Synthesis of a sulfonic acid functionalized acidic ionic liquid modified silica catalyst and applications in the hydrolysis of cellulose

Article abstract of DOI:10.1016/j.catcom.2010.05.012

A sulfonic acid functionalized acidic ionic liquid modified silica catalyst was prepared in 68% overall yield from 3-chloropropyl silica by a simple two step method involving nucleophilic substitution of chlorine with imidazole, then condensation with 1,3-propanesultone and acidification using HCl. This silica supported acid catalyst was shown to be effective in the hydrolysis of cellulose (DP ～ 450) dissolved in 1-n-butyl-3-methylimidazolium chloride at 70 °C, producing glucose and total reducing sugars in 26 and 67% yields respectively.

Full text of DOI:10.1016/j.catcom.2010.05.012

Catalysis Communications 11 (2010) 1072–1075
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Synthesis of a sulfonic acid functionalized acidic ionic liquid modified silica catalyst
and applications in the hydrolysis of cellulose
⁎
Ananda S. Amarasekara , Onome S. Owereh
Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A sulfonic acid functionalized acidic ionic liquid modified silica catalyst was prepared in 68% overall yield
from 3-chloropropyl silica by a simple two step method involving nucleophilic substitution of chlorine with
imidazole, then condensation with 1,3-propanesultone and acidification using HCl. This silica supported acid
catalyst was shown to be effective in the hydrolysis of cellulose (DP∼450) dissolved in 1-n-butyl-3-
methylimidazolium chloride at 70 °C, producing glucose and total reducing sugars in 26 and 67% yields
respectively.
Received 27 February 2010
Received in revised form 17 May 2010
Accepted 19 May 2010
Available online 26 May 2010
Keywords:
Published by Elsevier B.V.
Ionic liquid
Acid catalyst
Modified silica
Cellulose
Hydrolysis
1. Introduction
mercaptopropyltrimethoxysilane modified silica gel by free radical
polymerization using azodiisobutyronitrile (AIBN) as the initiator.
Functionalized ionic liquids are a significant evolution in the ionic
liquid arena and these materials have been developed for various special
applications like catalysts, fossil fuel desulfurization reagents, lubricants
and as monomers for the synthesis of ionic polymers [1–4]. The sulfonic
acid group functionalized acidic ionic liquids are an important sub-
group in the ionic liquid based catalysts class, and have been used in
numerous types of reactionsincluding, esterification [5,6], alkylation [7],
nitration of aromatic compounds [8], Beckmann rearrangement [9], and
alkene polymerizations [10]. These acidic ionic liquids can be used in
neat or in a solvent in the catalysis applications and requires the
separation of the catalyst during the isolation of the product.
Immobilization of the functionalized ionic liquids have the benefits of
combining the ionic liquid characteristics with the common advantages
of immobilizations, such as ease of recycling and improved selectivity in
applications involving catalytic activity. There are several reports on the
preparation of ionic liquid covalently grafted silicas as chiral catalysts
[11], esterification catalysts [12,13], and functionalized hydroxyapatite-
encapsulated-Fe2O3 magnetic nanoparticles [14]. Yokoyama et al. has
recently reported [15] the immobilization of a sulfonic acid functiona-
lized ionic liquid catalyst and application of this catalyst for the
esterification and nitration of aromatic compounds. In this study, vinyl
substituted imidazolium ionic liquid has been grafted on to 3-
Among various interesting properties of ionic liquids, their ability
to dissolve cellulose and lignocellulosic biomass has attracted the
attention of a number of researchers in recent times [16,17]. In this
regard imidazolium ion based 1-n-butyl-3-methylimidazolium chlo-
ride (BMIMCl) and 1-allyl-3-methylimidazolium chloride (AMIMCl)
are well known for their cellulose dissolution capabilities. However,
hydrolysis and degradation of cellulose in the ionic liquid medium is a
relatively less explored application for these systems. Zhao et al. has
recently reported [18,19] the homogeneous phase hydrolysis of
cellulose using concentrated sulfuric acid as the catalyst in
1-n-butyl-3-methylimidazolium chloride medium without any pre-
treatment. Furthermore, solid acid catalysts like HY-Zeolite are also
known to be effective in the hydrolysis of cellulose in BMIMCl
medium [20]. Recently, we have shown [21] that cellulose dissolved in
Brönsted acidic ionic liquids 1-(1-propylsulfonic)-3-methylimidazo-
lium chloride and 1-(1-butylsulfonic)-3-methylimidazolium chloride
can be hydrolyzed at 70 °C under atmospheric pressure by the
addition of 2.0 equivalents of water per glucose unit to give glucose
along with other reducing sugars. In these experiments, the hydrolysis
of Sigmacell cellulose (DP∼450) in 1-(1-propylsulfonic)-3-methyli-
midazolium chloride produced the highest total reducing sugar (62%)
and glucose (14%) yields, and was attained with 1 h of preheating at
70 °C and 30 min heating at 70 °C, after adding water.
Our interest in the use of ionic liquid medium for the acid catalyzed
hydrolysis of cellulose into glucose and fermentable sugars for the
production of cellulosic-ethanol has led us to study the immobilization
⁎
Corresponding author. Tel.: +1 936 261 3107; fax: +1 936 261 3117.
E-mail address: asamarasekara@pvamu.edu (A.S. Amarasekara).
1566-7367/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.catcom.2010.05.012

A.S. Amarasekara, O.S. Owereh / Catalysis Communications 11 (2010) 1072–1075
1073
of sulfonic acid functionalized ionic liquids on silica surface. In this
communication we report an efficient and simple two step preparation
of imidazolium based acidic ionic liquid modified silica using 3-
chloropropyl silica as shown in Fig. 1, and application of this catalyst
for the hydrolysis of cellulose in BMIMCl medium.
2.4. Synthesis of sulfonic acid functionalized acidic ionic liquid modified
silica (3)
3-(1-Imidazole)propyl silica (2) (3.78 g, 2.61 mmol of imidazole
group) was suspended in 5 mL of dry toluene and 1,3-propanesultone
(0.320 g, 2.62 mmol) was added. The resulting mixture was stirred at
100 °C in an oil bath for 6 h, and then cooled to room temperature, filtered
and washed with dry toluene (3×10 mL) to give imidazolium salt grafted
silica. The intermediate was then treated with 36% w/w concentrated
hydrochloric acid (0.266 g, 2.62 mmol) and allowed to stand at room
temperature for 24 h. The resulting product was washed with ether
(3×20 mL), dried under vacuum at 100 °C for 24 h to give 3.96 g of
sulfonic acid functionalized acidic ionic liquid modified silica catalyst (3)
IR 463, 803, 1102, 1565, 1640, 2932, 3447 cm−1
2. Experimental
2.1. Materials and instrumentation
Fluka 60738 silica gel 60, 220–440 mesh (particle size 0.035–
0.070 mm) was used for the preparation of 3-chloropropyl silica (1).
(3-Chloropropyl)triethoxysilane, imidazole, 1,3-propanesultone, 1-n-
butyl-3-methylimidazolium chloride (BMIMCl), and Sigmacell® cel-
lulose (DP∼450) were purchased from Aldrich Chemical Co. FT-IR
spectra were recorded on a JASCO-470 PLUS IR spectrometer using
KBr pellets. Thermogravimetric analysis was carried out in air using
TA instruments TGA 2050 system. Elemental analysis was performed
at QTI laboratories, New Jersey. Total reducing sugars (TRS) and
glucose concentrations in aqueous solutions were determined using a
Carey 50 UV–Vis spectrophotometer and 1 cm quartz cells.
Found: C, 6.82; H, 1.49; N, 1.70%. Calc. for SiO2·0.0486(C9H16N2-
SO2Cl)·0.1H2O: C, 7.01; H, 1.32; N, 1.82%.
TGA: stable up to 220 °C, 17.8% weight loss with the decomposi-
tion of all organic structure at 660 °C. Sulfonic acid group loading was
calculated as 0.67 mmol/g by using thermogravimetric analysis data.
Determination of acidic sites: Titration of acidic ionic liquid modified
silica catalyst (3) with 5.05×10−2 M aq. NaOH, using phenolphthalein
as the indicator gave SO3H group loading as 0.677 mmol/g.
2.2. Synthesis of 3-chloropropyl silica (1)
2.5. Synthesis of sulfonated silica (4)
3-Chloropropyl silica (1) was prepared by using a modification of the
method used by Adam et al. [22]. A mixture silica (10.0 g) and
(3-chloropropyl)triethoxysilane (10.0 mL, 9.42 g, 42.5 mmol) in 15 mL
of toluene was prepared and magnetically stirred for 15 min at room
temperature, and then refluxed for 24 h. The reaction mixture was
cooled, and the product was filtered and repeatedly washed with
toluene (3×10 mL) and dried under reduced pressure at 100 °C for 8 h
to produce 3-chloropropyl silica 1 (10.9 g).
Sulfonated silica (4) was prepared by reacting silica with neat
chlorosulfonic acid at room temperature, as shown in Fig. 2, following
the literature procedure [23]. The acidic site loading in this sulfonated
silica (4) was determined as 2.52 mmol/g by titration with 5.05×
10−2 M aq. NaOH, using phenolphthalein as the indicator.
2.6. General experimental procedure for hydrolysis of cellulose using 3, 4,
or H2SO4 as catalyst
IR 465, 699, 802, 1096, 1638, 2963, 3453 cm−1
TGA: stable up to 270 °C, 5.5% weight loss with the decomposition
of all organic structure at 660 °C. Chloropropyl group loading was
calculated as 0.71 mmol/g by using thermogravimetric analysis data.
Cellulose (0.030 g, 0.185 mmol of glucose unit of cellulose) was
dissolved in 0.300 g of 1-n-butyl-3-methylimidazolium chloride
(BMIMCl) ionic liquid by heating at 80 °C for 4 h. Then catalyst
(5–10 mol%), and deionized water (6.7 μL, 2.0 equivalents/glucose
unit of cellulose) were added and the sample was warmed in a
thermostated water bath for a pre-determined time for the hydrolysis
of cellulose. Then the reaction was quenched by adding 10.0 mL of
deionized water and transferred to a centrifuge tube and centrifuged
at 3500 rpm for 6 min to precipitate and recover the acidic ionic liquid
modified silica catalyst. The clear supernatant was collected and the
acidic ionic liquid modified silica catalyst was repeatedly washed
(dispersion in 3 mL of deionized water followed by centrifugation,
three cycles) for recycling experiments. The supernatant and the
washings were combined and the total reducing sugar (TRS) and
glucose produced during the hydrolysis of cellulose were measured
using the procedures described in the Sections 2.6 and 2.7. The
average TRS and glucose yields produced in duplicate experiments are
shown in Table 1 as entries 1–8.
2.3. Synthesis of 3-(1-imidazole)propyl silica (2)
To a solution of imidazole (0.774 g, 11.38 mmol) in 25 mL of dry
benzene, 50% sodium hydride in mineral oil (0.546 g, 11.38 mmol) was
added and stirred under a nitrogen atmosphere at room temperature
for 30 min to give sodium imidazole. Then 3-chloropropyl silica (1)
(5.00 g, 3.55 mmol of Cl) was added and the mixture was refluxed
under a nitrogen atmosphere for 24 h. The resulting product was
filtered and washed with ethanol (3×20 mL) and dried under vacuum
at 100 °C for 24 h to give 4.93 g of 3-(1-imidazole)propyl silica (2).
IR 475, 797, 1093, 1513, 1638, 2962, 3453 cm−1
TGA: stable up to 224 °C, 7.5% weight loss with the decomposition of
all organic structure at550 °C. 3-(1-Imidazole)propylgroup loadingwas
calculated as 0.69 mmol/g by using thermogravimetric analysis data.
Fig. 1. Synthesis of sulfonic acid functionalized acidic ionic liquid modified silica catalyst (3).

1074
A.S. Amarasekara, O.S. Owereh / Catalysis Communications 11 (2010) 1072–1075
Table 2
Average percent yields of TRS and glucose produced in duplicate experiments
reutilizing catalyst (3) for the hydrolysis of untreated cellulose (DP∼450).
Run
Yield (%)
TRS
Glucose
Fig. 2. Synthesis of sulfonated silica (4).
1
2
3
4
5
67
65
64
60
60
26
26
24
25
22
2.7. Total reducing sugar (TRS) assay for cellulose hydrolyzate
2.50 mL of the clear solution from the centrifuge tube was
transferred to a vial and 0.50 mL of DNS reagent [24] was added.
The resulting solution was heated at 90 °C for 5.0 min to develop the
red-orange color. Then the absorbance was measured at 550 nm using
1 cm quartz cells and Carey 50 UV–Vis spectrophotometer. TRS
Concentration in the solution was calculated by employing a standard
curve prepared using glucose.
0.030 g of cellulose (DP∼450) dissolved in 0.300 g of 1-n-butyl-3-methylimidazolium
chloride (BMIMCl), and 10 mol% catalyst were used in all experiments. Reaction time:
6 h. Temperature: 70 °C and 6.7 μL of H2O was added before the hydrolysis.
3-Chloropropylsilica (1) was first prepared by reaction of (3-chlor-
opropyl)triethoxysilane with silica and then nucleophilic substitution of
thechlorine with imidazolegroup gave 3-(1-imidazole)propylsilica(2).
Condensation of 3-(1-imidazole)propyl silica (2) with 1,3-propanesul-
tone produced the imidazole salt, which was treated with one
equivalent of hydrochloric acid to give the sulfonic acid functionalized
acidic ionic liquid modified silica (3). This is a simple two step linear
synthesis of acidic ionic liquid grafted silica when compared with the
earlier approach for the preparation of a similar catalyst [15].
2.8. Glucose assay for cellulose hydrolyzate
1.0 mL of the clear solution from the centrifuge tube was
transferred to a vial, and 2.0 mL of glucose assay reagent [25]
(Sigma-Aldrich Glu 20 kit) was added. The resulting solution was
heated at 37 °C for 30 min, and reaction was quenched by adding
2.0 mL of 12 N H2SO4 to give a pink solution. Then the absorbance was
measured at 540 nm using 1 cm quartz cells and Carey 50 UV–Vis
spectrophotometer. Glucose concentration in the solution was
calculated by employing a standard curve prepared using glucose.
3.2. Structural characterization of sulfonic acid functionalized acidic
ionic liquid modified silica catalyst (3)
3.2.1. Elemental analysis and TG–DTG
Acidic ionic liquid modified silica (3) was characterized using
elemental analysis, thermogravimetric–derivative thermogravimetric
(TG–DTG) analysis, and FT-IR spectroscopy. The thermogravimetric
analysis (TG) curve showed (Fig. 3) an initial weight loss of 2.4% up to
110 °C due to surface silanol groups and the adsorbed water in silica.
Complete loss of all the covalently attached organic structure is seen
in the 220–660 °C temperature range leaving SiO2, and the organic
fraction corresponds to 17.8% weight of the acidic ionic liquid
modified silica. This decomposition of the organic structure occurs
in two steps and the two peaks in the DTG curve at 390 and 532 °C, are
related to 13.0 and 4.8% weight losses. The immobilized acid catalyst is
apparently stable up to about 220 °C. By using the thermogravimetric
data, the empirical formula of the catalyst can be calculated as
SiO2·0.0486 (C9H16N2SO3Cl)·0.1H2O. Structural characterization for
the catalyst 3 was supported by elemental analysis data shown in
Section 2.4. These CHN analysis results are within 0.19% of the CHN
percentages calculated using thermogravimetric analysis based
formula: SiO2·0.0486 (C9H16N2SO3Cl)·0.1H2O.
2.9. Reuse of the sulfonic acid functionalized acidic ionic liquid modified
silica catalyst (3)
The washed acidic ionic liquid modified silica catalyst (3) was
collected and dried in an oven at 105 °C for 24 h. This recycled catalyst
was used for the hydrolysis of cellulose using procedure 2.5 and the
hydrolyzate was analyzed using procedures 2.6 and 2.7. The results of
reusing the same catalyst sample in five cycles are shown in Table 2.
3. Results and discussion
3.1. Synthesis of sulfonic acid functionalized acidic ionic liquid modified
silica (3)
Sulfonic acid functionalized acidic ionic liquid modified silica (3)
was synthesized in two steps, and 68% overall yield as shown in Fig. 1.
Table 1
Average percent yields of TRS and glucose produced in duplicate experiments of
hydrolysis of untreated cellulose using sulfonic acid functionalized acidic ionic liquid
modified silica (3), H2SO4, sulfonated silica (4), and 1-(1-propylsulfonic)-3-methyli-
midazolium chloride (PSMIMCl).
3.2.2. FT-IR
The FT-IR spectrum of the functionalized acidic ionic liquid
modified silica (3) is shown in Fig. 4. The peak at 1565 cm−1 can be
Entry
Catalyst
(mol%)
Temperature
(°C)
Time
(h)
Yield (%)
Ref.
TRS
Glucose
1
2
3
4
5
6
7
8
3(5)a
70
70
70
70
70
90
70
70
70
70
70
2.0
4.0
4.0
6.0
10.0
6.0
6.0
6.0
0.5
1.0
4.0
26
28
52
67
55
29
38
24
62
42
29
10
10
15
26
19
12
6
11
14
7
This work
This work
This work
This work
This work
This work
This work
This work
[21]
3(5)a
3(10)a
3(10)a
3(10)a
3(10)a
H2SO4(10)a
4(10)a
9
10
11
PSMIMClb
PSMIMClb
PSMIMClb
[21]
[21]
4
a
0.030 g of cellulose (DP∼450) dissolved in 0.300 g of 1-n-butyl-3-methylimidazolium
chloride (BMIMCl), and 6.7 μL of H2O were added before the hydrolysis.
b
0.030 g of cellulose (DP∼450) dissolved in 0.300 g of 1-(1-propylsulfonic)-3-methyli-
midazolium chloride (PSMIMCl), and 6.7 μL of H2O were added before the hydrolysis.
Fig. 3. TG–DTG of sulfonicacidfunctionalized acidic ionic liquidmodifiedsilicacatalyst (3).

A.S. Amarasekara, O.S. Owereh / Catalysis Communications 11 (2010) 1072–1075
1075
TRS in 0.5 h) using PSMIMCl medium, glucose yields are comparatively
lower in these experiments. Furthermore, longer reaction times like
4 h (entry 11) resulted in much lower TRS and glucose values.
3.4. Reuse of the catalyst (3)
The reusability of the catalyst (3) was tested by using the same
catalyst sample in five catalytic cycles using 10 mol% of catalyst in the
hydrolysis of cellulose at 70 °C for 6 h, and the results of these
experiments are shown in Table 2. This catalyst recycling experiment
shows that sulfonic acid functionalized acidic ionic liquid modified
silica catalyst (3) can be reused for the hydrolysis of cellulose in
BMIMCl medium with a small loss of catalytic activity.
4. Conclusion
In summary, we have demonstrated that sulfonic acid functiona-
lized acidic ionic liquid modified silica (3) can be prepared in 68%
overall yield from 3-chloropropyl silica (1), in a simple two step
method by using nucleophilic substitution reaction of chlorine with
imidazole anion and then condensation of the alkylimidazole silica
with 1,3-propane sultone followed by acidification with HCl. The
immobilized acidic ionic liquid catalyst (3) is shown to be effective in
the hydrolysis of cellulose dissolved in 1-n-butyl-3-methylimidazo-
lium chloride at 70 °C and at atmospheric pressure. Additionally, the
catalyst can be recovered through a simple separation protocol and
can be reused with a small loss in the activity.
Fig. 4. FT-IR spectrum of sulfonic acid functionalized acidic ionic liquid modified silica
catalyst (3).
assigned to the imidazolium ring, and Yokoyama has reported [15] a
similar absorption at 1563 cm−1 for an imidazolium system attached
to a silica surface through alkyl sulfide tether in their preparation of
ionic liquid modified silica. The Si–O absorption of silica can be
observed as a strong peak at 1102 cm−1, whereas the weak absorption
at 2932 cm−1 can be assigned to the C–H bonds in 3. Furthermore, the
broad absorption in the region of 3447 cm−1 is due to the Si–OH
groups on the surface of the silica.
Acknowledgments
3.3. Comparison of the catalytic activity of acidic ionic liquid modified
silica catalyst 3 with other catalysts
Authors would like to thank Center for Environmentally Beneficial
Catalysis (CEBC) — University of Kansas, American Chemical Society-PRF
grant UR1-49436 and NSF grant CBET-0929970 for financial support.
Hydrolysis of cellulose using acidic ionic liquid modified silica
catalyst produced glucose and other reducing sugars. The TRS and
glucose yields produced in these hydrolysis experiments are shown in
Table 1. Initial experiments using 5 mol% of the catalyst 3 (entries 1
and 2) gave relatively low TRS and glucose yields, therefore the
catalyst loading was increased to 10 mol% in the subsequent set of
experiments. The highest TRS and glucose yields of 67 and 27%
respectively were obtained in the experiment using 10 mol% of
catalyst 3, at 70 °C after a 6 h reaction time (entry 4). Heating the
sample at 70 °C for a longer time (entry 5) or at a higher temperature
(entry 6) failed to give better yields of TRS and glucose, indicating the
importance of reaction time and temperature in the hydrolysis
reaction. Furthermore, longer heating times (entry 5) and higher
temperatures (entry 6) produced excessive charring of the sample,
giving black residues, and thus lowering the TRS and glucose yields.
The experiment using 10 mol% H2SO4 catalyst (entry 7) produced 38%
TRS yield and low 6% glucose yield. Sulfonated silica (4) produced only
24% TRS and 11% glucose (entry 8), showing the importance of the
imidazolium chloride tether. The higher catalytic activity of acidic
ionic liquid modified silica catalyst (3) may be due to hydrogen
bonding interactions of chloride ion with the hydrogens of cellulose
OH groups. Entries 9–11 shows the results from our previous work on
hydrolysis of cellulose using 1-(1-propylsulfonic)-3-methylimidazo-
lium chloride (PSMIMCl) as the solvent as well as the catalyst. Even
though, good TRS values are obtained in shorter times (entry 9; 62%
References
[1] J.H. Davis, Chem. Lett. 33 (2004) 1072–1077.
[2] Z.F. Fei, T.J. Geldbach, D.B. Zhao, P.J. Dyson, Chem. Eur. J. 12 (2006) 2122–2130.
[3] X. Li, D. Zhao, Z. Fei, L. Wang, Sci. China Ser. B 49 (2006) 385–401.
[4] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal., A 373 (2010) 1–56.
[5] D. Jiang, Y.Y. Wang, L.Y. Dai, React. Kinet. Catal. Lett. 93 (2008) 257–263.
[6] Y. Zhao, J. Long, F. Deng, X. Liu, Z. Li, C. Xia, J. Peng, Catal. Commun. 10 (2009)
732–736.
[7] K. Qiao, C. Yokoyama, Chem. Lett. 33 (2004) 472–473.
[8] K. Qiao, C. Yokoyama, Chem. Lett. 33 (2004) 808–809.
[9] K. Qiao, Y. Deng, C. Yokoyama, H. Sato, M. Yamashina, Chem. Lett. 33 (2004)
1350–1351.
[10] Y. Gu, F. Shi, Y. Deng, Catal. Commun. 4 (2003) 597–601.
[11] B. Gadenne, P. Hesemann, V. Polshettiwar, J.J.E. Moreau, Eur. J. Inorg. Chem.
(2006) 3697–3702.
[12] W. Hui, J. Zhang, G. Guan, Shiyou Huagong 38 (2009) 134–138.
[13] J. Zhang, W. Hui, G. Guan, HuaxueFanying Gongcheng Yu Gongyi24(2008)503–508.
[14] Y. Zhang, Y. Zhao, C. Xia, J. Mol. Catal. A: Chem. 306 (2009) 107–112.
[15] K. Qiao, H. Hagiwara, C. Yokoyama, J. Mol. Catal. A: Chem. 246 (2006) 65–69.
[16] A. Pinkert, K.N. Marsh, S. Pang, M.P. Staiger, Chem. Rev. 109 (2009) 6712–6728.
[17] L. Feng, Z.L. Chen, J. Mol. Liq. 142 (2008) 1–5.
[18] C. Li, Z.K. Zhao, Adv. Synth. Catal. 349 (2007) 1847–1850.
[19] C. Li, Q. Wang, Z.K. Zhao, Green Chem. 2 (2008) 177–182.
[20] Z. Zhang, Z.K. Zhao, Carbohydr. Res. 344 (2009) 2069–2072.
[21] A.S. Amarasekara, O.S. Owereh, Ind. Eng. Chem. Res. 48 (2009) 10152–10155.
[22] F. Adam, H. Osman, K.M. Hello, J. Colloid Interface Sci. 331 (2009) 143–147.
[23] H.R. Shaterian, M. Ghashang, M. Feyzi, Appl. Catal., A 345 (2008) 128–133.
[24] C. Breuil, J.N. Saddler, Enzyme Microb. Technol. 7 (1985) 327–332.
[25] H.U. Bergmeyer, E. Bernt, in: H.U. Bergmeyer (Ed.), Methods of Enzymatic Analysis,
Academic Press, New York, 1974, pp. 1205–1212.