5.2 IR study of the effect of temperature on the chitin deacetylation


Table 6 shows the effect of temperature on deacetylation; the more the temperature increases, the more the deacetylation increases, but the total conversion of

Table 5. Evolution of DD as a function of the concentration of the base at different temperatures [3].


#### Table 6.

Evolution of DD in relation to temperature at different concentrations of the base [3, 23].

chitin to chitosan is only carried out at the temperature of 80°C and for high concentrations of the base [23]. Analysis of the results of the measurement of DD shows that it has increased approximately 8% (66–74% DD), at high concentration of NaOH (10 M) and a thermal increase of 20°C (100–120°C). This variation of DD seems less important, only +2% (26–28% DD) at low concentration of NaOH (7.5 M). This discrepancy can be considered as an error of analysis. However, this increase in DD is particularly significant (13%) with a base concentration increase of 2.5 M (10–12.5 M). The temperature in this step is a factor that weakens the binding of the acetyl groups and accelerates the deacetylation reaction. Ahlafi and al [3] also found that the DDs increase as the NaOH concentration and temperature increase as the time of reaction increases. These reached a maximum of 63% and 71% for CNaOH = 12 N at T = 80 and 120°C, respectively. On the other hand, it can be observed that the critical time decreases when the temperature and the concentration of NaOH increase. It can be seen in the case of CNaOH = 12 N and T = 120°C. The increase of DD values with these parameters can be attributed to the change of the structure during deacetylation of chitin at height temperature, as confirmed by FTIR studies. These results confirm the hypothesis that the interaction of concentration and thermal energy are the main criteria to be taken into account for the deacetylation reaction.

The rate of deacetylation therefore depends not only on the concentration of NaOH used but also on the temperature for a breakdown of the acetyl bonds. However, a high temperature leads to a degradation of chitosan, which causes the viscosity η to drop and the molecular weight M (η = kM a) to drop, which affects the solubility of chitosan. The variation of the concentration of NaOH does not affect its

T(°C) 25 80 120

) 2nd step 1.70 <sup>10</sup><sup>5</sup> 2.23 <sup>10</sup><sup>4</sup> <sup>22</sup> <sup>10</sup><sup>4</sup>

Evolution of DD as a function of time at different concentrations of the base [3, 23].

Quantitative Analysis by IR: Determination of Chitin/Chitosan DD

DOI: http://dx.doi.org/10.5772/intechopen.89708

[23] 120 °C [3]

C mol/l/t mn 8.75 10 11.25 12.5 C mol/l/t mn 8 12 8 23 42 45 50 40 50 13 60 70 78 100 48 52.5 18 75 81 86 150 53 54 23 82 86 91 200 55 67 26.5 84 88 92 250 64 68 30 86 90 93 300 65 70

5.4 Determination of apparent rate constant k and energetic activation (Ea)

The chitin deacetylation process followed the pseudo–first-order kinetics for all the temperatures studied (25, 80, and 120°C) and at the same alkaline concentration (12 N) [3]. The Table 8 showed that the values decreased as a function of temperature. This indicates that the speed of the deacetylation reaction is faster at the beginning of the reaction t < 60 min, but it was very slow at the end of reaction. The apparent activation energy was estimated at about 48.76 kJ/mol from the straight line of the Arrhenius plot (in k vs. (1/T)) [3]. This value is in the same order of magnitude as that found by other authors for heterogeneous N-deacetylation performed between 80 and 120°C [10, 12, 22, 26]. The concentration of NaOH significantly influences the variation of the reaction temperature. Rinaud and al. [14] mention that at a NaOH concentration of 10–15 M, the energetic activation (Ea) of the deacetylation is, respectively, about 22–50 kJ/mol, which makes it possible to increase the degree of deacetylation. The results indicated that the reaction at higher concentration and temperature proceeded easier than that at their

The reagent diffusion mechanism represents the second step in deacetylation of chitin. Recently, Sarhan et al. [28] have proposed a mechanism in which heterogeneous N-deacetylation is controlled by both reaction and diffusion: the first step involves the reaction of the onium salt, designated (Q<sup>+</sup> X), with NaOH to give the

molecular weight [76].

Apparent rate constant [3].

Table 7.

Table 8.

k2 (min<sup>1</sup>

lower values.

117

5.5 Mechanism of the chitin deacetylation

### 5.3 FTIR study of the effect of time on the chitin deacetylation

The analysis of the results (Table 7) shows that the best deacetylation started after 60 min, for the three concentrations of NaOH studied. At this time, the activation energy for deacetylation can be reached and the chemical bond break occurs. However, this breakdown of the acetyl groups is significant only at the level of the high alkaline concentration (> 10 M). Diffusion of the alkaline solution on the shell substrate increases with time. These results confirm the hypothesis that there is an interaction between the concentration of NaOH and the reaction time on DD.

At the lowest NaOH concentration (7.5 M), the percentage of deacetylation increases from about 60°C but remains low even at 120°C (Table 7). The extracted product remains predominantly in the form of chitin. The deacetylation is terminated after a period ranging from 90 to 120min depending on the concentration of NaOH. Moreover, for a lower concentration of NaOH (8.75 M), this reaction is incomplete even after 3 h for different authors [74]. Therefore, in the case of a low concentration of NaOH (less than 10 M), the deacetylation does not occur completely. Thus, to obtain a good-quality chitosan, it is necessary to resume the treatment several times using NaOH solution [75, 76]. This reprocessing scheme is technically not easy and economically unviable on a large scale. So, for better deacetylation, 1 h is sufficient for concentrations greater than 10 M.


### Quantitative Analysis by IR: Determination of Chitin/Chitosan DD DOI: http://dx.doi.org/10.5772/intechopen.89708

Table 7.

chitin to chitosan is only carried out at the temperature of 80°C and for high concentrations of the base [23]. Analysis of the results of the measurement of DD shows that it has increased approximately 8% (66–74% DD), at high concentration of NaOH (10 M) and a thermal increase of 20°C (100–120°C). This variation of DD seems less important, only +2% (26–28% DD) at low concentration of NaOH (7.5 M). This discrepancy can be considered as an error of analysis. However, this increase in DD is particularly significant (13%) with a base concentration increase of 2.5 M (10–12.5 M). The temperature in this step is a factor that weakens the binding of the acetyl groups and accelerates the deacetylation reaction. Ahlafi and al [3] also found that the DDs increase as the NaOH concentration and temperature increase as the time of reaction increases. These reached a maximum of 63% and 71% for CNaOH = 12 N at T = 80 and 120°C, respectively. On the other hand, it can be observed that the critical time decreases when the temperature and the concentration of NaOH increase. It can be seen in the case of CNaOH = 12 N and T = 120°C. The increase of DD values with these parameters can be attributed to the change of the structure during deacetylation of chitin at height temperature, as confirmed by FTIR studies. These results confirm the hypothesis that the interaction of concentration and thermal energy are the main criteria to be taken into account for the

Evolution of DD in relation to temperature at different concentrations of the base [3, 23].

Ref. [23] Ref. [3]

Modern Spectroscopic Techniques and Applications

T°C/CNaOH mol/l 7.5 10 12.5 CNaOH mol/l T°C DD — 1 2 8 180 38 10 14 20 10 120 45 24 60 67 12 60 55 26 68 78 8 180 38 28 74 90 10 120 45

5.3 FTIR study of the effect of time on the chitin deacetylation

The analysis of the results (Table 7) shows that the best deacetylation started after 60 min, for the three concentrations of NaOH studied. At this time, the activation energy for deacetylation can be reached and the chemical bond break occurs. However, this breakdown of the acetyl groups is significant only at the level of the high alkaline concentration (> 10 M). Diffusion of the alkaline solution on the shell substrate increases with time. These results confirm the hypothesis that there is an interaction between the concentration of NaOH and the reaction time on DD. At the lowest NaOH concentration (7.5 M), the percentage of deacetylation increases from about 60°C but remains low even at 120°C (Table 7). The extracted product remains predominantly in the form of chitin. The deacetylation is terminated after a period ranging from 90 to 120min depending on the concentration of NaOH. Moreover, for a lower concentration of NaOH (8.75 M), this reaction is incomplete even after 3 h for different authors [74]. Therefore, in the case of a low

concentration of NaOH (less than 10 M), the deacetylation does not occur completely. Thus, to obtain a good-quality chitosan, it is necessary to resume the treatment several times using NaOH solution [75, 76]. This reprocessing scheme is technically not easy and economically unviable on a large scale. So, for better

deacetylation, 1 h is sufficient for concentrations greater than 10 M.

deacetylation reaction.

116

Table 6.

Evolution of DD as a function of time at different concentrations of the base [3, 23].


#### Table 8.

Apparent rate constant [3].

The rate of deacetylation therefore depends not only on the concentration of NaOH used but also on the temperature for a breakdown of the acetyl bonds. However, a high temperature leads to a degradation of chitosan, which causes the viscosity η to drop and the molecular weight M (η = kM a) to drop, which affects the solubility of chitosan. The variation of the concentration of NaOH does not affect its molecular weight [76].
