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Brönsted acidic ionic liquids as novel catalysts for the hydrolyzation of soybean isoflavone glycosides
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Catalysis Communications 9 (2008) 1307–1311 www.elsevier.com/locate/catcom
Bro¨nsted acidic ionic liquids as novel catalysts for the hydrolyzation of soybean isoflavone glycosides Qiwei Yang, Zuojun Wei, Huabin Xing, Qilong Ren * National Laboratory of Secondary Resources Chemical Engineering, Zhejiang University, Hangzhou 310027, China Received 19 August 2007; received in revised form 18 November 2007; accepted 21 November 2007 Available online 26 December 2007
Abstract In this work, the hydrolyzation of soybean isoflavone glycosides using Bro¨nsted acidic ionic liquids (ILs) as novel catalysts was studied. The experimental results show that those ILs have good catalytic activities to the hydrolyzation reactions, and their activities are mainly dependent on the types of their anions. The SO3H-functional ILs with HSO4 as anion have the best catalytic activities which are similar to that of sulfuric acid, with the conversion of glycitin more than 90%. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ionic liquids; Hydrolyzation; Isoflavone glycosides; Catalyst
1. Introduction Ionic liquids (ILs), being recognized as environmentally benign media, have been widely applied in many reactions as catalysts or dual catalyst–solvents, such as alkylation, esterification, Michael addition, oligomerization and rearrangement [1–7]. However, the study of hydrolyzation reactions using ILs as catalysts was few. It might be due to the limitations of the traditional ILs that either are water unstable [8] or just have very weak acidities [9], therefore making it hard to obtain good catalytic activities in hydrolyzation reactions using ILs as catalysts. In fact, although some articles have studied the effects of ILs in hydrolysis reactions, those ILs, such as [bmim]Cl and [bmim]BF4, were only used as solvents or additives to enhance the enzymatic or acid-catalyzed processes [10–12]. Recently, the introduction of Bro¨nsted acidic functional groups into cations or anions of the ILs, especially the SO3H-functional groups, which has obviously enhanced their acidities and water solubili*
Corresponding author. Tel./fax: +86 571 8795 2773. E-mail address: [email protected] (Q. Ren).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.023
ties [13–16], gives great promise of using ILs as green catalysts in hydrolyzation reactions with good catalytic activities. Isoflavones are a group of diphenolic secondary metabolites extracted mainly from soybeans [17]. The major types of isoflavones in soybean are daidzein, glycitein and genistein, which have been proved as important health-enhancing compounds [18]. However, most of those isoflavones in soybean exist in the form of glycosides which have only poor biological effects and are hard to be absorbed in the intestines, indicating the strong need for being hydrolyzed to their corresponding aglycones [19]. The most popular methods of hydrolyzation of soybean isoflavone glycosides include enzymatic processes and acid-catalyzed processes [20–22], but problems still exist. The former are often restricted by the relatively high cost of highly efficient enzymes and the latter often involve hydrochloric acid or sulfuric acid as catalyst, bringing a large amount of acidic effluent that may do much damage to the environment. Therefore, in the present study, we for the first time use the water-stable Bro¨nsted acidic functional ionic liquids catalyzing the hydrolyzation of soybean isoflavone glycosides, in order
1308
Q. Yang et al. / Catalysis Communications 9 (2008) 1307–1311
to supply an alternative approach to the green hydrolyzation and extend the application of ILs.
MS detections of IL I–IV were done to ensure their quality (supporting information).
2. Experimental
2.2. Hydrolyzation of soybean isoflavone glycosides (Fig. 2)
N-Methyl imidazole and 1,4-butane sulfonate were purchased from Acros Organics (USA). 1,3-Propane sulfonate was purchased from TCI Chemical Co. (Japan). The crude soybean isoflavones powder was supplied by Heilongjiang Jiusan Oil and Fat Co. (China) and recrystallized four times before hydrolyzation, with the content of isoflavones elevated from 41.1% to about 99% which contains 58.49% daidzin, 20.13% glycitin and 20.23% genistin. All other chemicals (AR grade) were commercially available and used as received unless otherwise stated. 1H NMR Spectra was recorded on a Bruker Advance DMX-500 spectrometer at 500 MHz. ESI-MS was carried out with Agilent 1100 series LC/MSD Trap SL mass spectrometer. Hydrolysis products were analyzed by HPLC (Waters 1525 binary pump, Waters 2487 dual k absorbance detector, 254 nm) with a C18 column.
In general, 30 mg soybean isoflavones powder, 0.6 mmol IL and 30 ml glycol–water solution (50%, v/v) were added to a three-necked flask equipped with a reflux condenser. Then, the mixture was heated to the desired temperature and stirred for 8 h, and then cooled to room temperature to end the reaction. The resulting mixture was a transparent solution and samples were taken directly for HPLC analysis. In recovery experiments of ILs, hydrolyzation reaction was carried out in water and the resulting mixture was a solid–liquid biphasic system, and the aglycone distributed mainly in the solid phase. After filtration, samples of each phase were taken for HPLC analysis. The filtrate, IL-water solution, was then evaporated under reduced pressure to get a mixture of IL and glucose. Ethanol (20 ml) was then added to dissolve the IL selectively, followed by filtration to remove the undissolved glucose. Then, IL was reused after the vacuum evaporation of the filtrate IL-ethanol solution.
2.1. Preparations of Bro¨nsted acidic ionic liquids [bmim]Br, [bmim]BF4 [23] and five SO3H-functional ILs [24,25] were synthesized according to previous literatures. Their structures are shown in Fig. 1. A typical procedure of the functional ILs’ synthesis is given by IL I: N-methyl imidazole and equimolar 1,4-butane sulfonate were mixed without other solvent and stirred magnetically for 12 h at 40 °C. Then, a white solid zwitterion was formed and washed repeatedly with ether. Then, equimolar concentrated sulfuric acid (98%) was added dropwise to the zwitterion and the mixture was stirred magnetically for 6 h at 80 °C. The resulting liquid was washed repeatedly with toluene and dried in a high vacuum at 120 °C for 12 h. The residual water contents of these ILs were determined by Karl Fisher Titrator (ZKF-1) to be about 2%. NMR and
3. Results and discussion 3.1. Synthesis of Bro¨nsted acidic ILs Most of the synthesized ILs are viscous colorless or light– brown transparent liquids at room temperature. [sbmim][H2PO4] and [sbmim][p-TS] are transparent solids and will slowly transform into white solids at room temperature after long times (e.g. several weeks). This phenomenon is consistent with the behavior of some other ILs, such as SO3H-functional pyridine-based ILs [25,26]. All produced ILs are water-stable and miscible with water, so they have the potential to be applied in water-containing reactions.
I: n=4, X- = HSO4N
N
n X-
SO3H II: n=4, X = H2PO4 III: n=4, X- = H3C
[sbmim][HSO4] [sbmim][H2PO4] SO3- [sbmim][p-TS]
IV: n=3, X- = HSO4-
[spmim][HSO4]
SO3H N
V: X-= HSO4-
[sbpr][HSO4]
VI: X-= Br-
[bmim]Br
VII: X-= BF4-
[bmim]BF4
X-
N
N X-
Fig. 1. The structure of ionic liquids used in this study.
Q. Yang et al. / Catalysis Communications 9 (2008) 1307–1311 CH2OH O
O
O O
Ionic liquid
HO H OH
OH
H2O or H2O-glycol
R1 R2
O
1309
HO R1 R2
O
OH
OH
R1=H, R2=H: daidzin
R1=H, R2=H: daidzein
R1=OCH3, R2=H: glycitin
R1=OCH3, R2=H: glycitein
R1=H, R2=OH: genistin
R1=H, R2=OH: genistein
Fig. 2. Hydrolyzation of soybean isoflavone glucoside.
3.2. Hydrolyzation of soybean isoflavone glycosides The hydrolyzation results are shown in Table 1. The reason why we used the mixture of glycol and water as reaction media is that the existence of glycol can greatly improve the solubilities of isoflavones, which makes sure the reactions take place in single phase. From Table 1 one can see that these SO3H-functional ILs have good catalytic activities for the hydrolyzation of soybean isoflavone glycosides (Table 1, entries 1–5). The well-studied ILs [bmim]Br and [bmim]BF4, which were often used as solvents or additives in other relative articles, could only cause very low conversions (entries 6 and 7). It should be pointed out that the isoflavone aglycones will transform into other chemicals when heated to high temperature (e.g. 100 °C) in an acidic environment [27,28], resulting in lower yields compared to conversions. It appears clearly that the IL’s catalytic activity is dependent on its constitution. As entries 1, 4 and 5 show, when the anions are same, the differences in cations that the little change of the sulfonic side chain or whether the ring is imidazole or pyridine just cause slight differences in reaction activities. For example, the conversion of glycitin catalyzed by [sbmim][HSO4] is 97.6% and that by [sbpr][HSO4] is 95.7%. Otherwise, when the cations are same ([(CH2)4SO3HMIm] +, entries 1–3), the IL’s anion shows a more significant impact. First, the hydrolyzation reaction shows a better conversion using [sbmim][HSO4] as catalyst than using the other two, because the anion [HSO4] has a much higher Bro¨nsted acidity than [H2
PO4] and [p-TS] . Second, unexpectedly, the conversion using [sbmim][p-TS] as catalyst is much higher than that using [sbmim][H2PO4]. Since the anion [p-TS] has a similar acidity to [H2PO4] , this phenomenon indicates that the acidity is not the only factor affecting the IL’s catalytic activity. Considering [p-TS] is more lipophilic than [H2PO4] , a probable explanation could be that because isoflavones have comparatively large molecular sizes as bioactive chemicals, the lipophilicity of the catalyst may have an influence on the reaction consequently. In addition, the activities of these ILs were compared to that of sulfuric acid (entry 8). The reaction activities are similar, for example, the conversion of glycitin catalyzed by [sbmim][HSO4] is 97.6% and that of H2SO4 is 99.8%. The results of glycosides conversions at different temperatures show that the increase in temperature can result in obvious acceleration of the hydrolyzation (Table 1, entries 1, 9 and 10). Besides, as the results in Table 2 show, the three kinds of isoflavone glycosides have different reaction activities. Glycitin exhibits a higher reactivity than genistin and daidzin, as a result of their subtle structure differences. Although the existence of glycol could bring better solubilities and higher reaction temperature, it also caused difficulty to the separation of products. The high boiling point of glycol (197.5 °C) might be unfavorable to the stability of aglycones during the distillation of products solution to form a solid–liquid biphasic system. Therefore, the hydrolyzation reactions were carried out using pure water as both reactant and solvent afterwards (Table
Table 1 Hydrolyzation of soybean isoflavone glycosidesa Entry
1 2 3 4 5 6 7 8 9 10 a
Catalyst
I II III IV V VI VII H2SO4 I I
T (°C)
100 100 100 100 100 100 100 100 110 80
Conversion (%)
Yield (%)
Daidzin
Glycitin
Genistin
Daidzein
Glycitein
Genistein
76.4 29.0 69.1 75.6 69.5 3.1 0.9 78.6 90.1 36.7
97.6 52.7 93.3 98.6 95.7 6.5 3.8 99.8 100.0 51.8
78.6 34.6 70.3 77.4 71.6 4.0 2.0 80.1 92.6 39.0
40.6 11.3 34.2 41.3 41.0 1.4 0.3 42.5 53.8 19.7
62.4 29.8 52.2 62.4 64.7 4.0 2.2 63.3 62.8 40.2
41.7 12.2 36.7 42.3 42.1 1.3 1.2 44.9 54.6 23.5
Reactions were run in 30 ml 50% (v/v) glycol–water solution for 8 h. Isoflavone glycosides were 30 mg, ILs and sulfuric acid were all 0.6 mmol.
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Q. Yang et al. / Catalysis Communications 9 (2008) 1307–1311
Table 2 Reaction activities of different isoflavone glycosidesa Entry
Time (min)
1 2 3
40 200 480 a
Conversion (%)
Yield (%)
Daidzin
Glycitin
Genistin
Daidzein
Glycitein
Genistein
6.8 38.1 76.4
26.9 74.7 97.6
9.5 40.3 78.6
6.2 23.8 40.6
18.0 54.7 62.4
7.3 27.1 41.7
Reactions were run in 30 ml 50% (v/v) glycol–water solution at 100 °C catalyzed by IL I. Isoflavone glycosides 30 mg, IL 0.6 mmol.
Table 3 Reaction activities in different liquid mediaa Entry
Media
Conversion (%) Daidzin
Glycitin
Genistin
Daidzein
Glycitein
Genistein
1 2 3b
50% glycol–water Water Water
76.4 61.9 42.2
97.6 91.7 78.9
78.6 64.3 55.0
40.6 32.9 24.3
62.4 58.8 49.5
41.7 34.4 29.6
a b
Yield (%)
Reactions were run in 30 ml liquid media at 100 °C for 8 h catalyzed by IL I. Isoflavone glycosides 30 mg, IL 0.6 mmol. The third cycle.
3), and the results showed it had a similar reaction efficiency to that in the glycol case. Then, research about the reusability of IL was made subsequently. As the result shows, after two cycles, the catalytic activity lowered probably resulting from the loss of IL during recovery and the accumulation of impurities (Table 3, entry 3). The decrease in activities of functional ILs has been mentioned in many previous literatures [29] and thus we plan to immobilize functional ILs on solid supports to reduce the loss and simplify the separation. Relative researches are underway. 4. Conclusions In this study, several ILs were prepared and those SO3H-functional Bro¨nsted acidic ILs showed good catalytic activities in the hydrolyzation reactions of soybean isoflavone glycosides. The catalytic activities of these ILs are mainly dependent on their different acidities, and can be also influenced probably by the lipophilicities of the anions. Increasing reaction temperature will greatly improve the reaction activity. The SO3H-functional ILs with HSO4 as anion have the best catalytic activities which are similar to that of sulfuric acid, with the conversion of glycitin more than 90% at 100 °C. And then, the promising reusability of ILs can reduce the discharge of acidic effluent, indicating the potential of the further application of functional ILs in the research of environment-friendly hydrolyzation processes. Acknowledgement This work was financially supported by National Key Project of Scientific and Technical Supporting Programs Funded by Ministry of Science & Technology of China (No. 2006BAD27B03), National Natural Science Founda-
tion of China (No. 20506023), and a grant of Science and Technology Plan of Zhejiang Province (No. 2006C11026). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/ j.catcom.2007.11.023. References [1] K.S. Ya, V.V. Namboodiri, R.S. Varma, P.G. Smirniotis, J. Catal. 222 (2004) 511. [2] T. Joseph, S. Sahoo, S.B. Halligudi, J. Mol. Catal. A: Chem. 234 (2005) 107. [3] B.C. Ranu, S. Banerjee, Org. Lett. 7 (2005) 3049. [4] R.D. Tilve, M.V. Alexander, A.C. Khandekar, S.D. Samant, V.R. Kanetkar, J. Mol. Catal. A: Chem. 223 (2004) 237. [5] T. Welton, Chem. Rev. 99 (1999) 2071. [6] V.I. Parvulescu, C. Hardacre, Chem. Rev. 107 (2007) 2615. [7] J. Ranke, S. Stolte, R. Stormann, J. Arning, B. Jastorff, Chem. Rev. 107 (2007) 2183. [8] M.H. Valkenberg, C. deCastro, W.F. Holderich, Green Chem. 4 (2002) 88. [9] J.S. Wilkes, Green Chem. 4 (2002) 73. [10] C.Z. Li, Z.K. Zhao, Adv. Synth. Catal. 349 (2007) 1847. [11] C. Chiappe, E. Leandri, S. Lucchesi, D. Pieraccini, B.D. Hammock, C. Morisseau, J. Mol. Catal. B: Enzym. 27 (2004) 243. [12] S.S. Mohile, M.K. Potdar, J.R. Harjani, S.J. Nara, M.M. Salunkhe, J. Mol. Catal. B: Enzym. 30 (2004) 185. [13] A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weaver, D.C. Forbes, J. James, H. Davis, J. Am. Chem. Soc. 124 (2002) 5962. [14] A. Arfan, J.P. Bazureau, Org. Process Res. Dev. 9 (2005) 743. [15] K. Qiao, C. Yokoyama, Catal. Commun. 7 (2006) 450. [16] P. Wasserscheid, M. Sesing, W. Korth, Green Chem. 4 (2002) 134. [17] P. Chuankhayan, T. Rimlumduan, J. Svasti, J.R. Cairns, J. Agr. Food Chem. 55 (2007) 2407. [18] M. Messina, O. Kucuk, J.W. Lampe, J. Aoac. Int. 89 (2006) 1121. [19] Y. Kawakami, W. Tsurugasaki, S. Nakamura, K. Osada, J. Nutr. Biochem. 16 (2005) 205. [20] Y.B. Choi, K.S. Kim, J.S. Rhee, Biotechnol. Lett. 24 (2002) 2113.
Q. Yang et al. / Catalysis Communications 9 (2008) 1307–1311 [21] W.D. Chiang, C.J. Shih, Y.H. Chu, Food Chem. 72 (2001) 499. [22] E.A. Utkina, S.V. Antoshina, A.A. Selishcheva, G.M. Sorokoumova, E.A. Rogozhkina, V.I. Shvets, Russ. J. Bioorg. Chem. 30 (2004) 385. [23] C.P. Fredlake, J.M. Crosthwaite, D.G. Hert, S.N. Aki, J.F. Brennecke, J. Chem. Eng. Data 49 (2004) 954. [24] J. Gui, X. Cong, D. Liu, X. Zhang, Z. Hu, Z. Sun, Catal. Commun. 5 (2004) 473. [25] H. Xing, T. Wang, Z. Zhou, Y. Dai, Ind. Eng. Chem. Res. 44 (2005) 4147.
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[26] C.P. Fredlake, J.M. Crosthwaite, D.G. Hert, S.N. Aki, J.F. Brennecke, J. Chem. Eng. Data 49 (2004) 954. [27] F.C. Stintzing, M. Hoffmann, R. Carle, Mol. Nutr. Food Res. 50 (2006) 373. [28] H.H. Huang, H.H. Liang, K.C. Kwok, J. Sci. Food Agr. 86 (2006) 1110. [29] K. Qiao, H. Hagiwara, C. Yokoyama, J. Mol. Catal. A: Chem. 246 (2006) 65.
Catalysis Communications 9 (2008) 1307–1311 www.elsevier.com/locate/catcom
Bro¨nsted acidic ionic liquids as novel catalysts for the hydrolyzation of soybean isoflavone glycosides Qiwei Yang, Zuojun Wei, Huabin Xing, Qilong Ren * National Laboratory of Secondary Resources Chemical Engineering, Zhejiang University, Hangzhou 310027, China Received 19 August 2007; received in revised form 18 November 2007; accepted 21 November 2007 Available online 26 December 2007
Abstract In this work, the hydrolyzation of soybean isoflavone glycosides using Bro¨nsted acidic ionic liquids (ILs) as novel catalysts was studied. The experimental results show that those ILs have good catalytic activities to the hydrolyzation reactions, and their activities are mainly dependent on the types of their anions. The SO3H-functional ILs with HSO4 as anion have the best catalytic activities which are similar to that of sulfuric acid, with the conversion of glycitin more than 90%. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ionic liquids; Hydrolyzation; Isoflavone glycosides; Catalyst
1. Introduction Ionic liquids (ILs), being recognized as environmentally benign media, have been widely applied in many reactions as catalysts or dual catalyst–solvents, such as alkylation, esterification, Michael addition, oligomerization and rearrangement [1–7]. However, the study of hydrolyzation reactions using ILs as catalysts was few. It might be due to the limitations of the traditional ILs that either are water unstable [8] or just have very weak acidities [9], therefore making it hard to obtain good catalytic activities in hydrolyzation reactions using ILs as catalysts. In fact, although some articles have studied the effects of ILs in hydrolysis reactions, those ILs, such as [bmim]Cl and [bmim]BF4, were only used as solvents or additives to enhance the enzymatic or acid-catalyzed processes [10–12]. Recently, the introduction of Bro¨nsted acidic functional groups into cations or anions of the ILs, especially the SO3H-functional groups, which has obviously enhanced their acidities and water solubili*
Corresponding author. Tel./fax: +86 571 8795 2773. E-mail address: [email protected] (Q. Ren).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.023
ties [13–16], gives great promise of using ILs as green catalysts in hydrolyzation reactions with good catalytic activities. Isoflavones are a group of diphenolic secondary metabolites extracted mainly from soybeans [17]. The major types of isoflavones in soybean are daidzein, glycitein and genistein, which have been proved as important health-enhancing compounds [18]. However, most of those isoflavones in soybean exist in the form of glycosides which have only poor biological effects and are hard to be absorbed in the intestines, indicating the strong need for being hydrolyzed to their corresponding aglycones [19]. The most popular methods of hydrolyzation of soybean isoflavone glycosides include enzymatic processes and acid-catalyzed processes [20–22], but problems still exist. The former are often restricted by the relatively high cost of highly efficient enzymes and the latter often involve hydrochloric acid or sulfuric acid as catalyst, bringing a large amount of acidic effluent that may do much damage to the environment. Therefore, in the present study, we for the first time use the water-stable Bro¨nsted acidic functional ionic liquids catalyzing the hydrolyzation of soybean isoflavone glycosides, in order
1308
Q. Yang et al. / Catalysis Communications 9 (2008) 1307–1311
to supply an alternative approach to the green hydrolyzation and extend the application of ILs.
MS detections of IL I–IV were done to ensure their quality (supporting information).
2. Experimental
2.2. Hydrolyzation of soybean isoflavone glycosides (Fig. 2)
N-Methyl imidazole and 1,4-butane sulfonate were purchased from Acros Organics (USA). 1,3-Propane sulfonate was purchased from TCI Chemical Co. (Japan). The crude soybean isoflavones powder was supplied by Heilongjiang Jiusan Oil and Fat Co. (China) and recrystallized four times before hydrolyzation, with the content of isoflavones elevated from 41.1% to about 99% which contains 58.49% daidzin, 20.13% glycitin and 20.23% genistin. All other chemicals (AR grade) were commercially available and used as received unless otherwise stated. 1H NMR Spectra was recorded on a Bruker Advance DMX-500 spectrometer at 500 MHz. ESI-MS was carried out with Agilent 1100 series LC/MSD Trap SL mass spectrometer. Hydrolysis products were analyzed by HPLC (Waters 1525 binary pump, Waters 2487 dual k absorbance detector, 254 nm) with a C18 column.
In general, 30 mg soybean isoflavones powder, 0.6 mmol IL and 30 ml glycol–water solution (50%, v/v) were added to a three-necked flask equipped with a reflux condenser. Then, the mixture was heated to the desired temperature and stirred for 8 h, and then cooled to room temperature to end the reaction. The resulting mixture was a transparent solution and samples were taken directly for HPLC analysis. In recovery experiments of ILs, hydrolyzation reaction was carried out in water and the resulting mixture was a solid–liquid biphasic system, and the aglycone distributed mainly in the solid phase. After filtration, samples of each phase were taken for HPLC analysis. The filtrate, IL-water solution, was then evaporated under reduced pressure to get a mixture of IL and glucose. Ethanol (20 ml) was then added to dissolve the IL selectively, followed by filtration to remove the undissolved glucose. Then, IL was reused after the vacuum evaporation of the filtrate IL-ethanol solution.
2.1. Preparations of Bro¨nsted acidic ionic liquids [bmim]Br, [bmim]BF4 [23] and five SO3H-functional ILs [24,25] were synthesized according to previous literatures. Their structures are shown in Fig. 1. A typical procedure of the functional ILs’ synthesis is given by IL I: N-methyl imidazole and equimolar 1,4-butane sulfonate were mixed without other solvent and stirred magnetically for 12 h at 40 °C. Then, a white solid zwitterion was formed and washed repeatedly with ether. Then, equimolar concentrated sulfuric acid (98%) was added dropwise to the zwitterion and the mixture was stirred magnetically for 6 h at 80 °C. The resulting liquid was washed repeatedly with toluene and dried in a high vacuum at 120 °C for 12 h. The residual water contents of these ILs were determined by Karl Fisher Titrator (ZKF-1) to be about 2%. NMR and
3. Results and discussion 3.1. Synthesis of Bro¨nsted acidic ILs Most of the synthesized ILs are viscous colorless or light– brown transparent liquids at room temperature. [sbmim][H2PO4] and [sbmim][p-TS] are transparent solids and will slowly transform into white solids at room temperature after long times (e.g. several weeks). This phenomenon is consistent with the behavior of some other ILs, such as SO3H-functional pyridine-based ILs [25,26]. All produced ILs are water-stable and miscible with water, so they have the potential to be applied in water-containing reactions.
I: n=4, X- = HSO4N
N
n X-
SO3H II: n=4, X = H2PO4 III: n=4, X- = H3C
[sbmim][HSO4] [sbmim][H2PO4] SO3- [sbmim][p-TS]
IV: n=3, X- = HSO4-
[spmim][HSO4]
SO3H N
V: X-= HSO4-
[sbpr][HSO4]
VI: X-= Br-
[bmim]Br
VII: X-= BF4-
[bmim]BF4
X-
N
N X-
Fig. 1. The structure of ionic liquids used in this study.
Q. Yang et al. / Catalysis Communications 9 (2008) 1307–1311 CH2OH O
O
O O
Ionic liquid
HO H OH
OH
H2O or H2O-glycol
R1 R2
O
1309
HO R1 R2
O
OH
OH
R1=H, R2=H: daidzin
R1=H, R2=H: daidzein
R1=OCH3, R2=H: glycitin
R1=OCH3, R2=H: glycitein
R1=H, R2=OH: genistin
R1=H, R2=OH: genistein
Fig. 2. Hydrolyzation of soybean isoflavone glucoside.
3.2. Hydrolyzation of soybean isoflavone glycosides The hydrolyzation results are shown in Table 1. The reason why we used the mixture of glycol and water as reaction media is that the existence of glycol can greatly improve the solubilities of isoflavones, which makes sure the reactions take place in single phase. From Table 1 one can see that these SO3H-functional ILs have good catalytic activities for the hydrolyzation of soybean isoflavone glycosides (Table 1, entries 1–5). The well-studied ILs [bmim]Br and [bmim]BF4, which were often used as solvents or additives in other relative articles, could only cause very low conversions (entries 6 and 7). It should be pointed out that the isoflavone aglycones will transform into other chemicals when heated to high temperature (e.g. 100 °C) in an acidic environment [27,28], resulting in lower yields compared to conversions. It appears clearly that the IL’s catalytic activity is dependent on its constitution. As entries 1, 4 and 5 show, when the anions are same, the differences in cations that the little change of the sulfonic side chain or whether the ring is imidazole or pyridine just cause slight differences in reaction activities. For example, the conversion of glycitin catalyzed by [sbmim][HSO4] is 97.6% and that by [sbpr][HSO4] is 95.7%. Otherwise, when the cations are same ([(CH2)4SO3HMIm] +, entries 1–3), the IL’s anion shows a more significant impact. First, the hydrolyzation reaction shows a better conversion using [sbmim][HSO4] as catalyst than using the other two, because the anion [HSO4] has a much higher Bro¨nsted acidity than [H2
PO4] and [p-TS] . Second, unexpectedly, the conversion using [sbmim][p-TS] as catalyst is much higher than that using [sbmim][H2PO4]. Since the anion [p-TS] has a similar acidity to [H2PO4] , this phenomenon indicates that the acidity is not the only factor affecting the IL’s catalytic activity. Considering [p-TS] is more lipophilic than [H2PO4] , a probable explanation could be that because isoflavones have comparatively large molecular sizes as bioactive chemicals, the lipophilicity of the catalyst may have an influence on the reaction consequently. In addition, the activities of these ILs were compared to that of sulfuric acid (entry 8). The reaction activities are similar, for example, the conversion of glycitin catalyzed by [sbmim][HSO4] is 97.6% and that of H2SO4 is 99.8%. The results of glycosides conversions at different temperatures show that the increase in temperature can result in obvious acceleration of the hydrolyzation (Table 1, entries 1, 9 and 10). Besides, as the results in Table 2 show, the three kinds of isoflavone glycosides have different reaction activities. Glycitin exhibits a higher reactivity than genistin and daidzin, as a result of their subtle structure differences. Although the existence of glycol could bring better solubilities and higher reaction temperature, it also caused difficulty to the separation of products. The high boiling point of glycol (197.5 °C) might be unfavorable to the stability of aglycones during the distillation of products solution to form a solid–liquid biphasic system. Therefore, the hydrolyzation reactions were carried out using pure water as both reactant and solvent afterwards (Table
Table 1 Hydrolyzation of soybean isoflavone glycosidesa Entry
1 2 3 4 5 6 7 8 9 10 a
Catalyst
I II III IV V VI VII H2SO4 I I
T (°C)
100 100 100 100 100 100 100 100 110 80
Conversion (%)
Yield (%)
Daidzin
Glycitin
Genistin
Daidzein
Glycitein
Genistein
76.4 29.0 69.1 75.6 69.5 3.1 0.9 78.6 90.1 36.7
97.6 52.7 93.3 98.6 95.7 6.5 3.8 99.8 100.0 51.8
78.6 34.6 70.3 77.4 71.6 4.0 2.0 80.1 92.6 39.0
40.6 11.3 34.2 41.3 41.0 1.4 0.3 42.5 53.8 19.7
62.4 29.8 52.2 62.4 64.7 4.0 2.2 63.3 62.8 40.2
41.7 12.2 36.7 42.3 42.1 1.3 1.2 44.9 54.6 23.5
Reactions were run in 30 ml 50% (v/v) glycol–water solution for 8 h. Isoflavone glycosides were 30 mg, ILs and sulfuric acid were all 0.6 mmol.
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Table 2 Reaction activities of different isoflavone glycosidesa Entry
Time (min)
1 2 3
40 200 480 a
Conversion (%)
Yield (%)
Daidzin
Glycitin
Genistin
Daidzein
Glycitein
Genistein
6.8 38.1 76.4
26.9 74.7 97.6
9.5 40.3 78.6
6.2 23.8 40.6
18.0 54.7 62.4
7.3 27.1 41.7
Reactions were run in 30 ml 50% (v/v) glycol–water solution at 100 °C catalyzed by IL I. Isoflavone glycosides 30 mg, IL 0.6 mmol.
Table 3 Reaction activities in different liquid mediaa Entry
Media
Conversion (%) Daidzin
Glycitin
Genistin
Daidzein
Glycitein
Genistein
1 2 3b
50% glycol–water Water Water
76.4 61.9 42.2
97.6 91.7 78.9
78.6 64.3 55.0
40.6 32.9 24.3
62.4 58.8 49.5
41.7 34.4 29.6
a b
Yield (%)
Reactions were run in 30 ml liquid media at 100 °C for 8 h catalyzed by IL I. Isoflavone glycosides 30 mg, IL 0.6 mmol. The third cycle.
3), and the results showed it had a similar reaction efficiency to that in the glycol case. Then, research about the reusability of IL was made subsequently. As the result shows, after two cycles, the catalytic activity lowered probably resulting from the loss of IL during recovery and the accumulation of impurities (Table 3, entry 3). The decrease in activities of functional ILs has been mentioned in many previous literatures [29] and thus we plan to immobilize functional ILs on solid supports to reduce the loss and simplify the separation. Relative researches are underway. 4. Conclusions In this study, several ILs were prepared and those SO3H-functional Bro¨nsted acidic ILs showed good catalytic activities in the hydrolyzation reactions of soybean isoflavone glycosides. The catalytic activities of these ILs are mainly dependent on their different acidities, and can be also influenced probably by the lipophilicities of the anions. Increasing reaction temperature will greatly improve the reaction activity. The SO3H-functional ILs with HSO4 as anion have the best catalytic activities which are similar to that of sulfuric acid, with the conversion of glycitin more than 90% at 100 °C. And then, the promising reusability of ILs can reduce the discharge of acidic effluent, indicating the potential of the further application of functional ILs in the research of environment-friendly hydrolyzation processes. Acknowledgement This work was financially supported by National Key Project of Scientific and Technical Supporting Programs Funded by Ministry of Science & Technology of China (No. 2006BAD27B03), National Natural Science Founda-
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