Dissolution of Bamboo (Gigantochloa scortechinii) Using Ionic Liquids
In todays world the energy crisis is the main concern. It has been tried to develop substitute materials of petroleum and other decreasing natural fuel resources. Biomass is an abundant and renewable resource and it impelled researcher to develop suitable and proficient ways for converting it to valuable chemicals, biofuels and biomaterials. Due to their several advantageous properties ionic liquid was used for the dissolution of biomass. Biomass after dissolution in ionic liquids was regenerated with reconstitutes solvents (acetone/water). The regenerated material was characterized by FT-IR, XRD and SEM and compared with untreated biomass. The regenerated material was found to exhibit more homogeneity in the microstructure and the fibrous nature of material disappeared after dissolution. The crystallinity of cellulose was changed from cellulose I to amorphous during dissolution and regeneration from ionic liquids.
Received: January 28, 2010;
Accepted: March 06, 2010;
Published: June 01, 2010
Biomass is the total quantity or weight of plants and animals in a particular
area or volume (Hornby, 2000) and is one of renewable
resources. Plant biomass includes dead trees and branches, yard clippings and
wood chips and also includes plant used for production of fibers, chemicals
or fuel. Biomass is mainly consisted of cellulose, lignin and hemicellulose.
Cellulose, the main part of wood, constitutes 45-50%, Hemicellulose constitutes
15-25% (Browning, 1975) and lignin constitutes 23-33%
in softwood and 16-25% in hardwoods (Rowell, 1984). Dissolution
of biomass in common solvents is practically difficult due to the three dimensional
network structures of lignin which binds the plant cells together providing
mechanical strength to structure. Recently various types of solvents have been
used for processing of cellulose. For example in paper making, wood is treated
with alkaline solution (Elton, 2006) and in the preparation
of cellulose xanthate carbon disulphide is used as the solvent (Blanco,
1926). But the conventional solvents used for cellulose, suffer from some
drawbacks related to volatility, generation of toxic gas and difficulty in recovery
for recycle use (Ohno and Fukaya, 2009). Ionic liquids
are organic salts with melting point less than 100°C (Plentnev
et al., 2004). They are considered as green solvent because of their
several benign properties like negligible vapor pressure, non-flammability,
on-explosiveness, electrochemical stability, thermal stability, high conductivity
and ease of recycling (Li, 2005). Because of their advantageous
properties, ionic liquids attract great attention in many fields (Liu
et al., 2008; Fukumoto et al., 2005;
Muldoon et al., 2007; Xiaohui
et al., 2004). Graenacher (1934) worked on
the dissolution of cellulose in n-ethylpyridinium chloride, in the presence
of nitrogen containing bases. This was the initial use of ionic liquids for
cellulose dissolution. Later on Rogers and his group carried out comprehensive
studies on cellulose dissolution and its regeneration (Swatloski
et al., 2002). Kilpelainen et al. (2007)
observed the dissolution of hard and soft woods in ionic liquids of imidazolium
base cations. To improve the dissolution properties of ionic liquids, a large
no of anions were combined with imidazolium-based cations. Swatloski
et al., 2002 studied the effect of anion on the dissolution of wood.
When 1-butyl-3-methylimidazolium cation was combined with Cl¯, PF6¯,
Br¯, SCN and BF4 anions, it was observed that only Cl¯
SCN and Br¯ anion containing ionic liquids were able to dissolve wood at
100-110°C. Erdmenger et al. (2007) studied
on the effect of alkyl chain length on 1-alkyl-3-methylimidazolium cations combined
with chloride anion on cellulose dissolution. Both even and odd numbers of alkyl
chain, length i.e., C2-C10 was investigated by the authors.
It was observed that even the number of alkyl chain length had a great effect
on the dissolution of cellulose. Luo et al. (2005)
introduced -OH groups on the imidazole cation and synthesized 1-(2-hydroxyethyl-3-methyl
imidazolium chloride (HEmimCl) ionic liquid. The synthesized ionic liquid was
found to have better dissolution capacity and cellulose dissolution reached
to 6.8% at 70°C in it. For biomass dissolution it is necessary, to break
this network among its constituents. Ionic liquids which have greater ability
to make hydrogen bond (hydrogen bond basicity β) are good solvents for
biomass dissolution. Fukaya et al. (2006) prepared
a series of 1, 3-dialkylimidazolium formate ionic liquids, which had low viscosity,
polarity and were free of halogen. These ionic liquids have efficient potential
for the dissolution of polysaccharides. It has been attributed to its low viscosity
66 c P at 25°C and high hydrogen bond bascity β (0.99) as compared
to 1-allyl-3-methylimidazolium chloride (AmimCl) and 1-butyl-3-methylimidazolium
chloride (BmimCl), which had 0.83 and 0.84, respectively. In another work, both
BmimCl and EmimOAc ionic liquids were used for the dissolution of hard and soft
wood. It was observed that EmimOAc was more effective in dissolution as BmimCl
ionic liquid and it was also attributed to high hydrogen bond basicty of EmimOAc
ionic liquid (Sun et al., 2009). Swatloski
et al., 2002 observed that the solubility of cellulose in ionic liquids
could be improved by microwave heating. Under similar conditions the microwave
heating was found to be more effective compared to general heating, as it is
related to internal heating of the material while general heating, relates to
conduction of heat. Vesa and Reijo (2005) worked on the
dissolution of wood, straw, plywood, softwood in BmimCl ionic liquid under microwave
irradiation and pressure. Swatloski et al., 2002
observed that the solubility of cellulose decreased in the presence of water
in ionic liquids. It was related to the competition of water hydrogen bonds
and cellulose hydrogen bonds for anion part of ionic liquid. Rayne
and Mazza (2007) pointed out that dissolution in BmimCl on large scale has
practical limitation due to the hygroscopic nature of BmimCl.
In this study some ionic liquids have been synthesized and were used for the
dissolution of biomass. To confirm the dissolution effect of ionic liquids on
biomass, the regenerated material from ionic liquids has been characterized
by TGA, FTIR, XRD and SEM.
MATERIALS AND METHODS
Materials: 1, 3-propane sulfone (purchased from Merck), 1-methylimidazole (purchased from Sigma), H2SO4 (purchased from Fisher), toluene (Merck), Sodium hydroxide (Merck), ethanol amine (Merck), acetic acid (Merck) were used as received. Tri-octyl-tetra decyl phosphonium chloride (TTPCl) ionic liquid was synthesized in our laboratory. Bamboo plant was received from local market and it was grind and pass through sieve for different particle size (250 μm to 1 mm).
Synthesis of ionic liquids
Synthesis of 3-(1-methylimidazolium-3-yl) propane-1-sulfonate ionic liquid:
Synthesis of 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogen sulfate ((PSmim)
HSO4), ionic liquid: 1, 3-propane sulfone was charged in three-necked
flask and was dissolved in toluene. 1-methylimidazole was added drop wise to
the cold mixture and stirred it for 2 h. The resultant mixture was filtered
to get a white precipitate. The precipitate was washed three times with diethyl
ether and dried using rotary under vacuum at 80°C for 6 h. The white powder
MIM-PS zwitterions ionic liquid was obtained (Liu et
3-(1-methylimidazolium-3-yl) propane-1-sulfonate ionic liquid (MIM-PS) was
dissolved in deionized water and equal-mole of sulfuric acid was added drops
wise slowly at room temperature. The mixture was stirred at 25°C for 30
min and then heated in an oil-bath at about 90 °C for 2 h. The water of
the mixture was evaporated under vacuum by using rotary for 6 h at 90 °C
and vacuum oven for 2 days at 80°C; the colorless viscous liquid of (HSO3-pmim)
HSO4 was obtained (Liu et al., 2008).
Synthesis of 2-hydroxyethanaminium acetate ionic liquid:
0.125 moles of MEA was dissolved in 25 mL ethanol and was loaded in 250 mL
round bottom flask equipped with reflux condenser, a magnetic stirrer and inlet
and out let for N2 gas. The flask was mounted in ice. A mixture of
0.125 moles of acetic acid dissolved in 25 mL ethanol was added drop wise to
cold MEA containing flask in about 70 min. The reaction mixture was heated with
stirring for 24 h at 37°C. The resultant brown and viscous product was distilled
in rotary vacuum pump to remove solvent. Then it was washed three times with
acetonitrile. Keep in vacuum oven for 24 h to remove any remaining solvent.
The ionic liquid 2-hydroxyethanaminium acetate will be kept under nitrogen gas
until use (Yuan et al., 2007).
Dissolution and regeneration of biomass: 5 wt. % solutions of dried biomass (moisture content 3% by mettler Toledo HR 73 Halogen Moisture Analyzer ) were made in above-mentioned ionic liquids. In case of 3-(1-methylimidazolium-3-yl) propane-1-sulfonate ionic liquid the solution was heated at 165°C for 12 h while for 1-methyl-3- (3-sulfopropyl)-imidazolium hydrogen sulfate, Tri-octyl-tetra decyl phosphonium chloride (TTPCl) and 2-hydroxyethanaminium acetate it was heated for 24 h in oil bath at 105°C with magnetic stirring of 500 rpm.
After dissolution, the dissolved material was regenerated by pouring the bamboo/ILs solution in a beaker containing mixture of acetone/water (3:2 v/v). The beaker was sealed with parafilm and the mixture was stirred at room temperature for 30 min. The precipitated cellulosic-rich materials were separated by vacuum filtration through nylon filter paper, while the lignin was precipitated by evaporation of acetone and separated by vacuum filtration. Ionic liquids were recycled by evaporation of water (Sun et al., 2009).
Characterization: FTIR Spectra for the samples were taken by using SHIMDZU 8400S in wave number range of 400-4000 cm-1.
The crystallinity of bamboo powder and regenerated cellulose-rich materials were monitored by powder X-ray diffraction (PXRD), using a Bruker D8 Advance horizontal X-ray diffractometer equipped with Cu anode, at room temperature. The samples were scanned within 5.00-40.00° 2θ in step mode with a step of 0.01° and a rate of 1° min-1.
SEM images of the sample were taken at 3000x magnification using a LEO VP1430 SEM instrument which was operated at 15 kV accelerating voltage. Prior to imaging, the samples were sputter-coated with gold to make the fibers conductive, avoiding degradation and build up of charge on the specimen.
The sample bamboo wood (125 μm particle size) was loaded in two fractions of 5-wt % ionic liquids of 3-(1-methylimidazolium-3-yl) propane-1-sulfonate (MIM-PS), 1-methyl-3- (3-sulfopropyl)-imidazolium hydrogen sulfate ((PSmim) HSO4), 2-hydroxyethanaminium acetate and Tri-octyl-tetra decyl phosphonium chloride (TTPCl). During dissolution process it was observed that only 3-(1-methylimidazolium-3-yl) propane-1-sulfonate (MIM-PS) ionic liquid has the ability to dissolve biomass completely while in case of 1-methyl-3- (3-sulfopropyl)-imidazolium hydrogen sulfate ((PSmim) HSO4) ionic liquid partial dissolution was observed. The others two ionic liquids i.e., 2-hydroxyethanaminium acetate and Tri-octyl-tetra decyl phosphonium chloride (TTPCl) was found ineffective in the dissolution process of bamboo biomass.
||FT-IR spectra for bamboo powder (A) and regenerated cellulosic-rich
material from MIM-PS ionic liquid
||FT-IR spectra of lignin alkali (A) and regenerated lignin
from MIM-PS ionic liquid
FTIR spectra of untreated bamboo and regenerated material from MIM-PS ionic liquid showed the same basic structure as shown in Fig. 1. There was a strong broad O-H stretching absorption around 3425 cm-1 and a prominent C-H stretching absorption around 2939 cm-1. The non-conjugated C-O stretch was observed at 1730 cm-1, in spectra (A) that was absent in the spectra (B). In spectra (B), peak around 1693 cm-1 was observed which was absent in spectra (A) while other peaks at 1604, 1512 cm-1, 1330, 1161, 1037 and 894 cm_1 was observed in both spectra.
The spectra of lignin, recovered from 3-(1-methylimidazolium-3-yl) propane-1-sulfonate
(MIM-PS) ionic liquid treatment and lignin alkali standard (purchased form sigma)
has been shown in Fig. 2. It was observed that both the spectra
showed similar pattern of peaks.
XRD spectra were collected for untreated bamboo powder as shown in Fig. 3a and for regenerated cellulose rich materials from Tri-octyl-tetra decyl phosphonium chloride (TTPCl) and 2-hydroxyethanaminium acetate ionic liquid as shown in 3b and 3-(1-methylimidazolium-3-yl) propane-1-sulfonate and 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogen sulfate ionic liquids as shown in Fig. 3b. In figures and two typical diffraction peaks (at 2θ = 16.3° and 22.2°) were observed for all spectra. In Fig. 3c., the upper spectra showed one broad peak at 2θ = 22.2° while no such peak was observed in lower peak of Fig. 3c.
SEM images of the morphology of the Bamboo and regenerated materials from 3-(1-methylimidazolium-3-yl)
propane-1-sulfonate, 1-methyl-3- (3-sulfopropyl)-imidazolium hydrogen sulfate,
Tri-octyl-tetra decyl phosphonium chloride (TTPCl) and 2-hydroxyethanaminium
acetate ionic liquids have been shown in Fig. 4a-e, respectively.
From SEM images it has been observed that regenerated materials from ionic liquids
have different morphology as compared to untreated material of bamboo. Again
from the microstructure it was further apparent that different ionic liquids
have different dissolving tendency for bamboo biomass.
Dissolution of biomass in ionic liquid depends on initial sample load, particle
size, temperature water content and high hydrogen bond basicity of ionic liquid
(Sun et al., 2009).
||(a) Diffractograms of (a) Bamboo powder. (b) (a) regenerated
material from 2-hydroxyethanaminium acetate Ionic Liquid (b) regenerated
material from Tri-octyl-tetra decyl phosphonium chloride (TTPCl) ionic liquid
and (c) Diffractograms of regenerated material from (a) ((PSmim) HSO4)
ionic liquid and (b) MIM-PS ionic liquid
||SEM images of bamboo fiber, (a) regenerated materials, (b)
from MIM-PS, (c) ((PSmim) HSO4), (d) TTPCl and (e) HEA ionic
The sample of bamboo wood (125 μm particle size) was loaded in two fractions
of the 5 wt. % solution to the ionic liquid. The increased dissolution of smaller
wood particles is likely due to increased surface area and more mechanical grinding
is required to obtained smaller particles. In this study 5 wt. % of the sample
was loaded for rapid dissolution at 105°C. This high temperature along with
ease of dissolution reduced the effect of water content on dissolution. If the
water content exceeds more then 1-wt %, it would impair the dissolution process
as hydrogen bonding in water would compete for hydrogen bonding in cellulose
for the anionic part of ionic liquids (Swatloski et al., 2002). During
dissolution of biomass in MIM-PS and ((PSmim) HSO4) it was observed
that MIM-PS ionic liquid due to its zwitterions effect has the ability to dissolve
biomass. However it required more energy due its high melting point. While ((PSmim)
HSO4) ionic liquid has high acidity it was not so much effective
in dissolution of biomass because of the presence of extra hydrogen, which interrupted
in the dissolution process. Tri-octyl-tetra decyl phosphonium chloride (TTPCl)
was found not effective in dissolution of biomass due to the bulky substituents
on phosphonium, which reduce chloride content and also non-aromatic nature of
phosphonium, which does not interact with lignin aromaticity during dissolution
of biomass. In 2-hydroxyethanaminium acetate ionic liquid, although acetate
ion has high hydrogen bond basicity (the ability to make hydrogen) for the dissolution
of biomass but due to the presence of OH group and non-aromatic nature of cation,
it was found ineffective in the dissolution of biomass.
From FTIR spectra as shown in Fig. 1, in the fingerprint
region between 800 and 1800 cm-1, many well-defined peaks were observed
which provided abundant information on various functional groups present in
bamboo and regenerated material from MIM-PS ionic liquid. For example, the non-conjugated
C-O stretch (in hemicellulose) was observed at 1730 cm-1, in spectra
(A) that was absent in the spectra (B). It was due to loss of hemicellulose
during dissolution and regeneration from ionic liquid (Sun
et al., 2009). However in spectra B, peak around 1693 cm-1
appeared which was related to carbohydrates. It showed the increase in carbohydrates
content of regenerated material. The aromatic skeletal vibration in lignin appeared
at 1604 and 1512 cm-1, while the peaks around 1330, 1161, 1037 and
894 cm-1 mainly related to the carbohydrates were observed in both
spectra as shown in Fig. 1 (Xiaoqing and
Haiqing, 2008). However, from spectra of regenerated material it has been
observed that ionic liquid treatment partially removed lignin. The peaks of
lignin still persist in the regenerated material.
The close similarity of spectra (B) to the spectra (A) as shown in Fig. 2, showed that the recovered material from MIM-PS ionic liquid after evaporation of acetone was lignin. The typical peaks of lignin were observed in both spectra.
XRD spectra (Fig. 3a, b) showed two typical
diffraction peaks (at 2θ = 16.3° and 22.2°) for cellulose I. While
in Fig. 3c., the upper spectra showed one broad peak at 2θ
= 22.2° which revealed the partial dissolution in this case. However the
lower spectra in Fig. 3c. showed amorphous nature of material,
which confirmed the dissolution of material. It was reported that during the
dissolution of biomass using ionic liquid, crystallinity of cellulose change
from cellulose I to cellulose II (Takahashi and Matsunaga,
1991; Raymond et al., 1995; Kolpak
and Blackwell, 1976; Zhang et al., 2009).
However reappearance of cellulose II crystallinity during regeneration depends
upon way of stirring of dissolved material with non-solvent (water). It has
been reported that rapid stirring of dissolved biomass with non-solvent cause
the formation of amorphous nature of material, which has been observed in this
work too as shown in Fig. 3c (Kilpelainen
et al., 2007).
From SEM images it has been observed that before dissolution of biomass in ionic liquid, the fibers were present as shown in Fig. 4a. However in Fig. 4b the fibers were completely dissolved and changed into particle nature of material, which confirmed the dissolution of biomass. Figure 4c showed a partial dissolution of bamboo biomass due to presence of some fibers. Figure 4d and e showed the presence of fibers, which revealed no dissolution.
Ionic liquids have been used for dissolution of bamboo biomass. It has been observed that only 3-(1-methylimidazolium-3-yl) propane-1-sulfonate (MIM-PS) ionic liquid due to its zwitterions effect has the potential to dissolve biomass. From FTIR spectra of regenerated material it has been concluded that lignin could be partially removed by ionic liquid treatment. From XRD analysis it has been observed that during dissolution of biomass in ionic liquid the crystallinity of cellulose changed from cellulose I to its amorphous state. From SEM analysis it has been observed that fibrous nature of material disappeared and it was converted into particle nature of material.
This research was supported by Ionic Liquid Laboratory, Department of Chemical Engineering, Universiti Teknologi PETRONAS, Malaysia.
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