Selective catalytic dehydration of furfuryl alcohol to 2, 2'-difurfuryl ether using a polyoxometalate catalyst.

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.

Introduction

class="Chemical">Furfuryl alcohol (FA) is coclass="Chemical">nsidered as aclass="Chemical">n importaclass="Chemical">nt template chemicclass="Chemical">n class="Chemical">al for the production a range of useful chemicals, such as levulinic acid[1], alkyl levulinate[2] and various other useful polymer products[3,4]. 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 applications[5]. Several molecular structures of OFAs and PFAs have been proposed using a combination of NMR[6], IR[7], UV–Vis[8], Raman spectroscopy[9], XRD[10] and DFT calculations[11]. 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 dimer[12,13]. 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 product[14]. 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 existence[14].

A particularly vclass="Chemical">aluable chemicclass="Chemical">n class="Chemical">al 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 odours[16,17]. 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.

class="Chemical">DFE is syclass="Chemical">nthesised from FA iclass="Chemical">n a two-step process comprised of bromiclass="Chemical">natioclass="Chemical">n followed by class="Chemical">n class="Chemical">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 priority[19,20]. Interestingly, DFE can also be obtained as a side-product during FA oligomerization reactions over heterogeneous catalysts[21]. 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 catalyst[22]. 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.

Figure 1

The two-step synthesis of DFE from FA via bromination and etherification reactions.

The two-step synthesis of n class="Chemical">DFE from FA via bromiclass="Chemical">natioclass="Chemical">n aclass="Chemical">nd class="Chemical">n class="Chemical">etherification reactions.

Results and Discussion

With respect to FA oligomerization reactions, the catclass="Chemical">alyst class="Chemical">n class="Chemical">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 products[23-25]. 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 materials[26].

As with class="Chemical">all catclass="Chemical">n class="Chemical">alysis, 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 [27-31]. 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.

Table 1

Synthesis of the catalysts.

Entry	Catalyst 4	Chemical compositions of catalyst	Yield (%)	IR (cm−1)
1	4a	Na3PW12O40	81	1079,976,895, 802
2	4b	FePW12O40	85	1063,968,897, 807
3	4c	K3PW12O40	79	1079,976,895, 802
4	4d	AlPW12O40	82	1076,981,897, 803
5	4e	Na3PMo12O40	80	1063,964,893, 802
6	4 f	FePMo12O40	86	1067,961,893, 802
7	4 g	K3PMo12O40	77	1092,964,893, 802
8	4 h	AlPMo12O40	80	1064,961,869, 782
9	4i	{[(CH3) 4N][H2PW12O40]}	88	2922,1851,1635,1486,1079,976,895,802
10	4j	{[(CH3) 3C16H33N][H2PW12O40]}	80	2922,2851,1623,1481, 1062,959,879,803
11	4k	{[C5H5NC16H33][H2PW12O40]}	76	2922,2851,1635,1486, 1079,976,895,802
12	4 l	{[(CH3CH2 CH2 CH2)4N][H2PW12O40]}	83	2971,2867,1615,1474, 1080,976,894, 816
13	4 m	{[(CH3) 4N][H2P Mo12O40]}	88	2922,2851,1635,1471, 1062,956,880,798
14	4n	{[(CH3) 3C16H33N][H2P Mo12O40]}	72	2922,2851,1671,1471, 1080,977,897, 805
15	4o	{[C5H5NC16H33][H2PMo12O40]}	82	2922,2851,1635,1486, 1079,976,895, 802
16	4p	{[(CH3CH2 CH2 CH2)4N][H2P Mo12O40]}	70	2922,2851,1671,1471, 1080,977,897, 805
17	4q	{[C5H5NC16H33]2[HPMo12O40]}	78	2921,2851,1640,1478, 1062, 961, 879, 794
18	4r	{[(CH3CH2CH2CH2)4N]2[SMo12O40]}	75	2921,2851,1634,1488, 1079, 976, 895, 799

Table 2

Optimization of the catalyst.

Entry	Catalyst	DFE yield (%)	Entry	Catalyst	DFE yield (%)
1	4a	2.08	10	4j	7.62
2	4b	5.41	11	4k	6.46
3	4c	7.85	12	4 l	1.41
4	4d	13.30	13	4 m	17.92
5	4e	3.55	14	4n	16.46
6	4 f	3.97	15	4o	16.73
7	4 g	4.90	16	4p	14.06
8	4 h	14.58	17	4q	14.20
9	4i	13.09	18	4r	26.90

*Reaction conditions: FA (10 mmol), catalyst (0.1 mmol), toluene (10 mL), 100 °C and 7 h. GC yield.

Synthesis of the catn class="Chemical">alysts.

Optimization of the catn class="Chemical">alyst.

*Reaction conditions: FA (10 mmol), catclass="Chemical">alyst (0.1 mmol), class="Chemical">n class="Chemical">toluene (10 mL), 100 °C and 7 h. GC yield.

Whilst class="Chemical">POMs were kclass="Chemical">nowclass="Chemical">n as efclass="Chemical">n class="Chemical">fective catalysts, reports generally focus on their chemical oxidation, electrochemical oxidation, reduction reactions, photochemical oxidation, base catalysed reactions, acid catalysis and other reaction potential[32]. 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 class="Chemical">DFE usiclass="Chemical">ng the class="Chemical">n class="Chemical">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].

Table 3

Optimization of the Reaction Conditions using the 4r catalyst.

Entry	Catalyst amount (mmol)*	Yield (%)	Entry	Reaction time **	Yield (%)
1	0.01	8.29	11	1 h	10.58
2	0.03	14.16	12	2 h	10.99
3	0.05	15.72	13	3 h	15.51
4	0.07	20.65	14	4 h	16.28
5	0.09	23.16	15	5 h	18.47
6	0.1	26.90	16	6 h	19.40
7	0.2	26.12	17	7 h	26.90
8	0.3	26.08	18	8 h	27.24
9	0.4	25.24	19	9 h	34.50
10	0.5	25.07	20	10 h	34.25

*Reaction conditions: FA (10 mmol), catalyst 4r (relative equiv.), toluene (10 mL), 100 °C and 7 h. GC yield. **Reaction conditions: FA (10 mmol), catalyst 4r (0.01 equiv.), toluene (10 mL), 100 °C and 10 h. GC yield.

Optimization of the Reaction Conditions using the n class="Chemical">4r catclass="Chemical">n class="Chemical">alyst.

*Reaction conditions: FA (10 mmol), catclass="Chemical">alyst class="Chemical">n class="Chemical">4r (relative equiv.), toluene (10 mL), 100 °C and 7 h. GC yield. **Reaction conditions: FA (10 mmol), catalyst 4r (0.01 equiv.), toluene (10 mL), 100 °C and 10 h. GC yield.

As per previous literature preparations of class="Chemical">DFE[25], other compouclass="Chemical">nds appear iclass="Chemical">n the oligomerizatioclass="Chemical">n reactioclass="Chemical">n (Fig. 2, Figure S1), as determiclass="Chemical">ned by GC/MS. As showclass="Chemical">n iclass="Chemical">n Table 4, these iclass="Chemical">nclude: compouclass="Chemical">nd 5 (5–furfuryl–class="Chemical">n class="Chemical">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.

Figure 2

The selective catalytic dehydration process converting furfuryl alcohol to 2,2′-difurfuryl ether using a polyoxometalate (POM) catalyst.

Table 4

The yields for the oligomerization reaction using the 4r catalyst.

Entry	Reaction time	Compound 5 yield (%)	Compound 6 yield (%)	Compound 7 yield (%)	FA conversion (%)
1	1 h	5.85	6.84	1.63	28.42
2	2 h	6.61	6.33	2.81	30.70
3	3 h	13.97	7.20	5.47	43.27
4	4 h	14.18	11.72	6.53	50.59
5	5 h	18.7	17.03	6.75	57.4
6	6 h	20.2	23.20	10.1	75.1
7	7 h	20.3	20.60	10.86	80.9
8	8 h	17.47	13.80	8.94	82.56
9	9 h	14.20	8.94	6.93	89.81
10	10 h	13.10	6.87	6.81	89.82

*Reaction conditions: FA (10 mmol), catalyst 4r (0.01 equiv.), toluene (10 mL) and 100 °C. GC yield.

The selective catclass="Chemical">alytic class="Chemical">n class="Disease">dehydration process converting furfuryl alcohol to 2,2′-difurfuryl ether using a polyoxometalate (POM) catalyst.

The yields for the oligomerization reaction using the n class="Chemical">4r catclass="Chemical">n class="Chemical">alyst.

*Reaction conditions: FA (10 mmol), catclass="Chemical">alyst class="Chemical">n class="Chemical">4r (0.01 equiv.), toluene (10 mL) and 100 °C. GC yield.

As shown in Tables 3 and 4, the reaction time has an obvious influence on the yields of compound 4 (class="Chemical">DFE), compouclass="Chemical">nd 5, compouclass="Chemical">nd 6 aclass="Chemical">nd compouclass="Chemical">nd 7. As expected, the yields of compouclass="Chemical">nd 5, compouclass="Chemical">nd 6 aclass="Chemical">nd compouclass="Chemical">nd 7 decrease aclass="Chemical">nd yields of compouclass="Chemical">nd 4 iclass="Chemical">ncreases with reactioclass="Chemical">n time. The yield of compouclass="Chemical">nd 5 iclass="Chemical">ncreased graduclass="Chemical">n class="Chemical">ally 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 class="Chemical">POMs catclass="Chemical">n class="Chemical">alysts 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

class="Chemical">All of the catclass="Chemical">n class="Chemical">alysts a-h were synthesised by the same approach. This method is illustrated following for catalyst 4a as an example.

A solution of class="Chemical">H3PW12O40 (2.88 g, 1 mmol) iclass="Chemical">n deioclass="Chemical">nized class="Chemical">n class="Chemical">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.

Cclass="Chemical">alculated for 4a class="Chemical">n class="Chemical">Na3PW12O40: Na, 2.34; P, 1.05; W, 74.88%. Found: Na, 2.37; P, 1.11; W, 74.79%.

Cclass="Chemical">alculated for 4b class="Chemical">n class="Chemical">FePW12O40: Fe, 1.90; P, 1.06; W, 75.22%. Found: Fe, 1.88; P, 1.09; W, 75.29%.

Cn class="Chemical">alculated for 4c class="Chemical">n class="Chemical">K3PW12O40: K, 3.92; P, 1.03; W, 73.68%. Found: K, 3.95; P, 1.07; W, 73.69%.

Cclass="Chemical">alculated for 4d class="Chemical">n class="Chemical">AlPW12O40: Al, 0.93; P, 1.07; W, 75.97%. Found: Al, 0.90; P, 1.07; W, 76.01%.

Cclass="Chemical">alculated for 4e class="Chemical">n class="Chemical">Na3PMo12O40: Na, 3.65; P, 1.64; Mo, 60.88%. Found: Na, 3.61; P, 1.69; Mo, 60.81%.

Cclass="Chemical">alculated for 4 f class="Chemical">n class="Chemical">FePMo12O40: Fe, 2.97; P, 1.65; Mo, 61.30%. Found: Fe, 2.93; P, 1.60; Mo, 61.41%.

Cn class="Chemical">alculated for 4 g K3Pclass="Chemical">n class="Chemical">Mo12O40: K, 6.05; P, 1.60; Mo, 59.36%. Found: K, 6.11; P, 1.58; Mo, 59.43%.

Cclass="Chemical">alculated for 4 h class="Chemical">n class="Chemical">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 catn class="Chemical">alysts was illustrated by the syclass="Chemical">nthesis of catclass="Chemical">n class="Chemical">alyst 4n.

class="Chemical">H3P class="Chemical">n class="Chemical">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.

Cclass="Chemical">alculated for 4i {class="Chemical">n class="Chemical">[(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%.

Cclass="Chemical">alculated for 4j {class="Chemical">n class="Chemical">[(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%.

Cclass="Chemical">alculated for 4k {[C5class="Chemical">n class="CellLine">H5NC16H33][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%.

Cclass="Chemical">alculated for 4 l {[(class="Chemical">n class="CellLine">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%.

Cclass="Chemical">alculated for 4 m {class="Chemical">n class="Chemical">[(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%.

Cclass="Chemical">alculated for 4class="Chemical">n {class="Chemical">n class="Chemical">[(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%.

Cclass="Chemical">alculated for 4o {[C5class="Chemical">n class="CellLine">H5NC16H33][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%.

Cclass="Chemical">alculated for class="Chemical">n class="Chemical">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%.

Cclass="Chemical">alculated for 4q {[C5class="Chemical">n class="CellLine">H5NC16H33]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 class="Chemical">Na2MoO4·2H2O (6.05 g, 25 mmol) iclass="Chemical">n deioclass="Chemical">nized class="Chemical">n class="Chemical">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 catclass="Chemical">alysts were employed, respectively, for this reactioclass="Chemical">n aclass="Chemical">nd the overclass="Chemical">n class="Chemical">all synthetic conditions are illustrated following using 4r as an example (Fig. 3).

Figure 3

The synthesis of 2,2′-difurfuryl ether using catalyst 4r [(C4H9)4N]2SMo12O40.

The synthesis of 2,2′-difurfuryl class="Chemical">ether usiclass="Chemical">ng catclass="Chemical">n class="Chemical">alyst 4r [(C4H9)4N]2SMo12O40.

FA (0.98 g, 10 mmol), class="Chemical">4r (0.23 g, 0.1 mmol, 1% equiv.) aclass="Chemical">nd class="Chemical">n class="Chemical">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%).

class="Chemical">1H class="Chemical">n class="Chemical">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).

class="Chemical">13C class="Chemical">n class="Chemical">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).

Supplementary information