SEM helps to reveal the morphological and topographical features of the precursors and the resulting carbon composites. The E-SEM images of cellulose before and after the oxidation are presented in Fig. 4a (before) and 4b (after). Before the oxidation and the degradation, the cellulose particles resembled blocks and logs of wood. After degradation, the original crystallinity of the cellulose particles is lost. The results show that the morphology structure of cellulose crystals has changed after the oxidation which indicates substantiate the TGA results on the successful degradation of the cellulose.
3.4. X-ray diffraction (XRD)
To further determine the change in crystallinity, the XRD patterns of microcrystalline cellulose and degraded cellulose are shown in Fig. 5. For the MCC there are crystallographic diffraction peaks at about 2θ = 34.45°, 2θ = 22 and 2θ = 15 (Pang et al. 2014), indicating the characteristic cellulose I crystal structure. After the oxidation and degradation, the crystalline peaks near 22° and 35° disappeared. Also the peak around 2θ = 15 gets more wide and less sharp. These results demonstrate that the crystallinity of cellulose was noticeably decreased. The peak at 30° belongs to the holder, as it can be seen by the curves of the empty holder, that it contains peaks around 2θ = 30° and 2θ = 41°. This result is in accordance with the SEM results, confirming that the oxidation reaction decomposes the whole cellulose polymer and diminishes its crystallinity.
3.5. X-ray photoelectron spectroscopy (XPS)
XPS analysis provides an information about the surface elemental composition and functional groups. The images in Fig. 6, obtained from XPS patterns of the MCC and the DMCC which present the difference in the functional groups that appear in the product. Both of them consist mainly of carbon and oxygen. The cellulose polymer had two types of functional group, the multiple hydroxyl groups, per unit of the polymer had one primary alcohol, two secondary alcohol, and one ether group. Figure 6a shows the C1s spectra of MCC which is deconvoluted into two peaks located at 285 and 286.3 eV, representing C–C/C–H and C–O–C /C–OH groups. However, in the C1s spectra of the DMCC, reveal additional peaks were found at 288.04 eV and 289.3 eV, corresponding to C = O and COO− as well as a shoulder at 286.e eV assigned to C-OH functional group. These peaks belong to the organic acids after the degradation (FA, AA and GA). Also in Fig. 6c,d appears the comparison of the two oxygen (O1s) spectra of the MCC and DMCC, these results reveal the new bonds in the product (DMCC) that didn’t appear in the original pattern of the MCC. The peaks here in the O1s spectra of the DMCC disclosed three main classes of oxygen bonds:
Bond
|
Peak value (eV)
|
O-(C = O*)-C bonds peaks
|
532.2 which exist in GA and AA
|
O*-(C = O)-C bonds peaks
|
533.7 which exist in GA and AA
|
Carbonyl, Organic C = O bonds
|
532.6
|
All the characterization experiments lead to the same conclusion the DMCC is a product different from the cellulose. In this work, we found that only potassium chlorate was able to oxidize the MCC. None of the other nine well-known oxidizing agents was successful in the same oxidation process neither by Microwave nor hydrothermally. Even for the Chlorate ions, negative results were obtained when the reaction was carried out hydrothermally. We are therefore left with two major questions 1) why only ClO3 − 1 can oxidize the MCC under MW radiation and secondly why these products are obtained only by Microwave irradiation?
At first, we tried to correlate the oxidizing power to the standard oxidation potential of these ten oxidizing agents which are presented in Table 4. The table indicates evidently that the chlorate is not the strongest oxidizing agent. However, to this table, we must add the basic conditions under which the reaction is performed, namely, pH ~ 11. Under these basic conditions, the chlorate is the strongest oxidizing agent, while for example, K2S2O8, decomposes fast under these conditions to HSO4−. Potassium permanganate is also a stronger oxidizing agent than ClO3 − 1 but this is only true for acidic medium, it is a weak oxidant in neutral and alkaline medium (Peroxide et al. 2019). Potassium dichromate acts as an oxidizing agent only in an acidic medium. It doesn't act as an oxidizing agent in basic medium because as it involves in non-redox reaction. It forms chromate ion in basic medium. In a basic medium. Cr has (+ 6) oxidation state in dichromate as well as the as in the chromate form. It reacts according to the following reaction.
K2Cr2O7+2NaOH K2CrO4+Na2CrO4+H2O
Table 4
Standard Oxidation potential values of the reagents
Reagent
|
Oxidation potential
|
K2S2O8
|
2.1
|
KMnO4
|
1.49
|
KClO4
|
1.39
|
NH4ClO4
|
1.39
|
K2Cr2O7
|
1.36
|
KClO3
|
1.15
|
NaIO4
|
-0.7
|
KIO4
|
-1.650
|
K2CrO4
|
-6.5
|
The main products in the one-step oxidation of cellulose by the chlorate ions were formic acid, glycolic acid and acetic acid.
To comprehend the mechanism of the cellulose’s degradation, we have examined whether the degradation mechanism involves first the decomposition of the cellulose to glucose. This was done by a control experiment in which identical conditions were used to oxidize the glucose as for the cellulose. The NMR results (fig.S1) display different spectrum comparing with the peaks in Fig. 1b. The NMR peaks indicate that different products such as glucuronic acid and glucose are obtained in the oxidation of glucose. Moreover, unlike with cellulose the glucose was not completely degraded. These results show that the degradation of cellulose does not proceed via the glucose. Returning to the second question of why MW radiation. Microwave is a well-known mean for accelerating chemical reactions especially for reactants or catalysts with a dipole moment. The ability to accelerate chemical reaction is either attributed to heating effect in which the “real” temperature is higher than the measured temperature. This is frequently termed superheating (Tao et al. 2021) or the existence of hot spots (Liu et al. 2020). The other explanation is due to a drastically reduced activation energy offering the reaction to progress along a new mechanistic route. This is a special useful when the transition state has a dipole moment which will strongly interact with the MW radiation.
In the current reaction these two factors play an important role in accelerating the reaction. The presence of many ions ClO3 − 1 and OH− 1 plus their counter ions help to absorb the MW radiation and lead to hot spots. In addition, the transition state of the reaction is also having a dipole moment and speeding up this chemical reaction. The first step; the reaction endures the base hydrolysis forms the glucose. The next step is the glucose oxidizes by KClO3 and forms a different product with MW condition which is provided in the scheme 2. The formation of the acids was confirmed by NMR techniques (Wang et al. 2018a).