Production of Levoglucosan from in High Temperature Water On-line Number 242 Hiroe Satoh,1 Kenji Takahashi,2 Harumi Kaga3 1 Dept. Chem. & Chem. Eng., Kanazawa University, Kanazawa 920-8667, Japan, E-mail: [email protected] 2 Dept. Chem. & Chem. Eng., Kanazawa University, Kanazawa 920-8667, Japan, E-mail: [email protected] 3 National Institute of Advanced Industrial Science and Technology (AIST), Sapporo 062-8517, Japan, E- mail:[email protected]

ABSTRACT Application of high temperature water to produce levoglucosan from glucose was studied. The reactions were performed using a tube reactor, in which the temperature of glucose solution can rapidly increase by mixing with high temperature water. Kinetics of glucose decomposition in high temperature water was followed by a first order reaction. The temperature dependence of the reaction rate in the glucose decomposition was the Arrhenius type up to 360 °C. However, above 360 °C, the reaction rate decreased with temperature. The yields of levoglucosan were examined in a temperature range from 200 to 410 °C. A range of pressure examined is 5 to 26 MPa. The instantaneous fractional yields of levoglucosan were between 5 to 32 mol%. The yield of levoglucosan increased with decreasing the pressure. Based on the alcohol dehydration mechanism, a possible reaction mechanisms to produce levoglucosan in high temperature water were proposed.

KEYWORDS glucose, levoglucosan, kinetics, reaction mechanism, dehydration

INTRODUCTION

Recentry hyperbranched polysaccharides have been successfully produced by polymerization of levoglucosan (1,6-anhydro-β-D-glucopyranose). Figure 1 shows a model of the hyperbranched polysaccharides proposed by Kakuchi and Sato (Satoh, 2003). Physical properties of the hyperbranched polysaccharides are quite different from linear polysaccharides. For eaxmple, the viscosities of hyperbranched polysaccharides are significantly less than that of liner polysaccharides. Levoglucosan is also an important starting material for the synthesis of stereoregular polysaccharides possessing biological activities, such as anti-human immunodeficiency virus and blood anticoagulant activities (Hattori, 1998). The derivatives of stereoregular polysaccharides can be used for the chiral stationary phase in high-performance liquid chromatography (Kusuno, 2002). It is well known that levoglucosan can be produced by a fast of cellulosic material. Miura et al. reported that levoglucosan was produced with 3% yield by microwave heating of wood block (Miura, 2004). Very recently Saka et al have demonstrated the conversion into levoglucosan in a polar solvent, sulfolane (Kawamoto, 2003). Within 1 min, levoglucosan is formed in 34.5% yield, corresponding to approximately 60% on the basis of the reacted celluose. A maximum yield of 36.0% was observed at 2 min of reaction time.

1 O O O HO O HO O O O OH O O OH O O O O O O O O OH O OH OH HO OH OH O O O O O O OH HO O O O OH O O O O OH O HO OH O O O O O HO HO OH O 1, 6- Anhydr o- - D-glucopyranose O OH O hyper br anched pol ysacchari des

Figure 1. Structures of levoglucosan (1,6-anhydro-β-D-glucopyranose) and hyperbranched polysaccharides.

Arai et al. have studied glucose decomposition in subcritical and supercritical water (Kabeyemela, 1997). The decomposition products include levoglucosan. However, the yield of levoglucosan was quite small according to the HPLC chromatographs. They didn't examine the effect of experimental conditions on the yield of levoglucosan. There are a few advantages to use glucose as a source of levoglucosan. One advantage is an easy handling of the water soluble material. One doesn't need any special pumps to flow the material. Hence the continuous operation can be achieved. Another advantage is that glucose is very cheap. In this report we present a method to produce levoglucosan with a high yield from glucose using high temperature water. Effects of temperature and pressure on the yields of levoglucosan will be examined. We will propose reaction mechanisms in the production of levoglucosan based on the alcohol dehydration.

EXPERIMENTAL

Apparatus and Procedure

Thermocouple An experimental apparatus was Back-Pressure P Regulator shown in figure 2. The reactor was 1/16" Reactor tube (Hastelloy C-276 alloy). The inside diameter of reactor is 0.5 mm. Two Heater Heater  Cooling HPLC pumps were used to flow glucose Pressure Water solution and water, respectively. The Gauge water and glucose solutions were Pump1 Pump2 continuously purged with ambient pressures of Ar. The glucose solution was merged with high temperature water at the inlet of the tube reactor. The water Water Aqueous Solution was heated using two electric heaters so of Glucose that the temperature of merged solution is desired one. The temperatures at inlet Figure 2. Experimental apparatus of high temperature water and outlet of the reactor were also reaction system.

2 controlled by an electric heater so that the both temperatures are identical. The pressure was controlled using a back pressure regulator. To terminate the reaction, the glucose solution which passed the reactor was immediately cooled down by a heat exchanger. The residence time of glucose solution was ranged from 0.5 to 15 sec. The residence time was calculated by τ = Vρ/F, where F is the mass flow rates, V reactor volume, and ρ the density of glucose solution. The density was assumed to be the density of pure water because the sample concentration is very dilute.

Products Analysis and Identification of Levoglucosan

The products were analyzed by the HPLC chromatogram with a KS-801 (Shodex) column operated at 50 °C with a 1 ml/min flow of water solvent. The detector used was a refractive index (Shimadzu) detector. Typical results from HPLC analysis are shown in figure 3. The peak of levoglucosan was identified by the comparison of the peak retention time with glucose levoglucosan the standard solution of the pure material obtained from Tokyo Chemical. Levoglucosan was also analyzed as a trimethylsilyl derivative using gas chromatography (GC: Shimadzu GC14A; column: Shimadzu CBP10, 25 m x 0.32 mm ID x 0.5 µm; column temperature: 110-260°C; heating rate: 7.5 °C/min; glass capillary column: 25 m; He; FID-detector). Figure 3.Typical results from HPLC analysis. RESULTS AND DISCUSSION

Kinetics of Glucose Decomposition 0

Kabyemela et al reported that glucose -1 decomposition kinetics follows a first order reaction (Kabyemela, 1997). We have confirmed )

0 -2 the first order kinetics as shown in figure 4. Temperature /C Although glucose decays with the first order, the f 200°C decomposition path is not only one. Kabyemela et -3 250°C ln(C 300°C al suggested two decomposition paths. One is a 320°C path that will produce fructose. Another path 360°C -4 370°C leads to unknown products. 380°C The first-order reaction rates are plotted as 390°C a function of temperature in figure 5. The rate -5 400°C constant at 25 MPa essentially shows Arrhenius 0 5 10 15 behavior up to 350 °C. Above 360 °C, the rate Reaction Time [sec] constant gradually decreases. A similar temperature dependence has been reported for Figure 4. First order plot of glucose decomposition reaction.

3 different reactions in high temperature water including supercritical conditions (Marrone, 1998).

-3 4 6x10 25MPa 25MPa 2 5MPa 360°C 5 1 8 380°C 320°C 6 4 400°C 300°C 4 200°C 3 250°C 2 250°C 300°C 320°C Reaction rate [1/s] 2 0.1 360°C 8 370°C 6 200°C 380°C 4 1 Concentration of Levoglucosan 390°C 400°C 2 -3 0 1.4 1.6 1.8 2.0 2.2x10 -1 0 5 10 15 1/T [K ] Reaction Time [sec] Figure 5. Arrhenius plot for glucose Figure 6. Change of levoglucosan concentration as a decomposition. function of time.

Formation and Decomposition of Levoglucosan

The concentrations of levoglucosan as a function of time are plotted in figure 6. At 250°C, the levoglucosan concentration increases with time and reaches a maximum value, and then decrease. At 1.2 25MPa higher temperatures, the concentration maximum 1.0 appeared at early time. These results suggest the following reaction scheme. 0.8

→ → 0 Glu LG R (1) /C 0.6 f C 300°C where Glu is the glucose, LG the levoglucosan, and R 0.4 320°C 360°C unknown products. To increase the yield of 370°C 0.2 380°C levoglucosan, the decomposition of levoglucosan to the 390°C unknown products must be prevented. Therefore the 400°C 0.0 reactions of levoglucosan in high temperature water 0 2 4 6 8 10 were examined. The results are shown in figure 7. The Residence Time [sec] reaction of levoglucosan didn't follow the first order Figure 7. Decomposition of levoglucosan in reaction. At least two reaction products were detected high temperature water. by the HPLC with RI detector. We are analyzing the decomposition kinetics. The instantaneous fractional yields of levoglucosan were examined as a function of water

4 0.4 200°C 1.0 250°C + 280°C 0.9 [H ]=3.1E-17 0.3 320°C + [H ]=2.5E-6 360°C 0.8 380°C 0

0.2 /C f

C 0.7

+ 0.1 [H ]=3.0E-6 250°C 5MPa 0.6 250°C 25MPa + Yield of Levoglucosan [H ]=2.9E-6 280°C 5MPa 280°C 25MPa 0.0 0.5 0 10 20 30 0 10 20 30 40 50 Pressure [MPa] Reaction Time [sec] Figure 8. Effect of water pressure on the yield Figure 9. Effect of concentration of hydronium of levoglucosan. ion on decay kinetics of levoglucosan.

pressure at several different temperatures (200 to 380°C). The residence time was kept at 1 sec. Figure 8 shows the results. At high density of water, the yields of levoglucosan are 10 to 17 wt%, depending on the temperature. However, under lower pressure conditions (P < 8MPa), the yields of levoglucosan increased significantly with the decreasing of water density. The maximum yield was obtained with the water pressure of 7 MPa at 320°C. It is known that levoglucosan reacts with hydronium ions. Then we thought that the concentration of hydronium ions is an important factor to increase the yield of levoglucosan. To examine the effect of hydronium ions concentration on the levoglucosan decomposition, the concentrations of hydronium ions were controlled by changing the pressure. Figure 9 shows the decay kinetics of levoglucosan under different hydronium concentrations. At 250°C, the decay rate at 5 MPa is almost the same as that at 25 MPa because the concentrations of hydronium ions at the both pressures are almost identical, 3.0µM at 25 MPa and 2.5 µM at 5 MPa. However, at 280°C, the decay rate at 5 MPa is significantly slower than that at 25 MPa. This is due to the low concentration of hydronium ions at 5 MPa and 280°C.

Reaction Modeling

Based on the above results and the dehydration reaction of alcohol, we assumed a combination of a radical reaction model and an acid catalyzed ionic dehydration model. The dehydration of alcohol is a classic organic reaction that has been extensively studied. The established mechanisms are E1cB, E1 and E2. These notations stand for elimination ("E") with a rate- limiting step that is either unimolecular ("1") or bimolecular ("2"). In the presence of a strong base and a weak acid, alcohol dehydration occurs primarily via a carbanion intermediate, which is formed when the base abstracts a proton from the β (E1cB model). In the presence of a strong acid and a weak base, alcohol dehydration occurs primarily via a carbocation intermediate, which is formed by a loss of the protonated hydroxy grouop as water (E1 model). In the E2 model, alcohol dehydration occurs by β elimination, in which the proton on the β carbon and the hydroxy group are eliminated in a concerted fashion. The reaction is bimolecular because a nucleophile is required to initiate elimination. In pure water, no base is present that is strong enough to abstract a proton from the carbon backbone. Therefore, the E1cB mechanism is unlikely. The remaining candidates are acid-catalyzed E1 and E2 mechanisms. Figure 10 shows the reaction model for acid-catalyzed glucose dehydration.

5 Step1 OH OH H O Fast O OH O H H OH OH O+ H OH - HO HO OH OH Step 2 OH OH H O H Slow O OH O+ H OH C+ O H HO HO H OH OH carbocation Step 3 O H O O O Fast O O OH C+ OH- OH C+ H OH OH H H HO HO HO OH OH OH

Figure 10. Reaction modeling for dehydration of levoglucosan according to E1 mechanism.

A key intermediate is carbocation in the E1 mechanism. In high temperature and high density water, the carbocation is stable because of the solvation energy. Hence in the high density hot water, E1 mechanism could be a dominant reaction channel to produce levoglucosan. However, in the low density hot water including supercritical water, the carbocation will be unstable. In the low density hot water, a radical reaction mechanism is favorable. Recently, Takahashi et al proposed a mechanism of alcohol dehydration in which a biradical is an intermediate (Takahashi, 2004).

6 H OH 5 O H O O O OH O O 4 OH C OH C H OH OH H OH HO 1 HO HO 3 OH OH OH 2 biradical levoglucosan

Figure 11. A reaction mechanism for dehydration of glucose

Based on their dehydration mechanism, we assumed the following dehydration mechanism of glucose as shown in figure 11. In this mechanism, two water molecules near a glucose molecule play an important role. Firstly hydrogen atom will be transfered from hydroxyl group(6) to the water molecule near glucose. The water molecule will path the excess hydrogen to the next water molecule. Finally the hydrogen atom will be transfered to the hydroxyl group(1) of glucose, and a water molecule will eliminate from the glucose molecule. And then resultant biradical will be formed and its intramolecular termination reaction may lead to levoglucosan.

6 CONCLUSIONS

We have studied dehydration of glucose in high temperature water including supercritical conditions. Levoglucosan was successfully produced. The reaction kinetics of glucose decomposition was followed by the first order kinetics. The Arrhenius plot shows a linear relationship up to 360°C. However, above 360°C, the reaction rates decreased with temperature. The kinetics of levoglucosan decomposition was not followed by the first order kinetics. The effect of pressure on the instantaneous fractional yields of levoglucosan was examined. The pressure effect was significant. The instantaneous fractional yields increased with decreasing the pressure. The maximum yield of 32 mol% was obtained at 320°C and 7 MPa. Based on the dehydration of alcohol, at high temperature and high density conditions, we assumed unimolecular dehydration of glucose in which the carbocation is an intermediate. Whereas in low density hot water including supercritical water, a radical reaction mechanism, in which a biradical of glucose is an intermediate, may occur.

ACKNOWLEDGMENTS

The authors wish to thank the Hokuriku Industrial Advancement Center (Kanazawa, Japan) for financial support.

REFERENCES

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