Aerobic oxidation of 5-hydroxymethylfurfural (HMF) effectively catalyzed by a Ce0.8Bi0.2O2-δ supported Pt catalyst at room temperature

Article abstract of DOI:10.1039/c4ra16968a

The oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) was efficiently catalyzed when Pt nanoparticles (NPs) were supported onto a Ce_0.8Bi_0.2O_2-δ solid solution. 98% yield of FDCA was achieved within 30 min at room temperature and the catalyst was reused five times without much loss of FDCA selectivity. It is the first report on the oxidation of HMF, an alcohol and an aldehyde, effectively catalyzed by a ceria-based material supported Pt catalyst. The individual properties of the Pt NPs and the ceria-based support were retained and not affected after their combination. The superior oxygen activation ability of the Bi-doped ceria thoroughly changed the performance of the ceria supported Pt catalyst. Pt NPs were responsible for the formation of the Pt-alkoxide intermediate, followed by β-H elimination with the help of hydroxide ions. Bi-containing ceria accelerated the oxygen reduction process because of the presence of a large amount of oxygen vacancies and the cleavage of the peroxide intermediate promoted by bismuth. These specific functions were well incorporated during the catalytic oxidation cycle, leading to the generation of the highly efficient Pt/Ce_0.8Bi_0.2O_2-δ catalyst for HMF oxidation at room temperature. This journal is

Full text of DOI:10.1039/c4ra16968a

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Aerobic oxidation of 5-hydroxymethylfurfural
(HMF) effectively catalyzed by a Ce0.8Bi0.2O2ꢀd
supported Pt catalyst at room temperature
Cite this: RSC Adv., 2015, 5, 19823
ac
Zhenzhen Miao,ab Tianxiao Wu,a Jingwei Li,ab Ting Yi,ab Yibo Zhang
*
ac
*
and Xiangguang Yang
The oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) was efficiently
catalyzed when Pt nanoparticles (NPs) were supported onto a Ce0.8Bi0.2O2ꢀd solid solution. 98% yield of
FDCA was achieved within 30 min at room temperature and the catalyst was reused five times without
much loss of FDCA selectivity. It is the first report on the oxidation of HMF, an alcohol and an aldehyde,
effectively catalyzed by a ceria-based material supported Pt catalyst. The individual properties of the Pt
NPs and the ceria-based support were retained and not affected after their combination. The superior
oxygen activation ability of the Bi-doped ceria thoroughly changed the performance of the ceria
supported Pt catalyst. Pt NPs were responsible for the formation of the Pt–alkoxide intermediate,
followed by b-H elimination with the help of hydroxide ions. Bi-containing ceria accelerated the oxygen
reduction process because of the presence of a large amount of oxygen vacancies and the cleavage of
the peroxide intermediate promoted by bismuth. These specific functions were well incorporated during
the catalytic oxidation cycle, leading to the generation of the highly efficient Pt/Ce0.8Bi0.2O2ꢀd catalyst
for HMF oxidation at room temperature.
Received 24th December 2014
Accepted 9th February 2015
DOI: 10.1039/c4ra16968a
www.rsc.org/advances
oxidized and 5-hydroxymethyl-2-furancarboxylic acid (HFCA)
was obtained at the initial reaction stage. The following oxida-
Introduction
tion of the alcohol group to aldehyde and thus the production of
5-formyl-2-furancarboxylic acid (FFCA) was the rate-limiting
step.9–12 The oxidation of alcohols is also a pivotal reaction in
organic chemistry, affording versatile carbonyl compounds.
Using molecular oxygen instead of stoichiometric inorganic
oxidants as terminal oxidant is a preferable alternative for both
economic and environmental benets.13–16 Though various
catalysts have been applied successfully in HMF oxidation to
FDCA, long reaction time, high temperature and high molar
ratio of the precious metal to HMF were oen demanded to
obtain a high FDCA yield. It remains a major task to develop
efficient catalysts for HMF oxidation using molecular oxygen as
Using the abundant biomass resources to replace petrochemi-
cals is an urgent demand for sustainable chemistry.1–3
5-Hydroxymethylfurfural (HMF), a key biomass-derived plat-
form chemical that can be obtained by dehydration of carbo-
hydrates in acidic media, and has great potential as a versatile
chemical intermediate.4 The focus of research on the selective
oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) has
increased notably in recent years. FDCA is a possible alternative
to substitute fossil-based terephthalic, isophthalic and adipic
acids for preparation of polymers. It is also a promising
chemical intermediate for the preparation of ne chemicals,
pharmaceuticals and other materials.5–7 Considering these
applications, FDCA was listed as one of the top twelve value
added chemicals from biomass by the US department of energy
in 2004.2
The oxidation of HMF to FDCA comprises an alcohol
oxidation and an aldehyde oxidation (Scheme 1).8,9 In most
catalytic systems, the aldehyde side chain of HMF was easily
aState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China.
E-mail: xgyang@ciac.ac.cn; yibozhang@ciac.ac.cn; Tel: +86-431-8526-2228
bUniversity of Chinese Academy of Sciences, Beijing, 100039, China
cLaboratory of Green Chemistry and Process, Changchun Institute of Applied
Scheme 1 The oxidation of biomass-derived HMF to FDCA.
RSC Adv., 2015, 5, 19823–19829 | 19823
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terminal oxidant in eco-friendly aqueous medium and at were obtained from Aladdin Chemicals Co. Ltd (Shanghai).
moderate reaction conditions. Concentrated H2SO4, concentrated HNO3, NaOH, citric acid
For a redox reaction including the electron gain or loss, monohydrate and ethylene glycol were obtained from Beijing
reducible oxide was a suitable selection as the catalyst or Chemicals Co. Ltd (Beijing). H2PtCl6$6H2O was achieved from
the catalyst carrier.17–19 Ceria-based substance containing the Shanghai Shi Wu Chemical Reagent Science and Technology
CeIV/CeIII redox couple and oxygen vacancies was a promising Co. Ltd (Shanghai).
catalytic material and has drawn much attention in the eld of
The powder X-ray diffraction (XRD) patterns were carried out
catalytic oxidation.19–21 In our previous work, Ce1ꢀxBixO2ꢀd on
a
Bruker D8 Advance X-ray diffractometer using a
ꢀ
(0.08 # x # 0.5) were found to catalyze HMF to HFCA and Cu Ka radiation source (l ¼ 1.5406 A). The powder samples
2,5-bishydroxymethylfuran (BHMF) in the presence of oxygen in were placed onꢀa1 glass slide and scanned from 20ꢁ to 80ꢁ at a
alkaline aqueous solution. While as excellent support for Au rate of 10ꢁ min or 1ꢁ minꢀ1
.
nanoparticles (NPs), Ce1ꢀxBixO2ꢀd (0.08 # x # 0.2) catalyzed
Transmission electron microscopy (TEM) was performed
HMF oxidation to FDCA.22 Despite the Bi-doped ceria which using a FEI Tecnai G2 S-Twin instrument with a eld emission
existed alone was inactive towards catalyzing the conversion of gun operating at 200 kV. The powder was ultrasonically
the alcohol group in HMF, the oxygen activation and hydride dispersed in ethanol and the suspension was deposited on a
transfer were enhanced by the presence of the large amount of copper grid coated with a porous carbon lm.
oxygen vacancies and the specic Bi during the catalytic cycle.
Elemental contents were measured by ICP-OES (iCAP
The Bi-doped ceria should also be a superior support for other 6300 Thermo Scientic USA). Typically, a sample with an
components which could afford the active sites for the activa- amount of 0.1 g was subjected to microwave assisted digestion
tion of the C–H bond in the alcohol group, resulting in the with 6 mL HCl and 2 mL HNO3 at 200 ꢁC for 1 h, then diluted by
generation of active catalysts for HMF oxidation.
deionized water and analyzed along with known solutions.
The performances of the Au and Pt catalysts towards HMF
oxidation at conditions of 295 K, 690 kPa O2, 0.15 M HMF and
0.3 M NaOH have been compared previously.8 Though the
activity of the Au catalysts was much higher than that of the Pt
catalysts, the Pt catalysts showed better selectivity to FDCA. The
initial turnover frequency of HMF conversion was an order of
magnitude greater on Au catalysts compared to Pt. However, the
major product with the Au catalysts was HFCA. While longer
time was needed for Pt catalyst to achieve the complete
conversion of HMF, the desired diacid product was obtained in
majority. The results implied that the dehydrogenation of the
alcohol group and thus the achievement of the nal production
of FDCA were energetically more favorable with a Pt catalyst,
which had also been shown for other alcohols oxidation.13,23 In
addition, the stability of Au catalysts limited its industrial
application.9,22 In this work, Pt NPs were combined with the
Ce1ꢀxBixO2ꢀd support for catalyzing HMF oxidation to FDCA.
Gratifyingly, Pt/CeO2 showed poor catalytic activity just as a
previous report,24 while Pt/Ce0.8Bi0.2O2ꢀd catalyst showed
excellent performance at room temperature. Unlike ceria sup-
ported Au catalyst, the Pt NPs and the ceria-based support
remained their natural properties aer combination, the
excellent catalytic oxidation activity of the Pt/Ce0.8Bi0.2O2ꢀd
catalyst was derived from a well incorporation of their specic
functions. To the best of our knowledge, it is the rst report
Catalyst preparation
Porous Ce1ꢀxBixO2ꢀd (x ¼ 0, 0.1, 0.2, 0.5) samples were prepared
by the citrate method and the specic process was described in
our previous report.22
Pt was loaded onto Ce1ꢀxBixO2ꢀd via the sol–impregnation
method. For the preparation of Pt NPs stabilized by PVP,25
0.72 g PVP (K30) was dissolved in 27.05 mL of 0.024 M H2PtCl6
aqueous solution and the solution was mixed with 243.5 mL
ethylene glycol. Then, the reaction mixture was reuxed at
200 ꢁC for 1.5 h. Within minutes, the color of the solution
changed from orange to dark brown, showing the formation of
Pt NPs. The as-synthesized colloid were puried and collected
by adding a large excess of acetone (nearly triple volume) and
stirring overnight. Aer discarding the supernatant, the oc-
culated Pt NPs were re-dispersed in a specied volume of
ethanol. Pt sol of perfectly known concentration was obtained.
1 wt% Pt NPs on Ce1ꢀxBixO2ꢀd was prepared by wet impregna-
tion of Ce1ꢀxBixO2ꢀd with Pt sol. The suspension was evaporated
ꢁ
ꢁ
by heating at 70 C and then dried at 70 C overnight without
further treatment. The loading amount of Pt was veried by
ICP-OES analysis.
HMF oxidation
about HMF, a kind of alcohol and aldehyde, oxidation effi- The aqueous phase catalytic oxidation of HMF was performed
ciently catalyzed by a ceria-based material supported Pt catalyst. in a 50 mL autoclave equipped with a PTFE liner and a magnetic
stirrer. In a typical process, 6 mL of 0.15 M HMF and
0.6 M NaOH solution was added to the reactor along with
appropriate amount of catalyst. The autoclave was ushed with
O2 3 times and pressurized at 1.0 MPa. Aer reacting for
Experimental
Chemicals and characterization
All the chemicals were used without further purication. HMF a certain time at room temperature under stirring, the sample
(98%) and BHMF (98%) were obtained from InnoChem Science was centrifuged, acidied with sulfuric acid and diluted with
& Technology Co. Ltd (Beijing). FDCA (97%), HFCA and FFCA water. The resulting solution was ltered using a syringe
(98%) were purchased from J & K Co. Ltd (Beijing). Poly- lter (0.2 mm) before analysis in a high performance liquid
vinylpyrrolidone (PVP), Bi(NO3)3$5H2O and Ce(NO3)3$6H2O chromatograph (HPLC) equipped with a Waters 2487 Dual l
19824 | RSC Adv., 2015, 5, 19823–19829
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absorbance detector. The HPLC utilized a Bio-Rad Aminex HPꢀX1-
87H column at 35 ꢁC and 5 mM H2SO4 owing at 0.5 mL min
to perform the separation. The identication and quantication
of HMF, HFCA, FFCA, DFF and FDCA were carried out by
injecting known concentrations. The yield of BHMF was deter-
mined by gas chromatography (GC) analysis.
Results and discussion
Monodisperse Pt NPs with
a narrow size distribution
of (3.5 ꢂ 1) nm (Fig. 1a) were prepared following the ethylene
glycol reduction method under the protection of PVP. The
Pt NPs were well dispersed and remained nearly unchanged
aer supporting on the Ce1ꢀxBixO2ꢀd samples via the conven-
tional wet impregnation method (Fig. 1b). Two lattice fringes of
0.23 nm and 0.32 nm ascribed to the d spacing values of the
{111} planes of Pt and Bi-doped ceria could be seen from the
HRTEM result of the Pt/Ce0.8Bi0.2O2ꢀd sample (inset of
Fig. 1b).26 The proportion of the larger particles that were more
than 3.5 nm was increased, which was perhaps caused by the
interaction between the Pt NPs and the support or some of the
small Pt particles in the pores of the support.
Fig. 2 XRD patterns of (a) Pt/CeO2, (b) Pt/Ce0.9Bi0.1O2ꢀd, (c) Pt/
Ce0.8Bi0.2O2ꢀd and (d) Pt/Ce0.5Bi0.5O2ꢀd samples (inset: XRD of
samples scanned at low speed).
performance of the Pt sol was also found. All the Bi-contained
catalysts showed preferable catalytic activities compared
to the Pt sol and the Pt/CeO2 catalyst, especially the Pt/Ce0.8
-
Bi0.2O2ꢀd which afforded a 100% conversion of HMF and a 98%
XRD patterns of the four Pt/Ce1ꢀxBixO2ꢀd (x ¼ 0, 0.1, 0.2, 0.5)
samples were also taken and the results are shown in Fig. 2.
Only peaks ascribed to the supports were found. The absence of
the diffraction line for the Pt phase, such as the strongest
diffraction peak at 39.75ꢁ (inset of Fig. 2), also indicated that
platinum was highly dispersed throughout all the supports.
HMF oxidation was then carried out with the Pt sol or the
obtained Pt/Ce1ꢀxBixO2ꢀd (x ¼ 0, 0.1, 0.2, 0.5) samples at room
temperature. The results are shown in Fig. 3. The Pt sol alone
presented as catalyst in the reaction system displayed poor
activity towards HMF oxidation to FDCA. Only 17% conversion
of HMF with 1% and 15% yields of FDCA and HFCA were
obtained aer reaction for 30 min. It indicated that the Pt sol
was merely a little active for the oxidation of the aldehyde group
but inactive for the oxidation of the alcohol group in HMF at
these moderate reaction conditions. The Pt/CeO2 showed a
similar catalytic performance with the Pt sol, implying that the
CeO2 was an inactive support for Pt NPs. It was identical with a
previous report.24 That the CeO2 support did not inuence the
yield of FDCA at the same conditions. The catalytic performance
of all these catalysts followed the order: Pt sol z Pt/CeO2
ꢃ
Pt/Ce0.9Bi0.1O2ꢀd Pt/Ce0.5Bi0.5O2ꢀd Pt/Ce0.8Bi0.2O2ꢀd.
<
<
A control experiment without catalyst was also carried out under
the same conditions. Nearly 10% conversion of HMF was
obtained and the major products were derived from the
degradation of HMF. To achieve a high yield of FDCA, active
catalyst was needed to convert HMF quickly at the beginning of
the reaction period, thus inhibiting the degradation of HMF
and keeping a high selectivity to the desired oxidation products.
The catalytic oxidation of HMF to FDCA in water using PVP
stabilized Pt NPs as catalyst has previously been investigated by
Siankevich et al., 100% conversion of HMF and 95% yield of
FDCA were obtained with molecular oxygen as oxidant.27
However, a high catalyst loading of 5 mol% and a long reaction
time of 24 h were needed even the reaction was conducted at
Fig. 1 TEM images and Pt particle size distributions of (a) PVP stabi-
lized Pt sol and (b) Pt/Ce0.8Bi0.2O2ꢀd (inset of (b): HRTEM of Pt/ Fig. 3 HMF conversion, FDCA and HFCA yields on different studied
Ce0.8Bi0.2O2ꢀd). N indicated the number of particles included in each catalysts (Pt : HMF ¼ 5 ꢄ 10ꢀ3 mol molꢀ1, 4 equiv. NaOH, 1.0 MPa O2,
particle size distribution.
23 ꢁC, 30 min).
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80 ꢁC. The results above emphasized the importance of between them were retained at the same time. It urged us to
combining noble metal NPs with active support for the prepa- know that the Pt NPs and the ceria-based support did not
ration of highly effective catalyst. In addition, the Pt NPs catalyst inuence their mutual functions when coexisting in the reac-
could be recovered easily by ltration or centrifuging aer tion system. The exclusive roles of the Pt NPs and the Bi-doped
supporting on bulk material.
ceria were synergistic well during the catalytic oxidation cycle.
Under the same conditions as Fig. 3 except for the presence
The fact that the catalytic activity was nearly unchanged
when Pt sol was deposited on CeO2 while increased sharply of base, reaction resulted in very low conversion of HMF with
when deposited on Ce1ꢀxBixO2ꢀd (x ¼ 0.1, 0.2, 0.5) samples was the Pt/Ce0.8Bi0.2O2ꢀd catalyst. Only DFF, with nearly negligible
to some degree identical with our previous work. In the previous amount, was detected besides HMF in the solution aer reac-
work, Ce1ꢀxBixO2ꢀd (x ¼ 0.1, 0.2, 0.5) exhibited excellent cata- tion. It indicated that base played an essential role during the
lytic performance on converting HMF to HFCA and BHMF in reaction process and this result was identical with previous
alkaline aqueous solution without degradation of HMF, while reports.30–32 Zope et al.30 investigated the reaction mechanism of
CeO2 was inactive for the reaction and led to the serious ethanol and glycerol selective oxidation to acids over supported
degradation of HMF at the same conditions.22 Then, the metal catalysts in aqueous phase by using density functional
Ce1ꢀxBixO2ꢀd (x ¼ 0.1, 0.2, 0.5) catalysts were further examined theory calculations and labeling experiments with 18O2 and
for the conversion of HMF at room temperature to identify the H218O. They suggested that the cleavage of the O–H and
difference of their catalytic activities.
C–H bond in alcohols and thus the production of aldehyde were
As shown in Fig. 4, HMF was efficiently converted to HFCA generally facilitated by the noble metal catalyst and the
and BHMF with the presence of all the three Bi-doped ceria hydroxide ions which presented in the solution or as the surface
catalysts. The catalytic activity followed an order of Ce0.9Bi0.1
-
bound hydroxide intermediates. Oxygen atoms incorporated
O2ꢀd < Ce0.5Bi0.5O2ꢀd < Ce0.8Bi0.2O2ꢀd, which was in the exact into the acids products were originated from hydroxide ions
same sequence with Pt/Ce1ꢀxBixO2ꢀd catalyzing HMF oxidation instead of molecular oxygen. Molecular oxygen participated in
at the same conditions (Fig. 3). This result indicated that the the catalytic cycle by removing the electrons depositing into the
natural difference among the three Ce1ꢀxBixO2ꢀd supports was metal particles. This O2 reduction process regenerated
remained aer supporting Pt NPs and the Pt NPs did not hydroxide ions via the formation and dissociation of peroxide
inuence the ceria-based support playing its own role during intermediates. Moreover, the mechanism above was also found
the catalytic oxidation cycle. For the conversion of HMF to suitable for the selective oxidation of HMF to FDCA.32
HFCA and BHMF catalyzed by Bi-doped CeO2, BHMF and HFCA
The Pt NPs in all the Pt/Ce1ꢀxBixO2ꢀd catalysts were the same
were obtained equally from the Cannizzaro reaction of HMF and the reaction conditions, including the alkali concentration
and HFCA was also product from the oxidation of the aldehyde were identical (Fig. 3). The dramatic difference among the
group in HMF. The differences between the yields of HFCA and catalysts should be rooted in the oxygen activation ability of the
BHMF (Fig. 4) implied that the catalytic oxidation activity of the supports. Theoretical studies by Teng et al.19 showed that
Bi-doped ceria also followed the same order. It was identical CeO2 could serve as an electronic repository, gain and store
with previous works in which doping 20 mol% of bismuth in electrons from the supported metal cluster and release them
ceria showed the best performance towards oxidation reaction when the metal particle interacted with molecular O2. This
or oxygen storage.28,29
function was related to the reducible property of CeO2. The
As analyzed above, the inactive CeO2 support did not change incorporation of Bi2O3 into CeO2 lattice changed its dynamic
the catalytic oxidation activity of the Pt sol (Fig. 3) aer their redox activity at low temperature drastically.28 We can reason-
combination. The active Bi-doped ceria samples showed pref- ably deduce that the Bi-doped ceria captured the electrons from
erable activities aer depositing Pt NPs and the discrimination the Pt NPs more easily during the O–H and C–H bond activation
processes. The Pt NPs was recovered quickly when combining
with the Bi-doped ceria support. Then, O2 reduction, thus
regenerating hydroxide ions and closing the catalytic cycle,
would be achieved by the Bi-doped ceria. There existed a large
amount of oxygen vacancies in the Bi-doped ceria.22,29 O2 tended
to adsorb on these sites and became peroxide (OOH*) and
hydrogen peroxide (HOOH*) intermediates through capturing
hydrogen from H2O.33,34 The decomposition of HOOH* to
hydroxide could be catalyzed by the Pt NPs.30 Moreover, some
works indicated that bismuth was an active species towards the
cleavage of the peroxide species.35,36 Despite the precise role of
bismuth which was accompanied with the oxygen vacancy was
not clear, it should be an important reason for the essential
particularity of the Bi-doped ceria. Finally, electrons were
captured by the hydroxide intermediates which were derived
from the dissociation of the peroxide species and hydroxide
ions were regenerated.30,32
Fig. 4 HMF conversion and product yields as a function of reaction time
during HMF reaction in aqueous solution over Ce1ꢀxBixO2ꢀd (x ¼ 0.1, 0.2,
0.5) catalysts (0.1 g catalyst, 4 equiv. NaOH, 1.0 MPa O2, 23 ꢁC).
19826 | RSC Adv., 2015, 5, 19823–19829
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The effect of the O2 concentration on the reaction over the
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Pt/Ce0.8Bi0.2O2ꢀd catalyst was examined and the results are
shown in Fig. 5. Reaction under 1.0 MPa N2 afforded a certain
amount of products obtained from the oxidation of the alcohol
group (FDCA, FFCA and DFF), which might ascribed to the
presence of large amount of stoichiometric oxidant sites in the
Ce0.8Bi0.2O2ꢀd support.37 The yields of the oxidation products
increased with the increasing concentration of O2 and
were limited with the low O2 contents. It indicated that the O2
reduction was a fast reaction process and the oxygen transfer
limited this process when the O2 concentration was low. For
HMF oxidation catalyzed by the PVP stabilized Pt sol, O2
concentration was identied not rate limiting, which matched
the low reaction rate and catalytic activity.27 In the presence of
the Bi-doped ceria, the reaction rate increased greatly and high
concentration O2 was needed to capture the produced electrons
and recover the catalyst timely.
Scheme 2 The proposed reaction mechanism for the oxidation of
HMF in alkaline aqueous solution. CeBi* represented the oxygen
vacancy accompanied with the bismuth.
Thus, the Pt NPs provided the active sites for the generation
of metal–alkoxide intermediate and then b-H elimination ach-
The reaction mechanism was proposed in Scheme 2. With the ieved. However, oxygen activation could not accomplish without
Pt NPs and the Bi-doped ceria presented in the reaction system an active support. The poor activity of the alcohol oxidation was
simultaneously, Pt–alkoxide intermediate was rstly formed with resulted with Pt sol or Pt/CeO2. The Bi-doped ceria has the
the help of the hydroxide ion in alkaline aqueous solution capacity to transfer hydride and activate oxygen. But it could not
(Scheme 2a). Then, b-H elimination was achieved when the activate the C–H bond in the alcohol group, which only resulted
surface bound hydroxide intermediate captured the hydride in the conversion of the aldehyde group and the production of
which transferred from the C–H bond. These processes produced HFCA and BHMF. When combining the Bi-doped ceria and Pt
the aldehyde product and deposited two electrons on the Pt NPs. NPs, their functions were tly combined and the efficient
The electrons further transferred to molecular oxygen which catalyst for HMF oxidation to FDCA was generated. Under the
adsorbed on the oxygen vacancy accompanied by bismuth. same conditions as Fig. 3, the same amounts of the Pt sol and
Finally, O2 converted to hydroxide ions through the formation Ce0.8Bi0.2O2ꢀd as that of the previous Pt/Ce0.8Bi0.2O2ꢀd catalyst
and dissociation of peroxide intermediate (Scheme 2b). The were added into the reaction system separately, the reaction
catalytic cycle completed and the catalyst recovered. This oxygen also resulted in a high 96% yield of FDCA aer 30 min. It further
reduction process could not occur with the CeO2 at the moderate identied that when catalyzing the oxidation of HMF to FDCA,
reaction conditions. CeO2 showed a lower redox activity at low the Pt sol and the Ce0.8Bi0.2O2ꢀd support played their own
temperature with a small amount of oxygen vacancies and the specic roles and their functions combined well.
dissociation of the O–O bond of the formed peroxide interme-
The 98% high yield of FDCA was obtained only aer 30 min
diates might be inactive without bismuth. In addition, the over the most active Pt/Ce0.8Bi0.2O2ꢀd catalyst at room temper-
aldehyde group of HMF, HFCA and FFCA underwent the similar ature. Therefore, the loading amount of Pt was reduced to
oxidation processes aer it hydrated to the geminal diol inter- 0.1 wt% to further investigate the time course of the reaction.
mediate and FDCA was obtained nally (Scheme 2c).
The result is shown in Fig. 6. Although the molar ratio of HMF
to Pt was as high as 2000 in the reaction system, the catalyst
showed excellent activity towards the oxidation of HMF. 98%
conversion of HMF and 69% yield of FDCA were achieved only
aer 10 min. The TOF was calculated from the moles of HMF
consumed in 5 min, normalized by the Pt atoms in the reactor.
The value was 5.45 sꢀ1. It was much higher than the commercial
Pt/C catalyst, which was examined in the oxidation of HMF in a
previous work.8 HMF was rstly oxidized to HFCA, DFF and
FFCA which were further oxidized to FDCA (Fig. 6). There exis-
ted large amounts of HFCA and FFCA aer 5 min, indicating
that the oxidation of the alcohol group and the aldehyde group
in HMF proceeded simultaneously and was both energetically
favorable with the catalyst. In our previous work, however, a
little FFCA and no DFF was detected in the reaction course over
the Au/Ce0.9Bi0.1O2ꢀd catalyst.22 It suggested that the oxidation
of the alcohol group over the Pt catalyst was much easier than
that over the Au catalyst.
Fig.
5 HMF conversion and product yields at different reaction
atmospheres over the Pt/Ce0.8Bi0.2O2ꢀd catalyst (Pt : HMF ¼ 5 ꢄ 10ꢀ3
Despite the conversions of HMF, HFCA and FFCA were very
quick at the rst 10 min, HFCA hardly transformed to FDCA
mol molꢀ1, 4 equiv. NaOH, 23 ꢁC, 30 min).
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Fig. 6 HMF conversion and product yields as a function of reaction
time at 23 ꢁC for 30 min and 60 ꢁC for another 30 min over 0.1 wt% Pt/
Ce0.8Bi0.2O2ꢀd catalyst (Pt : HMF ¼ 5 ꢄ 10ꢀ4 mol molꢀ1, 4 equiv.
NaOH, 1.0 MPa O2).
Fig. 8 Reusability study of 1 wt% Pt/Ce0.8Bi0.2O2ꢀd catalyst in the
oxidation of HMF in aqueous solution (Pt : HMF ¼ 5 ꢄ 10ꢀ3 mol molꢀ1
,
4 equiv. NaOH, 1.0 MPa O2, 23 ꢁC, 30 min; HMF: black, FDCA: red,
HFCA: blue).
aer 20 min and a 73% yield of FDCA was nally obtained aer
30 min. The reactor was then placed in an oil pot at 60 C and
also the generation of nearly 20% yield of BHMF was occurred.
BHMF was the product of the Cannizzaro reaction of HMF and
Ce0.8Bi0.2O2ꢀd showed excellent catalytic activity towards this
reaction.22 It further implied that the natural properties of the
Ce0.8Bi0.2O2ꢀd support were remained aer depositing Pt NPs. The
Ce0.8Bi0.2O2ꢀd support was capable for transferring hydride and
activating oxygen and thus an active catalyst for converting HMF
to HFCA and BHMF. However, when the loading amount of Pt was
no less than 0.1 wt%, the oxidation reaction which also included
the hydride transfer and oxygen activation procedures was the
dominant reaction. The Cannizzaro reaction was inhibited and no
BHMF was produced.
ꢁ
reacted for another 30 min. The relative yields of HFCA and
FDCA were nearly unchanged. It seemed that the catalyst
became inactive aer reaction. Then, the catalyst was recovered
by centrifuging aer reaction 30 min at room temperature and a
new set of HMF oxidation experiment was carried out. HMF was
also readily oxidized to the products within 30 min, suggesting
that the catalyst was still active for the oxidation reaction aer
one catalytic cycle. The certain amount of HFCA which was
inevitable in the nal products might derive from some of the
active sites in the catalyst being blocked by the products, which
inhibited further oxidation of the alcohol group.
Finally, the catalyst recycling was studied for the 1 wt%
Pt/Ce0.8Bi0.2O2ꢀd catalyst. Aer a reaction cycle, the catalyst was
recovered by centrifuging and washing with water three times
and used for another reaction cycle. The Pt/Ce0.8Bi0.2O2ꢀd
catalyst could be reused for ve times without signicant loss of
the activity under identical reaction conditions (Fig. 8). HMF
was totally converted to oxidation products for all the reaction
cycles and the FDCA yields were 98%, 98%, 97%, 95% and 97%
in the 1st, 2nd, 3rd, 4th and 5th cycles, respectively. The ltrate
aer the rst reaction cycle was analyzed by ICP analysis, the Pt
content was 0.74 mg mLꢀ1, which corresponded nearly 0.5% Pt
leaching. The amount was low and might be caused by the
residual solid particles in the ltrate. Aer using ve times,
TEM analysis was conducted to examine the Pt morphology. No
aggregation of Pt occurred and the size of Pt NPs was main-
tained well. All these results conrmed the superiority of the
Pt/Ce0.8Bi0.2O2ꢀd catalyst for the oxidation of HMF.
The inuence of the loading amount of Pt (as well as the HMF/
Pt molar ratio) on the product distribution was then investigated.
The results shown in Fig. 7 illustrated that the selectivity to FDCA
was improved by increasing the loading amount of Pt. It high-
lighted the viewpoint that Pt afforded the active sites for the
generation of metal–alkoxide intermediate and then the oxidation
of the alcohol group was achieved with the help of the hydroxide
ions and the Bi-doped ceria. In addition, it was noteworthy that
not only the conversion of HMF and the yield of FDCA decreased
greatly when the loading amount of Pt reduced to 0.075%, but
As the oxidation of HMF to FDCA contained an alcohol
oxidation and an aldehyde oxidation, and the Pt/Ce0.8Bi0.2O2ꢀd
catalyst was efficient for the reaction at room temperature, we
could imagine that the catalyst was also active for the oxidation
of other alcohols or aldehydes.
Conclusions
Fig. 7 HMF conversion and product yields over the Pt/Ce0.8Bi0.2O2ꢀd
catalysts with different loading amounts of Pt (0.0878 g catalyst,
4 equiv. NaOH, 1.0 MPa O2, 23 ꢁC, 30 min).
In this work, Pt sol stabilized by PVP and the Ce1ꢀxBixO2ꢀd
(x ¼ 0, 0.1, 0.2, 0.5) supported Pt catalysts obtained by the sol–
19828 | RSC Adv., 2015, 5, 19823–19829
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Acknowledgements
The authors thank the National Natural Science Foundation of
China. This work was supported by the National Natural Science
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