o
Article history: Received 13 October 2015 Received in revised form 14 March 2016 Accepted 18 March 2016 Available online xxx Keywords: Chitosan-Zn complex Thermal degradation Pyrolysis Kinetic parameter Volatile compounds
a b s t r a c t The thermal degradation kinetics and pyrolysis characteristics of a chitosan-Zn complex have been studied by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and pyrolysis gas-chromatography/mass spectrometry (Py-GC/MS). FTIR analysis showed that the absorptions associated with the O H, N H, C O and C C bonds shifted to lower frequencies and became much less intense at high temperatures. Furthermore, several new peaks appeared at 3049, 2332, 2202 and 600–900 cm−1 under these conditions. TGA experiments indicated that the thermal degradation of the chitosan-Zn complex occurred over two stages. The kinetic triplets of the second thermal degradation stage were estimated by Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose and Coats–Redfern methods, −2 and the most plausible kinetic model was F3 ([(1 − ˛) − 1]/2), i.e., a third-order reaction. The Py-GC/MS results indicated that the pyrolysis of the chitosan-Zn complex was very complicated, producing many volatile products, including carbon dioxide, acetic acid and nitrogen-containing aromatic heterocyclic compounds. Compared with chitosan, the decomposition of the chitosan-Zn complex was easier and more thorough. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Dwindling fossil fuel reserves and the increasing energy demands of an ever-growing population represent serious environmental problems, and there is therefore an urgent need for the development of sustainable energy sources to support future generations. Biomass, which is the third largest energy resource in the world, has received considerable interest from numerous researchers working in a variety of different fields during the last decade. Chitin can be obtained from various sources, including the shells of crustaceans (e.g., crab and shrimp), the cuticles of insects and the cell walls of fungi, and is one of the most abundant polysaccharides available from nature after cellulose [1]. Chitosan (CS), which is a linear copolymer of -(1 → 4)-linked 2-acetamido-2-deoxy-d-glucopyranose and 2-amino-2-deoxy--d-glucopyranose units [2], can be readily derived from chitin via the base-mediated Ndeacetylation of this material [3]. CS contains a large number of
∗ Corresponding author. E-mail address: [email protected] (S. Li).
metal ion binding groups (e.g., -NH2 and -OH) in its macromolecular structure, allowing it to coordinate to metal salts to form the corresponding metal complexes [4,5]. Chitosan-metal ion complexes have many applications in medicine [6], antimicrobial agents [7,8], wastewater treatment [9] and heterogeneous catalysis [10]. However, most of the chitosan-metal compounds used in these cases are disposed of as solid waste once they have fulfilled their purpose. There are therefore growing concerns about the environmental impact associated with the disposal of these waste materials. One treatment option for the disposal of solid wastes involves the use of a sanitary backfill. However, this disposal route is not advisable because it could lead to the contamination of groundwater supplies. An alternative disposal option involves the direct conversion of these solid wastes to fine chemical products using a variety of physical and chemical methods. Biomass pyrolysis is a fundamental thermo chemical conversion process that holds a special place in both industrially and ecologically. Biomass pyrolysis generally proceeds via a series of extremely complex reactions, with the outcome of this process being influenced by several important factors such as the heating rate, temperature and feedstock composition [11]. Pyrolysis allows for the biomass macromolecules to be broken down into
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low molecular weight gas, liquid and solid char. Chitin biomass has attracted growing interest from numerous researchers during the last few years because CS can be converted to carbon fiber derivatives by oxidation and pyrolysis processes [12]. Furthermore, CS can be pyrolyzed to give nitrogen-containing aromatic heterocyclic compounds such as pyrazines, pyridines, pyrroles and furans [13], as well as providing access to strictly defined carbonaceous materials [14]. The flash pyrolysis of chitin leads to the production of several volatile compounds, including carbon dioxide, acetamide, furfuryl alcohol, 2-furfural, 2-pyrrolealdehyde and 3-acetamidefunan [15]. Chitin biomass and its derivatives can also be convert to 5-hydroxymethylfurfural and levulinic acid [16,17]. Notably, 5-(chloromethyl) furfural (45%) and levulinic acid (29%) have both been prepared by the degradation of chitin in a biphasic reaction system [18]. However, there have been very few reports pertaining to the use of a chitosan-metal compound as a potentially important type of chitin biomass [19]. In this study, we have evaluated the thermal degradation and pyrolysis characteristic of a chitosan-Zn complex (CS-Zn). The aim of this study was to obtain the kinetic parameters for the decomposition of this complex using the Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods. We also wanted to determine the degradation mechanism of CS-Zn using the Coats–Redfern (C–R) method and identity any volatile compounds generated during its pyrolysis by gas chromatography mass spectrometry (GC/MS). It was envisaged that a deeper knowledge of the thermal pyrolysis kinetics of CS-Zn would enhance our understanding of this material and enable us to plan industrial process for specific applications with greater accuracy.
2. Materials and methods 2.1. Synthesis of CS-Zn CS was purchased from Shanghai Greenbird Science and Technology Development Co., Ltd. (Shanghai, China) The viscosity average molecular weight and degree of deacetylation of this material were 180 kDa and >90%, respectively. A mixture of 1% (v/v) acetic acid in water (100 mL) and CS (1.0 g) was magnetically stirred in a glass beaker at 298 K for 2 h. A specific amount of zinc chloride (corresponding to a molar ratio of 4:1 compared with the CS residue) was dissolved in distilled water (60 mL), and the resulting solution was slowly added to the glass beaker. The resulting mixture was stirred for 5 h at 323 K, cooled to ambient temperature and treated with a 1% (w/v) solution of sodium carbonate in water, which was slowly added to the reaction mixture until the pH of the solution reached 8.0. This process stopped the reaction, leading to the formation of a white precipitate, which was filtered and washed several times with water and ethanol before being dried under vacuum in an oven at 60 ◦ C.
ducted in triplicate to obtain accurate results and the resulting curves were normalized. 2.2.2. Powder X-ray scattering (PXRD) Powder X-ray diffraction (PXRD) analyses were conducted on freshly made samples using a D/max IIIA diffractometer (Rigaku, Japan) with a Cu K␣ target. Standard runs were carried out using a voltage of 40 kV, a current of 100 mA and a scanning rate of 8◦ min−1 over a 2 range of 2–60◦ with a step size of 0.04◦ and a counting time of 0.5 s step−1 . 2.2.3. Thermogravimetric analysis Thermogravimetric analysis (TGA) experiments were conducted on a PerkinElmer TG/DTA6300 system, and the measuring accuracy of the sample temperature was checked against the onset fusion temperatures of indium (156.6 ± 0.2 ◦ C) and selenium (232 ± 0.2 ◦ C) with heating/cooling dynamic segments. The samples used in this study were 5–6 mg in size. Nitrogen was used as the carrier gas at a flow rate 50 mL min−1 and the experiments were carried out at temperatures in the range of 298–1073 K at various heating rates ( = 10, 15, 20 K min−1 ) to record the TGA and DTG curves. All of these experiments were conducted in triplicate to confirm the repeatability and authenticity of the resulting data. 2.2.4. Py-GC/MS analysis The fast pyrolysis of CS-Zn was achieved in a CDS 5200 analytical pyrolyzer using 1.0 ± 0.02 mg of sample in a silica tube. Kinetic analysis showed that CS-Zn rapidly decomposed at temperatures in the range of 473–673 K. Based on the high heating rate of the pyrolyzer and the poor thermal conductivity of the sample, the optimal condition for analyzing the product distribution resulting from the fast pyrolysis of CS-Zn were determined to be 773 K for 20 s. Moreover, a series of fast pyrolysis reactions were carried out at different temperatures (673, 773 and 873 K) each for 20 s, and the products were held for different reaction times (5, 10, and 20 s) at 773 K, with the same heating rate of 2.0×104 K s−1 and Py-GC/MS transfer line temperature of 553 K. The volatile products resulting from the pyrolysis were analyzed by GC/MS (Agilent 7890A/5975C) using an injector temperature of 558 K. The chromatographic separation of the materials was achieved over an HP-5ms capillary column (30m × 0.25mm × 0.25 m). Helium (99.999%) was used as the carrier gas with a constant flow rate 1 mL min−1 , and a split ratio of 1:60. The oven temperature was increased from 323 K (where it was held for 1 min) to 565 K (where it was held for 2 min) at a heating rate 8 K min−1 . The temperature of the GC/MS interface was held at 568 K and the mass spectrometer was operated in the electrospray ionization mode at 70 eV covering m/z values in the range of 35–550 amu at a scan rate of 2337 amu s−1 . All of the volatile products were qualitatively and quantitatively analyzed against the NIST MS library. All of these experiments were conducted in triplicate to confirm the reproducibility f the results.
2.2. Characterization of sample
3. Results and discussion
2.2.1. FTIR analysis FTIR spectra were recorded on a PerkinElmer Spectrum-GX-1 infrared spectrometer (PerkinElmer, USA) equipped with DiffusIR accessory. A total accumulation of 24 scans and a resolution of 4 cm−1 provided high signal-to-noise spectra in the range of 4000–400 cm−1 . Variable temperature infrared spectra were recorded at temperatures in the range of 298–873 K at 100 K increments by placing around 5.0 mg of sample on a heating stage. This material was subsequently heated at a rate of 10 K min−1 with a thermocouple until the temperature reached the specified value, where it was held for 10 min. All of these experiments were con-
3.1. FTIR analysis Fig. 1A and B shows the FTIR spectra of CS and CS-Zn at different temperatures. Prior to being heated, a comparison of the FTIR spectra of CS and CS-Zn revealed that there were several clear differences between these two materials. For example, the wide peak corresponding to the stretching vibrations of the NH2 and OH groups in CS at 3510 cm−1 [20] shifted to a higher frequency in CSZn (3607 cm−1 ). However, the peak corresponding to vibration of the carbonyl group in CS at 1656 cm−1 had a similar shift to that of CS-Zn (1657 cm−1 ). The bending vibration of the amine groups at
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Fig. 1. FTIR spectra of CS (A) and CS-Zn (B) at different temperatures: 1–298 K, 2–423 K, 3–573 K, 4–673 K, 5–773 K, 6–873 K.
1595 cm−1 in the FTIR spectrum of CS shifted to a higher frequency in CS-Zn (1609 cm−1 ) [21]. Furthermore, the N H wagging mode in the FTIR spectrum of CS at 665 cm−1 shifted to 678 cm−1 [22]. It was therefore obvious that the NH2 and OH groups of CS
were involved in the coordination of the metal ion [23]. The band (1046 cm−1 ) assigned to the second OH group in CS [8] showed a significant shift to a lower wave number (1003 cm−1 ) as well as an increased intensity in CS-Zn [7]. These data therefore suggested
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3.3. TG analysis
Fig. 2. X-ray diffraction patterns of CS and CS-Zn.
that the second OH group was also involved in the coordination of the Zn ion. The structures of CS and CS-Zn were also evaluated by FTIR analysis after they had been heated. Notably, there were no significant changes in the structure of CS when it was heated up to 423 K [24]. However, further increases in the temperature beyond this point led to a variety of different changes, especially in the region of 1660–900 cm−1 , which were associated with changes to the groups connected to the pyranose and aromatic rings, where there was a general shift towards shorter wave numbers and reduced intensities with increasing temperature [14,25,26]. More changes were observed for CS-Zn than CS during the heating process. The wide peak in the FTIR spectrum of CS-Zn at 3607 cm−1 shifted to a lower frequency and became increasingly narrow as the treatment temperature increased. Furthermore, the intensities of several peaks, including the C O vibrations associated with the pyranose ring at 1166 cm−1 , the N H bending vibration at 1609 cm−1 and the peak for the carbonyl groups at 1657 cm−1 , became weaker with increasing temperature, and were completely lost at 773 K. The loss of these signals could be attributed to the degradation of the chitosan chain and the decomposition of pyranose ring. It is noteworthy that several new peaks appeared at 3049, 2332, 2202 and 600–900 cm−1 , which were attributed to the formation of an aromatic ring. The FTIR spectra of the thermal degradation residues of CS and CS-Zn showed that the degradation of CS was a gradual process, which was accompanied by the formation of aromatic rings with increasing temperature [12]. These results also showed that zinc played a very important role in the cracking of the pyranose rings and the formation of new aromatic heterocyclic compounds.
3.2. Powder X-ray scattering analysis X-ray diffraction analysis was used for the characterization of the chitosan macromolecule. Fig. 2 shows the different patterns for CS and CS-Zn. The X-ray diffraction pattern for CS showed two characteristic peaks, including a small, broad peak at 2 = 9.85◦ and the main reflection peak at 2 = 19.70◦ , which provided evidence of the partial crystallinity of the chitosan specimen. The CS-Zn complex was characterized by several smaller peaks at 12.64◦ , 22.67◦ , 27.94◦ and 32.62◦ , which indicated that the complexation of zinc to CS had led to pronounced changes in the morphological characteristics of CS.
3.3.1. Effect of ˇ on the process of thermal degradation of CS-Zn Fig. 3A and B shows the TG and DTG curves for the thermal degradation of CS and CS-Zn at three different heating rates in nitrogen. Fig. 3 shows that there were two peaks in the DTG curves of these materials, indicating that the thermal decompositions of CS and CSZn were both two-stage processes. The first of these peaks appeared round 373 K, which was attributed to the loss of water from these materials. The second of these peaks appeared at a temperature in the range of 473–673 K, which was attributed to the thermal degradation of the main chain of CS. However, the second peak in the DTG curve of CS-Zn was accompanied with an extra “shoulder” peak at 250 ◦ C, which might be caused by metal ion catalyzing degradation the main chain of CS. The characteristic temperatures for the second thermal degradation stages of CS and CS-Zn are given in Table 1, where Tst is the temperature of the initial weight loss and Tmax is the temperature at the maximum rate of weight loss (the peak temperature on a differential thermogravimetry curve). All of the characteristic temperatures increased in a linear manner with increasing heating rate, which indicated that the degradation temperatures were mainly affected by the heating rate as a result of the heat hysteresis [27]. The characteristic temperatures of CS were higher than those of CS-Zn, which indicated that CS-Zn degraded much more readily than CS. 3.3.2. The determination of the activation energies for the thermal degradations of CS and CS-Zn using isoconversional methods The activation energy (E) can be accurately determined using isoconversional methods without the prior assumption of a reaction model, as recommended by the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC Kinetics Committee) [28]. In this work, the activation energies were calculated using the FWO and KAS methods. The FWO method was based on the following equation [29]: ln ˇ = ln(AE ⁄Rg(˛)) − 5.335 − 1.0516E ⁄RT
(1)
The KAS method can be described by the following equation [30]: ln ˇ⁄T 2 = ln(AR⁄Eg(˛)) − E ⁄RT
(2)
where ␣ is the conversion rate, ˇ is the heating rate (K min−1 ), A is the pre-exponential factor (s−1 ), E is the activation energy (kJ mol−1 ), R is the gas constant (8.314 J mol−1 K−1 ), T is the absolute temperature (K) and g(˛) is the integral algebraic expression used for the mechanisms of solid-state processes. For a given ␣ value, it is possible to estimate the values of E based on the gradients of the lines obtained by plotting ln ˇ versus 1⁄T (FWO) and ln ˇ⁄T 2 versus 1⁄T (KAS), respectively. The calculated activation energies (E) and correlation coefficients (r 2 ) are given in Table 2. These data clearly show that the E values calculated using the FWO method were slightly higher than those obtained using the KAS method. Furthermore, all of the correlation coefficients (r 2 ) were in the range of 0.9947–0.9999 (CS) and 0.9871–0.9980 (CS-Zn), and the E value of CS was in good agreement with the value reported by Tao (2011), which meant that the values obtained from these two methods were credible and reasonable. 3.3.3. Determination of kinetic mechanism and kinetic parameters for the thermal degradation of CS-Zn The Coats–Redfern method [31] is a good choice of method for investigating the processes underlying the thermal degradation of CS and CS-Zn. This method is based on the following equation: ln g(˛)⁄T 2 = ln AR⁄ˇE − E ⁄RT
(3)
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Fig. 3. TG and DTG curves of the thermal degradation of CS (A) and CS-Zn (B).
Table 1 Characteristic temperatures and kinetic parameters during the second thermal degradation stage of CS and CS-Zn. ˇ (K min−1 ) CS
CSZn
10 15 20 10 15 20
Tst (K) 434.3 440.9 448.5 413.4 418.6 428.5
Tmax (K) 563.7 571.7 576.9 550.6 558.1 562.2
E (kJ mol−1 )
A (s−1 )
137.7 143.4 146.1 121.4 122.5 121.5
5.0 × 10 1.2 × 1011 1.4 × 1011 3.7 × 109 4.8 × 109 3.6 × 109
Depending of the different degradation processes, it is possible to obtain E and A from a plot of ln g(˛)⁄T 2 versus 1⁄T using the g(˛) values listed in Table 3 [32]. It is also possible to obtain a valid reaction mechanism in this way. The activation energy values shown in Tables 2 and 3 indicated that the decomposition of CS followed an F2 mechanism and that the optimal mechanism for this reaction was a second-order reaction ((1 − ˛)−1 − 1). In contrast, the results for the decomposition
11
H (kJ mol−1 )
S (J mol−1 K−1 )
G (kJ mol−1 )
142.4 148.2 150.9 126.0 127.2 126.2
−26.3 −38.3 −37.1 −66.9 −64.7 −67.1
157.2 170.1 172.3 162.8 163.3 164.0
of CS-Zn suggested that it could follow a D2 , D4 or F3 mechanism, although the correlation coefficient for F3 was the largest (r = 0.9958) of all of these mechanism, suggesting that the optimal mechanism was a third-order reaction ([(1 − ˛)−2 − 1]/2). To fully elucidate the mechanism of this thermal decomposition process, we also calculated the activation entropy (S), activation enthalpy (H) and the activation Gibbs energy (G) [33], and the result are listed in Table 1.
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Table 2 Activation energy and correlation coefficient obtained from FWO and KAS methods for the second thermal degradation stage of CS and CS-Zn. ␣
FWO method
KAS method
CS
CS-Zn
E (kJ mol 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average
−1
)
128.8 128.2 136.3 139.7 142.3 148.1 164.6 166.6 146.1
r
2
0.9999 0.9999 0.9998 0.9987 0.9992 0.9986 0.9998 0.9952
E (kJ mol
CS −1
)
110.9 121.8 117.6 117.6 119.8 124.4 123.7 109.6 118.2
r
CS-Zn −1
2
E (kJ mol
0.9977 0.9983 0.9976 0.9940 0.9947 0.9973 0.9924 0.9893
)
126.7 125.8 134.2 137.6 140.2 146.2 163.5 165.3 144.2
r
2
0.9999 0.9999 0.9998 0.9985 0.9991 0.9984 0.9998 0.9947
E (kJ mol−1 )
r2
108.2 119.4 114.7 114.6 116.7 121.4 120.5 105.3 115.1
0.9973 0.9980 0.9972 0.9930 0.9938 0.9968 0.9911 0.9871
Table 3 Integral algebraic expressions for g(˛) for the most frequently used mechanisms of solid-state process. Symbols
g(˛) (Mechanisms)
E (kJ mol−1 ) CS
Random nucleation and nuclei growth 1/2 A2 [− ln(1 − ˛)] (Two-dimensional) 1/3 A3 [− ln(1 − ˛)] (Three-dimensional) Limiting surface reaction between both phases ˛ (one-dimension) R1 1⁄2 R2 [1 − (1 − ˛) ] (Two-dimensions) 1⁄3 [1 − (1 − ˛) R3 ] (Three-dimensions)
CS-Zn
ˇ = 10
ˇ = 15
ˇ = 20
ˇ = 10
ˇ = 15
ˇ = 20
46.8 28.1
49.0 29.6
49.8 30.0
29.7 16.7
29.9 16.8
29.4 16.5
76.1
79.7
80.8
49.9
50.2
49.5
88.5
92.5
93.9
58.4
58.9
58.9
93.1
97.3
98.7
61.6
62.0
61.3
161.5 176.9
168.7 184.7
171.1 187.3
108.8 119.4
109.6 120.3
108.3 119.0
195.5
203.9
206.8
132.3
133.3
131.9
D4
˛2 (One-way transport) (1 − ˛) ln(1 − ˛) + ˛(Two- way transport) 1⁄3 2 [1 − (1 − ˛) ] (Three- way transport) 2⁄3 Ginstling-Brounshtein equation [1 − (2⁄3)˛] − (1 − ˛)
183.1
191.0
193.8
123.7
124.6
123.3
Order of reaction F1 F2 F3
− ln(1 − ˛)(First-order) −1 (1 − ˛) − 1(Second-order) −2 [(1 − ˛) − 1]/2 (Third-order)
102.9 137.7 179.7
107.4 143.4 186.9
109.0 146.1 189.7
68.4 92.4 121.4
68.9 93.2 122.5
68.1 92.3 121.5
Exponential nucleation P2 P3 P4
˛1/2 (Power law n = 1/2) ˛1/3 (Power law n = 1/3) ˛1/4 (Power law n = 1/4)
33.4 19.2 12.1
35.1 20.3 12.9
35.7 20.6 13.1
20.4 10.6 5.7
20.5 10.6 5.6
20.1 10.3 5.6
Diffusion D1 D2 D3
Kinetic thermal analysis showed that the E values obtained using the Coats-Redfern method were in good agreement with those obtained using the FWO and KAS methods, in that all of the E, H and G values were positive and all of the S values were negative. These results indicated that the second step involved in the decomposition of CS and CS-Zn was a non-spontaneous process. It is noteworthy, however, that the activation energy for the thermal degradation of CS-Zn was lower than that of CS, which also indicated that CS-Zn degraded more readily than CS. 3.4. Analysis by Py-GC/MS 3.4.1. Analysis of pyrolysis products from CS-Zn Fig. 4A and B shows the gas chromatograms of the volatiles generated during the pyrolysis of CS-Zn at various temperatures and times. These volatiles are also listed in Table 4. A total of 14 volatile compounds were identified, including small linear molecules such as carbon dioxide, acetaldehyde and acetic acid; several nitrogencontaining aromatic heterocyclic compounds such as pyrazines, pyrroles and pyridines, with C4 and C5 predominating [13,34]; and numerous long chain fatty acids such as n-hexadecanoic acid. It was not possible to measure the release of NH3 or H2 O using this
instrument. The major compounds identified in this way were carbon dioxide, acetic acid and pyrazine (listed in decreasing order). However, pyrazine compounds were the most important products in the pyrolysis of chitosan, because they play an important role as intermediates for perfumes, pharmaceuticals and agricultural chemicals. 3.4.2. Effect of reaction conditions on the pyrolysis products of CS-Zn To develop a deeper insight into the relationship between the volatile compounds formed during the pyrolysis and the pyrolysis temperatures, we investigated the pyrolysis of CS-Zn at temperatures in the range of 673–873 K for a pyrolysis time of 20 s (Fig. 4A and Table 4). The results of these experiments revealed that whilst the majority of the products were similar, the product distributions were different at different pyrolysis temperatures. For example, the use of a high temperature led to a large number of volatile products, such as butane and pyridine, whereas the use of a lower temperature led to the formation of pentanal. The amounts of carbon dioxide and pyrazine resulting from the pyrolysis of CSZn changed considerably depending on the temperature, with the values initially increasing and then decreasing with increasing tem-
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Table 4 Main products from fast pyrolysis of CS at 773 K and CS-Zn at 673 K, 773 K, 873 K for 20 s (% in relative area). RT (min)
compound
Formula
Pyrolysis temperature
Reference
CS-Zn
1.32 1.37 1.50 1.52 1.78 2.00 2.77 2.83 2.98 3.08 3.97 4.05 4.45 5.71 6.06 7.86 22.46
Carbon dioxide Acetaldehyde Butane Pentanal 2-ethyl-Butanal Acetic acid Pyrazine 1,3-Diazine Pyridine Pyrrole 2-methyl-Pyridine 2-methyl-Pyrazine 2-methyl-1H-Pyrrole ethyl-Pyrazine 2-ethyl-1-butanol acetylpyrazine n-Hexadecanoic acid
CO2 C2 H4 O C4 H10 C5 H10 O C6 H14 C2 H4 O2 C4 H4 N2 C4 H4 N2 C5 H5 N C4 H5 N C6 H7 N C5 H6 N2 C5 H9 N C6 H9 N2 C6 H14 O C6 H7 O C16 H32 O2
CS
673 K
773 K
873 K
773 K
48.8 4.6 0 3.9 0 18.2 11.5 0 0 6.3 0 5.7 0 0 0 0 0.9
31.6 4.9 0 6.9 0 18.5 5.7 0 2.0 7.2 0 7.2 3.8 1.0 0 0 5.6
33.8 5.5 9.5 0 1.7 17.8 6.0 2.5 2.6 7.1 1.9 6.6 1.4 1.4 0 0 2.3
26.9 5.4 0 4.6 0 9.1 14.4 0 3.7 8.1 3.0 7.5 2.83 1.2 2.3 5.5 4.5
Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib
Fig. 5. Gas chromatography of volatile compounds generated from pyrolysis of CS and CS-Zn at 773 K for 20 s.
perature. In contrast, those of acetaldehyde and acetic acid changed only slightly with increasing temperature, and the total amount of nitrogen-containing aromatic heterocyclic compounds remained almost the same at temperatures of 773 and 873 K. CS-Zn was pyrolyzed at 773 K for 5, 10 and 20 s (Fig. 4B) to investigate the relationship between the volatile compounds formed during the pyrolysis and the pyrolysis times. The results showed that there were no discernible differences in the types of product formed at the different temperature. However, the relative amounts of these products did change slightly with increasing reaction time. For example, the amount of carbon dioxide decreased, whereas the total amount of nitrogen-containing aromatic heterocyclic compounds increased.
Fig. 4. Gas chromatography of volatile compounds generated during the pyrolysis of CS-Zn (A) at vavious temperatures for 20 s, (B) at various reaction times at 773 K.
3.4.3. Stability analysis of CS and CS-Zn To compare the thermal stabilities of CS and CS-Zn, and better track changes in the volatile compounds resulting from the pyrolysis of these materials, we investigated the pyrolysis of CS at 773 K for 20 s (Fig. 5). This result revealed that the pyrolysis of CS resulted in more types of volatile product than the pyrolysis of CS-Zn. For
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shorter retention times (0–6 min), the pyrolysis of CS led to the same pyrolysis gases as CS-Zn, including carbon dioxide, acetaldehyde, acetic acid, pyridine, pyrazines and pyrroles, but the relative amounts of these materials were different. For longer retention times (6–24 min), the pyrolysis of CS led to the appearance of several new volatile compounds, such as 2-ethyl-1-butanol (RT, 6.06; yield, 2.27%) and acetylpyrazine (RT, 7.86; yield, 5.50%). Based on the above results, it is clear that the processes involved in the pyrolysis of CS-Zn and CS are very complicated, and the pyrolysis mechanisms of these materials are different. CS is a linear copolymer consisting of randomly distributed N-acetylated and N-deacetylated units in the main chains. The thermal degradation process of CS could potentially involve the random degradation of its main chains and the splitting of side chains [35]. The former starting with the scission of a C O C bond, which was supported by the fact that the bond energy of C O C was smaller compared with that of C C [36], generates carbon dioxide, oligomers, dimmers of glucosamine and solid char [14,37]; the latter releases ammonia and acetic acid. Glucosamine with ␣-amino carbonyl structure can soon be condensed to form pyrazine compounds via dehydration and deprotonation [38,39], which is exothermic. Pyridines and pyrroles are generated from the pyrolysis of glucosamine [13]. Increasing the pyrolysis temperature forms a series of substituted heterocyclic compounds. The FTIR, XRD and TGA results for CS-Zn showed that the coordination of zinc to CS formed a unique structure. This structure clearly favored the degradation of CS, making the pyrolysis of CS-Zn much easier and more thorough than that of CS. In addition, condensation of glucosamine may be catalyzed by the granular zinc in the solid char [40]. For pyrolysis of CS-Zn, the reaction temperature and pyrolysis time had a pronounced effect on the product distribution, and the processes of the formation of carbon dioxide and nitrogen-containing aromatic heterocyclic compounds may have competed with each other under these conditions. When the pyrolysis was conducted at temperatures in the range of 673–873 K for 5–20 s, the major volatile products were carbon dioxide and acetic acid, with lower pyrolysis temperature and longer reaction times affording more pyrazine compounds. The pyrolysis of CS-Zn produced about 15% carbonizate. We measured the iodine adsorption capacity of carbonizate using GB7702-87 standard and found that the carbonizate produced by CS-Zn exhibited stronger adsorption capacity (1043.2 mg g−1 ) than that of CS (617.1 mg g−1 ), representing an interesting opportunity for future studies, because it suggests that the transition-metal complexes of CS could be used as promising sources for the production of N-doped carbons and coals for catalysis, adsorption and ion storage batteries.
4. Conclusions The thermogravimetric analysis of CS-Zn in nitrogen revealed the presence of a two-stage degradation process. Kinetic analysis showed that the most plausible model for the second thermal degradation stage of CS-Zn was F3 ([(1 − ˛)−2 − 1]/2), i.e., a thirdorder reaction. The thermal degradation residues generated by the pyrolysis of these materials were analyzed by FTIR, whereas the volatile compounds generated during the same process were measured by GC/MS. The results indicated that the decomposition of CS-Zn was very complicated, leading to the production of many volatile products such as carbon dioxide, acetic acid and nitrogencontaining aromatic heterocyclic compound. Compared with CS, the pyrolysis of CS-Zn occurred much more readily and proceeded to a much greater extent. Notably, the reaction conditions had a considerable impact on the type and distribution of the volatile compounds formed during the pyrolysis.
Acknowledgements We gratefully acknowledge the financial support of National Natural Science Foundation of China (51403104, 81274095, 31271938), Natural Science Foundation of Jiangsu Province (BK20131414, BK20141465), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] W. Ding, Q. Lian, R.J. Samuels, M.B. Polk, Synthesis and characterization of a novel derivative of chitosan, Polymer 44 (2003) 547–556. [2] M. Morimoto, H. Saimoto, H. Usui, Y. Okamoto, S. Minami, Y. Shigemasa, Biological activities of carbohydrate-branched chitosan derivatives, Biomacromolecules 2 (2001) 1133–1136. [3] N.V. Majeti, K. Ravi, A review of chitin and chitosan applications, React. Funct. Polym. 46 (2000) 1–27. [4] R.A.A. Muzzarelli, G. Jenuiaux, G.W. Goodt, Chitin in Nature and Technology, Plenum Press, New York, 1995. [5] A.J. Varma, S.V. Deshpande, J.F. Kennedy, Metal complexation by chitosan and its derivatives: a review, Carbohydr. 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Study on the thermal degradation kinetics and pyrolysis characteristics of chitosan-Zn complex Chunyan Ou a , Song Chen b , Yonghai Liu a , Jiangjuan Shao a , Sidong Li b,∗ , Tingming Fu c , Wenling Fan a , Hui Zheng a , Qin Lu a , Xiaolin Bi a a
School of Pharmacy, Nanjing University of Chinese Medicine, Nanjing 210023, China School of Chemistry and Environment, Guangdong Ocean University, Zhanjiang 524088, China c Jiangsu Collaborative Innovation Center of Chinese Medicinal Resources Industrialization, Nanjing 210023, China b
a r t i c l e
i n f o
Article history: Received 13 October 2015 Received in revised form 14 March 2016 Accepted 18 March 2016 Available online xxx Keywords: Chitosan-Zn complex Thermal degradation Pyrolysis Kinetic parameter Volatile compounds
a b s t r a c t The thermal degradation kinetics and pyrolysis characteristics of a chitosan-Zn complex have been studied by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and pyrolysis gas-chromatography/mass spectrometry (Py-GC/MS). FTIR analysis showed that the absorptions associated with the O H, N H, C O and C C bonds shifted to lower frequencies and became much less intense at high temperatures. Furthermore, several new peaks appeared at 3049, 2332, 2202 and 600–900 cm−1 under these conditions. TGA experiments indicated that the thermal degradation of the chitosan-Zn complex occurred over two stages. The kinetic triplets of the second thermal degradation stage were estimated by Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose and Coats–Redfern methods, −2 and the most plausible kinetic model was F3 ([(1 − ˛) − 1]/2), i.e., a third-order reaction. The Py-GC/MS results indicated that the pyrolysis of the chitosan-Zn complex was very complicated, producing many volatile products, including carbon dioxide, acetic acid and nitrogen-containing aromatic heterocyclic compounds. Compared with chitosan, the decomposition of the chitosan-Zn complex was easier and more thorough. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Dwindling fossil fuel reserves and the increasing energy demands of an ever-growing population represent serious environmental problems, and there is therefore an urgent need for the development of sustainable energy sources to support future generations. Biomass, which is the third largest energy resource in the world, has received considerable interest from numerous researchers working in a variety of different fields during the last decade. Chitin can be obtained from various sources, including the shells of crustaceans (e.g., crab and shrimp), the cuticles of insects and the cell walls of fungi, and is one of the most abundant polysaccharides available from nature after cellulose [1]. Chitosan (CS), which is a linear copolymer of -(1 → 4)-linked 2-acetamido-2-deoxy-d-glucopyranose and 2-amino-2-deoxy--d-glucopyranose units [2], can be readily derived from chitin via the base-mediated Ndeacetylation of this material [3]. CS contains a large number of
∗ Corresponding author. E-mail address: [email protected] (S. Li).
metal ion binding groups (e.g., -NH2 and -OH) in its macromolecular structure, allowing it to coordinate to metal salts to form the corresponding metal complexes [4,5]. Chitosan-metal ion complexes have many applications in medicine [6], antimicrobial agents [7,8], wastewater treatment [9] and heterogeneous catalysis [10]. However, most of the chitosan-metal compounds used in these cases are disposed of as solid waste once they have fulfilled their purpose. There are therefore growing concerns about the environmental impact associated with the disposal of these waste materials. One treatment option for the disposal of solid wastes involves the use of a sanitary backfill. However, this disposal route is not advisable because it could lead to the contamination of groundwater supplies. An alternative disposal option involves the direct conversion of these solid wastes to fine chemical products using a variety of physical and chemical methods. Biomass pyrolysis is a fundamental thermo chemical conversion process that holds a special place in both industrially and ecologically. Biomass pyrolysis generally proceeds via a series of extremely complex reactions, with the outcome of this process being influenced by several important factors such as the heating rate, temperature and feedstock composition [11]. Pyrolysis allows for the biomass macromolecules to be broken down into
http://dx.doi.org/10.1016/j.jaap.2016.03.021 0165-2370/© 2016 Elsevier B.V. All rights reserved.
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low molecular weight gas, liquid and solid char. Chitin biomass has attracted growing interest from numerous researchers during the last few years because CS can be converted to carbon fiber derivatives by oxidation and pyrolysis processes [12]. Furthermore, CS can be pyrolyzed to give nitrogen-containing aromatic heterocyclic compounds such as pyrazines, pyridines, pyrroles and furans [13], as well as providing access to strictly defined carbonaceous materials [14]. The flash pyrolysis of chitin leads to the production of several volatile compounds, including carbon dioxide, acetamide, furfuryl alcohol, 2-furfural, 2-pyrrolealdehyde and 3-acetamidefunan [15]. Chitin biomass and its derivatives can also be convert to 5-hydroxymethylfurfural and levulinic acid [16,17]. Notably, 5-(chloromethyl) furfural (45%) and levulinic acid (29%) have both been prepared by the degradation of chitin in a biphasic reaction system [18]. However, there have been very few reports pertaining to the use of a chitosan-metal compound as a potentially important type of chitin biomass [19]. In this study, we have evaluated the thermal degradation and pyrolysis characteristic of a chitosan-Zn complex (CS-Zn). The aim of this study was to obtain the kinetic parameters for the decomposition of this complex using the Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods. We also wanted to determine the degradation mechanism of CS-Zn using the Coats–Redfern (C–R) method and identity any volatile compounds generated during its pyrolysis by gas chromatography mass spectrometry (GC/MS). It was envisaged that a deeper knowledge of the thermal pyrolysis kinetics of CS-Zn would enhance our understanding of this material and enable us to plan industrial process for specific applications with greater accuracy.
2. Materials and methods 2.1. Synthesis of CS-Zn CS was purchased from Shanghai Greenbird Science and Technology Development Co., Ltd. (Shanghai, China) The viscosity average molecular weight and degree of deacetylation of this material were 180 kDa and >90%, respectively. A mixture of 1% (v/v) acetic acid in water (100 mL) and CS (1.0 g) was magnetically stirred in a glass beaker at 298 K for 2 h. A specific amount of zinc chloride (corresponding to a molar ratio of 4:1 compared with the CS residue) was dissolved in distilled water (60 mL), and the resulting solution was slowly added to the glass beaker. The resulting mixture was stirred for 5 h at 323 K, cooled to ambient temperature and treated with a 1% (w/v) solution of sodium carbonate in water, which was slowly added to the reaction mixture until the pH of the solution reached 8.0. This process stopped the reaction, leading to the formation of a white precipitate, which was filtered and washed several times with water and ethanol before being dried under vacuum in an oven at 60 ◦ C.
ducted in triplicate to obtain accurate results and the resulting curves were normalized. 2.2.2. Powder X-ray scattering (PXRD) Powder X-ray diffraction (PXRD) analyses were conducted on freshly made samples using a D/max IIIA diffractometer (Rigaku, Japan) with a Cu K␣ target. Standard runs were carried out using a voltage of 40 kV, a current of 100 mA and a scanning rate of 8◦ min−1 over a 2 range of 2–60◦ with a step size of 0.04◦ and a counting time of 0.5 s step−1 . 2.2.3. Thermogravimetric analysis Thermogravimetric analysis (TGA) experiments were conducted on a PerkinElmer TG/DTA6300 system, and the measuring accuracy of the sample temperature was checked against the onset fusion temperatures of indium (156.6 ± 0.2 ◦ C) and selenium (232 ± 0.2 ◦ C) with heating/cooling dynamic segments. The samples used in this study were 5–6 mg in size. Nitrogen was used as the carrier gas at a flow rate 50 mL min−1 and the experiments were carried out at temperatures in the range of 298–1073 K at various heating rates ( = 10, 15, 20 K min−1 ) to record the TGA and DTG curves. All of these experiments were conducted in triplicate to confirm the repeatability and authenticity of the resulting data. 2.2.4. Py-GC/MS analysis The fast pyrolysis of CS-Zn was achieved in a CDS 5200 analytical pyrolyzer using 1.0 ± 0.02 mg of sample in a silica tube. Kinetic analysis showed that CS-Zn rapidly decomposed at temperatures in the range of 473–673 K. Based on the high heating rate of the pyrolyzer and the poor thermal conductivity of the sample, the optimal condition for analyzing the product distribution resulting from the fast pyrolysis of CS-Zn were determined to be 773 K for 20 s. Moreover, a series of fast pyrolysis reactions were carried out at different temperatures (673, 773 and 873 K) each for 20 s, and the products were held for different reaction times (5, 10, and 20 s) at 773 K, with the same heating rate of 2.0×104 K s−1 and Py-GC/MS transfer line temperature of 553 K. The volatile products resulting from the pyrolysis were analyzed by GC/MS (Agilent 7890A/5975C) using an injector temperature of 558 K. The chromatographic separation of the materials was achieved over an HP-5ms capillary column (30m × 0.25mm × 0.25 m). Helium (99.999%) was used as the carrier gas with a constant flow rate 1 mL min−1 , and a split ratio of 1:60. The oven temperature was increased from 323 K (where it was held for 1 min) to 565 K (where it was held for 2 min) at a heating rate 8 K min−1 . The temperature of the GC/MS interface was held at 568 K and the mass spectrometer was operated in the electrospray ionization mode at 70 eV covering m/z values in the range of 35–550 amu at a scan rate of 2337 amu s−1 . All of the volatile products were qualitatively and quantitatively analyzed against the NIST MS library. All of these experiments were conducted in triplicate to confirm the reproducibility f the results.
2.2. Characterization of sample
3. Results and discussion
2.2.1. FTIR analysis FTIR spectra were recorded on a PerkinElmer Spectrum-GX-1 infrared spectrometer (PerkinElmer, USA) equipped with DiffusIR accessory. A total accumulation of 24 scans and a resolution of 4 cm−1 provided high signal-to-noise spectra in the range of 4000–400 cm−1 . Variable temperature infrared spectra were recorded at temperatures in the range of 298–873 K at 100 K increments by placing around 5.0 mg of sample on a heating stage. This material was subsequently heated at a rate of 10 K min−1 with a thermocouple until the temperature reached the specified value, where it was held for 10 min. All of these experiments were con-
3.1. FTIR analysis Fig. 1A and B shows the FTIR spectra of CS and CS-Zn at different temperatures. Prior to being heated, a comparison of the FTIR spectra of CS and CS-Zn revealed that there were several clear differences between these two materials. For example, the wide peak corresponding to the stretching vibrations of the NH2 and OH groups in CS at 3510 cm−1 [20] shifted to a higher frequency in CSZn (3607 cm−1 ). However, the peak corresponding to vibration of the carbonyl group in CS at 1656 cm−1 had a similar shift to that of CS-Zn (1657 cm−1 ). The bending vibration of the amine groups at
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Fig. 1. FTIR spectra of CS (A) and CS-Zn (B) at different temperatures: 1–298 K, 2–423 K, 3–573 K, 4–673 K, 5–773 K, 6–873 K.
1595 cm−1 in the FTIR spectrum of CS shifted to a higher frequency in CS-Zn (1609 cm−1 ) [21]. Furthermore, the N H wagging mode in the FTIR spectrum of CS at 665 cm−1 shifted to 678 cm−1 [22]. It was therefore obvious that the NH2 and OH groups of CS
were involved in the coordination of the metal ion [23]. The band (1046 cm−1 ) assigned to the second OH group in CS [8] showed a significant shift to a lower wave number (1003 cm−1 ) as well as an increased intensity in CS-Zn [7]. These data therefore suggested
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3.3. TG analysis
Fig. 2. X-ray diffraction patterns of CS and CS-Zn.
that the second OH group was also involved in the coordination of the Zn ion. The structures of CS and CS-Zn were also evaluated by FTIR analysis after they had been heated. Notably, there were no significant changes in the structure of CS when it was heated up to 423 K [24]. However, further increases in the temperature beyond this point led to a variety of different changes, especially in the region of 1660–900 cm−1 , which were associated with changes to the groups connected to the pyranose and aromatic rings, where there was a general shift towards shorter wave numbers and reduced intensities with increasing temperature [14,25,26]. More changes were observed for CS-Zn than CS during the heating process. The wide peak in the FTIR spectrum of CS-Zn at 3607 cm−1 shifted to a lower frequency and became increasingly narrow as the treatment temperature increased. Furthermore, the intensities of several peaks, including the C O vibrations associated with the pyranose ring at 1166 cm−1 , the N H bending vibration at 1609 cm−1 and the peak for the carbonyl groups at 1657 cm−1 , became weaker with increasing temperature, and were completely lost at 773 K. The loss of these signals could be attributed to the degradation of the chitosan chain and the decomposition of pyranose ring. It is noteworthy that several new peaks appeared at 3049, 2332, 2202 and 600–900 cm−1 , which were attributed to the formation of an aromatic ring. The FTIR spectra of the thermal degradation residues of CS and CS-Zn showed that the degradation of CS was a gradual process, which was accompanied by the formation of aromatic rings with increasing temperature [12]. These results also showed that zinc played a very important role in the cracking of the pyranose rings and the formation of new aromatic heterocyclic compounds.
3.2. Powder X-ray scattering analysis X-ray diffraction analysis was used for the characterization of the chitosan macromolecule. Fig. 2 shows the different patterns for CS and CS-Zn. The X-ray diffraction pattern for CS showed two characteristic peaks, including a small, broad peak at 2 = 9.85◦ and the main reflection peak at 2 = 19.70◦ , which provided evidence of the partial crystallinity of the chitosan specimen. The CS-Zn complex was characterized by several smaller peaks at 12.64◦ , 22.67◦ , 27.94◦ and 32.62◦ , which indicated that the complexation of zinc to CS had led to pronounced changes in the morphological characteristics of CS.
3.3.1. Effect of ˇ on the process of thermal degradation of CS-Zn Fig. 3A and B shows the TG and DTG curves for the thermal degradation of CS and CS-Zn at three different heating rates in nitrogen. Fig. 3 shows that there were two peaks in the DTG curves of these materials, indicating that the thermal decompositions of CS and CSZn were both two-stage processes. The first of these peaks appeared round 373 K, which was attributed to the loss of water from these materials. The second of these peaks appeared at a temperature in the range of 473–673 K, which was attributed to the thermal degradation of the main chain of CS. However, the second peak in the DTG curve of CS-Zn was accompanied with an extra “shoulder” peak at 250 ◦ C, which might be caused by metal ion catalyzing degradation the main chain of CS. The characteristic temperatures for the second thermal degradation stages of CS and CS-Zn are given in Table 1, where Tst is the temperature of the initial weight loss and Tmax is the temperature at the maximum rate of weight loss (the peak temperature on a differential thermogravimetry curve). All of the characteristic temperatures increased in a linear manner with increasing heating rate, which indicated that the degradation temperatures were mainly affected by the heating rate as a result of the heat hysteresis [27]. The characteristic temperatures of CS were higher than those of CS-Zn, which indicated that CS-Zn degraded much more readily than CS. 3.3.2. The determination of the activation energies for the thermal degradations of CS and CS-Zn using isoconversional methods The activation energy (E) can be accurately determined using isoconversional methods without the prior assumption of a reaction model, as recommended by the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC Kinetics Committee) [28]. In this work, the activation energies were calculated using the FWO and KAS methods. The FWO method was based on the following equation [29]: ln ˇ = ln(AE ⁄Rg(˛)) − 5.335 − 1.0516E ⁄RT
(1)
The KAS method can be described by the following equation [30]: ln ˇ⁄T 2 = ln(AR⁄Eg(˛)) − E ⁄RT
(2)
where ␣ is the conversion rate, ˇ is the heating rate (K min−1 ), A is the pre-exponential factor (s−1 ), E is the activation energy (kJ mol−1 ), R is the gas constant (8.314 J mol−1 K−1 ), T is the absolute temperature (K) and g(˛) is the integral algebraic expression used for the mechanisms of solid-state processes. For a given ␣ value, it is possible to estimate the values of E based on the gradients of the lines obtained by plotting ln ˇ versus 1⁄T (FWO) and ln ˇ⁄T 2 versus 1⁄T (KAS), respectively. The calculated activation energies (E) and correlation coefficients (r 2 ) are given in Table 2. These data clearly show that the E values calculated using the FWO method were slightly higher than those obtained using the KAS method. Furthermore, all of the correlation coefficients (r 2 ) were in the range of 0.9947–0.9999 (CS) and 0.9871–0.9980 (CS-Zn), and the E value of CS was in good agreement with the value reported by Tao (2011), which meant that the values obtained from these two methods were credible and reasonable. 3.3.3. Determination of kinetic mechanism and kinetic parameters for the thermal degradation of CS-Zn The Coats–Redfern method [31] is a good choice of method for investigating the processes underlying the thermal degradation of CS and CS-Zn. This method is based on the following equation: ln g(˛)⁄T 2 = ln AR⁄ˇE − E ⁄RT
(3)
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Fig. 3. TG and DTG curves of the thermal degradation of CS (A) and CS-Zn (B).
Table 1 Characteristic temperatures and kinetic parameters during the second thermal degradation stage of CS and CS-Zn. ˇ (K min−1 ) CS
CSZn
10 15 20 10 15 20
Tst (K) 434.3 440.9 448.5 413.4 418.6 428.5
Tmax (K) 563.7 571.7 576.9 550.6 558.1 562.2
E (kJ mol−1 )
A (s−1 )
137.7 143.4 146.1 121.4 122.5 121.5
5.0 × 10 1.2 × 1011 1.4 × 1011 3.7 × 109 4.8 × 109 3.6 × 109
Depending of the different degradation processes, it is possible to obtain E and A from a plot of ln g(˛)⁄T 2 versus 1⁄T using the g(˛) values listed in Table 3 [32]. It is also possible to obtain a valid reaction mechanism in this way. The activation energy values shown in Tables 2 and 3 indicated that the decomposition of CS followed an F2 mechanism and that the optimal mechanism for this reaction was a second-order reaction ((1 − ˛)−1 − 1). In contrast, the results for the decomposition
11
H (kJ mol−1 )
S (J mol−1 K−1 )
G (kJ mol−1 )
142.4 148.2 150.9 126.0 127.2 126.2
−26.3 −38.3 −37.1 −66.9 −64.7 −67.1
157.2 170.1 172.3 162.8 163.3 164.0
of CS-Zn suggested that it could follow a D2 , D4 or F3 mechanism, although the correlation coefficient for F3 was the largest (r = 0.9958) of all of these mechanism, suggesting that the optimal mechanism was a third-order reaction ([(1 − ˛)−2 − 1]/2). To fully elucidate the mechanism of this thermal decomposition process, we also calculated the activation entropy (S), activation enthalpy (H) and the activation Gibbs energy (G) [33], and the result are listed in Table 1.
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Table 2 Activation energy and correlation coefficient obtained from FWO and KAS methods for the second thermal degradation stage of CS and CS-Zn. ␣
FWO method
KAS method
CS
CS-Zn
E (kJ mol 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average
−1
)
128.8 128.2 136.3 139.7 142.3 148.1 164.6 166.6 146.1
r
2
0.9999 0.9999 0.9998 0.9987 0.9992 0.9986 0.9998 0.9952
E (kJ mol
CS −1
)
110.9 121.8 117.6 117.6 119.8 124.4 123.7 109.6 118.2
r
CS-Zn −1
2
E (kJ mol
0.9977 0.9983 0.9976 0.9940 0.9947 0.9973 0.9924 0.9893
)
126.7 125.8 134.2 137.6 140.2 146.2 163.5 165.3 144.2
r
2
0.9999 0.9999 0.9998 0.9985 0.9991 0.9984 0.9998 0.9947
E (kJ mol−1 )
r2
108.2 119.4 114.7 114.6 116.7 121.4 120.5 105.3 115.1
0.9973 0.9980 0.9972 0.9930 0.9938 0.9968 0.9911 0.9871
Table 3 Integral algebraic expressions for g(˛) for the most frequently used mechanisms of solid-state process. Symbols
g(˛) (Mechanisms)
E (kJ mol−1 ) CS
Random nucleation and nuclei growth 1/2 A2 [− ln(1 − ˛)] (Two-dimensional) 1/3 A3 [− ln(1 − ˛)] (Three-dimensional) Limiting surface reaction between both phases ˛ (one-dimension) R1 1⁄2 R2 [1 − (1 − ˛) ] (Two-dimensions) 1⁄3 [1 − (1 − ˛) R3 ] (Three-dimensions)
CS-Zn
ˇ = 10
ˇ = 15
ˇ = 20
ˇ = 10
ˇ = 15
ˇ = 20
46.8 28.1
49.0 29.6
49.8 30.0
29.7 16.7
29.9 16.8
29.4 16.5
76.1
79.7
80.8
49.9
50.2
49.5
88.5
92.5
93.9
58.4
58.9
58.9
93.1
97.3
98.7
61.6
62.0
61.3
161.5 176.9
168.7 184.7
171.1 187.3
108.8 119.4
109.6 120.3
108.3 119.0
195.5
203.9
206.8
132.3
133.3
131.9
D4
˛2 (One-way transport) (1 − ˛) ln(1 − ˛) + ˛(Two- way transport) 1⁄3 2 [1 − (1 − ˛) ] (Three- way transport) 2⁄3 Ginstling-Brounshtein equation [1 − (2⁄3)˛] − (1 − ˛)
183.1
191.0
193.8
123.7
124.6
123.3
Order of reaction F1 F2 F3
− ln(1 − ˛)(First-order) −1 (1 − ˛) − 1(Second-order) −2 [(1 − ˛) − 1]/2 (Third-order)
102.9 137.7 179.7
107.4 143.4 186.9
109.0 146.1 189.7
68.4 92.4 121.4
68.9 93.2 122.5
68.1 92.3 121.5
Exponential nucleation P2 P3 P4
˛1/2 (Power law n = 1/2) ˛1/3 (Power law n = 1/3) ˛1/4 (Power law n = 1/4)
33.4 19.2 12.1
35.1 20.3 12.9
35.7 20.6 13.1
20.4 10.6 5.7
20.5 10.6 5.6
20.1 10.3 5.6
Diffusion D1 D2 D3
Kinetic thermal analysis showed that the E values obtained using the Coats-Redfern method were in good agreement with those obtained using the FWO and KAS methods, in that all of the E, H and G values were positive and all of the S values were negative. These results indicated that the second step involved in the decomposition of CS and CS-Zn was a non-spontaneous process. It is noteworthy, however, that the activation energy for the thermal degradation of CS-Zn was lower than that of CS, which also indicated that CS-Zn degraded more readily than CS. 3.4. Analysis by Py-GC/MS 3.4.1. Analysis of pyrolysis products from CS-Zn Fig. 4A and B shows the gas chromatograms of the volatiles generated during the pyrolysis of CS-Zn at various temperatures and times. These volatiles are also listed in Table 4. A total of 14 volatile compounds were identified, including small linear molecules such as carbon dioxide, acetaldehyde and acetic acid; several nitrogencontaining aromatic heterocyclic compounds such as pyrazines, pyrroles and pyridines, with C4 and C5 predominating [13,34]; and numerous long chain fatty acids such as n-hexadecanoic acid. It was not possible to measure the release of NH3 or H2 O using this
instrument. The major compounds identified in this way were carbon dioxide, acetic acid and pyrazine (listed in decreasing order). However, pyrazine compounds were the most important products in the pyrolysis of chitosan, because they play an important role as intermediates for perfumes, pharmaceuticals and agricultural chemicals. 3.4.2. Effect of reaction conditions on the pyrolysis products of CS-Zn To develop a deeper insight into the relationship between the volatile compounds formed during the pyrolysis and the pyrolysis temperatures, we investigated the pyrolysis of CS-Zn at temperatures in the range of 673–873 K for a pyrolysis time of 20 s (Fig. 4A and Table 4). The results of these experiments revealed that whilst the majority of the products were similar, the product distributions were different at different pyrolysis temperatures. For example, the use of a high temperature led to a large number of volatile products, such as butane and pyridine, whereas the use of a lower temperature led to the formation of pentanal. The amounts of carbon dioxide and pyrazine resulting from the pyrolysis of CSZn changed considerably depending on the temperature, with the values initially increasing and then decreasing with increasing tem-
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Table 4 Main products from fast pyrolysis of CS at 773 K and CS-Zn at 673 K, 773 K, 873 K for 20 s (% in relative area). RT (min)
compound
Formula
Pyrolysis temperature
Reference
CS-Zn
1.32 1.37 1.50 1.52 1.78 2.00 2.77 2.83 2.98 3.08 3.97 4.05 4.45 5.71 6.06 7.86 22.46
Carbon dioxide Acetaldehyde Butane Pentanal 2-ethyl-Butanal Acetic acid Pyrazine 1,3-Diazine Pyridine Pyrrole 2-methyl-Pyridine 2-methyl-Pyrazine 2-methyl-1H-Pyrrole ethyl-Pyrazine 2-ethyl-1-butanol acetylpyrazine n-Hexadecanoic acid
CO2 C2 H4 O C4 H10 C5 H10 O C6 H14 C2 H4 O2 C4 H4 N2 C4 H4 N2 C5 H5 N C4 H5 N C6 H7 N C5 H6 N2 C5 H9 N C6 H9 N2 C6 H14 O C6 H7 O C16 H32 O2
CS
673 K
773 K
873 K
773 K
48.8 4.6 0 3.9 0 18.2 11.5 0 0 6.3 0 5.7 0 0 0 0 0.9
31.6 4.9 0 6.9 0 18.5 5.7 0 2.0 7.2 0 7.2 3.8 1.0 0 0 5.6
33.8 5.5 9.5 0 1.7 17.8 6.0 2.5 2.6 7.1 1.9 6.6 1.4 1.4 0 0 2.3
26.9 5.4 0 4.6 0 9.1 14.4 0 3.7 8.1 3.0 7.5 2.83 1.2 2.3 5.5 4.5
Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib Lib
Fig. 5. Gas chromatography of volatile compounds generated from pyrolysis of CS and CS-Zn at 773 K for 20 s.
perature. In contrast, those of acetaldehyde and acetic acid changed only slightly with increasing temperature, and the total amount of nitrogen-containing aromatic heterocyclic compounds remained almost the same at temperatures of 773 and 873 K. CS-Zn was pyrolyzed at 773 K for 5, 10 and 20 s (Fig. 4B) to investigate the relationship between the volatile compounds formed during the pyrolysis and the pyrolysis times. The results showed that there were no discernible differences in the types of product formed at the different temperature. However, the relative amounts of these products did change slightly with increasing reaction time. For example, the amount of carbon dioxide decreased, whereas the total amount of nitrogen-containing aromatic heterocyclic compounds increased.
Fig. 4. Gas chromatography of volatile compounds generated during the pyrolysis of CS-Zn (A) at vavious temperatures for 20 s, (B) at various reaction times at 773 K.
3.4.3. Stability analysis of CS and CS-Zn To compare the thermal stabilities of CS and CS-Zn, and better track changes in the volatile compounds resulting from the pyrolysis of these materials, we investigated the pyrolysis of CS at 773 K for 20 s (Fig. 5). This result revealed that the pyrolysis of CS resulted in more types of volatile product than the pyrolysis of CS-Zn. For
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shorter retention times (0–6 min), the pyrolysis of CS led to the same pyrolysis gases as CS-Zn, including carbon dioxide, acetaldehyde, acetic acid, pyridine, pyrazines and pyrroles, but the relative amounts of these materials were different. For longer retention times (6–24 min), the pyrolysis of CS led to the appearance of several new volatile compounds, such as 2-ethyl-1-butanol (RT, 6.06; yield, 2.27%) and acetylpyrazine (RT, 7.86; yield, 5.50%). Based on the above results, it is clear that the processes involved in the pyrolysis of CS-Zn and CS are very complicated, and the pyrolysis mechanisms of these materials are different. CS is a linear copolymer consisting of randomly distributed N-acetylated and N-deacetylated units in the main chains. The thermal degradation process of CS could potentially involve the random degradation of its main chains and the splitting of side chains [35]. The former starting with the scission of a C O C bond, which was supported by the fact that the bond energy of C O C was smaller compared with that of C C [36], generates carbon dioxide, oligomers, dimmers of glucosamine and solid char [14,37]; the latter releases ammonia and acetic acid. Glucosamine with ␣-amino carbonyl structure can soon be condensed to form pyrazine compounds via dehydration and deprotonation [38,39], which is exothermic. Pyridines and pyrroles are generated from the pyrolysis of glucosamine [13]. Increasing the pyrolysis temperature forms a series of substituted heterocyclic compounds. The FTIR, XRD and TGA results for CS-Zn showed that the coordination of zinc to CS formed a unique structure. This structure clearly favored the degradation of CS, making the pyrolysis of CS-Zn much easier and more thorough than that of CS. In addition, condensation of glucosamine may be catalyzed by the granular zinc in the solid char [40]. For pyrolysis of CS-Zn, the reaction temperature and pyrolysis time had a pronounced effect on the product distribution, and the processes of the formation of carbon dioxide and nitrogen-containing aromatic heterocyclic compounds may have competed with each other under these conditions. When the pyrolysis was conducted at temperatures in the range of 673–873 K for 5–20 s, the major volatile products were carbon dioxide and acetic acid, with lower pyrolysis temperature and longer reaction times affording more pyrazine compounds. The pyrolysis of CS-Zn produced about 15% carbonizate. We measured the iodine adsorption capacity of carbonizate using GB7702-87 standard and found that the carbonizate produced by CS-Zn exhibited stronger adsorption capacity (1043.2 mg g−1 ) than that of CS (617.1 mg g−1 ), representing an interesting opportunity for future studies, because it suggests that the transition-metal complexes of CS could be used as promising sources for the production of N-doped carbons and coals for catalysis, adsorption and ion storage batteries.
4. Conclusions The thermogravimetric analysis of CS-Zn in nitrogen revealed the presence of a two-stage degradation process. Kinetic analysis showed that the most plausible model for the second thermal degradation stage of CS-Zn was F3 ([(1 − ˛)−2 − 1]/2), i.e., a thirdorder reaction. The thermal degradation residues generated by the pyrolysis of these materials were analyzed by FTIR, whereas the volatile compounds generated during the same process were measured by GC/MS. The results indicated that the decomposition of CS-Zn was very complicated, leading to the production of many volatile products such as carbon dioxide, acetic acid and nitrogencontaining aromatic heterocyclic compound. Compared with CS, the pyrolysis of CS-Zn occurred much more readily and proceeded to a much greater extent. Notably, the reaction conditions had a considerable impact on the type and distribution of the volatile compounds formed during the pyrolysis.
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Please cite this article in press as: C. Ou, et al., Study on the thermal degradation kinetics and pyrolysis characteristics of chitosan-Zn complex, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.03.021