O R I G I N A L R E S E A R C H
Alkaline deep eutectic solvent: a novel green solvent for lignocellulose pulping
Wei-Lun Lim.Ahmad Anas Nagoor Gunny.Farizul Hafiz Kasim. Inas Muen AlNashef.Dachyar Arbain
Received: 22 February 2018 / Accepted: 24 February 2019 ÓSpringer Nature B.V. 2019
Abstract This work studied the feasibility of potas- sium carbonate-glycerol deep eutectic solvent (K2CO3-Gly DES) as a potential green solvent applied in lignocellulose pulping. Cellulose fibers were extracted from rice straw via novel alkaline DES pulping technique using 1:7 molar ratio of K2CO3-Gly DES. Optimum pulping parameters were determined using the one-factor-at-a-time (OFAT) method. The cellulose fibers were characterized for chemical com- position of cellulose, hemicellulose, lignin and extrac- tives. Changes in physical structure, chemical structure, morphological structure, functional groups and crystallinity index (CrI) were investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results revealed that the optimum pulping
temperature at 140°C, reaction time of 100 min and 1:10 rice straw to DES mass ratio produced the highest cellulose content of 73.8% for unbleached DES treated pulp. Chemical composition analysis and FTIR veri- fied that this alkaline DES pulping method was able to achieve partial removal of hemicellulose and lignin from lignocellulosic matrix. Moreover, XRD result demonstrated that the CrI of cellulose fiber increased from 52.8 to 60.0% after pretreatment. The cellulose fibers had diameters ranging from 3.58 to 5.68 lm.
This study proved that the specifically-designed K2CO3-Gly DES could successfully isolate cellulose from lignocellulosic biomass through alkaline DES pulping.
W.-L. LimF. H. KasimD. Arbain
School of Bioprocess Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia
e-mail: [email protected] F. H. Kasim
e-mail: [email protected] D. Arbain
e-mail: [email protected] A. A. N. Gunny (&)F. H. Kasim
Centre of Excellence for Biomass Utilization, School of Bioprocess Engineering, Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia
e-mail: [email protected]
A. A. N. Gunny
Department of Chemical Engineering Technology, Faculty of Engineering, Universiti Malaysia Perlis, Kampus UniCITI Alam, Sungai Chuchuh, 02100 Padang Besar, Perlis, Malaysia I. M. AlNashef
Department of Chemical Engineering, Khalifa University of Science and Technology, Masdar City Campus, PO Box 54224, Abu Dhabi, United Arab Emirates e-mail: [email protected]
D. Arbain
Center for Renewable Energy, STT-PLN, Jalan Lingkar Luar Barat Kosambi, Jakarta 11750, Indonesia https://doi.org/10.1007/s10570-019-02346-8(0123456789().,-volV)( 0123456789().,-volV)
Graphical abstract
Keywords Deep eutectic solventPotassium carbonateGlycerolCellulose pulpAlkaline DES pulping
Introduction
To date, pulping processes are still the most common techniques used to treat lignocellulosic biomass resources for the production of paper, fiberboard and membranes, as well as to convert cellulose-enriched pulp into nanocellulose (Garcı´a et al.2016; Pilate et al.
2002). Among all the available pulping processes, kraft pulping appears to be most dominant, with 90%
of total production capacity at the global scale (Azadi et al.2013). In kraft pulping, white liquor containing sodium hydroxide (NaOH) and sodium sulfide (Na2S) is generally used to dissolve lignin and hemicellulose at high temperature and pressure, while leaving cellulose relatively intact (Brandt et al.2013). Despite its efficiency and popularity, application of white liquor in kraft pulping releases volatile sulphur compounds such as hydrogen sulphide (H2S), sulphur dioxide (SO2), dimethyl sulphide (C2H6S) and metha- nethiol (CH4S) into the atmosphere, apart from contaminating water sources (Sun et al.2009).
During the past several decades, numerous alter- native pulping processes have been studied such as organsolv pulping (Jime´nez et al. 2002), acidic magnesium-based sulphite pulping (Marques et al.
2009) and sulphur-free soda pulping (Francis et al.
2006), which involved a variety of aqueous organic and inorganic solvents to degrade lignin and hemicel- lulose while retaining cellulose. At the beginning of the twenty-first century, Swatloski et al. (2002) discovered the potential of hydrophilic ionic liquids (ILs) for dissolving cellulose, thereby gaining sub- stantial interest from scientists in this field. Neverthe- less, there are several drawbacks in IL pretreatment of lignocellulose such as high synthesis cost, high toxicity, low biodegradability, long-term recyclability aspect of ILs and partial separation of cellulose from lignocellulose (Sun et al. 2011; Vigier et al. 2015).
Recently, DESs have been acknowledged as promis- ing green solvents in biological and chemical appli- cations. DES was first introduced by Abbott et al.
(2003). DES refers to a homogenous mixture com- posed of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) with melting point lower than its individual compounds (Choi et al. 2011; Dai et al.
2013). The charge delocalization that occurs through hydrogen bonding between HBD and HBA in the mixture is responsible for the decrease in melting point of DES (Abbott et al. 2001). Gorke et al. (2008) reported that DESs not only share similar physico- chemical properties with ILs, but are also environ- mental-friendly and relatively inexpensive. DESs are easy to synthesize in high purity and fine-tune for specific applications based on the compositions of HBD and HBA to produce low cost, low toxicity and biodegradable solvents (Garcı´a et al. 2015). DESs
have been widely applied in organic synthesis (Azizi and Manocheri 2012), materials dissolution (Abbott et al.2011), biomass hydrolysis (Gunny et al.2015) and DES pulping (Sˇkulcova´ et al. 2017). Although DES has shown the ability to dissolve lignocellulosic components, its application in biomass processing or pulping is still scarce (Kumar et al. 2016; Xu et al.
2016).
This study focused on lignocellulose pulping using DES, which is composed of an alkali metal-based salt and a polyol compound. Glycerol, a polar, non-toxic, colorless, odorless and viscous liquid, commonly generated as a major by-product in the biodiesel industry. Glycerol has three hydroxyl groups that facilitate dissolution of inorganic salts, acids, bases and transition metal complexes (Mjalli et al. 2014;
Wolfson et al.2009). It has been successfully utilized as a green solvent and hydrogen donor in catalytic transfer hydrogenation-dehydrogenation reactions (Wolfson et al. 2009), potential hydrogen donor solvents in biomass liquefaction (Isa et al.2018) and as an alternative organic solvent in lignocellulosic biomass pretreatment prior to enzymatic hydrolysis (Sun and Chen 2008). Although glycerol has been applied in various pretreatment methods in the past few years, Lynam and Coronella (2014) reported that glycerol alone was unable to remove lignin and hemicellulose from the lignocellulosic matrix at low pretreatment temperatures (\110°C). However, rais- ing the temperature over 110°C and mixing small amounts of acid, alkali or IL with glycerol may result in an effective delignification (Ebrahimi et al.2017).
Owing to this, glycerol has been successfully used as IL co-solvent for biomass pretreatment and HBD in synthesis of choline chloride-based DES (Lobo et al.
2012). Potassium carbonate (K2CO3), a white salt that is readily soluble in water to form alkaline solution (Mjalli et al.2014). K2CO3has been used for ages to produce soaps due to its basicity in nature and synthesis of agricultural fertilizer since potassium element is an essential micronutrient for plant growth.
K2CO3 has low heat capacity and deliquescence properties to function as promoters for primary or secondary amines in bulk carbon dioxide (CO2) removal. Numerous studies have been conducted by mixing diamine piperazine with K2CO3 to enhance CO2absorption due to its low cost, large capacity, ease of handling and regeneration capability (Park et al.
1998). Several research groups have utilized K CO as
base-catalyst in organosolv lignin depolymerization process (Toledano et al.2012) and HBA in synthesis of alkali metals-based salts DES that analogues to IL.
Nonetheless, the discovery of K2CO3-Gly DES by Mjalli et al. (2014) has yet to be tested in any chemical or biological application.
This study probed into the potential of alkaline DES with K2CO3as HBA and glycerol as HBD on the first attempt in lignocellulose pulping. K2CO3-Gly DESs with varied molar ratios were prepared and character- ized by measuring relevant physicochemical proper- ties such as pH, viscosity and thermal stability.
Optimum pulping parameters for DES pulping such as pulping temperature, reaction time and rice straw to DES mass ratio were determined by integrating the OFAT method. Chemical compositional analyses of rice straw, unbleached DES treated pulp and bleached DES treated pulp were performed to compare the contents of cellulose, hemicellulose, lignin and extractives. The chemical structure, functional groups, crystallinity index and surface morphology were assessed using FTIR, XRD and SEM, respectively.
Materials and methods
Materials
Rice straw was collected from local paddy field (Perlis, Malaysia) and used as lignocellulosic biomass feedstock to produce cellulose pulp. It was thoroughly washed with distilled water and dried in an oven at 105 °C for 24 h. Oven-dried rice straw was ground using a high speed grinder and sieved to pass an 80 mesh (0.2 mm opening). Potassium carbonate (99%), anthrone (97%), glycerol ([98%) and sulphuric acid (95–97%) were purchased from Sigma-Aldrich and were used without further purification.
Preparation and physicochemical properties measurement of K2CO3-Gly DES
Potassium carbonate (K2CO3) and glycerol were used to prepare 7 samples of K2CO3-Gly DESs with different molar ratios as shown in Fig.1. K2CO3and glycerol were mixed homogenously at 80°C for 120 min until transparent colorless mixture was formed. DESs were synthesized at atmospheric pres- sure and under tight control of moisture content. DESs
were poured into universal bottles and placed inside a vacuum desiccator to prevent structural changes and humidity effects that may affect the physicochemical properties (Mjalli et al.2014). DESs were dried under vacuum for overnight to eliminate excessive moisture prior to measuring pH, viscosity and thermal stability.
The pH was measured using a pH meter (OHAUS STARTER 300) while the viscosity was determined using a Brookfield DV-I Prime Viscometer with spindle type S14 and thermal stability was character- ized using a Mettler Toledo Thermogravimetric Ana- lyzer (TGA/SDTA 851).
Screening of optimum pulping parameters level using the OFAT method
The influences of pulping temperature, reaction time and rice straw to DES mass ratio were investigated using the OFAT method. The variable parameters were determined for different levels at a time while keeping the other factors constant (Czitrom 1999).
This method determined the optimum pulping param- eters level in producing pulp for further optimization studies. Rice straw was treated with K2CO3-Gly DES at pulping temperature between 110°C and 150°C, reaction time between 40 and 120 min and rice straw to DES mass ratio between 1:8 and 1:12.
Isolation of cellulose fibers from rice straw
Weights of rice straw and K2CO3-Gly DES were reported based on the solid to liquid mass ratio.
K2CO3-Gly DES was preheated using a paraffin oil bath to the desired temperature. About 10 g of screened rice straw was slowly added into 100 g of preheated DES with stirring at 300 rpm. The solid–
liquid mixture was heated with optimized parameters level obtained from the OFAT method. At the end of DES pulping, the solid residue was filtered and washed repeatedly with distilled water until the pH of the filtrate was close to neutral. The washed solid residue was then transferred into a weighing dish and beaten into a thin pulp sheet. The unbleached DES treated pulp sheet was air-dried for 2 days prior to character- ization and chemical compositional analysis. About 5 g of the thin unbleached DES treated pulp sheet was finely ground using a high speed blender for further bleaching. The screened unbleached DES treated pulp was treated with 1.7 wt% sodium chlorite (NaClO )
solution and acetate buffer (NaOH and acetic acid) at 80°C for 2 h. The bleached solid residue was washed repeatedly with distilled water until the pH of the filtrate reached neutral before transferring into a weighing dish and beaten into a thin pulp sheet. The bleached DES treated pulp sheet was air-dried for 2 days before the following characterization and chemical compositional analysis.
Chemical compositional analysis
The extractives content of rice straw, unbleached DES treated pulp and bleached DES treated pulp were determined using the Soxhlet method. About 2.5 g of sample was loaded into the cellulose thimble. Approx- imately 150 ml of acetone was used as the solvent for extraction process. The acetone was heated at tem- perature of 70 °C for 4 h. The sample was air dried at room temperature before drying in an oven at 105 °C for 24 h. The extractives content of sample was estimated gravimetrically (Ayeni et al. 2013). For hemicellulose content, about 0.1 g of extractives free sample was added into a 250 ml Erlenmeyer flask filled with 10 ml of 0.5 mol/l NaOH solution. The mixture was heated for 3.5 h in a boiling water bath. It was then cooled, filtered through vacuum filtration and washed with distilled water until the pH of filtrate approached neutral. The solid residue was dried to a constant weight at 105 °C. The solid residue was then cooled in a desiccator and weighed. The hemicellulose content was estimated gravimetrically (Lin et al.
2010). Lignin content of extractives free sample was determined by treating the sample with diluted H2SO4 solution. About 0.1 g of sample was weighed and added into a test tube. One ml of 72% H2SO4 was introduced into the test tube, which was placed in water bath at 30 °C. After 1 h, the mixture was diluted and transferred quantitatively into a screw cap bottle with 28 ml of distilled water. Next, the mixture was autoclaved at 121 °C for 1 h and filtered using a fritted glass crucible. The residue was washed with hot distilled water to remove excessive acid. The fritted glass crucible containing lignin residue was dried to a constant weight at 105°C. The lignin content was estimated gravimetrically (Sluiter et al. 2008). The cellulose content of extractives free sample was determined using the Updegraff method (Updegraff 1969). About 0.1 g of sample was added into a test tube filled with 3 ml of acetic/nitric acid reagent. The
test tube was submerged in a boiling water bath for 30 min. The test tube was left to cool before centrifuging for 40 min at 3000 rpm. The supernatant was discarded and the residue was rinsed with distilled water. Ten ml of 67% H2SO4was added into a test tube before transferred into a 50 ml round bottle flask. The mixture was stirred at room temperature for 60 min.
One ml of the mixture was pipetted into a volumetric flask and diluted to 100 ml with distilled water. The volumetric flask was shaken well and 1 ml of the diluted mixture was pipetted into another test tube.
Ten ml of freshly prepared anthrone reagent was introduced into the test tube and placed in a boiling water bath for 10 min. The test tube was left to cool to room temperature and the color intensity of the mixture was measured using a UV–Vis spectropho- tometer at 630 nm wavelength with anthrone reagent set as blank sample. The absorbance value of each sample was recorded and the cellulose content was determined (Updegraff1969).
Analytical methods
FTIR spectra were measured using a Spectrum 65 FTIR spectrometer (Perkin Elmer) at ambient condi- tions. Samples were analyzed using a KBr pellet containing 1% finely ground sample. The spectra were recorded in the range of 4000–450 cm-1 using an accumulation of 16 scans. XRD patterns were obtained using a Desktop D2 Phaser X-ray diffrac- tometer (Bruker Corporation, USA) with CuKaradi- ation that was operated at 30 kV and 10 mA. The
measurements of each sample were collected at 2h angles between 10°and 40°with a scan rate of 0.5 s per step. The CrI was calculated based on diffraction intensity of crystalline and amorphous regions using the following equation (Segal et al.1959) as shown in Eq. 1:
CrI = ðI200IamÞ I200
ð Þ ð1Þ
where I200 is the peak intensity at (200) plane (crystalline and amorphous portions) and Iam is the minimum intensity between the (110) and (200) planes (amorphous portion). The crystallite size (L200) was evaluated using the Scherrer equation (Nazir et al.
2013) as shown in Eq. 2:
D = Kk
b1=2cosh ð2Þ
where K is the Scherrer constant (0.94), k is the wavelength of X-ray radiation (1.54056 A˚ ),b1/2is the full width at half maximum (FWHM) of the diffraction peaks and h is the Bragg angle. The surface mor- phologies were examined via SEM. The samples were mounted on aluminum sample holder with conductive carbon tape and sputtered with platinum using a JEOL JFC-1600 Auto Fine Coater. The coated samples were observed and imaged using a JEOL JSM-6460LA SEM at 10 kV with 509, 5009and 6000 9mag- nifications, respectively.
Fig. 1 7 samples of K2CO3-Gly DESs with different molar ratios
Results and discussion
In this study, 7 samples of K2CO3-Gly DESs were prepared at a fixed amount of K2CO3while varying the molar amount of glycerol. It can be seen from Fig.1 shows that a white precipitate was formed in DES-G1, DES-G2 and DES-G3 while homogenous colorless liquid without precipitation was observed in DES-G4, DES-G5, DES-G6 and DES-G7. The presence of white precipitate indicates that the molar amount of K2CO3in DES-G1, DES-G2 and DES-G3 is greater than glycerol, and therefore failed to achieve good hydrogen bonds between HBA and HBD (Mjalli et al.
2014). However, substantial addition of glycerol in K2CO3-Gly DES seemed to improve the solubility of white salt under mild condition. In this case, DES-G1, DES-G2, and DES-G3 were not studied further due to unsuccessful formation of homogenous colorless mixture. Physicochemical properties such as pH, viscosity and thermal stability of DES-G4, DES-G5, DES-G6 and DES-G7 are presented in Table1.
Physicochemical properties of K2CO3-Gly DESs Measurement of pH is important in studying the corrosiveness of solvent. In fact, DESs that were composed of K2CO3and glycerol recorded pH higher than 7. Table 1displays the pH values of K2CO3-Gly DESs as a function of molar ratios varied in the range between 12 and 14. DES-G4 generally had the highest pH at 13.6 while DES-G7 had the lowest pH at 12.3. It can be observed that the pH value decreased gradually as the ratio amount of glycerol in K2CO3-Gly DES was increased. A similar observation was reported by Mjalli et al. (2014), who explained that K2CO3 is responsible for the high alkalinity in K2CO3-Gly DES.
For viscosity, solvent with lower viscosity is prefer- able in the pulping industry as it minimizes operational cost in mixing and pumping the solvent (Ruß and Ko¨nig2012). Table1presents the viscosity of K2CO3- Gly DESs with varying molar ratios at ambient
temperature. From the table, DES-G4 recorded the greatest viscosity value of 25,000 cP, followed by DES-G5 (17,613 cP), DES-G6 (7838 cP) and lastly, DES-G7 (6638 cP). Increment in the molar amount of glycerol in K2CO3-Gly DES seemed to substantially reduce the viscosity, wherein a similar trend was also reported by Mjalli et al. (2014). Nevertheless, this effect decreased at very high glycerol compositions.
For thermal stability, solvent with high thermal stability has greater resistance towards decomposition at elevated temperature. Thermal stability of solvent is often characterized by onset temperature (Tonset), which can be determined via step tangent method (Cao and Mu2014; Chancelier et al. 2014). Table1 shows the onset temperature (Tonset) of K2CO3-Gly DESs at various molar ratios. DES-G7 exhibited the highest Tonsetat 277.2°C in comparison to DES-G4 at 262.1°C. From industrial application point of view, a solvent must be able to withstand high temperature for certain period of time. In this case, DES-G7 was selected as the most suitable DES for the following lignocellulose pulping in this work due to its lower pH value, lower viscosity and higher thermal stability compared to the other DESs. Overall, the molar ratio of HBA and HBD did affect the physicochemical properties of the resulting DESs. These results are in agreement with those reported by Mjalli et al. (2014) and other research groups. Figure2 displays the proposed schematic illustration of the K2CO3-Gly DES formation mechanism.
Optimization of pulping conditions using the OFAT method
Effect of pulping temperature
The effect of pulping temperature on cellulose content was analyzed under 60 min of reaction time and 1:9 rice straw to DES mass ratio, as portrayed in Fig.3.
Preliminary screening involving several sets of exper- imental work indicated that 60 min of reaction time
Table 1 Physicochemical properties of K2CO3-Gly DESs at different molar ratios
Abbreviation Molar ratio pH Viscosity (cP) Onset temperature (°C)
DES-G4 1:4 13.6 25,000 262.1
DES-G5 1:5 12.9 17,613 268.4
DES-G6 1:6 12.6 7838 273.8
DES-G7 1:7 12.3 6638 277.2
and 1:9 rice straw to DES mass ratio displayed the ability to dissolve lignin and hemicellulose between 110 and 150°C. Therefore, 60 min of reaction time and 1:9 rice straw to DES mass ratio were set as the initial fixed parameters level for the first set of the OFAT experiment to determine the optimum pulping temperature. It can be seen from Fig. 3shows that the cellulose content of unbleached DES treated pulp increased with increment in pulping temperature. The highest cellulose content of 59.7% was obtained at 140 °C. Nonetheless, the cellulose content started to decline after 140°C. Interestingly, a similar observa- tion was reported by Song et al. (2016), where pulp yield decreased considerably when pulping tempera- ture exceeded 140°C. Li et al. (2014) discovered that the bamboo pulp yield decreased steadily from 95.6 to 73.7% when the pulping temperature was raised from 140 to 170 °C. According to Xie et al. (2007), rice straw lignin is composed of phenolic hydroxyl and carboxyl groups, as well as p-hydroxyphenyl units, which connect with other lignin structure through ester
bonds. Failure of ester bond in lignin and partial dissociation ofa-ether bond allowed the straw lignin to be fragmented or dissolved during DES pulping at elevated temperature. Thus, pulping temperature at 140 °C was selected as the fixed parameter to study the subsequent effect of reaction time on cellulose content.
Effect of reaction time
The effect of reaction time on cellulose content was determined under 140°C of pulping temperature and 1:9 rice straw to DES mass ratio as illustrated in Fig.4.
The cellulose content of unbleached DES treated pulp increased with increment in reaction time. The highest cellulose content of 69.4% was recorded at 100 min.
However, the cellulose content decreased marginally after 100 min. According to Liu et al. (2015), the cellulose yield hit a plateau over 60 min of reaction time and the variance beyond that was insignificant. A longer period of reaction time may have reduced the cellulose yield, apart from increasing energy con- sumption (Song et al.2016). Thus, 100 min of reaction time and 140°C of pulping temperature were selected as fixed parameters to study the subsequent effect of rice straw to DES mass ratio on cellulose content.
Effect of rice straw to DES mass ratio
The effect of rice straw to DES mass ratio on cellulose content was studied under 140 °C of pulping temper- ature and 100 min of reaction time as illustrated in Fig.5. The cellulose content of unbleached DES treated pulp increased with increment of rice straw to DES mass ratio until it reached a maximum at 1:10, Fig. 2 Proposed schematic illustration of the K2CO3-Gly DES formation mechanism
0 10 20 30 40 5060 70 80 90 100
110 120 130 140 150
Cellulose content (%)
Pulping temperature (°C)
Fig. 3 Effect of pulping temperature on cellulose content with rice straw to DES mass ratio of 1:9 and reaction time of 60 min
attaining the highest cellulose content of 73.8%. At lower rice straw to DES mass ratio, rice straw was not completely infiltrated by DES, thus failing to dissolve both lignin and hemicellulose completely. However, the cellulose content started to decrease slightly when rice straw to DES mass ratio exceeded 1:10. Liu et al.
(2015) reported that solid to liquid mass ratio at 1:9
was the optimum parameter for IL pulping. In this case, rice straw to DES mass ratio at 1:10 was selected for lignocellulose pulping to prevent wastage of solvent. Based on the OFAT experiment, 140°C of pulping temperature, 100 min of reaction time and 1:10 rice straw to DES mass ratio were selected as the optimum pulping parameters that generated the high- est cellulose content in the following study of chem- ical composition and physicochemical properties analyses.
Chemical composition analysis
The chemical compositions of rice straw, unbleached DES treated pulp and bleached DES treated pulp are presented in Table 2. Cellulose, hemicellulose and lignin are the main compositions of lignocellulose, which account for 86.3% of the total dry matter in rice straw. This experimental result is close to the values reported by Rodrı´guez et al. (2010) and Ibrahim et al.
(2013). The composition change of rice straw is an important indicator to evaluate the effectiveness of its pretreatment methods. It can be seen that the cellulose content in rice straw increased from 35.6 to 73.8%, while hemicellulose, lignin and extractives contents decreased from 26.3 to 19.9%, 24.4 to 2.8% and 7.8 to 2.3%, respectively after DES pulping. These changes were explained by Pavlostathis and Gossett (1985), which included fiber swelling, removal of the associ- ation of lignin with carbohydrate, degradation and dissolution of lignin and hemicellulose, resulting improved accessibility of cellulose towards reagent under alkaline pretreatment condition. According to Fan et al. (2013), 83% of cellulose content in rice straw pulp was obtained after treated using potassium hydroxide and acetic acid for 5 h. Ibrahim et al.
(2013) obtained 64.7% and 66.2% cellulose in rice straw pulp after treated with alkaline-acid pulping with 10% NaOH and followed by 5% H2SO4, as well 0
10 20 30 40 50 60 70 80 90 100
40 60 80 100 120
Cellulose content (%)
Reaction time (min)
Fig. 4 Effect of reaction time on cellulose content with rice straw to DES mass ratio of 1:9 and pulping temperature at 140°C
100 2030 4050 6070 8090 100
1:8 1:9 1:10 1:11 1:12
Cellulose content (%)
Rice straw to DES mass ratio Fig. 5 Effect of rice straw to DES mass ratio on cellulose content with pulping temperature at 140°C and reaction time of 100 min
Table 2 Chemical compositions of rice straw, unbleached DES treated pulp and bleached DES treated pulp
Sample Cellulose (%) Hemicellulose (%) Lignin (%) Extractives (%)
Rice straw 35.6±0.636 26.3±0.100 24.4±0.200 7.8±0.004
Unbleached DES treated pulp 73.8±0.417 19.9±0.115 2.8±0.361 2.3±0.013
Bleached DES treated pulp 81.9±0.241 12.9±0.100 1.8±0.404 2.1±0.046
as acid-alkaline pulping with 5% H2SO4and followed by 10% NaOH, respectively. For bleached DES treated pulp, hemicellulose, lignin and extractives were substantially removed while leaving cellulose intact after bleaching treatment. The light brown unbleached DES treated pulp was significantly bleached into pure white pulp after delignification and removal of hemicellulose and lignin. The resultant white residue was further confirmed to be cellulose by FTIR spectra.
FTIR analysis
The FTIR spectra of rice straw, unbleached DES treated pulp and bleached DES treated pulp were recorded in wavelength ranging between 4000 and 450 cm-1as illustrated in Fig.6. A strong and broad band observed at around 3410 cm-1was assigned to – OH stretching vibrations of hydroxyl functional groups. The region that lies between 3800 and 3000 cm-1 covered the sum of the vibration of valence bands in hydrogen bond for –OH group, as well as the bands of intramolecular and intermolecular hydrogen bonds (Hinterstoisser and Salme´n 1999).
The –OH compounds basically include absorbed water, aliphatic primary and secondary alcohols that were found in cellulose, hemicellulose, lignin and extractives (Ibrahim et al.2011). The absorbance peak at 2917 cm-1 was associated with C-H stretching vibrations of alkyl groups. The shoulder peak at 1730 cm-1 in rice straw was attributed to (C=O) stretching vibrations of carbonyl groups. Carbonyls usually exist in the side chains of lignin structural units, wherein the disappearance of such band indi- cates the rupture of side chains linkage in lignin (He et al.2008). The absorption peak at 1640 cm-1was associated with deformation vibrations of H–OH in absorbed water, which decreased significantly due to the partial removal of hemicelluloses from lignocel- lulosic matrix (Chen et al. 2011; He et al. 2008).
Nevertheless, the reduction in relative absorption peak at 1515 cm-1 corresponded to aromatic skeletal vibrations (C=C), indicating the removal of lignin.
The absorbance band at 1466 cm-1was ascribed to C-H deformations of asymmetric in methyl, methy- lene and methoxyl groups. The absorbance peaks at 797 cm-1and 470 cm-1in rice straw were ascribed to Si–O–Si bending and Si–O–Si symmetric stretching
Fig. 6 FTIR spectra:arice straw,bunbleached DES treated pulp,cbleached DES treated pulp
vibrations, respectively. These peaks were diminished thoroughly, signifying the removal of silica from unbleached DES treated pulp and bleached DES treated pulp. According to Ibrahim et al. (2011), the absorbance peaks at 1430 cm-1, 1373 cm-1, 1320 cm-1, 1163 cm-1, 1059 cm-1 and 897 cm-1 were represented by typical cellulose. Therefore, these FTIR results verified that the extractives, hemicellu- lose and lignin were removed from the rice straw fibers through DES pulping and bleaching.
XRD analysis
XRD patterns for rice straw, unbleached DES treated pulp and bleached DES treated pulp showed similar diffraction patterns with three diffraction peaks at 2h= 14.6°, 16.7° and 22.5° as depicted in Fig.7.
These peaks were attributed to (1-10), (110) and (200) planes, respectively, all of which were in agreement with the characteristic diffraction peaks of cellulose I (Gong et al.2017; Lu and Hsieh2012). The CrI of rice straw, unbleached DES treated pulp and bleached DES treated pulp are calculated using Eq. (1) and the results are shown in Table3. It was found that the CrI of cellulose increased from 52.8% for rice straw to 54.5% for unbleached DES treated pulp and further increased to 60.0% for bleached DES treated pulp. The increase in CrI was attributed to the removal of not only non-cellulosic components (hemicellulose and lignin), but also amorphous regions of cellulose as well as structural changes in organization and align- ment to form highly ordered crystalline bundle (Oun and Rhim2016; Rahnama et al.2013). The crystallite size (L200) of rice straw was 3.19 nm and those for the
Table 3 Crystallinity index and crystallite size of rice straw, unbleached DES treated pulp and bleached DES treated pulp
Sample Crystallinity index (%) Crystallite size (nm)
Rice straw 52.8 3.19
Unbleached DES treated pulp 54.5 3.23
Bleached DES treated pulp 60.0 3.21
Fig. 7 X-ray patterns:
arice straw,bunbleached DES treated pulp,
cbleached DES treated pulp
unbleached DES treated pulp and bleached DES treated pulp were 3.23 nm and 3.21 nm, respectively.
The similar crystallite sizes of unbleached DES treated pulp and bleached DES treated pulp indicate that these dimensions approaching the elemental crystallite size of rice straw cellulose (Lu and Hsieh2012).
SEM analysis
SEM micrographs of rice straw, unbleached DES treated pulp and bleached DES treated pulp are shown in Fig.8. The SEM results indicate that the DES pulping and bleaching affect the morphological struc- ture of rice straw. According to the above discussion, non-cellulosic components such as lignin, hemicellu- lose and extractive substances were removed from rice straw, while retaining cellulose. From morphological analysis, a rough surface of the fiber can be observed
in rice straw before pretreatment. However, the morphology of unbleached DES treated pulp and bleached DES treated pulp was completely different from rice straw, depicted from Fig.8d–i. The smooth surface of isolated cellulose fibers was examined with average diameter of 3.58–5.68 lm. The thin sheet DES treated pulp consisted multiple well aligned bundles of cellulose fibers with obvious boundaries, which can be further broken down into individual cellulose fibers.
Conclusion
In this work, K2CO3-Gly DES was successfully applied as novel green solvent in lignocellulose pulping. The physicochemical properties of K2CO3- Gly DES including pH, viscosity and thermal stability Fig. 8 SEM images:a–crice straw,d–funbleached DES treated pulp,g–ibleached DES treated pulp under950,9500 and96000 magnifications, respectively
were measured and reported as functions with varying HBD molar ratios. As a result, 1:7 molar ratio of K2CO3-Gly DES exhibited the lowest pH, the lowest viscosity and the highest thermal stability, thereby selected as the most appropriate and potential pulping solvent. Preliminary studies on the optimization using the OFAT method were carried out to determine desirable pulping parameters. Pulping temperature of 140 °C, reaction time of 100 min and rice straw to DES mass ratio of 1:10 emerged as optimum pulping parameters. Chemical composition analysis showed that alkaline DES pulping was able to achieve 73.8%
of cellulose content under optimum pulping parame- ters. FTIR spectra displayed that several absorption bands were diminished in corresponding to the functional groups of lignin and hemicellulose. CrI of cellulose increased from 52.8 to 60% after DES pulping and bleaching. Hence, it is worth mentioning that the combination of non-toxic polyol compounds, such as glycerol and white salt (e.g. K2CO3) can successfully synthesize novel deep eutectic mixture for dissolution process. The rationale of studying alkaline DES in lignocellulose pulping arises from the need of developing a biodegradable, environmental- friendly and greener solvent that can substitute conventional organic or inorganic solvents used in the pulping industries.
Acknowledgments The authors would like to thank the School of Bioprocess Engineering, Universiti Malaysia Perlis (UniMAP) for their support in this research.
Compliance with ethical standards
Conflict of interest The authors declare no conflict of interest in the publication.
References
Abbott AP, Capper G, Davies DL, Munro HL, Rasheed RK, Tambyrajah V (2001) Preparation of novel, moisture- stable, Lewis-acidic ionic liquids containing quaternary ammonium salts with functional side chains. Chem Com- mun.https://doi.org/10.1039/b106357j
Abbott AP, Capper G, Davies DL, Rasheed RK, Tambyrajah V (2003) Novel solvent properties of choline chloride/urea mixtures. Chem Commun. https://doi.org/10.1039/
b210714g
Abbott AP, Frisch G, Hartley J, Ryder KS (2011) Processing of metals and metal oxides using ionic liquids. Green Chem 13:471–481.https://doi.org/10.1039/c0gc00716a
Ayeni AO, Hymore FK, Mudliar SN, Deshmukh SC, Satpute DB, Omoleye JA, Pandey RA (2013) Hydrogen peroxide and lime based oxidative pretreatment of wood waste to enhance enzymatic hydrolysis for a biorefinery: process parameters optimization using response surface method- ology. Fuel 106:187–194. https://doi.org/10.1016/j.fuel.
2012.12.078
Azadi P, Inderwildi OR, Farnood R, King DA (2013) Liquid fuels, hydrogen and chemicals from lignin: a critical review. Renew Sust Energy Rev 21:506–523.https://doi.
org/10.1016/j.rser.2012.12.022
Azizi N, Manocheri Z (2012) Eutectic salts promote green synthesis of bis(indolyl) methanes. Res Chem Intermed 38:1495–1500. https://doi.org/10.1007/s11164-011-0479- 4
Brandt A, Gra¨svik J, Hallett JP, Welton T (2013) Deconstruction of lignocellulosic biomass with ionic liquids. Green Chem 15:550–583.https://doi.org/10.1039/c2gc36364j
Cao Y, Mu T (2014) Comprehensive investigation on the ther- mal stability of 66 ionic liquids by thermogravimetric analysis. Ind Eng Chem Res 53:8651–8664.https://doi.org/
10.1021/ie5009597
Chancelier L, Diallo AO, Santini CC, Marlair G, Gutel T, Mailley S, Len C (2014) Targeting adequate thermal sta- bility and fire safety in selecting ionic liquid-based elec- trolytes for energy storage. Phys Chem Chem Phys 16:1967–1976.https://doi.org/10.1039/C3CP54225D Chen X, Yu J, Zhang Z, Lu C (2011) Study on structure and
thermal stability properties of cellulose fibers from rice straw. Carbohyd Polym 85:245–250. https://doi.org/10.
1016/j.carbpol.2011.02.022
Choi YH, van Spronsen J, Dai Y, Verberne M, Hollmann F, Arends IWCE, Witkamp G-J, Verpoorte R (2011) Are natural deep eutectic solvents the missing link in under- standing cellular metabolism and physiology? Plant Physiol 156:1701–1705. https://doi.org/10.1104/pp.111.
178426
Czitrom V (1999) One-factor-at-a-time versus designed exper- iments. Am Stat 53:126–131. https://doi.org/10.1080/
00031305.1999.10474445
Dai Y, van Spronsen J, Witkamp G-J, Verpoorte R, Choi YH (2013) Natural deep eutectic solvents as new potential media for green technology. Anal Chim Acta 766:61–68.
https://doi.org/10.1016/j.aca.2012.12.019
Ebrahimi M, Villaflores OB, Ordono EE, Caparanga AR (2017) Effects of acidified aqueous glycerol and glycerol car- bonate pretreatment of rice husk on the enzymatic digestibility, structural characteristics, and bioethanol production. Bioresour Technol 228:264–271. https://doi.
org/10.1016/j.biortech.2016.12.106
Fan G, Wang M, Liao C, Fang T, Li J, Zhou R (2013) Isolation of cellulose from rice straw and its conversion into cellu- lose acetate catalyzed by phosphotungstic acid. Carbohyd Polym 94:71–76. https://doi.org/10.1016/j.carbpol.2013.
01.073
Francis RC, Shin S-J, Omori S, Amidon TE, Blain TJ (2006) Soda pulping of hardwoods catalyzed by anthraquinone and methyl substituted anthraquinones. J Wood Chem Technol 26:141–152.https://doi.org/10.1080/0277381060 0701737
Garcı´a G, Aparicio S, Ullah R, Atilhan M (2015) Deep eutectic solvents: physicochemical properties and gas separation applications. Energy Fuel 29:2616–2644.https://doi.org/
10.1021/ef5028873
Garcı´a A, Gandini A, Labidi J, Belgacem N, Bras J (2016) Industrial and crop wastes: a new source for nanocellulose biorefinery. Ind Crop Prod 93:26–38. https://doi.org/10.
1016/j.indcrop.2016.06.004
Gong J, Li J, Xu J, Xiang Z, Mo L (2017) Research on cellulose nanocrystals produced from cellulose sources with various polymorphs. RSC Adv 7:33486–33493.https://doi.org/10.
1039/C7RA06222B
Gorke JT, Srienc F, Kazlauskas RJ (2008) Hydrolase-catalyzed biotransformations in deep eutectic solvents. Chem Com- mun.https://doi.org/10.1039/b716317g
Gunny AAN, Arbain D, Nashef EM, Jamal P (2015) Applica- bility evaluation of deep eutectic solvents–cellulase system for lignocellulose hydrolysis. Bioresour Technol 181:297–302. https://doi.org/10.1016/j.biortech.2015.01.
057
He Y, Pang Y, Liu Y, Li X, Wang K (2008) Physicochemical characterization of rice straw pretreated with sodium hydroxide in the solid state for enhancing biogas produc- tion. Energy Fuel 22:2775–2781.https://doi.org/10.1021/
ef8000967
Hinterstoisser B, Salme´n L (1999) Two-dimensional step-scan FTIR: a tool to unravel the OH-valency-range of the spectrum of cellulose I. Cellulose 6:251–263.https://doi.
org/10.1023/A:1009225815913
Ibrahim MM, El-Zawawy WK, Abdel-Fattah YR, Soliman NA, Agblevor FA (2011) Comparison of alkaline pulping with steam explosion for glucose production from rice straw.
Carbohyd Polym 83:720–726. https://doi.org/10.1016/j.
carbpol.2010.08.046
Ibrahim MM, El-Zawawy WK, Ju¨ttke Y, Koschella A, Heinze T (2013) Cellulose and microcrystalline cellulose from rice straw and banana plant waste: preparation and characteri- zation. Cellulose 20:2403–2416. https://doi.org/10.1007/
s10570-013-9992-5
Isa KM, Abdullah TAT, Ali UFM (2018) Hydrogen donor sol- vents in liquefaction of biomass: a review. Renew Sust Energy Rev 81:1259–1268.https://doi.org/10.1016/j.rser.
2017.04.006
Jime´nez L, Pe´rez I, Lo´pez F, Ariza J, Rodrı´guez A (2002) Ethanol-acetone pulping of wheat straw. Influence of the cooking and the beating of the pulps on the properties of the resulting paper sheets. Bioresour Technol 83:139–143.
https://doi.org/10.1016/S0960-8524(01)00196-1
Kumar AK, Parikh BS, Pravakar M (2016) Natural deep eutectic solvent mediated pretreatment of rice straw: bioanalytical characterization of lignin extract and enzymatic hydrolysis of pretreated biomass residue. Environ Sci Pollut Res 23:9265–9275. https://doi.org/10.1007/s11356-015-4780- 4
Li G, Fu S, Zhou A, Zhan H (2014) Improved cellulose yield in the production of dissolving pulp from bamboo using acetic acid in prehydrolysis. BioResources 10:877–886.
https://doi.org/10.15376/biores.10.1.877-886
Lin L, Yan R, Liu Y, Jiang W (2010) In-depth investigation of enzymatic hydrolysis of biomass wastes based on three major components: cellulose, hemicellulose and lignin.
Bioresour Technol 101:8217–8223. https://doi.org/10.
1016/j.biortech.2010.05.084
Liu Y, Zhang L, Li Y, Xu Y (2015) The influence of chloride- based 1-butyl-3-methylimidazolium ionic liquid on the pulping process of wheat straw cooking. J Mol Liq 208:
196–201.https://doi.org/10.1016/j.molliq.2015.04.037 Lobo HR, Singh BS, Shankarling GS (2012) Deep eutectic
solvents and glycerol: a simple, environmentally benign and efficient catalyst/reaction media for synthesis ofN-aryl phthalimide derivatives. Green Chem Lett Rev 5:487–533.
https://doi.org/10.1080/17518253.2012.669500
Lu P, Hsieh Y-L (2012) Preparation and characterization of cellulose nanocrystals from rice straw. Carbohyd Polym 87:564–573. https://doi.org/10.1016/j.carbpol.2011.08.
022
Lynam JG, Coronella CJ (2014) Glycerol as an ionic liquid co- solvent for pretreatment of rice hulls to enhance glucose and xylose yield. Bioresour Technol 166:471–478.https://
doi.org/10.1016/j.biortech.2014.05.086
Marques AP, Evtuguin DV, Magina S, Amado FML, Prates A (2009) Structure of lignosulphonates from acidic magne- sium-based sulphite pulping of Eucalyptus globulus.
J Wood Chem Technol 29:337–357. https://doi.org/10.
1080/02773810903207762
Mjalli FS, Naser J, Jibril B, Al-Hatmi SS, Gano ZS (2014) Ionic liquids analogues based on potassium carbonate. Ther- mochim Acta 575:135–143.https://doi.org/10.1016/j.tca.
2013.10.028
Nazir MS, Wahjoedi BA, Yussof AW, Abdullah MA (2013) Eco-friendly extraction and characterization of cellulose from oil palm empty fruit bunches. BioResources 8:
2161–2172.https://doi.org/10.15376/biores.8.2.2161-2172 Oun AA, Rhim J-W (2016) Isolation of cellulose nanocrystals
from grain straws and their use for the preparation of car- boxymethyl cellulose-based nanocomposite films. Carbo- hyd Polym 150:187–200. https://doi.org/10.1016/j.
carbpol.2016.05.020
Park S-B, Lee H, Lee K-H (1998) Solubilities of carbon dioxide in aqueous potassium carbonate solutions mixed with physical solvents. Int J Thermophys 19:1421–1428.https://
doi.org/10.1023/A:1021935619427
Pavlostathis SG, Gossett JM (1985) Modeling alkali consump- tion and digestibility improvement from alkaline treatment of wheat straw. Biotechnol Bioeng 27:345–354.https://doi.
org/10.1002/bit.260270320
Pilate G, Guiney E, Holt K, Petit-Conil M, Lapierre C, Leple´
J-C, Pollet B, Mila I, Webster EA, Marstorp HG, Hopkins DW, Jouanin L, Boerjan W, Schuch W, Cornu D, Halpin C (2002) Field and pulping performances of transgenic trees with altered lignification. Nat Biotechnol 20:607–612.
https://doi.org/10.1038/nbt0602-607
Rahnama N, Mamat S, Shah UKM, Ling FH, Rahman NAA, Arif AB (2013) Effect of alkali pretreatment of rice straw on cellulase and xylanase production by local trichoderma harzianum SNRS3 under solid state fermentation. BioRe- sources 8:2881–2896.https://doi.org/10.15376/biores.8.2.
2881-2896
Rodrı´guez A, Sa´nchez R, Requejo A, Ferrer A (2010) Feasibility of rice straw as a raw material for the production of soda cellulose pulp. J Clean Prod 18:1084–1091.https://doi.org/
10.1016/j.jclepro.2010.03.011
Ruß C, Ko¨nig B (2012) Low melting mixtures in organic syn- thesis-an alternative to ionic liquids? Green Chem 14:2969–2982.https://doi.org/10.1039/c2gc36005e Segal L, Creely JJ, Martin AE, Conrad CM (1959) An empirical
method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer. Text Res J 29:786–794. https://doi.org/10.1177/004051755902901 003
Sˇkulcova´ A, Majova´ V, Sˇima J, Jablonsky M (2017) Mechanical properties of pulp delignified by deep eutectic solvents.
BioResources 12:7479–7486
Sluiter A, Hames B, Ruiz R, Scarlata C, Sluiter J, Templeton D, Crocker D (2008) Determination of structural carbohy- drates and lignin in biomass. Lab Anal Proced 1617:1–6 Song W, Deng Y, Zhu H (2016) Research on wheat straw
pulping with ionic liquid 1-ethyl-3-methylimidazole bro- mide. Kem Ind 65:579–585.https://doi.org/10.15255/KUI.
2016.032
Sun F, Chen H (2008) Organosolv pretreatment by crude glyc- erol from oleochemicals industry for enzymatic hydrolysis of wheat straw. Bioresour Technol 99:5474–5479.https://
doi.org/10.1016/j.biortech.2007.11.001
Sun N, Rahman M, Qin Y, Maxim ML, Rodrı´guez H, Rogers RD (2009) Complete dissolution and partial delignification of wood in the ionic liquid 1-ethyl-3-methylimidazolium acetate. Green Chem 11:646–655.https://doi.org/10.1039/
b822702k
Sun N, Rodrı´guez H, Rahman M, Rogers RD (2011) Where are ionic liquid strategies most suited in the pursuit of chemi- cals and energy from lignocellulosic biomass? Chem Commun 47:1405–1421. https://doi.org/10.1039/C0CC0 3990J
Swatloski RP, Spear SK, Holbrey JD, Rogers RD (2002) Dis- solution of cellose with ionic liquids. J Am Chem Soc 124:4974–4975.https://doi.org/10.1021/ja025790m Toledano A, Serrano L, Labidi J (2012) Organosolv lignin
depolymerization with different base catalysts. J Chem Technol Biotechnol 87:1593–1599. https://doi.org/10.
1002/jctb.3799
Updegraff DM (1969) Semimicro determination of cellulose inbiological materials. Anal Biochem 32:420–424.https://
doi.org/10.1016/S0003-2697(69)80009-6
Vigier KDO, Chatel G, Je´roˆme F (2015) Contribution of deep eutectic solvents for biomass processing: opportunities, challenges, and limitations. ChemCatChem 7:1250–1260.
https://doi.org/10.1002/cctc.201500134
Wolfson A, Dlugy C, Shotland Y, Tavor D (2009) Glycerol as solvent and hydrogen donor in transfer hydrogenation-de- hydrogenation reactions. Tetrahedron Lett 50:5951–5953.
https://doi.org/10.1016/j.tetlet.2009.08.035
Xie H, King A, Kilpelainen I, Granstrom M, Argyropoulos DS (2007) Thorough chemical modification of wood-based lignocellulosic materials in ionic liquids. Biomacromole- cules 8:3740–3748.https://doi.org/10.1021/bm700679s Xu G-C, Ding J-C, Han R-Z, Dong J-J, Ni Y (2016) Enhancing
cellulose accessibility of corn stover by deep eutectic sol- vent pretreatment for butanol fermentation. Bioresour Technol 203:364–369. https://doi.org/10.1016/j.biortech.
2015.11.002
Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
