The catalysts have been characterized by using powdered X-ray diffraction (PXRD), to know the crystalline nature of the samples and its purity. The diffraction peaks of Fe3O4@C appeared at Bragg’s angle at 2θ ~ 30.3, 35.3, 43.1, 53.1, 57.1, 62.7, 74.2 corresponding to (220), (311), (400), (422), (511), (440) and (533) planes respectively. These data is attributed to Fe3O4 face centred cubic structure [JCPDS card No. 89–0950]. In the XRD pattern of Fe3O4@C, no additional peaks were observed except the peak broadening, which is attributed to due to the presence of carbon on iron oxide (Fig. 1). The crystallite size was calculated by using Debye-Scherer equation and found that 10.7 and 11.8 nm respectively for pure magnetite and Fe3O4@C respectively (Fig. 1).
Since Fe3O4 has been successfully used in many oxidation reactions with good selectivity towards the desired product [41], initially the same catalyst has been evaluated in the aerobic oxidation of 5-HMF in the presence of various solvents. A variety of carbon materials including carbon nanotubes (CNTs), carbon nanofiber (CNF), graphene, graphene oxide (GO) and fullerenes have been extensively used in various biological and environmental applications [42–44]. Quite recently, carbon nanoparticles (CNPs) are being implemented for various applications due to their non-toxicity [45]. Synthesis of CNPs using candle soot is a simple technique, inexpensive and also another advantage is it take short period of time to synthesize [46–49]. In toluene, the oxidation of HMF has been carried out and observed that after 72 h very little conversion of HMF took place, either in the presence of a base or in the absence of a base at 100 °C (Table 1, Entry 1 & 2). However, in the presence of an oxidant like hydrogen peroxide 56% conversion of HMF was observed with 55.4% selectivity towards DFF in toluene using only Fe3O4 as a catalyst (Table 1, Entry 3). There was no reaction either in the absence of base or absence of oxidant (Table 1, Entry 4). Remarkably, in the presence of base in aqueous medium, 58% conversion of HMF took place with 94.8% selectivity towards DFF. The other by products observed in this case are FDCA, FFCA and levulinic acid (Table 1, Entry 5). In the presence of hydrogen peroxide, though selectivity remains constant, but conversion increases to 74.8% using Fe3O4 as a catalyst at 100℃ whereas at 80℃, 68.4% conversion of 5-HMF was observed (Table 1, Entry 6 & 7). In the oxidation of 5-HMF, obviously (as expected) CNPs are quite inactive in aqueous medium in the presence of a base. To improve the catalytic activity, we incorporated CNPs inside magnetite and tested for the oxidation of HMF. It was found that in the absence of base, there was no oxidation of HMF took place using CNP/magnetite (Table 1, Entries 9–14). The rate of the reaction was triggered by the presence CNPs on magnetite. Notably, in the presence of oxidant like hydrogen peroxide, 93% conversion was observed after 72h using K2CO3 as a base with 78.7% selectivity (Table 1, Entry 15). However, in the presence of oxygen (1.0 atm), using Fe3O4@C, corresponding DFF product gave with 95.6% selectivity against 98% HMF conversion (Table 1, Entry 16). Temperature has a predominant effect on the conversion and selectivity in the oxidation of HMF.
Table 1
Oxidation of 5-hydroxymethylfurfural to DFF using various solvents and catalysts
Entry
|
Catalyst
|
Base
|
Solvent
|
Oxidant
|
Temp.
(℃)
|
Time
(h)
|
HMF
Conversion (%)
|
DFF
Selectivity
(%)
|
1
|
Fe3O4
|
-
|
Toluene
|
Air
|
100
|
72
|
< 10%
|
-
|
2
|
Fe3O4
|
K2CO3
|
Toluene
|
Air
|
100
|
72
|
< 10%
|
-
|
3
|
Fe3O4
|
K2CO3
|
Toluene
|
H2O2
|
100
|
72
|
56%
|
55.4%
|
4
|
Fe3O4
|
-
|
Water
|
Air
|
100
|
72
|
-
|
-
|
5
|
Fe3O4
|
K2CO3
|
Water
|
Air
|
100
|
72
|
58%
|
94.8%
|
6
|
Fe3O4
|
K2CO3
|
Water
|
H2O2
|
100
|
72
|
74.8%
|
95.4%
|
7
|
Fe3O4
|
K2CO3
|
Water
|
H2O2
|
80
|
72
|
86.4%
|
95.2%
|
8
|
CNPs
|
K2CO3
|
Water
|
Air
|
100
|
72
|
-
|
-
|
9
|
Fe3O4@C
|
-
|
Toluene
|
Air
|
100
|
12
|
< 1
|
< 1
|
10
|
Fe3O4@C
|
-
|
Toluene
|
Air
|
100
|
12
|
< 1
|
< 1
|
11
|
Fe3O4@C
|
-
|
water
|
Air
|
100
|
12
|
< 1
|
< 1
|
12
|
Fe3O4@C
|
-
|
DMF
|
Air
|
100
|
12
|
< 1
|
< 1
|
13
|
Fe3O4@C
|
-
|
DMSO
|
Air
|
100
|
12
|
< 1
|
< 1
|
14
|
Fe3O4@C
|
-
|
water
|
H2O2
|
100
|
12
|
< 1
|
< 1
|
15
|
Fe3O4@C
|
K2CO3
|
Water
|
H2O2
|
80
|
72
|
93
|
78.7
|
16
|
Fe3O4@C
|
K2CO3
|
Water
|
O2
|
80
|
72
|
98
|
95.6
|
17
|
Fe3O4@C
|
K2CO3
|
Water
|
H2O2
|
RT
|
72
|
35
|
11
|
18
|
Fe3O4@C
|
-
|
Acetonitrile
|
TEMPO
|
100
|
24
|
6
|
< 1
|
19
|
Fe3O4@C
|
K2CO3
|
Acetonitrile
|
TEMPO
|
100
|
72
|
63
|
74.6
|
20
|
Fe3O4@C
|
-
|
Water
|
TEMPO
|
80
|
72
|
< 1
|
< 1
|
21
|
Fe3O4@C
|
K2CO3
|
Water
|
TEMPO
|
80
|
72
|
95
|
95
|
Reaction Condition: substrate (0.25mmol), Catalyst (5.0 mg), Base (27.5mg), solvent (5.0 mL). Conversion and selectivity are calculated based on HPLC |
At room temperature, only 35% conversion took place with 11% selectivity (Table 1, Entry 17). In general oxidation of 5-HMF in low boiling aprotic solvent like acetonitrile gave the product in excellent yields with heterogeneous catalysts using TEMPO as an oxidant. In our case, we have also tested the potentiality of our catalyst in acetonitrile gave low conversion less than 6% along with other products FDCA, DFF and LA using TEMPO as an oxidant in the absence of a base and gave 63% conversion with 74.6% selectivity towards DFF in the presence of a base. (Table 1, Entry 18 and 19). Interestingly, in the presence of TEMPO as an oxidant using Fe3O4@C, conversion of 5-HMF has been observed 95% with 98% selectivity at 80 ℃ (Table 1, Entry 20 & 21). These results suggested that the introduction of CNP into metal oxide not only improve the catalytic activity, but also enhance the selectivity of DFF.
Notably, the reaction did not produce any product under catalyst free conditions. The candle soot supported iron salts confirms the Fe3O4@C hybrid as one of the best non-precious catalyst for the consecutive dehydration and oxidation reaction. The higher activity may be due to hydrophobic nature of carbon from the candle soot, resulted in a better Fe3+ adsorption and enhanced dispensability. The higher activity of CNP/Fe3O4 may be due to the facile redox reaction of Fe3+ to Fe2+ occurs in an easier way because of the synergetic effects of CNP on inverse spinel such as Fe3O4 and thus reaction proceeds smooth way.
The heterogeneous catalyst Fe3O4@C has characterized by XPS, which confirms the presence of carbon incorporated on magnetite. (Fig. 1, SI). The doublet profile of Fe2p spectra into distinct Fe2p3/2 and Fe2p1/2 orbit at binding energies at 710.7 eV and 724.1 eV respectively. The stoichiometric magnetite Fe3O4 of cubic close packed oxygen sublattice can be alternatively expressed as FeO•Fe2O3. Therefore, it consists of both iron ions Fe2+ and Fe3+ occupying the tetrahedral and octahedral interstices of cubic spinel type structure. The O 1s binding energy at 532.22 eV corresponds to O2– of Fe3O4 showing only one kind of oxygen species. It was identified to be the bridge oxygen (O) between octahedral iron (Fe3+) and the tetrahedral (Fe2+) lattice. Further binding energy for C 1s was observed at 284.12 eV, 293.15 eV and 295.62 eV respectively, which is indicating the presence of different carbon species. The peaks at low binding energies nearly at 284 eV, are ascribed to the lattice carbon atoms, The binding energies corresponding to CNPs are 293 eV and at 296 eV, oxidized carbons, which may be present (Fig. 1, SI).
In addition, Fe3O4@C has also been characterized by Raman spectroscopy. It was employed to determine the nature of the iron oxide core (magnetite), where the Raman effect is caused by the molecular effects produced from certain energy irradiated on the sample. In the present case, the Raman spectrum peaks of iron oxide (Fe3O4) were investigated and found that translator movement (T2g1) corresponding to FeO4 are changed from 193 cm-1 of Fe3O4 to 214 cm-1 for CNP/Fe3O4 (Fig. 2). However, the asymmetric stretching (T2g2) and bending of Fe-O (T2g3) are remains same before and after incorporation of CNPs in the magnetite. Significantly, the symmetric bending of Fe-O of magnetite corresponding to Eg has been shifted from 319 to 279 cm-1 after incorporation of CNPs [50]. In addition, the peak at 680cm-1 was identified as a band characteristic which was present on magnetite, however it has been disappeared in Fe3O4@C, which clearly indicates that hydrophobic CNPs have accommodated inside the magnetite, leads to the formation of crystal defects (Table 2).
Table 2
Raman Shift Vibrations of Different Bondings in the on Fe3O4 and CNP/Fe3O4
S.No
|
Vibrational Mode
|
Fe3O4 (Cm− 1)
|
Fe3O4@C (Cm− 1)
|
1
|
T2g1 (Translatory movement of FeO4)
|
193
|
214
|
2
|
T2g2 (Asymmetric stretch of Fe-O)
|
488
|
485
|
3
|
T2g3 (Asymmetric bend of Fe-O)
|
538
|
538
|
4
|
Eg (Symmetric bend of Fe-O)
|
319
|
279
|
5
|
A1g (Symmetrical stretch of oxygen atoms along Fe-O)
|
683
|
670
|
6
|
Characteristic peak
|
1383
|
1286
|
Table 3
Conversion of Biomass into DFF Catalyzed by CNP/Fe3O4 Catalyst.
S.No
|
Substrate
|
Oxidant
|
HMF (%)
|
DFF (%)
selectivity
|
1
|
Glucose
|
H2O2
|
-
|
99.3
|
2
|
Glucose
|
Air
|
-
|
84.3
|
3
|
Fructose
|
H2O2
|
< 1%
|
73.8
|
4
|
Fructose
|
Air
|
< 2%
|
94.8
|
Reaction condition: Substrate (1.0 mmol), Catalyst (5.0 mg), K2CO3 (27.5mg), H2O (5.0 mL) |
The morphology and structural features of the Fe3O4@C has also been studied using scanning electron microscopy (SEM). Well defined spherical particles were observed with the nanometer size of approximately 20 nm and observed that all expected elements (Fe, O and C) are present on CNP/Fe3O4 NPs (Fig. 3).
We have further investigated and potentiality of our catalyst in the one pot synthesis of DFF either from glucose or fructose [50–52]. In the case of glucose in the aqueous medium in the presence of air at 80°C, DFF was obtained with 99.3% selectivity and found that HMF has been completely converted. Remarkably, in the presence of air, it was also found that there was no HMF remained in the flask after 72 h and DFF was obtained with 84.3% selectivity. The other products formed under aqueous conditions are FDCA, FFCA and LA. Significantly, DFF has been obtained with 94.8% selectivity with complete conversion of fructose at 80℃ with only 2% of HMF. This result clearly indicates that our developed catalyst CNP/Fe3O4 could acts as a bifunctional catalyst where the subsequently dehydration and oxidation reactions took place. Of particular note is that no catalytic conversion of glucose or fructose was observed using magnetite alone.
In summary, we developed a new heterogeneous catalyst, operable under mild conditions, for the selective oxidation of HMF to DFF using candle soot derived, Fe3O4@C catalyst. Under optimized reaction conditions, we used water as a greener solvent using the robust heterogeneous catalyst. The present system circumvents the use of green protocols for utilizing biomass into value added products. The absence of precious metals and with the simple preparation of catalyst makes our catalytic protocol attractive not only for the selective oxidation of HMF to DFF, but also for the conversion of glucose/fructose. Our work will stimulate a lot of further research on the design of new heterogeneous catalysts for catalysis and the elucidation of the underlying modes of action.