Ruthenium nanoparticles supported on N-containing mesoporous polymer catalyzed aerobic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF)

Article abstract of DOI:10.1016/j.apcata.2016.03.035

Ruthenium nanoparticles embedded on a mesoporous poly-melamine-formaldehyde material (Ru@mPMF) has been synthesized and characterized by powder XRD, HRTEM, TGA, SEM, EDS, UV-vis diffuse reflection spectroscopy (DRS), Raman spectroscopy, XPS and N₂ adsorption/desorption study. The Ru@mPMF catalyst was excellent for selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in toluene. Moreover, this catalyst is easily recoverable and it is reusable upto six cycles without notable decrease in its catalytic activity. Profoundly scattered and firmly bound Ru-NPs sites in mPMF might be engaged for the excellent performance of the material. Due to strong binding with the functional groups of the polymer, no indication of ruthenium leaching was there during the reaction. This suggests the actual heterogeneous nature of the material.

Full text of DOI:10.1016/j.apcata.2016.03.035

Applied Catalysis A: General 520 (2016) 44–52
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Ruthenium nanoparticles supported on N-containing mesoporous
polymer catalyzed aerobic oxidation of biomass-derived
5
-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF)
Kajari Ghosh , Rostam Ali Mollaa,1, Md. Asif Iqubalb,1, Sk. Safikul Islama,c
a
,
a,∗
Sk. Manirul Islam
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235 W.B., India
Department of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India
Department of Chemistry, Aliah University, Kolkata, West Bengal 700156, India
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Ruthenium nanoparticles embedded on
a
mesoporous poly-melamine-formaldehyde material
Received 31 December 2015
Received in revised form 16 March 2016
Accepted 17 March 2016
(Ru@mPMF) has been synthesized and characterized by powder XRD, HRTEM, TGA, SEM, EDS, UV–vis
diffuse reflection spectroscopy (DRS), Raman spectroscopy, XPS and N2 adsorption/desorption study.
The Ru@mPMF catalyst was excellent for selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-
diformylfuran (DFF) in toluene. Moreover, this catalyst is easily recoverable and it is reusable upto six
cycles without notable decrease in its catalytic activity. Profoundly scattered and firmly bound Ru-NPs
sites in mPMF might be engaged for the excellent performance of the material. Due to strong binding
with the functional groups of the polymer, no indication of ruthenium leaching was there during the
reaction. This suggests the actual heterogeneous nature of the material.
Available online 9 April 2016
Keywords:
Mesoporous
Poly-melamine-formaldehyde
RuNPs
HMF
© 2016 Elsevier B.V. All rights reserved.
DFF
Oxidation
1
. Introduction
The ever increasing demand and continuous exploitation have
and liquid fuels [9–11]. A variety of promising platform molecules
can be obtained, of which 5-hydroxymethylfurfural (HMF) is syn-
thesized by acid-catalyzed dehydration of C6-based carbohydrates
[12].
caused decrease in the availability of fossil feedstock which indi-
cates the rise in cost of such products. Thus a shifting towards
the production of sustainable alternative energy and chemical
feedstock has increased in recent years [1–4]. Renewable energy
resources like solar, geothermal and wind have gained much
attention. But these resources are unable to produce the organic
chemicals which are currently obtained from fossil fuels [5–8]. But,
renewable biomass that contain carbon molecules could serve as
potential and sustainable feedstock for the chemical industry.
The chemical industry view biomass as a promising feedstock in
future and thus biomass conversion into value added chemicals is
a matter of growing interest nowadays. The excessive consump-
tion and dependence on fossil fuels and thus its depletion has
led to severe impact on environment. Biomass, on the other hand
has emerged as the only carbon-containing alternative, valuable,
renewable and abundant feedstock that can provide bulk chemicals
HMF contains two different functional groups, hydroxyl- and
aldehyde groups which accounts for its versatility in the gener-
ation of pharmaceuticals [13], fine chemicals [14–16], antifungal
agents [17], plastics [18,19] and liquid fuels [20–22]. Thus,
selective functional group conversion of HMF via aerial oxi-
dation to get industrially important products, is an essential
one. 2,5-Diformylfuran is an important oxidised product of HMF
[23–30] which can be used as monomers for various polymer
synthesis. 2,5-furandicarboxylic acid is another vital oxidised
product of HMF [31–37]. However, several furan compounds
like 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-
2-furancarboxylic acid (FFCA), DFF, and FDCA can possibly be
formed during HMF oxidation [9,39–47]. So, selective HMF oxi-
dation to a particular product seems difficult. Now, the selective
oxidation of HMF to DFF is carried out by the oxidation of
hydroxyl group such that the aldehyde group (highly reactive)
is not attacked, or else 5-hydroxymethyl-2-furancarboxylic acid,
5
-formyl-2-furancarboxylic acid, and finally 2,5-furandicarboxylic
∗ Corresponding author.
acid will be formed. DFF offers a large number of industrial appli-
cations and used for the preparation of polymer resins, poly schiff
E-mail address: manir65@rediffmail.com (Sk.M. Islam).
1
These authors contribute equally to this work.
http://dx.doi.org/10.1016/j.apcata.2016.03.035
926-860X/© 2016 Elsevier B.V. All rights reserved.
0

K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52
45
◦
Scheme 1. Schematic diagram showing the formation of mesoporous Ru @mPMF.
bases, composites, antifungal agents, sealants, adhesives, foams,
binders, macrocyclic ligands, solvents, and organic conductors
metal. As ruthenium is less toxic thus use of it is an environmental
friendly pathway for conversion of biomass derived feedstock. The
conversion of HMF to DFF via base free selective oxidation using
molecular oxygen is a green alternative route.
[
17,47–49]. Several homogeneous and heterogeneous catalysts are
reported for HMF oxidation to DFF using O2 as green oxidizing
agent. Among the few works available, various vanadium-based
supported catalysts were examined for HMF conversion to DFF by
aerobic oxidation [23,39,48,50–53]. The reported catalysts mostly
suffer from several disadvantages like difficulty in separation due
to leaching of active species, excess by-product and low activity.
Thus there is a requirement to develop more efficient and recy-
clable catalysts for the above purpose. Selective HMF oxidation
using supported catalysts is an environment friendly approach.
Recently, Ebitani et al. reported Ru(OH)x/HT catalyst [54] that gave
2. Experimental
2.1. Chemicals
Melamine, paraformaldehyde from Sigma-Aldrich, dimethyl
sulfoxide (DMSO), RuCl3 from Spectrochem, India and other
reagents and solvents were purchased from Merck, India and were
used without further purification. Standard procedures were fol-
lowed for solvent drying and distillation.
9
2 percent DFF yield where DMF was used as solvent which is com-
parable to the yield of DFF using MnO -based catalysts in DMSO
2
[
3
55]. But, the reactivity of Ru(OH)x/HT catalyst is quite low (around
−
1
2.2. Synthesis of mesoporous poly-melamine-formaldehyde
polymer (mPMF)
.8 h
estimated at ≈100% conversion). Recycled catalyst gets
deactivated which may be for unstable HT surface and Ru leach-
ing [54]. Similarly heterogeneous catalysts like covalent triazine
frameworks-supported Ru catalysts [56] and an iron oxide encap-
sulated by ruthenium hydroxyapatite have also been used for the
synthesis of DFF [24].
Therefore, developing an efficient, selective, recyclable, stable
catalyst with high reactivity is still challenging. Herein, we demon-
strated a catalyst achieving higher reacting efficiency and excellent
recyclability at the same time. A nitrogen containing mesoporous
polymer with high surface was used as the support for Ru nanopar-
ticles, which also offers a way for 5-(hydroxymethyl) furfural to
enter the porous channels to interact with RuNPs in a diffusional
way. Here, we propose a safe process for the synthesis of DFF using
novel Ru nano grafted mesoporous poly-melamine-formaldehyde
3
mmol of melamine (0.378 g) and 5.4 mmol of paraformalde-
hyde (1.8 eq, 162 mg) were taken together in 3.36 mL DMSO (total
concentration is 2.5 molar). It was taken in autoclave container and
◦
was kept in an oven for 1 h at 120 C after which it was taken out
from oven for stirring on a magnetic stirrer plate. This was done to
have a homogeneous solution. The mixture was again kept in an
◦
oven for 72 h at 170 C. The reaction was then removed from oven
and cooled, after which it was filtered, washed with acetone then
◦
dried for 24 h at 80 C under vacuum.
2.3. Synthesis of Ru@mPMF catalyst
In typical synthesis, 500 mg mesoporous polymer in water
(
Ru@mPMF) material. The process is green and base free and thus
(10 mL) was stirred along with 40 mg RuCl3 at room temperature
environmentally friendly one. The catalyst, Ru@mPMF exhibited
remarkably high activity for the synthesis of DFF from selective
oxidation of HMF without the addition of homogeneous base. The
condensed network structural feature of Ru@mPMF comparable to
inorganic framework [57,58] containing a large number of nitro-
gen atoms is subjected to the superb performance of the catalytic
material and is beneficial for the stabilization of RuNPs. RuNPs have
larger surface areas and can be more active than the normal Ru
for 10 h. It was then evaporated to dryness in a rotary evaporator.
Ethylene glycol solution (20 mL) was added to the dry composite.
◦
It was then refluxed at 180 C for 6 h. When solid settled down,
it was washed with distilled water. Then the solution was filtered
and washed with methanol and distilled water (Scheme 1). The
◦
obtained material was dried for 12 h at 60 C. Ruthenium loading
to the mPMF was determined to be 4.20% by AAS.

4
6
K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52
Table 1
◦
Effect of solvent on HMF oxidation catalyzed by Ru @mPMF.
Entry Solvent Conversion HMF (%) DFF selectivity (%) FFCA selectivity (%)
1
3
4
5
ACN
Toluene 99.6
DMSO
H2O
35.5
67
85
76
69
8
3
14
23
84.3
88.2
Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL solvent, 2 MPa O2, 12 h,
◦
1
05 C.
(Fig. 2E) depicts the presence of the expected elements in the struc-
ture of the catalyst, namely ruthenium, carbon and nitrogen.
Fig. 3A and B shows the scanning electron micrographs of the
Ru@mPMF. The FE-SEM images indicate uniform submicron-sized
spherical morphology of the material. Large assembly of particles
are formed from aggregation of the spherical paticles. These are
again inter connected with each other. The presence of carbon and
nitrogen are indicated in elemental mapping (Fig. 3C and D).
The N2 adsorption-desorption isotherm of the Ru@mPMF
(Fig. 4A) exhibits a type IV isotherm that demonstrates the meso-
porous structure of the material. Again, the sharp increase in uptake
of nitrogen and the hysteresis loop (P/P0 above 0.80) indicated the
interparticle spaces. The BET surface area of the Ru@mPMF was
Fig. 1. Powder XRD pattern of mPMF and Ru@mPMF materials.
2
.4. Catalytic reaction
2
found to be 190 m /g. In comparison to the mPMF the surface area
2
−1
decreases from 536 (Fig. S8) to 190 m g , and the mesopore diam-
eter decreases from 6.2 nm to ≈ 1.4 nm. This may be due to the
deposition of ruthenium nanoparticles at the outer surface as well
as on the pores of mPMF.
The selective oxidation of HMF with molecular O2 was per-
formed in a 50 mL stainless-steel teflon-lined autoclave. Ru@mPMF
50 mg) and HMF (2 mmol) were added into the reactor that was
(
pre-charged with toluene (10 mL) followed by the introduction of
Fig. 5 displays the Raman spectra of Ru@mPMF where bands
◦
O2 pressure (2 MPa) at 105 C for 12 h. When the system reached
−1
−1
at 1412 and 1551 cm have been shown. The band at 1412 cm
the desired temperature, vigorous stirring was initiated and fixed at
00 rpm. Then after a fixed time, the reaction was brought to an end
is the D band showing the disorder or defect in carbon atom.
6
−1
The band at 1551 cm
corresponds to G band and depicts the
and the reactor was cooled to room temperature. After the reaction,
samples of the reaction mixture were taken and, after filtering off
the catalyst, analyzed by GC (Varian-430 with a flame ionization
detector) and HPLC (Agilent 1200 series, using a reverse-phase C18
column). The quantity of furan compounds was sensitive to a UV
detector at a wavelength of 280 nm. The mobile phase was com-
posed of acetonitrile and 0.1 wt% acetic acid aqueous solution in
a volume ratio of 30:70, and the flow rate of 1.0 mL min
employed to identify possible products according to calibration
with standard solutions of the products and reactants.
2
sp in plane vibration of carbon atom. Raman spectroscopy of
mesoporous Ru@mPMF showed different intensity peaks of each
component.
XPS spectrum of Ru@mPMF is depicted in Fig. 6. The spectra
show that ruthenium species in the Ru@mPMF is in the metallic
◦
form (Ru ). For Ru@mPMF, three peaks of Ru 3d5/2, Ru 3d3/2 and Ru
3
p3/2 are intensified at 281.8 eV, 286.2 eV and 461.9 eV respectively.
−
1
was
It is clear from the XPS data that the Ru(0) species are produced dur-
ing reduction and are dispersed in an uniform manner throughout
the mesoporous object. These binding energy values are agreeable
with the values for ruthenium (0) state accounted in the literature
[62].
3
. Results and discussion
3.1. Characterization of catalyst
3.2. Catalytic activity
Fig. 1 shows the powdered XRD of mPMF and Ru nanoparticles
Aerobic oxidation of HMF
dispersed on it (Ru@mPMF). Mesoporous mPMF shows its char-
◦
acteristic broad diffraction peak centered at 2␪= 21.75 [59]. The
decrease of the overall reflection intensity with respect to the par-
ent, Ru@mPMF is a consequence of the inclusion of guest ruthenium
nanoparticles in its pores and surfaces. In the XRD pattern of the
Recently, we have reported various mesoporous polymer sup-
ported catalysts exhibiting high performance for a great variety of
industrially relevant methods [63–65]. In polymer confined RuNPs
(Ru@mPMF) material in the field of selective oxidation of HMF using
molecular oxygen as the sole oxidant (Scheme 2).
Ru@mPMF, the additional prominent diffraction peaks at 2␪ = 39.4,
◦
4
6.4 and 77.3 represents (100), (101) and (110) FCC crystal planes
due to Bragg’s reflection respectively [60].
5-Hydroxymethylfurfural (HMF) is oxidized to get various prod-
ucts. Selectivity of a particular product is dependent on the reaction
parameters and nature of the catalyst used. Recently, we have
reported a catalytic system for aerobic oxidation of benzyl alcohols
where molecular oxygen was used as the sole oxidant [66]. Now we
have performed aerobic oxidation of HMF using ruthenium based
catalyst and oxygen as the sole oxidant. At first, HMF oxidation was
carried out by taking 0.2 mmol HMF, 50 mg Ru@mPMF in a reac-
HR-TEM images of Ru@mPMF is demonstrated in Fig. 2 which
suggests that the material have ordered foam-like interconnected
porous network structure containing high electron density dark
spots throughout the specimen [61]. The spherical natures of
the nanoparticles are revealed in a closer view of the individual
nanoparticles (Fig. 2B). These spherical particles are assigned to
ruthenium-nanoparticles in Ru@mPMF (Fig. 2C) and nanoparti-
cles were homogeneously dispersed with an average particle size
of ≈3.3 nm (Fig. 2D). TEM-EDX obtained from the nano material
◦
tor using acetonitrile (10 mL) as the solvent at 80 C under 1 MPa
pressure of O2 for 3 h, but the reaction did not progress beyond

K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52
47
Fig. 2. HR-TEM images (A–C), distribution plot (2-D), EDX (2E) of Ru@mPMF material.
3
8% conversion of HMF. The reaction was carried out in different
reactions like over oxidation can possibly occurr at higher polar
solvents [68]. Literature study shows that in polar solvent medium
favour the formation of FDCA than DFF. Remarkably, as evident
from entry 6 of Table 1, the selectivity was much higher in toluene
than in water. DFF selectivity in water even declined sharply to
69%, the corresponding FFCA selectivity being 23% and conversion
of HMF being 88.2%. This emulates that in HMF and DFF, the alde-
hyde group hydration to geminal diols [69] is facilitated by water
that subsequently gets oxidized to carboxylic acids (Scheme 3).
The aerobic oxidation of HMF was also carried out under dif-
ferent reaction time and catalyst amount to obtain the optimum
reaction conditions(Table 2). When the amount of catalyst was
10 mg in the above reaction, DFF (66%) was formed as a major prod-
uct along with the FFCA (2%) and HMF conversion was 19% at 5 h but
solvents, to select the best one for the same (Table 1).
Thus, the selective HMF oxidation was carried out in acetonitrile,
toluene, dimethylsulfoxide (DMSO) and water. Effects of various
solvents on the conversion of HMF and selectivity of DFF using
Ru@mPMF catalyst are summarized in (Table 1, entries 1–5). Selec-
tivity of DFF is depends on the polarity of solvents. As the polarity
of solvent increase gradually conversion of HMF to DFF decreases.
Among the various solvents, aromatic less polar solvent, toluene
was found to be the best for the transformation in comparison to
other solvents such as DMSO, DMF and water. Usually it is assumed
that the activity of the catalyst is affected by solvent polarity, but
there is no general agreement regarding this phenomenon on the
conversion and selectivity [67]. The reason might be that more side

4
8
K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52
Fig. 3. FE-SEM (A & B) images and elemental mapping (C & D) of Ru@mPMF.
Scheme 2. Selective oxidation of 5-hydroxymethylfurfural towards 2,5-diformylfuran (DFF).
when it was increased to 50 mg the conversion of HMF increased
upto 99.6% (Table 2, entry 5). The noted difference in activity may
be due to RuNPs loading that again, may be involved in forming
various active species. The active sites may possess varied reactiv-
ity and accessibility in the porous network of the mPMF support.
On the other hand, increasing the reaction time upto 12 h resulted

K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52
49
Scheme 3. Probable mechanism for the oxidation of HMF.
Ru@mPMF
D band
G Band
1
250
1500
1750
2000
-
1
Wavenumber (Cm
)
Fig. 5. Raman spectroscopy of Ru@-mPMF.
Table 2
Effect of reaction time and catalyst amount on HMF oxidation.
◦
Entry
Reaction time (h)
Ru @mPMF(mg)
Conversion(%)
1
2
3
4
5
6
5
8
10
12
12
14
10
20
30
40
50
60
19
32
59
81
99.6
99.6
Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL toluene, 2 MPa O2,
◦
105
C
The aerobic oxidation of HMF was carried out under various
temperatures (Fig. 7) where it was observed that the reaction tem-
perature affected the oxidation efficiency and also the products
◦
distribution. Under 70 C, HMF conversions were not more than
◦
6
5% in 12 h. Noticeably, as the temperature was raised from 70 C to
Fig. 4. N2 adsorption/desorption isotherms (A) and pore size distribution (B) of
Ru@mPMF.
◦
105 C, there was a gradual increase in HMF conversion upto 99.6%.
But beyond this temperature, selectivity (%) of the DFF decreased
with increasing the temperature from 105 C to 115 C and the DFF
yield decreased from 85% to 82.5%. Beyond 105 C FFCA began to
form. FFCA selectivity was 3% at 105 C temperature. These results
show that Ru @mPMF catalyst is worthwhile for selective oxidation
of HMF. Its catalytic activity can be increased at higher tempera-
tures without the loss of its high DFF selectivity.
◦
◦
in almost full conversion of HMF (99.6%), clearly indicating that
the conversion of the reaction depends on the reaction time. These
conditions did not give the full substrate conversion. To find the
appropriate reaction conditions reaction time and catalyst amount
were screened (Table 2). The identification of DFF in the reaction
product was confirmed by 1H NMR (Fig. S2).
◦
◦
◦

5
0
K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52
Table 3
Effect of oxidant on HMF conversion and selectivity.
Entry
Oxidant
(%) HMF conversion
DFF selectivity (%)
1
2
H2O2
TBHP
Air
O2
O2
11.4
100
51
69
99.6
38
9
68
73
85
a
3
4
a
b
5
Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL toluene, 2 MPa O2,12 h,
◦
a
b
1
05 C; (1 MPa), (2 MPa).
notable effect both on conversion of HMF and yield of DFF (Table 3).
HMF conversion reached only upto 11.4% (Table 3, entry 1) when
H O was used as the oxidant. H O2 decomposed to oxygen while
2
2
2
following the given condition. Upto 100% conversion of HMF was
obtained after 12 h when TBHP was used as the oxidant. But in this
case DFF yield was very low, only 9.0% (Table 3, entry 2). Moreover,
other oxidation products like FDCA and HMFCA were not observed,
which might be for the furan ring breaking by t-BuOOH [69]. Com-
paratively meager reactivity in air may be ascribed to the low O2
concentration. Admirable capability of O2 activation for selective
HMF oxidation to DFF was shown by the catalyst. Excellent conver-
sion of HMF (99.6%) and 85% of DFF yield were obtained.
Influence of the support
As we discussed before that mPMF support contain large num-
ber of nitrogen atoms which can bind metal on its surface and its
pore. Small sized RuNPs can disperse on the mPMF support. This
result clearly showed that the support plays a key role for the selec-
tive oxidation of HMF to DFF. Generally, a catalyst support may
offer better dispersions of active sites. Another important effect
of the support is the enrichment of the adsorption of the reacting
species and the reaction intermediates. This effect becomes partic-
ularly vital for the heterogeneous catalytic conversion of organic
molecules. Ru@mPMF has large surface area which may influ-
ence the adsorption of HMF; on the other hand, porous channels
of mPMF support enhanced the activity of the catalyst. Various
inorganic metal oxides have been used as a support, more acidic
support such as ZSM led to poorer activity, especially very little DFF
selectivity. More basic support may produce unidentified degrada-
tion and polymerization byproducts. Thus Nie et al. showed that
for the efficient synthesis of DFF by selective oxidation of HMF,
the dormant part of the support whose acidity and basicity are
weak gets favourable [38]. So, mPMF was chosen as support as
it contains high amount of aminal ( NH CH2 NH-) groups. The
dual roles of a Brønsted acid and Lewis base are played by the
triazine rings [61]. Catalytic activity of the nanoparticles greatly
depends on size of nanoparticles. The diameter of most reported
ruthenium nanoparticles were larger than 5 nm. The characteris-
tics of small nanoclusters, especially those below 5 nm, could be
considerably different from larger ones due to the quantum size
effects. The difficulty in the synthesis of small sized Ru clusters
makes it challenging. Successful synthesis of ruthenium nanoclus-
ters with controlled core size with different procedures [60] have
been reported recently. Here, we synthesized 3.2 ± 0.3 nm ruthe-
nium nanoparticles. On the other hand, we have used mPMF as the
support, which is highly porous. So, very small sized ruthenium
nanoparticles can be easily incorporated into it. Thus, Ru@mPMF
catalyst is highly reusable and effective for the oxidation of HMF
reaction. Porous nature of Ru@mPMF material increased the cata-
lyst novelty.
Fig. 6. X-ray photoelectron spectroscopy (XPS) analysis of Ru@mPMF.
HMF conversion (%)
1
00
DFF selectivity (%
)
9
8
7
6
0
0
0
0
7
0
80
90
100
0
110
120
Temperature ( C)
Fig. 7. Effect of temperature on oxidation of HMF.
Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL toluene, 2 MPa O2, 12 h.
Effect of oxidants
3.3. Catalyst reusability
For the identification of the optimum condition for the oxida-
tion of HMF, various oxidants were screened and they displayed a
The recyclability of Ru@mPMF material in selective oxidation
of HMF to DFF (Fig. 8) was studied as the stability and reusabil-

K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52
51
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