Extraction of Cellulose
The cellulose samples appeared white to off-white in color, having both rough and smooth appearances. This appearance is characteristic of crude cellulose samples, which is due to the presence of both amorphous and crystalline regions in the cellulose polymers.
Synthesis of Cellulose Derivatives
The cellulose samples (10 wt%) dissolved in IL and kept for 30 minutes from the start of the reaction. The maximum derivatizing temperature (100°C) led to minimal degradation. The cellulose triacetate appeared dirty white, characteristic of the triacetate derivative of cellulose. Ethyl cellulose and cellulose phthalate also appeared white to off-white. The percentage yield was calculated with respect to the final product obtained. From the equation below:
$$Yield in \%=\frac{experimental}{theoretical}*100$$
General equation of the reactions:
Cellulose Acetate Mechanism
The scheme 2 represents the proposed mechanism for the synthesis of cellulose acetate. In the reaction, the 1-butyl-3-methylimidazolium chloride [BMIM]Cl ionic liquid [IL] functions as a catalyst for the derivatization process and therefore initiates the reaction. The chloride ion in the IL acts as a nucleophile and attacks acetic anhydride, producing nucleophilic acetate as well as reactive acetic acid chloride 1. The nucleophilic acetate abstracts the hydrogen ion attached to the –OH group on the cellulose molecule, while the nucleohplic attack by –O of the cellulose molecule of the reactive acetic acid chloride produces 2, releasing the chloride ion back into catalytic cycle 3, while the derivatized product (cellulose acetate) is produced 4.
Cellulose Phthalate Mechanism
In this reaction (scheme 3), the proposed mechanism for the synthesis of cellulose phthalate is represented. In the reaction, the 1-butyl-3-methylimidazolium chloride [BMIM]Cl ionic liquid [IL] functions as a catalyst for the derivatization process and therefore initiates the reaction. The chloride ion in the IL acts as a nucleophile and attacks phthalic anhydride, producing nucleophilic phthalate as well as reactive phthalic acid chloride 1. The nucleophilic phthalate abstracts the hydrogen ion attached to the –OH group on the cellulose molecule, while the nucleohplic attack by –O of the cellulose molecule of the reactive phthalic acid chloride produces 2, releasing the chloride ion back into the catalytic cycle 3, while the derivatized product (cellulose phthalate) is produced 4.
Cellulose Ether Mechanism
In scheme 4, nucleophilic attack of bromine of ethyl bromide by chloride ions (Cl-) of the ionic liquid (IL) produces a more reactive ethyl chloride 1. This attack is possible because chlorine (Cl) is a stronger nucleophile than bromine (Br). The chloride ion attached to the etherification reagent consequently attacks the hydrogen (H) of the C-2, C-3 or C-6 of the hydroxyl (-OH) group of the cellulose, leading to the dissolution and consequent derivatization of the cellulose substrate via the SN2 mechanism 2. The product (ethylcellulose) formed with the elimination of hydrochloric acid (HCl) 3, a weak nucleophile but a good leaving group. The IL is regenerated through thermal evaporation of the solvent and excess etherification reagent 4.
Percentage Yield and Degrees of Substitution of Cellulose Derivatives
The percentage yield of the cellulose derivative was temperature dependent. The first five reactions showed that better yields when the reaction temperature was 100 °C. A higher temperature (120 °C) led to a decrease in the yield of the product. Therefore, the other reactions were carried out at 100 °C. Acetates, phthalates and ethyl derivatives of different cellulose samples samples were prepared with different degrees of substitution values. Cellulose triacetate was the product of acetylation of cellulose with acetic anhydride, which is evident from the appearance (colour), degree of substitution and solubility. The degrees of substitution of the derivatives are presented in the table 1 below. The results of the yields obtained as a function of temperature and the degrees of substitution are presented in the table 1.
In the table (1) presented, it is evident that at least one of the hydroxyl groups (of C-2, C-3 or C-6) has been substituted with the new groups (acetyl, phtrhaloyl or ethyl). The cellulose acetate prepared (R. H-OAc and DAN-OAc.) had a DS greater than two, indicating that at least two of the –OH groups are substituted with C-6, which is likely to be substituted preferentially because the –OH is attached to a primary carbon. The other derivatives (PLN-Ph., CTN.Ph., C.C.Eth., and FIC.Eth.) had a DS slightly above one, meaning at least one of the –OH groups had been substituted. Overall, it was evident that the IL used dissolved and functionalized the native cellulose, as evident from the various DS values as well as other spectroscopic techniques, which will be discussed later.
Furthermore, higher yields of the products were obtained when the reaction was carried out at 100 °C compared to a slightly higher temperature of 120 °C. This might be due to the possible decomposition of the products formed at temperatures higher than 100 °C, leading to the loss of products. It is concluded that the temperature for the optimal yield for cellulose dissolution and derivatization is 100 °C within 120 minutes.
Table 1: Percent Yields and Degrees of Substitution (DS) of Cellulose Derivatives
S/No.
|
Samples
|
Temperature (°C)
|
% Yield
|
D.S
|
1.
|
PLN-Ph.
|
100
|
50.00
|
1.20
|
2.
|
CTN-Ph.
|
100
|
61.00
|
1.10
|
3.
|
C.C-Ethyl
|
100
|
70.30
|
1.50
|
4.
|
FIC-Ethyl
|
100
|
75.10
|
1.45
|
5.
|
DAN-OAc
|
100
|
59.50
|
2.77
|
6.
|
R.H-OAc
|
120
|
45.00
|
2.70
|
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR spectroscopy of the products (cellulose and cellulose derivatives) revealed the successful extraction of cellulose and synthesis of cellulose derivatives in the IL. In the spectra presented (Fig.2a), the absorption bands at approximately 3300, 2900, 1430, 1374,1050, and 890 cm-1 are all associated with native cellulose.[20] The stretching of O-H groups and aliphatic saturated C-H bonds are responsible for absorbance peaks in the ranges of 3450–3300 cm-1 and 2900–2800 cm-1.[21], [22] The C-O-C pyranose ring skeletal vibrations are responsible for the peaks that appeared at approximately 1050 cm-1. The absorption bands at approximately 1374 cm-1 are responsible for the C-O asymmetric bridge stretching..[18] Furthermore, the spectral absorption peak at 1429 cm-1 is due to a symmetric CH2 bending vibration and is known as the "crystallinity band".[23] The crystallinity of the sample is further proven with the absorption bands at 897 cm-1.[24] All these regions found in all the spectra suggested that the cellulosic compositions are identical. The absorption peak at 1624 cm−1 is responsible for water absorption due to the hydrophilic nature of cellulosic material.[25], [26] The absence of peaks at 1512 cm-1 and 1735 cm-1, which are assigned to the C-C vibration as well as the aromatic C-O stretching in hemicelluloses and lignin, respectively, suggested that the pretreatment method adopted suitably eliminated the noncellulosic component of the raw materials.[27] The FTIR spectra of the prepared cellulose acetate are presented in Fig. 2b below. In the spectra, the absorption bands at approximately 3300, 2900, 1430, 1374, 1050, and 890 cm-1 are all associated with native cellulose, as elucidated above. However, there is evidence of acetylation of cellulose with the appearance of absorption bands at 1750, 1250 and 1100 cm-1. The carbonyl –C=O stretching of ester is responsible for the absorption band at 1750 cm-1. The absorption band at 1250 cm-1 was attributed to SP2 –CO asymmetric stretching in ester, while the SP3 –CO bending in ester of the cellulose acetate appeared at 1100 cm-1.[28], [29] All these results revealed the successful synthesis of cellulose acetate in the IL, leaving no traces of the ILs used in the final product. The FTIR spectra of the prepared cellulose phthalate are presented in Fig. 2c below. In the spectra, the absorption bands at approximately 3300, 2900, 1430, 1374,1050, and 890 cm-1 are all associated with native cellulose, as elucidated above. However, the evidence of acetylation of cellulose shows the appearance of absorption bands at 1730, 1577, 1272 and 1100 cm-1. The carbonyl –C=O stretching of ester is responsible for the absorption band at 1730 cm-1. The absorption band at 1250 cm-1 was attributed to SP2 –CO asymmetric stretching in ester, the SP3 –CO bending in ester of the cellulose acetate appeared at 1100 cm-1, and the aromatic –CH stretching of the phthalate group appeared at 1577 cm-1.[18] All these results revealed the successful synthesis of cellulose phthalate in the IL, leaving no traces of the ILs used in the final product. The FTIR spectra of the prepared ethyl cellulose are presented in Fig. 2d below. In the spectra, the absorption bands at approximately 3300, 2900, 1430, 1374,1050, and 890 cm-1 are all associated with native cellulose, as elucidated above. However, there is evidence of acetylation of cellulose with the appearance of absorption bands at 1730, 1577, 1272 and 1100 cm-1. R-C-O-CH2CH3 stretching appeared at 1720 cm-1. The SP3 –CO bending in R-C-O-CH2CH3 of the ethyl-cellulose appeared at 1100 cm-1.[4], [30] Furthermore, the absence of SP2 C-O absorption band at 1200-12500 cm-1further suggests the group which replaced the –H of cellulose is the –CH2CH3 forming ether (O-–CH2CH3) linkage at C-6. This proves that the absorption band at 1720 cm-1 is that of O-CH2CH3 of the ethyl cellulose derivatives prepared. Conclusively, these results revealed the successful synthesis of ethyl-cellulose in IL, leaving no traces of the ILs used in the final product.
X-Ray Diffraction (XRD) Spectroscopic Analysis of the Cellulose and the Cellulose Derivatives
The diffraction patterns of all the cellulose samples (Fig. 3a) showed peaks of the 2θ angles at approximately 14.5°, 17°, 22.7°, and 35.5° for all the samples, which are assigned to the typical reflection planes of cellulose I. The values obtained are identical to those reported by Hu et al. (2015) and Rahman et al. 2020 for carbon nanocellulose and microcrystalline cellulose, respectively.[9], [15], [31], [32] The crystallinity indices values of the samples ranged from 64.40% to 72.0%, indicating good values for cellulose. The crystallinity indices of the products are presented in table. The nondestructive nature of IL in the dissolution of cellulose evidenced in the spectra presented (Figs. 3b-3d): In the spectra, the diffraction for cellulose samples with 2θ values at 14.5°, 17°, 22.7°, and 35.5° are retained for the cellulose derivatives. In addition, the crystallinity index values of the derivatives are also evidence of the nondestructive nature of ILs in the dissolution of cellulose. More so, the crystallinity indices of the derivatives were a few times higher than those of the starting materials which is further proven by the glass transition temperature (Tg) of the derivatives being higher than those of the starting materials (respective cellulose samples) in the TGA thermogram. The crystallinity indices of the products are shown in the table 2:
Table 2: Crystallinity Indices (C. I) of the Cellulose
S/No.
|
Sample
|
C.I ( % )
|
1.
|
C.C-CELL.
|
66.67
|
2.
|
CTN-CELL.
|
70.50
|
3.
|
R.H-CELL.
|
64.40
|
4.
|
DAN-CELL.
|
67.80
|
5.
|
FIC-CELL.
|
67.53
|
6.
|
PLN-CELL.
|
66.50
|
7.
|
C.C-Ethyl
|
65.67
|
8.
|
FIC-Ethyl
|
70.00
|
9.
|
R.H-OAc.
|
64.67
|
10.
|
DAN-OAc.
|
68.99
|
11.
|
CTN-Ph.
|
72.30
|
12.
|
PLN-Ph.
|
69.88
|
Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS)
The SEM micrographs of cellulose samples (Figs. 4) revealed that they are composed of long fibre strands closely and irregularly packed together and have a rough appearance. Furthermore, the SEM images of the derivatives (-Ph, -Ethyl and -OAc) revealed longer but less ordered tubes than those of the MMCs but similar to those of cellulose. This implies that the dissolution solvent ([BMIM[]Cl) have minimal or nondestructive effects on the cellulose samples. On the other hand, EDS analysis showed that as expected, all products (cellulose and cellulose derivatives) contained carbon (C) and oxygen (O) with hydrogen (H) not shown in the spectra. The absence of elements such as sodium (Na), sulfur (S), chlorine (Cl) and nitrogen (N) is indicative of the pure nature of the products. This implies the absence of impurities from the cellulose extraction up to the dissolution and derivatization stage. However, the presence of Au and Nb elements on the EDS spectra is due to coating the samples with gold before being subjected to SEM/EDS analysis. The SEM appearance of cellulose revealed, as stated above, that the samples are fiber strands connected together, containing shorter fiber strands/tubes. The network of long fibre strands is the amorphous region, while the shorter fibres/tubes represent the crystalline region of the cellulose molecules. The SEM appearance of cellulose derivatives presented in the figures (Figs. 5) below revealed that the starting materials as well as the reaction temperatures contributed to the appearance of the products. The SEM micrograph of FIC-Ethyl, C. C-Ethyl, DAN-OAc., R.H-OAc., PLN-Ph., and CTN-Ph. showed a network of fibres (length of 7-8μm) connected together due to the appearance of the starting material (cellulose). All these appearances showed that using IL as the dissolution solvent did not disrupt the nature of the starting material and hence the products. On the other hand, the fibre strands of R. H-OAc appeared distorted, which might be attributed to the higher reaction temperature (120 °C). Generally, all other reactions took place at 100 °C. Therefore, the optimal temperature for the nondestructive dissolution and derivatization of cellulose in ILs are said to be 100 °C. The SEM appearance of the samples was similar, which is an indication that the extraction of cellulose from different sources was successful. The SEM appearance of cellulose derivatives presented in the figures below revealed that the starting materials as well as the reaction temperatures contributed to the appearance of the products. The SEM micrograph of FIC-Ethyl, C. C-Ethyl, DAN-OAc., R.H-OAc., PLN-Ph., and CTN-Ph. showed a network of fibres connected together due to the appearance of the starting material (cellulose). All these appearances showed that using IL as the dissolution solvent did not disrupt the nature of the starting material and hence the products. On the other hand, the fibre strands of R. H-OAc appeared distorted, which might be attributed to the higher reaction temperature (120 °C) at which acetylation took place. Generally, all other reactions took place at 100 °C. Therefore, the optimal temperature for the nondestructive dissolution and derivatization of cellulose in ILs can be said to be 100 °C.
EDS Analysis of Cellulose and Cellulose Derivatives
The EDS analysis of cellulose and its derivatives (Figs.6 and Figs. 7) revealed that all samples are relatively pure with little or no contaminants, from the cellulose extraction and purification stage up to cellulose dissolution, derivatization and regeneration. This is evident from the absence of elements aside from C and O on the EDS diffractogram.
Thermogravimetric Analysis/Differential Thermal Analysis (TGA/DTA)
The thermal spectra of cellulose and Cellulose Derivatives are presented in the appendices (supplimentary material). The thermogravimetric curve revealed a single-step thermal degradation pattern of the samples from 300 to 700 °C. The first degradation stage on the TGA curve was due to the evaporation of water and volatile solvents for all the samples. This is due to the hydrophilic nature of cellulosic samples. Differential thermal analysis (DTA) showed <70% weight loss at the glass transition temperature (Tg). The single-step thermal degradation exhibited by the samples confirmed the absence of impurities, hemicelluloses, lignin as well as little or no traces of water in its core structure. Therefore, the samples are thermally stable. The thermogravimetric analysis (TGA) curve revealed that the Tmax (temperature at which maximum weight loss occurs) was 600 °C as an average for all samples. Conclusively, these samples are thermally stable. The thermal properties for each of the samples are presented in the table 3:
Table 3: Thermal Information of Cellulose and Cellulose Derivatives
S/No.
|
Samples
|
Tg (°C)
|
%mass left at 495 (°C)
|
1.
|
R.H-CELL
|
327.97
|
2.57
|
2.
|
DAN-CELL
|
332.36
|
5.67
|
3.
|
PLN-CELL
|
333.70
|
11.96
|
4.
|
GME.-CELL
|
328.61
|
26.02
|
5.
|
C.C-CELL
|
327.48
|
17.84
|
6.
|
CTN-CELL
|
350.20
|
3.84
|
7.
|
PLN-Ph.
|
345.20
|
20.04
|
8.
|
CTN-Ph.
|
361.88
|
9.94
|
9.
|
C.C-Ethyl
|
346.60
|
8.91
|
10.
|
FIC-Ethyl
|
345.97
|
14.61
|
11.
|
DAN-OAc
|
333.67
|
17.26
|
12.
|
R.H-OAc
|
332.69
|
20.20
|
In the table, all samples in each category showed closer values for glass transition temperature (Tg). Tg is the temperature at which the samples crystallize. The Tg values of the derivatives are higher than those of the corresponding cellulose starting materials due to the possible breakdown of part or whole of the amorphous region of the cellulose in the process of dissolution in ionic liquids. The one-step degradation process of the derivatives, which is similar to those of the corresponding cellulose, revealed that ionic liquid is a nondestructive solvent in the dissolution of cellulose and that they (cellulose and derivatives) are relatively pure with little or no impurities. This further proved that the new products were synthesized and not a mere attachment of the groups on the cellulose backbone, as corroborated by the FTIR spectra of the derivatives presented earlier.