Thomas S.
Hansen
,
Jerrik
Mielby
and
Anders
Riisager
*
Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, DK-2800 Kgs. Lyngby,
Denmark. E-mail: ar@kemi.dtu.dk; thomh@kemi.dtu.dk; Tel: (+45) 45 25 22 33
First published on 30th November 2010
In combination with various salts boric acid, B(OH)3, was shown to be an efficient, weak Lewis acid catalyst in the aqueous dehydration of fructose to 5-hydroxymethylfurfural (HMF) due to strong complexation between the boron atom and the hexoses. In the dehydration of a highly concentrated aqueous fructose solution (30 wt%), a HMF yield of 60% was achieved at 92% fructose conversion and a HMF selectivity of 65% with 100 g L−1B(OH)3 and 50 g L−1sodium chloride in the aqueous phase and methyl-isobutylketone (MIBK) as extracting solvent. Furthermore, the dehydration of glucose resulted in a yield of 14% HMF at 41% glucose conversion after 5 h at similar conditions. These results are highly competitive with currently reported aqueous HMF dehydration systems. In combination with the non-corrosive and non-toxic nature of the boric acid, compared to other well known homogeneous catalysts applicable for the dehydration process (e.g., H2SO4 and HCl), clearly, the boric acid-salt mixture is a very attractive catalyst system.
An attractive compound that is obtainable directly from the dehydration of carbohydrates, such as fructose and glucose, is 5-hydroxymethylfurfural (HMF). The molecule constitutes a platform compound that can be converted into a wide range of industrially important chemicals.7,8 In this context, the oxidation product of HMF, 2,5-furandicarboxylic acid (FDA), is of specific interest as it can potentially replace terephthalic acid in polyester manufacturing (Scheme 1).9
Scheme 1 The reaction path from D-fructose to a terephthalic acid polyester analogue of 2,5-furandicarboxylic acid (FDA). |
Currently the most successful efforts to produce HMF have been done in ionic liquids,10–12 high-boiling organic solvents (e.g.dimethylsulfoxide, DMSO),13,14 by using strong and highly corrosive mineral acids such as HCl, H2SO4 and H3PO4 with an organic solvent to extract the HMF as it is produced in a two phase system5,15–17 or by employing microwaves as the heating source.18–20
Herein, we report our results from the development of a novel21 method to produce HMF from highly concentrated aqueous fructose solutions employing only a weak, non-toxic and non-corrosive acid, namely boric acid (pKa = 9.27, T = 293.15 K)22 as the dehydration catalyst.
During sucrose analysis the substrate was hydrolyzed to glucose and fructose on the HPLC column resulting in a poor elution profile on the HPLC chromatogram. Thus, in order to analyze experiments with sucrose, an aqueous 30 wt% solution was hydrolyzed at 50 °C for 6 h in 0.05 M HCl and subsequently analyzed viaHPLC. The resulting amount of glucose and fructose was used to determine the initial amount of sucrose.
Concentrations of products were determined from calibration curves obtained with reference samples and corrected with an internal standard, while product yields (%), product selectivities (%) and fructose conversions (%) were based on the initial concentration of sugars and calculated as:
The mass balances did not add up to 100% due to formation of insoluble polymers (humins), soluble polymers and other unidentified products during the dehydration process.
Scheme 2 Equilibria between boric acid and hexoses in water. |
In principle, the pH will gradually increase during the HMF dehydration reaction and return to a value determined by the boric acid-water equilibrium when all sugars are converted. However, by-product formation of formic acid (FA) and levulinic acid (LA) by competitive rehydration of HMF during the reaction27,28 prevent the pH from increasing. From a downstream HMF processing point of view, this phenomenon could prove very useful, as the isolation of LA and FA, which are also valuable by-products, results in a HMF product solution which is not nearly as acidic compared to using mineral acids. This is likely to make the overall HMF production process more economically attractive by reducing costs related to both neutralization and the use of corrosion resistant materials.
Fig. 1 Titration of a 1.88 M fructose solution and a 1.93 M glucose solution (both ∼30 wt%) with solid B(OH)3. Between additions of boric acid, the system was allowed sufficient time to stabilize before pH measurements. |
A rapid decrease in pH in the 30 wt% sugar solutions was observed upon the first boric acid addition which then levelled out as more B(OH)3 was added. The change was more pronounced for fructose than glucose suggesting a stronger complexation between B(OH)3 and fructose than with glucose, in accordance with previous findings.25
Fig. 2 Dehydration of 30 wt% fructose solutions with varied B(OH)3 concentrations and MIBK as extracting solvent (150 °C, 45 min, MIBK:aqueous volume ratio = 4:1). |
Using the reaction conditions where the intrinsic conversion to HMF was found to be minimal (i.e. 150 °C, 45 min, MIBK:aqueous volume ratio = 4:1), a set of experiments were conducted varying the catalyst amount in a 30 wt% fructose solution in order to investigate how the B(OH)3 concentration influenced the dehydration of highly concentrated fructose solutions (Fig. 2).
A clear relation between the B(OH)3 concentration and the fructose conversion was found, showing an increase in fructose conversion with increasing B(OH)3 concentration. The most commonly used catalysts for the dehydration process are strong acids, as previously mentioned, so increasing the amount of the weak Lewis acid B(OH)3 was expected to result in a higher fructose conversion and HMF yield.
Fig. 3 Dehydration of 30 wt% fructose solutions with MIBK as extracting solvent and no catalyst or added NaCl and/or B(OH)3 (150 °C, 45 min, MIBK:aqueous volume ratio = 4:1). |
The addition of 50 g L−1NaCl to the reaction mixture resulted in a HMF yield of 5% at 13% fructose conversion compared to 2% HMF at 5% fructose conversion without NaCl present. This weak catalytic effect is believed to arise from the slight acidification observed when salts are added to a concentrated solution of fructose. The dehydration reaction of fructose to HMF is highly dependent on the pH in solution, hence a small decrease in pH is expected to have a positive influence on the rate of fructose conversion and HMF formation.
Surprisingly the addition of NaCl in low concentrations together with B(OH)3 was found not only to increase the HMF yield, due to the salting-out effect as described by Román-Leshkov and Dumesic,16 but also to significantly increase the fructose conversion and HMF yield. Thus, reaction at 150 °C for 45 min with 50 g L−1NaCl, 100 g L−1B(OH)3 and MIBK as extracting solvent (MIBK:aqueous volume ratio = 4:1) resulted in a fructose conversion of 70% and 46% HMF yield (Fig. 3). The synergistic effect of using both B(OH)3 and NaCl is believed to occur as a result of the increased acidity of aqueous B(OH)3 solutions in combination with salts, suggesting that the salts stabilizes one or more of the charged intermediates and products shown in Scheme 2. The acidifying effect of different salts on boric acid has been correlated to the energy of hydration by Shishido29 and shows an increase in acidity with increasing hydration energy. As already mentioned, the addition of salt to a concentrated fructose solution also results in a slight acidification of the solution. Hence, several factors add up in the observed synergy and complex behavior of NaCl and B(OH)3 in the dehydration of aqueous fructose solutions to HMF.
An experiment was conducted in order to find the highest obtainable HMF yield from a 30 wt% aqueous fructose solution near full substrate conversion with the NaCl-containing catalytic system. Increasing the reaction time to 90 min subsequently resulted in a 92% fructose conversion, a 60% HMF yield and a HMF selectivity of 65%.
A range of other salts, primarily containing alkali or alkaline earth metal cations, were further employed in the dehydration of fructose in order to investigate the influence of the salts on the synergistic interplay with B(OH)3 (Table 1).
Entry | Salt | Fructose conversion (%) | HMF yield (%) | HMF selectivity (%) | R valueb (MIBK:aq) |
---|---|---|---|---|---|
a Reaction conditions: 100 g L−1B(OH)3, 0.87 M salt with respect to the anion, 45 min, 150 °C, MIBK:aqueous volume ratio = 4:1. b The R value is the HMF distribution obtained between the MIBK phase and the aqueous phase, i.e.[HMF]MIBK/[HMF]aq. | |||||
1 | LiCl | 69 | 45 | 66 | 1.1 |
2 | LiBr | 61 | 38 | 62 | 1.0 |
3 | LiNO3 | 49 | 21 | 42 | 0.9 |
4 | NaCl | 70 | 46 | 65 | 1.0 |
5 | NaBr | 60 | 38 | 64 | 0.9 |
6 | NaNO3 | 49 | 20 | 41 | 0.9 |
7 | Na2SO4 | 90 | 41 | 45 | 1.7 |
8 | KCl | 67 | 44 | 65 | 1.0 |
9 | KBr | 63 | 39 | 62 | 0.9 |
10 | KI | 56 | 35 | 63 | 0.7 |
11 | KNO3 | 49 | 20 | 40 | 0.8 |
12 | K2SO4 | 89 | 40 | 46 | 1.5 |
13 | MgCl2 | 81 | 52 | 65 | 1.1 |
14 | AlCl3 | 100 | 21 | 21 | 1.1 |
15 | FeCl3 | 99 | 36 | 36 | 1.1 |
A clear relationship between the nature of the anion and the resulting fructose conversion and HMF yield was observed. Generally, sulfates (entries 7 and 12) resulted in high fructose conversions and a high HMF distribution in the organic phase but low HMF selectivities and yields. The phase distribution selectivity is commonly expressed as the R value (calculated as [HMF]org/[HMF]aq), which quantify the extracting power of the organic solvent. In contrast, the nitrates (entries 3, 6 and 11) resulted in lower fructose conversions, HMF yields, HMF selectivities and R values compared to the halide salts, which were shown to be superior when comparing HMF yields and selectivities. The size of the halide anions did not seem to affect the selectivity towards HMF, whereas the fructose conversion rate decreased in the order: chloride > bromide > iodide.
The nature of the alkali cation did also not seem to have great importance on the dehydration reaction and only small variations in HMF yields and selectivities within the estimated experiment error were observed. Similar trends were observed in the control experiments conducted without B(OH)3 in order to test the activity of the salts alone (results not presented here).
Interestingly, the high R values, representing the distribution of HMF in the organic phase relative to the aqueous phase, observed in experiments with sulfate salts did not result in correspondingly high relative HMF yields and selectivities, as would be expected when the mean residence time of HMF in the aqueous solution was reduced due to diminished rehydration. The same conclusion was reached by Román-Leshkov and Dumesic16 in their investigation of fructose dehydration reactions employing different salts and a strong mineral acid as the dehydration catalyst, thus suggesting the sulfate anion has some influence on the dehydration reactivity, e.g. shifting the pyranose-furanose equilibrium.
In combination, the strong Lewis acidic salts, AlCl3 or FeCl3, and B(OH)3 (entries 14 and 15) resulted in formation of large amounts of polymers and the rehydration products FA and LA, demonstrating that HMF was no longer the favored product.
Fig. 4 Dehydration of 30 wt% fructose solutions with organic extraction solvents, 100 g L−1B(OH)3 and 50 g L−1NaCl (150 °C, 45 min unless otherwise mentioned, organic:aqueous volume ratio = 4:1). |
Reactions with THF as extraction solvent were found to be substantially slower than analogous reactions with MIBK. This could be attributed to an effective decrease in the fructose concentration at elevated temperatures as a result of miscibility of THF and saline aqueous mixtures, or an increased ability of THF to extract the produced organic acids compared to MIBK. However, when increasing the reaction time with THF as extracting solvent comparable HMF yields and selectivities to experiments with MIBK were obtained.
THF is an interesting extraction solvent for the dehydration reaction due to avoidance of humin formation which, if formed, would impose a severe drawback from a process point of view. Although the apparent HMF selectivity with THF was not significantly higher than experiments with visible humin formation, the selectivity was observed to increase with increasing reaction time, most likely due to reversion of isomeric and dimer forms of fructose. The disadvantage and concern of applying THF is obviously its low flashpoint (−14 °C), its tendency to form peroxides over time and the relatively high chemical aggressiveness of THF fumes. Unfortunately, the latter made longer term experiments (>75 min) impossible with our available apparatus.
Entry | Catalyst | Time (min) | Glucose conversion (%) | HMF yield (%) | HMF selectivity (%) |
---|---|---|---|---|---|
a Reaction conditions: 150 °C, MIBK:aqueous volume ratio = 4:1. b Reaction conditions: 100 g L−1B(OH)3, 50 g L−1NaCl, 150 °C, MIBK:aqueous volume ratio = 4:1. | |||||
1a | — | 45 | <1 | 0 | 0 |
2b | B(OH)3 + NaCl | 45 | 8 | 2 | 25 |
3a | — | 180 | 13 | 1 | 10 |
4b | B(OH)3 + NaCl | 180 | 36 | 10 | 27 |
5a | — | 300 | 24 | 3 | 13 |
6b | B(OH)3 + NaCl | 300 | 41 | 14 | 34 |
The dehydration of glucose is much more difficult than the dehydration of fructose, presumably because the first step in the Lobry de Bruyn-van Ekenstein transformation30 of glucose to fructose proceeds rapidly in basic media, but very slowly in acidic media where HMF production is possible. Furthermore the HMF selectivity increased over time indicating that some intermediate hexoses formed from glucose were able to revert back to fructose and dehydrate to HMF.
Boric acid in combination with NaCl was found to be less efficient for the dehydration of glucose in water, which is also generally considered to be a more challenging task to achieve. Hence, only poor HMF yields were accomplished. However, the catalytic system developed here performed much better than the un-catalyzed reaction and could also be applied for highly concentrated aqueous sucrose solutions.
This study is, to the best of our knowledge, the first to demonstrate the use of boric acid in the dehydration of sugars to HMF and to provide a detailed parameter study of the dehydration process for fructose. A simple modification of the catalytic system by addition of salt was shown to have a synergistic effect of the dehydration of fructose to HMF allowing good yields and selectivities of HMF to be reached.
Since boric acid is a weak Lewis acid, non-toxic, cheap and already widely used in industrial processes it is desirable from an industrial point of view compared to other mineral acids such as HCl and H2SO4, which are highly corrosive. This could clearly make future implementation of B(OH)3 in HMF production attractive compared to other acids.
Footnote |
† Electronic supplementary information (ESI) available: Tables with experimental values reported in the figures. See DOI: 10.1039/c0gc00355g |
This journal is © The Royal Society of Chemistry 2011 |