Selective Conversion of D-Fructose to 5-Hydroxymethylfurfural by Ion

A maximum 5-HMF selectivity of 85% with 89% fructose conversion was obtained. ... ionic liquid solvent (1-alkyl-3-methylimidazolium chloride) with met...
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Ind. Eng. Chem. Res. 2008, 47, 9234–9239

Selective Conversion of D-Fructose to 5-Hydroxymethylfurfural by Ion-Exchange Resin in Acetone/Dimethyl sulfoxide Solvent Mixtures Xinhua Qi,†,‡ Masaru Watanabe,*,† Taku M. Aida,† and Richard Lee Smith, Jr.*,† Research Center of Supercritical Fluid Technology, Tohoku UniVersity, 6-6-11 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan, and College of EnVironmental Science and Engineering, Nankai UniVersity, Tianjin 300071, China

Catalytic dehydration of D-fructose to 5-Hydroxymethylfurfural (5-HMF) in acetone/dimethyl sulfoxide solvent mixtures was studied in the presence of a strong acidic cation-exchange resin catalyst (DOWEX 50WX8-100) by microwave heating. The addition of acetone to the dimethyl sulfoxide (DMSO) solvent promoted the formation of 5-HMF from D-fructose. For a D-fructose conversion of 97.9%, the 5-HMF selectivity was 91.7% for a 20-min reaction time in 70:30 (w/w) acetone/DMSO solvent mixtures. Concentrations as high as 10 wt % D-fructose were studied, for which it was found that 5-HMF yields of 82.1% for a reaction time of 10 min could be obtained. The stability of the ion-exchange resin used as the catalyst was confirmed. Compared to pure DMSO solvent, the combination of low-boiling-point acetone with DMSO used as the reaction medium not only gives highly selective 5-HMF formation, but also improves the separation efficiency and reduces environmental risk. 1. Introduction Diminishing fossil fuel reserves and growing concerns about global warming are indications that sustainable sources of energy and chemicals will need to be developed. Biomass resources are a promising alternative for the sustainable supply of fuel and valuable chemicals,1-6 although it is clear that technologists will need to consider their management to both reduce greenhouse gases7 and avoid competition with food supplies. For example, 5-hydroxymethylfurfural (5-HMF), which is used in plastics, pharmaceuticals, and fine chemicals, is presently derived from petrochemicals but could be substituted by its biomass-derived counterpart.2 The most convenient method for the preparation of 5-HMF is the acid-catalyzed dehydration of fructose, and this process has received increasing attention.1,8-13 The dehydration of fructose to form 5-HMF has been conducted in water,9,14-22 organic solvents18,23-25 [dimethyl sulfoxide (DMSO)], organic/water mixtures,1,11,12,26 ionic liquids,8,10,13,27,28 and more recently, biphasic water/organic systems,1,29-31 using catalysts such as mineral acids,1,9,11,29,30 transition metal ions,8,14,18,25 H-form zeolites,21 and strong acid cation-exchange resins.1,13,24,31 For example, Roman-Leshkov et al.29 studied acid-catalyzed fructose dehydration in a twophase reactor system, where fructose was dehydrated in the aqueous phase with dimethyl sulfoxide (DMSO) and poly(1vinyl-2-pyrrolidinone) (PVP). The 5-HMF product was continuously extracted into an organic phase (methylisobutylketone) modified with 2-butanol to enhance partitioning from the reactive aqueous solution. A maximum 5-HMF selectivity of 85% with 89% fructose conversion was obtained. Zhao et al.8 studied the catalytic conversion of fructose to 5-HMF in an ionic liquid solvent (1-alkyl-3-methylimidazolium chloride) with metal halides such as chromium(II) chloride as catalysts, achieving a 73% yield of 5-HMF at a temperature of 120 °C for a reaction time of 3 h. * Corresponding author. Tel (Fax): (+81) 022-795-5864. E-mail: [email protected] (M.W.); [email protected] (R.L.S.) † Tohoku University. ‡ Nankai University

However, all of these catalytic systems exhibit some aspects that prevent their use. For example, aqueous processes are favored with respect to both ecological and technological factors but are, unfortunately, inefficient because 5-HMF readily reacts in water to form levulinic acid and formic acid. Ionic liquids are advanced solvents in view of their low vapor pressures, but they remain expensive and entail difficulties in terms of separation of products and possible byproducts. Homogenous acid catalytic processes are effective, but they also have serious drawbacks in terms of separation and recycling in addition to equipment corrosion. On the other hand, heterogeneous catalysts such as H-form zeolites can be recycled and have high selectivities (60-90%), but they have only low fructose conversions (30-60%) even at reaction times as long as 2 h.15,16 DMSO has been shown to be an effective solvent for the dehydration of fructose to 5-HMF because it can prevent the formation of byproducts such as levulinic acid and humins from 5-HMF. Although a high selectivity to 5-HMF of >90% has been obtained in DMSO as the solvent in both the absence and presence of ion-exchange resins,23,24 the corresponding question of product separation from DMSO must be raised because of the high boiling point of DMSO (196 °C). Therefore, it is essential to address other solvent systems that have low environmental risks and better separation characteristics than DMSO. Bicker et al.32 studied the acid-catalyzed dehydration of fructose to 5-HMF in sub- and supercritical methanol and subcritical acetic acid and obtained consecutive reaction products from fructose to 5-HMF to 5-methoxymethylfurfural (79% selectivity, 99% conversion) and 5-acetoxymethylfurfural (38% selectivity, 98% conversion), respectively. They examined the dehydration of fructose to 5-HMF in sub- and supercritical acetone/water mixtures and obtained the results that both the 5-HMF selectivity and the fructose conversion increased with decreasing water content and that the optimal selectivity to 5-HMF of 77% with 99% fructose conversion was obtained with acetone/water in a 90:10 v/v ratio.11 In a previous study,26 we found that the use of a 70:30 (w/w) acetone/water reaction medium resulted in yields of 5-HMF as high as 73.4% for 94% conversion at 150 °C in the presence of a strong cation-exchange resin as the catalyst under microwave heating. However, the

10.1021/ie801016s CCC: $40.75  2008 American Chemical Society Published on Web 11/01/2008

Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9235

formed 5-HMF still reacted with water and decomposed into acids with increasing reaction time, thus resulting in lower 5-HMF selectivities than would be expected. Furthermore, in that work,26 for high initial fructose concentrations of 20 wt %, the 5-HMF yield was 54.3% for 89.3% fructose conversion because of the decomposition of 5-HMF into levulinic acid and the formation of a solvent-soluble polymer. In the conversion of D-fructose to 5-HMF, acetone is considered to be essential to the reaction, because it promotes conversion of fructose to the furanose form.11 Unfortunately, the solubility of fructose in pure acetone is negligible (0.5 g/L at 25 °C),11 and thus, either water or another solvent is needed to allow conversion at a practical scale. The use of water as a solvent does increase the solubility of fructose in the solvent phase, but it also promotes side reactions such as humin formation and 5-HMF decomposition into acids. According to the literature,11 5-HMF selectivity is generally below 65% despite a maximum value of 77% being obtained for the best case in acetone/water solvent. To improve the selectivity of fructose to 5-HMF, it seems to be necessary to remove water from the solvent system. As described above, fructose has a good solubility in DMSO, and a high 5-HMF yield of up to 90% can be obtained in pure DMSO solvent. Acetone, on the other hand, is a low-boilingpoint solvent. The use of acetone as a promoter could possibly lead to a better solvent system than has previously been proposed. The catalyst remains to be chosen, and in this work, we consider ion-exchange resins because they can be easily recycled. Thus, the objective of this work was to study the dehydration of fructose to 5-HMF over a strong cation-exchange resin in acetone/DMSO mixtures. 2. Experimental Section 2.1. Materials. D-Fructose (99%), dimethyl sulfoxide (99%), acetone (99%), 5-HMF (100%), furfural (98%), levulinic acid (98%), and formic acid (90%) were purchase from Wako Pure Chemical Company and used without further purification. DOWEX 50WX8-100 ion-exchange resin was purchased from Sigma-Aldrich Corporation. Ultrapure water that was deionized (conductivity, 18 MΩ/cm) was used as obtained from a water distillation apparatus (Yamato Co., model WG-220). 2.2. Resin Properties. Strong acid cation-exchange resin DOWEX 50WX8-100 (50-100 mesh beads, gel, 40-70% water content) is insoluble in water and consists of a sulfonated copolymer of styrene and divinyl benzene in the hydrogen form. The matrix of the resin is styrene-divinylbenzene (DVB), and total exchange capacity is 1.7 mequiv/mL (H+) as stated by the manufacturer. 2.3. Typical Workup Procedure. The experimental procedure is similar to that used in a related work.26 A solution of fructose, DMSO, and acetone and a given amount of the ionexchange resin catalyst were loaded into a thick-wall Pyrex glass tube (volume, 10 mL; i.d., 11.6 mm; o.d., 18 mm; wall thickness, 3.2 mm) that had a maximum working pressure of 10 MPa. The glass tube was mounted in a polycarbonate (PC) tube that was sealed with poly(ether ether ketone) (PEEK) screw caps. This assembly was placed in a microwave oven (Shikoku Keisoku µReactor, SMW-087, 2.45 GHz, maximum power 700 W). Nitrogen gas was used for purging air inside the reactor at a pressure of about 1.4 MPa, which would produce calculated gas pressures of ca. 2.02 MPa assuming ideal gas behavior at a temperature of 150 °C. The use of N2 gas pressure increases the boiling point of the solvent mixtures and prevents boiling. At the given conditions, the solvent mixtures should not boil

below 150 °C according to calculations made with the solution activity model UNIFAC,33 and the change of liquid-phase composition resulting from acetone volatilization into the vapor phase should be small (below ca. 5%). The boiling of the solvent mixtures was not observed during any of the experiments. The reaction mixture was heated to 150 °C within 30 s with microwave irradiation. In the reaction analyses, zero time was taken to be the time at which the temperature reached 150 °C. After the desired reaction time had passed, the microwave irradiation was turned off, and a valve between the cooling water tank and the PC tube was opened to allow the cooling water to enter the PC tube and cool the reactor. The reactor was removed from the assembly, and the reaction solution was collected by washing the glass tube with a given amount (ca. 50 mL) of distilled water. 2.4. Analysis. A high-performance liquid chromatograph (Jasco) with an SH 1011 column (Shodex) and a refractive index detector (ERC-7571A) was used to analyze the liquid samples. Each sample was diluted with ultrapure water before analysis to prevent the overloading of the column with organic solvents. Elemental analysis (C, H, S) of the resin before and after use was carried out using Elementar Analysensysteme GmbH VarioEL instrument (Germany). Fructose conversion (X, mol %), product yield (Y, mol %), and product selectivity (S, mol %) are defined as follows X (mol %) ) 1 -

fructose concentration in the product fructose concentration in the loaded sample

Y (mol %) )

number of moles of carbon in the product number of moles of carbon loaded as fructose

S (mol %) )

yield of product fructose conversion

Further, R is defined as the weight ratio of the substrate to the ion-exchange resin catalyst. The carbon balance was between 85% and 95% and tended to decrease with increasing Rvalue or initial fructose concentration because of the formation of humic acids and soluble polymers that are common in carbohydrate chemistry. Humic acids and other byproducts that formed were not analyzed in this work. 3. Results and Discussion 3.1. Influence of the Reaction Medium’s Composition on the D-Fructose Conversion and 5-HMF Yield. DMSO has been shown to be an effective solvent for the dehydration of D-fructose to 5-HMF, but its high boiling point limits its application. Therefore, mixtures of acetone and DMSO were used as the solvent for the process, and the effect of the solvent composition was studied. The experimental results are shown in Figure 1. It can be seen that acetone/DMSO mixtures can effectively suppress the formation of levulinic acid, so that the 5-HMF yield increased for the reaction times studied. More importantly, the 5-HMF selectivity also increased with increasing D-fructose conversion. The D-fructose conversion, 5-HMF yield and selectivity were 97.9%, 89.8%, and 91.7%, respectively, for a 20-min reaction time for the 70:30 (w/w) acetone/ DMSO solvent mixture. Table 1 lists first-order reaction rate constants for D-fructose conversion to 5-HMF at solvent mixture weight ratios of acetone to DMSO ranging from 70:30 (w/w), corresponding to about 3:1 on a molar basis, to 0:100 (i.e., pure DMSO). It can be seen that the weight ratio between acetone and DMSO had some influence on the reaction rate of D-fructose conversion. The reaction rate of D-fructose increased with increasing proportion

9236 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008

Figure 1. Influence of solvent composition on (a) D-fructose conversion and (b) 5-HMF selectivity (2 wt % D-fructose, 150 °C, R ) 1). Acetone/ DMSO (w/w) ) (0) 70:30, (O) 50:50, (4) 20:80, (×) 0:100. Table 1. First-Order Reaction Rate Constants (k) of D-Fructose Conversion at Different Reaction Medium Compositions acetone/DMSO (w/w)

k (min-1)

correlation coefficient

70:30 50:50 20:80 0:100

0.1681 0.159 0.141 0.105

0.9878 0.9855 0.9930 0.9947

of acetone in the solvent mixture. The results demonstrate that acetone/DMSO mixtures are highly preferable to pure DMSO for the dehydration of D-fructose to 5-HMF. 3.2. Effect of the Reaction Temperature and Kinetics Analysis. Figure 2 shows the influence of the temperature on the D-fructose conversion and 5-HMF selectivity. Reaction temperature had a large effect on both the D-fructose conversion and the 5-HMF yield. When the reaction temperature was 100 °C, the D-fructose conversion was 65.7% with 83.9% 5-HMF selectivity for a 60-min reaction time. The D-fructose conversion and 5-HMF selectivity increased to 98% and 91.6%, respectively, at 150 °C for 20 min. The 5-HMF yield always increased with increasing reaction time for all temperatures. Levulinic acid yield was below 0.3% in all experiments. Kuster34 and Vogel11 reported a reaction order of 1 for the dehydration of D-fructose. In accordance with this result, a kinetic analysis of the dehydration of D-fructose in 70:30 (w/ w) acetone/DMSO was performed. Plots of ln(1 - X) versus reaction time (t) were made to obtain first-order kinetic constants. With those constants, an Arrhenius plot was generated, as shown in Figure 3. For comparison, the plot from our previous work [in which 70:30 (w/w) acetone/water was used and the other conditions were the same as in the present work]26 is also included in Figure 3. The activation energies and preexponential factors for acid-catalyzed fructose dehydration to 5-HMF obtained by different authors under different conditions are summarized in Table 2. It seems that there are substantial

Figure 2. Influence of temperature on (a) D-fructose conversion and (b) 5-HMF selectivity (acetone/DMSO ) 70/30 w/w, 2 wt % D-fructose, R ) 1). Temperature ) (0) 100, (O) 120, (4) 130, (3) 140, (f) 150 °C.

Figure 3. Arrhenius plot for the dehydration of D-fructose in (0) 70:30 (w/w) acetone/DMSO and (b) 70:30 (w/w) acetone/water.

differences among the pre-exponential factors from different processes, from 4.8 × 106 to 8.7 × 1011 min-1, depending on the solvents, catalysts, and even heating methods used, as well as for the activation energies, which varied from 60.4 to 160.6 kJ/mol. The values probably have a large variation because of the structural form of fructose in solution, and only the values from this work and ref 26 are discussed in detail. Compared to 70:30 (w/w) acetone/water as the reaction medium, the activation energy decreased from 103 to 60.4 kJ/mol when 70:30 (w/w) acetone/DMSO was used, and the pre-exponential factor decreased from 8.7 × 1011 to 4.8 × 106. The difference in the pre-exponential factors of the reactions between two kinds of reaction media can be attributed to the polarity difference between water and DMSO. The higher activity of water under

Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9237 Table 2. Comparison of Activation Energies and Pre-Exponential Factors for Acid-Catalyzed Fructose Dehydration to 5-Hydroxymethylfurfural Ea (kJ/mol)

A (min-1)

reaction solvent

catalyst

ref

60.4 103 99.0 80.0 65.8 ( 8 160.6

4.8 × 10 8.7 × 1011 3.9 × 1011 1.8 × 1010 6.5 × 106 -

70:30 (w/w) acetone/DMSO 70:30 (w/w) acetone/water 90:10 (v/v) supercritical acetone/water supercritical methanol aqueous solution aqueous solution

ion-exchange resin ion-exchange resin H2SO4 H2SO4 NbOPO4 HCl

this work 26 11 32 35 22

6

Table 3. Influence of the Catalyst Dosage (Ra) on D-Fructose Conversion, Product Yield, and 5-HMF Selectivityb

run

D-fructose concentration (wt %)

reaction time (min)

D-fructose Conversion (%)

1 2 3 4

2 2 2 2

1 3 5 20

62.1 78.9 88.2 99.0

5 6 7 8 9

2 2 2 2 2

1 3 5 7 10

41.7 65.6 78.0 84.4 87.9

10 11 12 13 14

2 2 2 2 2

1 3 5 10 20

37.7 54.6 65.2 79.6 88.6

15 16 17

10 10 10

10 20 30

96.4 99.0 99.4

2-furfural

product yields (%) levunilic formic acid acid

5-HMF

5-HMF selectivity (%)

R ) 0.5 0.81 1.42 0.99 1.37

0.68 0.35 0.27 0.91

1.09 1.04 1.05 1.07

56.5 69.2 78.9 87.4

82.9 87.6 89.6 88.3

0.22 0.17 0.25 0.29 0.33

1.54 1.04 1.14 1.45 1.07

33.2 56.2 68.1 73.1 77.8

79.7 85.7 87.3 86.6 88.6

0.19 0.25 0.39 0.29 0.38

0.80 0.57 0.98 0.92 1.32

26.4 41.4 51.6 66.4 77.1

70.1 75.8 79.3 83.4 87.0

0.56 1.09 1.59

0.52 0.67 1.14

82.1 83.2 81.6

85.2 84.1 82.1

R)1 0.23 1.01 1.02 1.07 1.14 R)2 0.59 0.77 1.35 1.54 1.65 R ) 2.5

a R ) weight of D-fructose/weight of resin (g/g). w, 5 g of solution, 150 °C.

b

1.48 1.67 1.51

Conditions: DOWEX 50WX8-100 strong acidic ion-exchange resin, acetone/DMSO ) 70/30 w/

microwave irradiation probably leads to a higher number of collisions and, thus, a larger pre-exponential factor. On the contrary, in solution, D-fructose exists in five different tautomeric forms noted in the literature as R-pyranoid, β-pyranoid, R-furanoid, β-furanoid, and open chain. The furanoid forms are best achieved in DMSO, whereas the pyranoid forms are preferred in water.32,36 According to the study of Vogel and co-workers, the concentrations of the furanoid forms of Dfructose in acetone mixtures are high because the acetone molecule has some chemical aspects similar to those of the DMSO molecule.32A study by Antal and co-workers suggested that 5-HMF is formed by dehydration of D-fructose from its furanose form.37 Therefore, higher furanoid form concentrations of D-fructose in acetone/DMSO mixtures might be the reason for the lower activation energy compared acetone/water mixtures. 3.3. Effect of the Catalyst Dosage on the D-Fructose Conversion and 5-HMF Yield. Table 3 shows the influence of the catalyst dosage on the D-fructose conversion and 5-HMF yield. The weight ratio of substrate (D-fructose) to catalyst (resin), R, was 2, 1, and 0.5. It can be seen from Table 3 that increasing the resin dosage led to a marked increase in D-fructose conversions, 5-HMF yields, and 5-HMF selectivities. For the range of catalyst dosages employed in this study, the 5-HMF yield and selectivity increased as the ratio of catalyst to substrate increased. 3.4. D-Fructose Conversion and 5-HMF Yield for a High Initial D-Fructose Concentration in Acetone/DMSO Mixtures. From a practical point of view, if higher concentrations of D-fructose can be used as feedstock, the acetone/DMSO

and strong acid ion-exchange resin combination is favorable. Dehydration of D-fructose to 5-HMF for a 10 wt % initial concentration of D-fructose was studied with this point in mind (Table 3, runs 15-17). It can be seen that the 5-HMF yield reached 82.1% with a D-fructose conversion of 96.4% for a 10-min reaction time, and the corresponding values for the conversion and yield were 99.0% and 83.2%, respectively, at a 20-min reaction time. In addition, the 5-HMF yield decreased little with increasing reaction time, showing that there was only a small loss in 5-HMF yield due to successive reaction. In aqueous systems, 5-HMF easily undergoes cross-polymerization and humin formation between itself, water, and D-fructose to produce acids. It has been reported that, in aqueous systems including aqueous mixture systems, losses due to humin formation amount to 35% for 18 wt % fructose solution, decreasing to 20% for 4.5 wt % fructose solutions.34 3.5. Resin Catalyst Recycle. Generally, strong acid sulfonated copolymer resins are thought to be usable only below 130 °C,34 but our experimental results showed that the resin could work well above 130 or even 150 °C. In green engineering, recycling of the catalyst is very important, because catalyst synthesis can require considerable resources. Thus, the recycle of the resin was tried five times to study its stability and activity. The results are reported in Table 4. From Table 4, one can see that the catalytic activity and selectivity of the resin for the dehydration of D-fructose to 5-HMF had a constant activity after being reused five times at 150 °C for a 20-min reaction time in each case. The D-fructose conversion and 5-HMF yield remained at about 97% and 87%, respectively, for all runs.

9238 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 4. Effect of Catalyst Recyclea no. of recycles

fructose conversion (%)

5-HMF yield (%)

1 2 3 4 5b

97.6 97.3 97.0 97.0 96.9

88.1 88.0 88.0 86.6 86.3

a Conditions: 2 wt % D-fructose; reaction medium, 70:30 (w/w) acetone/DMSO; reaction time, 20 min; temperature, 150 °C; R ) 0.5. b Elemental analysis results (C, H, S) for resin before use and after being reused five times (new resin/used resin): C (%), 42.5/49.1; H (%), 5.38/5.97; S (%), 13.7/13.5.

To examine whether the resin was damaged or had reacted after being used, the resin that had been reused five times was collected, rinsed with pure water and acetone, and dried for 24 h in a vacuum drying oven; this solvent wash treatment was also used for some new resin. Then, elemental analysis (C, H, S) of both of these resins was performed, and the results are included in Table 4. It can been seen that the C and H contents in the reused resin increased by about 6.6% and 0.59%, respectively, and the S content decreased by about 0.2% compared to the levels in the new resin. The increase in C and H contents was most likely due to adsorbed products that could not be removed from the resin by acetone/water washing. The S content in the reused resin decreased by 0.2%, and this probably can be attributed to some sulfonate loss in the resin or the increasing of C and H because of the deposition of humins or other organic residues. The resin was stable upon being reused five times and also provided stable D-fructose conversions and 5-HMF yields, which demonstrates the possibility for its continual use. 4. Conclusions Catalytic dehydration of fructose to 5-HMF using a strong acid cation-exchange resin as the catalyst in acetone/DMSO mixtures by microwave heating was investigated. Acetone/ DMSO mixtures were shown to be effective for the dehydration of fructose to 5-HMF. A high 5-HMF yield of 89.8% (selectivity of 91.7%) was obtained for a 20-min reaction time in 70:30 (w/w) acetone/DMSO mixtures at 150 °C. In acetone/DMSO mixtures at 150 °C and 20-min reaction time, both the 5-HMF yield and the reaction rate increased with increasing proportion of acetone in the mixture. For the range of catalyst dosages employed, the 5-HMF yield and selectivity remained stable. The reaction solvent mixture and the method were also effective for high D-fructose concentrations (ca. 10 wt %) for which the D-fructose conversion and 5-HMF yield were 96.4% and 82.1%, respectively, at a reaction time of 10 min. The resin maintained its activity at 150 °C, and its catalytic activity remained stable after the resin had been reused five times. Acknowledgment The authors gratefully acknowledge the financial support by the Japan Society for the Promotion of Science (JSPS). X.Q. would also like to thank the National Science Foundation of China for support (No. 20806041). Literature Cited (1) Roman-Leshkov, Y.; Chheda, J. N.; Dumesic, J. A. Phase modifiers promote efficient production of hydroxymethylfurfural from fructose. Science 2006, 312, 1933. (2) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals. Angew. Chem., Int. Ed. 2007, 46, 7164.

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ReceiVed for reView June 30, 2008 ReVised manuscript receiVed August 21, 2008 Accepted September 4, 2008 IE801016S