Characterization of ionic liquid
The laboratory synthesized ionic liquids were analyzed with FTIR to characterize the target products. The important FTIR spectrum peaks for [Bmim]Cl and [Amim]Cl were: 3150 cm− 1 (C-H of imidazole ring), 3,110 cm− 1 (= C-H of imidazole ring), 1,430 cm− 1 (C-H of = CH2 structure), 1,570 cm− 1 (C-N of imidazole ring) and 621 cm− 1 (C-Cl). Additional of peaks 1,650 cm− 1 (C = C, allyl) and 951 cm− 1 (R-CH = CH-H) for [Amim]Cl (Xing et al., 2014).
[Bmim]Cl spectrum at Fig. 3(a) presented wave number 3110 cm− 1, 1570 cm− 1 and 1461 cm− 1 were the imidazole ring skeletons vibration absorption peaks. The characteristic bands at 1168 cm− 1 was assigned to the methyl hydrogen’s deformation absorption while the skeleton vibration of C-C was at 1569 cm− 1. Peak at 1168 cm− 1 and 620 cm− 1 were the inner and outer bending vibration of C-H and C-Cl respectively. The peaks of wave number 3460 cm-1 and 3360 cm-1 which were attribute to the absorption of n-butyl hydrogen. Determination of the peaks were indicated that the [Bmim]Cl was synthesized accordingly.
The functional groups and their wavelengths of [Amim]Cl were showed in Fig. 3(b). Imidazole ring skeletons vibration absorption peaks indicated at peak 3150 cm− 1 (alkyl group), 1430 cm− 1and 1570 cm− 1 (C = C stretching vibration) and 1165 cm− 1 (HCC). 1639 cm− 1 was the peak for1 C = C, allyl group while peak at 951 cm− 1 was the R-CH = CH-H. According to the analysis results, the specific functional groups are consistent with the results analyzed by and the results proved that the target product was successfully synthesized.
Comparison of biomass composition by Van Soest and TGA method
The biomass composition analysis by Van Soest and TGA were compared and showed at Table 2. The result was described as composition of moisture, cellulose/hemicellulose and lignin, ash and other. Cellulose and hemicellulose contents were presented together as sugar sources is come from both of them. The experiment result showed that, among the other biomass components, cellulose/hemicellulose content was the largest composition for both BSW and SCB. Cellulose/hemicellulose content of SCB, showed 2% significant difference which were 74.03% by Van Soest and 71.99% by TGA method. Lower cellulose/hemicellulose content from Van Soest of SCB may because that certain percentage of soluble hemicellulose might have been dissolved in neutral detergent solution (Hindrichsen et al., 2006). Nevertheless, BSW shown very near cellulose/hemicellulose content by both Van Soest and TGA method. BSW contained 66.12% and 65.13% of cellulose/hemicellulose as identified by Van Soest and TGA method respectively. The finding showed that TGA was a comparative method with Van Soest on quantifying cellulose/hemicellulose content of lignocellulose biomass even though there were a slight different. Moisture content determination by both methods were consistent as the results were very close for Van Soest and TGA method. Biomass composition analysis by TGA method seems to be quite reliable because the value where close to the data from Van Soest method.
Table 2
Biomass waste compositions by Van Soest and TGA measurement
Composition (%) | Bamboo stem wall | Sugar cane bagasse |
Van Soest | TGA | Van Soest | TGA |
Moisture content | 5.69 ± 0.39 | 5.66 ± 0.02 | 7.16 ± 0.10 | 6.49 ± 0.43 |
Cellulose & hemicellulose | 66.12 ± 1.45 | 65.13 ± 1.86 | 74.03 ± 1.77 | 77.63 ± 1.07 |
Lignin, ash & others | 28.20 ± 0.62 | 26.33 ± 0.71 | 18.81 ± 0.62 | 21.53 ± 0.43 |
Total | 100 | 100 | 100 | 100 |
Biomass waste composition by TGA method
All the biomass waste’s chemical compositions were analyzed with TGA method. According to the literature on pyrolysis lignocellulose components, hemicellulose started the weight loss at temperature 220–315 °C, follow by cellulose that degraded at temperature from 315 to 400 °C, while lignin have widest temperature range of degradation which at 100 °C to 900 °C. Figure 4(a) shows the pyrolysis curve of scrap paper cups (SPC) in TGA analysis. The first weight loss of SPC occurred at room temperature (RT) to 100 °C, it accounts for 7.52% of total weight, which indicate the water content in the SPC. From the pyrolysis curve, it was determined the hemicellulose and cellulose composition of SPC which are 58.50% and 17.17% respectively. There might be some weight of lignin together with the composition of cellulose and hemicellulose because of the wide degradation of lignin. Nonetheless, as the degradation of lignin is very low mass loss rate (< 0.14 wt%/°C) the amount can be negligible. The SPC pyrolysis curve has comparable study of Dikobe and Luyt (2010) which reported on the properties of PP/LLDPE/wood composites polymer. There were identified on the SPC pyrolysis curve, 11.46 wt% weight loss at 400–500 °C and this signal is visibly known as linear low-density polyethylene (LLDPE) material. Above 500 °C, some residual was decomposed and identified as ash. The composition of scrap paper cup, rice husk, sugar cane bagasse and oil palm empty fruit bunch were shown in Table 3. From the result analyzed by TGA method, bamboo stem wall contains 27.35% cellulose, 43.14% hemicellulose, 19.39% lignin and the rest of 6.94% was the ash and others. The cellulose, hemicellulose and lignin contents were congruent with chemical composition of bamboo studied by (Sharma et al., 2018) and (Sulaiman et al., 2016) which the analysis was performed in TAPPI and NREL method respectively. The studies found that the SCB chemically composed of 27.35%, cellulose, 28.71% hemicellulose, 26.29% lignin and others. The cellulose content was low if compare to study did by Rocha et al., (2012), while the hemicellulose and lignin content is quite similar with the research finding even though the analysis was performed by NREL method. Oil palm empty fruit bunch contains cellulose 16.11%, hemicellulose 45.36%, lignin 22.61% and 7.55% of ash or others. The cellulose content was also low compared to OPEB chemical composition found by (Nur Farahin et al., 2018)(Abdul et al., 2016) (Tajuddin et al., 2019) which the cellulose content is around 31–42% of total composition. The hemicellulose composition in this study was determined high compare to other findings. Nevertheless, proximate analysis of OPEFB pyrolysis by TGA performed by Alias et al. (2014) shown similar result with this findings. The difference may due to the influence of lignin, because the pyrolysis range of lignin covers the entire pyrolysis process.
Biomass waste dissolution and recovery
The experiment result showed that the laboratory prepared ionic liquids [Amim]Cl and [Bmim]Cl were able to dissolved completely the SPC, BSW, SCB and OPEFB biomass waste at temperature 120 °C. The complete dissolution of the biomass waste was confirmed though scanning electron micrograph (SEM) images as shown at Fig. 5. Dissolution mechanism of cellulose in ionic liquids is based on capability of ionic liquid’s anions to effectively break the extensive intra and inter molecular hydrogen bonding network in cellulose. [Amim]Cl ionic liquid is composed of [Amim]+ cation and Cl− anion. While, [Bmim]Cl has [Bmim]+ cation and also Cl− anion. Cl− anion of the ionic liquid acts as the hydrogen bond acceptor in dissolution where it interacts specifically with the hydroxyl protons of the cellulosic materials and facilitates the formation of hydrogen bonds between cellulose and ionic liquid (Gogoi & Hazarika, 2017; Gupta & Jiang, 2015; Hou et al., 2017; Liu et al., 2019). Ionic liquid with strong hydrogen bonds are effective in weakening the hydrogen bonding network of the polymer chain. The cation of ionic liquid also can indirectly influence the dissolving ability by impacting their physical properties, such as melting point, density and viscosity. The researcher also found that weak hydrogen bond present between the oxygen atom of cellulose and the acidic hydrogen ring ofcation. Small van der Waals interaction formation between of glucose and cations.
Table 3
Biomass waste compositions by TGA measurement
Composition (%) | Scrap paper cup | Bamboo stem wall | Sugar cane bagasse | Oil palm empty fruit bunch |
Moisture content | 7.52 | 5.66 ± 0.02 | 6.49 ± 0.43 | 8.37 ± 0.23 |
Cellulose & hemicellulose | 75.67 | 65.13 ± 1.86 | 71.99 ± 1.07 | 61.47 ± 2.50 |
LLDPE | 11.46 | - | - | - |
Lignin, Ash and others | 4.47 | 29.21 ± 1.43 | 21.53 ± 0.43 | 30.16 ± 0.97 |
Total | 100 | 100 | 100 | 100 |
After complete dissolution process of biomass waste, the mixture was appeared dark brown in color which is imparted by the dissolved lignin of the lignocellulosic matrix in the ionic liquid. Researchers were reported that the ionic liquid [Bmim]Cl and [Amim]Cl also capable to dissolve lignin (Hou et al., 2017; Moniruzzaman & Goto, 2018). Some amount of water was added for the fractionation of the lignocellulosic components. Two layer of mixtures were form as shown at Fig. 6.
Lignin has well-known properties as water impermeability, hydrophobicity and film-forming ability. Once the lignocellulose biomass was dissolved, it can be fractionated to its principle component by adding anti-solvent. In this experiment, water was used as the anti-solvent. The dissolution of lignin is more tolerant to water compared to the dissolution of cellulose. High solubility of lignin can be achieved in the mixture of ionic liquids and water, while the presence of water has negative impact on the dissolution ability of cellulose in ionic liquid. They also found that addition of applicable amount of water to ionic liquid can significantly increase the lignin solubility. This is due to the increased interaction probability between lignin and free ions, which the amount and mobility were remarkably increased after the addition of water (Hou et al., 2017). Thus, addition of water in a complete dissolution of lignocellulose with ionic liquid is applicable for fractionation of cellulose, lignin and the ionic liquid recyclability. The lignin separated from lignocellulose biomass is not only a desirable for preparation of lignin-free renewable feedstock for the production of bioenergy, but the extracted lignin also can be a promising starting material for production of novel materials. Therefore, the extraction of lignin from biomass has greatly improve the biorefinery productivity and profitability.
Further washing of cellulose solution resulted a smooth and transparent layer of regenerated cellulose as shown at Fig. 5 (a). Theoretically, the regeneration of cellulose was started when water is added in the dissolve cellulose with ionic liquid. This is because, the hydrogen bonds that formed during dissolution process, between the hydroxyl group of cellulose and Cl− anions are dismissed by adding more H2O. The regenerated cellulose was converted to cellulose II from cellulose I in original raw biomass. Addition of water was promoted formation of hydrogen bond of water with anions Cl− and resulted the hydrogen bonds between cellulose are connected again leading to precipitation (Gupta & Jiang, 2015). It was observed that the surface morphology and cellulose structure of the cellulose was change significantly. Subsequently, relative homogeneous textures were displayed as shown in SEM images (Fig. 5b, d, f, h).
Table 4 showed the results of cellulose recovery from the ionic liquid and the ionic liquid recovery. The cellulose recovery percentage of SPC, BSW, SCB and OPEFB were 96.00 wt%, 91.34 wt%, 87.16 wt%, and 91.14 wt% respectively. SPC recorder the highest cellulose recovery compare to other biomass because it contained lesser amount of lignin component (less that 5 wt%) as reported previously at Table 1. It was supported by Hou et al., that dissolution of polysaccharide which are not bonded with lignin or fewer lignin composition is easier that the dissolution of supermolecular network of complex entanglement lignocellulose compounds (Hou et al., 2017).
As much as ionic liquid cost is important, the ionic liquid recycling rate is a key variable for the process economics perspective. In this experiment, the recycled ionic liquids were highly recovered by vacuum distillation. The ionic liquid recovery rate after dissolution of SPC, BSW, SCB and OPEFB were 94.0%, 95.04%, 97.12% and 98.15% individually. The purity of the recycle ionic liquids were proof by FTIR analysis at Fig. 7. The result shows that the FTIR spectra of recycled ionic is principally consistent with the original.
Table 4
The cellulose/hemicellulose and ionic liquid recovery percentage
Biomass waste | Ionic liquids | Cellulose/hemicellulose recovery (wt%) | Ionic liquids recovery (wt%) |
Scrap paper cups | [Bmim]Cl | 96.00 | 94.00 |
Bamboo stem wall | [Bmim]Cl | 94.34 | 95.04 |
Sugar cane bagasse | [Amim]Cl | 87.16 | 97.12 |
Oil palm empty fruit bunch | [Amim]Cl | 91.14 | 98.15 |
The SPC, IL-SPC, IL-BSW, IL-SCB and IL-OPEFB were hydrolyzed by 10 M sulfuric acid at temperature 65 °C to produce fermentable reducing sugar. In the acid hydrolysis process, glycosidic bonds between the cellulose molecules are break to form monosaccharides. Literatures reported that, compare to other method of hydrolysis, acid hydrolysis promising higher sugar yield and good reproducibility. However, the method will produce large amount of degradation products from the monosaccharids such as furfural generated from pentose and hydroxymethylfurfural produced from hexose which further degrade into formic acid. The composition of the biomass waste hydrolysate including the sugars and degradation products were shown at Table 5. From the experimental data, IL-SPC hydrolysate contained higher glucose compare to SPC. It is because, IL-SPC cellulose has undergo ionic liquid dissolution which resulted higher surface area and disintegration of the cellulosic fibril structure then provide better accessibility for the hydrolysis process (Jeong et al., 2018; Loow et al., 2016). Almost negligible amount of xylose (0.29 g/L) was found and neither furfural nor HMF were detected in the hydrolysate of IL-SPC compare to SPC’s hydrolysate. It shows that, the ionic liquid dissolution was removed most of hemicellulose components from the SPC which also contribute to high glucose conversion from IL-SPC (Kim, 2018). This result also supported by the result of biomass hydrolysis (IL-BSW, IL-SCB and IL-OPEFB) where their total sugar are higher that the SPC even though, their cellulose content are lower that the SPC. Besides glucose, sucrose, arabinose and xylose, biomass waste hydrolysate contained other unknown soluble sugar. The unknown soluble sugar may consist of other monomers of hemicellulose such as galactose and mannose. Besides monomer, the soluble sugar can also be cello-oligomer and xylo-oligomers. Oligomer is a molecule that consists of a few monomers units which can be dimer, trimer, and tetramer, depends on the number of monomers, two, three or four. Dimer can be digest by the bacteria but trimer oligomers are cannot (Nivea et al., 2006).