Carbon Dioxide Promoted Hydrolysis of Xylose to Furfural Using 1,1,3

Article pubs.acs.org/IECR

Carbon Dioxide Promoted Hydrolysis of Xylose to Furfural Using 1,1,3,3-Tetramethyl Guanidinium Hydrogen Sulfate: A Remarkable Enhancement in Reaction Rate Subodh Kumar, Jitendra Kumar, Savita Kaul, and Suman L. Jain* Chemical Sciences Division, CSIRIndian Institute of Petroleum Mohkampur, Dehradun 248005 India S Supporting Information *

ABSTRACT: 1,1,3,3-Tetramethylguanidinium hydrogen sulfate (TMG·HSO4) catalyzed conversion of xylose to furfural in water enriched with carbon dioxide at high temperature is described. The presence of carbon dioxide exhibited significant enhancement in the reaction rates and produced product yield more than 90%. The probable reasons for this rate enhancement are the following: (i) carbon dioxide might be reducing the viscosity of the reaction medium for better mass transfer and therefore enhancing the reaction; (ii) the reaction between carbon dioxide and water at higher temperature produces carbonic acid, which subsequently dissociates to increase the hydronium ion concentration and in turn the acidity of the medium. Henry reaction,15 one-pot synthesis of pyran,16 the synthesis of 3,4-dihydropyridin-2-(1H)-ones,17 and direct Aldol reaction.18 In continuation of our ongoing research on biomaterials, we herein report for the first time a simple, efficient, and reusable catalytic system, i.e., carbon dioxide enriched water containing 1,1,3,3-tetramethylguanidinium hydrogen sulfate (TMG·HSO4) for the conversion of xylose into furfural. The addition of carbon dioxide enhanced the reaction rate significantly and produced furfural in 90.2% yield at 160 °C for a reaction time of 2 h. The TMG·HSO4 could be recycled, and the catalyst exhibited similar activity over five cycles of evaluation (Figure 5).

1. INTRODUCTION Continuous depletion in fossil fuel reserves, global warming, and other environmental concerns suggest the urgent need for sustainable sources of energy in the near future.1 Carbohydrates represent 75% of the annual renewable biomass which led to the development of the chemistry of furanic compounds from carbohydrates for the preparation of value-added chemicals to be of tremendous importance.2−4 In this context, furfural is one of the key compounds, used as an intermediate for the preparation of value-added products5−7 including plastics (furan-based polymers), pharmaceuticals, fuels, and fine chemicals (Scheme 1). Current commercial processes use concentrated inorganic acids (H2SO4 and HCl) as catalysts for the hydrolysis of hemicellulosics to xylose, followed by dehydration of the latter at higher temperature (200−225 °C) to form furfural.8,9 However, under these conditions the selectivity of furfural does not exceed 40−50%. Furthermore, these acids are highly toxic, corrosive, and difficult to handle. Another major drawback of using mineral acids is the occurrence of many side reactions with longer reaction times that reduce the furfural yield. To overcome these limitations, Sako et al.10 reported the continuous extraction of furfural with supercritical CO2 to increase the yield of furfural to 80% in the hydrolysis of xylose. Moreau et al.11 reported that the selectivity of furfural was found to be 90% over microporous solid acid catalysts at 170 °C from xylose. Zhang and Zhao12 studied microwave-assisted conversion of lignocellulosic biomass into furans in ionic liquids, but the yield of furfural was about 30%, and it was difficult to separate furfural from the reaction system. Tao et al.13 reported the use of 1-(4-sulfonic acid) butyl-3methylimidazolium hydrogen sulfate ionic liquid for the conversion of xylose to furfural under mild conditions. Recently, protic ionic liquids, especially those based on 1,1,3,3-tetramethylguanidine (TMG) and synthesized through simple neutralizing of equimolar TMG with acids, have been received considerable interest due to their low cost and facile synthesis.14 Such ionic liquids have been extensively used as catalysts for various organic transformations including the © 2014 American Chemical Society

2. EXPERIMENTAL SECTION Synthesis of TMG·HSO4. TMG·HSO4 was prepared according to the literature method.14 In a typical experiment, 100 mL of ethanol and 23 g of TMG (0.2 mol) were loaded into a 250 mL flask in a water bath of 0 °C. Then 0.2 mol of H2SO4 in 35 mL of ethanol was added into the flask under stirring, and the stirring was continued for 2 h. The reaction mixture was evaporated under reduced pressure. Then, the product was dried under vacuum at 100 °C for 48 h. The synthesized catalyst was characterized by elemental analysis and 1 H NMR spectroscopy. General Procedure for the Conversion of Xylose to Furfural. Experiments were carried out in a stainless steel autoclave of 15 mL capacity; the vessel was charged with xylose (25 mmol, 3.75 g), TMG·HSO4 (5−25 mol %, 0.2−1.4 g), water (1.0 g), and carbon dioxide at 20 bar of pressure. The reaction mixture was heated to 160 °C for 2 h, and then the reactor was removed and quickly quenched in a cool water bath. Furfural was extracted from the water phase with ethyl Received: Revised: Accepted: Published: 15571

July 1, 2014 September 10, 2014 September 11, 2014 September 11, 2014 dx.doi.org/10.1021/ie502614z | Ind. Eng. Chem. Res. 2014, 53, 15571−15575

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Scheme 1. Synthesis of Furfural from Xylose

acetate (10 mL) five times and analyzed by HPLC using calibration curves generated with commercially available standards. The aqueous layer was concentrated at reduced pressure to remove water and residual ethyl acetate. The recovered catalyst containing unreacted xylose was added in methanol; TMG·HSO4 was dissolved in methanol and unreacted xylose was precipitated out. The methanol layer was separated and concentrated under reduced pressure, and the recovered catalyst was subsequently used for the recycling experiments.

3. CALCULATIONS The following equations were used to calculate the conversion of xylose, the yield of furfural, and the selectivity of furfural. mol formed of all products conversion of xylose (%) = ·100 mol used of xylose yield of furfural (%) =

Figure 1. TGA of TMG·HSO4.

mol formed of furfural ·100 mol converted of xylose

selectivity of furfural (%) =

Scheme 3. CO2 Promoted Hydrolysis of Xylose Using TMG· HSO4 Catalyst

mol formed of furfural ·100 mol formed of all products

4. RESULTS AND DISCUSSION During the present study, the required catalyst TMG·HSO4 was synthesized as shown in Scheme 2. The synthesized compound Scheme 2. Synthesis of TMG·HSO4

be in good agreement with the calculated values: C = 35.79%, H = 6.57%, N = 14.46%, and S = 9.58%. The thermal stability of the synthesized compound was determined by thermogravimetric analysis (Figure 1). As shown in Figure 1, an initial weight loss of 2.4% in the range 100−110 °C is probably due to the evaporation of water or volatile solvent molecules. After this, the prepared compound was found to be stable up to 250 °C and then exhibited rapid decomposition between 250 and 300 °C. Furthermore, the successful synthesis and identity of the synthesized compound were confirmed by 1H NMR spectroscopy [(400 MHz, D2O) 2.88(s); Supporting Information, Figure S1]. The synthesized catalyst was dried under vacuum at 80 °C for 12 h and tested for the hydrolysis of xylose using water

was characterized using elemental analysis, thermogravimetric analysis (TGA), and 1H NMR spectroscopy. Elemental analysis of the synthesized compound reveals the following: C (%), 35.71; H (%), 6.59; N (%)14.12; S (%) 9.53. This was found to 15572

dx.doi.org/10.1021/ie502614z | Ind. Eng. Chem. Res. 2014, 53, 15571−15575

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Table 1. Effect of Temperature and Time on Hydrolysis of Xylose in CO2 Enriched Watera no.

temp (°C)

time (h)

conv (%)

yield (%)

1 2 3 4 5

100 140 160 180 160

1 2 2 2 4

35 85 94.5 96 96.5

32.1 80.5 90.2 82 80

a

Conditions: 5 mmol of xylose, 1 g of ionic liquid, and 1.0 g of water under 20 bar pressure of CO2.

Figure 2. UV−vis spectra of ionic liquid in water, without CO2 and with CO2.

Figure 5. Results of recycling experiments.

Table 2. Comparison of TMG·HSO4 with Literature Methodsa no.

solvent

yield (%)

ref

83

20

56 73

21 22

52

23

NaCl

60

24

CO2

90.2

catalyst

additive NaCl or FeCl3 LiBr metal chloride −

1

H2O

H2SO4

2 3

N,N-DMA MIBK

CrCl3 ChCl−citric acid

4

H2O

5

H2O

6

H2O

mesoporous zirconium phosphate MCM-41-supported niobium oxide TMG·HSO4

Figure 3. Effect of catalyst amount on furfural yield.

a

N,N-DMA, N,N-dimethylacetamide; MIBK, methyl isobutyl ketone; ChCl, choline chloride.

determined by the weight of the unreacted xylose after the reaction, and the selectivity of the furfural was determined by HPLC. Other possible side products which might be formed due to the dehydration of xylose and degradation of furfural could not be identified. However, in the absence of CO2 the yield of the furfural was found to be 52.5% under identical reaction conditions. These results suggested that the addition of carbon dioxide to the reaction medium resulted in a remarkable enhancement in furfural yield. The presence of CO2 might be reducing the viscosity of the medium for better mass transfers which in turn enhance the reaction rate. Further, as suggested in the literature,19 the reaction between CO2 and water at higher temperature produces carbonic acid, which subsequently dissociates and enhances the acidity of the medium. To ascertain the effect of CO2 on the acidity of the reaction

Figure 4. Effect of water content on furfural yield.

enriched with carbon dioxide as a reaction medium at high temperature (Scheme 3). In a typical experiment, 25 mmol of xylose and TMG·HSO4 (20 mmol, 1.0 g) were taken in water (1 g) and the resulting mixture was heated at 160 °C and 20 bar of CO2 pressure for 2 h. The maximum yield of the furfural under these conditions was found to be 90.2%. The conversion of the xylose was 15573

dx.doi.org/10.1021/ie502614z | Ind. Eng. Chem. Res. 2014, 53, 15571−15575

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Scheme 4. Plausible Mechanism of the Reaction

with the furfural yield of 90.2%. As listed in Table 1, by prolonging the reaction time, the conversion of xylose was increased but the selectivity for furfural was decreased significantly, which is probably due to the further degradation of furfural to byproducts. Recycling of the catalyst is an essential step toward developing a green and sustainable catalytic system for chemical transformations. We also studied the recycling of the developed catalyst as shown in Figure 5. After completion of the reaction, the product furfural was extracted from the water layer by extraction with ethyl acetate. The aqueous phase containing acid catalyst was concentrated under reduced pressure to remove the water and residual ethyl acetate. The recovered catalyst was then used directly for the next run by adding xylose and water under identical conditions. The recovered catalyst was reused for five subsequent runs. In all cases similar conversions of xylose and similar yields of furfural were obtained. These studies indicated that the developed catalyst can be recycled efficiently for several runs without any significant loss in catalytic activity. The comparison of the catalytic efficiency of the TMG·HSO4 catalyst with the known methods is shown in Table 2. As shown in Table 2, most of the methods have used expensive additives and toxic acid catalysts for the hydrolysis of xylose to give moderate to high yields of the furfural. In contrast, the present methodology utilizes an easily accessible metal-free catalyst by using carbon dioxide as additive to give a higher yield of the furfural. These facts established the superiority of the developed methodology over the existing ones. The mechanism of conversion of xylose to furfural using acidic catalysts is well documented in the literature.25 In analogy to the existing reports, we assume that the reaction probably involves the isomerization to xylose to xylulose followed by dehydration of xylulose to furfural. As suggested in the literature, Lewis acids are particularly important for the isomerization step; however, Brønsted acidity is required for the dehydration step. In the present study, TMG·HSO4 contains both Lewis and Brønsted acid sites, which probably take part in the reaction and enhance the reaction rate as shown in Scheme 4.

medium, we determined the acidity of the ionic liquid in water at higher temperature before and after the addition of CO2, taking 4-nitroaniline as an indicator, by using UV−visible spectroscopy. The spectra of the ionic liquid and 4-nitroaniline in water with or without adding CO2 are shown in Figure 2. It is evident that the absorbance of 4-nitroanilinium ion is increased because of the protonation of 4-nitroaniline, indicating the higher acidity of the reaction mixture in the presence of CO2. However, the spectrum of the ionic liquid in the absence of CO2 shows a significant decrease in the absorbance of 4-nitroanilinium ion, which clearly indicates the lower protonation of the indicator. To study the effect of catalyst concentration as shown in Figure 3, we varied the catalyst amount from 5 to 25 mol % (0.2−1.4 g) and studied the hydrolysis of xylose under identical experimental conditions. Initially the reaction was found to be increased with the catalyst amount increasing from 0.2 to 1.0 g (Figure 3). However, with further increase in the amount beyond 1 g, the reaction did not show any significant improvement. Thus, we have used 1.0 g (20 mol %) catalyst as the optimum amount for the present reaction. In the absence of catalyst, under identical reaction conditions, no xylose conversion was observed even after prolonged time (3.5 h). As the synthesized acidic catalyst is completely miscible with water, the influence of water content on xylose dehydration was studied as shown in Figure 4. When the water content in the system was increased from 1 to 2 g, it had little effect on the furfural yield of 88%. However, as the amount of water increased from 2 to 4.0 g, the furfural yield decreased to 75%. On the basis of these results we have selected a 1:1 weight ratio of water and catalyst as the optimum one. The effects of reaction temperature and time on xylose conversion are shown in Table 1. It can be seen that the reaction was highly dependent on the reaction temperature and time. When the reaction temperature was 100 °C at 20 bar of CO2 pressure, with a 60 min reaction time, the xylose conversion was 35% and a furfural yield of 32.1% was obtained. The furfural yield increased from 35 to 80.5% with increase of temperature from 100 to 140 °C for 2 h reaction time. A further increase in reaction temperature from 160 to 180 °C for 2 h produced the maximum conversion of xylose, i.e., 94.5%, 15574

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(10) Sako, T.; Taguchi, T.; Sugeta, T.; Nakazawa, N.; Okubo, T.; Hiaki, T. J. Kinetic study of furfural formation accompanying supercritical carbon dioxide extraction. Chem. Eng. Jpn. 1992, 25, 372−377. (11) Moreau, C.; Durand, R.; Peyron, D.; Duhamet, J.; Rivalier, P. Selective preparation of furfural from xylose over microspores solid acid catalysts. Ind. Crops Prod. 1998, 7, 95−99. (12) Zhang, Z.; Zhao, Z. K. Microwave-assisted conversion of lignocellulosic biomass into furans in ionic liquid. Bioresour. Technol. 2010, 101, 1111−1114. (13) Tao, F.; Song, H.; Chou, L. Efficient process for the conversion of xylose to furfural with acidic ionic liquid. Can. J. Chem. 2011, 89, 83−87. (14) Gao, H. X.; Han, B. X.; Li, J.; Jiang, T.; Liu, Z. M.; Wu, W. Z.; Chang, Y. H.; Zhang, J. Preparation of Room Temperature Ionic Liquids by Neutralization of 1,1,3,3-Tetramethylguanidine with Acids and their use as Media for Mannich Reaction. Synth. Commun. 2004, 34, 3083−3089. (15) Jiang, T.; Gao, H. X.; Han, B. X.; Zhao, G. Y.; Chang, Y. H.; Wu, W. Z.; Gao, L.; Yang, G. Y. Ionic Liquid Catalyzed Henry Reactions. Tetrahedron Lett. 2004, 45, 2699−2701. (16) Shaabani, A.; Samadi, S.; Badri, Z.; Rahmati, A. Ionic Liquid Promoted Efficient and Rapid One-Pot Synthesis of Pyran Annulated Heterocyclic Systems. Catal. Lett. 2005, 104, 39−43. (17) Shaabani, A.; Rahmati, A. Ionic Liquid Promoted Efficient Synthesis of 3,4-Dihydropyrimidin-2-(1H)-ones. Catal. Lett. 2005, 100, 177−179. (18) Zhu, A. L.; Jiang, T.; Han, B. X.; Huang, J.; Zhang, J. C.; Ma, X. M. Study on Guanidine-based Task-Specific Ionic Liquids as Catalysts for Direct Aldol Reactions Without Solvent. New J. Chem. 2006, 30, 736−740. (19) Hunter, S. E.; Savage, P. E. Acid-Catalyzed Reactions in Carbon Dioxide-Enriched High-Temperature Liquid Water. Ind. Eng. Chem. Res. 2003, 42, 290−294. (20) Rong, C.; Ding, X.; Zhu, Y.; Li, Y.; Wang, L.; Qu, Y.; Ma, X.; Wang, Z. Production of Furfural from Xylose at Atmospheric Pressure by Dilute Sulfuric acid and Inorganic Salts. Carbohydr. Res. 2012, 350, 77−80. (21) Binder, J. B.; Bland, J. J.; Cefali, A. V.; Raines, R. T. Synthesis of Furfural from Xylose and Xylan. ChemSusChem 2010, 3, 1268−1272. (22) Zhang, L.; Yu, H. Conversion of Xylan and Xylose into Furfural in Bio renewable Deep Eutectic Solvent with Trivalent Metal Chloride Added. BioResources 2013, 8, 6014−6025. (23) Cheng, L.; Guo, X.; Song, C.; Yu, G.; Cui, Y.; Xue, N.; Peng, L.; Guo, X.; Ding, W. High Performance Mesoporous Zirconium Phosphate for Dehydration of Xylose to Furfural in Aqueous Phase. RSC Adv. 2013, 3, 23228−23235. (24) Garcia-Sancho, C.; Sadaba, I.; Moreno-Tost, R.; Merida-Robles, J.; Santamaria-Gonzalez, J.; Lopez-Granados, M.; Maireles-Torres, P. Dehydration of Xylose to Furfural over MCM-41-Supported NiobiumOxide Catalysts. ChemSusChem 2013, 6, 635−642. (25) Choudhary, V.; Sandler, S. I.; Vlachos, D. G. Conversion of Xylose to Furfural Using Lewis and Brønsted Acid Catalysts in Aqueous Media. ACS Catal. 2012, 2 (9), 2022−2028.

5. CONCLUSION 1,1,3,3-Tetramethyl guanidinium hydrogen sulfate (TMG· HSO4) was found to be an efficient acid catalyst for the preparation of furfural through the dehydration reaction of xylose using carbon dioxide enriched water as a reaction medium. The addition of carbon dioxide enhanced the acidity of the reaction medium and therefore exhibited higher reaction rates. Under the developed process, the maximum conversion of xylose of 94.5% with a 90.2% furfural yield was obtained when the reaction was carried out at 160 °C for a reaction time of 2 h. The catalyst ionic liquid could be recycled almost without loss of activity five times after the product furfural was separated by extraction with ethyl acetate. Because of the ability of carbon dioxide to increase the acidity of the reaction medium in an environmentally benign manner, the developed process may open new possibilities to developing more sustainable and green methodologies for acid catalyzed organic transformations.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectrum of the synthesized catalyst TMG·HSO4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We kindly acknowledge the Director of IIP for his kind permission to publish these results. The Analytical Science Division of the Institute is acknowledged for providing analyses of the samples. Subodh Kumar is thankful to CSIR, New Delhi, for his research fellowship.

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REFERENCES

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dx.doi.org/10.1021/ie502614z | Ind. Eng. Chem. Res. 2014, 53, 15571−15575