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Brønsted acid ionic liquid catalyzed formation of pyruvaldehyde dimethylacetal from triose sugars
Catalysis Today 200 (2013) 94–98
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Brønsted acid ionic liquid catalyzed formation of pyruvaldehyde dimethylacetal from triose sugars Shunmugavel Saravanamurugan, Anders Riisager ∗ Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e
i n f o
Article history: Received 15 April 2012 Received in revised form 25 June 2012 Accepted 4 July 2012 Available online 10 August 2012 Keywords: Ionic liquid Sulfonic acid Dihydroxyacetone Pyruvaldehyde dimethylacetal
a b s t r a c t A series of sulfonic acid functionalized ionic liquids (SO3 H-ILs) have been synthesized, characterized and investigated as catalysts for the conversion of the triose sugars, 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA), to pyruvaldehyde dimethylacetal (PADA) in methanol. Depending on the reaction conditions and the applied SO3 H-ILs a good yield of up to 52% of PADA was obtained. Under identical reaction conditions the derivative of PADA, 1,1,2,2-tetramethoxy propane (TMP), could be obtained in yields up to 49% using another SO3 H-IL. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Carbohydrates contained in biomass have in the last decade been established as important renewable feedstocks to make carbonaceous chemicals and fuels. Glucose is the most important compound in carbohydrates and it is readily available in abundance as cellulose polymer. Hence, transformations of glucose into bio-platform chemicals have in particular been extensively investigated. Isomerization of glucose to fructose is a key reaction in the industrial process to produce high-fructose corn syrup as sweetener and pave also a way to make a wide range of chemicals and fuels directly [1–4]. The direct conversion of glucose/fructose to lactic acid and 5-hydroxymethyl furfural (HMF) are important transformations because these bio-platform chemicals have a variety of applications. For example, HMF can be selectively oxidized into furan-2,5-dicarboxylic acid, which is an alternative chemical to terephthalic acid [5]. Similarly, transformation of C5 sugars to fuels and chemicals are also important, for example, dehydration of xylose to furfural and subsequent conversion into the value-added chemical tetrahydrofuran [1,6]. Another important transformation in carbohydrate chemistry is the conversion of triose sugars to pyruvic acid (in water) or pyruvaldehyde acetal (in alcohol). Acetals are generally important in organic synthesis where they are used as protecting groups for carbonyl compounds, since they are stable under basic conditions and in the presence of oxidizing and reducing agents
∗ Corresponding author. Tel.: +45 4525 2233. E-mail address: [email protected] (A. Riisager). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.07.001
[7–9]. They may also be important chemicals themselves like, for example, acetaldehyde diethylacetal which is used as a flavoring compound in distilled beverages. The typical synthetic route to acetals involves reaction of an oxo-containing compound with a dissolved acid catalyst in alcohol or the orthoformate compound [10]. However, recently it has been shown that ordinary solid acid zeolites are active catalysts for the dehydration of the trioses 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA) to form pyruvaldehyde (PA), which reacted further – depending on the nature of the acid sites on the catalyst – to form lactic acid in water or methyl lactate and pyruvaldehyde dimethylacetal (PADA) in methanol [11]. Ionic liquids (ILs) are low-melting organic salts that can be attractive alternatives to common organic solvents due to their non-measurable vapor pressure, relatively high thermal stability, tunable acidity or basicity as well as combined catalyst-solvent properties [12–15]. Importantly, the solubility of the polysaccharide cellulose can also be much higher in ILs than in common organic solvents – a feature that has intensively increased the use of IL catalyst-solvent systems for making bio-platform chemicals from carbohydrates [16–19]. Since classical ILs lack chemical functionality, new concepts have emerged where functional groups are integrated in the IL ions. In this context, Davis and coworkers reported in 2002 the first amine functionalized imidazolium-based ILs (task-specific) for CO2 capture [20]. Subsequently, sulfonic acid functionalized imidazolium-based ILs were also reported and investigated for Fischer esterification and pinacol rearrangement reactions with excellent reusability [21]. Since this pioneer work several other reports on the synthesis and application of new acid and base functionalized ILs have followed [22–25]. In our
S. Saravanamurugan, A. Riisager / Catalysis Today 200 (2013) 94–98
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Scheme 1. The structures of the SO3 H-ILs.
previous reports, we have shown that sulfonic acid functionalized ILs were promising catalyst for the dehydration of fructose and glucose to make ethyl levulinate and ethyl-d-glucopyranoside in ethanol, respectively [3]. In the present study, we report the synthesis of a series of sulfonic acid functionalized imidazolium-, pyridinium- and ammonium-based ILs (SO3 H-ILs) with the cations: 1-methyl-3(4-sulfobutyl)-imidazolium ([BMIm-SO3 H][X], X = HSO4 , NTf2 , OMs and TfO), 1-(4-sulfobutyl)pyridinium ([BPyr-SO3 H][HSO4 ]) and N,N,N-triethyl-4-sulfobutanaminium ([NEt3 B-SO3 H][HSO4 ]). The structures of the SO3 H-ILs are depicted in Scheme 1. The SO3 H-ILs were tested as catalysts in the conversion of the trioses DHA and GLA into PADA using methanol as solvent under autogenic pressure. The influence of reaction parameters such as reaction time, temperature and concentration of DHA was optimized. 2. Experimental 2.1. Synthesis and characterization of SO3 H-ILs 1-methylimidazol (99%, Sigma–Aldrich) or pyridine (>99%, Sigma–Aldrich) or triethylamine (>99.5%, Fluka) (0.2 mol) and 1,4butanesultone (99%, Aldrich, 0.2 mol) were charged in a 100 ml round bottomed flask. The mixture was then stirred at 40–80 ◦ C for 10 h. The solid zwitterion formed was recovered by filtration, washed repetitively with diethyl ether until all unreacted reactants were completely removed (confirmed by NMR) and then dried under reduced pressure (15 mbar, 50 ◦ C) overnight. A stoichiometric amount of acid (98% H2 SO4 , >98% TfOH, 95% HNTf2 or >99.5% MsOH, Sigma–Aldrich) was subsequently added drop wise to the respective zwitterion and the mixture stirred at 80 ◦ C for 6 h. The obtained viscous ionic liquids were finally purified by extractive washing with diethyl ether and finally dried under reduced pressure (15 mbar, 50 ◦ C) overnight. Yields were above 95% for all ILs. The identity of the synthesized sulfonic acid functionalized ionic liquids was confirmed by NMR (Bruker AM360 NMR spectrometer, 25 ◦ C). The thermal decomposition temperature (Td ) of the SO3 HILs were measured by TGA analysis (TGA/DSC 1 apparatus, Mettler Toledo) by heating the ionic liquid (9–18 mg) in an aluminum sample holder from 40 ◦ C to 800 ◦ C with a heating ramp of 20 ◦ C/min under nitrogen atmosphere. 1-Methyl-3-(4-sulfobutyl)imidazolium trifluoromethanesulfonate ([BMIm-SO3 H][OTf]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.7 (s, 3H; N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D O): ı/ppm = 20.5, 27.9, 35.5, 49.0, 50.0, 121.9, 2 123.4, 131.0, 135.5; Td > 200 ◦ C. 1-Methyl-3-(4-sulfobutyl)imidazolium hydrogensulfate ([BMImSO3 H][HSO4 ]): 1 H NMR (300 MHz, D2 O) ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.7 (s, 3H; N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.5, 27.9, 35.5, 49.0, 50.0, 121.9, 123.4, 135.5; Td > 300 ◦ C. methanesulfonate 1-Methyl-3-(4-sulfobutyl)imidazolium ([BMIm-SO3 H][OMs]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H),
3.7 (s, 3H; N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.7, 27.9, 35.5, 38.3, 48.6, 49.9, 121.9, 123.4, 135.7; Td > 300 ◦ C. 1-Methyl-3-(4-sulfobutyl)imidazolium bis((trifluoromethyl)sulfonyl)amide ([BMIm-SO3 H][NTf2 ]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.7 (s, 3H, N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.7, 27.9, 35.2, 48.6, 49.8, 117.0, 121.9, 123.3, 135.3; Td > 200 ◦ C. N,N,N-triethyl-4-sulfobutaneammonium hydrogensulfate ([NEt3 B-SO3 H][HSO4 ]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.0–1.2 (t, 9H; 3CH3 ), 1.5–1.8 (m, 4H; 2CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.0–3.2 (m, 8H; 4CH2 N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 6.5, 19.9, 21.0, 49.9, 52.3, 55.9; Td > 300 ◦ C. 1-(4-Sulfobutyl)pyridinium hydrogensulfate ([BPyrSO3 H][HSO4 ]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.7 (m, 2H; CH2 ), 1.9–2.1 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 4.4–4.6 (t, 3H; N CH2 ), 7.8–8.0 (t, 2H; 2CH), 8.4 (t, 2H; 2CH), 8.7 (d, 1H; CH); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.5, 28.9, 49.6, 60.7, 127.9, 143.8, 145.3; Td > 300 ◦ C. 2.2. Catalytic testing The catalytic reactions were carried out in 15 ml ace pressure tubes. 139.3 mg (1.5 mmol) of DHA (97%, Sigma–Aldrich) or 142.2 mg (1.5 mmol) of GLA (95%, Sigma–Aldrich), 0.11 mmol of SO3 H-IL, 30 mg of naphthalene (internal reference) and 4 g of methanol (>99.9%, Sigma–Aldrich) were charged into the ace pressure tube and heated under stirring at 120 ◦ C (oil bath temperature) for 24 h. 2.3. Reactant and product analysis The reaction mixtures were subjected to GC-FID analysis (Agilent 6890N instrument, HP-5 capillary column 30.0 m × 320 m × 0.25 m) as well as HPLC-RI analysis (Agilent 1200 series, 30 cm Aminex© HPX-87H column, 0.005 M H2 SO4 eluent, flow rate 0.6 ml/min). A GC–MS system (Agilent 6850 GC coupled with Agilent 5975C mass detector) was used for qualitative analysis. Conversion of the triose sugars DHA and GLA as well as yields of PA were determined by HPLC using standards made from commercial samples. The yield of pyruvaldehyde dimethylacetal (PADA) and 1,1,2,2 tetramethoxypropane (TMP) were calculated from GC results on series of PADA and TMP standards with naphthalene as internal standard. 3. Results and discussion The conversion of DHA and GLA to PADA was carried out with SO3 H-ILs as catalysts in methanol at 120 ◦ C and the results are presented in Table 1. Reaction of DHA with [BMIm-SO3 H][OMs] (Table 1, entry 1) yielded 41% of PADA and 45% of TMP along with more than 99% conversion of DHA with PA being observed as intermediate by HPLC. The yields of PADA and TMP were equally good or
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Table 1 Conversion of DHA and GLA to PADA and TMP.a Entry
Ionic liquid
Triose
Conversion (%)
PADA yield (%)
TMP yield (%)
1 2 3 4 5 6 7 8b
[BMIm-SO3 H][OMs] [BMIm-SO3 H][HSO4 ] [BMIm-SO3 H][OTf] [BPyr-SO3 H][HSO4 ] [BMIm-SO3 H][NTf2 ] [NEt3 B-SO3 H][HSO4 ] [BMIm-SO3 H][OMs] [BMIm-SO3 H][OMs]
DHA DHA DHA DHA DHA DHA GLA DHA
>99 >99 >99 >99 >99 >99 46 >99
41 44 52 46 52 39 6 67
45 41 35 45 38 49 9 11
See Section 2 for reaction conditions. The results obtained after three runs by adding fresh DHA after each run.
better (PADA: 39–52%, TMP: 35–49%) when using the other SO3 H-IL catalysts (Table 1, entries 2–6), and the highest yield of PADA (52%) was observed with [BMIm-SO3 H][OTf] and [BMIm-SO3 H][NTf2 ] ILs. As shown in Scheme 2, a plausible reaction pathway involves dehydration of DHA to PA which reacts with methanol to form PADA, which may further be reacted with methanol to form the methylated derivative TMP. Since the sulfonic acid functionalized ILs have strong Brønsted acid sites they are expected to easily dehydrate DHA to form PADA (as also observed), and in the presence of strong acid sites PADA can partially be converted further into TMP. High yields of PADA have been observed for fluorinecontaining SO3 H-ILs compared to other SO3 H-ILs due to the strong acid strength generated by inclusion of the very weakly basic [NTf2 ]− and [OTf]− anions. In our previous report, a similar effect has been observed for SO3 H-ILs for the conversion of fructose to ethyl levulinate in ethanol [3,26]. The triose sugar, GLA, has also been tested in the reaction with [BMIm-SO3 H][OMs] as catalyst under identical reaction conditions as DHA (Table 1, entry 7). Here, a low yield of PADA (6%) and TMP (9%) was observed along with only 46% conversion of GLA. These results reveal that the GLA did not isomerise effectively into DHA which is required in order to facilitate formation of the intermediate PA by dehydration, thus lowering the yield of PADA and TMP. Due to the ineffective isomerization, GLA might be degrading slowly in methanol in the presence of [BMIm-SO3 H][OMs] and the formation of unidentified products was observed in both GC and HPLC. This could account for the rest of the converted GLA apart from a low yield of PADA (6%) and TMP (9%). The combined yields of the two acetal products, PADA and TMP, remained in all reactions with the different SO3 H-ILs between 86% and 91%. Since no significant difference in combined yields were observed and to avoid fluorine containing SO3 H-ILs, [BMImSO3 H][OMs] was chosen as the IL for further optimization of other reaction parameters. In general, fluorine-containing ionic liquids are relatively more toxic compared to non-fluorine containing ILs [27,28]. The influence of reaction temperature on the product distribution has thus been studied with [BMIm-SO3 H][OMs] catalyst between 60 ◦ C and 120 ◦ C and the results are shown in Table 2. A relatively low yield of PADA (10%) and TMP (19%) was observed along with 61% conversion of DHA at 60 ◦ C (Table 2, entry 1). However, a significant amount of intermediate PA (22%) was observed, thus suggesting that even at moderate temperature the IL was able to catalyze the dehydration of DHA to PA. At a temperature of 80 ◦ C, Table 2 Influence of temperature on the yields of PADA and TMP.a Entry
Temperature (◦ C)
Conversion (%)
PADA yield (%)
TMP yield (%)
1 2 3 4
60 80 100 120
61 96 >99 >99
10 32 35 41
19 53 56 45
a
Triose = DHA, IL = [BMIm-SO3 H][OMs]. See Section 2 for reaction conditions.
the yield of PADA and TMP increased to 32% and 53%, respectively, and the DHA reach close to full conversion of 96% (Table 2, entry 2). Hence, at this temperature the PA, once formed, readily reacted with methanol to form PADA which further was transformed into TMP. A further increase in temperature to 100 ◦ C did only induce a very small change in the yields of PADA (35%) and TMP (56%) corresponding to all DHA being converted (Table 2, entry 3). At 120 ◦ C the yield of PADA increased to 41% whereas the yield of TMP decreased to 45%, implying that TMP could be hydrolyzed and the reaction reversed to form PADA at this high temperature. The influence of reaction time on the product distribution has also been studied with the [BMIm-SO3 H][OMs] IL and the results are illustrated in Fig. 1. After 10 min of reaction, the conversion of DHA was 36% and a significant amount of PA was formed (20%). The yields of PADA and TMP were 3% and 10%, respectively. This suggested that once PADA formed from PA, it immediately reacted with methanol to form TMP. The conversion of DHA increased to 60% along with 22% PA after 30 min, whereas the yield of PADA increased to a significant amount of 15% and TMP increased to 25%. After 1 h of reaction the yield of PADA and TMP further increased to 29% and 40%, respectively, while a significant amount of PA remained, thus suggesting the dehydration of DHA to PA to occur rapidly. When the reaction time was prolonged to 24 h the conversion of DHA was quantitative (>99%) and the yield of PADA (41%) and TMP (45%) were increased at the expense of PA. The [BMIm-SO3 H][OMs] IL catalyst was also applied when investigating the influence of DHA concentration on the product distribution at 120 ◦ C (Fig. 2). When the concentration of DHA was increased from 3.3 to 5 wt%, the yield of PADA increased from 40% to 55% but the yield of TMP decreased from 45% to 20% while the conversion of DHA remained high at 99%. At higher concentrations of DHA a further decrease in the yield of TMP was found
100
Conversion / Yield (%)
a b
DHA PA PADA TMP
80 60 40 20 0 0
1
2
3
4
5
6
24
Time (h) Fig. 1. Time-course study on the conversion of DHA to PADA and TMP (reaction conditions: DHA = 139.3 mg, [BMIm-SO3 H][OMs] = 0.11 mmol, naphthalene = 30 mg, methanol = 4.0 g, temperature = 120 ◦ C).
S. Saravanamurugan, A. Riisager / Catalysis Today 200 (2013) 94–98
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Scheme 2. The plausible pathway for the formation of PADA and TMP from DHA and GLA.
resulting in less than 5% TMP when using 18 wt% DHA. In contrast, the yield of PA increased when more concentrated DHA solutions were used, while the PADA yield remained almost constant. Hence, when relatively large amounts of PA accumulated in reactions with high DHA loadings it was rather easily converted into PADA, while the subsequent methylation to TMP was limited, possibly due to unavailability of active sites. Interestingly, even at higher DHA concentration the [BMIm-SO3 H][OMs] IL catalyst was able to convert all DHA. In our previous work, we have shown that [BMIm-SO3 H][OMs] could be recycled in at least three runs without loss of any catalytic activity for the conversion of fructose to ethyl levulinate in ethanol. In the present study, however, the reusability of [BMImSO3 H][OMs] IL was investigated by adding fresh DHA after each run in methanol and the results are shown in Table 1. After the third run (Table 1, entry 8) the product distribution of PADA and TMP was significantly changed compared to the first run (Table 1, entry 1) as the yield of TMP was drastically reduced from 45% to 11% and the yield of PADA increased from 41% to 67%. This clearly indicated that the conversion reaction of PADA to TMP proceeded slowly after the first run thus lowering the yield of TMP. Even though there was a significant change in product distribution, the conversion of DHA was more than 99% and 14% of the intermediate PA was also formed
DHA
PA
PADA
TMP
4. Conclusions SO3 H-ILs are demonstrated to possess excellent catalytic activity in the conversion of the triose sugar DHA to PADA and the derivative TMP in methanol. The highest yield of PADA (52%) was found with [BMIm-SO3 H][OTf] and [BMIm-SO3 H][NTf2 ] ILs. A poor yield of PADA and TMP were, however, found for the triose isomer GLA due to low isomerization reactivity of the SO3 H-ILs toward DHA where from PADA was formed. At higher DHA concentrations the yield of TMP decreased while the yield of PADA remained good at 54% to 58%. Reuse of the [BMIm-SO3 H][OMs] catalyst revealed that the product distribution of PADA and TMP was significantly changed after a third run with the IL, but the conversion of DHA remained high at more than 99% and a significant amount of PA was still formed. Acknowledgement We thank the Danish Council for Independent ResearchTechnology and Production Sciences (project no. 10-081991) for financial support of the work. References [1] [2] [3] [4] [5]
100
Conversion / Yield (%)
implying that [BMIm-SO3 H][OMs] was able to dehydrate all DHA even after the third run.
80 60
[6] [7] [8] [9] [10]
40 20
[11]
0 3.3
5.0
9.0
13.0
18.0
DHA (wt %) Fig. 2. Influence of concentration of DHA on the yield of PADA and TMP (reaction conditions: DHA = 139.3 mg, [BMIm-SO3 H][OMs] = 0.11 mmol, naphthalene = 30 mg, methanol = 4.0 g, temperature = 120 ◦ C, time = 24 h).
[12] [13] [14] [15] [16] [17] [18]
A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411. M.S. Holm, S. Saravanamurugan, E. Taarning, Science 328 (2010) 602. S. Saravanamurugan, O.N.V. Buu, A. Riisager, ChemSusChem 4 (2011) 723. S. Bhosale, M. Rao, Microbiol. Rev. 60 (1996) 280. Y.Y. Gorbanev, S.K. Klitgaard, J.M. Woodley, C.H. Christensen, A. Riisager, ChemSusChem 2 (2009) 672. V. Choudhary, A.B. Pinar, S.I. Sandler, D. Gvlachos, R.F. Lobo, ACS Catal. 1 (2011) 172. E. Wenkert, T.E. Goodwin, Synth. Commun. 7 (1977) 409. A. Hassner, R. Wiederkehr, A.J. Kascheres, J. Org. Chem. 35 (1970) 1962. S.A. Patwardhan, S. Dev, Synthesis (1974) 348. A.F.B. Cameron, J.S. Hunt, J.F. Oughton, P.A. Wilkinson, B.M. Wilson, J. Chem. Soc. (1953) 3864. R.M. West, M.S. Holm, S. Saravanamurugan, J.M. Xiong, Z. Beversdorf, E. Taarning, C.H. Christensen, J. Catal. 269 (2010) 122. J.H. Huang, T. Ruther, Aus. J. Chem. 62 (2009) 298. M. Haumann, A. Riisager, Chem. Rev. 108 (2008) 1474. A. Riisager, R. Fehrmann, S. Flicker, R. Van Hal, M. Haumann, P. Wasserscheid, Angew. Chem. Int. Ed. 44 (2005) 815. T. Thananathanachon, T.B. Rauchfuss, Angew. Chem. Int. Ed. 49 (2010) 6616. H. Zhao, J.E. Holladay, H. Brown, Z.C. Zhang, Science 316 (2007) 1597. G.W. Huber, R.D. Cortright, J.A. Dumesic, Angew. Chem. Int. Ed. 43 (2004) 1549. G.W. Huber, A. Corma, Angew. Chem. Int. Ed. 46 (2007) 7184.
98
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[19] T. Ståhlberg, W. Fu, J.M. Woodley, A. Riisager, ChemSusChem 4 (2011) 451. [20] E.D. Bates, R.D. Mayton, J.H. Davis, J. Am. Chem. Soc. 124 (2002) 926. [21] A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weaver, D.C. Forbes, J.H. Davis, J. Am. Chem. Soc. 124 (2002) 5962. [22] A. Winkel, P.V.G. Reddy, R. Wilhelm, Synthesis (2008) 999. [23] Z. Du, Z. Li, Y. Deng, Synth. Commun. 35 (2005) 1343. [24] X. Li, D. Zhao, Z. Fei, L. Wang, Sci. China Ser. B: Chem. 49 (2006) 385.
[25] A. Fernicola, S. Panero, B. Scrosati, M. Tamada, H. Ohno, Chem. Phys. Chem. 8 (2007) 1103. [26] C. Thomazeau, H. Olivier-Bourbigou, L. Manga, S. Luts, B. Gilert, J. Am. Chem. Soc. 125 (2003) 5264. [27] C.W. Cho, T.P.T. Pham, Y.-C. Jeon, Y.-S. Yun, Green Chem. 10 (2008) 67. [28] S. Stolte, J. Arning, U. Botton-Weber, M. Matzke, F. Stock, K. Thiele, M. Uerdingen, U. Welz-Biermann, B. Jastorft, J. Ranke, Green Chem. 8 (2006) 621.
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Brønsted acid ionic liquid catalyzed formation of pyruvaldehyde dimethylacetal from triose sugars Shunmugavel Saravanamurugan, Anders Riisager ∗ Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e
i n f o
Article history: Received 15 April 2012 Received in revised form 25 June 2012 Accepted 4 July 2012 Available online 10 August 2012 Keywords: Ionic liquid Sulfonic acid Dihydroxyacetone Pyruvaldehyde dimethylacetal
a b s t r a c t A series of sulfonic acid functionalized ionic liquids (SO3 H-ILs) have been synthesized, characterized and investigated as catalysts for the conversion of the triose sugars, 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA), to pyruvaldehyde dimethylacetal (PADA) in methanol. Depending on the reaction conditions and the applied SO3 H-ILs a good yield of up to 52% of PADA was obtained. Under identical reaction conditions the derivative of PADA, 1,1,2,2-tetramethoxy propane (TMP), could be obtained in yields up to 49% using another SO3 H-IL. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Carbohydrates contained in biomass have in the last decade been established as important renewable feedstocks to make carbonaceous chemicals and fuels. Glucose is the most important compound in carbohydrates and it is readily available in abundance as cellulose polymer. Hence, transformations of glucose into bio-platform chemicals have in particular been extensively investigated. Isomerization of glucose to fructose is a key reaction in the industrial process to produce high-fructose corn syrup as sweetener and pave also a way to make a wide range of chemicals and fuels directly [1–4]. The direct conversion of glucose/fructose to lactic acid and 5-hydroxymethyl furfural (HMF) are important transformations because these bio-platform chemicals have a variety of applications. For example, HMF can be selectively oxidized into furan-2,5-dicarboxylic acid, which is an alternative chemical to terephthalic acid [5]. Similarly, transformation of C5 sugars to fuels and chemicals are also important, for example, dehydration of xylose to furfural and subsequent conversion into the value-added chemical tetrahydrofuran [1,6]. Another important transformation in carbohydrate chemistry is the conversion of triose sugars to pyruvic acid (in water) or pyruvaldehyde acetal (in alcohol). Acetals are generally important in organic synthesis where they are used as protecting groups for carbonyl compounds, since they are stable under basic conditions and in the presence of oxidizing and reducing agents
∗ Corresponding author. Tel.: +45 4525 2233. E-mail address: [email protected] (A. Riisager). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.07.001
[7–9]. They may also be important chemicals themselves like, for example, acetaldehyde diethylacetal which is used as a flavoring compound in distilled beverages. The typical synthetic route to acetals involves reaction of an oxo-containing compound with a dissolved acid catalyst in alcohol or the orthoformate compound [10]. However, recently it has been shown that ordinary solid acid zeolites are active catalysts for the dehydration of the trioses 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA) to form pyruvaldehyde (PA), which reacted further – depending on the nature of the acid sites on the catalyst – to form lactic acid in water or methyl lactate and pyruvaldehyde dimethylacetal (PADA) in methanol [11]. Ionic liquids (ILs) are low-melting organic salts that can be attractive alternatives to common organic solvents due to their non-measurable vapor pressure, relatively high thermal stability, tunable acidity or basicity as well as combined catalyst-solvent properties [12–15]. Importantly, the solubility of the polysaccharide cellulose can also be much higher in ILs than in common organic solvents – a feature that has intensively increased the use of IL catalyst-solvent systems for making bio-platform chemicals from carbohydrates [16–19]. Since classical ILs lack chemical functionality, new concepts have emerged where functional groups are integrated in the IL ions. In this context, Davis and coworkers reported in 2002 the first amine functionalized imidazolium-based ILs (task-specific) for CO2 capture [20]. Subsequently, sulfonic acid functionalized imidazolium-based ILs were also reported and investigated for Fischer esterification and pinacol rearrangement reactions with excellent reusability [21]. Since this pioneer work several other reports on the synthesis and application of new acid and base functionalized ILs have followed [22–25]. In our
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Scheme 1. The structures of the SO3 H-ILs.
previous reports, we have shown that sulfonic acid functionalized ILs were promising catalyst for the dehydration of fructose and glucose to make ethyl levulinate and ethyl-d-glucopyranoside in ethanol, respectively [3]. In the present study, we report the synthesis of a series of sulfonic acid functionalized imidazolium-, pyridinium- and ammonium-based ILs (SO3 H-ILs) with the cations: 1-methyl-3(4-sulfobutyl)-imidazolium ([BMIm-SO3 H][X], X = HSO4 , NTf2 , OMs and TfO), 1-(4-sulfobutyl)pyridinium ([BPyr-SO3 H][HSO4 ]) and N,N,N-triethyl-4-sulfobutanaminium ([NEt3 B-SO3 H][HSO4 ]). The structures of the SO3 H-ILs are depicted in Scheme 1. The SO3 H-ILs were tested as catalysts in the conversion of the trioses DHA and GLA into PADA using methanol as solvent under autogenic pressure. The influence of reaction parameters such as reaction time, temperature and concentration of DHA was optimized. 2. Experimental 2.1. Synthesis and characterization of SO3 H-ILs 1-methylimidazol (99%, Sigma–Aldrich) or pyridine (>99%, Sigma–Aldrich) or triethylamine (>99.5%, Fluka) (0.2 mol) and 1,4butanesultone (99%, Aldrich, 0.2 mol) were charged in a 100 ml round bottomed flask. The mixture was then stirred at 40–80 ◦ C for 10 h. The solid zwitterion formed was recovered by filtration, washed repetitively with diethyl ether until all unreacted reactants were completely removed (confirmed by NMR) and then dried under reduced pressure (15 mbar, 50 ◦ C) overnight. A stoichiometric amount of acid (98% H2 SO4 , >98% TfOH, 95% HNTf2 or >99.5% MsOH, Sigma–Aldrich) was subsequently added drop wise to the respective zwitterion and the mixture stirred at 80 ◦ C for 6 h. The obtained viscous ionic liquids were finally purified by extractive washing with diethyl ether and finally dried under reduced pressure (15 mbar, 50 ◦ C) overnight. Yields were above 95% for all ILs. The identity of the synthesized sulfonic acid functionalized ionic liquids was confirmed by NMR (Bruker AM360 NMR spectrometer, 25 ◦ C). The thermal decomposition temperature (Td ) of the SO3 HILs were measured by TGA analysis (TGA/DSC 1 apparatus, Mettler Toledo) by heating the ionic liquid (9–18 mg) in an aluminum sample holder from 40 ◦ C to 800 ◦ C with a heating ramp of 20 ◦ C/min under nitrogen atmosphere. 1-Methyl-3-(4-sulfobutyl)imidazolium trifluoromethanesulfonate ([BMIm-SO3 H][OTf]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.7 (s, 3H; N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D O): ı/ppm = 20.5, 27.9, 35.5, 49.0, 50.0, 121.9, 2 123.4, 131.0, 135.5; Td > 200 ◦ C. 1-Methyl-3-(4-sulfobutyl)imidazolium hydrogensulfate ([BMImSO3 H][HSO4 ]): 1 H NMR (300 MHz, D2 O) ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.7 (s, 3H; N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.5, 27.9, 35.5, 49.0, 50.0, 121.9, 123.4, 135.5; Td > 300 ◦ C. methanesulfonate 1-Methyl-3-(4-sulfobutyl)imidazolium ([BMIm-SO3 H][OMs]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H),
3.7 (s, 3H; N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.7, 27.9, 35.5, 38.3, 48.6, 49.9, 121.9, 123.4, 135.7; Td > 300 ◦ C. 1-Methyl-3-(4-sulfobutyl)imidazolium bis((trifluoromethyl)sulfonyl)amide ([BMIm-SO3 H][NTf2 ]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.6 (m, 2H; CH2 ), 1.8–1.9 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.7 (s, 3H, N CH3 ), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.7, 27.9, 35.2, 48.6, 49.8, 117.0, 121.9, 123.3, 135.3; Td > 200 ◦ C. N,N,N-triethyl-4-sulfobutaneammonium hydrogensulfate ([NEt3 B-SO3 H][HSO4 ]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.0–1.2 (t, 9H; 3CH3 ), 1.5–1.8 (m, 4H; 2CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 3.0–3.2 (m, 8H; 4CH2 N); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 6.5, 19.9, 21.0, 49.9, 52.3, 55.9; Td > 300 ◦ C. 1-(4-Sulfobutyl)pyridinium hydrogensulfate ([BPyrSO3 H][HSO4 ]): 1 H NMR (300 MHz, D2 O): ı/ppm = 1.5–1.7 (m, 2H; CH2 ), 1.9–2.1 (m, 2H; CH2 ), 2.7–2.8 (t, 2H; CH2 -SO3 H), 4.4–4.6 (t, 3H; N CH2 ), 7.8–8.0 (t, 2H; 2CH), 8.4 (t, 2H; 2CH), 8.7 (d, 1H; CH); 13 C NMR (75.5 MHz, D2 O): ı/ppm = 20.5, 28.9, 49.6, 60.7, 127.9, 143.8, 145.3; Td > 300 ◦ C. 2.2. Catalytic testing The catalytic reactions were carried out in 15 ml ace pressure tubes. 139.3 mg (1.5 mmol) of DHA (97%, Sigma–Aldrich) or 142.2 mg (1.5 mmol) of GLA (95%, Sigma–Aldrich), 0.11 mmol of SO3 H-IL, 30 mg of naphthalene (internal reference) and 4 g of methanol (>99.9%, Sigma–Aldrich) were charged into the ace pressure tube and heated under stirring at 120 ◦ C (oil bath temperature) for 24 h. 2.3. Reactant and product analysis The reaction mixtures were subjected to GC-FID analysis (Agilent 6890N instrument, HP-5 capillary column 30.0 m × 320 m × 0.25 m) as well as HPLC-RI analysis (Agilent 1200 series, 30 cm Aminex© HPX-87H column, 0.005 M H2 SO4 eluent, flow rate 0.6 ml/min). A GC–MS system (Agilent 6850 GC coupled with Agilent 5975C mass detector) was used for qualitative analysis. Conversion of the triose sugars DHA and GLA as well as yields of PA were determined by HPLC using standards made from commercial samples. The yield of pyruvaldehyde dimethylacetal (PADA) and 1,1,2,2 tetramethoxypropane (TMP) were calculated from GC results on series of PADA and TMP standards with naphthalene as internal standard. 3. Results and discussion The conversion of DHA and GLA to PADA was carried out with SO3 H-ILs as catalysts in methanol at 120 ◦ C and the results are presented in Table 1. Reaction of DHA with [BMIm-SO3 H][OMs] (Table 1, entry 1) yielded 41% of PADA and 45% of TMP along with more than 99% conversion of DHA with PA being observed as intermediate by HPLC. The yields of PADA and TMP were equally good or
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Table 1 Conversion of DHA and GLA to PADA and TMP.a Entry
Ionic liquid
Triose
Conversion (%)
PADA yield (%)
TMP yield (%)
1 2 3 4 5 6 7 8b
[BMIm-SO3 H][OMs] [BMIm-SO3 H][HSO4 ] [BMIm-SO3 H][OTf] [BPyr-SO3 H][HSO4 ] [BMIm-SO3 H][NTf2 ] [NEt3 B-SO3 H][HSO4 ] [BMIm-SO3 H][OMs] [BMIm-SO3 H][OMs]
DHA DHA DHA DHA DHA DHA GLA DHA
>99 >99 >99 >99 >99 >99 46 >99
41 44 52 46 52 39 6 67
45 41 35 45 38 49 9 11
See Section 2 for reaction conditions. The results obtained after three runs by adding fresh DHA after each run.
better (PADA: 39–52%, TMP: 35–49%) when using the other SO3 H-IL catalysts (Table 1, entries 2–6), and the highest yield of PADA (52%) was observed with [BMIm-SO3 H][OTf] and [BMIm-SO3 H][NTf2 ] ILs. As shown in Scheme 2, a plausible reaction pathway involves dehydration of DHA to PA which reacts with methanol to form PADA, which may further be reacted with methanol to form the methylated derivative TMP. Since the sulfonic acid functionalized ILs have strong Brønsted acid sites they are expected to easily dehydrate DHA to form PADA (as also observed), and in the presence of strong acid sites PADA can partially be converted further into TMP. High yields of PADA have been observed for fluorinecontaining SO3 H-ILs compared to other SO3 H-ILs due to the strong acid strength generated by inclusion of the very weakly basic [NTf2 ]− and [OTf]− anions. In our previous report, a similar effect has been observed for SO3 H-ILs for the conversion of fructose to ethyl levulinate in ethanol [3,26]. The triose sugar, GLA, has also been tested in the reaction with [BMIm-SO3 H][OMs] as catalyst under identical reaction conditions as DHA (Table 1, entry 7). Here, a low yield of PADA (6%) and TMP (9%) was observed along with only 46% conversion of GLA. These results reveal that the GLA did not isomerise effectively into DHA which is required in order to facilitate formation of the intermediate PA by dehydration, thus lowering the yield of PADA and TMP. Due to the ineffective isomerization, GLA might be degrading slowly in methanol in the presence of [BMIm-SO3 H][OMs] and the formation of unidentified products was observed in both GC and HPLC. This could account for the rest of the converted GLA apart from a low yield of PADA (6%) and TMP (9%). The combined yields of the two acetal products, PADA and TMP, remained in all reactions with the different SO3 H-ILs between 86% and 91%. Since no significant difference in combined yields were observed and to avoid fluorine containing SO3 H-ILs, [BMImSO3 H][OMs] was chosen as the IL for further optimization of other reaction parameters. In general, fluorine-containing ionic liquids are relatively more toxic compared to non-fluorine containing ILs [27,28]. The influence of reaction temperature on the product distribution has thus been studied with [BMIm-SO3 H][OMs] catalyst between 60 ◦ C and 120 ◦ C and the results are shown in Table 2. A relatively low yield of PADA (10%) and TMP (19%) was observed along with 61% conversion of DHA at 60 ◦ C (Table 2, entry 1). However, a significant amount of intermediate PA (22%) was observed, thus suggesting that even at moderate temperature the IL was able to catalyze the dehydration of DHA to PA. At a temperature of 80 ◦ C, Table 2 Influence of temperature on the yields of PADA and TMP.a Entry
Temperature (◦ C)
Conversion (%)
PADA yield (%)
TMP yield (%)
1 2 3 4
60 80 100 120
61 96 >99 >99
10 32 35 41
19 53 56 45
a
Triose = DHA, IL = [BMIm-SO3 H][OMs]. See Section 2 for reaction conditions.
the yield of PADA and TMP increased to 32% and 53%, respectively, and the DHA reach close to full conversion of 96% (Table 2, entry 2). Hence, at this temperature the PA, once formed, readily reacted with methanol to form PADA which further was transformed into TMP. A further increase in temperature to 100 ◦ C did only induce a very small change in the yields of PADA (35%) and TMP (56%) corresponding to all DHA being converted (Table 2, entry 3). At 120 ◦ C the yield of PADA increased to 41% whereas the yield of TMP decreased to 45%, implying that TMP could be hydrolyzed and the reaction reversed to form PADA at this high temperature. The influence of reaction time on the product distribution has also been studied with the [BMIm-SO3 H][OMs] IL and the results are illustrated in Fig. 1. After 10 min of reaction, the conversion of DHA was 36% and a significant amount of PA was formed (20%). The yields of PADA and TMP were 3% and 10%, respectively. This suggested that once PADA formed from PA, it immediately reacted with methanol to form TMP. The conversion of DHA increased to 60% along with 22% PA after 30 min, whereas the yield of PADA increased to a significant amount of 15% and TMP increased to 25%. After 1 h of reaction the yield of PADA and TMP further increased to 29% and 40%, respectively, while a significant amount of PA remained, thus suggesting the dehydration of DHA to PA to occur rapidly. When the reaction time was prolonged to 24 h the conversion of DHA was quantitative (>99%) and the yield of PADA (41%) and TMP (45%) were increased at the expense of PA. The [BMIm-SO3 H][OMs] IL catalyst was also applied when investigating the influence of DHA concentration on the product distribution at 120 ◦ C (Fig. 2). When the concentration of DHA was increased from 3.3 to 5 wt%, the yield of PADA increased from 40% to 55% but the yield of TMP decreased from 45% to 20% while the conversion of DHA remained high at 99%. At higher concentrations of DHA a further decrease in the yield of TMP was found
100
Conversion / Yield (%)
a b
DHA PA PADA TMP
80 60 40 20 0 0
1
2
3
4
5
6
24
Time (h) Fig. 1. Time-course study on the conversion of DHA to PADA and TMP (reaction conditions: DHA = 139.3 mg, [BMIm-SO3 H][OMs] = 0.11 mmol, naphthalene = 30 mg, methanol = 4.0 g, temperature = 120 ◦ C).
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Scheme 2. The plausible pathway for the formation of PADA and TMP from DHA and GLA.
resulting in less than 5% TMP when using 18 wt% DHA. In contrast, the yield of PA increased when more concentrated DHA solutions were used, while the PADA yield remained almost constant. Hence, when relatively large amounts of PA accumulated in reactions with high DHA loadings it was rather easily converted into PADA, while the subsequent methylation to TMP was limited, possibly due to unavailability of active sites. Interestingly, even at higher DHA concentration the [BMIm-SO3 H][OMs] IL catalyst was able to convert all DHA. In our previous work, we have shown that [BMIm-SO3 H][OMs] could be recycled in at least three runs without loss of any catalytic activity for the conversion of fructose to ethyl levulinate in ethanol. In the present study, however, the reusability of [BMImSO3 H][OMs] IL was investigated by adding fresh DHA after each run in methanol and the results are shown in Table 1. After the third run (Table 1, entry 8) the product distribution of PADA and TMP was significantly changed compared to the first run (Table 1, entry 1) as the yield of TMP was drastically reduced from 45% to 11% and the yield of PADA increased from 41% to 67%. This clearly indicated that the conversion reaction of PADA to TMP proceeded slowly after the first run thus lowering the yield of TMP. Even though there was a significant change in product distribution, the conversion of DHA was more than 99% and 14% of the intermediate PA was also formed
DHA
PA
PADA
TMP
4. Conclusions SO3 H-ILs are demonstrated to possess excellent catalytic activity in the conversion of the triose sugar DHA to PADA and the derivative TMP in methanol. The highest yield of PADA (52%) was found with [BMIm-SO3 H][OTf] and [BMIm-SO3 H][NTf2 ] ILs. A poor yield of PADA and TMP were, however, found for the triose isomer GLA due to low isomerization reactivity of the SO3 H-ILs toward DHA where from PADA was formed. At higher DHA concentrations the yield of TMP decreased while the yield of PADA remained good at 54% to 58%. Reuse of the [BMIm-SO3 H][OMs] catalyst revealed that the product distribution of PADA and TMP was significantly changed after a third run with the IL, but the conversion of DHA remained high at more than 99% and a significant amount of PA was still formed. Acknowledgement We thank the Danish Council for Independent ResearchTechnology and Production Sciences (project no. 10-081991) for financial support of the work. References [1] [2] [3] [4] [5]
100
Conversion / Yield (%)
implying that [BMIm-SO3 H][OMs] was able to dehydrate all DHA even after the third run.
80 60
[6] [7] [8] [9] [10]
40 20
[11]
0 3.3
5.0
9.0
13.0
18.0
DHA (wt %) Fig. 2. Influence of concentration of DHA on the yield of PADA and TMP (reaction conditions: DHA = 139.3 mg, [BMIm-SO3 H][OMs] = 0.11 mmol, naphthalene = 30 mg, methanol = 4.0 g, temperature = 120 ◦ C, time = 24 h).
[12] [13] [14] [15] [16] [17] [18]
A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411. M.S. Holm, S. Saravanamurugan, E. Taarning, Science 328 (2010) 602. S. Saravanamurugan, O.N.V. Buu, A. Riisager, ChemSusChem 4 (2011) 723. S. Bhosale, M. Rao, Microbiol. Rev. 60 (1996) 280. Y.Y. Gorbanev, S.K. Klitgaard, J.M. Woodley, C.H. Christensen, A. Riisager, ChemSusChem 2 (2009) 672. V. Choudhary, A.B. Pinar, S.I. Sandler, D. Gvlachos, R.F. Lobo, ACS Catal. 1 (2011) 172. E. Wenkert, T.E. Goodwin, Synth. Commun. 7 (1977) 409. A. Hassner, R. Wiederkehr, A.J. Kascheres, J. Org. Chem. 35 (1970) 1962. S.A. Patwardhan, S. Dev, Synthesis (1974) 348. A.F.B. Cameron, J.S. Hunt, J.F. Oughton, P.A. Wilkinson, B.M. Wilson, J. Chem. Soc. (1953) 3864. R.M. West, M.S. Holm, S. Saravanamurugan, J.M. Xiong, Z. Beversdorf, E. Taarning, C.H. Christensen, J. Catal. 269 (2010) 122. J.H. Huang, T. Ruther, Aus. J. Chem. 62 (2009) 298. M. Haumann, A. Riisager, Chem. Rev. 108 (2008) 1474. A. Riisager, R. Fehrmann, S. Flicker, R. Van Hal, M. Haumann, P. Wasserscheid, Angew. Chem. Int. Ed. 44 (2005) 815. T. Thananathanachon, T.B. Rauchfuss, Angew. Chem. Int. Ed. 49 (2010) 6616. H. Zhao, J.E. Holladay, H. Brown, Z.C. Zhang, Science 316 (2007) 1597. G.W. Huber, R.D. Cortright, J.A. Dumesic, Angew. Chem. Int. Ed. 43 (2004) 1549. G.W. Huber, A. Corma, Angew. Chem. Int. Ed. 46 (2007) 7184.
98
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[19] T. Ståhlberg, W. Fu, J.M. Woodley, A. Riisager, ChemSusChem 4 (2011) 451. [20] E.D. Bates, R.D. Mayton, J.H. Davis, J. Am. Chem. Soc. 124 (2002) 926. [21] A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weaver, D.C. Forbes, J.H. Davis, J. Am. Chem. Soc. 124 (2002) 5962. [22] A. Winkel, P.V.G. Reddy, R. Wilhelm, Synthesis (2008) 999. [23] Z. Du, Z. Li, Y. Deng, Synth. Commun. 35 (2005) 1343. [24] X. Li, D. Zhao, Z. Fei, L. Wang, Sci. China Ser. B: Chem. 49 (2006) 385.
[25] A. Fernicola, S. Panero, B. Scrosati, M. Tamada, H. Ohno, Chem. Phys. Chem. 8 (2007) 1103. [26] C. Thomazeau, H. Olivier-Bourbigou, L. Manga, S. Luts, B. Gilert, J. Am. Chem. Soc. 125 (2003) 5264. [27] C.W. Cho, T.P.T. Pham, Y.-C. Jeon, Y.-S. Yun, Green Chem. 10 (2008) 67. [28] S. Stolte, J. Arning, U. Botton-Weber, M. Matzke, F. Stock, K. Thiele, M. Uerdingen, U. Welz-Biermann, B. Jastorft, J. Ranke, Green Chem. 8 (2006) 621.