A. Kumar, R. Srivastava
MolecularCatalysis465(2019)68–79
Table 3
increased and it was reached to 100% at 413 K. However, beyond
413 K, the selectivity of DFF dropped down. Therefore, 413 K was
chosen as the best temperature. Then, the role of catalyst amount in the
range of 50–130 mg was investigated (Fig. 5b). Increasing the catalyst
amount from 50 mg to 110 mg, HMF conversion was increased and
maximum HMF conversion and maximum selectivity for DFF were
obtained using 110 mg catalyst. Finally, the influence of reaction time
was optimized (Fig. 5c). During the initial duration, the rate was high
the maximum HMF conversion and the highest DFF selectivity were
obtained for the reaction occurring after 24 h. Therefore, 24 h reaction
time was optimized for the selective conversion of HMF to DFF. Having
optimized the reaction condition for the selective oxidation of HMF to
DFF, various aromatic and aliphatic alcohols were converted to alde-
hyde products, selectively. Benzyl alcohol was converted to benzaldyde
exclusively in just 5 h under the optimum reaction condition (Table 5).
Even cinnamyl alcohol was converted to cinnamaldehyde in 5 h with
98% yield (Table 5). However, aliphatic alcohols such as hexanol and
(Table 5). Moreover, cyclohexanol was converted to cyclohexanone in
just 5 h with > 99% yield. Under the optimum condition, furfural al-
cohol was also converted selectively to furfural but with low yield as
Transformation of HMF and carbohydrates to EMF using various catalysts in-
vestigated in this study.
S. No.
Catalyst
Reactant Conv. (%)
Product select. (%)
1
2
3
4
5
6
HMF conv. < 1 %
HMF conv. < 1 %
HMF conv. 96.3%
HMF conv. 94.1 %
Sucrose conv. 75.2 %
Fructose conv. 86.3 %
EMF (100)
EMF (100)
EMF (87), FA (3)
EMF (85), FA (15)
EMF (76), FA (24)
EMF (84), FA (16)
a
Reaction condition: Reactant (1 mmol), catalyst (50 mg), ethanol (5 mL),
temperature (363 K), time (6 h).
b
time = 24 h.
reaction, 86% fructose conversion and 84% EMF selectivity were ob-
Third, the reaction condition was optimized for HMF to DFF
transformation. Since the final aim of this study is to convert sucrose to
DFF via HMF as an intermediate, therefore, this study was conducted in
DMSO as a solvent which exhibited the best activity in the conversion of
sucrose to HMF. The role of catalysts was investigated. In the absence of
catalyst, a minute amount of DFF was obtained (Table 4, Entry 1). Using
PANI and S-PANI as catalysts, very low yields of DFF were obtained
(Table 4, Entries 2, 3). When FeVO4 was used as a catalyst, a very high
selectivity were obtained. This clearly shows that FeVO4 facilitated the
selective oxidation reaction leading to DFF. When the similar reaction
was carried out using O2 (1 atm, balloon) then in that case also DFF was
selectively obtained but in this case, low HMF conversion was obtained
when compared with O2 in flow condition (10 mL/min) (Table 4,
compare entries 4,5). Therefore, remaining of the reaction was con-
ducted in O2 flow condition. Finally, nanocomposites were investigated
to catalyze the selective oxidation of HMF to DFF conversion. By
varying the S-PANI-FeVO4 nanocomposites, HMF conversion varies but
in all the cases, DFF was obtained in exceptionally high selectivity. HMF
conversion varies by varying the S-PANI content in the nanocomposite.
Using S-PANI-FeVO4(11), 100% HMF conversion with > 99% DFF se-
lectivity were obtained. Based on this study, it can be confirmed that S-
PANI-FeVO4(11) has the ability to provide high yield and selectivity for
DFF. Very small loading of FeVO4 in the catalyst confirmed that finely
dispersed small size FeVO4 is more suitable for the oxidation reaction
when compared to bulk FeVO4. Having optimized S-PANI-FeVO4(11) as
the best nanocomposite, the detailed investigation was carried out to
optimize the reaction parameters (Fig. 5). HMF to DFF conversion was
(Fig. 5a). Increasing temperature of the reaction, HMF conversion was
highly active catalyst S-PANI-FeVO4(11). In the first step of the one-pot
tandem reaction, sucrose (1 mmol), DMSO (5 mL), and catalyst
(120 mg) were heated at 393 K under N2 flow (10 mL min−1) for 6 h to
achieve the maximum conversion of sucrose to HMF. The HMF for-
mation was monitored by 1H NMR after a regular time interval. After
the first-step, N2 flow was stopped and the temperature was rose to
413 K. Then O2 flow was started and the reaction was carried out for
next 24 h in the same pot. 1H NMR of the reaction mixture was recorded
after 24 of the reaction. Fig. 6 shows the 1H NMR spectra recorded
during one-pot synthesis of DFF from sucrose. Moreover, DFF was iso-
lated with 80% yield from the reaction medium using MIBK as ex-
traction medium as described in the experimental section. Using fruc-
tose as reactant, one-pot two step conversion led to give an isolated
yield of 91% DFF.
In order to explain the catalytic role of S-PANI-FeVO4(11) in the
dehydration and oxidation steps in the one-pot reaction, a potential
pathway is proposed for the direct transformation of sucrose to DFF via
HMF as the intermediate (Scheme 2). The first step of one-pot reaction
catalyzed by the acid sites of the catalyst. Then glucose isomerisation
into fructose takes place using FeVO4. In the next step, the most basic
hydroxyl group in fructose attached to the anomeric carbon is hydro-
lysed by the Brönsted acid sites (—SO3H group) of the S-PANI. This
results into enol form which immediately tautomerizes into most stable
keto form by the loss of water molecule. This is followed by three
successive dehydration steps assisted by S-PANI and DMSO leading to
the formation of HMF as an intermediate. Finally, the selective oxida-
tion of HMF to DFF is catalyzed by FeVO4 present in the nanocompo-
site. The HMF is adsorbed on the catalyst surface through the interac-
tion between the hydroxyl group of HMF and oxidation sites, i.e., Fe,
and V, on the catalyst surface. This interaction results in the formation
of intermediate (I), which after β-elimination affords DFF as the se-
lective product. In the final step, the catalyst is regenerated by the
dissolved oxygen present in the reaction medium with the elimination
of a water molecule. The catalyst S-PANI-FeVO4(11) is more effective
for the selective production of DFF and this is the reason that some
amount of DFF was also formed in the first step of one-pot reaction even
under N2 atmosphere as confirmed from 1H NMR (Fig. 6). This may be
due to the oxidation of HMF by lattice oxygen (V+5eOeFe+3), in the
presence of a minute amount of molecular oxygen (O2) present in the
reaction vessel according to the Mars-van Krevelen mechanism [38,39].
The catalyst was separated from the reaction mixture using
Table 4
Selective oxidation of HMF to DFF using various catalysts investigated in this
study.
S. No. Catalyst
Oxidant
HMF Conv. DFF select.
(%)
(%)
1
2
3
4
5
None
O2 flow
O2 flow
O2 flow
O2 flow
O2 (1 atm,
balloon)
O2 (1 atm,
balloon)
O2 flow
O2 flow
O2 flow
1.3
100
100
100
100
100
PANI
1.6
S-PANI
FeVO4
FeVO4
1.5
84.3
46.2
6
FeVO4 treated with
Chlorosulfonic acid
S-PANI-FeVO4(37)
S-PANI-FeVO4(11)
S-PANI-FeVO4(73)
100
88
7
8
9
94.4
100
100
> 99
> 99
> 99
Reaction condition: HMF (1 mmol), catalyst (120 mg), DMSO (5 mL), tem-
perature (413 K), time (24 h).
75