Efficient aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid on Ru/C catalysts

Article abstract of DOI:10.1007/s11426-016-0489-3

2,5-Furandicarboxylic (FDCA) is a potential substitute for petroleum-derived terephthalic acid, and aerobic oxidation of 5-hydroxymethylfurfural (HMF) provides an efficient route to synthesis of FDCA. On an activated carbon supported ruthenium (Ru/C) catalyst (with 5 wt% Ru loading), HMF was readily oxidized to FDCA in a high yield of 97.3% at 383 K and 1.0 MPa O₂ in the presence of Mg(OH)₂ as base additive. Ru/C was superior to Pt/C and Pd/C and also other supported Ru catalysts with similar sizes of metal nanoparticles (1–2 nm). The Ru/C catalysts were stable and recyclable, and their efficiency in the formation of FDCA increased with Ru loadings examined in the range of 0.5 wt%–5.0 wt%. Based on the kinetic studies including the effects of reaction time, reaction temperature, O₂ pressure, on the oxidation of HMF to FDCA on Ru/C, it was confirmed that the oxidation of HMF to FDCA proceeds involving the primary oxidation of HMF to 2,5-diformylfuran (DFF) intermediate, and its sequential oxidation to 5-formyl-2-furancarboxylic acid (FFCA) and ultimately to FDCA, in which the oxidation of FFCA to FDCA is the rate-determining step and dictates the overall formation rate of FDCA. This study provides directions towards efficient synthesis of FDCA from HMF, for example, by designing novel catalysts more efficient for the involved oxidation step of FFCA to FDCA.

Full text of DOI:10.1007/s11426-016-0489-3

SCIENCE CHINA
Chemistry
doi: 10.1007/s11426-016-0489-3
• ARTICLES •
· SPECIAL ISSUE · Green Chemistry
Efficient aerobic oxidation of 5-hydroxymethylfurfural to
2,5-furandicarboxylic acid on Ru/C catalysts
Lufan Zheng1, Junqi Zhao1, Zexue Du1, Baoning Zong1* & Haichao Liu2*
1Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
2Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Stable and Unstable Species,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Received November 23, 2016; accepted January 5, 2017; published online May 9, 2017
2,5-Furandicarboxylic (FDCA) is a potential substitute for petroleum-derived terephthalic acid, and aerobic oxidation of
5-hydroxymethylfurfural (HMF) provides an efficient route to synthesis of FDCA. On an activated carbon supported ruthenium
(Ru/C) catalyst (with 5 wt% Ru loading), HMF was readily oxidized to FDCA in a high yield of 97.3% at 383 K and 1.0 MPa
O2 in the presence of Mg(OH)2 as base additive. Ru/C was superior to Pt/C and Pd/C and also other supported Ru catalysts with
similar sizes of metal nanoparticles (1–2 nm). The Ru/C catalysts were stable and recyclable, and their efficiency in the formation
of FDCA increased with Ru loadings examined in the range of 0.5 wt%–5.0 wt%. Based on the kinetic studies including the
effects of reaction time, reaction temperature, O2 pressure, on the oxidation of HMF to FDCA on Ru/C, it was confirmed that the
oxidation of HMF to FDCA proceeds involving the primary oxidation of HMF to 2,5-diformylfuran (DFF) intermediate, and
its sequential oxidation to 5-formyl-2-furancarboxylic acid (FFCA) and ultimately to FDCA, in which the oxidation of FFCA
to FDCA is the rate-determining step and dictates the overall formation rate of FDCA. This study provides directions towards
efficient synthesis of FDCA from HMF, for example, by designing novel catalysts more efficient for the involved oxidation step
of FFCA to FDCA.
aerobic oxidation, 5-hydromethylfurfural, 2,5-furandicarboxylic, supported Ru catalyst, base additives, reaction
mechanism
Citation:
Zheng L, Zhao J, Du Z, Zong B, Liu H. Efficient aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid on Ru/C catalysts. Sci
China Chem, 2017, 60, doi: 10.1007/s11426-016-0489-3
1 Introduction
such as fructose, glucose and cellulose [3–5], and converted
to a variety of bulk chemicals and intermediates, such as
2,5-furandicarboxylic acid (FDCA) and 2,5-diformylfuran
(DFF) [6–8].
FDCA has a similar conjugated structure to terephthalic
acid (PTA), and it is considered as a potential substitute for
PTA towards the sustainable production of polyamides and
polyesters [9]. In attempts to achieve efficient aerobic oxi-
dation of 5-hydroxymethylfurfural (HMF) to FDCA, a large
number of homogeneous (e.g. Co/Mn/Br) and heterogeneous
catalysts (e.g. Pt, Au, Pd and Ru-based catalysts) have been
explored [10–19]. Pt, Au and Pd-based catalysts usually re-
Biomass is renewable and largely available in nature, and
it provides the most viable alternative to fossil fuels for
sustainable production of liquid fuels and organic chemi-
cals [1,2]. 5-Hydroxymethylfurfural (HMF), as one of the
important biomass-based platform molecules, has attracted
increasing attention over the past decade. It can be syn-
thesized by acid-catalyzed dehydration of carbohydrates,
*Corresponding authors (email: zongbn.ripp@sinopec.com; hcliu@pku.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2017
chem.scichina.com link.springer.com

2
Zheng et al. Sci China Chem
quire aqueous alkali solutions to obtain high yields of FDCA.
For example, Ait Rass et al. [17] reported an almost quantita-
tive yield of FDCA on Pt-Bi/C catalyst at 373 K and 4 MPa air
in a Na2CO3 solution. On Au-Cu/TiO2 catalyst, Pasini et al.
[18] obtained FDCA yields of 90%–99% at 368 K and 1 MPa
O2 in the presence of NaOH. The presence of alkali or other
base additives can prevent strong adsorption of FDCA onto
the noble metal surfaces and their consequent deactivation,
which can also neutralize and stabilize FDCA. Moreover, the
basic conditions can facilitate the hydration of aldehyde in-
termediates to the corresponding germinal diol intermediates
and their oxidation to FDCA [17].
Compared with Pt, Pd and Au, Ru is much cheaper.
Ru-based catalysts exhibit excellent performance in the
selective oxidation of alcohols, and have now been applied
to the aerobic oxidation of HMF to FDCA. Gorbanev et
al. [20] loaded Ru(OH)x on hydrotalcite (HT), showing
95% yield of FDCA after HMF oxidation at 413 K for 6 h
in water without addition of alkali additive. However, the
catalyst tended to deactivate due to the decomposition of HT
support and Ru leaching. Artz and Palkovits [21] compared
supported Ru catalysts on covalent triazine frameworks and
commercial Ru/C catalysts, and found the superiority of
Ru/CTF-c to Ru/C in term of the FDCA yield (77.6% vs.
62.8%) under the same reaction conditions. Interestingly,
they also found that for the commercial Ru/C catalysts, with-
out being washed with dimethyl sulfoxide (DMSO) before
use, the FDCA yield decreased significantly (from 62.8%
to 41.4%). Zhang and coworkers [22] reported base-free
HMF oxidation to FDCA in 88% yield on commercial Ru/C
catalysts (HMF/Ru (molar ratio)=10) after reaction at 393 K
for 10 h, notwithstanding the observed activity loss imposed
by strong adsorption of FDCA on the Ru surfaces.
2 Experimental
2.1 Catalyst preparation and characterization
Activated carbon supported Ru, Pt, Pd catalysts were pre-
pared by an incipient wetness impregnation method. Briefly,
activated carbon (Sinopharm Chemical, China) was dried
at 393 K overnight in air, into which aqueous solutions of
RuCl3·nH2O (Sinopharm Chemical, China), H2PtCl6·6H2O
(Sinopharm Chemical, China), PdCl2 (Sinopharm Chemical,
China; with 0.5 mL of concentrated aqueous HCl solution)
were added in turn at 298 K. After impregnation for 6 h,
the catalysts were dried overnight in air at 383 K, and then
reduced in a flowing gas of 20% H2/N2 at 673 K for 4 h.
Following the similar impregnation procedures, metal
oxide-supported Ru catalysts were also prepared. The sup-
ports included ZrO2 (Sinopharm Chemical, China), Al2O3
(Sinopharm Chemical, China) and TiO2 (J&K Scientific,
China), and they were calcined at 673 K in air prior to use.
Base additives, including MgO, Al2O3, Al(OH)3, Mg(OH)2,
CaCO3, NaOH, Na2CO3 and La2O3 were purchased from
Sinopharm Chemical, China. Hydrotalcite samples with
different Mg/Al molar ratios were prepared by a homogenous
co-precipitation method, as described in our previous work
[25].
Transmission electron microscope (TEM) images were
taken on a Philips Tecnai F30 FEG-TEM (Netherlands) at
300 kV. The catalysts were dispersed in ethanol uniformly
and then placed on carbon-coated Cu grids. The average
sizes and the size distribution of metal particles were ob-
tained by counting at least 300 particles randomly from the
TEM images.
2.2 Selective oxidation of HMF to FDCA
Recently, we reported that Ru/C efficiently catalyzed the
oxidation of HMF to DFF with a high yield of 96% in toluene
[23], and to FDCA with a moderate yield of 78% in water
upon addition of hydrotalcite (HT) base (Mg/Al=3) [24,25].
This work presents a systematic study on the synthesis of
FDCA from the aerobic oxidation of HMF on Ru/C in the
presence of base additives. We compared the performances of
Ru/C and C-supported Ru, Pt and Pd catalysts and also differ-
ent supported Ru catalysts. To better understand the functions
of base additives, we investigated some representative solid
oxides and hydroxides with basicity (MgO, Mg(OH)2, La2O3,
Al2O3, Al(OH)3, and also HT with different Mg/Al ratios) as
well as alkaline compounds (NaOH, CaCO3, and Na2CO3).
We also examined the effects of reaction parameters includ-
ing reaction temperature, O2 pressure and reaction time on
the oxidation of HMF to FDCA. Finally, we discussed the
reaction pathways for the oxidation of HMF to FDCA, and
confirm DFF instead of 5-hydroxymethyl-2-furancarboxylic
acid (HMFCA) as the key intermediate for the formation of
FDCA.
Oxidation reactions of HMF were carried out in a Teflon-lined
stainless steel autoclave (100 mL), equipped with a mechani-
cal stirrer and temperature measurement gauges. In a typical
run, 1 mg HMF (98%, Alfa Aesar, USA), 0.04 g Ru/C and 0.2
g Mg(OH)2 were introduced into an autoclave (100 mL) con-
taining 20 mL deionized H2O. The autoclave was then pres-
surized to 1.0 MPa with oxygen, and heated from 298 K to
383 K with vigorous stirring at a speed of 700 r/min during
the reaction to eliminate the mass-transfer limitation. After
the reaction, the catalysts were removed by filtration, and the
reactants and products were analyzed by a high-performance
liquid chromatography (HPLC; Shimadzu LC-20A, Japan)
using an UV detector and an Alltech OA-1000 (USA) organic
acid column (with a mobile phase of 0.005 M H2SO4, at a flow
rate of 0.6 mL/min and an oven temperature of 343 K). In the
recycling experiments, the used catalysts were washed thor-
oughly with deionized water and dried in air at 343 K before
the next cycle. After each cycle, about 30% more Mg(OH)2
was added to compensate the consumption from its reaction

Zheng et al. Sci China Chem
3
with the carboxylic acid products. The HMF conversion and
product selectivity are calculated as follows:
tion on activated carbon (C)-supported Ru, Pt and Pd catalysts
in the presence of Mg(OH)2 as a base additive at 383 K and 1.0
MPa O2. For comparison, ZrO2, TiO2 and Al2O3-supported
Ru catalysts were also examined. While C is frequently used
as an inert support for noble metal catalysts in HMF oxida-
tion [26], ZrO2, TiO2 and Al2O3 are widely used supports in
alcohol oxidation reactions [27]. TEM images and size distri-
butions for these catalysts are displayed in Figure 1, showing
that their metal particles were well dispersed and possessed
similar mean diameters of 1.1–2.1 nm.
Conversion (%)
mol of reactant charged mol of reactant left
=
× 100
× 100
mol of reactant charged
Selectivity (%)
mol of product
mol of reactant charged mol of reactant left
=
For the selective oxidation of HMF, it is known that FDCA
is a secondary oxidation product via 2,5-diformylfuran (DFF)
intermediate, and its selectivity generally increases with in-
creasing the HMF conversions in the presence of base addi-
tives [25]. Therefore, the different catalysts in Table 1 were
compared at full conversion of HMF, in order to rigorously
3 Results and discussion
3.1 Activity and selectivity in HMF oxidation reaction
Table 1 shows the conversion and selectivity of HMF oxida-
Table 1 HMF conversion and product selectivity of activated carbon-supported Pt, Pd and different supported Ru catalysts in aerobic oxidation of HMF a)
Selectivity (%)
b)
Entry
Catalyst
HMF conversion (%)
Carbon balance (%)
FDCA
97.3
73.4
33.1
82.2
80.8
86.1
FFCA
0
1
2
3
4
5
6
Ru/C
Pt/C
100
100
100
100
100
100
97.3
91.6
85.4
86.3
86.1
88.0
18.2
52.3
4.1
Pd/C
c)
c)
Ru/ZrO2
Ru/TiO2
Ru/Al2O3
5.3
c)
1.9
a) Reaction conditions: 383 K, 8 h, 1.0 MPa O2, 0.1 g HMF, HMF/metal=40:1 (molar ratio), 20 mL H2O, 0.2 g Mg(OH)2; b) 5.0 wt% metal loading; c)
HMF/metal=30:1 (molar ratio).
Figure 1 TEM images (scale bar=20 nm) and size distributions of different metal catalysts. (a) Ru/C; (b) Pt/C; (c) Pd/C; (d) Ru/ZrO2; (e) Ru/TiO2; (f) Ru/Al2O3.

4
Zheng et al. Sci China Chem
show their efficiency in the formation of FDCA. As shown
in Table 1 (entries 1–3), Ru/C was selective to FDCA at
383 K and 1.0 MPa O2, and as the only product detected by
HPLC, its selectivity reached as high as 97.3% (entry 1) at
100% HMF conversion after reaction for 8 h, corresponding
to its yield of 97.3%. The FDCA selectivity was much higher
than that on Pt/C and Pd/C, being 73.4% and 33.1%, respec-
tively, under the identical reaction conditions. Pt/C and Pd/C
also led to the formation of 5-formyl-2-furancarboxylic acid
(FFCA) with a selectivity of 18.2% and 52.3%, respectively
(entries 2 and 3), revealing that Pt/C and Pd/C are inferior to
Ru/C in the selective oxidation of HMF to FDCA.
To examine the support effect, similar Ru nanoparticles
(1.1–1.2 nm) with narrow size distributions, as shown in
Figure 1, were prepared on the four different supports.
Ru/ZrO2, Ru/TiO2 and Ru/Al2O3 exhibited lower activity,
and thus in order to reach the full conversion of HMF, they
required lower HMF/metal molar ratio of 30/1, relative to
40/1 used for Ru/C (Table 1, entries 1 and 4–6). Moreover,
they were less selective for the formation of FDCA, and the
FDCA selectivities fell in the range of 81%–86%. Ru/Al2O3
achieved a higher FDCA selectivity (86.1%) than Ru/ZrO2
(82.2%) and Ru/TiO2 (81.1%). In addition, it is noted that
the ZrO2, TiO2, and Al2O3 supports led to lower carbon
balance (86.1%–88.0% vs. 97.3%), although the underlying
reason was not clear, which may be attributed to the favored
formation of unidentified degradation and polymerization
byproducts on their surfaces, compared with the inert C
surface [28]. Such comparison reflects that C is the most
preferable support, at least, in the oxidation of HMF.
HMF conversion. This result is different from that in the
presence of Mg(OH)2, as discussed above, and FDCA was
the only detected product with a selectivity of 97.3% (entry
2). HT was found to be efficient for the HMF oxidation to
FDCA in our previous work [25], and the selectivity of FDCA
reached 78.2% in the presence of HT (Mg/Al=3), similar to
the result (76.4%, entry 4) in this work. When the Mg/Al
ratio of HT increased to 4 (entry 3), the FDCA selectivity
increased to 89.6%, and no FFCA was detected. However,
when the Mg/Al ratio decreased to 2 (entry 5), the FDCA se-
lectivity decreased to 73.3% with 5.1% FFCA selectivity. Us-
ing Al2O3 and Al(OH)3 instead, the FDCA selectivities were
47.1% and 62.3% respectively (entries 6 and 7). The pres-
ence of La2O3 led to the dominant formation of FFCA with
a selectivity of 60.5% while the FDCA selectivity was only
25.1% (entry 8). Contrary to La2O3, CaCO3 with stronger ba-
sicity offered the FDCA and FFCA selectivities were 61.9%
and 23.3%, respectively (entry 9). When NaOH and Na2CO3
were added (entries 10 and 11), FDCA was solely formed but
with low selectivities of around 70%. Meanwhile, the low
carbon balances (~70%) and the brown color of the reaction
solutions reflect that their strong basicity facilities the forma-
tion of humins from HMF through its ring-opening and poly-
merization reactions [29,30]. Based on these results, it is clear
that the base additives act as not only neutralizers to stabilize
FDCA, but also promotors for the oxidation of HMF and par-
ticularly FFCA intermediate to FDCA.
It is noted that the efficiency of the Ru/C catalyst in the
formation of FDCA strongly depends on its Ru loading
(Figure 2). To examine such dependence more clearly, the
HMF reaction time was shortened to 4 h from 8 h, the time
generally employed in this work. C support was inactive
for the HMF oxidation, and the conversion and FDCA yield
were less than 1% after 4 h. Under the identical conditions,
loading 0.5 wt% Ru on C support led to a 100% HMF
conversion, and dominant formation of FDCA and FFCA in
Using Ru/C (with 5 wt% Ru loading) as the catalyst, differ-
ent base additives were examined, in addition to Mg(OH)2,
in the selective oxidation of HMF to FDCA. As shown in
Table 2, in the absence of any bases (entry 1), FDCA and
FFCA were formed in selectivities of 52.2% and 38.4%, re-
spectively, and the carbon balance was around 90.6% at 100%
Table 2 Effect of base additives on HMF conversion and product selectivity of Ru/C in aerobic oxidation of HMF a)
Selectivity (%)
Entry
Base additive
HMF conversion (%)
Carbon balance (%)
FDCA
52.2
97.3
89.6
76.4
73.3
47.1
62.3
25.1
61.9
72.3
68.5
FFCA
38.4
0
none
Mg(OH)2
HT (Mg/Al=4)
HT (Mg/Al=3)
HT (Mg/Al=2)
Al2O3
1
2
100
100
100
100
100
100
100
100
100
100
100
90.6
97.3
89.6
76.4
78.4
62.4
66.8
85.6
85.2
72.3
69.7
3
0
4
0
5
5.1
15.3
4.5
60.5
23.3
0
6
7
Al(OH)3
8
La2O3
9
CaCO3
10
11
NaOH
Na2CO3
1.2
a) Reaction conditions: 383 K, 8 h, 1.0 MPa O2, 0.1 g HMF, HMF/metal=40:1 (molar ratio), 20 mL H2O, 0.2 g base.

Zheng et al. Sci China Chem
5
tivity declined to 54.7% with the increase in the FFCA selec-
tivity to 29.1%, close to the values of Ru/C with 0.5 wt% Ru
loading. Such negative effect of excess C support appears to
be due to its strong adsorption of FFCA intermediate and thus
the prohibited oxidation of FFCA on Ru surface to FDCA.
Taken together, these results suggest that the higher Ru load-
ings, while maintaining the higher dispersion of Ru nanopar-
ticles, on C support are beneficial to the oxidation of HMF to
FDCA.
The stability and recyclability of the Ru/C catalysts were
also examined in the HMF oxidation. Figure 4 shows the rep-
resentative results on Ru/C with 5 wt% Ru loading at 383 K
and 1.0 MPa O2 in the presence of Mg(OH)2. The HMF con-
version remained to be 100%, and FDCA was the only de-
tected product and its selectivity hardly changed over the six
cycles. Inductively coupled plasma-atomic emission spec-
trometry (ICP-AES) analysis showed no detectable leaching
of Ru species in the reaction solution after six cycles. Char-
acterization of the catalyst by TEM (Figure 5) shows no es-
sential change in the size distribution and the mean size of
the Ru nanoparticles (1.1±0.3 nm) after six cycles, compared
with the fresh Ru/C catalyst (1.1±0.2 nm, Figure 1(a)). These
results demonstrate that Ru/C is stable and recyclable under
the reaction conditions in this work.
Figure 2 Effect of Ru loading of Ru/C catalysts on their catalytic perfor-
mances in the aerobic oxidation of HMF to FDCA. Reaction conditions: 383
K, 4 h, 1.0 MPa O2, 0.1 g HMF, HMF/metal=40:1 (molar ratio), 20 mL H2O,
0.2 g Mg(OH)2.
47.8% and 34.1% selectivity, respectively. With increasing
the Ru loadings from 0.5 wt% to 5.0 wt%, the FDCA selectiv-
ity increased from 47.8% to 82.7%, while the FFCA selectiv-
ity concurrently decreased from 34.1% to 11.4% (Figure 2).
For these Ru/C catalysts with different Ru loadings, the Ru
nanoparticles were well dispersed on the C support surface
and the mean sizes were similar (1.0–1.1 nm), as shown in
Figures 1(a) and 3. Therefore, the difference in the FDCA
selectivity for these catalysts is clearly not due to the differ-
ence in the size of the Ru nanoparticles, and the consequent
effect on their oxidation activity. Further studies indicate that
the observed lower FDCA selectivity at lower Ru loadings ap-
pears to be related to the presence of excess C support when
compared at a given amount of Ru. One such experiment was
carried out using a physical mixture of Ru/C (with 5 wt% Ru
loading) and C support. By keeping the total C amount equal
to that for Ru/C with 0.5 wt% Ru loading, the FDCA selec-
3.2 Effects of reaction parameters on selective oxidation
of HMF to FDCA
Figure 6 shows the evolution of the HMF conversion and
product selectivity with the reaction time at 383 K and 1.0
MPa O2. It is noted that at the very beginning, the HMF con-
version reached 73.2% with the dominant formation of DFF
(51.2%) and FFCA (31.3%), and FDCA was not detected.
Such observed HMF conversion was contributed from the
Figure 3 TEM images (scale bar=20 nm) and size distributions of Ru/C catalysts with different Ru loadings. (a) 0.5 wt%; (b) 1.0 wt%; (c) 2.0 wt%; (d) 3.0
wt%.

6
Zheng et al. Sci China Chem
from 90.5% to 0. Further increase in the temperature to 423
K led to a slight decrease in the FDCA selectivity to 89.3%,
most likely due to the poor stability of HMF and its favored
condensation and degradation reactions to some unidentified
byproducts at higher temperatures.
Figure 8 shows that the HMF conversion remained to be
100% in the O2 pressure range 0.2–1.5 MPa. With increasing
the O2 pressure from 0.2 MPa to 1.0 MPa, the FDCA selec-
tivity increased from 52.0% to 82.7%, and the FFCA selec-
tivity decreased from 41.5% to 11.4%, which then remained
the same up to 1.5 MPa.
Figure 4 HMF conversion and FDCA selectivity for six reaction cycles on
Ru/C (with 5 wt% Ru loading). Reaction conditions: 383 K, 8 h, 1.0 MPa
O2, 0.1 g HMF, HMF/metal=40:1 (molar ratio), 20 mL H2O, 0.2 g Mg(OH)2.
3.3 Reaction pathways for aerobic oxidation of HMF to
FDCA
For the aerobic oxidation of HMF to FDCA on different sup-
ported metal catalysts, it can proceed following two alter-
native routes, differing in the primary oxidation of the hy-
droxyl group of HMF to DFF or its aldehyde group to 5-hy-
droxymethyl-2-furancarboxylic acid (HMFCA) intermediate
[31–36]. As shown in Figure 6, HMFCA was not detected,
and DFF formed as the primary product on Ru/C in the pres-
ence of Mg(OH)2. Taken together with the observed effect
of reaction temperature and O2 pressure (Figures 7 and 8),
we tentatively propose the reaction pathways for the aerobic
oxidation of HMF to FDCA, as depicted in Scheme 1, in-
volving DFF intermediate, rather than HMFCA, which then
undergoes sequential oxidation to FFCA and FDCA. Com-
paring the evolution trends of DFF, FFCA and FDCA with
time (Figure 6), obviously, the formation of DFF and its ox-
idation to FFCA occurred much faster than the oxidation of
FFCA to FDCA, which indicates that the conversion of FFCA
to FDCA is the rate-limiting step in the oxidation of HMF.
To verify such proposition, the individual reaction steps in-
volved in the oxidation of HMF to FDCA were studied on
heating process that lasted about 20 min to reach 383 K from
298 K. After reaction for 0.5 h at 383 K, the HMF conver-
sion increased from 73.2% to 100%, and the DFF selectivity
decreased sharply from 51.2% to 0. Differently, the FFCA se-
lectivity first increased from 31.3% to 76.1% during the first
0.5 h, and then decreased to 11.4% after 4 h, and ultimately
to 0 when the reaction time was further prolonged to 8 h. The
FDCA selectivity increased rapidly from 0 to 82.7% within
4 h, and then gradually approached the maximum value of
97.9% after reaction for 12 h. These results suggest that
DFF is formed as the primary product for the HMF oxida-
tion, which is subsequently oxidized to FFCA; FFCA is the
key intermediate for FDCA.
Higher reaction temperatures and O2 pressures can accel-
erate the oxidation of FFCA to FDCA on Ru/C. As shown
in Figure 7, the HMF conversion remained to be 100% at
343–423 K. However, when the temperature increased from
343 K to 403 K, the FDCA selectivity increased from 2.9% to
92.6% concurrently with the decrease in the FFCA selectivity
Figure 5 TEM image (scale bar=20 nm) and size distribution of Ru/C after six reaction cycles.

Zheng et al. Sci China Chem
7
Figure 8 Effect of O2 pressure on the catalytic performances of Ru/C. Re-
action conditions: 383 K, 4 h, 0.1 g HMF, HMF/metal=40:1 (molar ratio),
20 mL H2O, 0.2 g Mg(OH)2.
Figure 6 Effect of the reaction time on the catalytic performances of Ru/C
in aerobic oxidation of HMF. Reaction conditions: 383 K, 1.0 MPa O2, 0.1
g HMF, HMF/metal=40:1 (molar ratio), 20 mL H2O, 0.2 g Mg(OH)2.
much smaller than those of HMF oxidation to DFF (9.15×10−5
s−1) and DFF to FFCA (8.05×10−5 s−1). Accordingly, the ox-
idation of FFCA to FDCA possessed the higher activation
energy (52.5 kJ/mol), compared with the other two oxida-
tion steps of HMF to DFF (34.2 kJ/mol) and DFF to FFCA
(38.6 kJ/mol). Therefore, the oxidation of FFCA to FDCA as
the rate-determining step dictates the formation rate of FDCA
from HMF on Ru/C.
4 Conclusions
Activated carbon supported Ru nanoparticles (~1 nm) are ef-
ficient and stable in the aerobic oxidation of HMF to FDCA
in the presence of Mg(OH)2 as a base additive, offering a high
FDCA yield of 97% at 383 K and 1.0 MPa O2. Ru/C is supe-
rior to Pt/C and Pd/C and also the other supported Ru catalysts
(including Ru/ZrO2, Ru/TiO2 and Ru/Al2O3) for the forma-
tion of FDCA under the same reaction conditions. Higher Ru
loadings on C support facilitate the HMF oxidation to FDCA
Figure 7 Effect of reaction temperature on product selectivity on Ru/C at
100% HMF conversion. Reaction conditions: 383 K, 4 h, 1.0 MPa O2, 0.1 g
HMF, HMF/metal=40:1 (molar ratio), 20 mL H2O, 0.2 g Mg(OH)2.
Ru/C (with 5 wt% Ru loading). As shown in Table 3, the rate
constant of FFCA oxidation to FDCA (2.49×10−6 s−1) was
Scheme 1 Reaction pathways for aerobic oxidation of HMF to FDCA on Ru/C.
Table 3 Rate constant and apparent activation energy for aerobic oxidation of HMF, DFF and FFCA on Ru/C catalyst a)
b)
c)
Reaction
k (s−1)
Ea (kJ/mol)
34.2
Oxidation of HMF to DFF
Oxidation of DFF to FFCA
Oxidation of HMF to FDCA
9.15×10−5
8.05×10−5
2.49×10−6
38.6
52.5
a) Reaction conditions: 1.0 MPa O2, 0.1 g substrate, substrate/metal=40:1 (molar ratio), 20 mL H2O, 0.2 g Mg(OH)2; b) reaction at 313 K (reaction rate
constants were estimated by assuming a first-order reaction in HMF and DFF); c) reaction at 303–333 K (activation energy was calculated by the Arrhenius
equation).

8
Zheng et al. Sci China Chem
in the range of 0.5 wt%–5.0 wt%. Base additives act as not
only neutralizers for FDCA, but also promotors for the oxi-
dation of HMF to FDCA. It is confirmed that the oxidation of
HMF to FDCA is preceded by oxidation of its hydroxyl group
to DFF intermediate, and the oxidation of FFCA to FDCA is
the rate-determining step.
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