Abstract
The spice flavour compound 2, 2′-difurfuryl ether (DFE) is widely utilised in the food industry as it has a coffee-like, nutty, earthy, mushroom-like odour. However, despite intensive research efforts, to date, an environmentally friendly and practical synthetic preparation technique for 2, 2′-difurfuryl ether is still unavailable. Here, we investigate a new approach using polyoxometalate catalysts to selectively catalytically dehydrate furfuryl alcohol to 2, 2′-difurfuryl ether. We have successfully applied this methodology using the polyoxometalate (POMs) catalyst {[(CH3CH2CH2CH2)4N]2[SMo12O40]} to produce 2,2′-difurfuryl ether in a 30.86% isolated yield.
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Introduction
Furfuryl alcohol (FA) is considered as an important template chemical for the production a range of useful chemicals, such as levulinic acid1, alkyl levulinate2 and various other useful polymer products3,4. FA is synthesised by a selective hydrogenation process from furfural and its conversion into oligomer (Oligomerized FA, OFA) and polymer (Polymerized FA, PFA) products has been widely explored owing to their utility in a range of applications5. Several molecular structures of OFAs and PFAs have been proposed using a combination of NMR6, IR7, UV–Vis8, Raman spectroscopy9, XRD10 and DFT calculations11. From these studies a variety of dimer products have been proposed including: 2,2′-difurfuryl ether (DFE); 2,2′-difurylmethane (DFM); 2,2′-difuryl-ethylene (DFEt) and a hydroxyl-carbon bridge dimer12,13. Further examples include, 4-furfuryl-2-pentenoic acid γ-lactone (PAL) which can be produced over γ-alumina during FA polymerisation and 2-hydroxymethyl-5(5-furfuryl) furan (HFF) which is a themaleic anhydride product14. However, studies reveal that HFF and PAL cannot co-exist in either acid-polymerized or γ-alumina-polymerized FA, although analytical results were not enough to support PAL existence14.
A particularly valuable chemical product of FA is 2,2′-difurfuryl ether (DFE)15, which is a spice flavour compound with an aroma described as a mixture of coffee and mushroom scents combined with nutty and earthy odours16,17. It can be eaten according to the Flavour Extract Manufacturers′ Association (FEMA), Joint FAO/WHO Expert Committee on Food Additives (JECFA) and National Health and Family Planning Commission of PRC (NHFPC) regulation guidelines. DFE is referred to by the FEMA No. 3337, the JECFA No. 1522 and the Chinese Standards for Food Additives No. S1108.
DFE is synthesised from FA in a two-step process comprised of bromination followed by etherification (Fig. 1)18. However, this particular synthetic method poses significant environmental hazards, such as pollution, and thus, a search for an alternative cleaner, safer and more environmentally friendly approach is a key priority19,20. Interestingly, DFE can also be obtained as a side-product during FA oligomerization reactions over heterogeneous catalysts21. Polyoxometalate (POMs) catalysts are one example of a heterogeneous catalyst which could be used for this purpose; however, to the best of our knowledge, very little quantitative analysis information is available on the presence of DFE during such FA oligomerization reactions. Indeed, we have previously reported, the successful synthesis of another flavour compound (−)-Ambrox, which was prepared using (−)-sclareol as a starting material which was oxidised using hydrogen peroxide in the presence of the POMs catalyst {[C5H5NC16H33][H2PMo12O40]}, which is a quaternary ammonium phosphomolybdate catalyst22. Therefore, in this study, we investigate the feasibility of using selective catalytic dehydration of furfuryl alcohol in the presence of various POM catalysts to produce 2, 2′-difurfuryl ether - thus producing a more environmentally friendly synthetic approach.
The two-step synthesis of DFE from FA via bromination and etherification reactions.
Results and Discussion
With respect to FA oligomerization reactions, the catalyst tungsten oxide in the liquid phase (100 °C) has been successfully employed to produce a range of OFAs. These include: five dimers (2,2′-difurylmethane, 2-(2-furylmethyl)-5-methylfuran, difurfuryl ether, 4-furfuryl-2-pentenoic acid γ-lactone and 5-fufuryl-furfuryl alcohol) and two trimers (2,5-difurfurylfuran and 2,2′-(furylmethylene)-bis(5-methylfuran)) were observed, difurfuryl ether and 5-Furfuryl-furfuryl alcohol were the dominant products23,24,25. Another class of catalysts are POMs, which are discrete metal-oxide clusters containing W, Mo, V or Nb that have attracted increasing interest owing to their multi-electronic redox activities, and photochemical, acidic and magnetic properties. Importantly, there are a wide range of potential applications that POMs can be envisaged for, such as catalysts and functional materials26.
As with all catalysis, the first step in utilising POMs for the selective catalytic dehydration of furfuryl alcohol to 2, 2′-difurfuryl ether, will be to choose an appropriate POM catalyst. For thus, a series of POMs catalysts were prepared as summarised in Table 1 27,28,29,30,31. In order to relatively assess the utility of these synthetic catalysts a set of standard experimental conditions was employed (i.e., in toluene at 100 °C for 7 h). The results are given in Table 2, revealing catalytic activities in the following order: sulfo-polyoxometalates > quaternary ammonium phosphomolybdates > quaternary ammonium phosphotungstates and heteropolyacid salts. With respect to the heteropolyacid salts, the catalysts 4 d and 4 h showed greater yields (entry 4, 8 Table 2) than the other heteropolyacid salt catalysts (entry 1–8 Table 2). We also found that the heteropolyacid Al3+ salts showed a much better catalytic ability than the Na+, K+ and Fe3+ salts. Furthermore, of the quaternary ammonium phosphomolybdates with the same phosphomolybdic group, we found that the character of the quaternary ammonium cation groups have a very limited influence on the catalytic activity (entries 13–17, Table 2). Moreover, although Mo and W belong to the same main group, they display difference catalytic activities in this reaction. We also found that the quaternary ammonium phosphomolybdates usually displayed better catalytic ability (entries 13–17, Table 2) than the quaternary ammonium phosphotungstates (entries 9–12, Table 2). Overall, of all the POMs tested, the sulfo-polyoxometalate catalyst 4r ({[(CH3CH2CH2CH2)4N]2 [SMo12O40]}) gave the best yield (26.90%; entry 18, Table 2), and the product was readily isolated and purified.
Whilst POMs were known as effective catalysts, reports generally focus on their chemical oxidation, electrochemical oxidation, reduction reactions, photochemical oxidation, base catalysed reactions, acid catalysis and other reaction potential32. In this study, the reasons these different POMs catalysts showed different activities on this selective catalytic dehydration reaction were unclear.
In order to optimise the synthetic conditions for DFE using the 4r POMs catalyst, we systematically varied the parameters of catalyst quantity and reaction time. The amount of catalyst 4r in the reaction was optimised firstly (entries 1–10, Table 3). We found that DFE was produced in the highest yield (26.90%) when 1% equivalent of the catalyst was used (entry 6, Table 3). The yield decreased significantly, from 26.90% to 8.29%, when the catalyst loading was lowered from 1% to 0.1% equivalents, whereas the yield did not increase with incremental catalyst loading from 1% to 5% equivalents. We subsequently optimised the reaction time, the results were shown in Table 3 (entries 11–20). We found that the DFE yield increased gradually with extended reaction times from 1 h to 9 h (entries 11–19, Table 3), however, the yield did not increase furthermore up to 10 h (entries 19, 20, Table 3). Overall, the optimised conditions for DFE synthesis are a reation time of 9 h at 100 °C with a 1% equivalent of 4r catalyst, resulting in a yield of 34.50% (entries 19, Table 3). The reaction was repeated under the above optimised conditions and 2,2′-difurfuryl ether (DFE) was obtained in an average isolated yield of 30.86%16,17.
As per previous literature preparations of DFE25, other compounds appear in the oligomerization reaction (Fig. 2, Figure S1), as determined by GC/MS. As shown in Table 4, these include: compound 5 (5–furfuryl–furfuryl alcohol, Figure S5); compound 6 (2, 2′–difurylmethane, Figure S6) and compound 7 (2, 5–difurfurylfuran, Figure S7). Although other compounds have been proposed as side-products in such reactions, we found no evidence of them under our experimental and equipment conditions.
The selective catalytic dehydration process converting furfuryl alcohol to 2,2′-difurfuryl ether using a polyoxometalate (POM) catalyst.
As shown in Tables 3 and 4, the reaction time has an obvious influence on the yields of compound 4 (DFE), compound 5, compound 6 and compound 7. As expected, the yields of compound 5, compound 6 and compound 7 decrease and yields of compound 4 increases with reaction time. The yield of compound 5 increased gradually with extended reaction times from 1 h to 7 h (entries 1–7, Table 4), but decreased with reaction time from 7 h to 10 h (entries 7–10, Table 4). Compound 5 was obtained in the highest yield of 20.30% after 7 h (entries 7, Table 4). The yield of compound 6 increased gradually with extended reaction times from 1 h to 6 h (entries 1–6, Table 4), but the yield decreased with reaction time from 6 h to 10 h (entries 6–10, Table 4). Compound 6 has the highest yield of 23.20% after 6 h (entries 6, Table 4). The yield of compound 7 increased gradually with extended reaction times from 1 h to 7 h (entries 1–7, Table 4), but the yield decreased with the increment of reaction time from 7 h to 10 h (entries 7–10, Table 4). Compound 7 has highest yield of 10.86% after 7 h (entries 7, Table 4). Therefore, it was fortunate that compound 4 (DFE) was obtained in the highest yield of 34.50% after 9 h (entries 19, Table 3). These results clearly illustrate that catalyst 4r was a strong candidate as a heterogeneous catalyst for the selective catalytic dehydration of FA to DFE.
Conclusions
In this paper, a comprehensive study on the utility of POMs catalysts for the selective catalytic dehydration of furfuryl alcohol to 2, 2′-difurfuryl ether has successfully been carried out. Through assessing a range of potential POMs catalysts, we found that {[(CH3CH2CH2CH2)4N]2[SMo12O40]} was the most effective, accomplishing the reaction in an overall 30.86% yield. Thus, we have present a novel synthetic avenue for the efficient and environmentally benign synthesis of 2, 2′-difurfuryl ether, which employs a inexpensive and simple POMs catalyst. Further studies are underway to further improve the yield of 2, 2′-difurfuryl ether using other POMs catalysts and various synthetic conditions.
Methods
Synthesis of the catalysts a-h
All of the catalysts a-h were synthesised by the same approach. This method is illustrated following for catalyst 4a as an example.
A solution of H3PW12O40 (2.88 g, 1 mmol) in deionized water (10 mL) was added into a 50 mL beaker. The reaction mixture was stir for 5 min at 25 °C, and Na2CO3 (1.06 g, 10 mmol) in deionized water (10 mL) was added over 5 min. After addition, the mixture was stir for 1 h at 25 °C, then filtered and washed with deionized water and dried in vacuo and subsequently calcined at 450 °C for 2 h to afford 4a as a white solid (2.38 g, 81%)33. The elemental analysis data for the purified salts were as follows.
Calculated for 4a Na3PW12O40: Na, 2.34; P, 1.05; W, 74.88%. Found: Na, 2.37; P, 1.11; W, 74.79%.
Calculated for 4b FePW12O40: Fe, 1.90; P, 1.06; W, 75.22%. Found: Fe, 1.88; P, 1.09; W, 75.29%.
Calculated for 4c K3PW12O40: K, 3.92; P, 1.03; W, 73.68%. Found: K, 3.95; P, 1.07; W, 73.69%.
Calculated for 4d AlPW12O40: Al, 0.93; P, 1.07; W, 75.97%. Found: Al, 0.90; P, 1.07; W, 76.01%.
Calculated for 4e Na3PMo12O40: Na, 3.65; P, 1.64; Mo, 60.88%. Found: Na, 3.61; P, 1.69; Mo, 60.81%.
Calculated for 4 f FePMo12O40: Fe, 2.97; P, 1.65; Mo, 61.30%. Found: Fe, 2.93; P, 1.60; Mo, 61.41%.
Calculated for 4 g K3PMo12O40: K, 6.05; P, 1.60; Mo, 59.36%. Found: K, 6.11; P, 1.58; Mo, 59.43%.
Calculated for 4 h AlPMo12O40: Al, 1.46; P, 1.67; Mo, 62.26%. Found: Al, 1.51; P, 1.69; Mo, 62.20%.
Synthesis of the catalysts 4i-q
Synthesis of catalysts was illustrated by the synthesis of catalyst 4n.
H3P Mo12O40 (1.82 g, 1 mmol) and deionized water (10 mL) were combined in a 50 mL three-neck flask. The mixture was stirred for 5 min at 25 °C and further cetylpyridinium chloride (0.36 g, 1 mmol) in deionized water (10 mL) was added after 5 min, then the mixture was stirred for 3 h at 25 °C. When filtered, the filtrate cake was washed with liquid and dried by vacuum to produce 4n (1.76 g, 82%) as a dark green solid. The elemental analysis data for the purified salts were as follows.
Calculated for 4i {[(CH3)4N][H2PW12O40]}: C, 1.63; H, 0.48; N, 0.47; P, 1.05; W, 74.70%. Found: C, 1.59; H, 0.47; N, 0.50; P, 1.09; W, 74.73%.
Calculated for 4j {[(CH3)3C16H33N][H2PW12O40]}: C, 7.21; H, 1.40; N, 0.44; P, 0.98; W, 69.73%. Found: C, 7.20; H, 1.43; N, 0.41; P, 1.02; W, 69.71%.
Calculated for 4k {[C5H5NC16H33][H2PW12O40]}: C, 7.92; H, 1.27; N, 0.44; P, 0.97; W, 69.30%. Found: C, 7.94; H, 1.29; N, 0.43; P, 0.96; W, 69.34%.
Calculated for 4 l {[(CH3CH2CH2CH2)4N][H2PW12O40]}: C, 6.16; H, 1.23; N, 0.45; P, 0.99; W, 70.67%. Found: C, 6.16; H, 1.25; N, 0.44; P, 0.98; W, 70.63%.
Calculated for 4 m {[(CH3)4N][H2PMo12O40]}: C, 2.53; H, 0.74; N, 0.74; P, 1.63; Mo, 60.65%. Found: C, 2.51; H, 0.77; N, 0.75; P, 1.62; Mo, 60.69%.
Calculated for 4n {[(CH3)3C16H33N][H2PMo12O40]}: C, 10.82; H, 2.10; N, 0.66; P, 1.47; Mo, 54.59%. Found: C, 10.78; H, 2.07; N, 0.64; P, 1.50; Mo, 54.55%.
Calculated for 4o {[C5H5NC16H33][H2PMo12O40]}: C, 11.85; H, 1.89; N, 0.66; P, 1.46; Mo, 54.08%. Found: C, 11.81; H, 1.92; N, 0.65; P, 1.44; Mo, 54.12%.
Calculated for 4p {[(CH3CH2CH2CH2)4N][H2PMo12O40]}: C, 9.30; H, 1.85; N, 0.68; P, 1.50; Mo, 55.71%. Found: C, 9.34; H, 1.83; N, 0.69; P, 1.53; Mo, 55.69%.
Calculated for 4q {[C5H5NC16H33]2[HPMo12O40]}: C, 20.74; H, 3.19; N, 1.15; P, 1.27; Mo, 47.33%. Found: C, 20.70; H, 3.16; N, 1.17; P, 1.26; Mo, 47.37%.
Synthesis of the catalyst 4r
A solution of Na2MoO4·2H2O (6.05 g, 25 mmol) in deionized water (200 mL) was added into a 500 mL beaker. The reaction mixture was stir for 5 min at 25 °C, and then NH4VO3 (0.6 g, 5.1 mmol) in H2SO4 (50 mL, 2 mol/L) was added. The reaction mixture was stir for 5 min, then CH3COCH3 (250 mL) was added. After stirring for 1 h at 25 °C, tetrabutylammonium bromide (10 g, 31 mmol) was added. After addition, the mixture was stir for 0.5 h at 25 °C, then filtered, washed with deionized water, ethanol and acetonitrile, and dried in vacuo to afford 4r as a yellow solid (3.60 g, 75%)29. The elemental analysis data for the purified salts were as follows. Calculated for 4r {[(CH3CH2CH2CH2)4N]2[SMo12O40]}: C, 16.65; H, 3.14; N, 1.21; S, 1.39; Mo, 49.88%. Found: C, 16.66; H, 3.12; N, 1.23; S, 1.42; Mo, 49.87%.
Synthesis of the DFE
Each of the catalysts were employed, respectively, for this reaction and the overall synthetic conditions are illustrated following using 4r as an example (Fig. 3).
The synthesis of 2,2′-difurfuryl ether using catalyst 4r [(C4H9)4N]2SMo12O40.
FA (0.98 g, 10 mmol), 4r (0.23 g, 0.1 mmol, 1% equiv.) and toluene (10 mL) were added into a 50 mL three-neck flask. The mixture was stirred for 9 h at 100 °C. The toluene was subsequently removed under reduced pressure. The residue was extracted with ether, the organic phases were then washed with a saturated solution of Na2CO3 and brine and then dried over MgSO4. After solvent removal, the residue was purified by flash chromatography on silica gel (petroleum/EtOAc, 40:1) to afford DFE as a colourless liquid (0.55 g, 30.86%).
1H NMR (300 MHz, CDCl3) δ: 4.48 (4 H, s, -CH2-O), 6.34(4 H, s, -CH = CH-), 7.42(2 H, d, J = 0.9 Hz, C = CH-O) (Figure S2).
13C NMR (75 MHz, CDCl3) δ: 63.38, 109.54, 110.19, 142.81, 151.30 (Figure S3).
MS (ESI), m/z: 178.1 [M] +, 147.0, 119.0, 91.1, 53.1 (Figure S4).
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Acknowledgements
This work was supported by National key research and development program (2016YFD0400801), General funding project of Beijing Education Commission Research Project (SQKM201610011003), the National Natural Science Foundation of China (31571886).
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S.X.Y., H.Y.T. and B.G.S. onceived of the project and designed the experiments. S.X.Y. wrote most of the paper. Y.F.H., J.L.W., Y.M.Z., H.W. and Y.G.L.carried out the experiments, analysed the data. All authors discussed the results and commented on the manuscript.
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Yang, S., Hao, Y., Wang, J. et al. Selective catalytic dehydration of furfuryl alcohol to 2, 2′-difurfuryl ether using a polyoxometalate catalyst. Sci Rep 7, 12954 (2017). https://doi.org/10.1038/s41598-017-13472-3
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DOI: https://doi.org/10.1038/s41598-017-13472-3
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