Efficient Conversion of Cellulose to Glucose, Levulinic Acid

Optimization of SO 2 -catalyzed hydrolysis of corncob for xylose and xylitol production. Journal of Chemical Technology & Biotechnology 2014, ...
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Efficient Conversion of Cellulose to Glucose, Levulinic Acid, and Other Products in Hot Water Using SO2 as a Recoverable Catalyst Weina Liu,† Yucui Hou,‡ Weize Wu,*,† Zhenyu Liu,† Qingya Liu,† Shidong Tian,† and Kenneth N. Marsh*,§ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China § Centre for Energy, School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, Western Australia 6009, Australia ‡

S Supporting Information *

ABSTRACT: Cellulose is the most widely distributed source of biomass, and its efficient conversion to a variety of chemicals is important for a sustainable future. In this work, sulfur dioxide (SO2) dissolved in hot water has been demonstrated to be an efficient catalyst for the selective conversion of cellulose to chemicals such as glucose and levulinic acid. The selectivity of products can be tuned by the SO2 concentration, temperature, and reaction time. SO2 acts both as a supply of H+ ions through ionization of H2SO3 when dissolved in water and as a Lewis acid catalyst that breaks the hydrogen bonds in cellulose. Importantly, SO2 in the reaction mixture can be recovered completely by stream stripping, thus avoiding the formation of acidic wastewater. This work provides a new, efficient, and environmentally benign way to convert cellulose to chemicals.

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INTRODUCTION Cellulose is the most abundant and widely distributed natural biomass that can be used as an alternative to fossil resources for sustainable production of fuels and chemicals.1 Currently, there are intensive investigations on the conversion of cellulose to chemicals.2−4 However, it is still a challenge to efficiently convert cellulose by a simple and environmentally friendly process because of its robust crystalline structure. For example, enzyme hydrolysis5,6 results in limited pollution and few byproducts but is of low efficiency and high cost. Mineral acidcatalyzed processes7,8 are efficient and technically mature but require acid recovery, which results in environmental pollution and equipment corrosion. Certain ionic liquids9−14 show a high dissolving capacity for cellulose, as well as good thermal stability, but are difficult to recover from the products because of their low volatility and poor recovery from added solvents such as water, which increases the overall cost for their use. Hot water15−17 is an unpoluting solvent, but the concentration of H+ generated is too low, making the addition of a catalyst necessary. Orozco and his co-workers7 reported that H3PO4, a medium-strong acid, could act as a catalyst for the conversion of cellulose to chemicals because its acidity is just sufficient to break the bonds of cellulose and not too strong to control the reaction process. The disadvantage is that H3PO4 is difficult to separate from the reaction solution, similar to H2SO4. SO2 can react with water to generate sulfurous acid, a medium-strong acid with properties similar to those of H3PO4. The application of SO2 as a catalyst in the pretreatment process of lignocellulose has been reported by many researchers.18−20 Conner et al.21 reported that aqueous SO2 pretreatment could dramatically reduce the degree of polymerization of celluose before enzymatic hydrolysis. Bura et al.22 found that the monomeric sugar yield obtained by enzymatic hydrolysis can be © 2012 American Chemical Society

enhanced considerably by a SO2-catalyzed steam explosion pretreatment process. All of the above results indicate that SO2 in water has the potential to break down cellulose to useful chemicals. Moreover, our work has confirmed that SO2 can be totally swept out by steam stripping from an aqueous solution, and the recovered SO2 can be reused after collection. The detailed reaction conditions on the choice of products and the SO2 recovery process have not been reported previously. In this work, SO2 was used as a catalyst to directly convert cellulose in hot water. The effects of the temperature, reaction time, and SO2 content on the conversion of cellulose to chemicals were studied, and a mechanism of cellulose conversion to chemicals by SO2 in hot water is postulated. SO2 was found to be an efficient catalyst for cellulose conversion to chemicals and can be readily recovered by steam stripping for recycling.

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MATERIALS AND METHODS Chemicals. Microcrystalline cellulose, D-(+)-cellobiose, anhydrous D-glucose, and D-fructose with mass fraction purity of 0.96, 0.98, 0.998, and 0.98, respectively, were purchased from Bio-Basic Inc. (Canada). Furfural, levulinic acid (LA), and 5(hydromethyl)furfural (HMF) with mass fraction purity of 0.99, 0.99, and 0.98, respectively, were provided by Aladdin Reagent Inc. (Shanghai, China). SO2 and N2 with a volume fraction purity of 0.9995 were supplied by Beijing Haipu Gases Ltd. (Beijing, China). All reagents and solvents above were of analytical grade and were used without further purification. Received: Revised: Accepted: Published: 15503

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Conversion of Cellulose. The conversion of cellulose was carried out in a 25 cm3 stainless steel reactor (with a maximum pressure of 30 MPa) containing a magnetic stirrer. The reactor was heated by a furnace, whose temperature was controlled by a temperature controller and measured with an uncertainty of ±0.5 °C, and the pressure was monitered by a calibrated pressure transducer, with an uncertainty of ±0.025 MPa. In a typical procedure, 0.5 g of cellulose and 15.0 cm3 of H2O were loaded into a reactor, which was then sealed and purged at least three times with inert gas (N2) to remove the air inside the reactor. SO2 was then added from a sampling bomb, and the mass of added SO2 was calculated from the mass difference of the sampling bomb before and after charging SO2 into the reactor. When both the samples and SO2 were loaded, the reactor was submerged in a furnace at the desired temperatures (from 190 to 220 °C) and stirred for a known time (the reaction time was considered to have started when the reactor was placed in the furnace; the time of heating of the reactor from room temperature to a required temperature was about 12 min). The pressures measured in this process are shown in Table S1 in the Supporting Information (SI). The reaction was quenched by placing the reactor in cold water. When the temperature was about 100 °C, the valves on both ends of the reactor were opened and the solution was stripped by saturated steam at about 100 °C until the concentration of SO2 in the liquid phase was less than 0.01 g·dm−3, determined by a sample analyzed by high-performance liquid chromatography (HPLC) or an iodometric method. At the same time, the stripped-out SO2 was collected at the outlet of the reactor. Finally, the solution was filtered using a 0.45 μm membrane, the residual solid was washed three times with water, and the total solution obtained was analyzed by HPLC to determine the yield of products including fructose, furfural, glucose, HMF, and LA. The solid material obtained by filtration was dried and weighed to calculate the conversion of cellulose. When this process was performed using H2SO4 as the catalyst, 0.5 g of cellulose, 15.0 cm3 of H2O, and a known mass of concentrated H2SO4 were loaded into the reactor, which was then sealed and purged with N2. After reaction, the solution was filtered, washed, and analyzed as described above. Analysis of Products. A high-performance liquid chromatograph (Waters 2695, USA) with a Shodex SH 1011 column (Shodex, Tokyo) and a differential refractive index detector (Waters, USA) was employed for analysis of the liquid samples. The column oven temperature was 55 °C, and the mobile phase was a 0.01 mol·dm−3 aqueous sulfuric acid solution at a flow rate of 0.5 cm3·min−1. Recovery of SO2. The aim of this work was to show that the catalyst SO2 could be separated from the reaction system by steam stripping to achieve an inexpensive environmentally friendly process. For all experiments with added SO2, SO2 in the product mixture was removed by steam stripping before the mixture was analyzed by HPLC. In order to accurately quantify the removal rate of SO2, the following experiment was used as an example. The apparatus for the process is shown in detail in Figure S1 in the SI. In this experiment, 1.541 g of SO2, 0.500 g of cellulose, and 15 cm3 of H2O were added to the reactor and the cellulose was converted for 30 min at 190 °C. When the reactor had been cooled to about 100 °C, the valves on both ends of the reactor were opened, the gas in the reactor was released, and the solution was stripped with steam delivered into the liquid mixture. SO2 in the liquid mixture was then brought out, together with the

steam. When SO2 and the steam passed through a condenser, the steam condensed to a liquid and SO2 remained as a gas because SO2 has a very low solubility in water at room temperature. Therefore, SO2 could be easily separated from water. Samples of the product mixture were withdrawn at intervals, and the concentration of residual SO2 in the product mixture was measured by an iodometric method.23

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RESULTS AND DISCUSSION

Conversion of Cellulose in the H2O−SO2 System. The effects of the temperature, reaction time, and SO2 concentration on the conversion of cellulose are shown in Figures 1 and 2. The results reveal that cellulose can be effectively converted to chemicals, sequentially from cellulose to glucose, HMF, and LA with an increase in the temperature, reaction

Figure 1. Effect of the SO2 concentration CSO2 on the conversion α of cellulose and yields Y of glucose and LA at different temperatures: ■, 190 °C; ●, 200 °C; ▲, 210 °C; ★, 220 °C. Reaction conditions: cellulose, 0.5 g; H2O, 15 g; reaction time, 30 min. 15504

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in the temperature promotes the breakage of hydrogen bonds in cellulose, leading to an increase of the conversion and glucose yield. Subsequently, the high temperature also accelerates the further dehydration of glucose, resulting in an increase in the LA yield. Thus, the glucose yield goes through a maximum. Moreover, less SO2 is needed to reach the highest glucose yield at a higher temperature. The reason is that high temperatures are favorable to break both the inter- and intramolecular hydrogen bonds in cellulose and the −O− bonds beween the glucose units. Figure 2 shows that the reaction time is crucial for the distribution of products. When cellulose is converted at 210 °C for 15 min, the main products are glucose, with small amounts of fructose, furfural, and HMF as intermediate products. After 45 min, the main product is LA with a maximum yield of 45.0%. However, when the reaction time is 45 min, the conversion of cellulose does not change significantly when the SO 2 concentration is more than 0.027 g·cm−3. This is due to the formation of a dark insoluble polymeric substance, generally called humin.25 The results above indicate that the conversion of cellulose and yields of products can be controlled by the concentration of SO2, temperature, and reaction time. Generation Process of Humin. It is noted that the conversion of cellulose is always higher than the total yield of products, and the difference value (D-value) is increased with an increase in the temperature, reaction time, and SO2 concentration. From the literature, we know that the formation of humin from cellulose in an acid hydrolysis system can lead to a decrease of product yields.26 In order to further increase the yield of products, it is important to analyze the process of humin generation. The results are listed in Table 1. Cellobiose is the basic repeating unit of cellulose and is formed by a β-1,4-glycosidic linkage of two glucose units. When the products of cellobiose only include glucose and fructose, the D-value is only 0.1%, but this value sharply increases to 10.7% when minor amounts of HMF and LA appear (Table 1, entries 1 and 2). This indicates that the D-value mainly comes from further dehydration of glucose. The conversion of glucose is low, but the D-value is relatively large (Table 1, entries 3 and 4). Stability studies of HMF and LA show that they are both stable in a water solution without SO2 (Table 1, entries 5 and 7), and LA is not very reactive in an acidic solution (Table 1, entry 8), while HMF is very reactive when SO2 is added (Table 1, entry 6). However, when a mixture of LA and HMF was tested, an increase of solid residues (humin) was noted. The LA yield (Table 1, entry 10) was less than that obtained in the presence of pure HMF (Table 1, entry 6), and it even became negative in aqueous solution (Table 1, entry 9). These results indicate that HMF interacts with LA under these conditions. A similar conclusion was made by Li et al., who stated that HMF and glucose were the main precursors for humin formation in aqueous solution.26 Therefore, the key to reducing the formation of humin and improving the yield of products is to reduce the production of HMF or protect the active functional group of HMF. Mechanism of Cellulose Conversion to Chemicals in the H2O−SO2 System. It is well-known that SO2 can dissolve in water to form sulfurous acid, so H+ from sulfurous acid can serve as the catalyst similar to H2SO4 or HCl, and the conversion process can be written as follows:

Figure 2. Effect of the SO2 concentration CSO2 on cellulose conversion α and product yields Y at different reaction times: (a), 15 min; (b), 30 min; (c), 45 min. Reaction conditions: cellulose, 0.5 g; H2O, 15 g; temperature, 210 °C.

time, and SO2 concentration, which is similar to the products obtained from other acid-catalyzed reactions.24 As expected, the conversion of cellulose increases considerably with an increase of the SO2 concentration when the reaction temperature and reaction time are fixed. SO2 dissolved in an aqueous solution can form sulfurous acid and generate H+ ions. The more SO2 added to the aqueous solution, the higher the concentration of H+ ions, which acts as an effective catalyst for cellulose conversion to chemicals. Figure 1 also shows that the temperature has a great influence on the conversion of cellulose and yields of glucose and LA. With an increase in the temperature, the conversion of cellulose increases greatly. The yield of glucose first increases and then decreases, while the yield of LA increases. An increase

V1

V2

cellulose → glucose → LA + others 15505

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Table 1. Transformation and Stability of Products from Cellulosea Y/% entry

stock

1b 2b

cellobiose cellobiose

entry

stock

c

glucose glucose HMF HMF LA LA HMF + LA HMF + LA

3 4c 5c 6c 7c 8c 9c 10c a

t/min

CSO2/g·cm 15 15

−3

0.011 0.147

t/min 30 30 15 15 15 30 15 15

α/%

glucose

fructose

87.2 96.7

86.0 82.9

1.1 0.8

LA 0 0.6 Y/%

D-value 0.1 10.7

CSO2/g·cm−3

α/%

HMF

LA

0 0.008 0 0.016 0 0.016 0 0.016

37.4 42.9 4.9 84.5 0.7 3.7 15.9 100

17.2 10.3

0.5 4.3 2.7 29.2

19.7 28.3 2.2 55.3

−12.0 12.3

27.9 87.7

D-value

Reaction conditions: stocks, 0.5 g; H2O, 15 g. bTemperature = 180 °C. cTemperature = 210 °C.

Figure 3. Effect of the (a) H2SO4 concentration CH2SO4 and (b) SO2 concentration CSO2 on the conversion α of cellulose and yields Y of products: ■, conversion; □, yield of LA; ○, yield of HMF; △, yield of glucose. Conditions: cellulose, 0.5 g; H2O, 15 g; temperature, 210 °C; time, 30 min.

where Vi is the reaction rate for the different steps of cellulose conversion to chemicals. In order to further verify the catalytic mechanism of H2O + SO2 for cellulose, the product distributions of cellulose catalyzed by SO2 and H2SO4 were compared. The results, shown in Figure 3, indicate that the products are rather different when catalyzed by SO2 and H2SO4 under the same conditions. In the H2O−SO2 system, the dominant product is glucose when the acid concentration is low, and then glucose is gradually converted to LA. However, in the H2O−H2SO4 system, the major product is always LA, and only minor amounts of glucose are produced. The dissociation constant of H2SO3 is very low (5.7 × 10−3 at 50 °C10), so there must exist a large amount of molecular SO2 in the H2O−SO2 mixture. To test this hypothesis, the interaction between cellulose and SO2 in dimethyl sulfoxide (DMSO), an aprotic solvent, was studied by Fourier transform infrared (FT-IR) spectroscopy (see Figure 4). The absorption band of a cellulose −OH stretching vibration shifts from 3345 cm−1 in the absence of SO2 to 3420 cm−1 in the presence of SO2. This shift demonstrates that the hydrogen bonds existing in cellulose are broken, thus deconstructing the compact structure of cellulose. However, HPLC analysis shows that no product is detected in a DMSO + SO2 solution, which suggests that molecular SO2 has no ability to convert cellulose to glucose or glucose to LA and that H+ ions are essential in the process of cellulose conversion to glucose and LA.

Figure 4. FT-IR spectra of transmittance at wavenumber ν for (a) pure cellulose, (b) regenerated cellulose in a DMSO solution at 210 °C for 30 min, and (c) regenerated cellulose in the DMSO−SO2 system at 210 °C for 30 min. Note the band at 3345 cm−1 in parts a and b shifts to 3420 cm−1 in part c.

From the above results, it is concluded that a H2O + SO2 solution that has a relatively low concentration of H+ ions is sufficient for the conversion of cellulose to glucose but not 15506

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sufficient to further convert glucose. These two observations make the conversion rates of cellulose in the H2O−SO2 and H2O−H2SO4 systems follow the orders of V1 > V2 and V2 > V1, respectively. This conclusion is consistent with the results of Figures 1 and 2. Therefore, one of the advantages of SO2 over H2SO4 is that one can obtain relatively high yields of glucose by controlling the amount of added SO2 in the H2O−SO2 system. Recovery of SO2. Importantly, we found that the catalyst SO2 in all of the product mixture could be easily recovered by steam stripping. Figure 5 shows the residual concentration of

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.W.). ken.marsh@uwa. edu.au (K.N.M.). Tel/Fax: +86 10 6442 7603 (W.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21176020), the Beijing Natural Science Foundation (Grant 2082017), and the Program for New Century Excellent Talents in University (Grant NCET08-0710).

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Figure 5. Effect of the steam stripping time t on the concentration of residual SO2 CSO2 in a product mixture.

NOMENCLATURE D-value =difference value between cellulose conversion and total product yields, % CSO2 =concentration of SO2 in the reaction system, g·cm−3 CH2SO4 =concentration of H2SO4 in the reaction system, g·cm−3 α =conversion of stocks, % t =reaction time, min Y =yield of products in mole fraction, % REFERENCES

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SO2 in a product mixture as a function of the stripping time. In 5 min, the concentration of SO2 in solution decreased from 6.73 to 0.31 g·dm−3. In 20 min, it decreased to 0.01 g·dm−3, and after 30 min, it was lower than 0.007 g·dm−3 (limit of detection). This can be understood from two aspects. First, an increase of the temperature reduces the solubility of SO2 in an aqueous solution. Second, the steady flow of steam purges SO2 from the gas phase. SO2 removed by the steam from the product mixture was collected, and the amount of SO2 was determined by an iodometric method, which indicates that SO2 in the reaction mixture can be easily recovered and used for further reaction. In this work, the product mixtures of all experiments with SO2 added were swept for 30 min using the same method before the residues were analyzed by HPLC, and no SO2 or SO32− was detected by HPLC analysis.

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CONCLUSION In conclusion, SO2 in hot water can act as a catalyst to efficiently convert cellulose to glucose and then to LA and other species. Molecular SO2 and H+ ions via ionization of H2SO3 act together as catalysts, leading to high yields of products and less corrosion to the reactor than that using dilute sulfuric acid. The conversion of cellulose and distribution of products are greatly influenced and can be controlled by the temperature, reaction time, and amount of SO2. A high glucose yield of 48.0% and a high LA yield of 45.0% have been obtained. SO2 in the product mixture can be completely recovered by steam stripping, which supplies an environmentally benign process for cellulose conversion to chemicals.

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Article

ASSOCIATED CONTENT

S Supporting Information *

Schematic diagram of the apparatus for SO2 recovery and pressures measured in the H2O−SO2 system. This material is available free of charge via the Internet at http://pubs.acs.org. 15507

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