Abstract
Green synthesis of room temperature ionic liquids (RTILs), are presented as friendly and challenging solvents for the effective dissolution of oil palm-lignocellulosic biomass. A series of Bronsted acidic-ionic liquids were prepared by the direct neutralization of diethyl dimethyl ammonium hydroxide with several (economical and environmental friendly) Bronsted acids as RTILs. The structural and physicochemical characterization was performed by applying various techniques as Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR), thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC), zeta-nanosizer and dynamic light scattering (DLS) respectively, to state the effect of anion on the extended cellulose dissolution capabilities of the synthesized RTILs under mild conditions. As a polysaccharide solvent, diethyl dimethyl ammonium phosphate (A1P) showed the extreme capability to extract 65 % of cellulose from biomass without any pretreatment for 30 min. The present study could be a significant step toward the synthesis of efficient RTILs and generating upgraded cellulose for Hi-tech engineered composites and energy concerns.
Introduction
RTILs, also termed as liquid salts at room temperature, are thermodynamically favored because the size of constituent ions resulting in small lattice enthalpies and large entropy changes that favor the liquid state [1], [2]. Due to their low volatility and extraordinary thermal stability, RTILs are highly engaged as reusable “green” solvents for several industrial developments such as dissolution, extraction, catalysis and synthesis [3], [4], [5]. The prospective of RTILs to dissolve a large number of biomacromolecules such as cellulose, silk fibroin, lignin, starch, zein protein, chitin/chitosan and wool keratin with high efficiency is demanding day by day. Regarding cellulose dissolution, the interest in room temperature ionic liquids (RTILs) must be addressed due to the low cost of preparation, accepted synthetic procedure, refining, biodegradability and the use of environmental friendly acids [6], [7], [8]. The recent research has mainly focused on RTILs composed of asymmetric diethyl dimethyl ammonium cations associated with a variety of Bronsted-acidic anions. The effect of cations on the cellulose dissolution capabilities of ILs has been studied in many literature but the current study report to highlight the effect of anions on the overall dissolution of cellulosic biomass. Various halogenated-ILs such as alkylimidazolium chloride with undesirable viscosity corrosiveness, toxicity and disapproving environmental effects are discovered for better cellulose dissolution than caustic or any other organic solvents [9], [10]. This problem can be cracked by modification in respective anions to tune the properties mentioned as stability, H-bonding, charge density, liquid range, viscosity and miscibility to overcome the above problems [11], [12], [13]. The low viscosities of RTILs must be addressed to facilitate the cellulose dissolution at room temperature and to recover activated and highly arranged arrays of cellulose [14].
During the dissolution of oil palm lignocellulosic biomass in RTILs, swelling in cell-wall of plant occurs in the presence of ionic liquid due to extended H-bonding and breaking of existing bonds in the lignocellulose matrix [15]. As a result interaction take place between the hydrogen and oxygen atoms of cellulose with anions of RTILs [16]. Thus, various interactions occur due to presence of the crystalline and residual cellulose, hemicellulose and lignin, which increases the viscosity of solution and decreases the rate of RTIL for effective dissolution of cellulose. Consequently, a mixture of water and acetone was used as antisolvent to stop the direct interaction of cellulose with RTILs [17].
The key objectives of this study include synthesis of a series of ammonium based RTILs, investigating the effect of anion on the structural and physicochemical properties, and cellulose dissolution capabilities of these newly synthesized RTILs under mild conditions.
The previous literature is focusing on the effect of different cations, halogenated anions and high temperature treatment of cellulose [18], [19], [20], [21]. The effect of ammonium based RTILs with environmental-friendly anions must be addressed by focusing on the basicity of anions, its ability to accept hydrogen bonding, their physicochemical performance and their ability to disrupt the interpolymeric holds in lignocellulosic biomass. The synthesized RTILs act as hydrophilic salts which result in regeneration of cellulose simply by the addition of water. Further, the dissolved cellulose was properly quantified and spectroscopically analyzed. Thus, the effect of selective anions on structural and physicochemical properties of synthesized RTILs in term of thermal stability, zeta potential, conductivity, ionic mobility and viscosity is addressed in accordance to their cellulose dissolution capabilities. The effect of pH and temperature on physicochemical and electrokinetic properties of synthesized RTILs is addressed as a tool for further tasks regarding industrial and biochemical applications.
Materials and methods
Materials
Oil-palm biomass samples below 250 microns were processed from nearby plantations around Universiti Sains Malaysia. Diethyl dimethyl ammonium hydroxide (A1H), ascorbic acid (A), phosphoric acid (P), acetic acid (Ac), formic acid (F), citric acid (C), gluconic acid (G), toluene and co-solvent dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich (Germany) and Acros (Geel, Belgium) respectively. All chemicals were of analytical grade and used as received without further purification. The apparatus was dried in oven at 120°C overnight. The cation as diethyl dimethyl ammonium (A1), derivatives were synthesized by using chemical procedures as per discussion.
Analysis of oil palm lignocellulosic biomass
Prior to dissolution, the main components of biomass known as cellulose, hemicellulose and lignin were calculated by using analytical methods as per reported literature.
Analysis of waxes and moisture contents
Initially the biomass samples (B0) were leached in a mixture of benzene/ethanol (2:1 v/v) at constant temperature for 1 h. Then the temperature was raised gently up to 120°C to maintain a steady mass followed by cooling the residue in a desiccator at room temperature for further analysis. The residue dried mass (W1) and the extractive (B1) wt.% is calculated by using Eq. 1 [22].
Analysis of hemicellulose contents
The residue dried mass (B1) was added in a 250 mL flask containing 150 mL NaOH solution (20 g/L). The mixture was boiled for 3 h with recycled water assembly followed by continuous filtering and washing until no more Na+ and then dried to gain a continuous mass. The residue is then allowed to cool and placed in a desiccator and weight (B2). The percentage of hemicellulose (W2) is calculated by using Eq. 2
Analysis of lignin contents
A fix amount of residue (B1) was taken in a 100 mL flask containing 30 mL of sulfuric acid (70 %) and was stored for 48h at 15oC. The mixture was transferred to a flask containing 500 mL deionized distilled water, followed by boiling for 30 min with recycled water. After thoroughly filtering and continuously washing with distilled water to ensure the complete removal of SO42− ions, the residue was dried and kept at room temperature in a desiccator and weight (B3). The lignin wt.% (W3) is calculated as
Analysis of cellulose contents
The cellulose content (W4) is calculated as
Analysis of cellulose recovery
The formula applied to determine the percentage of recovered cellulose after pretreatment is the same as applied before treatment analysis. A critical quantification before and after treatment was done in order to calculate the percentage of cellulose recovered after treating with selective RTILs.
Dissolution of biomass
Oil palm frond samples were first ground and sieved by using a Retch SM 100 series grinder to ensure particle size below 250 microns. Further, 0.1 g of oil-palm biomass was added into 0.8 g of the selected RTIL and incubated at room temperature for 15 min. The dissolution conditions were controlled at 25°C under magnetic stirring at 400 rpm for 30 min with addition of DMSO to ensure the dissolution process [9], [23]. The cellulose flocks were collected by a vacuum filtration, followed by a thorough washing and drying with distilled water and acetone by vigorous stirring for 45 min. The anti-solvent acetone/water was removed from the supernatant by using a rotary evaporator. Afterward, the precipitated solid residue was separated out by decantation and washed several times with distilled water to ensure complete removal of residual RTIL and co-solvent DMSO. The wet fiber was dried in an air-circulating oven at 65°C for 12h. The obtained yield of cellulose was measured as (A1P) 65 %, (A1Ac) 62 %, A1F (58 %) and (A1G) 52 %, (A1C) 53 %, (A1A) 48 % respectively. After mixing, the mixture was centrifuged (Clay Adams® Brand DYNAC centrifuge, Bohemia, NY, USA) for 05 min. The solution was then separated from the residue by vacuum filtration. The recovered cellulose flocks were further washed with deionized water many times and dried at 65°C overnight to calculate the yield [24]. The elemental compositions of highly pure cellulose flocks (without any ionic liquid, DMSO or water content) were studied using a Field Emission Scanning Electron Microscope (S-4800) coupled with an Ametek Energy Dispersive X-ray (EDX) spectrometer. (FT-IR) spectra of the precipitates were recorded over a spectral range of 4000–400 cm−1 with 20 accumulations.
Characterization of RTILs
Fourier transform infrared (FT-IR) absorption spectra of the representative RTILs were measured in liquid form by using (Waltman, MA, USA) spectrum for the identification of various functionalities in the synthesized RTILs. All the spectra were obtained in the range of 4000–400 cm−1 in triplicate. The IR peak signal/ noise ratio was kept 20 000:1 for a scan time of 2 min at room temperature.
The liquid state 13C NMR spectra at 50.28 MHz were obtained using a Bruker AMX-200 spectrometer equipped with an HP amplifier at 200.13 MHz. In all the measurements, the spin rate was kept constant at 8 kHz. In order to obtain the best signal-to-noise ratio. For all the C signals and to minimize the artifacts, the proper contact time was chosen according to a previously reported procedure [25]. In order to avoid signal saturation, a recycle time of 40 s and a 13C pulse width of 3 μs, corresponding to a flip angle of 60° were used.
Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) scans were carried out (using Netzsch TG 209 C Iris system and DSC 204 F1, Netzsch, Germany) respectively. The experiments were performed as follows: 25 mg samples were sealed in aluminum pans and pressing the pans together, cooled down to 0°C and then heated from 0 to 400°C at the rate of 10°C/min. The open aluminum cell was swept with N2 at 20 mL/min during the analysis. The TGA and DSC data were plotted as Temperature versus weight %, from which the onset and final decomposition temperatures were calculated [26].
Field emission scanning electron microscope (FESEM-JEOL JSM6360LV, Japan) was used to determine the morphologies of recovered celluose and their flocks as described in Section “Characterization of the recovered cellulose”. The dried samples were placed on an aluminum mount, sputtered with gold and palladium and then scanned at an accelerating voltage of 5.0 kV.
NANOZS Zetasizer (Malvern Instruments, Malvern, UK, using He-Ne Laser with 633 nm wavelength and a detector angle of 173°) was used to investigate the stability (zeta potential), conductivity and ionic mobility of the synthesized RTILs. The pH of all solution was measured using (Weilheim, Germany) inoLab pH 720 pH meter. All solutions were prepared in the desired buffer as per experimental requirements. Before each reading, each solution was passed through Millipore Millex-HV filter (pore size≈0.45 μm) in order to remove any entrapped dust particles. The required solutions were first equilibrated for 20 min. Each reported value is the average of three measurements taken on the cylindrical cell at stationary level. The standard deviation was <4 % for every measurement. The correlation functions were analyzed by the constrained regularized CONTIN method to obtain distribution decay rates. The decay rates gave the distribution of apparent mutual diffusion coefficient
where n is the refractive index of DMSO. The scattering intensities at different time, pH range (1.68–12.45), temperature (25–65°C), were measured for synthesized RTILs. The correlation functions obtained from DLS were analyzed to get distribution decay rates, zeta potential, conductivities, ionic mobilities and viscosities respectively.
Synthesis of diethyl dimethyl ammonium Brønsted acidic-ionic liquids
Neutralization, quaternization and metathesis reactions are reported in a variety of ionic liquid processing schemes. We proposed neutralization reaction to synthesize several low-cost, sustainable and environmental friendly Bronsted acidic-ionic liquids where the proton transfer taking place between the acid base moieties [29], [30], [31]. The reaction mechanism is shown in Fig. 1.
Synthetic pathway of Bronsted-acidic RTILs.
The detailed synthetic procedure is given as follows. A particular quantity of base (diethyl dimethyl ammonium hydroxide) and toluene was taken in a 50 mL double necked round bottom flask with a pressure equalizing dropping funnel. The stoichiometric quantity of particular protic acid was added drop-wise through the dropping funnel at room temperature. The solution was then stirred at room temperature for 48 h. The immiscible layers were separated by decanting the toluene and the synthesized ionic liquid was washed with n-hexane several times to remove unreacted constituents. Finally, each product was tightly sealed and placed in vacuum oven to remove trace amount of water and to become stable for further analysis [29]. The feed composition of all the reactants and curing parameters are shown in Table 1. The physical data of respective RTILs including molecular weight and elemental analysis are given in Section “NMR analysis of synthesized RTILs”.
Feed composition of diethyl dimethyl ammonium Bronsted acidic-ionic liquids.
| S. no | RTILs | Sample code | A1 (g) | Acid (g) | Curing time (h) | Curing temp (°C) | Setting time (h) | Obtained yield (%) |
|---|---|---|---|---|---|---|---|---|
| (1) | A1-ascorbate | A1A | 0.4 | 0.4 | 48 | 25 | 8 | 81 |
| (2) | A1-phosphate | A1P | 1.2 | 0.4 | 48 | 25 | 8 | 85 |
| (3) | A1-acetate | A1Ac | 0.4 | 0.4 | 48 | 25 | 8 | 81 |
| (4) | A1-formate | A1F | 0.4 | 0.4 | 48 | 25 | 8 | 80 |
| (5) | A1-citrate | A1C | 1.2 | 0.4 | 48 | 25 | 8 | 76 |
| (6) | A1-gluconate | A1G | 0.4 | 0.4 | 48 | 25 | 8 | 72 |
Results and discussion
FT-IR spectra of synthesized RTILs
In order to quantify the selected functionalities, Fig. 2, presents the FT-IR spectra of various ammonium based Bronsted-acidic ionic liquids. The absorption band at 1090–1150 cm−1 is corresponded to (νC–N) stretching of the ammonium linkages to alkyl groups in the overall synthesized ionic liquids. The broad peaks in the range of 3100–3600 cm−1 assigned to axial (νO–H) groups present in the synthesized ionic liquids AIA and AIG, but the respective stretching is absent in A1P and A1Ac, giving a direct clue to the successful replacement of hydroxide by phosphate and acetate moieties. The peaks in the range of 1650–1750 cm−1 assigned to (νC=O) is absent in A1P, but strongly appear in AIA, AIAc and AIG respectively which also verified the effective synthesis of ionic liquids with various acid group moieties [31], [32], [33].
Representative FT-IR spectra of synthesized RTILs.
NMR analysis of synthesized RTILs
The NMR data was recorded on a Bruker ARX of 400 MHz spectrometer at School of Chemical Sciences, Universiti Sains Malaysia to check the purity of the RTILs. The experimentally-determined 13C NMR spectra for representative A1C and A1G are shown in Fig. 3, and the obtained peaks were confirmed with the reported literature as shown in Table 2. Similarly, the 1HNMR spectrum of A1A showed signal at 16.71 ppm correspond to OH group which is attached to double bond while peaks at 3.89 and 3.78 ppm is related to OH attached to –CH bonds. The peak at 166 ppm of –C=O confirms the formation of ionic liquid AIA. The shifting of peaks occurs in AIP due to phosphate group while the peaks at 177, 169 and 178 for carbonyl groups in 13C spectra for ionic liquids AIAc, AIF, AIC and AIG verified the synthesis. Elemental analysis showed that the values of synthesized ionic liquids are in close agreement with the theoretical values.
13C NMR conformational spectra of representative A1C and A1G synthesized RTILs.
13C NMR conformational peaks in representative A1C and A1G synthesized RTILs.
| Peak assignment | Chemical groups | Standard peaks | Experimental peaks |
|---|---|---|---|
| (1) | C–NH2 | 30–37 | 25–32 |
| (2) | C–OH | 40–45 | 48–52 |
| (3) | –CH2 | 50–52 | 45–55 |
| (4) | –C–N | 60–75 | 80–95 |
| (5) | C=O | 185–195 | 180–190 |
N,N-diethyl-N,N-dimethylammonium ascorbate (A1A)
Yield: 81 %: mol. wt.: 277.31: Anal. Calc. for C12H23NO6: C, 51.95 (51.97); H, 8.34 (8.29); N, 4.97 (5.03); O, 34.57 (34.65): 1H NMR (ppm): 16.71 (s, 1H, C–OH), 3.89 (t, 1H, –CH–OH), 3.78 (d, 2H, –CH–CH2), 3.65 (s, 1H, CH2–OH), 3.31 [s, 6H, N–(CH3)2], 3.25 [m, 4H, N–(CH2)2–], 1.21 (t, 6H, 2CH3), 13C NMR (ppm): 171 (C–O−), 168 (C–OH), 166 (–C=O), 74, 62 (2CH2–OH).
N,N-diethyl-N,N-dimethylammonium phosphate (A1P)
Yield: 85 %: mol. wt.: 401.34: Anal. Calc. for C18H48N3O4P: C, 53.81 (53.79); H, 12.01 (12.05); N, 10.49 (10.42); O, 15.91 (15.95); P, 7.65 (7.71): 1H NMR (ppm): 3.33 (s, 18H, N–CH3), 3.29 (m, 12H, N–CH2), 1.22 (t, 18H, –C–CH3), 13C NMR (ppm): 50 (N–6CH3) 59, 7.8 (N–6CH2–6CH3).
N,N-diethyl-N,N-dimethylammonium acetate (A1Ac)
Yield: 81 %: mol. wt.: 161.24: Anal. Calc. for C8H19NO2: C, 59.52 (59.58); H, 11.89 (11.93); N, 8.71 (8.69); O, 19.79 (19.85): 1H NMR (ppm): 3.30 [s, 6H, N–(CH3)2], 3.24 [m, 4H, N–(CH2)2–CH3], 1.25 (t, 6H, 2CH3), 2.21 (S, 3H, CO–CH3), 13C NMR (ppm): 53 (N–2CH3), 61, 7.9 [N–2(CH2–CH3)], 177 (C=O), 22 (COCH3).
N,N-diethyl-N,N-dimethylammonium formate (A1F)
Yield: 80 %: mol. wt.: 147.22: Anal. Calc. for C7H17NO2: C, 57.09 (57.14); H, 11.61 (11.64); N, 9.49 (9.53); O, 21.74 (21.70): 1H NMR (ppm): 3.30 [s, 6H, N–(CH3)2], 3.28 [q, 4H, N–(CH2)2–CH3], 1.26 [t, 6H, N–2(CH2–CH3)], 13C NMR (ppm): 51 (N–2CH3), 61, 7.8 [N–2(CH2–CH3)], 169 (C=O).
N,N-diethyl-N,N-dimethylammonium citrate (A1C)
Yield: 76 %: mol. wt.: 495.69: Anal. Calc. for C24H53N3O7: C, 58.15 (58.09); H, 10.78 (10.75); N, 8.48 (8.39); O, 22.58 (22.49): 1H NMR (ppm): 3.30 [s, 18H, N–6(CH3)], 3.27 [m, 12H, N–6(CH2)], 1.25 [t, 18H, 6(CH3)], 2.84 [s, 4H, 2(CH2–C=O)], 13C NMR (ppm): 49.39 (N–6CH3), 58.58, 8.11 [N–6(CH2–CH3)], 207.88 (C=O), 31.10 [2(CH2–C=O)].
N,N-diethyl-N,N-dimethylammonium gluconate (A1G)
Yield: 72 %: mol. wt.: 297.18: Anal. Calc. for C12H27NO7: C, 48.47 (48.39); H, 9.15 (9.03); N, 4.71 (4.67); O, 37.67 (37.58): 1H NMR (ppm): 3.31 [s, 6H, N–(CH3)2], 3.25 [q, 4H, N–(CH2)2–], 1.27 (t, 6H, 2CH3), 3.65 (s, OH), 3.57 (s, 3OH), 2.79 (s, 1H, OH–CH–C=O), 4.16 [s, 2H, –CH2–C=O], 13C NMR (ppm): 49.35 (N–2CH3), 49.32, 8.03 [N–2(CH2–CH3)], 176.46 (C=O), 58.58 (CH2–OH), 58.56, 58.54, 56.54, 49.35 (CH–OH).
TGA analysis of synthesized RTILs
A narrow range of melting points (25–30°C) was observed for the synthesized RTILs. With increase in temperature from 150°C up to 350°C, two step degradation was observed in the representative RTILs as shown in Fig. 4. Thermal degradation such as combustion and the decomposition of respective groups were revealed by all samples. Further, no additional thermal event was observed in the synthesized RTILs, specifying that no impurities and by-product could be observed. The observed initial weight loss up to 150°C is attributed to loss of unreacted cations and the evaporation of residual water retained in ILs due to hydrophilic contacts. Initially, a weight loss of almost 45 % was observed in A1Ac and AIP, which is attributed to the loss of hydroxyl and alkyl moieties. A weight loss of 23 % was observed at 150°C in case of A1C due to presence of covalent points in the citrate ions, but at a later stage, a sharp weight loss up 85 % was noticed up to 350°C, where the anionic citrate ions were completely degraded but still seen more thermally stable than AIAc and AIP respectively [34], [35]. Further, the degradation of A1Ac and A1P moieties were completed from 255°C until 275°C. The TGA and DSC results showed that the synthesized RTILs are thermally stable following the order of stability as A1C>A1P>A1Ac which is attributed to the organic molecular architecture and aliphatic characters of the constituent moieties.
TGA analysis of representative A1C, A1Ac and A1P synthesized RTILs.
DSC analysis of synthesized RTILs
Differential scanning calorimetry (DSC) is the best analytical technique to find the thermophysical properties of RTILs. In this technique a sample would heat or cool at linear intervals of temperature and measure the particular temperature and energy accompanied by any thermal event. Still, there is a no data available on the DSC thermogram description of ammonium based Bronsted-acidic ionic liquids. Figure 5, show DSC results of A1C, A1Ac and A1P respectively. The broad endothermic peak up to 150°C in the overall spectra is due to decomposition of unreacted cations, and the evaporation of residual water retained in ILs due to hydrophilic contacts [36]. The peaks appeared in the range of 255–285°C in A1Ac and A1P are due to the decomposition of acetate and phosphate anions, where incase of AIC, the degradation of citrate is observed till 350°C which is in close agreement with the TGA analysis. During the combination of reactive conjugate-anions with diethyl dimethyl ammonium cations, various interactions takes place which results in the appearance of peaks fairly different from each other. A decreased peak area was observed in A1C, which is attributed to the amorphous phase related to entanglement and stronger H-bonding in A1C and the respective solvent.
DSC analysis of representative A1C, A1Ac and A1P synthesized RTILs.
Physicochemical properties of RTILs in the presence of DMSO and buffer solutions
Dynamic light scattering (DLS) was performed to study the effect of various stimuli (pH and temperature) on the physicochemical performance of selective RTILs containing various anions. Regarding the cellulose dissolution capabilities of ionic liquids, still there is no report to state the effect of various stimuli on internal and external parameters. The physicochemical parameters including zeta potential (stability), conductivity, viscosity and ionic mobilities are addressed to reflect the optimum parameters for dissolution process. The effect of respective anions in term of basicity, H-bonding, polarizability, variation in conductivities and stabilities are addressed to reflect the structure-property relationship on effective dissolution. The physicochemical investigations revealed lower stability and conductivity values at acidic conditions (pH≈1.68), but a significant increase in conductivity and stability was observed for the RTILs at higher temperature (T=65°C) [37]. Thus, how and why the internal parameters responded to specific pH and temperature is addressed, which is still a challenging task. The proposed study suggested that RTILs can work efficiently at higher pH and temperature. The selective anionic moieties certified the dual-responsive nature of synthesized RTILs with a potential for hi-tech targeted applications including cellulose dissolution, biocatalysis, biofuel processing, bio-purification and biochemical sensing applications.
Effect of pH on physicochemical performance of RTILs
There is an urgent need to intensify the biomass dissolution with a system responding to a number of stimuli due to the presence of ionizable-pendant groups present in respective anions under mild conditions. In order to verify the effect of the ionizable groups of anions on physicochemical performance of selective RTILs, the effect of pH on the stability, conductivity and ionic mobility is addressed to reflect the effective dissolution process. As a variety of pH exist in the surrounding, so RTIL must be identified at specified pH (with pKa of pendant groups) for targeted applications. One such system aims to target the oil palm lignocellulosic biomass is proposed in this work, which effectively works in the pH range of 1.68–12.45.
The presence of either acidic (carboxylic and phosphoric) or basic (ammonium, and amino) moieties in RTILs form the basis of donating or accepting the protons when exposed to a certain pH [38]. These ionizable groups enhance the dissolution capabilities of RTILs by promoting the H-bonding. During the reaction, these pendent groups suffer from electrostatic interactions (either attraction or repulsion) with conformational changes in response to pH. This behavior actually decide the dissolution of different biomass species in RTILs under mild conditions. The stability parameter known as zeta potential is the key criteria that controls the electrostatic interactions and strength during a chemical reaction. When the particle moves, ions within the boundary also travel, thus the ions beyond the boundary stay with the bulk dispersant. The potential at this boundary is called the zeta potential. The zero value of ZP represents the isoelectric point where positively ions (ammonium) are balanced by negatively charged (Bronsted-acidic) anions. This value is typically considered as the point where the colloidal system is less stable. It is clear from the discussion that ZP measurement not only gives the quantitative and qualitative information about the charges but also explore the stability of RTILs for effective dissolution of cellulose [39]. The success of proposed work has been verified from the Fig. 6, where in case of (A1A) as representative, the ZP values shifted from −5 to −41.1 mV when the pH was increased from 1.68 to 12.45 respectively. This reflected the compatibility nature of A1A where the pendant groups present in anions were extremely ionized at higher pH due to pKa interference. Consequently, the increase in Coulombic attraction and H-bonding between hydroxyl moities in cellulosic biomass and A1A was verified in term of pH effect. At specific pH, a high degree of ionizable pendant groups are required in RTILs for effective dissolution of biomass, where the degree of ionization is based on the nature of interacting groups, specified pKa and pH of the system [27]. So RTILs with ionizable anions should be considered which can increase the respective ionic mobilities and conductivities at specific conditions. The studies also addressed the factors including diffusional motion, ionic mobility, frictional forces and charge-density and molecular architecture of respective cations and anions to decide the conductivities ionic mobilities and hence the capabilities for effective dissolution of cellulose. The overall order of conductivity of synthesized ionic liquids are A1P>A1Ac>A1G>A1A respectively. It is very clear that higher conductivity in case of A1P justifies the high charge-density, diffusional motion and ionic mobilities due to the smaller anionic size.
Plot of zeta potential, conductance and ionic mobility as a function of pH at 25°C.
Temperature effect
RTILs exhibit a variety of groups for H-bonding and Vander’s waals interactions, forming the basis of cellulose dissolution at definite temperature. There is a lot of data available on dissolution of cellulose from several biomasses using ionic liquids [17], [40], but still, ammonium based Bronsted acidic-ionic liquids are not addressed to show the art of cellulose dissolution at mild conditions. Furthermore, there is no report to state the effect of temperature on physicochemical properties like stability, conductivity and electrophoric mobility of RTILs. Therefore, the effect of temperature on the physicochemical properties of RTILs is presented as a guideline for further catalytic applications. The DLS analysis revealed a direct relation between the temperature and molecular and diffusional motion of ionic liquid moieties. It is important to mention that molecular architecture of constituent cations and anions decide the diffusional and relative interfaces between the liquid with cellulose. Thus, smaller size result in higher diffusional and H-bonding capabilities for effective dissolution. By increasing temperature, a successive amplitude in interfaces tend to rise the respective conductivities in order A1P>A1Ac>A1G>A1A as shown in Fig. 7. The increase in the zeta potential of the selected RTIL is attributed to the increased conductivity at higher temperature, confirming the high thermal stability of selected RTILs. We are interested to describe the physicochemical and electrokinetic parameters at room temperature which is more important for consideration in the field of dissolution of cellulose under mild conditions.
Plots of zeta potential, conductance and ionic mobility as a function of temperature at 25–65°C for specified RTILs.
A linear increase in conductivity of selective RTILs at higher temperature is attributed to the fact that at higher temperature, the effective collisions and ionic mobility increases which result in the increased conductivity. It was declared that viscosity and the dielectric constant for the specified ionic liquids were not significantly affected by the temperature as we are dealing with dilute solutions. It is observed from the Fig. 7, that the negative zeta potential values exist at lower temperature shows that anionic groups are available for communicating with the hydroxyl groups of cellulose through H-bonding, van der Waal and Coulombic interactions [38], [41]. The ZP shifted from negative to positive at elevated temperature because, the anions due to pKa interference were observed at low temperature. This reason justify that at room temperature the anions were fully available for interaction with the cellulosic biomass through H-bonding and van der Waal interactions. The overall physicochemical properties as a function of temperature are listed in Table 3. Consequently, the acceptable temperature-dependent nature of synthesized ionic liquids needs a greater responsiveness for applications in cellulose dissolution, depolymerization, biofuel processing, bio-purification and chemical sensing applications.
Effect of temperature on the physicochemical properties by dynamic light scattering.
| Temp (°C) | Sample code | A1A (1) | A1P (2) | A1Ac (3) | A1F (4) | A1C (5) | A1G (6) |
|---|---|---|---|---|---|---|---|
| 25 | ZP (mV) | −29.3 | −12.45 | −31.45 | −32.21 | −32.14 | −12.10 |
| Cond (mS/cm) | 18.21 | 24.36 | 14.56 | 14.56 | 12.25 | 10.36 | |
| I.mob (μm·cm/Vs) | −0.61 | −0.86 | −0.82 | −0.82 | −0.25 | 1.121 | |
| Viscosity (Cp) | 41.14 | 38.75 | 30.13 | 32.14 | 38.62 | 52.431 | |
| 65 | ZP (mV) | 13.54 | 25.45 | 10.25 | −32.21 | 31.58 | −12.45 |
| Cond (mS/cm) | 62.14 | 74.36 | 72.65 | 14.56 | 65.01 | 10.36 | |
| I.mob (μm·cm/Vs) | 61.14 | 4.264 | 5.241 | 4.521 | 3.012 | 2.214 | |
| Viscosity (Cp) | 38.12 | 32.21 | 28.12 | 30.45 | 33.64 | 46.251 |
Dissolution of biomass and quantification of the residue
The crude lignocellullosic oil palm biomass with 25.5 % cellulose, 40.0 % hemicellulose and 22.8 % lignin with traces of waxes and moisture contents was analyzed as per reported literature [42] . The percentage mass of extracted cellulose was properly quantified as mass of recovered cellulose from the mass of total cellulose present in biomass. Also, the change in color of treated samples were observed after treating with selective RTIL in DMSO solution, which verified the successive removal of fatty acids, waxes, surface moieties and especially lignin contents. For each trial, 0.1 g biomass powder was added to 0.8 g of synthesized ionic liquid in a 100 mL two neck round bottom flask. The flask was placed in a temperature controlled assembly for 30 min with magnetic stirring at 400 rpm at 25°C. The synthesized RTIL was not insufficient for complete dissolution of the biomass, so 5 % (v/v) of DMSO was added to the reaction mixture in order to facilitate the dissolution process. The effect of DMSO is crucial to separate cellulose from biomass by extending the interactions with anti-solvent and breaking the interaction in IL moieties through multi solvation-effect [43]. Consequently, inter and intra-molecular hydrogen bonding in cellulose and lignocellulosic biomass were disrupted resulting in the complete dissolution of cellulose in selective medium, while the residual lignin, wax and fatty acid contents were retained in IL solution (IL=DMSO). A mixture of water: acetone was used as antisolvent for complete precipitation of cellulose from the solution. In this step complete displacement of solute takes place where bipolar and columbic interactions as well as hydrogen bonding between water and ionic liquid take place resulting in precipitation of cellulose from the solution [24]. After mixing, the mixture was centrifuged (Clay Adams® Brand DYNAC centrifuge, Bohemia, NY) for 05 min. The solution was then separated from the residue by vacuum filtration. The recovered cellulose flocks were further washed with deionized water many times and dried at 65°C overnight to calculate the percentage yield. Summary of the investigated RTILs for cellulose dissolution from oil palm lignocellulosic biomass are listed in the Table 4.
Comparative cellulose dissolution studies of synthesized RTILs.
| S. no. | Sample code | Raw material | Cellulose recovery (%) | Conditions | Ref. |
|---|---|---|---|---|---|
| 1 | A1A | Oil palm biomass | 48 | 25°C, 06 h | This work |
| 2 | A1P | – | 65 | 25°C, 06 h | – |
| 3 | A1Ac | – | 62 | 25°C, 06 h | – |
| 4 | A1F | – | 58 | 25°C, 06 h | – |
| 5 | A1C | – | 53 | 25°C, 06 h | – |
| 6 | A1G | – | 52 | 25°C, 06 h | – |
| 7 | [Emim][Ac] | Poplar biomass | 52 | 90°C, 12 h | [44] |
| 8 | [Mmim][DMP] | – | 36 | 90°C, 12 h | [44] |
| 9 | [Mmim][M3PPh] | – | 90°C, 12 h | [44] | |
| 10 | [C4C1im][HCOO]− | Palm biomass | 08 | 160°C, 3–16 h | [6] |
| 11 | [Bmim][HSO4]− | Pine biomass | 55 | 120°C, 12 h | [45] |
| 12 | [Emim][CH3COO]− | – | 38 | 155°C, 03 h | [45] |
| 13 | [Emim][CH3COO]− | Red oak biomass | 48 | 110°C, 16 h | [45] |
| 14 | [Emim][CH3COO]− | Corn biomass | 29.9 | 140°C, 03 h | [45] |
Characterization of the recovered cellulose
The recovered cellulose obtained from different RTILs was compared with standard cellulose as shown in Fig. 8. The peaks at 990cm−1 and 1098cm−1 (referring to the crystallinity of cellulose) are very prominent, indicating the crystalline nature of recovered cellulose [19]. The characteristic lignin peak at 1510 cm−1 used for normalization is absent in the overall spectra further confirming the effective removal of lignin from the recovered cellulose after treatment with selective RTILs [46]. In the selective spectra, the corresponding increase in band intensities from 1900 cm−1 to 2000 cm−1, further confirm that material recovered is rich in cellulose having no lignin as compared to untreated palm lignocellulosic biomass. This is attributed to the successful modification and settled geometry of the recovered cellulose after treating with selective RTILs.
FT-IR spectra of recovered cellulose using A1P, A1Ac, A1C and standard cellulose.
The scanning electron microscopic (SEM) images of the recovered cellulose from crude oil palm lignocellulosic biomass revealed highly organized arrays. The recovered cellulose was characterized at different magnifications (5000×, 15 000× and 30 000×) as shown in Fig. 9. The surface of the untreated oil palm lignocellulosic biomass was highly coarse and asymmetrical due to the existence of lignin and hemicellulosic additives, but after dissolution in synthesized RTILs, the cellulose-rich surface appeared which demonstrated that these RTILs could alter the biomass structure. The SEM analysis revealed that the packed structure of biomass was distorted by ionic liquid treatment due to successive removal of non-cellulosic contents. Thus, the cellulose fibrils were separated from the non-cellulosic matrix and the surface roughness of ionic liquid treated fibers was improved [47], [48]. Figure 6 (d–i), shows the regular arrays of recovered cellulose which can find potential applications in enzymatic processes, energy concerns and in cellulose reinforced polymer composites.
![Fig. 9: SEM micrographs of oil palm biomass (OPB) before and after RTILs treatment: [(a) Raw OPB (surface 5000×), (b) (surface 15 000×), (c) (cross-sectional 30 000×)], [(d) A1Ac- treated OPB (surface 5000×), (e) (surface 15 000×), (f) (cross-sectional 30 000×)], [(g) A1P-treated OPB (surface 5000×), (h) (surface 15 000×), (i) (cross-sectional 30 000×)].](/document/doi/10.1515/pac-2017-0315/asset/graphic/j_pac-2017-0315_fig_009.jpg)
SEM micrographs of oil palm biomass (OPB) before and after RTILs treatment: [(a) Raw OPB (surface 5000×), (b) (surface 15 000×), (c) (cross-sectional 30 000×)], [(d) A1Ac- treated OPB (surface 5000×), (e) (surface 15 000×), (f) (cross-sectional 30 000×)], [(g) A1P-treated OPB (surface 5000×), (h) (surface 15 000×), (i) (cross-sectional 30 000×)].
Conclusion
Current investigation deals with the effective synthesis of newly introduced room temperature ionic liquids (RTILs) based on diethyl dimethyl ammonium as a cation source with the anions of several environmental friendly Bronsted acids. Various techniques including FTIR, NMR, TGA, DSC, zeta-nanosizer and dynamic light scattering, (DLS), explored the effect of anion on the structure and physicochemical properties of synthesized RTILs. The synthesis, characterization and physicochemical investigations addressed the new features of selective RTILs in term of stability, ionic mobility, conductivity and cellulose dissolution capabilities. The effect of pH and temperature on the physicochemical properties recognized a new platform for demanding industrial and biochemical applications. Oil palm lignocellulosic biomass was treated with RTILs to investigate the maximum dissolution of cellulose. Among the synthesized RTILs, diethyl dimethyl ammonium Phosphate (A1P) certified the extreme capability to extract 65 wt.% of cellulose from crude palm lignocellulosic biomass without any pretreatment for 30 min. Present study could be a significant step toward RTILs processing and producing upgraded cellulose for engineered composites materials and energy concerns.
Article note:
A collection of invited papers based on presentations at the 25th POLYCHAR 2017 World Forum on Advanced Materials Kuala Lumpur, Malaysia, October 9–13, 2017.
Acknowledgements
The author wishes to acknowledge School of Material and Mineral Source Engineering, Engineering campus, Universiti Sains Malaysia, for sponsoring this research under project Grant FRGS-203/PBAHAN/6071337.
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