Non-isothermal kinetics of thermal degradation of chitosan
© Georgieva et al.; licensee Chemistry Central Ltd. 2012
Received: 14 May 2012
Accepted: 27 June 2012
Published: 2 August 2012
Chitosan is the second most abundant nitrogen containing biopolymer in nature, obtained from the shells of crustaceans, particularly crabs, shrimp and lobsters, which are waste products of seafood processing industries. It has great potential application in the areas of biotechnology, biomedicine, food industries, and cosmetics. Chitosan is also capable of adsorbing a number of metal ions as its amino groups can serve as chelation sites. Grafted functional groups such as hydroxyl, carboxyl, sulfate, phosphate, and amino groups on the chitosan have been reported to be responsible for metal binding and sorption of dyes and pigments. The knowledge of their thermal stability and pyrolysis may help to better understand and plan their industrial processing.
Thermogravimetric studies of chitosan in air atmosphere were carried out at six rates of linear increasing of the temperature. The kinetics and mechanism of the thermal decomposition reaction were evaluated from the TG data using recommended from ICTAC kinetics committee iso-conversional calculation procedure of Kissinger-Akahira-Sunose, as well as 27 mechanism functions. The comparison of the obtained results showed that they strongly depend on the selection of proper mechanism function for the process. Therefore, it is very important to determine the most probable mechanism function. In this respect the iso-conversional calculation procedure turned out to be the most appropriate.
Chitosan have excellent properties such as hydrophilicity, biocompatibility, biodegradability, antibacterial, non-toxicity, adsorption application. The thermal degradation of chitosan occurs in two stages. The most probable mechanism function for both stages is determined and it was best described by kinetic equations of n-th order (Fn mechanism). For the first stage, it was established that n is equal to 3.0 and for the second stage – to 1.0 respectively. The values of the apparent activation energy E, pre-exponential factor A in Arrhenius equation, as well as the changes of entropy ΔS≠, enthalpy ΔH≠ and free Gibbs energy ΔG≠ for the formation of the activated complex from the reagent are calculated.
A number of reviews [1–9] and articles [10–15] have been dedicated to chitin and the products obtained from its chemical treatment at different conditions. Chitosan is the second most abundant nitrogen containing biopolymer in nature, obtained from the shells of crustaceans, particularly crabs, shrimp and lobsters, which are waste products of seafood processing industries [13, 15].
Chitin and chitosan are of commercial interest due to their high percentage of nitrogen (6.89%) compare to synthetically substituted cellulose (1.25%). In this respect, chitin and chitosan are recommended as suitable functional materials, because these natural polymers have excellent properties such as hydrophilicity, biocompatibility, biodegradability, antibacterial, non-toxicity, adsorption properties, and remarkable affinity for many biomacromolecules. Chitosan has been widely studied for biosensors, tissue engineering, separation membrane, waste water treatment, because of its multiple functional groups .
Recently, much attention has been paid to chitin and chitosan as a potential polysaccharide resource. Due to their special chemical and biological properties and widespread availability chitins and their derivatives have extensive applications in many industrial, and agricultural fields [15, 16]. Chitosan have great potential application in the areas of biotechnology, biomedicine, food industries, and cosmetics. Chitosan is also capable of adsorbing a number of metal ions as its amino groups can serve as chelation sites. Due to their high nitrogen content and porosity, chitosan-based sorbents have exhibited relatively high sorption capacities and kinetics for most heavy metal ions. This could be easily explained by the presence of amino groups in the polymer matrix, which can interact with metal ions in the solution by ion exchange and complexation reactions [8, 15]. The high content of amino groups also makes possible many chemical modifications in the polymer with the purpose of improving its sorptive features, such as selectivity and adsorption capacity. Grafted functional groups such as hydroxyl, carboxyl, sulfate, phosphate, and amino groups on the chitosan have been reported to be responsible for metal binding and sorption of dyes and pigments [17–22]. The knowledge of their thermal stability and pyrolysis may help to better understand and plan their industrial processing.
Studies on mechanism and kinetics of reactions involving solid compounds is challenging and difficult task with complexity results from the great variety of factors with diverse effects, e.g. reconstruction of solid state crystal lattice, formation and growth of new crystallization nuclei, diffusion of gaseous reagents or reaction products materials heat conductance, static or dynamic character of the environment, physical state of the reagents – dispersity, layer thickness, specific area and porosity, type, amount and distribution of the active centers on solid state surface, etc. . Recently, the methods of thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are quite useful, since they provide reliable information on the physico-chemical parameters, characterizing the processes of transformation of solids or participation of solids in processes of isothermal or non-isothermal heating [23–28]. The kinetic triplet (i.e., E A and f(α)) is typical outcome of the regular kinetic analysis. Practically, the kinetic triplet is needed to provide mathematical description of the process. If the kinetic triplet is determined correctly, it can be used reproduce the original kinetic data as well as to predict the process kinetics outside the experimental temperature region. For example, may be predicting at any desired temperature, the time to reach any extent of conversion . The results, obtained on this basis can be directly applied in materials science for the preparation of various metals and alloys, cements, ceramics glasses, enamels, glazes, synthetic or natural polymers and composite materials . In the case of chitosan, several studies concerning the thermal degradation by means of TG, DTA and DSC techniques were reported and different kinetic values were obtained depending on the experimental conditions in which essay were performed [12–14, 30, 31]. The knowledge of thermal degradation kinetic may help a better understanding and planning of processes to recover metals or metal oxides previously adsorption on chitosan.
The aim of this paper is to assess the kinetic models of chitosan pyrolysis and to estimate the kinetics triplet (E, A and the shape of the most appropriate f(α)-function) of this process by means of thermogravimetric data, using different calculation procedures.
The materials used for thermal degradation chitosan obtained from crab shells were commercially obtained from SIGMA-ALDRICH (Cat. No. C3646), deacetylated with ≥75% and were used without further purification. Before using, chitosan was vigorously grounded in agate mortar and dried in air at 60°C for 4 hours.
The thermogravimetrical measurements (TG-DTG-DTA) were carried out in a flow of synthetic air at a rate of 25 cm3 min–1 under non-isothermal conditions on an instrument STA 449 F3 Jupiter (Nietzsch, Germany) with its high temperature furnace. Samples of about 7 ±0.1 mg mass were used for the experiments varied out at hearing rates of 3, 6, 9, 12, 15 and 18°C min-1 up to 800°C. The samples were loaded without pressing into an open 6 mm diameter and 3 mm high platinum crucible, without using of a standard reference material. The TG, DTG and DSC curves were recorded simultaneously with 0.1 mg sensitivity.
Theoretical approach and calculation procedures
where m n and p are empirically obtained exponent factors, one of them always being zero. The combination of different values of m n and p make it possible to describe various probable mechanisms [23, 25, 32].
The solution of left-hand side integral depend on the explicit expression of the function f(α) and are denoted as g(α). The formal expression of the function g(α) depend on the conversion mechanism and its algebraic expression. The latter usually represents the limiting stage of the reaction – the chemical reactions; random nucleation and nuclei growth; phase boundary reaction or diffusion. The algebraic expression of functions of the most common reaction mechanisms operating in solid-state reactions are presented in some papers [23–25, 27, 29].
Coats-Redfern calculation procedure
For most values of E and for the temperature range over which reactions generally occurs, and because 2RT/E is much lower than unity, we have reason to write the right side of Eq. (7). If the correct g(α) function is used, a plot of lng(α)/T2 against 1/T should give a straight line from which the values of the activation energy E and the pre-exponential factor A in Arrhenius equation can be calculated. They can be calculated from the slope and intercept respectively. The model that gives the best linear fit is selected as the chosen model. The integral method of Coats-Redfern has been mostly and successfully used for studying of the kinetics of dehydration and decomposition of different solid substances when 20 < E/RT < 60 [23, 24, 26–28]. This approach is applied and for single TG curves.
Plotting lng(α) versus lnq and using linear regressive of least square method, if the mechanism studied conforms to certain g(α) function, the slope of the straight line should be equal to −1.0000 and the linear correlation coefficient R2 should be equal to unity. Obviously, the values of E and A do not influenced on the shape of this obtained straight lines.
The values of ΔS≠, ΔH≠ and ΔG≠ were calculated at T = Tp (Tp is the peak temperature at the corresponding stage), since this temperature characterizes the highest rate of the process, and therefore, is its important parameter [23, 24, 26, 27].
Estimation of lifetime
where the value of the reaction order n is obtained previously. With these equations, the time to equivalent damage at different temperatures can be calculated.
Results and discussion
Two steps can be observed in the TG-curve: the first one at about 90°C connected with 5.8% mass loss was accompanied by endothermic effect and was attributed to the evaporation of water absorbed in the inner polymer. The second one, beginning at about 245°C and ending at 580°C was connected with 78.5% mass loss and was indicated for vaporization and burning of volatile compounds produced from the thermal degradation of polymeric chain. The pyrolysis of polysaccharides structure starts by a random split of the glycosidic bonds followed by a further decomposition to form acetic and butyric acids and a series of lower fatty acids where C2, C3 and C6 predominate . At temperatures above 380°C, significant change was observed in the course of the TG-curve. It is may be due to the change of the structure of the material and the change of the mechanism of its thermal degradation process. In the DTG and DTA-curves, two peaks were observed – at 294.5 and 544.5°C, respectively. These two stages are strongly exothermic. The first stage ending at 400°C is connected with 43.5% mass loss while the second one – with 35% mass loss. According to some authors [12–14] the first stage is connected with deacetylation and depolymeryzation of chitosan. The second one corresponds to the residual cross-liked degradation chitosan .
Effect of the heating rate on the kinetic parameters for the thermal decomposition of chitosan calculated according Coats-Redfern procedure
Heating rate (K min–1)
E (kJ mol-1)
E (kJ mol-1)
As can be seen from Table 1 the average value of E and A for the first stage of the thermal degradation of chitosan is higher than these for the second one. Because the obtained value of the apparent activation energy E for the second stage are near to 100 kJ mol-1, may be say that the kinetic of this stage is diffusion controlled process.
As can be seen from Figure 4, the TG and DTA-curves are shifted to higher temperatures with the increase of the heating rates. Furthermore, two exo-effects were observed, the second of them stronger. These effects correspond to a step which abruptly changes its course at temperatures above 400°C. It means that the kinetic model describing the thermal decomposition (the algebraic expression of f(α)-function) will also change.
The apparent activation energy E was directly evaluated from the slope of these plots and frequency factor A – from the cut-off from the ordinate axis respectively.
Lines with slope equal to −1.0000 and linear correlation coefficient R2 close to unity were obtained with Fn mechanism functions at different values of n. For the first stage, it was established that n is equal to 3.0 and for the second stage - to 1.0 respectively. A reaction order n > 2 is mathematically equivalent to Gamma distribution of frequency factor. The interpretation of the high values of reaction order has been discussed in the literature . However, the reason why this parameter has such a high value in relation to the described phenomena has not been completely explained.
Kinetic parameters of the thermal degradation of chitosan
E (kJ mol–1)
–Δ S ≠(J mol–1 K–1)
Δ H ≠, (kJ mol–1)
Δ G ≠, (kJ mol–1)
As can be see from Table 2, the average value of the apparent activation energy E and pre-exponential factor A in the Arrhenius equation is higher for the first stage of the thermal degradation of chitosan. The values of the pre-exponential factor for a solid phase reactions are expected to be in a wide range (six or seven orders of magnitude), even after the effect of surface area is taken into account . For first order reactions, the pre-exponential factor may vary from 105 to 1016 min-1. The low factors will often indicate a surface reaction, but if the reactions are not dependent on surface area, the low factor may indicate a “tight” complex. The high factors will usually indicate a “loose” complex . Even higher factors (after correction for surface area) can be obtained for complexes having free translation on the surface. Since the concentrations in solids are not controllable in many cases, it would have been convenient if the magnitude of the preexponential factor indicated for reaction molecularity. However, this appears to be true only for non-surface-controlled reactions having low (<108 min-1) pre-exponential factors. Such reactions (if elementary) can only be bimolecular.
The change of entropy for the formation of the activated complex from the reagent ΔS≠ reflects how close the system is to its own thermodynamic equilibrium. Lower activation entropy means that the material has just passed through some kind of physical or chemical rearrangement of the initial structure, bringing it to a state close to its own thermodynamic equilibrium. In this situation, the material shows little reactivity, increasing the time necessary to form the activated complex. On the other hand, when high activation entropy values are observed, the material is far from its own thermodynamic equilibrium. In this case, the reactivity is higher and the system can react faster to produce the activated complex, and consequently, short reaction times are observed. In particular, for example, the negative values of ΔS≠ would indicate that the formation of activated complex is connected with decrease of entropy, i.e. the activated complex is “more organized” structure compared to the initial substance and such reactions are classified as “slow” . The negative values of ΔS≠ obtained for the second stage of the thermal degradation of chitosan showed that its structure is far from its own thermodynamic equilibrium, comparing with the second one.
Comparison of the kinetic parameters obtained with the most probable mechanism function g( α ) for non-isothermal degradation of chitin and chitosan
E (kJ mol–1)
–ΔS≠ (J mol–1 K–1)
ΔH≠ (kJ mol–1)
ΔG≠ (kJ mol–1)
As can be seen from Table 3 chitin is more stable than chitosan. The values of the apparent activation energy and pre-exponential factor for both stages of thermal degradation of chitin are higher than these of chitosan. The same tendency was established from other authors [12, 49]. The negative values of ΔS≠ for both compounds showed that the formation of the activated complex from the reagents is connected with a decreasing of entropy, i.e. the activated complex is “more organized” structure and the formation process may be classify as “slow” .
It is obvious from Figure 7 that the lifetime is a parameter strongly depending on the temperature and decreases exponentially with the increase of the temperature. The lifetime is more sensitive concerning the temperature for the first stage of the thermal decomposition of chitin. The same tendency was established for the lifetime of chitin .
Chitosan have excellent properties such as hydrophilicity, biocompatibility, biodegradability, antibacterial, non-toxicity, adsorption application. The thermal degradation of chitosan occurs in two stages. The kinetics and mechanism of the thermal decomposition reaction were established using iso-conversional calculation procedure. The most probable mechanism function for both stages is determined and it was best described by kinetic equations of n-th order (Fn mechanism). For the first stage, it was established that n is equal to 3.0 and for the second stage – to 1.0 respectively. The values of the apparent activation energy E, pre-exponential factor A in Arrhenius equation, as well as the changes of entropy ΔS≠, enthalpy ΔH≠ and free Gibbs energy ΔG≠ for the formation of the activated complex from the reagent are calculated.
The authors would like to express their gratitude to Operating Programme Human Resources Development; Grant agreement: BG051PO001/3.3-05-001 “Science and Business” Funded by: EC, European Social Fund for their financial support.
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