Characterization of Synthesized Cellulose Derivatives in 1-Butyl-3-Methylimidazolium Chloride Ionic Liquid

DOI: https://doi.org/10.21203/rs.3.rs-1893762/v1

Abstract

To obtain pure holocellulose, cellulosic wastes were chemically pretreated. Depending on the derivatizing agents used, the dissolution and functionalization of various holocelluloses obtained in 1-butyl-3-methylimidazolium chloride [BIMIM]Cl ionic liquid using different derivatizing agents such as cellulose acetate, cellulose phthalates, and cellulose ether. Cold acetone and dichloromethane were used to regenerate the dissolved product in ionic liquids, and the resulting ionic liquids were reused.The degree of substitution (DS) of the products obtained ranged from 1.00 to 2.77 at 100°C. FTIR analysis revealed important absorption bands which include: (C = O at ~ 1750 cm− 1, SP3 –CO at ~ 1250cm− 1, SP2 –CO at 1100cm− 1, for esters of cellulose acetate and phthalate, Aromatic –CH stretching at 1577cm− 1 for cellulose phthalate and –CO-CH2CH3 at 1720cm− 1 with the absence of SP2 bending frequency at 1250cm− 1 which confirms the successful synthesis of ethyl cellulose). XRD showed values at (2θ= ~14.5°, 17.0°, 22.7° and 34.5°) for all samples. Comparison of SEM images of the cellulose and derivatives showed little or no destruction of the fibre strands while EDS revealed C and O as (Elemental composition of all samples. Thermogravimetric analysis (TGA/DTA) showed the derivatives possessed higher thermal stability that the starting materials thus, producing materials with better application. All these revealed a successful extraction and purification of cellulose from wastes as well as the synthesis of cellulose derivatives.

Introduction

Agricultural wastes or byproducts are a source of pollution to the environment. Therefore, utilizing these wastes and turning them into finished products that would serve as a source of income to man, which in turn would reduce the rate at which the wastes litter the environment, is very crucial.[1] Several attempts have been made into the usage of agricultural wastes, and these include their uses as sources of activated carbon for high-performance capacitors,[2] composting for multiple agricultural sources,[3] enzymes[4] and cellulose sources for various applications in biomedicals,[5] pharmaceuticals,[6] cosmetics, and food industries[7]. Most of these agricultural wastes are either byproducts of forestry, agricultural crops, animals, microorganisms, or domestic and industrial practices such as food processing, wood and rice milling.[8] Examples of these agricultural wastes include rice husks, corncobs, wood dusts, cow and chicken dungs, and kitchen wastes, and the numbers are numerous. It is worth noting that most of these wastes, especially plant- and microorganism-based wastes, are mainly composed of cellulose. [9].

Cellulose is named one of the most abundant and useful polymers on earth owing to its availability in reasonable percentages in plants and secreted by microorganisms such as oocyte bacteria. It has been estimated that cotton, which contains the purest and largest percentage of cellulose, has close to 99% cellulose composition. Hard wood contains approximately 50%, soft wood contains approximately 45%, while other plant parts can contain as much as 30%. Usually, celluloses are found in combination with other components, such as lignin and hemicellulose.[9]

Cellulose and its numerous derivatives have been used for several applications due to its special qualities, including biocompatibility, good mechanical properties, high thermal properties, low density and very good tunable properties.[10] The importance of cellulose in its various forms cannot be overemphasized, as it has found its way into all areas of life, and it is being projected to replace most of the non-biodegradable petroleum feed stocks in the future.[11]

The past two decades have seen an improvement in the processing of cellulose as a raw material for further use with the introduction of nonaqueous, nonvolatile, thermally stable, high electrochemical windows, low viscosity values, moderate melting temperatures and reusable classes of solvents named ‘ionic liquids’ (ILs) for their homogenous dissolution and further processing. All these properties are responsible for the ability of ILs to dissolve cellulose homogenously with little or no side reactions.[12]

Several reports published the use of Ionic liquids (IL) for agricultural wastes cellulose processing focused mainly on the structural characteristics of the ILs that lead to cellulose solvation, D.S and structural modification. [9], [13]–[16] Reports on the non-destructive role it (IL) played in the homogenous dissolution and functionalization of the cellulose samples are scarce. More so, it is widely reported that 1-butyl-3-methyl Immidazolium chloride ionic liquid can only dissolve 10wt% cellulose efficiently. However, in this report, up to 20 wt% cellulose samples were efficiently dissolved and functionalized. More so, full reaction mechanisms of various derivatives prepared are presented.

This research focused on the derivatization of cellulose from native agricultural wastes generated from, Nigeria, western Africa. Cellulose was extracted from local agricultural wastes, purified and treated; Ether, acetate and phthalate derivatives of the native cellulose were also synthesized by dissolving the cellulose homogenously in 1-butyl-3-methylimidazolium chloride [BMIM]Cl- ionic liquid with dimethyl sulfoxide (DMSO) used as a co-solvent to enhance cellulose dissolution. Desired results were obtained by optimizing various process parameters, and it was shown that with the discovery of ILs, more attention needed to be paid to the conversion of cellulosic wastes to beneficial and cheaper raw materials for laboratory and industrial use. This practice will not only eliminate or reduce wastes from the environment it also involves giving back to nature in a more beneficial way, what was taken from it (Nature) so that nothing becomes useless. More so, unlike other conventional solvents, Ionic liquids dissolve cellulose without destroying the cellulose structure. This is evident in the thermal stabilities and X-ray diffraction patterns of the derivatives prepared in this study.

Experimental

Sample Collection and Identification:

Rice husks were collected at a rice mill in the northcentral part of Nigeria, West Africa. Corn cobs were also collected in the same part of Nigeria but were picked at different locations because they were lying randomly around. Wood dust was collected at a local saw mill, and the plant sources of this dust were documented in local language. The botanical names of the plant species from which the wood dust came from were sourced at the herbarium of the Department of Plant Biology, University of Ilorin, Nigeria.

Materials: Agricultural wastes (rice husk (R. H), Corncob (C. C), Wood dust of Daniella olivera (DAN) and Planatus occidentalis (PLN.), Ficus platyphylla (FIC.), Cotton (CTN)).

Reagents: acetic anhydride, ethyl bromide, phthalic anhydride, 0.1 N HCl

Solvents: Dimethyl sulfoxide (DMSO), ethanol, acetone, dichloromethane, 1-butyl-3-methylimidazolium chloride (ionic liquid), distilled water.

All reagents and solvents used were purchased from Central Drug House, India and are all of analytical grade and used as purchased.

 Extraction and Purification of Cellulose

In a manner analogous to Arowona et al.,[17] the extraction of cellulose from various agricultural sources with slight modifications is analysed below:

Cellulosic (100 g) was weighed into a three-necked flask. One hundred grams of the sample was weighed and transferred into a 1000 mL beaker containing a solution of 100 g sodium chlorite adjusted to pH 4.0 using 10% acetic acid at 75 °C for 1 h for delignification. Then, 100 g of sodium chlorite was added while stirring continuously for another 1 h at the same temperature to further delignify the biomass material. The residue was washed with distilled water and ethanol until its pH became neutral, the resulting solution was filtered, and the residue was thereafter washed and dried in an oven at 60 °C for 5 h. Afterwards, the dried residue was treated with 10% potassium hydroxide for 6 h to complete the delignification process. The cellulose was washed with distilled water and ethanol and dried for 3 h at 60 °C. Thereafter, it was treated with sodium hypochlorite, washed with distilled water and ethanol and dried in an oven at 60 °C for 3 h. The cellulose samples were weighed, stored in sample bottles and kept in a cool dry place until further analysis.

Cellulose Dissolution and Functionalization

In a procedure similar to that reported by Liu et ai., 2007[18][19] with some modifications;

Approximately 15 g IL (1-butyl-3-methyl imidazolium chloride) was added to a three-necked quick fit flask, placed on a hot plate/magnetic stirrer and allowed to melt. Cellulose sample (3 g, 0.0185 mol of AGU unit and 20 w% of IL) was added to the melted ionic liquid in the flask. This mixture was stirred under a N2 atmosphere for 20 min at 100 °C. Approximately 10 mL of DMSO was added as a cosolvent to aid the homogeneous dissolution of cellulose while stirring continuously under N2. Corresponding amounts (0.0185 mol) of derivatizing agents (specifically, 1.9 mL of OAC2, 1.143 mL of CH3CH2Br, ands 2.74 g of PhAn) were added five times to aid the functionalization process. The reaction was stopped 2 hrs after the beginning of the reaction.

Regeneration of Cellulose Derivatives

The functionalized cellulose regenerated from the solution by pouring the whole reaction mixture into approximately 30 mL of cold acetone or dichloromethane. The Erlenmeyer flask containing the mixture was placed in an ice bath and triturated with a glass rod to facilitate the precipitation of the product. The precipitated solids (Cell. derivatives) and filtered under vacuum, the acetone/IL/DMSO mixture was set aside, and the residue was washed thoroughly with distilled water and ethanol to remove excess reagents (acetic anhydride, phthalic anhydride or ethyl bromide). Solvent (acetone or dichloromethane) was removed in vacuo from the IL-DMSO mixture. The IL-DMSO mixture was reused for another (up to four times) batch of cellulose dissolution and functionalization.

Calculation of the Percentage Yield of Cellulose Derivatives

 The percent yield of the derivatives was calculated with respect to the final product obtained. In this procedure, the molar equivalence of cellulose derivatives was determined from a balanced equation of reaction. A 3 g (0.0185 mol anhydroglucoe unit of cellulose) of cellulose produces 0.0185 mol of the product theoretically. This is the ‘theoretical yield’. The ‘experimental yield’ was calculated from the corresponding number of moles of mass of the product obtained. The %yield is calculated as follows equation (1):

  

Equations of Reaction

Cellulose acetate

Cellulose (3 g, 0.0185 mol of AGU) + 5Acetic anhydride 5(1.9 g, 0.0185 mol)---- Cellulose acetate (xg, xmol)

Cellulose Phthalate

Cellulose (3 g, 0.0185 mol of AGU) + 4 Phthalic Anhydride(4(2.7 g, 0.0185 mol)----   Cellulose Phthalate (xg, xmol)                                           

Ethyl Cellulose

Cellulose (3 g, 0.0185 mol of AGU)+ 4Ethylbromide 4(5.72 mls, 0.0185 mol)---- EthylCellulose (xg, xmol)

 

Calculation of Degree of Substitution

The degree of substitution was determined to investigate the number of hydroxyl groups (-OH) that have been substituted. The possible sites of derivatization on the cellulose molecule include C-2, C-3 and C-6, with the possible order of derivatization being C-6 > C2 > C-3. This determination was in a manner similar to Liu et al., 2007 with slight modification. In this method, a desired weight of the sample was dissolved in 10 mL of 0.1 M NaOH by stirring at 50°C for 30 mins. The mixture was allowed to cool at room temperature, after which 6 drops of phenolphthalein indicator was added to the sample and titrated against standard 0.025 M HCl solution. The sample preparation and titration were repeated three times, and the average volume of HCl consumed was used for the calculation.

The degree of substitution was calculated from the Eq. (2a) below:

$$DS=\frac{162 *nCOOH}{m}-\left(X*nCOOH\right)--------------\left(2a\right)$$

m = mass of sample taken, X = the net increase in the mass of an AGU for each acetyl, phthaloyl and –ethyl substituted.

nCOOH = the amount of COOH calculated from the obtained value of the equivalent volume of known molarity HCl from the equation below;

$$nCOOH=\frac{\left[\left(MV\right)NaOH-\left(MV\right)HCl\right]}{2}-----------------\left(2b\right)$$

(MV)NaOH = number of moles of NaOH; (MV)HCl = number of moles of HCl

Fourier Transform Infrared Spectroscopy

FT-IR spectroscopy was used to identify the chemical composition of each sample by defining the functional groups. To achieve this, a few milligrams (mg) of dried cellulose and microcrystalline cellulose (MCC) samples were mixed with potassium bromide (1:90) and compressed into transparent tablets using a hydraulic press (M-15, Technosearch) to enable electromagnetic radiation to pass through easily. Then, in the range of 4000–400 cm-1, the FT-IR machine (ALPHA-II, Bruker, Germany) was used to analyse the transparent tablets.

X-ray Powder Diffraction (XRD) Spectroscopy

The degree of crystallinity of the samples was determined by X-ray powder diffraction spectroscopy. The X-ray diffractometer machine (Make: PANalytical, Model: X’pert PRO, Netherlands) powered by a 40 kilovolt X-ray generator at an input of 30 Ma with Cu K alpha radiation was used to analyse the samples.

Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS)

Scanning electron microscopy (SEM) was employed to determine the surface morphology coupled with EDS to determine the elemental composition. A few milligrams of the samples were coated with gold to make them conductive to obtain clear images. The SEM images were obtained with the aid of an SEM machine (Smart SEM, Version 6.07, service pack 10, ZEI 33, Germany). 

Thermogravimetric/Differential Thermal Analysis (TGA/DTA) and Differential Scanning Calorimetry (DSC)

Thermogravimetric analysis (TGA) was carried out on the samples to monitor the thermal degradation pattern, while DTA was used to measure the residual weight loss. TGA measurements were performed using a STA449 instrument (F3, Netzsch, Germany) under a nitrogen atmosphere (40 mL/min), and the samples were heated at 10°C/min from 50°C to 850°C. The residual weight loss was evaluated by measuring the residual weight loss at 850°C. DSC measures the heat flow into and out of the sample with respect to temperature.

Results And Discussion

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 Tvalues 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.

Conclusion

Cellulosic agricultural wastes were obtained from different sources and purified. FTIR, XRD and SEM/EDS analyses revealed similarities in their functional groups, X-ray diffraction patterns and crystallinity indices as well as their appearance and elemental compositions. The preparation of derivatives (cellulose acetate, cellulose phthalate and ethyl cellulose) followed the extraction and purification of cellulose from their respective sources in the IL. Spectroscopic studies of the prepared derivatives revealed the absence of residual IL by the nonappearance of an –NH absorption band as a sharp peak at approximately 3300 cm− 1 in the FTIR absorption spectra as well as the absence of elemental nitrogen (N) peaks in the EDS diffractogram of the derivatives. It is worthy of note that cellulosic agricultural wastes are economic sources of good grade cellulose for use in diverse areas of endeavors such as in biomedicals, wastes water treatment, pharmaceuticals and so on. More so, processing the cellulosics in Ionic liquid is an easy, non-destructive and environmentally friendly method of conversion of cellulosic wastes into useful raw materials (regardless of their (wastes’) species or origin).

Declarations

ACKNOWLEDGMENT

The authors acknowledge the Council for Scientific and Industrial Research-Institute of Minerals and Materials Technology (CSIR-IMMT), Bhubaneswar, Odisha India for providing funds to purchase the reagents needed for this research and giving access to the equipment used for the analysis throughout the course of this research. The corresponding author also thanks The Director, CSIR-IMMT and Head of the Environment and sustainability and Dr. T. Das (Senior Principal Scientist) for giving the needed support in the course of the research. Special appreciation to TWAS-UNESCO for granting the fellowship, 2021 (FR number: 3240319608) by providing travel funds and monthly stipend.

CONFLICT OF INTEREST

All authors declare that they have no conflict of interest.

AUTHORS CONTRIBUTION 

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by; Baker Mariam Temitope, Oguntoye Steven Olubunmi and Ramasamy Boopathy. Baker Mariam Temitope wrote the first draft of the manuscript and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Schemes 1-4

Schemes 1-4 are available in the Supplementary Files section.