Dissolution of lignocellulose in ionic liquids and its recovery by nanofiltration membrane

Separation and Purification Technology 97 (2012) 123–129

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Dissolution of lignocellulose in ionic liquids and its recovery by nanofiltration membrane Swapnali Hazarika ⇑, N.N. Dutta, P.G. Rao Chemical Engineering Division, CSIR-North East Institute of Science and Technology, Jorhat 785 006, Assam, India

a r t i c l e

i n f o

Article history: Available online 30 April 2012 Keywords: Lignocellulose Dissolution Ionic liquid Nanofiltration Pore flow model

a b s t r a c t The dissolution kinetics of lignocellulose from a suitable source of north east India has been studied in ionic liquid at room temperature. The measured profiles of time versus concentration of lignocelluloses were interpreted from a kinetic model and the rate constant was obtained by regression analysis of the data and the values of the rate constant was found to be 54.79  108 cm/s. The dissolution profiles indicate clear prospect of separating the constituents by suitable methods. Recovery of ionic liquid has also been studied by membrane technology using commercial nanofiltration membrane. The effect of applied pressure gradient (DP) and concentration of lignocellulose on the rejection and membrane fouling was studied over a range of pressure and concentration of 2–5 bar and 0.01–0.03 mmole L1 respectively. The solution flux increases with pressure in the pressure range studied indicate the effect of concentration polarization is not significant in this range. The permeation phenomenon has been analyzed on the basis of pore flow transport model and is well fitted with the experimental values. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Lignocellulosic biomass is renewable, environmentally friendly and abundant in the natural world. It is mainly consisted of cellulose, lignin and hemicelluloses. With lignin, cellulose occurs as main part of every lignocellulosic biomass. North East region of India is rich in natural resources and biomass which provides feedstock for fuels, fine chemicals, speciality chemicals, foods and functional foods, nutraceuticals etc. Some reported work indicates that coir fibre of this region contains large amount of lignin which we considered as the lignocellulosic biomass for present study [1]. The typical composition of this biomass is, cellulose 45–56%, hemicelluloses 10–25% and lignin 18–30%. Lignocellulose is the major structural component of woody and non woody plants and represents a major source of renewable organic matter as reported by Howard et al. [2]. Lignocellulose consists of lignin, hemicelluloses and cellulose. Large amount of lignocellulose waste are generated through forestry and agricultural practices, paper and pulp industries, timber industries and many agro industries. Among potential alternative bioenergy resources, lignocellulosics have been identified as the prime source of biofuels and other value added products [3]. Cellulose is the most abundant renewable resource in the world. Cellulose containing materials and their derivatives has been widely

⇑ Corresponding author. Tel.: +91 3762370012; fax: +91 3762370011. E-mail address: [email protected] (S. Hazarika). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.04.026

used in our society and hence it can be extracted from its primitive resources for common use. Traditional cellulose dissolution processes are often cumbersome or expensive and require the use of unusual solvents, typically with high ionic strength and use relatively harsh conditions [4]. These processes sometimes cause serious environmental problems because these solvents cannot be recovered and reused. Thus selection of an efficient solvents itself is an important research from technological perspectives. Ionic liquids are a group of new organic salts that exist as liquids at a relatively low temperature (<100 oC). They have many attractive properties, such as chemical and thermal stability, non-flammability and immeasurably low vapor pressure [5]. In contrast to traditional volatile organic compounds, they are called green solvent and have been used widely [6,7]. Since the solubility of cellulose in ionic liquid is appreciably high, significant progress has been made to develop technological knowledgebase and knowhow based on favorable thermodynamic parameter. This has provided a new platform for ‘‘green’’ comprehensive utilization of cellulose resources. Zavrel et al. [8] studied the high throughput screening for ionic liquids dissolving (lingo) cellulose and confirmed that some ionic liquids are most effective for dissolving wood chips. In our work we have reported the dissolution of lignocelluloses obtained from north east India in ionic liquid and try to establish the dissolution mechanism considering the regeneration and reuse of ionic liquids. Due to the high cost, recovery and recycle of ionic liquids is required for commercial use. Cellulosic content of the biomass can be recovered from the ionic liquid solution by addition of an anti

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solvent [8]. Alternatively ionic liquids have been recovered by adding an aqueous solution containing a kosmotropic anion such as phosphate, carbonate or sulfate [8]. Gutowski et al. [9] reported the formation of an aqueous biphasic system based on [Bmim]Cl, water and K3PO4. Phase diagrams for a variety of IL/water/salt systems have been reported by other researcher also [10,11] and most of these are based on ionic liquids containing imidazolium cation and anions. In our study we have applied membrane based technology for recovery of ionic liquid after precipitation of cellulose by adding water. Ionic liquids could be used to dissolve cellulose, however, this was thought of little practical value because the concept of ionic liquids had not been put forward at the time [3]. Recently, the value of dissolution of cellulose with ionic liquids is re-evaluated based on the understanding of ionic liquids. In 2002, Robin Rogers applied ionic liquids to the dissolution of cellulose [12]. With using 1-butyl-3-methyl-imidazolium chloride he was the first to be able to dissolve cellulose in technically useful concentrations by physical dissolution in an inert solvent without using any auxiliaries. The dissolution process is mainly driven by the anion of the ionic liquid [13]. Anions such as halides, carboxylates or phosphates are able to very effectively break down interchain hydrogen bonds within the cellulose structures. The other important criteria are: the anion must be a good hydrogen bond acceptor, the cation should be a moderate hydrogen bond donator because the cation has the most moderate activated hydrogen for forming hydrogen bonding with oxygen atoms of the hydroxyls of cellulose and the size of the cation should not be too large [14–16]. If the dissolved cellulose oligomer content increases the ionic liquid solutions become viscous [15]. It is recognized that high solvating properties of RTIL have been exploited in the dissolution of cellulose, lignin and even whole wood [17]. Several 1-alkyl-3-methylimidazolium cations and inorganic anions have been reported for cellulose dissolution. However the observation was that [BMIM]Cl was found to be the most effective RTIL capable of dissolving up to 25% w/w cellulose. The decreasing solubility dissolution power of various inorganic anions can be arranged in the order Cl > Br > SCN > BF4 > PF6 [8]. Chloride based as well as [EMIM]OAc ILs have known to be effective solvents for cellulose. The rate of dissolution was found to be high at 50 oC and 80 oC and the dissolution capacity depends

on the strong interaction through hydrogen bonding and disruption of the cross lattice structure of the polysaccharides. Interaction of RTILs and cellulose cause swelling of the cellulose fibres. Transmission optical microscopic image shows division of cellulose fibres in segments followed by fragmentation of fibres [12]. Confocal fluorescence imaging has been applied to provide swelling behavior which substantially contributes to dissolution rate. Time course of images of ionic liquids pretreated lignocellulosic biomass has been interpreted to gel insight of the solubilization process qualololized [18]. Based on NMR results, a mechanism of solubilization has been prepared recently [19]. The mechanism involves hydrogen bonding between the carboxylate hydroxyl proteins and the ionic liquid anion and is critical in ionic liquid solubilization process. Ionic liquid treatment effectively weakens the Vander Waal’s interaction between cell wall polymers. Hemicellulose forms covalent linkages with lignin through ferulic acid and cellulose vs pectins. 2. Experimental 2.1. Materials For our study we used coir fibre obtained from NE region of India and it contains: Cellulose: 35–36%, Lignin: 36–40%, Ash content: 2.10%, Silica: 0.02% as reported earlier [1]. 1-n-Butyl-3-methylpyridinium tetrafluoroborate, was obtained from Sigma Aldrich, Mumbai, India. Nanofiltration Membrane, NF 270–400 obtained from M/s FilmTech, USA as gift sample. According to the manufacturer, these membranes are polyamide thin-film composite membrane viable for operation at pH from 3–10 and temperatures up to 45 oC. 2.2. Methods 2.2.1. Dissolution experiment Dissolution experiments were carried out by adding 0.6 g fibre in 6 ml ionic liquid and stirred in a shaker with 200 rpm stirring speed at 30 oC. The change in colour of ionic liquid was observed every 2 h interval and the colour intensity was assessed from light transmission microscopy. After ten hours the dissolved compo-

Fig. 1. Flow diagram of permeation experiment. 1Membrane cell 2. Magnetic stirrer 3. Magnetic capsule 4. Membrane 5. feed tank 6. Peristaltic pump 7. N2 gas 8. Gas valve 9. Gas valve 10.Water vessel 11. Sample collecting valve 12. Pressure gauze 13. Pressure gauze.

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Fig. 2. (a) Cellulose before treatment, (b) 1 h (c) 2 h (d) 3 h (e) 4 h (f) 5 h (g) 6 h (h) 7 h (i) 8 h (j) 9 h (k) 10 h (after treatment).

nents were separated out and precipitated the cellulose by adding water in ionic liquid. The residue is separated out and the filtrate is used for membrane permeation experiment for recovery of ionic liquids. 2.2.2. Recovery and recycle of ionic liquid A two compartment membrane cell was used for the study with the experimental system shown in Fig. 1. Volume of each compartment of the cell was 50 ml. The polymeric membrane was placed

between the compartments with silicone–rubber packing and the cell was connected with a reservoir of 500 ml. The solution of dissolved lignocelluloses in ionic liquid was stirred continuously and circulated by peristaltic pump that was connected to the reservoir. Concentration of ionic liquid solution ranging from 0.01– 0.03 mmole L1 was used for the work. The sample solutions were collected from the permeate side in every 30 min interval for 6 h and analyzed by UV–VIS spectrometer (Shimadzu, Model 160A) which was calibrated at 280 nm. Analysis was done in triplicate

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3. Results and discussions

5.4

0% (w/w) 4% (w/w) 6% (w/w) 8% (w/w) 10% (w/w)

5.2 5.0

Scattered light [a.u.])

4.8 4.6

3.1. Dissolution of lignocellulose From Fig. 2 it is seen that after ten hours of ionic liquid treatment the cellulose content is high in the solution as is evident from the change in colour of ionic liquid in the photograph, which is only a qualitative study. Dissolution of lignocelluloses in ionic liquid is found up to 10% at room temperature as shown in Fig. 3. The SEM photograph of untreated and treated cellulose is shown in Fig. 4.

4.4 4.2 4.0 3.8 3.6

3.1.1. Kinetic model for dissolution It is perhaps now believed cellulose hydrolysis minimise the solubilisation of lignocelluloses and the kinetics of the process. The mechanistic pathway of cellulose dissolution was proposed by Feng and Chen [16] and is shown in Fig. 5. General kinetic model available for hydrolysis of cellubiose in ionic liquid may be useful for dissolution kinetics. The hydrolysis reaction is reported to confirm first order kinetic model given in Eq. (3) [20]

3.4 3.2 3.0 0

5

10

Time (hr) Fig. 3. Dissolution profiles of different weight percentage of lignocelluloses in ionic liquid.

and the reproducibility was within ±5%. The major emphasis was to determine permeation flux in terms of pertinent variables. The permeation flux was calculated by the equation,

J¼

V DC A Dt

ð1Þ

Where V is the volume of permeate (ml) in time t (sec), A Membrane area (cm2), DC the concentration variation in the corresponding aqueous solution (mmolL1) at the time interval Dt. The rejection percentage was defined as

R% ¼

Cf  Cp  100 Cf

ð2Þ

where Cf and Cp are the concentration in feed and permeate respectively.

D½glucose =dt ¼ r1ðcellulose hydrolysisÞ  r2ðglucose degradationÞ

ð3Þ

Where r1 = 2  2k1[cellobiose]; r2 = k2[glucose] So as a function of time:

½glucose=2½cellobioseo ¼ ð2k1 =ðk2  k1 ÞÞ  ðe22k  ek Þ t 2t

ð4Þ

Eqs. (3) and (4) were studied and initial rates of cellobiose hydrolysis r1o and glucose degradation r2o were directly measured and both expressed in lmolglucose s1cm3. The maximum yield in glucose achieved during the course of each experiment increases as twice the ratio of the rate constants k1/k2, which is equal to the ratio of the initial rates, r1o/r2o. For cellulose hydrolysis, there is only one glucose unit per b(1?4) link so the optimum will depend directly on the ratio of the rate constant k1/k2. If cellulose is soluble in [C2mim]Cl, the hydrolysis will be a homogeneous reaction with each link equally accessible to the acid catalyst. Thus the model used for hydrolysis of cellulose is based on a first order random chain scission followed

Fig. 4. SEM photograph of lignocelluloses, (a) (b) (c): before treatment, (d) (e) (f): After treatment.

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127

Fig. 5. Mechanistic pathway of cellulose dissolution proposed by Feng & Chen [16].

Integration within limit t = 0, Co = 0 and t = t, Ct,

6

Vp

Z

Ct

C

dC ¼ kA1 dtto ðC   C t Þ

ln(c*-c)X10

-6

) Vp lnðC  CtÞ ¼ kt

ð8Þ

4

A plot of ln (C⁄Ct) vs time shown in Fig. 6 is a straight line and the slope gives the value of k (54.79  108 cm/s). This value is typical of mass transfer co-efficient for cellulose dissolution and is a lumped parameter suitable for process application [21,22].

2

3.2. Recovery and recycle of ionic liquids

4

8

12

16

20

24

time (hr) Fig. 6. Plot of ln (C⁄–Ct) vs time for calculating K value.

by a first order glucose degradation [20]. Glucose concentration as a function of time and may be represented by the following Eq. (5) .

½glucose=½glucose=celluloseo ¼

t ð2k2  k1 Þ  ek1 ðk1  k2 Þ ðk2  k1 Þ þ  e2k1 t ðk2  2k1 Þ ðk1  k1 Þ þ  ek2 t ðk2  k1 Þ  ðk2  2k1 Þ

ð5Þ

The rate of hydrolysis reaction, k1 is assumed to be constant, regardless of the chain length of the substrate [20]. The rate of glucose degradation, k2, has to be determined. After numerical analysis it was found that for cellulose hydrolysis the value of k1/k2 is found to be 2.8 which is comparable with the reported value of 2.4 [20]. A more generalized mathematical model involving mass transfer may be obtained for dissolution kinetic in terms of the mass balance equation is shown below. For dissolution mass balance may be based as follows

Vp

ð7Þ

dc ¼ kA1 ðC  CÞ dt

Due to the current high cost of ionic liquid, recovery and recycle of ionic liquids is required for commercial use. We have done this exercise after dissolution of lignocelluloses in ionic liquid and subsequent precipitation of lignocelluloses by adding water. After separation of cellulose, water containing ionic liquid is obtained and used for separation of ionic liquid by membrane technology using nanofiltration membrane. Nanofiltration is a relatively new membrane separation technique which is a pressure driven membrane process normally applicable for separation of dissolved components. Most NF membranes are composite in nature, with a selective layer on the top of the microphorous substrate. In our study we have used commercial NF membrane for recovery of ionic liquid from water. 3.2.1. Effect of applied pressure and feed concentration Effect of applied pressure on recovery of ionic liquid was studied at a range of pressure 2–5 bar. Fig. 7 shows the variation of percentage rejection as a factor of pressure and concentration (0.01–0.03 mmol/L). The percentage rejection increases with increasing pressure and decreases with increase in concentrations. With increase in pressure, convective transport becomes dominant causing rejection to increase. However, concentration polarization will also increase with increase of pressure which results in decrease in rejection. The counteracting contribution of increased convective transport and increased concentration polarization will result in nearly constant rejection at high pressure range [23].

ð6Þ

where, C⁄ = solubility (gm/cm3), C = concentration (changing with time), k = transfer rate coefficients (cm/s), Vp = particle suspension volume (cm3), A1 = external surface area of particles (cm2).

3.2.2. Effect of concentration polarization on permeate flux Fig. 8 shows that the solvent flux increases with applied pressure indicating little effect of concentration polarization or fouling in the pressure range under study [23]. This may be due to the

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8

90

Rejection (%)

80

Symbol Pressure 3.0 bar 2.5 bar 2.0 bar

7

-1 -5 -2 Jv×10 (Lm sec )

85

Symbol Concentration 0.01 mmole/L 0.02 mmole/L 0.03 mmole/L

75 70

6

5

4

65 3 60 2 0

55 2.0

2.5

3.0

3.5

4.0

4.5

-5

-2 -1

JvX10 (Lm s )

Symbol Concentration 0.01 mmole/L 0.02 mmole/L 0.03 mmole/L

12

Jv ¼

8

10

0 4.0

4.5

DP  Dp l Rt

ð10Þ

where l is the water viscosity and Rt = Rm + Rg + Rads. The permeability coefficient, L’P, is defined as 1/lRt. The value of Rm is calculated from the pure water permeability coefficient, Lp = 1/lRm. This model provides the best fit of the experimental data for recovery of ionic liquid from aqueous solution. The osmotic pressure of ionic liquid solution can be determined using the Van’t– Hoff equation according to which

4

3.5

8

where DP is the applied pressure (bar) across the membrane, Dp is the osmotic pressure (bar) across the membrane and can be estimated from Van’t Hoff equation and L’p is the solvent permeability coefficient (ms1 bar1). Permeate flux, Jv, can be predicted considering, besides osmotic pressure, a total resistance to flow, Rt, that is the sum of the intrinsic membrane resistance, Rm, and additional resistances like those caused by concentration polarization/gel formation, Rg, and Rads. The permeate flux is given by [25].

20

3.0

6

Fig. 9. Variation of flux with time at different pressures at constant lignocellulose concentration of 0.01 mmole L1.

Fig. 7. Rejection of lignocellulose at different operating pressure.

2.5

4

Time (hr)

Pressure (bar)

16

2

5.0

5.0

Pressure (bar) Fig. 8. Variation of solvent flux with pressure at different lignocellulose concentration.

J v ¼ L0p ðDP  DpÞ

ð9Þ

16

-2 -1

Symbol Concentration 0.01 mmole/L 0.02 mmole/L 0.03 mmole/L

12

-5

3.2.3. Flux model fitting and validation The performance of pressure-driven membrane processes, namely nanofiltration, is associated to phenomena as concentration polarization and fouling caused by solute adsorption or pore blocking [25]. Concentration polarization occurs due to the accumulation of solute that is being retained at the membrane interface. These results in a concentrated layer less permeable to the solvent associated to higher osmotic pressure (p) at the membrane interface which leads to a decrease of the effective driving force. The solvent flux, Jv, is proportional to the effective applied pressure (DPDp) as given in the eq. (9)

20

JvX10 (Lm s )

hydrophilicity of the membrane. NF 270–400 is hydrophilic in nature [24] and has better fouling resistance capacity than hydrophobic membrane [25]. Fig. 9 shows that the decrease in permeate flux with time is insignificant. This implies that in the range of pressure studied, the membrane does not suffer much compaction effects which would reduce its pores and consequently the permeate flux [26].

8

4

0 2.5

3.0

3.5

4.0

4.5

5.0

Pressure (bar) Fig. 10. Comparison between experimental and theoretical curves of flux versus pressure, closed symbol: experimental, open symbol: theoretical.

S. Hazarika et al. / Separation and Purification Technology 97 (2012) 123–129

P¼

nRT CRT V

ð11Þ

where, P is the osmotic pressure across the membrane, n is the number of moles of lignocellulose taken, R is the universal gas constant, T is the temperature at which experiment was performed and V is the volume of the solution taken, C is the concentration. The intrinsic membrane resistance, Rm, was calculated from the slope of the linear plot of Jw versus DP. The corresponding Rm values obtained membranes is 3.93  105cm1 in the pressure and concentration range under study. The pore flow model providing the lowest error variance corresponds to best fit of the experimental data (Fig. 10) with R2 value 0.984. 4. Conclusion A unstructured kinetic model has been proposed for dissolution of lignocelluloses in certain ionic liquids and the value of lumped parameter was established from experimental data. The probable mechanism of the dissolution process was elucidated. However, in order to obtain a more realistic kinetic model, the pathway should be discussed and a mechanistic model would perhaps be more appropriately formulated. Recovery of ionic liquid was studied with nanofiltration membrane in which the ionic liquid was obtained as the reject in high concentration and more than 50% rejection was obtained. The permeation flux was interpreted from a model which accounts for convective flow, osmotic gradient and pertinent resistance. Further work is in progress in order to design actual transport mechanism. Acknowledgements Dr. Swapnali Hazarika is grateful to Council of Scientific and Industrial Research (CSIR), New Delhi for financial support to attend ILSEPT 2011 under Young Scientist Scheme. References [1] T. Goswami, final Project Report on ‘‘Utilization of Coir Fibre for development of Wood Substrate Building Material in the NE India’’ a collaborative project from COIR Board, Ministry of MSME, Govt of India, Alppuazha, Ref. No. CPP/ CLP-283/09/PR. [2] R.L. Howard, E. Abotsi, E.L. Jansen van Rensburg, S. Howard, Lignocellulose biotechnology: issues of bioconversion and enzyme production, African Journal of Biotechnology 2 (12) (2003) 602–619. [3] S.H. Yoo, J.Y. Jho, J. Won, H.C. Park, Y.S. Kang, Journal of Industrial Engineering Chemistry 6 (2) (2000) 129–134. [4] Z. Shengdong, W. Yuanxin, C. Qiming, Y. Ziniu, W. Cinwen, J. Shiwei, D. Yigang, W. Gang, Dissolution of cellulose with ionic liquids and its application: a mini review, Green Chemistry 8 (2006) 325–327. [5] I C Kim, K-H Lee J, Synthesis and characterization of polyethersulfone membranes Applied Polymer Science 89 (2003) 2562–2571. [6] M. Bodzek, S. Koter, K.W. Owska, Application of membrane techniques in a water softening process, Desalination 145 (2002) 321–328.

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