Modification of cellulose for high glucose generation

Bioresource Technology 104 (2012) 473–479

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Modification of cellulose for high glucose generation Xue Jiang a,b,⇑, Jian Gu a, Xiuzhi Tian a, Yali Li a, Dan Huang a a b

Key Laboratory of Eco-Textiles of Ministry of Education, Jiangnan University, Wuxi 214122, PR China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, PR China

a r t i c l e

i n f o

Article history: Received 25 April 2011 Received in revised form 22 August 2011 Accepted 25 October 2011 Available online 4 November 2011 Keywords: Microcrystalline cellulose Cyanuric chloride Crystal index d-Spacing Glucose yield

a b s t r a c t The influence of introduction of cyanuric chloride on glucose’s yield (Y) in acid-catalyzed hydrolysis of microcrystalline cellulose (MCC) has been studied. The content of cyanuric chloride (C) in modified MCCs was determined by X-ray photoelectric spectroscopy. The chemical structures of modified MCCs were analyzed by Fourier transformation-infrared spectroscopy and cross polarization/magic angle spinning 13 C nuclear magnetic resonance. Crystal index (CI) and the ratio (R) representing the sum of content of  0Þ and (1 1 0) to that of (2 0 0) were calculated based on diffraction intensity in wide angle X-ray difð1 1 fraction (WAXD). Hydrolysis experiment and WAXD show that Y, CI and R vary with C. The modified MCC containing 3.9 mol% of cyanuric chloride has the highest Y, the highest R and the lowest CI. Variations of CI and R show that the chemical modification changed the proportion of crystal/amorphous and crystal planes, both of which influence glucose’s generation in hydrolysis of cellulose. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Cellulose is the most significant component of lignocellulosic materials (35–50%) composing b-1, 4-linked D-glucose units ((C6H10O5)n). It has been recognized as a potential renewable source for biofuels and bio-based chemicals. Many industrially important chemicals or fuels such as ethanol, hydrocarbons, ketones and carboxylic acids can be produced from the hydrolysis of cellulose and the subsequent ferment of saccharides (Ferreira et al., 2011; Klemm et al., 2005; O’Sullivan, 1997; Ragauskas et al., 2006). However, the chemicals produced from cellulose currently may cost far more than those from petrochemical industry or even more than other green sources such as starch, mainly because of the low yield of saccharide in hydrolysis of cellulose. The low yield of saccharide is generally due to the high stability and resistance of cellulose to chemical attack (Alvira et al., 2010; Heinze and Liebert, 2001). As is well known, cellulose has a great amount of intra- and intermolecular hydrogen bonds as well as relatively high crystallinity (65–75%). Therefore, pretreatments of cellulose are necessary before hydrolysis, and they have been studied widely to achieve high yield of fermentable saccharides. Different pretreatments of lignocellulosic materials have been compared (Gupta et al., 2011; Ingram et al., 2011; Zhang et al., 2011) and investigated, including the use of ionic liquid combined with microwaves (Ha et al., 2011), alkali (Wu et al., 2011), different solvent ⇑ Corresponding author at: Key Laboratory of Eco-Textiles of Ministry of Education, Jiangnan University, Wuxi 214122, PR China. Tel.: +86 510 8591 2007; fax: +86 510 8591 2009. E-mail address: [email protected] (X. Jiang). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.10.091

systems such as organic solvents or ionic liquids (Wang et al., 2011; Yang et al., 2010), (super- or sub-) critical solvents (Ju et al., 2011; Kim and Hong, 2001). Nevertheless, the current pretreatments require substantial chemicals that add to huge cost of reagent and result in serious pollution of environment (Fukuoka and Dhepe, 2006; Sun and Cheng, 2002; Taherzadeh and Karimi, 2008). It has been reported that cellulose becomes unstable and thereby easier to be hydrolyzed after certain chemical modifications (Borsa et al., 1990; Karst et al., 2006; Karst and Yang, 2007). For example, Borsa et al. mentioned that the carboxymethyl cellulose with a low degree of substitution can be hydrolyzed more easily than the original cellulose. Besides, the hydrolysis of b-1, 4-glycosidic linkages is increased by the covalently bonding of reactive dyes with cellulose, which induces an abundant loss of strength and a decreasing degree of polymerization. This phenomenon is so-called ‘‘reactive tendering’’ in the textile industry. Whereafter, by molecular modeling, Karst et al. found that grafting various groups onto cellulose could increase the acid hydrolysis of b-1, 4glycosidic linkages, and the degree of hydrolysis increased with an increase in the size of the substituent groups. Further, a substituent on the C6 of glucose residue, of which the structure with numbered carbon atoms will be shown in NMR analysis section, can benefit the hydrolysis more than the substituent on the C3 or C2 (Karst et al., 2006; Karst and Yang, 2007). As discussed above, chemical modification is useful to increase the yield of saccharides in hydrolysis of cellulose. As a result, the modification of cellulose makes it possible for the cellulose-based ethanol to be economically competitive to other bio-based fuels and green chemicals. In this paper, cyanuric chloride was chosen to modify microcrystalline cellulose to increase the yield of glucose

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in hydrolysis of cellulose. Microcrystalline cellulose is known as the most crystallized cellulose, thus it was chosen to determine the influences of chemical modification on the crystal structure of cellulose. Moreover, the yield of glucose is also discussed.

C frequency of 100 MHz, using the combined techniques of proton dipolar decoupling (DD), magic angle spinning, and cross polarization (CP/MAS 13C SSNMR). 13C and 1H field strengths of 100 kHz corresponding to 90° pulses of 4 ms were used for the matched spin-lock cross-polarization transfer. The spinning speed was set at 3 kHz. The contact time was 1 ms, and the acquisition time was 10 ms. A typical number of 10,000 scans were acquired for each spectrum. Chemical shifts were referred to tetramethylsilane (TMS) as the internal standard.

2. Methods 2.1. Materials Microcrystalline cellulose (MCC) with an average particle size of 0.050 mm was purchased from Sigma–Aldrich (St. Louis, USA). Cyanuric chloride and acetone were purchased from Acros organic (Germany). NaOH was purchased from TCI chemical company (Japan). All of the chemicals were used as received.

2.2.5. WAXD analysis and determination of crystalline index Crystal structures of the original MCC and cyanuric chloridemodified MCCs were analyzed by wide-angle X-ray diffraction on a D 8 Advance instrument (WAXD, Bruker AXS Company, Germany) with a scan speed of 4o/min. The cellulose powder was laid on the glass sample holder (35  50  5 mm) and analyzed under plateau conditions. Ni-filtered Cu Ka radiation (k = 0.154 nm) generated at a voltage of 40 kV and current of 40 mA, a scan speed of 4o/min for 2h ranging from 3o to 60o were utilized. The crystalline index (CI) which indicates the relative amount of amorphous and crystalline regions in the sample was calculated based on the height of the crystalline and amorphous diffraction peaks according to Eq. (1). A CI value of zero means that the material has equal amount of crystalline and amorphous regions, and a positive CI indicates that the cellulose has higher content of crystalline regions than amorphous regions (Oh et al., 2005; Sun and Cheng, 2002).

2.2. Methods 2.2.1. Modification of microcrystalline cellulose The reaction between cyanuric chloride (trichloro-s-triazine) and cellulose is presented in Scheme 1. The substitution was restricted to just one of the three chlorines in cyanuric chloride by controlling the reaction temperature (Lenfeld et al., 1995; Pardal et al., 2001). Specifically, 2 g of MCC and 8 mL of acetone were mixed in a three-neck flask and stirred for 80 min under N2 atmosphere at 20 °C. Later, 4 mL of aqueous NaOH solution (4%) was added. The mixture was stirred for another 1 h, cooled down to 0 °C. Cyanuric chloride dissolved in acetone was introduced dropwise and then the reaction system was kept at 0 °C for 4 h. After that, the mixture was centrifugally separated, and the product (chemically modified cellulose) was washed with 40 mL of acetone and 40 mL of ice-cold water, centrifuged again and vacuum-dried at 50 °C for 12 h, and then stored in a vacuum drier (Lenfeld et al., 1995; Pardal et al., 2001).

  ham  100 % CI ¼ 1  hcr

where CI is the crystal index, (hcr) represents the crystalline scatter of the (2 0 0) reflection at 2h of 22.5o for cellulose I, and the amorphous height (ham) indicates the height of the amorphous reflection at 2h of 18o for cellulose I. 2.2.6. Hydrolysis of cellulose and determination of glucose yield About 0.5 g sample was charged into an autoclave with 10 mL of sulfuric acid solution (8% based on weight of cellulose), and then purged with N2 flow. The suspension was heated to 373 K and maintained for 5 h with constant stirring at an atmospheric pressure of approximately 0.1 MPa. After the reaction, the mixture was centrifuged to separate solid from water-soluble products in aqueous solution. The composition of the obtained solution was analyzed by high-performance liquid chromatography (LC-20AT, Shimazi, Japan) at 298 K equipped with a UV–Vis detector (Prominence SPD-20A/20AV UV–Vis), where the mobile phase was 0.005 M sulfuric acid and the flow rate was 0.5 mL/min. The extent of hydrolysis was calculated in terms of the % glucose yield based on Eq. (2).

2.2.2. Determination of cyanuric chloride content in modified MCC by XPS X-ray photoelectron spectra and the content of cyanuric chloride (C, mol%) of the modified MCC were recorded by a Kratos AXIS Ultra DLD spectrometer employing a monochromated Al Ka X-ray source (hm = 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multi-channel plate and delay line detector (DLD). All X-ray photoelectron spectra were recorded using an aperture slot of 300  700 lm2. 2.2.3. FT-IR spectroscopy Pellets of ca. 2 mg of the original MCC and cyanuric chloridemodified MCCs were prepared by mixing with 200 mg of spectroscopic grade KBr. FT-IR spectra were recorded on a Nicolet nexus 470 spectrometer (Thermo Electric, USA) with detector at 4 cm1 resolution and 64 scans per sample.

% Glucose Yield ¼

ð2Þ The zero content of cyanuric chloride was obtained using the same reaction conditions but without cyanuric chloride in microcrystalline cellulose.

Cl N

Cl O

O HO

O OH

+

N

Cl N

Cl

Glucose obtainedðmolÞ moles of glucose per anhydroglucose unit  100

2.2.4. Solid-state 13C NMR spectroscopy Solid state 13C NMR analysis of the original MCC and cyanuric chloride-modified MCC were performed with a 400 MHz NMR spectrometer (AVANCE III, Brooker company, Switzerland) at a

HO

ð1Þ

Alkali Temperature control

Cl

N N

N O O

O HO

Scheme 1. Reaction between MCC and cyanuric chloride.

O OH

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3. Results and discussion 3.1. Chemical structure of modified MCCs The chemically modified MCCs were obtained from the reaction between a chlorine in cyanuric chloride and the –OH on C6 of glucose residue in MCC, as exhibited in Scheme 1. 3.1.1. Analysis of elements by XPS The XPS spectra and the curve of content-dosage of cyanuric chloride are shown in Fig. 1. Wherein, the XPS curves of three samples are shown in Fig. 1a, and the relationship between the content of cyanuric chloride in modified MCC and the dosage of cyanuric chloride at the beginning of the reaction is shown in Fig. 1b, in which both content and dosage are mole percent. The dosage of cyanuric chloride in three samples in Fig. 1a was 4.8%, 9.1%, and 16.7%, respectively. The binding energy and atomic contents of N, O, and C were measured by the symbol’s strength of 1 s electron

Intensity (CPS)

6000

Intensity (CPS)

6000

Intensity (CPS)

(a)

6000

in XPS analysis. According to the linear relationship between the contents and the added percentages of cyanuric chloride in Fig. 1b, the actual content of cyanuric chloride in the modified MCCs (mol%) is 2.1%, 2.7%, 3.9%, 7.2%, 12.9%, 15.3%, 19.1%, 25.3%, and 37.9%, respectively. It can be seen from Fig. 1b that only 76% of cyanuric chloride has been attached to MCC. Except for the reaction with cellulose, cyanuric chloride can be hydrolyzed to form cyanuric acid in basic aqueous solution. These are two competing reactions. The reaction with cellulose is more than the hydrolysis in our experiment, of which the proportion of the cyanuric chloride that reacts with cellulose is about 76% according to the XPS test. The other 24% is probably hydrolyzed. 3.1.2. FT-IR spectra FT-IR spectrum of cyanuric chloride is shown in Fig. 2a, while FTIR spectra of the original and cyanuric chloride-modified MCCs are shown in Fig. 2b. Eight characteristic peaks of cyanuric chloride

O 1s

4000

C 1s

2000

N 1s

16.7%

0

4000

9.1%

2000 0

4000 2000

4.8%

0 600

500

400

300

200

100

0

Binding Energy (eV)

(b)

45

y = 0.7616x

40

Content of

cyanuric chloride (%mol )

2

R = 0.9997

35 30 25 20 15 10 5 0 0

10

20

30

40

50

Percentage of cyanuric chloride added before reaction (%mol) Fig. 1. XPS spectra of the modified MCCs obtained at different reaction concentration of cyanuric chloride (a). The relationship between the contents of cyanuric chloride in modified MCCs and the mole concentrations of cyanuric chloride added before reaction (b).

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1270

1500 2000

1800

1600

793

852

1440

879

1320

1360

(a)

1400

1200

1000

800

600

400

-1

Wave number (cm )

1576

25.3%

1710

(b)

12.9%

1440 1360 1320 1270

MCC

2000

829 789

3.9%

1800

1600

1400

1200

1000

800

600

-1

Wave number (cm ) Fig. 2. FT-IR spectra of pure cyanuric chloride (a), the original MCC and the cyanuric chloride-modified MCCs containing 3.9%, 12.9% and 25.3% of cyanuric chloride, respectively (b).

are clearly observed in Fig. 2a at 1500 cm1, 1440 cm1, 1360 cm1, 1320 cm1, 1270 cm1, 879 cm1, 852 cm1, and 793 cm1. The first five absorptions are due to the vibration of the triazine frame, and the last three are ascribed to the vibration of C–Cl (Gubeau et al., 1954). In Fig. 2b, the absorptions at 1576 cm1, 1440 cm1, 1360 cm1, 1320 cm1, and 1270 cm1 appear in the samples with 3.9% and 12.9% of cyanuric chloride. Compared with Fig. 2a, the absorption at 1500 cm1 moves to 1576 cm1, which is probably due to the vibration of the reacted triazine frame. However, the absorption at 1576 cm1 disappeared while that at 1710 cm1 appeared, when the content of cyanuric chloride in the modified MCC was increased to 25.3%. It could be explained by the hydrolysis of cyanuric chloride and the succeeding rearrangement of the produced cyanuric acid. The probable mechanisms of hydrolysis of cyanuric chloride and the subsequent rearrangement are exhibited in Scheme 2. The C=O group, which corresponded to the absorption peak at 1710 cm1, formed outside the triazine cycle by the rearrangement of HO– C=N– in cyanuric acid originated from the hydrolysis of cyanuric chloride, as seen in Scheme 2I. The absorption at 1710 cm1 only appears in FT-IR spectra of the samples with higher contents of cyanuric chloride (P25.3%) according to Fig. 2. It implies that, the

hydrolysis of cyanuric chloride cannot be neglected when a large amount of cyanuric chloride is utilized to react with cellulose. Moreover, the absorption peaks ranging from 700 cm1 to 900 cm1, which are due to the C–Cl bond in cyanuric chloride, appear in the FT-IR spectra of the modified MCCs with relatively low content of cyanuric chloride such as 3.9% or 12.9%. However, they faded away when the content of cyanuric chloride was increased to 25.3%. The disappearing of those absorption peaks in the modified MCC with 25.3% of cyanuric chloride indicated that all of the chlorine had been reacted. The reaction of chlorine in cyanuric chloride includes hydrolysis, reaction with cyanuric acid originated in hydrolysis and thereby cross-linking with cellulose, as shown in Scheme 2II. The cross-linking would not take place if the content of cyanuric chlorides in cellulose were not large enough, because the triazine cycle was too small to react with the –OH groups on two glucose residues synchronously. As the content of cyanuric chloride increased, the dimers or trimers produced by the reaction between cyanuric chloride and cyanuric acid made the cross-linking reaction of chains of cellulose possible. Therefore, the hydrolysis, reaction of cyanuric chloride and cyanuric acid and their cross-linking with cellulose led to a disappearance of chlorine when a large amount of cyanuric choride was introduced into the modified MCCs. Nevertheless, the unreacted chlorine, which was detected by FT-IR spectra in two modified MCCs, will probably count against hydrolysis of cellulose and ferment of saccharide. During acid hydrolysis of cellulose, the disadvantage of chlorine is that it would be hydrolyzed and thereby varying the pH value of reactant mixture. As a result, the yield of saccharides lowers. However, not any variation of pH value in process of hydrolysis was detected in our experiment, which indicated that the hydrolysis was not influenced. It can be attributed to that the chlorine in modified MCC is too scant to vary the pH value of the reactant mixture. During ferment of saccharides, the unreacted chlorine probably deactivates catalysts and thereby the yield of ferment decreases. However, the chemicals containing chlorine can be eliminated easily because they are insoluble while the fermentable saccharides are well soluble in water. Therefore, the remained chlorine cannot trouble either hydrolysis of cellulose or ferment of saccharide in cellulose–ethanol conversion. 3.1.3. CP/MAS 13C SSNMR CP/MAS 13C SSNMR of the original MCC (curve a) and the modified MCC with 3.9% of cyanuric chloride (curve b) were characterized, as well as the structure with numbered carbons are shown in Fig. 3. Peaks assigned to the carbons in the cellulose backbone at 105.26 (C1), 89.12 and 84.46 (C4), 75.21, 72.60 and 71.81 (C2, C3), 65.36 (C5), and 63.12, 62.67 (C6), respectively, were evidently detected (VanderHart and Atalla, 1987) in both curves a and b. The introduction of cyanuric chloride onto cellulose’s molecular chains is confirmed by appearance of the peaks at about 172 ppm in curve b (Witter et al., 2006), which is ascribed to the carbons (C-7, C-8 and C-9) in triazine cycle. The uniformity of main part of curves a and b shows that the backbone of cellulose has not been broken by the modification of cyanuric chloride. However, the changes of peaks of C6 further prove that cyanuric chloride has been introduced into MCC (VanderHart and Atalla, 1987; Wada et al., 2004). As shown in the expanded view in Fig. 3, there are two parts in the peaks of C6 ranging from 60 ppm to 70 ppm. The higher part is ascribed to the C6 atoms in crystal regions, and the lower part that is symbolized as N.C. can be attributed to the C6 atoms in amorphous regions (N.C. means non-crystal) (VanderHart and Atalla, 1987; Wada et al., 2004). The peaks of C6 were changed in two ways. Firstly, there are two subdivided peaks in the N.C. part of 13 C NMR of original MCC, while there is only one peak in the same part of 13C NMR of chemically modified MCC. Secondly, there is a shoulder peak in the higher part of the 13C NMR of the original

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N

N

rearrangement

N

N

N

NH

OH

O

I.

Cl N

Cl N

Cl N

alkaline a)

Cl

HO

N

Cl

N N

O

N N

O

N N

N

O

cellulose chains

OH

b)

N

O

N N

N OH

N

N

Cl

N O

N

HO

O

N N

O trimer

amorphous region of cellulose

II. Scheme 2. Rearrangement of ketone-enol exchange and cross-linking of cellulose. I. rearrangement of ketone-enol: between HO–C=N– and O=C–NH– groups in cyanuric acid produced by hydrolysis of cyanuric chloride; II. Cross-linking of celluloses by the oligomers of cyanuric chloride and its hydrolysis products. (a) Hydrolysis and self reaction of cyanuric chloride; (b) reaction between amorphous region of cellulose and oligomer of cyanuric chloride.

expanded view of C6

C2,3,5 Cl

8 N

9 Cl

N

N

7 O N.C.

O

6 5 4 HO

C1 N.C.

1 O

3

2

O OH

C6

b)

C4 a)

70

68

66

64

62

60

δ(ppm)

C7,8,9

b) a)

200

180

160

140

120

100

80

60

40

20

0

δ(ppm) Fig. 3. CP/MAS

13

C SSNMR of the original MCC (a) and the cyanuric chloride-modified microcrystalline cellulose (MCC) containing 3.9% of cyanuric chloride (b).

MCC (Rondeau-Mouro et al., 2011), which is indicated by an arrow in curve a, but the shoulder peak disappeared in the same area of 13 C NMR of chemically modified MCC (curve b). The variations in the expanded view indicate the chemical reaction on C6. 3.2. WAXD of modified MCCs WAXD patterns of the original MCC and several MCCs modified with cyanuric chloride are shown in Fig. 4. The four diffraction peaks at 2h of 22.5o, 34.0o, 16.0o and 14.0o in the spectrum of MCC corre 0Þ in spond to the four crystal planes (2 0 0), (4 0 0), (1 1 0) and ð1 1 MCC (cellulose I), respectively, and the diffraction peak at 2h of 18.5o is due to the amorphous regions in MCC (Atalla et al., 1977; Wada et al., 2004). The relative height of the diffraction peak of the amorphous region (18.5o) increases, while the relative height  0Þ, of the diffractions of the four crystal planes that are (1 1 0), ð1 1 (2 0 0) and (4 0 0) decreases, as the content of cyanuric chloride

increases from 0% to 3.9%, which are shown in Fig. 4. However, the change trend of the relative height of the diffraction peaks of the four crystal planes turned to opposite as the content of cyanuric chloride increased from 3.9% to 25.3%, while the relative height of the diffraction peak at 2h of 18.5o was slightly changed with the content of cyanuric chloride increasing from 3.9% to 25.3%. WAXD results indicate that the crystal structures of cyanuric chloride-modified MCCs were destructed when fewer cyanuric chloride was reacted with MCC (about lower than 3.9%). On the other hand, when the content of cyanuric chloride in the modified MCC was higher than 3.9%, the formed dimers, trimers or higher degree of the cyanuric chloride oligmers were too big to attack the glucose residue at crystal regions in cellulose, so cross-linking between different chains of cellulose was induced at amorphous regions only, as shown in Scheme 2II. Therefore, the diffractions in the WAXD of the modified MCCs with more than 3.9% of cyanuric chloride were changed little compared with the modified

478

X. Jiang et al. / Bioresource Technology 104 (2012) 473–479 o 22 (200) o

14 (110)

o

16 (110)

relative height of peaks

18.5

o

34 (400) 25.3%

o

12.9%

Scheme 3. Draft of different d-spacing values of the three main crystal planes that  0Þ, (1 1 0) and (2 0 0) in MCC, and comparison of their accessibilities to are ð1 1 hydronium.

3.9%

MCC

10

15

20

25

30

35

40

o

2θ( ) Fig. 4. WAXD patterns of the original MCC and cyanuric chloride-modified MCCs containing 3.9%, 12.9%, and 25.3% of cyanuric chloride, respectively.

Yield of glucose (%mol); Crystal index (%); Ratio of crystal plane

1.0

Yield of glucose/10 CI (Crystal Index)/100 R (Ratio of crystal planes)

0.9

0.8

0.7

0.6

0.5

0.4 0

5

10

15

20

25

30

35

40

Content of cyanuric chloride (%mol) Fig. 5. The yield of glucose in hydrolysis (j), crystal index of cellulose I (CI) (d) and  0Þ, (1 1 0) relative to (2 0 0) (N) of the original MCC the amount of crystal planes ð1 1 and modified MCCs with different contents of cyanuric chloride.

MCC with 3.9% of cyanuric chloride. In the case of the modified MCC with 25.3% or more of cyanuric chloride, the WAXD are very similar to the original MCC, thus the CI of the modified MCC with higher cyanuric chloride (P25.3%) is close to that of the original MCC. 3.3. Influence of crystal structure on hydrolysis of modified MCCs The yield of glucose from hydrolysis was measured, as shown in Fig. 5(j). The glucose’s yield of original MCC is 4.9%, and it becomes larger when the content of cyanuric chloride increases from 0% to 3.9%. The yield of glucose is 9.9% that is the highest when 3.9% of cyanuric chloride is contained in modified MCC. Then it decreased as the content of cyanuric chloride was increased beyond 3.9%. The crystal structure of cellulose has important effects on its hydrolysis or other properties based on prior studies (Hashaikeh and Abushammala, 2011; Rondeau-Mouro et al., 2011; Yeh et al., 2010). In order to quantitatively determine the extent of changes in crystal structure of cellulose, the CI of MCCs has been calculated

from WAXD data. The relationship between CI and content of cyanuric chloride in modified MCCs is shown in Fig. 5(d). Both the highest yield of saccharide and the lowest CI of modified MCC were attained when the content of cyanuric chloride in MCC was 3.9%. The CI of MCCs decreased from 79.7% to 55.7% as the content of cyanuric chloride increased from 0% to 3.9%, indicating that the proportion of crystal regions decreased. Consequently, the hydrolysis of modified MCCs is improved by the partially destruction of the crystal structure of MCC through its reaction with cyanuric chloride. The yield of glucose has an opposite trend compared with the CI of modified MCCs, which can be well explained according to the  0Þ, different proportion between the main crystal planes of ð1 1 (1 1 0) and (2 0 0). The reason lies in the different d-spacing of those three crystal planes. Actually, the d-spacing of MCC should have effects on acid hydrolysis of cellulose, because the location of water molecules in the crystal lattice is dependent on the d-spacing of crystal planes (Kobayashi et al., 2011). Each crystal plane has different d-spacing and different reactive properties. Thus, the proportion of the three  0Þ, (1 1 0) and (2 0 0), main crystal planes in cellulose, which are ð1 1 must have influences on the accessibility of cellulose to chemicals.  0Þ, (1 1 0) and (2 0 0) are 0.606 nm, The d-spacing values of ð1 1 0.537 nm and 0.395 nm, respectively based on WAXD analysis.  0Þ, (1 1 0) that are much larger Wherein, the d-spacing values of ð1 1 than that of (2 0 0), are closer to the diameter of hydronium ion that  0Þ, (1 1 0) planes are is 0.9 nm (Kielland, 1937). Therefore, the ð1 1 probably more accessible to hydronium ion than (2 0 0) plane, as shown in Scheme 3. Thereupon, an increase of the former two crystal planes encouraged the hydrolysis of cellulose because the larger d-spacing made cellulose more accessible. Namely, the ratio between these three main crystal planes would have effects on the hydrolysis ability  0Þ and (1 1 0) relative of cellulose. The amount of crystal planes ð1 1 to (2 0 0) (R) have been calculated by Eq. (3).

R¼

hð110Þ  þ hð110Þ hð200Þ

ð3Þ

 wherein, hð110Þ  , h(110), and h(200) are the height of ð1 1 0Þ, (1 1 0) and (2 0 0) planes, respectively. The results are shown in Fig. 5(N). The R of original MCC is 0.76, and then it increases to 0.96, 0.97 and 0.99 as the content of cyanuric chloride in the samples increases to 2.1%, 2.7% and 3.9%, respectively, based on Fig. 5(N). The R decreases with the content of cyanuric chloride increasing to 7.2% or higher. Especially, the R decreases to 0.74, which is lower than that of the original MCC, as the content of cyanuric chloride is 25.3%. The changing trend of R is coincident with that of glucose’s yield, as well as the changing trend of CI is. Accordingly, the following two facts are considered as explanations for the change of the glucose’s yield in the hydrolysis of the modified MCCs. Firstly, the increasing proportion of amorphous regions (or the increased CI) makes the modified MCCs more accessible, because the amorphous

X. Jiang et al. / Bioresource Technology 104 (2012) 473–479

structure is not arranged as well as the crystal regions, and the molecular interactions in amorphous regions are weaker than that in the crystal regions of cellulose (VanderHart and Atalla, 1987; Wada et al., 2004). Secondly, the proportions of crystal planes change after the reaction between cyanuric chloride and cellulose.  0Þ and (1 1 0) planes those have larger d-spacThe increasing of ð1 1 ing makes cellulose more accessible. As discussed above, the glucose’s yield from the acid-catalyzed hydrolysis of cellulose can be improved by cellulose’ chemical reaction with cyanuric chloride, but the reaction extent should be controlled carefully. When the content of cyanuric chloride in the sample is relatively high (>3.9%), the molecular rearrangement which results in the decreasing of the R and even cross-linking  0Þ and (1 1 0) planes decreasing, so that makes the content of ð1 1 the yield of glucose is depressed. To industrialize the cellulose–ethanol conversion, the costs of reagents and the pollutions of residues, which are due to the heavy use of chemicals and the low efficiency of the current hydrolysis process, are two main obstacles. The modification of cellulose in our study makes it possible to decline the amount of chemicals and to improve the efficiency of hydrolysis of cellulose theoretically. Furthermore, pilot study of the modification on a certain natural lignocellulosic biomass has been carried on in our group. The result indicates a charming prospect of improvement of celluloseglucose conversion. 4. Conclusions The impregnable crystalline structure of cellulose has been covalently attacked by cyanuric chloride. XPS results show that a certain content of cyanuric chloride has been introduced into celluloses. CP/ MAS 13C NMR and WAXD demonstrated that the crystal structure of cellulose was destructed. The proportions of crystal planes were changed dramatically by chemical reaction. These destructions and changes enhanced the dilute acid catalyzed hydrolysis of cellulose. Results of this work indicate that hydrolysis of cellulose with higher yield, lower cost, less resource consumption and more ecofriendly aspects can be achieved by chemical modification, which has a significant expectation of potential application. Acknowledgements The authors thank the National Natural Science Foundation of China (grant no. 20704018 and 31071318) and the Fundamental Research Funds for the Central Universities (grant no. JUSRP10902 and JUSRP10904), and the Open Project of State Key Lab of Pulp and Paper Engineering in South China University of Technology (201031) for financial support. References Alvira, P., Tomás-PejóE, Ballesteros, M., Negro, M.J., 2010. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: a review. Bioresour. Technol. 101, 4851–4861. Atalla, R.H., Dimick, B.E., Nagel, S.C., 1977. Studies on polymorphy in cellulose: cellulose IV and some effects of temperature. In: Jett, C., Arthur, J. (Eds.), Cellulose Chemistry and Technology (ACS Symposium Series). American Chemical Society, Washington, DC, pp. 30–41. Borsa, J., Tanczos, I., Rusznak, I., 1990. Acid hydrolysis of carboxymethylcellulose of low degree of substitution. Colloid Polym. Sci. 268, 649–657. Ferreira, S., Gil, N., Queiroz, J.A., Duarte, A.P., Domingues, F.C., 2011. An evaluation of the potential of Acacia dealbata as raw material for bioethanol production. Bioresour. Technol. 102, 4766–4773. Fukuoka, A., Dhepe, P.L., 2006. Catalytic conversion of cellulose into sugar alcohols. Angew. Chem. Int. Ed. 45, 5161–5163.

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