Base-free oxidation of 5-hydroxymethyl-2-furfural to 2,5-furan dicarboxylic acid over basic metal oxide-supported ruthenium catalysts under aqueous conditions

Article abstract of DOI:10.1007/s12039-018-1551-z

Abstract: Polyethylene terephthalate (PET) is the third-largest globally produced polymer, and many efforts have been made to replace PET with a renewable polymer. One renewable alternative to PET is polyethylene furanoate (PEF), which is prepared using 2,5-furan dicarboxylic acid (FDCA) as a precursor instead of terephthalic acid (TPA). Biomass-derived hydroxymethyl-2-furfural (HMF) can be converted to 2,5-furan dicarboxylic acid (FDCA) through multiple oxidation reactions. Metal oxide-supported Ru catalyst prepared by simple methods for the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furan dicarboxylic acid (FDCA) in the absence of a base under aqueous conditions is reported. This study clearly explains that the nature of basicity of the support has an important role on the selective oxidation of HMF to FDCA. Among the various materials studied magnesium oxide (MgO)-supported Ru catalyst afforded a 100% HMF conversion and more than 90% FDCA yield with 90 psi of O ₂ at 160 ^°C in 4?h and it could be used 5 times without a significant drop of FDCA yields. SYNOPSIS: Metal oxide-supported Ru catalyst prepared by simple methods for the oxidation of 5-Hydroxymethylfurfural (HMF) to 2,5-Furan dicarboxylic acid (FDCA) in the absence of a base under aqueous conditions are reported. Magnesium oxide (MgO)-supported Ru catalyst afforded a 100% HMF conversion and more than 90% FDCA yield.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-018-1551-z

J. Chem. Sci.
(2018) 130:156
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1551-z
REGULAR ARTICLE
Base-free oxidation of 5-hydroxymethyl-2-furfural to 2,5-furan
dicarboxylic acid over basic metal oxide-supported ruthenium
catalysts under aqueous conditions
CHURCHIL ANGEL ANTONYRAJa , NHAN THANH THIEN HUYNHa,b,
KYUNG WON LEEa,c, YONG JIN KIMa,b, SEUNGHAN SHINa,b, JONG SHIK SHINc and
JIN KU CHOa,b,∗
aGreen Chemistry and Engineering R&D Department, Korea Institute of Industrial Technology (KITECH),
89 Yangdaegiro-gil, Ipjang-myeon, Seobuk-gu, Cheonan, Chungnam 331-822, Korea
bDepartment of Green Process and System Engineering, Korea University of Science and Technology (UST),
89 Yangdaegiro-gil, Ipjang-myeon, Seobuk-gu, Cheonan, Chungnam 331-822, Korea
cDepartment of Biotechnology, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea
E-mail: jkcho@kitech.re.kr
MS received 10 May 2018; revised 13 August 2018; accepted 1 September 2018
Abstract. Polyethylene terephthalate (PET) is the third-largest globally produced polymer, and many efforts
have been made to replace PET with a renewable polymer. One renewable alternative to PET is polyethylene
furanoate (PEF), which is prepared using 2,5-furan dicarboxylic acid (FDCA) as a precursor instead of
terephthalic acid (TPA). Biomass-derived hydroxymethyl-2-furfural (HMF) can be converted to 2,5-furan
dicarboxylic acid (FDCA) through multiple oxidation reactions. Metal oxide-supported Ru catalyst prepared by
simple methods for the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furan dicarboxylic acid (FDCA) in
the absence of a base under aqueous conditions is reported. This study clearly explains that the nature of basicity
of the support has an important role on the selective oxidation of HMF to FDCA. Among the various materials
studied magnesium oxide (MgO)-supported Ru catalyst afforded a 100% HMF conversion and more than 90%
FDCA yield with 90 psi of O2 at 160 ◦C in 4h and it could be used 5 times without a significant drop of FDCA
yields.
Keywords. Ruthenium; metal oxides; base-free oxidation; aqueous conditions; 5-hydroxymethylfurfural
(HMF); 2,5-furan dicarboxylic acid (FDCA).
1. Introduction
supply of valuable intermediates to chemical industries
for the production of drugs and polymeric materials.2
“Roadmap of Biomass Technologies,” a report authored
by 26 leading experts, predicted that by 2030, 20%
of transportation fuels and 25% of chemicals will be
produced from biomass.3 Plant biomass is a labour-
intensive and land-intensive resource with a variable
economy due to rapidly growing utilization for the pro-
duction of fuels and chemicals.4 Plant biomass-derived
carbohydrates can be converted into fuels and chemi-
cals via fermentation to bioethanol, enzyme-catalyzed
conversion to hydrogen and anaerobic digestion to
The diminishing availability of petroleum resources is
an increasing problem for the production of energy
and chemicals that are essential for society. The world-
wide reserves of petroleum resources are expected to be
sufficient for another 40 years, which increased con-
cerns about rising oil prices.1 Biomass is the term
used to describe all biological matter produced by
photosynthesis using solar energy, and biomass is an
alternative to petroleum resources. Abundant biomass
resources are a promising alternative for the sustainable
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1551-z) contains
supplementary material, which is available to authorized users.
0123456789().: V,-vol

156 Page 2 of 9
J. Chem. Sci.
(2018) 130:156
Figure 1. Synthesis of PET and PEF polymers.
methane.5–7 Every process has its own advantages because complete oxidization to FDCA was impossible
and disadvantages during development, and research or ruthenium leaching caused the ruthenium to be unus-
is ongoing to further expand the use of biomass. The able.32,33 Recently, Ru/C commercial catalyst showed
conversion of carbohydrates to fuels and chemicals the base free oxidation of HMF to FDCA with an 88%
through 5-hydroxymethyl-2-furfural (HMF) is a cat- FDCA yield using high catalyst loading.34 However,
alytic method to utilize carbohydrate biomass in which such a Ru/C catalyst needs activation before reuse. More
oxidation leads to the replacement of polyethylene recently, our group developed a new class of MnCo2O4
terephthalate (PET), the third-largest globally produced spinel supported Ru catalyst for selective oxidation of
polymer.8 Several attempts have been made to replace HMF into DFF or FDCA according to solvents.35,36
PET using a renewable polymer, polyethylene fura-
Here we report exploration of basic metal oxide-
noate (PEF), because PEF is a carbon neutral bio-based supported Ru catalysts to use for this important reaction
material, and its physical performance, such as glass in the absence of a base.
transition temperature and gas permeability, is greater
than PET (Figure 1).9,10
HMF was identified as an intermediate chemical
with the potential to be converted to the PEF poly-
mer precursor, 2,5-furan dicarboxylic acid (FDCA).
FDCA is obtained by full oxidation of both the formyl
and hydroxymethyl groups of HMF to a carboxylic
acid. HMF can be oxidized to FDCA with quantitative
amounts of conventional oxidants such as KMnO4 or
HNO3.11 Recently, enzyme-mediated preparation meth-
ods of FDCA were reported.12–14 However, most studies
focus on heterogeneous catalysis using molecular oxy-
gen as an oxidant under aqueous conditions; further-
more, several studies supported Au and Pt catalysts for
2. Experimental
2.1 Materials
Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, 99%,
ACS reagent), aluminium nitrate nonahydrtate (Al(NO3)3·
9H2O, ≥98%, ACS reagent), sodium hydroxide (NaOH, ≥
97%, ACS reagent), sodium bicarbonate (NaHCO3, ≥ 99.7%,
ACS reagent), ruthenium(III) chloride (RuCl3, Ru content
45–55%), 5-hydroxymethyl-2-furfural (HMF, 99%), MgAl-
hydrotalcite (Mg6Al2(CO3)(OH)16·4H2O), MgO (97%, ACS
reagent), ZrO2 (powder, 99% trace metals basis), CeO2 (pow-
der, 99.995% trace metals basis) and La2O3 (99.99% trace
this purpose.15–17 Moreover, excess inorganic bases such metals basis) were purchased from Sigma-Aldrich (USA).
as NaOH or Na2CO3 were used in some processes.18–21
All chemicals were directly used without further purification
Hydrotalcite-supported Au nanoparticles gave 99%
unless otherwise mentioned.
FDCA yield in the absence of a base, but its yield was
significantly decreased during recycling.22 Several Pt
and Au based catalysts found to be successful for the
base free HMF oxidation but high cost of the precious
metals is still an issue.23–26 Spinel oxides with active
metal like cobalt and manganese were also successfully
employed in this selective oxidation reaction; but in
acetic acid solvent.27 Ruthenium is a potential oxidizing
metal; various ruthenium supported heterogeneous cata-
lysts were used for HMF oxidation to 2,5-diformylfuran
(DFF).28–31 However, ruthenium supported materials
were not successful for HMF oxidation to FDCA
2.2 Methods
Powder X-ray diffraction (PXRD) was carried out on a Bruker
M18XHF22 system using Cu Kα radiation (λ=1.54056 Å).
The operating voltage and current were 40 kV and 40 mA,
respectively with a scanning speed of 2 deg/min, for data col-
lection. The data were processed using the XPRESS (version
1.0.3) software. Transmission electron microscope (TEM)
images were taken with a Maker FEI, model Technai G2
microscope with an acceleration voltage of 200 keV using
carbon-coated 200 mesh copper grids. Total surface anal-
ysis of the materials obtained by NH3 and CO2 TPD was

J. Chem. Sci.
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Page 3 of 9 156
performed on an Autochem II 2920 (Micromeritics Corp.). fleshed with O2 for 3 times. Pressurized with O2 again and
Samples were pretreated using He at 100 ◦C for 1h, continued heated up to the desired temperature to achieve the operating
with adsorption of NH3 or CO2 at 100 ◦C for 1h, and followed pressure; all the reactions were carried for 4h with 650 rpm
◦
◦
◦
by gas desorption from 100 C to 800 C (rate 10 C/min). stirring. Immediately after the reaction reactor was cooled to
The specific surface area of the catalyst material was deter- room temperature and samples were analyzed with HPLC.
mined in a BET surface analyzer (Bruanuer-Emmett-Teller,
ASAP2010, Micromeritics, USA) using N2 as the adsorbent
at liquid nitrogen temperature (77 K) in a relative pressure
3. Results and Discussion
(P/P0) range of 0 to 0.25. The samples (in powder form) were
degassed in the air for over 12h at 100 ◦C prior to analysis.
Ruthenium was simply loaded on the metal oxides using
XPS measurements were carried out on a Thermo Fischer
Scientific K-alpha using monochromatized Al Kα radiation
wet impregnation method as reported previously.29
PXRD patterns of the synthesized Ru/metal oxides were
the same as the metal oxides reported in the literature.
(hν = 1486.6 eV) and processed using Thermo Avantage soft-
ware. NMR analysis was performed on a Bruker Spectrospin
300 (Bruker Corporation, Germany) in a DMSO-d6 solvent. No reflection was observed for ruthenium/ruthenium
The conversion of HMF and the yield of the products, includ-
ing FDCA were determined using high-performance liquid
chromatography (HPLC, Agilent 1200 series) equipped with
a refractive index (RI) detector and an Aminex HPX-87H
column (Bio-Rad Laboratories, Inc.). The HPLC analysis
conditions were as follows: eluent, 0.01 N H2SO4; flow rate,
oxide in these materials, implying that the ruthenium
is homogeneously loaded on the support. PXRD analy-
sis of Ru/MgAlO and Ru/MgO showed mixed metal
oxide and metal oxide phase and the right forma-
tion of mixed metal oxide MgAlO could be verified.
Ru/La2O3 contains pure La2O3 phase along with some
La(OH)3 impurity phase. Ru/ZrO2 and Ru/CeO2 con-
tain tetragonal zirconium phase and cubic CeO2 phase,
respectively (Figure 2). To understand the morphol-
ogy and nature of ruthenium loading TEM was carried
out and each material exhibited different morphology
(Figure S1, Supplementary Information). Ru/MgAlO
showed agglomerated small platelet materials form-
ing a sponge-like structure and Ru/MgO showed larger
platelet-like materials. Ru/La2O3 showed agglomerated
coral-like structure rather than the platelet and both
Ru/ZrO2 and Ru/CeO2 showed distorted sphere-like
structure. Presence of ruthenium over these materials
was confirmed by SEM-EDX analysis.
◦
0.6mL/min; and column temperature, 45 C. Prior to the
HPLC analysis, all samples were subjected to dilution with
water (HPLC grade) to prevent signal overload and damage
to the system.
2.3 Synthesis of MgAlO
The material was prepared by co-precipitation under low
supersaturation. In this method, two solutions, namely, solu-
tion (A) containing the desired amount of magnesium and alu-
minium nitrates and solution (B) having precipitating agents
(i.e., NaOH and Na2CO3) were added slowly (1mL/min) and
simultaneously using Schott electronic titrator, at a constant
pH 9.5 during addition under vigorous stirring at room tem-
perature. Theadditiontookapproximately90minandthefinal
pH was adjusted to 10 by adding few drops of precipitant solu-
tion. The samples were aged in the mother liquor at 65 ◦C for
18h in an oil bath, filtered, washed (until total absence of
nitrates and sodium in the washing liquids) and dried in an air
oven at 100–110◦C for 12h. This was calcined at 500 ◦C for
2h and named as MgAlO.
Oxidation of HMF was performed using synthesized
materials as a catalyst in a water medium (Table 1).19
HMF bears two oxidizable moieties corresponding
Ru/CeO2
Ru/ZrO2
Ru/La2O3
2.4 Ruthenium impregnation
Ru/MgO
Ru/MgAlO
Ruthenium was loaded on the support using wet
impregnation method like the calculated amount of RuCl3
(to achieve 2wt% metal) was dissolved in 100mL of water to
which 2 g of support (MgAlO prepared above or other metal
oxides purchased) was added and stirred at room temperature
for 3h. Filtered and washed with copious amount of water
and dried in a vacuum oven at 30 ◦C.
10
20
30
40
50
60
70
80
2.5 Catalytic reactions
2 Theta, deg
All the reactions were carried out in a 50mL Parr reactor;
HMF, solvent, and catalyst were added and the reactor was
Figure 2. Powder X-ray diffraction analysis of synthesized
materials (with 2wt% Ru loading).

156 Page 4 of 9
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Table 1. Conversion of HMF and selectivity of products in the oxidation of HMF
using a Ru catalyst supported on various metal oxides.a
Yield (%) b
Entry Catalyst
HMF conversion (%) b DFF HMFCA FFCA FDCA LA
1
2
3
4
5
6
Ru/MgAlO
100
88
4.6
89
100
100
0.5
2.8
0
45
86
7
0
0
0
0
0
0
0.2
21
1.4
35
14
26
99
58
0.8
7.0
0
0
Ru/MgO
Ru/La2O3
Ru/CeO2
Ru/ZrO2
5.4
2.4
3.2
0
c
RuCl3
18
30
aReaction conditions: HMF (252mg, 2mM), catalyst (200mg, 2wt% Ru), H2O
(20mL), O2 (90 psi), 140 ◦C, 4h.
bDetermined by HPLC.
cEquimolar to supported Ru.
Figure 3. Oxidation pathways from HMF to FDCA.
to formyl and hydroxymethyl groups. From HMF, Homogeneous ruthenium catalyzed a reaction in the
oxidation of the hydroxymethyl group yields DFF, while presence of RuCl3 prefers over oxidation of HMF to
oxidationoftheformylgroupyields5-(hydroxymethyl)- LA and FA rather than FDCA (Entry 6). In all sup-
2-furan carboxylic acid (HMFCA). Further oxidation of ports, HMFCA was not found as an intermediate, which
DFFandHMFCAcanyieldFDCAvia5-formyl-2-furan indicated that oxidation of HMF using metal oxides-
carboxylic acid (FFCA). Therefore, DFF, HMFCA, supported Ru catalyst proceeds via DFF.
FFCA can be obtained as intermediates during the oxi-
dation of HMF to FDCA (Figure 3).
From these results, the order of Ru catalyst activity for
HMF oxidation was determined according to supports:
Among the materials investigated, MgAlO support MgAlO > MgO > ZrO2 > CeO2 ꢀ La2O3. The
obtained from layered double hydroxide (LDH) was surface area of these materials was measured to find the
found to be highly active with a 100% HMF conver- reason the activity (Table 2). The order of BET surface
sion and a 99% FDCA yield (Entry 1).20 MgO supports area of the materials were in the order of Ru/MgAlO >
showed high activity towards HMF conversion, but the Ru/MgO ꢀ Ru/La2O3 > Ru/ZrO2 > Ru/CeO2 and
yield of FDCA was moderate (58%) (Entry 2). Instead, it confirms that the high activity of MgAlO and MgO
DFF and FFCA intermediates produced by the partial supports were due to its high surface area. But this anal-
oxidation of HMF still remained. La2O3 support has a ysis is not sufficient to explain the complete trend in the
very low HMF conversion of less than 5%, and a de- catalytic activity. Though these materials were different
formylated product, levulinic acid (LA), was obtained morphology as observed by TEM measurements reason
as a major product (Entry 3). ZrO2 support has a low for different catalytic activity might be due to different
yield for FDCA, but DFF and FFCA were obtained in basic nature of the support.
45% and 35% yields, respectively (Entry 4). CeO2 was
To find the basicity of these materials temperature
not sufficient for the oxidation of HMF up to FDCA, but programmed desorption (TPD) were investigated
CeO2 was converted mainly to DFF (86%) (Entry 5). (Table 3). TPD analysis of the catalysts indicates

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Table 2. Surface properties of synthesized materials.
Catalyst
BET surface area (m2/g)
Pore volume (cm3/g)
Aver. pore diameter (A)
Ru/MgO
Ru/MgAlO
Ru/La2O3
Ru/CeO2
Ru/ZrO2
73
200
13
3.9
4.5
0.372
0.394
0.051
0.019
0.018
203
79
156
191
157
Table 3. Temperature programmed desorption analysis of synthesized materials.
NH3 TPD CO2 TPD
Amount of
Maximum
desorption (◦C)
Maximum
HN3 (mmol/g) desorption (◦C)
Amount of
Total acidity
(mmol/g)
Total basicity
(mmol/g)
Name
CO2 (mmol/g)
Ru/MgAlO 279
1.66
195
217
346
156
221
340
325
384
484
–
4.40
0.22
1.40
0.08
0.11
5.90
3.40
0.30
1.40
–
6.02
6.09
5.10
1.66
3.10
Ru/MgO
293
354
1.30
1.80
Ru/La2O3
314
473
645
313
510
305
0.87
0.38
0.24
0.19
0.02
0.10
1.49
0.21
0.10
Ru/ZrO2
0
0
Ru/Ce2O3
–
–
that Ru/MgAlO, Ru/MgO, and Ru/La2O3 have total did not affect the product, i.e., the formed product was
basicity of 6.02, 6.09, and 5.10mmol/g, respectively stable under high-pressure conditions in presence of a
while Ru/ZrO2 and Ru/CeO2 show no basic property. catalyst. However, the Ru/MgAlO catalyst filtered after
In addition, the MgAlO and MgO support contain only the reaction washed with water and dried for reuse was
weak and moderate basic sites while La2O3 contains not completely active for the consecutive cycles, which
moderate and high basic sites. However, due to the pres- were also associated with a marginal amount of weight
ence of Lewis acidic character of lanthanum, La2O3 also loss (Figure 4). ICP analysis was done for catalyst
contains moderate and high acidic sites, which can be after first cycle which showed that 24.1% Ru reduc-
the cause for low catalytic activity. In general, the cata- tion which clearly indicated the leaching of the active
lyst supports with higher total basicity resulted in higher metal. This observation was different from materials
HMF conversion and FDCA yield.
reportedintheliterature, i.e., Ausupportedhydrotalcites
Variation studies of temperature and O2 pressure (MgAl-OH), which was found to have reusable multiple
were performed to understand the influence of reac- cycles.22 To solve this problem, the Ru/MgAlO catalyst
tion parameters on the oxidation of HMF to FDCA was calcined at different temperatures including 500 ◦C,
◦
◦
over Ru/MgAlO (Figure S2, Supplementary Informa- 700 C, and 900 C, and the reusability was verified
tion). Asthetemperatureincreased, theHMFconversion (Figure 4).
increased with FDCA yield, and it was clearly explained
As the calcination temperature increases the structure
that HMF oxidation passed through DFF and FFCA as of the oxide materials changes along with their acid/base
previously mentioned. It was also clarified that 140 ◦C properties. However, these catalysts were not found to
was the optimum temperature for the complete con- be reusable, and some catalyst weight loss was observed
version of intermediates to FDCA. Pressure variation after the first use. FDCA is a compound with high acidity
studies showed that 90 psi of O2 pressure was enough (pKa = 2.28), which can readily exchange the counter
for the complete conversion of HMF to FDCA; fur- cation. Therefore, it was understood that Ru might be
thermore, it was confirmed that increase in O2 pressure leached from the MgAlO support because Ru could be

156 Page 6 of 9
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(2018) 130:156
Figure 4. Reusability of the Ru/MgAlO catalyst with dif-
ferent calcinations temperatures. Reaction conditions: HMF
(252mg, 2mM), catalyst (200mg, 2wt% Ru), H2O (20mL),
O2 (90 psi), 140 ◦C, 4h.
Figure 6. Influence of catalyst amount on HMF oxidation
using Ru/MgO catalyst. Reaction conditions: HMF (252mg,
2mM), Ru/MgO, H2O (20mL), O2 (90 psi), 140◦C, 4h.
coordinated to the FDCA anion, as depicted in Figure 5. materials, and FDCA yield was reached to a
To confirm this phenomenon, commercial FDCA was maximum at a reaction temperature of 160 ◦C, whereas
◦
stirred with a Ru/MgAlO catalyst at 100 ◦C in a water temperature over 160 C induced decomposition of
solvent, and it was observed that the catalyst was dis- HMF took place to give considerable amounts of
solved in the FDCA solution.
levulinic acid and formic acid. Decomposition prod-
Since Ru/MgAlO failed to be reused for the oxidation ucts formation was also observed in reactions with
of HMF to FDCA, Ru/MgO with acceptable activity high reaction temperature and lower reaction time
was selected for further studies. Influence of catalyst (2 h).
loading was examined in order to increase HMF con-
To further study the reusability of these materials
version. Expectedly, the catalyst loading enhanced the immediately after the reaction, the catalyst was washed
HMF conversion along with FDCA yield, and 400mg with water then dried and used for the next cycle.
of the catalyst was sufficient for the complete conver- Figure 8 shows that these materials were reusable up
sion of HMF with greater than 90% of FDCA yield to 5 cycles with a relatively identical activity. To under-
using 90 psi of O2 in 4h. Further, it needs to be men- stand the nature of ruthenium species responsible for
tioned here that the yield of the FDCA was also high the oxidation XPS analysis was done (Figure S3, Sup-
(Figure 6).
plementary Information). It was noticeable that XPS
Temperature variation studies of Ru/MgO were analysis of Ru/MgO showed Ru 3d5/2 peak at 280.5 for
performed to understand the progress of the reaction the fresh catalyst while for the used one this reflection
(Figure 7). Its trend was similar to that of previous was at 279.5. This change in the peak position was due to
Figure 5. Proposed mechanism of leaching Ru and dissolving the metal oxide support.

J. Chem. Sci.
(2018) 130:156
Page 7 of 9 156
be used for polymerization reaction without any further
purification.
4. Conclusion
In conclusion, the base free oxidation of HMF to FDCA
using molecular oxygen was studied over different metal
oxide supported ruthenium catalysts under aqueous con-
ditions. Basicity of the support played an important role
on the complete conversion of HMF to FDCA further,
the presence of Lewis acid sites in the support leads to
de-formylation reaction and weak basic sites leads to
formation DFF. Among the 2wt% Ru containing basic
supports, MgAlO was found to be highly active; how-
ever, this support seriously dissolved during the reaction
due to the highly acidic nature of the formed FDCA. On
the other hand, a MgO-supported Ru catalyst, a bit less
active than Ru/MgAlO, could be used 5 times with 90 psi
of O2 at 160 ◦C in 4h without a significant drop of FDCA
yields. FDCA formed during the reaction can be easily
crystallized in water and which can be directly used for
additional polymerization studies without any further
purification.
Figure 7. Influence of temperature on HMF oxidation
using the Ru/MgO catalyst. Reaction conditions: HMF
(252mg, 2mM), Ru/MgO (200mg), H2O (20mL), O2
(90psi), 4 h.
Supplementary Information (SI)
Supplementary data is available for TEM, TEM-EDX, HPLC,
XPS, and NMR studies. Supplementary Information is avail-
able at www.iac.ac.in/chemsci.
Acknowledgements
We acknowledge financial support for this research by the
Internal Research Program (PEO18030) of Korea Institute
of Industrial Technology (KITECH) and this work was sup-
Figure 8. Reusability of the Ru/MgO catalyst on HMF oxi- ported by the National Research Council of Science &
dation. Reaction conditions: HMF (252mg, 2mM), Ru/MgO
Technology (NST) grant by the Korea government (MSIP)
(No. CMP-16-04-KITECH).
(400mg), H2O (20mL), O2 (90psi), 160◦C, 4h.
the electronic environment of the Ru metal was changed
during the reaction but not associated with any change
in the valance state of ruthenium.37
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