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Hydrolysis of cellulosic bamboo biomass into reducing sugars via a combined alkaline solution and ionic liquid pretreament steps
Renewable Energy 104 (2017) 177e184
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Hydrolysis of cellulosic bamboo biomass into reducing sugars via a combined alkaline solution and ionic liquid pretreament steps Samuel Kassaye a, Kamal K. Pant a, *, Sapna Jain b a b
Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110 016, India Department of Physical Science, Alabama State University, Montgomery, AL, 36104, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2016 Received in revised form 28 November 2016 Accepted 14 December 2016 Available online 18 December 2016
Dilute acid hydrolysis of cellulosic biomass is not only controlled by the reaction conditions such as temperature, concentration of acidic catalyst and hydrolysis time but also by changing the physical aspects of the reaction media. Therefore, overcoming the insolubility of cellulose by the use of effective solvent without having to derivatize their basic functional groups is of tremendous advantage in the utilization of lignocellulosic biomass. Ionic liquids are considered as the most suitable solvents to dissolve cellulosic biomass and overcome the recalcitrant nature of lignocellulosic biomass. This study investigates the valorisation of bamboo biomass regenerated from alkaline solution and ionic liquid pretreament steps followed by dilute sulphuric acid hydrolysis. Lignin removal from the biomass as a result of pretreatment steps was analysed by crystallinity index determination, surface morphology and thermal stability analysis. The solid biomass materials were characterized using FTIR, XDR, SEM, TGA and Elemental analysis techniques to investigate the effect of the pretreatment steps on the properties of the original bamboo biomass. Alkaline treatment was found to be effective against lignin and hemicellulose removal. However, it did not remove the complexity of the cellulosic portion of the biomass with equal success. The crystallinity of the recovered cellulosic biomass could be effectively reduced by using the ionic liquid pretreatment. Investigation revealed that the yield of total reducing sugars increased to 64% after alkaline solution pre-treatment in comparison to only 30% yield of reducing sugar in the untreated biomass sample. After both pretreatment steps, the yield of reducing sugar further increased to 80%. © 2016 Published by Elsevier Ltd.
Keywords: Bamboo biomass Pretreatment Ionic liquid TRS yield
1. Introduction The potential of biomass as feedstock for transportation fuels and platform chemical production had not been given enough attention because of the historic shift towards petroleum based resources in the twentieth century [1]. However, more recently, limited availability and environmental concern are the compelling factors in the pursuit of substituting fossil based energy dependency [2]. Lignocellulosic biomass is the ideal substitution for fossil based transportation fuel and platform chemicals production due to its abundance, low cost, even distribution and carbonneutrality [3] [4]. Lignocellulosic biomass is a porous microstructured composite which is composed of three biopolymers; lignin (15e25%), hemicelluloses (23e32%) and cellulose (38e50%) [5] [6]. These constituents form three-dimensional polymeric
* Corresponding author. E-mail address: [email protected] (K.K. Pant). http://dx.doi.org/10.1016/j.renene.2016.12.033 0960-1481/© 2016 Published by Elsevier Ltd.
composites to provide structural rigidity and stability to the cell wall of the plant. Cellulose and hemicellulose combined (holocellulose) forms the carbohydrate portion of the lignocellulosic biomass which can be depolymerized into simple sugars such as glucose and xylose. Lignin serves as a binder to hold together the cellulose and hemicellulose in the cell wall through the complex intra and inter-hydrogen bonding networks [7]. The complex nature of the lignocellulosic biomass creates a major challenge in developing a competitive process technology to convert lignocellulosic biomass into fuels and chemicals in economically feasible and environmentally friendly manner [8]. In addition, lignocellulosic biomass cannot be directly converted into bioethanol due to the presence of lignin and the recalcitrant nature of the cellulosic portion of the biomass [9] and these obstacles have been long standing challenges on the utilization of lignocellulosic biomass. However, more recently the discovery of ionic liquids for biomass transformation has opened a new area of research to utilize lignocellulosic biomass for intermediate chemical production such as sugars and furan derivatives. Ionic liquids are special
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solvents that can dissolve materials that are otherwise considered insoluble in conventional solvents. Ionic liquids have special properties such as broader liquid temperature, high thermal stability and negligible vapour pressure which are the vital properties required in the transformation lignocellulosic biomass [10]. In the conversion process of lignocellulosic biomass into transportation fuel and platform chemicals, pretreatment is most important step for conditioning biomass. Pretreatment plays an important role in fractionating the lignocellulosic biomass to its components and improve accessibility of cellulosic portion for downstream processes. However, it is the most energy intensive and economically expensive process [11]. Biomass pretreatment processes are categorized into different methods ranging from physical, physico-chemical, and chemical to biological methods based on the techniques applied. Alkaline solution pretreatment is a type of chemical pretreatment effective in the delignification of biomass without significantly affecting cellulosic structure of the biomass [12,13]. However, this method of pretreatment has limited effect on the reduction of the crystallinity of pretreated biomass at mild concentration and temperature to reduce the recalcitrant behaviour of cellulosic biomass rendering resistance in the hydrolysis process. Therefore, ionic liquid pretreatment which is also a type of chemical pretreatment method known for its specific purpose in dissolving and reducing the crystallinity of cellulose is incorporated following the alkaline solution pretreatment. This method reduce the crystallinity index of the cellulosic biomass so that the reactivity of the biomass could be enhanced during acid hydrolysis [15,16]. In this study, bamboo biomass was pretreated in alkaline solution and ionic liquid solvent for the purpose of improving acid hydrolysis and reducing the recalcitrant behaviour of the biomass through the combined effect of pretreatment steps to partially remove lignin and reduce crystallinity index. The biomass obtained from Haryana, India was studied for structural, morphological, chemical and physical changes exhibited as the result of the pretreatment steps. All the three bamboo biomass samples (original bamboo biomass, alkaline pretreated and ionic liquid pretreated) were subjected to dilute acid hydrolysis in ionic liquid solvent media (1-butyl 3-methylimidazolium chloride ([BMIM] Cl)). The total reducing sugar yield (TRS) was estimated by 3, 5dinitrisalcylic acid (DNS) array method using UVeVisible spectroscopy. The effects of hydrolysis temperature, time, acid concentration and substrate type on the TRS yield were studied in detail. The original bamboo biomass is referred as “Original bamboo biomass” (OBB), the solid biomass recovered from alkaline solution and [BMIM] Cl pretreatment was referred as “Recovered Biomass I” (RBI) and biomass obtained after acid treatment is referred as “Regenerated Biomass II” (RBII). 2. Materials, experimental and characterization 2.1. Materials Sulfuric acid (98%), N-methylimidazolium, 1-chlorobutane, acetonitrile and ethyl acetate were purchased from Spectrochem (Anand Bhuvan, Princess Street Mumbai, India). Bamboo biomass (Dendrocalamus Strictus) was obtained from Haryana, India. All chemicals were used without further purification. 2.2. Experimental 2.2.1. [BMIM] Cl synthesis 1-butyl 3-methylimidazolium chloride ([BMIM] Cl) was prepared form 1-chlorobutane and N-methyl imidazole in a toluene media as reported in literature [16] [17].
2.2.2. Ash content The ash content of the bamboo biomass was determined as part of the total composition of the bamboo biomass. The determination was performed using the National Renewable Energy Laboratory (NREL) procedure (Sluiter et al., 2008). In a typical ash content measurement, 2 g of the oven dried bamboo biomass was placed into weighed crucible and the sample was heated in muffle furnace (KHERA INSTRUMENTS PVT.LTD, INDIA) at 575 C for 5 h. After carefully removing the sample in the crucible, both the sample and the crucible weighed. The ash content of the bamboo biomass was estimated as percent of residue from the heating process and calculated from the Equation (1):
% Ash content ¼
Weightcrucible plus ash Weightcrucible * 100 % Oven dried Weightsample (1)
2.2.3. Extracts removal Three step extractive removal procedure was used to remove extracts from the bamboo biomass. Soxhlet extraction setup was used to remove the extracts while water, ethanol and hexane were used as extraction solvents as reported in literature [18]. In a typical extraction procedure, 5 g of original bamboo biomass was extracted for 2 h in each solvent and the weight change was measured after drying the extracted biomass after 24 h at 105 C in vacuum drying oven. Reduction in weight was observed after every treatment step as every step resulted in removal of some component of the biomass. 2.2.4. Pretreatment Extractive removed bamboo biomass sample weighing 1 g was treated with different concentration of sodium hydroxide solution in a round bottomed flask at predetermined temperature and time in 1:20 wt ratio of biomass to alkaline solution loading. The solid biomass was recovered by vacuum filtration using porcelain Bucher funnel and then washed three times with distilled water. The wet biomass material was dried at 105 C in vacuum dry oven for 24 h. Similarly, the precipitated lignin was separated using vacuum filtration and dried at 110 C for 24 h. The supernatant obtained from the partially dissolved biomass was adjusted to a pH of 5 using a dilute solution of HCl and recovered as dark brown precipitate of lignin (Silverstein et al., 2007). The effect of temperature, time and alkaline solution concentration was studied to determine degree of delignification and the crystallinity index of the recovered biomass. After determining the optimal condition for alkaline solution pretreatment, the biomass was treated in a larger volume batch reactor. Extracts removed biomass weighing 10 g was treated with 5% (W/V) of sodium hydroxide solution in a sealed AMAR Autoclave reactor for 30 min at 130 C and all the other posttreatment processes followed as described above. Finally, the biomass recovered from alkaline pretreament step was further treated using [BMIM] Cl to reduce the crystallinity of the alkaline treated biomass (RBI) and overcome the recalcitrant nature of the biomass. In this step 2.5 g of RBI was dissolve in 12.5 g of [BMIM] Cl and heated at 120 C for 6 h with continuous stirring. After completion of treatment time, 60 ml of Mill-Q water was added and stirred vigorously for 15 min to regenerate the cellulosic rich bamboo biomass (RBII). RBII was finally collected from the solution using vacuum filtration and washed with distilled water three times and then dried under vacuum at 105 C for 24 h.
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2.2.5. Biomass characterization Elemental composition of the biomass samples was performed to determine the carbon, hydrogen, nitrogen and sulfur content while oxygen content was determined by difference using a VarioEL V5.16.7.10, CHN Mode (Elemental Vario el Cube analyser from German) elemental composition analyser. The Fourier transform infrared (FTIR) of OBB, RBI and RBII were scanned from 4000 to 400 cm1 at a resolution of 4 cm1 to probe the change in functional groups due to the pretreatment steps. The translucent KBr discs were made from homogenized 5 mg biomass sample in 100 mg KBr using electrically operated automatic press (Model-AP-15 from Techno-Search Instruments, India) at 7 tons for 5 min. The TGA analysis of all the three samples (OBB, RBI and RBII) weighted, being 10 and 12 mg were carried out using TGA-Q600 instrument supplied by Waters LLC, USA with a continuous flow of N2 at flow rate of 40 ml/min and heating rate of 10 C/min for a temperature ranging from 25 to 600 C. In addition, the calorific value of the three bamboo biomass samples was determined using bomb calorimeter. Scanning electron microscopy (SEM) images of OBB, RBI and RBII were taken using a ZEISS EVO at 20 kV to study the change in the morphology of the bamboo biomass occurred due to the pretreatment steps. Eiko IB-3 Incoater was using in order to coat the samples with gold for improving the imaging of samples. The crystallinity indexes OBB, RBI and RBII were estimated using x-ray diffraction analysis. The analysis was performed with a Rigaku miniflex 600. The diffracted intensity of Cu K a radiation generated at 40 kV and 15 mA was measured in a 2q range between 5 and 80. The crystallinity indexes of the biomasses were determined using Segal's method [19] [20] as illustrated in Equation (2):
CrI ð%Þ ¼
ðI002 e Iam Þ *100% Iam
(2)
Where I00 and Iam I002 is the intensity of 002 peak at 2q ¼ 22.4 and Iam is the intensity of background scatter at 2q ¼ 18.
2.2.6. Acid hydrolysis The various cellulosic bamboo biomass samples (0.1 g) (OBB, RBI and RBII) were dissolved in [BMIM] Cl (4 g) at 175 C for 30 min followed with the addition of 10 ml of predetermined concentration of dilute H2SO4. After hydrolysis reaction, the mixture was quenched using ice bath for about 15 min and the pH adjusted to neutral using calcium carbonate solution. Then the hydrolysate was separated by vacuum filtration and further purified with centrifugation process.
2.2.7. TRS analysis The hydrolysate obtained from the acid hydrolysis was analysed using DNS array method [21]. The absorbance value of the hydrolysate reacting with DNS reagent was determined in UVeVis spectroscopy (CARY 100 Conc) at 540 nm wavelength. The TRS yield was determined from calibration curve formed using four point concentration of standard glucose solutions. The reducing sugar yield from the hydrolysis reaction was quantified using Eqn. (1) [22] as shown in Equation (3).
TRS Yield ð%Þ ¼
Reducing sugar weight 162 180 100 Dry cellulosic biomass weight
(3)
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3. Result and discussion 3.1. Extract removal The extracts in dry matter of lignocellulosic biomass such as nitrites, protein, ash, waxes, chlorophyll, etc were extracted using a Soxhlet apparatus and refluxed with three step solvent extractors namely: Milli-Q water, ethanol and hexane under heat for 2 h in each solvent case. The water was effective in removing inorganic materials and non-structural sugars while ethanol removed polar compounds such as chlorophyll, waxes and sterols. Finally, hexane was used to remove non-polar lipids and other hydrocarbons compounds. The weight of extract removed from the three steps were; 13.6 ± 0.8, 2.6 ± 0.6 and 5.2 ± 0.8 for water, ethanol and hexane, respectively. 3.2. Pretreatment The two step process of delignification and hemicellulose removal were carried out to efficiently regenerate cellulose rich biomass. Table 1 summarizes the change in mass observed in the process after uniform drying process. The effect of temperature, pretreatment time and alkaline solution concentration on the degree of delignification and biomass recovered was investigated. As can be seen from Table 1, the level of lignin removal from the alkaline solution pretreatment was dependent on the temperature while alkaline solution concentration and treatment time were kept constant at 5 wt % and 30 min, respectively. The amount of lignin recovery increased with increase in temperature while the solid biomass recovery decreased. The effect of pretreatment on the extent of lignin removed and biomass recovered from alkaline solution pretreatment was studied. It was found that the lignin residue recovered from the treatment increased as pretreatment time prolonged. Different concentrations of alkaline solution were considered while studying the effect of pretreatment dependency on reducing sugar yield. Based on the lignin recovery and biomass residue collected from the pretreatment process, it was observed that lignin recovery increased as the concentration of sodium hydroxide increased whereas amount of biomass recovered decreased as the concentration of alkaline solution increased due to the partial removal of lignin and hemicellulose in the pretreatment process. It was observed that the crystallinity index of the recovered biomass increased with temperature while maintaining the alkaline solution concentration and time of pretreatment fixed. This is probably due to the partial removal of hemicellulose, lignin and amorphous cellulose which increased the crystalline cellulose portion. However, the crystallinity index was reduced with increasing the duration of pretreatment and concentration of alkaline solution at constant temperature. The biomass recovered from pretreatment with alkaline solution and ionic liquid had higher crystallinity index due to the partial removal of lignin and hemicellulose In addition, [BMIM] Cl pretreatment enhanced the accessibility and digestibility of the biomass by overcoming the recalcitrant behaviour of the cellulosic biomass. 3.3. Ultimate and proximate analysis The OBB, RBI and RBII were analysed for ultimate analysis to quantify the elemental composition of sample substrates. The results presented in Table 2 include the crystallinity index and calorific values of the samples. As observed from the ultimate analysis, the carbon content of the bamboo biomass reduced from 45.2± 0.55% to 44.4± 0.52% for alkaline solution pretreatment and further reduced to 43.2 ± 0.46% after [BMIM] Cl pretreatment. The
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Table 1 Alkaline pretreatment. Effect of temperature, time and concentration on alkaline solution pretreatment of bamboo biomass. Temperature effect Temperature ( C)
NaOH (Wt. %)
Time (min)
Decrease in biomass (%)
Lignin residue (%)
Biomass residue (%)
CrI index (%)
90 110 130 150
5 5 5 5
30 30 30 30
12 18 22.4 28.4
8.4 8.5 9.1 10.2
88 82 77.6 71.6
58.8 60.7 60 60.1
Time effect Time (min)
NaOH (Wt. %)
Temperature ( C)
Decrease in biomass (%)
Lignin residue (%)
Biomass residue (%)
CrI index (%)
30 45 60 75
5 5 5 5
130 130 130 130
22.4 25.4 28.9 30.5
9.1 9.9 12.4 15.2
77.6 74.6 71.1 69.5
60 55.3 54.3 53.4
Concentration effect NaOH (Wt. %)
Time (min)
Temperature ( C)
Decrease in biomass (%)
Lignin residue (%)
Biomass residue (%)
CrI index (%)
5 7.5 10 12.5
30 30 30 30
130 130 130 130
22.4 34.8 35.6 37.9
9.1 12.6 13.2 14.8
77.6 65.2 64.4 62.1
60 60 56.2 46.4
reduction in carbon content observed is possibly attributed to the partial removal of hemicellulose, amorphous cellulose and lignin portion of the biomass due to the alkaline solution pretreatment. However, the hydrogen and oxygen content in the pretreated biomass increased after the pretreatment steps as compared to the original biomass. The ash content of the three biomass samples exhibited similar content for both the original and pretreated samples. However, the calorific values of RBI decreased from 17.33 to 16.25 MJ/kg and further decreased to 14.27 MJ/kg for RBII which can be accounted for the removal of carbon content from OBB.
3.5. SEM analysis The Fig. S1 shows the SEM images of the three samples (OBB, RBI and RBII). The images highlight the structural changes in biomass as a result of the pretreatment steps. OBB has more open, cylindrical and compact arrangement while RBI biomass sample exhibited a loss in hollow cylindrical shape of the OBB biomass and destruction in the structural rigidity is clearly observed. In case RBII sample a more destruction in the structure of the samples can be clearly seen. 3.6. FTIR analysis
3.4. XRD analysis Fig. 1 shows the XRD results of the three biomass samples. As explained above in the case of alkaline pretreatment process, the crystallinity index increased in RBI sample due to the removal of lignin, hemicellulose and amorphous cellulose at optimized pretreatment condition. However, [BMIM] Cl pretreatment step resulted in a significant decrease in the crystallinity of RBII sample due to breakage of the intra and inter-hydrogen bonding network. It is observed that the combined use of alkaline and [BMIM] Cl pretreatment was effective in removing lignin and hemicellulose portion of the biomass and reducing the crystallinity of the cellulosic bamboo biomass. It should be noted that the reduction in crystallinity index of the biomass is very crucial in enhancing the reactivity of the cellulosic biomass during enzymatic or acid hydrolysis. The overall crystallinity of all the three biomass samples as was measured and presented in Table 2, the crystallinity index of RBI is around 60% while OBB and RBII 56.5% and 39.5%, respectively.
The FTIR analysis of OBB, RBI and RBII was carried out in order to investigate the changes in structural features and functional groups change as a result of the pretreatment steps. The FTIR of the three samples is shown in the Fig. 2. Both the OBB and RBI samples have strong broad absorption band at 3350 cm1 due to the eOH stretching vibrations while RBII sample showed weak absorption band due to the elimination of the hydrogen bonding network and the release of eOH group. The prominent CeH stretching absorption was observed at 2910 cm1 in OBB and RBI completely disappeared from RBII. The absorbance at 1740 cm1 attributed to C] O stretching vibration accounted for the presence of hemicellulose showed a very weak absorption in the case of RBII sample implying the removal of hemicellulose. The aromatic skeletal vibration in lignin was evidenced in the absorption from 1600 to 1450 cm1 which is stronger in the case of OBB sample than RBII due to the partial removal of lignin while RBII sample exhibited the absence of aromatic skeletal vibration. The absorption peaks at 1740 and
Table 2 Ultimate and proximate analysis. Ultimate and proximate analysis of OBB, RBI and RBII. Ultimate analysis Elemental composition (Wt. %)
Ash content (%) Calorific value (MJ/Kg) CrI (%)
C H N S O e e e
OBB
RBI
RBII
45.2 ± 0.55 5.9 ± 0.15 0.52 ± 0.11 0.35 ± 0.01 48.03 ± 0.8 2.2 ± 0.2 17.338 56.5
44.4 ± 0.52 6.4 ± 0.16 0.06 ± 0.01 0.15 ± 0.02 48.99 ± 1.2 2.1 ± 0.3 16.251 60
43.2 ± 0.46 6.2 ± 0.2 0.25 ± 0.02 0.05 ± 0.01 50.3 ± 0.9 2.1 ± 0.1 14.276 39.5
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Fig. 1. XRD. XRD of OBB, RBI and RBII.
1245 cm1 are the result of the acetyl and uronic ester groups of the hemicellulose of the biomass and as can be observed from RBII sample, these peaks disappeared in comparison with OBB and RBI [23]. The intensity of the peak at 893 cm1 which is the characteristics of the presence of b-1,4 glycosidic bond linkages in the cellulose component of the biomass was observed in all the three samples signifying the pretreatment steps has not affected significantly the cellulose portion of the biomass.
3.7. TGA-DTA analysis The thermal property of OBB, RBI and RBII was analysed using thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) methods as shown in Fig. S2. The analysis provided important thermal property changes that occurred due to conditioning and pretreatment steps of the biomass. The weight changes of the biomass samples with respect to temperature can be used to estimate the composition of the samples in terms of the amount of cellulose, hemicellulose and lignin present in the sample. As reported in literature [24] and also observed from the analysis, the most reactive compound decomposes between 200 and 350 C, whereas cellulose decomposition occur from 305 to
Fig. 2. FTIR analysis. FTIR of OBB, RBI and RBII.
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375 C and lignin decompose at the temperature ranging from 250 to 500 C. In the TGA analysis, all the three samples showed similar pattern of thermal decomposition trend exhibiting three different regions as also reported in literature [25]. RBI and RBII showed higher thermal stability than OBB due to the removal of hemicellulose and other easily decomposing compounds from these samples. As observed from the TGA plot, in stage one which occurred in the temperature range of 100e175 C, the evaporation of moisture trapped in the biomass was eminent while in stage two which ranges from 175 to 430 C, the devolatilization of hydrocarbons was exhibited. Stage two mainly accounts for thermal decomposition of cellulose, hemicellulose and trace amount of lignin. Each sample of the analysed biomass represents different percentage of weight loss due to the different composition of the three polymeric materials. OBB lost 65% of its total weight in the decomposition process, while RBI and RBII samples weight loss was 70 and 77% in stage two, respectively. The difference in decomposition pattern and weight loss in different biomass samples originates from the composition of the samples under study. RBI and RBII contain more of cellulosic portion which decompose mostly in stage two as displayed in the TGA analysis pot. 3.8. Acid hydrolysis of RBII In the acid hydrolysis of bamboo biomass, it was possible to observe that prior dissolution of the substrates samples with [BMIM] Cl improved the yield of TRS as compared to the direct mixing of the reaction mixtures. Prior dissolution of the substrates overcomes recalcitrant nature of the cellulosic material by breaking the intra and inter-hydrogen bonding network which in turn reduce the crystallinity of the biomass while increasing the accessible surface area of the substrate to interact with the catalyst [26]. The reduction in crystallinity of the biomass was found to be significant in improving the penetration of hydronium ions of the sulphuric acid catalyst into the dissolved cellulosic substrate and catalyse the cleavage of the b-1,4-glycosidic bond [27]. [BMIM] Cl is considered as the most suitable ionic liquid in the transformation of biomass due to its valuable properties such as; low melting point,
Fig. 3. Effect of temperature. Effect of temperature on TRS yield from RBII substrate. Hydrolysis Conditions: Time 2 h, Acid conc. 5% v/v, substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
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Fig. 4. Effect of hydrolysis time. Effect of hydrolysis time on TRS yield from RBII substrate. Hydrolysis Conditions: Time 2 h, Acid conc. 5% v/v, substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
moderate viscosity and minimized corrosive property. In addition, its chloride ions form non-dimensional hydrogen bonds to link with cellulose molecules dissolved and hinder the reformation of the hydrogen bonds within cellulose chain and strands. [BMIM] Cl is known to interact with cellulosic biomass via ionic, p-p and hydrogen bonding interactions. During dissolution process, the p-p interaction plays an important role in the disruption of inter and intramolecular hydrogen bonds. 3.8.1. Effect of temperature In the present study, by limiting the maximum hydrolysis temperature at 220 C due to the minimum decomposition temperature of [BMIM] Cl (240 C), it was possible to hydrolyse RBII up to a maximum of 72% while 32% and 64% of TRS yield obtained for OBB and RBI, respectively at the similar hydrolysis temperature. The effect of temperature on the hydrolysis of RBII was studied in temperature range between 140 and 220 C with 10 C intervals as shown in Fig. 3. The effect of temperature on TRS yield showed a minor (only 2%) increase in yield as the temperature increased from 140 to 160 C. However, as the temperature changes from 160 to 170 C the yield of TRS increased up to 32% and reached to maxima (72%) at 180 C. This significant change in yield could possibly be the result of maximum dissolution and reduction in crystalline structure of RBII substrate which facilitated the contact between the substrate and the hydrogen ions in homogeneous ionic liquid hydrolysis media. Another interesting result is that TRS yield was started to decline as the temperature increased from 190 to 220 C in 40% yield. The decline in yield was mainly due to the decomposition of reducing sugars into some other form such as humins and furan derivative chemicals [28]. 3.8.2. Effect of time The total reducing sugar yield obtained from the hydrolysis of cellulosic bamboo biomass using dilute sulphuric acid catalyst in [BMIM] Cl media was found to be significantly dependent on the time of hydrolysis. The effect of hydrolysis time on the TRS yield was studied for a various set of temperature as shown in Fig. 4. TRS yield increased as the temperature increased for each set of temperature studied and reached maxima. However, after reaching
Fig. 5. Effect of acid concentration. Effect of acid concentration on TRS yield using RBII substrate. Hydrolysis Conditions: temperature 180 C, reaction time 2 h, substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
maximum yield, extended hydrolysis time resulted in declined yield as observed in the case of effect of temperature study due to the possible decomposition of sugar molecules produced. As shown in Fig. 4, the yield of TRS for all set of temperature studied have shown an increase in yield for some time during the course of the reaction and diminish thereafter implying decomposition of the sugar into other products. For instance in case of hydrolysis of RBII at 160 C, the maximum yield of TRS was obtained at 150 min and start decreasing as the time of hydrolysis further extended. The yield gradually increased after 30 min of hydrolysis time and continued increasing up to 150 min when the hydrolysis was carried out at 180 C. However, as the hydrolysis time extended further
Fig. 6. Effect of substrate type. Effect of substrate type on TRS yield. Hydrolysis Conditions: temperature 180 C, reaction time 60 min, 2 h, Acid Conc. 5% v/v, Substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
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from 150 min the TRS yield started declining as a result of product decomposition. Similarly, for hydrolysis taken at 200 C the maximum yield of TRS was obtained at 30 min marking minimum time of hydrolysis to reach maximum yield compared with the other temperatures, used in this study. From the general observation of experimental runs at different temperatures, it can be deduced that effective hydrolysis requires sufficient time between the substrate and hydrogen ions (Hþ) to obtain the maximum possible interaction that lead the catalyst to break the b-1,4 glycosidic bond to release sugar monomers from the depolymerization of cellulosic bamboo biomass [29]. 3.8.3. Effect of acid concentration Fig. 5 shows effect of dilute acid concentration sulphuric acid on yield of total reducing sugar. It is observed that the TRS yield was dependent on the amount of acid loaded (acid concentration). Increasing the acid concentration in the hydrolysis of RBII significantly affected the yield of TRS. As shown in the Fig. 5 it can be observed that the TRS yield increased as the concentration of acid increased for the given set of hydrolysis condition. The hydrolysis of cellulosic biomass is dependent on the concentration of hydrogen ion [27]. The higher the concentration of acid catalyst, there will be more number of hydrogen ions which can attack the glycosidic bond to release sugar molecules through subsequently protonation of the bond to form carbonium cation. Then, these carbonium cations easily hydrolyse to sugar molecules. As can be seen from the plot of TRS versus acid concentration, only 10% TRS yield was generated in pure water while over 40% increase in sugar yield was observed as the concentration of the acid increased from 2.5 to 5% v/v. The change in TRS yield increased up to a maximum of 80% for RBII substrate as the concentration of the acid increased to 7.5% v/v. 3.8.4. Effect of substrate type Different substrate (OBB, RBI and RBII) types was depolymerized and compared with TRS yield obtained from the hydrolysis of microcrystalline cellulose (MCC) to investigate the effect of the pretreatment steps on the effectiveness of depolymerisation process. All the four substrate samples were hydrolysed in the same reaction condition: temperature of 180 C, 3 h hydrolysis time and substrate to [BMIM] Cl weight ratio of 1:40. It was observed that the substrate sample which passed through the two subsequent pretreatment steps (RBII) resulted with TRS yield of 72% while the substrate pretreated only with alkaline solution (RBI) resulted in 64% TRS yield. The original bamboo biomass (OBB) yielded 32% while the microcrystalline cellulose used as model substrate hydrolysed to give 92% yield as shown in the Fig. 6. The results indicate that the TRS yield was significantly affected by the nature of the substrate which is related to the corresponding structural complexity, composition content, accessible surface area of the substrate and crystallinity index [27]. Thus, [BMIM] Cl ionic liquid used in this study contributed enormously in enhancing the dilute acid hydrolysis of the bamboo biomass by reducing the crystallinity of the substrate and facilitating contact between the hydrogen ions of the catalyst and the b-1, 4-glycosidic bond of the cellulose. 4. Conclusion Dilute sulphuric acid hydrolysis of different forms of bamboo biomass in [BMIM] Cl ionic liquid media was effective in terms of high yield of reducing sugars at relatively lower temperature (180 C) and pressure than the convention processes that take place at higher temperature (>250 C) and pressure. Hence, it was observed that the hydrolysis of RBII gives a better yield than OBB and RBI due to the relative reactivity of RBII substrate resulting from the influence of pretreatment steps in overcoming the
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recalcitrant behaviours of the biomass. This effect was evidenced from the lowest value of crystallinity index calculated for RBII and the corresponding high yield of sugars obtained. The two step pretreatment using alkaline solution and [BMIM] Cl ionic liquid resulted in removal of large portion of lignin and hemicellulose besides overcoming the structural complexity of bamboo biomass and resulting in easier dilute acid hydrolysis, evidenced with higher yield of reducing sugars. It is also observed that the combined pretreatment steps overcame two main challenges faced in the catalytic hydrolysis of lignocellulosic biomass into sugars, first by removing the lignin and hemicellulose which shield the accessibility of cellulose and second by reducing the recalcitrant behaviour of cellulose by eliminating structural complexity of lignocellulosic. Acknowledgement The authors acknowledge the financial support from the Department of Chemical Engineering, Indian Institute of Technology Delhi (NPN-05/BCHE). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.renene.2016.12.033. References [1] A. Brandt, J. Gr€ asvik, J.P. Hallett, T. Welton, Deconstruction of lignocellulosic biomass with ionic liquids, Green Chem. 15 (2013) 550, http://dx.doi.org/ 10.1039/c2gc36364j. [2] S. Nanda, P. Mohanty, K.K. Pant, S. Naik, J. a Kozinski, A.K. Dalai, Characterization of North American lignocellulosic biomass and biochars in terms of their candidacy for alternate renewable fuels, Bioenergy Res. 6 (2013) 663e677, http://dx.doi.org/10.1007/s12155-012-9281-4. [3] P. Weerachanchai, J.-M. Lee, Recyclability of an ionic liquid for biomass pretreatment, Bioresour. Technol. 169 (2014) 336e343, http://dx.doi.org/ 10.1016/j.biortech.2014.06.072. [4] J.-F. Liu, Y. Cao, M.-H. Yang, X.-J. Wang, H.-Q. Li, J.-M. Xing, Enhanced saccharification of lignocellulosic biomass with 1-allyl-3-methylimidazolium chloride (AmimCl) pretreatment, Chin. Chem. Lett. 25 (2014) 1485e1488, http://dx.doi.org/10.1016/j.cclet.2014.06.001. [5] N. Sun, H. Liu, N. Sathitsuksanoh, V. Stavila, M. Sawant, A. Bonito, K. Tran, A. George, K.L. Sale, S. Singh, B. a Simmons, B.M. Holmes, Production and extraction of sugars from switchgrass hydrolyzed in ionic liquids, Biotechnol. Biofuels 6 (2013) 39, http://dx.doi.org/10.1186/1754-6834-6-39. [6] H. Tadesse, R. Luque, Advances on biomass pretreatment using ionic liquids: an overview, Energy Environ. Sci. 4 (2011) 3913, http://dx.doi.org/10.1039/ c0ee00667j. [7] I.G. Baek, S.J. You, E.D. Park, Bioresource technology direct conversion of cellulose into polyols over Ni/W/SiO 2 -Al 2 O 3, Bioresour. Technol. 114 (2012) 684e690, http://dx.doi.org/10.1016/j.biortech.2012.03.059. [8] S. Behera, R. Arora, N. Nandhagopal, S. Kumar, Importance of chemical pretreatment for bioconversion of lignocellulosic biomass, Renew. Sustain. Energy Rev. 36 (2014) 91e106, http://dx.doi.org/10.1016/j.rser.2014.04.047. [9] H. Argun, G. Onaran, Delignification of vineyard pruning residues by alkaline peroxide treatment, Ind. Crops Prod. 74 (2015) 697e702, http://dx.doi.org/ 10.1016/j.indcrop.2015.05.031. [10] N.L. Mai, S.H. Ha, Y. Koo, Efficient pretreatment of lignocellulose in ionic liquids/co-solvent for enzymatic hydrolysis enhancement into fermentable sugars, Process Biochem. 49 (7) (2014) 1144e1151, http://dx.doi.org/10.1016/ j.procbio.2014.03.024. [11] K. Ninomiya, K. Inoue, Y. Aomori, A. Ohnishi, C. Ogino, N. Shimizu, K. Takahashi, Characterization of fractionated biomass component and recovered ionic liquid during repeated process of cholinium ionic liquidassisted pretreatment and fractionation, Chem. Eng. J. 259 (2015) 323e329, http://dx.doi.org/10.1016/j.cej.2014.07.122. [12] H. Rabemanolontsoa, S. Saka, Various pretreatments of lignocellulosics, bioresour, Technol 199 (2015) 83e91, http://dx.doi.org/10.1016/ j.biortech.2015.08.029. [13] J.S. Kim, Y.Y. Lee, T.H. Kim, A review on alkaline pretreatment technology for bioconversion of lignocellulosic biomass, Bioresour. Technol. 199 (2015) 42e48, http://dx.doi.org/10.1016/j.biortech.2015.08.085. [15] J. Zhang, L. Feng, D. Wang, R. Zhang, G. Liu, G. Cheng, Thermogravimetric analysis of lignocellulosic biomass with ionic liquid pretreatment, Bioresour. Technol. 153 (2014) 379e382, http://dx.doi.org/10.1016/j.biortech.2013. 12.004.
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Hydrolysis of cellulosic bamboo biomass into reducing sugars via a combined alkaline solution and ionic liquid pretreament steps Samuel Kassaye a, Kamal K. Pant a, *, Sapna Jain b a b
Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110 016, India Department of Physical Science, Alabama State University, Montgomery, AL, 36104, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2016 Received in revised form 28 November 2016 Accepted 14 December 2016 Available online 18 December 2016
Dilute acid hydrolysis of cellulosic biomass is not only controlled by the reaction conditions such as temperature, concentration of acidic catalyst and hydrolysis time but also by changing the physical aspects of the reaction media. Therefore, overcoming the insolubility of cellulose by the use of effective solvent without having to derivatize their basic functional groups is of tremendous advantage in the utilization of lignocellulosic biomass. Ionic liquids are considered as the most suitable solvents to dissolve cellulosic biomass and overcome the recalcitrant nature of lignocellulosic biomass. This study investigates the valorisation of bamboo biomass regenerated from alkaline solution and ionic liquid pretreament steps followed by dilute sulphuric acid hydrolysis. Lignin removal from the biomass as a result of pretreatment steps was analysed by crystallinity index determination, surface morphology and thermal stability analysis. The solid biomass materials were characterized using FTIR, XDR, SEM, TGA and Elemental analysis techniques to investigate the effect of the pretreatment steps on the properties of the original bamboo biomass. Alkaline treatment was found to be effective against lignin and hemicellulose removal. However, it did not remove the complexity of the cellulosic portion of the biomass with equal success. The crystallinity of the recovered cellulosic biomass could be effectively reduced by using the ionic liquid pretreatment. Investigation revealed that the yield of total reducing sugars increased to 64% after alkaline solution pre-treatment in comparison to only 30% yield of reducing sugar in the untreated biomass sample. After both pretreatment steps, the yield of reducing sugar further increased to 80%. © 2016 Published by Elsevier Ltd.
Keywords: Bamboo biomass Pretreatment Ionic liquid TRS yield
1. Introduction The potential of biomass as feedstock for transportation fuels and platform chemical production had not been given enough attention because of the historic shift towards petroleum based resources in the twentieth century [1]. However, more recently, limited availability and environmental concern are the compelling factors in the pursuit of substituting fossil based energy dependency [2]. Lignocellulosic biomass is the ideal substitution for fossil based transportation fuel and platform chemicals production due to its abundance, low cost, even distribution and carbonneutrality [3] [4]. Lignocellulosic biomass is a porous microstructured composite which is composed of three biopolymers; lignin (15e25%), hemicelluloses (23e32%) and cellulose (38e50%) [5] [6]. These constituents form three-dimensional polymeric
* Corresponding author. E-mail address: [email protected] (K.K. Pant). http://dx.doi.org/10.1016/j.renene.2016.12.033 0960-1481/© 2016 Published by Elsevier Ltd.
composites to provide structural rigidity and stability to the cell wall of the plant. Cellulose and hemicellulose combined (holocellulose) forms the carbohydrate portion of the lignocellulosic biomass which can be depolymerized into simple sugars such as glucose and xylose. Lignin serves as a binder to hold together the cellulose and hemicellulose in the cell wall through the complex intra and inter-hydrogen bonding networks [7]. The complex nature of the lignocellulosic biomass creates a major challenge in developing a competitive process technology to convert lignocellulosic biomass into fuels and chemicals in economically feasible and environmentally friendly manner [8]. In addition, lignocellulosic biomass cannot be directly converted into bioethanol due to the presence of lignin and the recalcitrant nature of the cellulosic portion of the biomass [9] and these obstacles have been long standing challenges on the utilization of lignocellulosic biomass. However, more recently the discovery of ionic liquids for biomass transformation has opened a new area of research to utilize lignocellulosic biomass for intermediate chemical production such as sugars and furan derivatives. Ionic liquids are special
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solvents that can dissolve materials that are otherwise considered insoluble in conventional solvents. Ionic liquids have special properties such as broader liquid temperature, high thermal stability and negligible vapour pressure which are the vital properties required in the transformation lignocellulosic biomass [10]. In the conversion process of lignocellulosic biomass into transportation fuel and platform chemicals, pretreatment is most important step for conditioning biomass. Pretreatment plays an important role in fractionating the lignocellulosic biomass to its components and improve accessibility of cellulosic portion for downstream processes. However, it is the most energy intensive and economically expensive process [11]. Biomass pretreatment processes are categorized into different methods ranging from physical, physico-chemical, and chemical to biological methods based on the techniques applied. Alkaline solution pretreatment is a type of chemical pretreatment effective in the delignification of biomass without significantly affecting cellulosic structure of the biomass [12,13]. However, this method of pretreatment has limited effect on the reduction of the crystallinity of pretreated biomass at mild concentration and temperature to reduce the recalcitrant behaviour of cellulosic biomass rendering resistance in the hydrolysis process. Therefore, ionic liquid pretreatment which is also a type of chemical pretreatment method known for its specific purpose in dissolving and reducing the crystallinity of cellulose is incorporated following the alkaline solution pretreatment. This method reduce the crystallinity index of the cellulosic biomass so that the reactivity of the biomass could be enhanced during acid hydrolysis [15,16]. In this study, bamboo biomass was pretreated in alkaline solution and ionic liquid solvent for the purpose of improving acid hydrolysis and reducing the recalcitrant behaviour of the biomass through the combined effect of pretreatment steps to partially remove lignin and reduce crystallinity index. The biomass obtained from Haryana, India was studied for structural, morphological, chemical and physical changes exhibited as the result of the pretreatment steps. All the three bamboo biomass samples (original bamboo biomass, alkaline pretreated and ionic liquid pretreated) were subjected to dilute acid hydrolysis in ionic liquid solvent media (1-butyl 3-methylimidazolium chloride ([BMIM] Cl)). The total reducing sugar yield (TRS) was estimated by 3, 5dinitrisalcylic acid (DNS) array method using UVeVisible spectroscopy. The effects of hydrolysis temperature, time, acid concentration and substrate type on the TRS yield were studied in detail. The original bamboo biomass is referred as “Original bamboo biomass” (OBB), the solid biomass recovered from alkaline solution and [BMIM] Cl pretreatment was referred as “Recovered Biomass I” (RBI) and biomass obtained after acid treatment is referred as “Regenerated Biomass II” (RBII). 2. Materials, experimental and characterization 2.1. Materials Sulfuric acid (98%), N-methylimidazolium, 1-chlorobutane, acetonitrile and ethyl acetate were purchased from Spectrochem (Anand Bhuvan, Princess Street Mumbai, India). Bamboo biomass (Dendrocalamus Strictus) was obtained from Haryana, India. All chemicals were used without further purification. 2.2. Experimental 2.2.1. [BMIM] Cl synthesis 1-butyl 3-methylimidazolium chloride ([BMIM] Cl) was prepared form 1-chlorobutane and N-methyl imidazole in a toluene media as reported in literature [16] [17].
2.2.2. Ash content The ash content of the bamboo biomass was determined as part of the total composition of the bamboo biomass. The determination was performed using the National Renewable Energy Laboratory (NREL) procedure (Sluiter et al., 2008). In a typical ash content measurement, 2 g of the oven dried bamboo biomass was placed into weighed crucible and the sample was heated in muffle furnace (KHERA INSTRUMENTS PVT.LTD, INDIA) at 575 C for 5 h. After carefully removing the sample in the crucible, both the sample and the crucible weighed. The ash content of the bamboo biomass was estimated as percent of residue from the heating process and calculated from the Equation (1):
% Ash content ¼
Weightcrucible plus ash Weightcrucible * 100 % Oven dried Weightsample (1)
2.2.3. Extracts removal Three step extractive removal procedure was used to remove extracts from the bamboo biomass. Soxhlet extraction setup was used to remove the extracts while water, ethanol and hexane were used as extraction solvents as reported in literature [18]. In a typical extraction procedure, 5 g of original bamboo biomass was extracted for 2 h in each solvent and the weight change was measured after drying the extracted biomass after 24 h at 105 C in vacuum drying oven. Reduction in weight was observed after every treatment step as every step resulted in removal of some component of the biomass. 2.2.4. Pretreatment Extractive removed bamboo biomass sample weighing 1 g was treated with different concentration of sodium hydroxide solution in a round bottomed flask at predetermined temperature and time in 1:20 wt ratio of biomass to alkaline solution loading. The solid biomass was recovered by vacuum filtration using porcelain Bucher funnel and then washed three times with distilled water. The wet biomass material was dried at 105 C in vacuum dry oven for 24 h. Similarly, the precipitated lignin was separated using vacuum filtration and dried at 110 C for 24 h. The supernatant obtained from the partially dissolved biomass was adjusted to a pH of 5 using a dilute solution of HCl and recovered as dark brown precipitate of lignin (Silverstein et al., 2007). The effect of temperature, time and alkaline solution concentration was studied to determine degree of delignification and the crystallinity index of the recovered biomass. After determining the optimal condition for alkaline solution pretreatment, the biomass was treated in a larger volume batch reactor. Extracts removed biomass weighing 10 g was treated with 5% (W/V) of sodium hydroxide solution in a sealed AMAR Autoclave reactor for 30 min at 130 C and all the other posttreatment processes followed as described above. Finally, the biomass recovered from alkaline pretreament step was further treated using [BMIM] Cl to reduce the crystallinity of the alkaline treated biomass (RBI) and overcome the recalcitrant nature of the biomass. In this step 2.5 g of RBI was dissolve in 12.5 g of [BMIM] Cl and heated at 120 C for 6 h with continuous stirring. After completion of treatment time, 60 ml of Mill-Q water was added and stirred vigorously for 15 min to regenerate the cellulosic rich bamboo biomass (RBII). RBII was finally collected from the solution using vacuum filtration and washed with distilled water three times and then dried under vacuum at 105 C for 24 h.
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2.2.5. Biomass characterization Elemental composition of the biomass samples was performed to determine the carbon, hydrogen, nitrogen and sulfur content while oxygen content was determined by difference using a VarioEL V5.16.7.10, CHN Mode (Elemental Vario el Cube analyser from German) elemental composition analyser. The Fourier transform infrared (FTIR) of OBB, RBI and RBII were scanned from 4000 to 400 cm1 at a resolution of 4 cm1 to probe the change in functional groups due to the pretreatment steps. The translucent KBr discs were made from homogenized 5 mg biomass sample in 100 mg KBr using electrically operated automatic press (Model-AP-15 from Techno-Search Instruments, India) at 7 tons for 5 min. The TGA analysis of all the three samples (OBB, RBI and RBII) weighted, being 10 and 12 mg were carried out using TGA-Q600 instrument supplied by Waters LLC, USA with a continuous flow of N2 at flow rate of 40 ml/min and heating rate of 10 C/min for a temperature ranging from 25 to 600 C. In addition, the calorific value of the three bamboo biomass samples was determined using bomb calorimeter. Scanning electron microscopy (SEM) images of OBB, RBI and RBII were taken using a ZEISS EVO at 20 kV to study the change in the morphology of the bamboo biomass occurred due to the pretreatment steps. Eiko IB-3 Incoater was using in order to coat the samples with gold for improving the imaging of samples. The crystallinity indexes OBB, RBI and RBII were estimated using x-ray diffraction analysis. The analysis was performed with a Rigaku miniflex 600. The diffracted intensity of Cu K a radiation generated at 40 kV and 15 mA was measured in a 2q range between 5 and 80. The crystallinity indexes of the biomasses were determined using Segal's method [19] [20] as illustrated in Equation (2):
CrI ð%Þ ¼
ðI002 e Iam Þ *100% Iam
(2)
Where I00 and Iam I002 is the intensity of 002 peak at 2q ¼ 22.4 and Iam is the intensity of background scatter at 2q ¼ 18.
2.2.6. Acid hydrolysis The various cellulosic bamboo biomass samples (0.1 g) (OBB, RBI and RBII) were dissolved in [BMIM] Cl (4 g) at 175 C for 30 min followed with the addition of 10 ml of predetermined concentration of dilute H2SO4. After hydrolysis reaction, the mixture was quenched using ice bath for about 15 min and the pH adjusted to neutral using calcium carbonate solution. Then the hydrolysate was separated by vacuum filtration and further purified with centrifugation process.
2.2.7. TRS analysis The hydrolysate obtained from the acid hydrolysis was analysed using DNS array method [21]. The absorbance value of the hydrolysate reacting with DNS reagent was determined in UVeVis spectroscopy (CARY 100 Conc) at 540 nm wavelength. The TRS yield was determined from calibration curve formed using four point concentration of standard glucose solutions. The reducing sugar yield from the hydrolysis reaction was quantified using Eqn. (1) [22] as shown in Equation (3).
TRS Yield ð%Þ ¼
Reducing sugar weight 162 180 100 Dry cellulosic biomass weight
(3)
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3. Result and discussion 3.1. Extract removal The extracts in dry matter of lignocellulosic biomass such as nitrites, protein, ash, waxes, chlorophyll, etc were extracted using a Soxhlet apparatus and refluxed with three step solvent extractors namely: Milli-Q water, ethanol and hexane under heat for 2 h in each solvent case. The water was effective in removing inorganic materials and non-structural sugars while ethanol removed polar compounds such as chlorophyll, waxes and sterols. Finally, hexane was used to remove non-polar lipids and other hydrocarbons compounds. The weight of extract removed from the three steps were; 13.6 ± 0.8, 2.6 ± 0.6 and 5.2 ± 0.8 for water, ethanol and hexane, respectively. 3.2. Pretreatment The two step process of delignification and hemicellulose removal were carried out to efficiently regenerate cellulose rich biomass. Table 1 summarizes the change in mass observed in the process after uniform drying process. The effect of temperature, pretreatment time and alkaline solution concentration on the degree of delignification and biomass recovered was investigated. As can be seen from Table 1, the level of lignin removal from the alkaline solution pretreatment was dependent on the temperature while alkaline solution concentration and treatment time were kept constant at 5 wt % and 30 min, respectively. The amount of lignin recovery increased with increase in temperature while the solid biomass recovery decreased. The effect of pretreatment on the extent of lignin removed and biomass recovered from alkaline solution pretreatment was studied. It was found that the lignin residue recovered from the treatment increased as pretreatment time prolonged. Different concentrations of alkaline solution were considered while studying the effect of pretreatment dependency on reducing sugar yield. Based on the lignin recovery and biomass residue collected from the pretreatment process, it was observed that lignin recovery increased as the concentration of sodium hydroxide increased whereas amount of biomass recovered decreased as the concentration of alkaline solution increased due to the partial removal of lignin and hemicellulose in the pretreatment process. It was observed that the crystallinity index of the recovered biomass increased with temperature while maintaining the alkaline solution concentration and time of pretreatment fixed. This is probably due to the partial removal of hemicellulose, lignin and amorphous cellulose which increased the crystalline cellulose portion. However, the crystallinity index was reduced with increasing the duration of pretreatment and concentration of alkaline solution at constant temperature. The biomass recovered from pretreatment with alkaline solution and ionic liquid had higher crystallinity index due to the partial removal of lignin and hemicellulose In addition, [BMIM] Cl pretreatment enhanced the accessibility and digestibility of the biomass by overcoming the recalcitrant behaviour of the cellulosic biomass. 3.3. Ultimate and proximate analysis The OBB, RBI and RBII were analysed for ultimate analysis to quantify the elemental composition of sample substrates. The results presented in Table 2 include the crystallinity index and calorific values of the samples. As observed from the ultimate analysis, the carbon content of the bamboo biomass reduced from 45.2± 0.55% to 44.4± 0.52% for alkaline solution pretreatment and further reduced to 43.2 ± 0.46% after [BMIM] Cl pretreatment. The
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Table 1 Alkaline pretreatment. Effect of temperature, time and concentration on alkaline solution pretreatment of bamboo biomass. Temperature effect Temperature ( C)
NaOH (Wt. %)
Time (min)
Decrease in biomass (%)
Lignin residue (%)
Biomass residue (%)
CrI index (%)
90 110 130 150
5 5 5 5
30 30 30 30
12 18 22.4 28.4
8.4 8.5 9.1 10.2
88 82 77.6 71.6
58.8 60.7 60 60.1
Time effect Time (min)
NaOH (Wt. %)
Temperature ( C)
Decrease in biomass (%)
Lignin residue (%)
Biomass residue (%)
CrI index (%)
30 45 60 75
5 5 5 5
130 130 130 130
22.4 25.4 28.9 30.5
9.1 9.9 12.4 15.2
77.6 74.6 71.1 69.5
60 55.3 54.3 53.4
Concentration effect NaOH (Wt. %)
Time (min)
Temperature ( C)
Decrease in biomass (%)
Lignin residue (%)
Biomass residue (%)
CrI index (%)
5 7.5 10 12.5
30 30 30 30
130 130 130 130
22.4 34.8 35.6 37.9
9.1 12.6 13.2 14.8
77.6 65.2 64.4 62.1
60 60 56.2 46.4
reduction in carbon content observed is possibly attributed to the partial removal of hemicellulose, amorphous cellulose and lignin portion of the biomass due to the alkaline solution pretreatment. However, the hydrogen and oxygen content in the pretreated biomass increased after the pretreatment steps as compared to the original biomass. The ash content of the three biomass samples exhibited similar content for both the original and pretreated samples. However, the calorific values of RBI decreased from 17.33 to 16.25 MJ/kg and further decreased to 14.27 MJ/kg for RBII which can be accounted for the removal of carbon content from OBB.
3.5. SEM analysis The Fig. S1 shows the SEM images of the three samples (OBB, RBI and RBII). The images highlight the structural changes in biomass as a result of the pretreatment steps. OBB has more open, cylindrical and compact arrangement while RBI biomass sample exhibited a loss in hollow cylindrical shape of the OBB biomass and destruction in the structural rigidity is clearly observed. In case RBII sample a more destruction in the structure of the samples can be clearly seen. 3.6. FTIR analysis
3.4. XRD analysis Fig. 1 shows the XRD results of the three biomass samples. As explained above in the case of alkaline pretreatment process, the crystallinity index increased in RBI sample due to the removal of lignin, hemicellulose and amorphous cellulose at optimized pretreatment condition. However, [BMIM] Cl pretreatment step resulted in a significant decrease in the crystallinity of RBII sample due to breakage of the intra and inter-hydrogen bonding network. It is observed that the combined use of alkaline and [BMIM] Cl pretreatment was effective in removing lignin and hemicellulose portion of the biomass and reducing the crystallinity of the cellulosic bamboo biomass. It should be noted that the reduction in crystallinity index of the biomass is very crucial in enhancing the reactivity of the cellulosic biomass during enzymatic or acid hydrolysis. The overall crystallinity of all the three biomass samples as was measured and presented in Table 2, the crystallinity index of RBI is around 60% while OBB and RBII 56.5% and 39.5%, respectively.
The FTIR analysis of OBB, RBI and RBII was carried out in order to investigate the changes in structural features and functional groups change as a result of the pretreatment steps. The FTIR of the three samples is shown in the Fig. 2. Both the OBB and RBI samples have strong broad absorption band at 3350 cm1 due to the eOH stretching vibrations while RBII sample showed weak absorption band due to the elimination of the hydrogen bonding network and the release of eOH group. The prominent CeH stretching absorption was observed at 2910 cm1 in OBB and RBI completely disappeared from RBII. The absorbance at 1740 cm1 attributed to C] O stretching vibration accounted for the presence of hemicellulose showed a very weak absorption in the case of RBII sample implying the removal of hemicellulose. The aromatic skeletal vibration in lignin was evidenced in the absorption from 1600 to 1450 cm1 which is stronger in the case of OBB sample than RBII due to the partial removal of lignin while RBII sample exhibited the absence of aromatic skeletal vibration. The absorption peaks at 1740 and
Table 2 Ultimate and proximate analysis. Ultimate and proximate analysis of OBB, RBI and RBII. Ultimate analysis Elemental composition (Wt. %)
Ash content (%) Calorific value (MJ/Kg) CrI (%)
C H N S O e e e
OBB
RBI
RBII
45.2 ± 0.55 5.9 ± 0.15 0.52 ± 0.11 0.35 ± 0.01 48.03 ± 0.8 2.2 ± 0.2 17.338 56.5
44.4 ± 0.52 6.4 ± 0.16 0.06 ± 0.01 0.15 ± 0.02 48.99 ± 1.2 2.1 ± 0.3 16.251 60
43.2 ± 0.46 6.2 ± 0.2 0.25 ± 0.02 0.05 ± 0.01 50.3 ± 0.9 2.1 ± 0.1 14.276 39.5
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Fig. 1. XRD. XRD of OBB, RBI and RBII.
1245 cm1 are the result of the acetyl and uronic ester groups of the hemicellulose of the biomass and as can be observed from RBII sample, these peaks disappeared in comparison with OBB and RBI [23]. The intensity of the peak at 893 cm1 which is the characteristics of the presence of b-1,4 glycosidic bond linkages in the cellulose component of the biomass was observed in all the three samples signifying the pretreatment steps has not affected significantly the cellulose portion of the biomass.
3.7. TGA-DTA analysis The thermal property of OBB, RBI and RBII was analysed using thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) methods as shown in Fig. S2. The analysis provided important thermal property changes that occurred due to conditioning and pretreatment steps of the biomass. The weight changes of the biomass samples with respect to temperature can be used to estimate the composition of the samples in terms of the amount of cellulose, hemicellulose and lignin present in the sample. As reported in literature [24] and also observed from the analysis, the most reactive compound decomposes between 200 and 350 C, whereas cellulose decomposition occur from 305 to
Fig. 2. FTIR analysis. FTIR of OBB, RBI and RBII.
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375 C and lignin decompose at the temperature ranging from 250 to 500 C. In the TGA analysis, all the three samples showed similar pattern of thermal decomposition trend exhibiting three different regions as also reported in literature [25]. RBI and RBII showed higher thermal stability than OBB due to the removal of hemicellulose and other easily decomposing compounds from these samples. As observed from the TGA plot, in stage one which occurred in the temperature range of 100e175 C, the evaporation of moisture trapped in the biomass was eminent while in stage two which ranges from 175 to 430 C, the devolatilization of hydrocarbons was exhibited. Stage two mainly accounts for thermal decomposition of cellulose, hemicellulose and trace amount of lignin. Each sample of the analysed biomass represents different percentage of weight loss due to the different composition of the three polymeric materials. OBB lost 65% of its total weight in the decomposition process, while RBI and RBII samples weight loss was 70 and 77% in stage two, respectively. The difference in decomposition pattern and weight loss in different biomass samples originates from the composition of the samples under study. RBI and RBII contain more of cellulosic portion which decompose mostly in stage two as displayed in the TGA analysis pot. 3.8. Acid hydrolysis of RBII In the acid hydrolysis of bamboo biomass, it was possible to observe that prior dissolution of the substrates samples with [BMIM] Cl improved the yield of TRS as compared to the direct mixing of the reaction mixtures. Prior dissolution of the substrates overcomes recalcitrant nature of the cellulosic material by breaking the intra and inter-hydrogen bonding network which in turn reduce the crystallinity of the biomass while increasing the accessible surface area of the substrate to interact with the catalyst [26]. The reduction in crystallinity of the biomass was found to be significant in improving the penetration of hydronium ions of the sulphuric acid catalyst into the dissolved cellulosic substrate and catalyse the cleavage of the b-1,4-glycosidic bond [27]. [BMIM] Cl is considered as the most suitable ionic liquid in the transformation of biomass due to its valuable properties such as; low melting point,
Fig. 3. Effect of temperature. Effect of temperature on TRS yield from RBII substrate. Hydrolysis Conditions: Time 2 h, Acid conc. 5% v/v, substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
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Fig. 4. Effect of hydrolysis time. Effect of hydrolysis time on TRS yield from RBII substrate. Hydrolysis Conditions: Time 2 h, Acid conc. 5% v/v, substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
moderate viscosity and minimized corrosive property. In addition, its chloride ions form non-dimensional hydrogen bonds to link with cellulose molecules dissolved and hinder the reformation of the hydrogen bonds within cellulose chain and strands. [BMIM] Cl is known to interact with cellulosic biomass via ionic, p-p and hydrogen bonding interactions. During dissolution process, the p-p interaction plays an important role in the disruption of inter and intramolecular hydrogen bonds. 3.8.1. Effect of temperature In the present study, by limiting the maximum hydrolysis temperature at 220 C due to the minimum decomposition temperature of [BMIM] Cl (240 C), it was possible to hydrolyse RBII up to a maximum of 72% while 32% and 64% of TRS yield obtained for OBB and RBI, respectively at the similar hydrolysis temperature. The effect of temperature on the hydrolysis of RBII was studied in temperature range between 140 and 220 C with 10 C intervals as shown in Fig. 3. The effect of temperature on TRS yield showed a minor (only 2%) increase in yield as the temperature increased from 140 to 160 C. However, as the temperature changes from 160 to 170 C the yield of TRS increased up to 32% and reached to maxima (72%) at 180 C. This significant change in yield could possibly be the result of maximum dissolution and reduction in crystalline structure of RBII substrate which facilitated the contact between the substrate and the hydrogen ions in homogeneous ionic liquid hydrolysis media. Another interesting result is that TRS yield was started to decline as the temperature increased from 190 to 220 C in 40% yield. The decline in yield was mainly due to the decomposition of reducing sugars into some other form such as humins and furan derivative chemicals [28]. 3.8.2. Effect of time The total reducing sugar yield obtained from the hydrolysis of cellulosic bamboo biomass using dilute sulphuric acid catalyst in [BMIM] Cl media was found to be significantly dependent on the time of hydrolysis. The effect of hydrolysis time on the TRS yield was studied for a various set of temperature as shown in Fig. 4. TRS yield increased as the temperature increased for each set of temperature studied and reached maxima. However, after reaching
Fig. 5. Effect of acid concentration. Effect of acid concentration on TRS yield using RBII substrate. Hydrolysis Conditions: temperature 180 C, reaction time 2 h, substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
maximum yield, extended hydrolysis time resulted in declined yield as observed in the case of effect of temperature study due to the possible decomposition of sugar molecules produced. As shown in Fig. 4, the yield of TRS for all set of temperature studied have shown an increase in yield for some time during the course of the reaction and diminish thereafter implying decomposition of the sugar into other products. For instance in case of hydrolysis of RBII at 160 C, the maximum yield of TRS was obtained at 150 min and start decreasing as the time of hydrolysis further extended. The yield gradually increased after 30 min of hydrolysis time and continued increasing up to 150 min when the hydrolysis was carried out at 180 C. However, as the hydrolysis time extended further
Fig. 6. Effect of substrate type. Effect of substrate type on TRS yield. Hydrolysis Conditions: temperature 180 C, reaction time 60 min, 2 h, Acid Conc. 5% v/v, Substrate to [BMIM] Cl 1: 40 ratio and stirring speed of 500 rpm.
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from 150 min the TRS yield started declining as a result of product decomposition. Similarly, for hydrolysis taken at 200 C the maximum yield of TRS was obtained at 30 min marking minimum time of hydrolysis to reach maximum yield compared with the other temperatures, used in this study. From the general observation of experimental runs at different temperatures, it can be deduced that effective hydrolysis requires sufficient time between the substrate and hydrogen ions (Hþ) to obtain the maximum possible interaction that lead the catalyst to break the b-1,4 glycosidic bond to release sugar monomers from the depolymerization of cellulosic bamboo biomass [29]. 3.8.3. Effect of acid concentration Fig. 5 shows effect of dilute acid concentration sulphuric acid on yield of total reducing sugar. It is observed that the TRS yield was dependent on the amount of acid loaded (acid concentration). Increasing the acid concentration in the hydrolysis of RBII significantly affected the yield of TRS. As shown in the Fig. 5 it can be observed that the TRS yield increased as the concentration of acid increased for the given set of hydrolysis condition. The hydrolysis of cellulosic biomass is dependent on the concentration of hydrogen ion [27]. The higher the concentration of acid catalyst, there will be more number of hydrogen ions which can attack the glycosidic bond to release sugar molecules through subsequently protonation of the bond to form carbonium cation. Then, these carbonium cations easily hydrolyse to sugar molecules. As can be seen from the plot of TRS versus acid concentration, only 10% TRS yield was generated in pure water while over 40% increase in sugar yield was observed as the concentration of the acid increased from 2.5 to 5% v/v. The change in TRS yield increased up to a maximum of 80% for RBII substrate as the concentration of the acid increased to 7.5% v/v. 3.8.4. Effect of substrate type Different substrate (OBB, RBI and RBII) types was depolymerized and compared with TRS yield obtained from the hydrolysis of microcrystalline cellulose (MCC) to investigate the effect of the pretreatment steps on the effectiveness of depolymerisation process. All the four substrate samples were hydrolysed in the same reaction condition: temperature of 180 C, 3 h hydrolysis time and substrate to [BMIM] Cl weight ratio of 1:40. It was observed that the substrate sample which passed through the two subsequent pretreatment steps (RBII) resulted with TRS yield of 72% while the substrate pretreated only with alkaline solution (RBI) resulted in 64% TRS yield. The original bamboo biomass (OBB) yielded 32% while the microcrystalline cellulose used as model substrate hydrolysed to give 92% yield as shown in the Fig. 6. The results indicate that the TRS yield was significantly affected by the nature of the substrate which is related to the corresponding structural complexity, composition content, accessible surface area of the substrate and crystallinity index [27]. Thus, [BMIM] Cl ionic liquid used in this study contributed enormously in enhancing the dilute acid hydrolysis of the bamboo biomass by reducing the crystallinity of the substrate and facilitating contact between the hydrogen ions of the catalyst and the b-1, 4-glycosidic bond of the cellulose. 4. Conclusion Dilute sulphuric acid hydrolysis of different forms of bamboo biomass in [BMIM] Cl ionic liquid media was effective in terms of high yield of reducing sugars at relatively lower temperature (180 C) and pressure than the convention processes that take place at higher temperature (>250 C) and pressure. Hence, it was observed that the hydrolysis of RBII gives a better yield than OBB and RBI due to the relative reactivity of RBII substrate resulting from the influence of pretreatment steps in overcoming the
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recalcitrant behaviours of the biomass. This effect was evidenced from the lowest value of crystallinity index calculated for RBII and the corresponding high yield of sugars obtained. The two step pretreatment using alkaline solution and [BMIM] Cl ionic liquid resulted in removal of large portion of lignin and hemicellulose besides overcoming the structural complexity of bamboo biomass and resulting in easier dilute acid hydrolysis, evidenced with higher yield of reducing sugars. It is also observed that the combined pretreatment steps overcame two main challenges faced in the catalytic hydrolysis of lignocellulosic biomass into sugars, first by removing the lignin and hemicellulose which shield the accessibility of cellulose and second by reducing the recalcitrant behaviour of cellulose by eliminating structural complexity of lignocellulosic. Acknowledgement The authors acknowledge the financial support from the Department of Chemical Engineering, Indian Institute of Technology Delhi (NPN-05/BCHE). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.renene.2016.12.033. References [1] A. Brandt, J. Gr€ asvik, J.P. Hallett, T. Welton, Deconstruction of lignocellulosic biomass with ionic liquids, Green Chem. 15 (2013) 550, http://dx.doi.org/ 10.1039/c2gc36364j. [2] S. Nanda, P. Mohanty, K.K. Pant, S. Naik, J. a Kozinski, A.K. Dalai, Characterization of North American lignocellulosic biomass and biochars in terms of their candidacy for alternate renewable fuels, Bioenergy Res. 6 (2013) 663e677, http://dx.doi.org/10.1007/s12155-012-9281-4. [3] P. Weerachanchai, J.-M. Lee, Recyclability of an ionic liquid for biomass pretreatment, Bioresour. Technol. 169 (2014) 336e343, http://dx.doi.org/ 10.1016/j.biortech.2014.06.072. [4] J.-F. Liu, Y. Cao, M.-H. Yang, X.-J. Wang, H.-Q. Li, J.-M. Xing, Enhanced saccharification of lignocellulosic biomass with 1-allyl-3-methylimidazolium chloride (AmimCl) pretreatment, Chin. Chem. Lett. 25 (2014) 1485e1488, http://dx.doi.org/10.1016/j.cclet.2014.06.001. [5] N. Sun, H. Liu, N. Sathitsuksanoh, V. Stavila, M. Sawant, A. Bonito, K. Tran, A. George, K.L. Sale, S. Singh, B. a Simmons, B.M. Holmes, Production and extraction of sugars from switchgrass hydrolyzed in ionic liquids, Biotechnol. Biofuels 6 (2013) 39, http://dx.doi.org/10.1186/1754-6834-6-39. [6] H. Tadesse, R. Luque, Advances on biomass pretreatment using ionic liquids: an overview, Energy Environ. Sci. 4 (2011) 3913, http://dx.doi.org/10.1039/ c0ee00667j. [7] I.G. Baek, S.J. You, E.D. Park, Bioresource technology direct conversion of cellulose into polyols over Ni/W/SiO 2 -Al 2 O 3, Bioresour. Technol. 114 (2012) 684e690, http://dx.doi.org/10.1016/j.biortech.2012.03.059. [8] S. Behera, R. Arora, N. Nandhagopal, S. Kumar, Importance of chemical pretreatment for bioconversion of lignocellulosic biomass, Renew. Sustain. Energy Rev. 36 (2014) 91e106, http://dx.doi.org/10.1016/j.rser.2014.04.047. [9] H. Argun, G. Onaran, Delignification of vineyard pruning residues by alkaline peroxide treatment, Ind. Crops Prod. 74 (2015) 697e702, http://dx.doi.org/ 10.1016/j.indcrop.2015.05.031. [10] N.L. Mai, S.H. Ha, Y. Koo, Efficient pretreatment of lignocellulose in ionic liquids/co-solvent for enzymatic hydrolysis enhancement into fermentable sugars, Process Biochem. 49 (7) (2014) 1144e1151, http://dx.doi.org/10.1016/ j.procbio.2014.03.024. [11] K. Ninomiya, K. Inoue, Y. Aomori, A. Ohnishi, C. Ogino, N. Shimizu, K. Takahashi, Characterization of fractionated biomass component and recovered ionic liquid during repeated process of cholinium ionic liquidassisted pretreatment and fractionation, Chem. Eng. J. 259 (2015) 323e329, http://dx.doi.org/10.1016/j.cej.2014.07.122. [12] H. Rabemanolontsoa, S. Saka, Various pretreatments of lignocellulosics, bioresour, Technol 199 (2015) 83e91, http://dx.doi.org/10.1016/ j.biortech.2015.08.029. [13] J.S. Kim, Y.Y. Lee, T.H. Kim, A review on alkaline pretreatment technology for bioconversion of lignocellulosic biomass, Bioresour. Technol. 199 (2015) 42e48, http://dx.doi.org/10.1016/j.biortech.2015.08.085. [15] J. Zhang, L. Feng, D. Wang, R. Zhang, G. Liu, G. Cheng, Thermogravimetric analysis of lignocellulosic biomass with ionic liquid pretreatment, Bioresour. Technol. 153 (2014) 379e382, http://dx.doi.org/10.1016/j.biortech.2013. 12.004.
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