Porous structured CuO-CeO2 nanospheres for the direct oxidation of cellobiose and glucose to gluconic acid

Article abstract of DOI:10.1016/j.cattod.2017.01.009

Porous-structured CuO-CeO₂ nanospheres were synthesized using a hydrothermal method and were tested as catalysts for the direct oxidation of cellobiose to gluconic acid. Catalytic reaction along with catalyst characterization results and ¹⁸O-oxygen isotope labeled experiments revealed that the surface lattice oxygen of CuO in CuO-CeO₂ nanospheres was consumed during the oxidation of cellobiose. This provides a direct evidence of our previous work (Amaniampong et al., Angew. Chem. Int. Ed. 54 (2015) 8928–8933). Characterization results further suggested that the lattice oxygen in CeO₂ did not participate in the oxidation; nonetheless, the addition of CeO₂ to CuO enhanced the surface area of the catalyst composite which was crucial for the reaction. The spent catalyst upon re-oxidation regained its activity. In addition, isotope labeled deuterium oxide (D₂O) experiments suggested that hydrogen exchange between the solvent and the substrate (glucose) are not involved in the mechanistic formation of gluconic acid and confirmed the solvent had no direct influence in the formation of gluconic acid.

Full text of DOI:10.1016/j.cattod.2017.01.009

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Porous structured CuO-CeO2 nanospheres for the direct oxidation of
cellobiose and glucose to gluconic acid
Prince Nana Amaniamponga,1, Quang Thang Trinhb, Kaixin Lia, Samir H. Mushrifa,
Yu Haoc,∗, Yanhui Yanga,∗
a School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore
b Cambridge Centre for Advanced Research and Education in Singapore (CARES), Nanyang Technological University, 1 Create Way, Singapore 138602,
Singapore
c School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous-structured CuO-CeO2 nanospheres were synthesized using a hydrothermal method and were
tested as catalysts for the direct oxidation of cellobiose to gluconic acid. Catalytic reaction along with
catalyst characterization results and 18O-oxygen isotope labeled experiments revealed that the surface
lattice oxygen of CuO in CuO-CeO2 nanospheres was consumed during the oxidation of cellobiose. This
provides a direct evidence of our previous work (Amaniampong et al., Angew. Chem. Int. Ed. 54 (2015)
8928–8933). Characterization results further suggested that the lattice oxygen in CeO2 did not participate
in the oxidation; nonetheless, the addition of CeO2 to CuO enhanced the surface area of the catalyst
composite which was crucial for the reaction. The spent catalyst upon re-oxidation regained its activity. In
addition, isotope labeled deuterium oxide (D2O) experiments suggested that hydrogen exchange between
the solvent and the substrate (glucose) are not involved in the mechanistic formation of gluconic acid
and confirmed the solvent had no direct influence in the formation of gluconic acid.
Received 27 September 2016
Received in revised form
30 November 2016
Accepted 3 January 2017
Available online xxx
Keywords:
C
H bond activation
Lattice oxygen
Nanospheres
Gluconic acid
Oxidation
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
the most active metal oxide of those explored for the catalytic incin-
eration of toluene with ␥-Al2O3 as support [11]. Copper-modified
Metal oxide catalysts are promising for oxidation processes, due
to their lower costs and higher stability, as compared to noble met-
als. The synthesis and characterization of transition metal oxides
with controlled shape, size, structure and uniform dimension have
attracted considerable interest because of their unique physico-
chemical properties and potential in catalysis [1,2]. Among these,
copper oxide (CuO) was reported to be suitable for various catalytic
applications [3–6], and the property of CuO has been documented
in literature [7] to be closely related to its microstructures, par-
ticularly its morphology, crystal size and orientation. Rajesh et al.
reported that CuO/Al2O3 was even more active than Pt/Al2O3 for the
complete oxidation of ethanol [8]. Larsson and Anderson attempted
CuOX/Al2O3 for the incineration of CO, ethanol and ethyl acetate
and found an excellent catalytic performance [9,10]. CuO was also
or copper promoted by ceria has been reported to exhibit enhanced
catalytic performance than copper only in the total oxidation of
carbon monoxide, catalytic wet oxidation of phenol and water
gas shift reactions [12–14]. The enhanced activity of the copper
oxide promoted by cerium oxide can be attributed to the structural
enhancement and improved thermal stability of the copper-cerium
mixed oxide [15].
The activation of C H bond was reported to be a key step in oxi-
dation reactions [16,17]. There has been considerable interest in
the oxidation of saturated hydrocarbons involving C H bond acti-
vation to manufacture valuable products such as the production of
oxygenates via the partial oxidation of light alkanes [18] through
the activation of C H bonds by under coordinated metal lattice
oxygen sites, with a concerted mechanism in which the metal
center inserted into the C H bond, and cleaved it. The produc-
tion of olefins via dehydrogenation or oxidative dehydrogenation
mechanisms over transition metal oxides has also become attrac-
tive [19]. Recently, great interest in the valorization of renewable
resources has led to extensive investigation on the catalytic oxi-
∗
Corresponding authors.
E-mail addresses: yuhao@scut.edu.cn (Y. Hao), yhyang@ntu.edu.sg (Y. Yang).
1
Current address: INCREASE (FR CNRS 3707), ENSIP, 1 rue Marcel Doré, TSA41105,
86073, Poitiers Cedex 9, France.
http://dx.doi.org/10.1016/j.cattod.2017.01.009
0920-5861/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: P.N. Amaniampong, et al., Porous structured CuO-CeO2 nanospheres for the direct oxidation of
cellobiose and glucose to gluconic acid, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2017.01.009

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dation of sugars and other carbohydrates. In spite of the great
interest in these processes, partially conflicting opinions regarding
their underlying mechanisms, were reported in the literature con-
cerning several details of the mechanism of oxidation over oxide
catalysts. For instance, the selective oxidation of glucose to glu-
conic acid, and the liquid phase oxidation at the alcoholic C OH
and carbonylic C O bonds have been reported to occur via dehydro-
genation mechanism [20,21]. In our previous study [6], oxidation of
glucose, cellobiose and cellulose was performed on a CuO catalyst,
in the form of nanoleaves, with excellent yield towards gluconic
acid. Using quantum mechanical calculations, it was revealed that
the surface lattice oxygen of CuO activated the formyl C H bond in
glucose and incorporated itself into the glucose molecule to oxidize
it to gluconic acid.
Herein, CuO-CeO2 nanospheres with a porous structure were
synthesized by a hydrothermal treatment, using urea as a precipi-
tating agent without the aid of surfactant to achieve a Ce-Cu binary
precursor and subsequent calcination of the precursor. The as-
synthesized CuO-CeO2 nanospheres were characterized by X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-
resolution transmission electron microscope (HR-TEM), transmis-
sion electron microscope (TEM), H2 temperature-programmed
reduction (H2-TPR), N2 adsorption-desorption techniques and
scanning electron microscope (SEM). The catalytic performance for
the selective oxidation of cellobiose and glucose was examined to
study the correlation between the catalytic performance and the
interaction at the Ce-Cu interface. It was anticipated that, high
oxygen mobility and redox properties of ceria oxide my further pro-
mote the catalytic properties of copper oxide in these reactions due
to the additional active sites generated from oxygen vacancies at
the Cu-Ce interfaces. Isotope labeling experiments with 18O (oxy-
gen) were carried out to study the reaction mechanism and also
to further confirm the findings in our previous study that, indeed
it was the lattice oxygen in CuO that incorporated into glucose to
form gluconic acid [6]. Deuterium labeled water (D2O) experiments
were also carried out to investigate the role of solvent.
equipped with TCD. In each run, approximately 50 mg of the cata-
lyst was pretreated at 300 ◦C under a flow of He (30 mL min−1), and
then heated to 700 ◦C with a ramp of 10 ◦C min−1 in the stream of
5 vol% H2/Ar (40 mL min−1). Surface area analysis was determined
by nitrogen physisorption on a Micromeritics TrisStar apparatus.
The specific area was calculated using the Brunauer-Emmett-Teller
(BET) equation. Raman measurements were performed on an inVia
reflex confocal microprobe Raman system (Renishaw Company).
Excitation with radiation of 514.5 nm was provided with an Ar+
laser.
2.2. Catalyst testing
Catalytic experiments were carried out using 0.050 g of CuO-
CeO2 catalyst and 0.205 g of cellobiose, unless otherwise stated.
A stainless-steel autoclave reactor, equipped with a Teflon liner
(50 mL) was employed to perform the oxidation of cellobiose. Typ-
ically, cellobiose, the catalyst and deionized water (15 mL) were
loaded into the reactor. The reactor was purged several times with
high-purity nitrogen, (unless otherwise stated) to eliminate any
traces of residue air present in the reactor. Reactions were allowed
to proceed at desired set temperatures with constant stirring at
800 rpm. After the reaction was completed, an Agilent 1100 HPLC
with a RID − 6A refractive index detector and a Hi-Plex H column
(300 × 6.5 mm) were used to analyze the reaction products, with
a mobile phase of 0.01 M H2SO4 buffer at a 1 mL min−1 flow rate.
For the isotopically labeled experiments, D2O was used as solvent
and O18-labeled oxygen was used for the spent catalyst regenera-
tion. Noteworthy, the detected amount of formic acid reported in
our study is the solubilized fraction of it, although the volatized
amount of formic acid at the analyze condition is small according
to its corresponding Henry’s law coefficient [23].
3. Results
3.1. Structural characterization
2. Experimental
CuO-CeO2 composites were obtained after calcining the Ce-Cu
binary precursors synthesized by the hydrothermal process. The
morphology and microstructure of as-synthesized CuO-CeO2 were
investigated by SEM, TEM and HR-TEM analysis. The low magni-
fication SEM image in Fig. 1a reveals that the samples consists of
spherical particles of 200–300 nm in diameter. The porous nature
of the as-synthesis CuO-CeO2 is shown in Fig. 1b (high magnifi-
cation SEM). The spent CuO-CeO2 catalyst (Fig. 1c and d) is also
sphere-like, indicating that the morphology of the fresh catalyst
is not destroyed after the catalytic reaction. Fig. 2 displays the
TEM and HR-TEM images of the as-synthesized and spent CuO-
CeO2 nanospheres. The TEM images (Fig. 2a) confirms our above
deduction that, the nanospheres consist of nanoparticles. These
small nanoparticles consisting of the nanospheres have a diame-
ter of ca. 10 nm. HR-TEM image shown in Fig. 2b of the CuO-CeO2
nanospheres give lattice spacing of 0.156 and 0.234 nm, corre-
sponding to (111) planes of cubic CeO2 phase and (111) planes
of monoclinic CuO phase, respectively [24,25]. The HR-TEM anal-
ysis reveals that the nanospheres are mainly composed of mixed
crystallites, preferentially exposing the disordered crystal phases
of CuO and CeO2 crystals. The TEM analysis of the spent catalyst
(Fig. 2c) confirms the sphere-like morphology of the spent CuO-
CeO2 catalyst as revealed by the SEM analysis. The HR-TEM image
in Fig. 2d also shows the presence of Cu (111) with lattice fringe
of 0.206 nm, and CeO2 (111) with lattice fringe of 0.156 nm. Sur-
face area analysis (BET) is performed to further investigate the
porous structure and surface area of the as-synthesized CuO-CeO2
nanospheres. The specific surface area estimated from the BET
2.1. Catalyst preparation and characterization
CuO-CeO2 nanospheres were prepared by a hydrothermal
method based on a previously reported two-step route [22].
In
a
typical synthesis process, Ce(NO3)3·6H2O (1 mmol) and
Cu(NO3)2·3H2O (1 mmol) were dissolved in 50 mL of deionized
water, followed by adding 8 mmol of urea dropwise. The trans-
parent solution attained after stirring the precursor solution, was
transferred to a Teflon-lined autoclave and then heated at 180 ◦C
for 100 min, resulting in the precipitation of the precursors. The
precipitate was centrifuged, washed several times with deionized
water and ethanol. After drying at ambient atmosphere, the pre-
cursor was finally calcined in air at 600 ◦C for 4 h. For the spent
catalyst regeneration, a specified amount of the CuO-CeO2 catalyst
recovered after reaction were thoroughly washed with DI water
and ethanol and dried at 60 ◦C. The dried catalyst was then placed
in a tube furnace and charged with O18-labeled oxygen and calcined
at 400 ◦C for 4 h.
Crystallographic analysis for the tested were performed by
means of XRD measurements in 2ꢀ mode on a Bruker AXS D8
diffractometer with CuK␣ (␭ = 0.154056 Å) radiation at 40 kV and
20 mA. XPS was performed on a Thermo Escalab 250 spectrometer.
The binding energy was calibrated using C1 s (284.6 eV) as a refer-
ence. The as-synthesized CuO-CeO2 morphology was also studied
by SEM (JEOL JSM 6700F field emission), TEM and HR-TEM (JEOL
JEM-2100F). H2-TPR was carried out in a quartz fixed-bed reactor
Please cite this article in press as: P.N. Amaniampong, et al., Porous structured CuO-CeO2 nanospheres for the direct oxidation of
cellobiose and glucose to gluconic acid, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2017.01.009

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3
Fig. 1. nanospheres (a) low magnification (b) high magnification SEM images of the as-synthesized CuO-CeO2 (c) low magnification (b) high magnification SEM of the spent
CuO-CeO2 catalyst.
method is 51 m2 g−1, which is evidently superior to that of pure
CuO (4 m2 g−1) and CeO2 (7 m2 g−1) nanospheres prepared using
the same hydrothermal method. The improved surface area for
the as-synthesized CuO-CeO2 may result from their highly porous
structure.
O 1s spectra of the CuO-CeO2 catalyst were shown in Fig. 4d, the
main peak centered at a binding energy around 528.9-529.8 eV was
characteristic of the lattice oxygen of CeO2 and CuO phases, while
the broad shoulder at around 531.5 eV may be assigned to the defect
oxide or absorbed oxygen with low coordination [27,30].
The lattice structure and lattice defects of the CuO-CeO2
nanospheres were further investigated by Raman spectroscopy.
The UV-Raman spectra of the CuO-CeO2 nanospheres (Fig. 5a)
revealed a broad band with relatively high intensity at about
453 cm−1, which was assigned to the F2g Raman vibration mode of
the cubic fluorite-structure of CeO2 in CuO-CeO2 [31,32]. Whereas
the peak at 301 cm−1 can be assigned to the Bg modes of CuO [33].
The bands appearing at about 588 and 1124 cm−1 can be linked to
oxygen vacancies in the CeO2 and CuO lattices and the presence of
CeO2 defects. The presence of oxygen vacancies was crucial to the
production and stability of Cu+ species. Noteworthy, the position of
the strong F2g band of the as-synthesized CuO-CeO2 nanospheres
deviated from that of pure CeO2 (462 cm−1) [34], suggesting that
the copper ions incorporated in CeO2 lattice and resulted in the
formation of the CuO-CeO2 nanospheres composites. The incor-
porated copper ions induced the lattice distortion of CeO2, which
influenced the polarizability of the symmetrical stretching mode
of the [Ce-O8] vibrational unit and led to the observed deviation
[35,36]. The UV-Raman spectra of the spent CuO-CeO2 catalyst
(Fig. 5b) revealed a broad band with relatively high intensity at
about 453 cm−1, similar to the observations made for the Raman
results of the as-synthesized CuOCeO2 fresh nanospheres. The new
peak at 600 cm−1 can be liked to oxygen vacancies in the CuOCeO2
lattices and the presence of Cu clusters on CeO2 surfaces [37]. More-
over, there was no Raman signature for the presence of CuO (Raman
peaks at 288–298, 330–345 and 621–632 cm−1)[38,39]. Further-
more, the small peak at 212 cm−1 can be assigned to Cu2O [40],
Analyses on the XRD pattern are performed on the fresh, spent
and oxidative regenerated CuO-CeO2 catalyst. The XRD pattern of
the CuO-CeO2 fresh composites is shown in Fig. 3a. All diffrac-
tion peaks can be assigned to the cubic phase of cerium (IV) oxide,
dominated by CeO2 (111) surface with lattice constant ␣ = 5.4138 Å
coexisting with the Tenorite monoclinic phase of copper (II) oxide
dominated by CuO (111) and CuO (−111), with typical reflections
centered at 35.6◦, 38.7◦, 61.5◦ and 66.2◦. Due to the high calcina-
tion temperature, it is possible that a small amount of CuO has been
incorporated into the CeO2 lattice. The XRD pattern of the spent
CuO-CeO2 (Fig. 3b) reveals diffraction peaks at 2ꢀ = 43.5, 50.6 and
74.1◦, attributed to the (111), (200), and (220) facets of pure cop-
per (Cu) face-centered cubic structure (fcc), respectively. While all
other indexed peaks are typical diffractions of CeO2. The XRD pat-
tern analysis of the oxidative regenerated CuO18-CeO2 with isotope
labeled oxygen (18O) catalyst (Fig. 3c) shows identical diffraction
peaks as the as-synthesized nanospheres composites.
The XPS spectra of Ce 3d and Cu 2p binding energies of CuO-CeO2
nanospheres were shown in Fig. 4. The peaks centered at 897.6
and 916.0 eV were ascribed to Ce4+ and those centered at 881.5
and 900.1 eV to Ce3+ (Fig. 4b), suggesting that Ce4+/Ce3+ redox pair
existed in the in the as-synthesized CuO-CeO2 nanospheres [26].
For the binding energy of Cu 2p (Fig. 4c) the presence of the shake-
up peak at about 941.6 eV and the lower Cu 2p3/2 binding energy
appearing at 932.9 eV were two major XPS characteristics of CuO
and Cu2O [27–29], clearly indicating the presence of Cu2+/Cu+ redox
pair in the as-synthesized CuO-CeO2 nanospheres composite. The
Please cite this article in press as: P.N. Amaniampong, et al., Porous structured CuO-CeO2 nanospheres for the direct oxidation of
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Fig. 2. (a) TEM image; (b) high-resolution TEM of the as-synthesized CuO-CeO2 nanospheres (c) TEM image (b) high-resolution TEM of the spent CuO-CeO2 nanospheres.
suggesting the reduction of CuO in the CuO-CeO2 fresh catalyst
after the reaction. This was consistent with our XRD, HR-TEM and
XPS results for the formation of CuO-CeO2 nanospheres compos-
ites, and also further revealed that the lattice oxygen in CuO, in the
CuO-CeO2 catalysts were consumed during the oxidation reaction.
The reducibility for the CuO-CeO2 nanospheres was further
studied by H2-TPR. It should be clarified that pure CeO2 showed
two well resolved reduction peaks (not shown for brevity). One
centered at 510 ◦C, attributed to the reduction of the surface cap-
ping oxygen of ceria, and the other at 790 ◦C was ascribed to the
reduction of bulk oxygen in ceria [41]. The as-synthesized CuO-
CeO2 nanospheres exhibited three major peaks in the temperature
at 141.8 (␣), 189.6 (␤), and 214.6 ◦C (␥), as shown in Fig. 6. The ␣
peak was attributed to the reduction of large CuO particles inter-
acting weakly with CeO2, while the ␤ peak was attributed to the
reduction of pure bulk CuO phase, and the ␥ peak was attributed
to the reduction of highly dispersed CuOx species on the surface
of CeO2, including isolated Cu2+ ions that weakly interacted with
CeO2 [42,43]. The reduction of pure bulk CuO has been reported
occur at temperature above 300 ◦C in the literature [44]. The signif-
icantly low reduction temperatures as compared to the individual
bulk catalysts, and high H2 consumption (2814 ␮mol/g) estimated
from the TPR-analysis, suggested the strong Cu-Ce interactions in
the as-synthesized CuO-CeO2 nanospheres. From these reduction
patterns obtained in this work, it can be concluded that a coop-
erative effect between CeO2 and CuO resulted in lower reduction
temperature of CuO.
3.2. Catalytic performance
The porous CuO-CeO2 nanospheres were tested for cellobiose
and glucose oxidation in the absence of molecular oxygen. It is
worth noting that, other metal oxides were also explored as catalyst
for the conversion of cellobiose to gluconic acid, under the same
reaction conditions and in the absence of molecular oxygen. The
reactor was purged with high purity nitrogen several times prior
to the reaction. Fig. 7 showed the distribution of reaction products
over the metal oxides investigated. Reactions performed over Fe2O3
and Fe3O4 revealed similar reaction product distributions with cel-
lobiose conversions of 24.1% and 22.2%, respectively. There was no
formation of gluconic acid, and the main products observed were
glucose and fructose with yields of 20.2 and 3.9%, respectively for
Fe2O3 catalysts, and 18.2 and 4.0%, respectively, for Fe3O4 catalyst,
suggesting in the poor oxidation activity of Fe2O3 and Fe3O4 at the
reaction temperatures used in this study [45]. Similar observations
were made for reactions performed over SiO2, with glucose and
fructose yields of 30.1 and 17.5%, though with slightly higher con-
version (47.6%). For reactions performed over CeO2 a gluconic acid
yield of 2.7% was observed, along with glucose (17.3%) and fructose
(2.7%) and a cellobiose conversion of 22.2%. For NiO catalyst, a con-
version of 16.7% was achieved, with a mixture of glucose, gluconic
acid and formic acid as the reaction products, with yields of 2.1, 7.8
and 6.7%, respectively, implying the ability of the NiO catalyst for
C
C bond cleavage and partial oxidation reactions [46].
Prior to the catalyst investigations using the CuO-CeO2
nanospheres, CuO-CeO2 catalyst loading was investigated. Catalytic
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Table 1
5
Time course for cellobiose conversion and gluconic acid yield. Reaction conditions:
0.050 g of catalyst, 0.205 g of cellobiose, 15 mL of H2O, 160 ◦C reaction temperature.
Reaction time
[min]
Catalystc
Conversion
[%]
Gluconic acid
yield [%]
30
60
CuO-CeO2 fresh
CuO-CeO2 fresh
CuO-CeO2 fresh
CuO-CeO2 fresh
CuO-CeO2 spent
34
43
67
68
68
n.d
20 0.2
42 1.0
51 0.8
120
180
180
180
180
240
240a
240b
8
0.2
48 1.8
4 0.5
CuO18-CeO2 reoxidized 69
CuO spent
54
73
70
100
CuO-CeO2 fresh
CuO-CeO2 fresh
CuO-CeO2 fresh
50 1.2
51 2.5
14 0.6
a
Reactor purged with high purity nitrogen.
Reaction temperature = 180 ◦C.
The fresh catalyst refers to the as-synthesized catalysts, reoxidized catalyst
b
c
refers to the catalyst which after one reaction cycle, is reoxidized for 240 min under
isotope labeled oxygen (18O). Spent catalyst refers to the catalyst which was recov-
ered after undergoing one cycle of reaction.
acid formed. Adding CuO-CeO2 catalyst increased the cellobiose
conversion from 5.7% observed for the blank reaction to 68.2%
with a gluconic acid yield of 50. 7%. Kinetic study of the reaction
with CuO-CeO2 fresh catalyst showed that, the conversion of
Fig. 3. (a) XRD pattern of fresh CuO-CeO2 nanospheres (b) XRD pattern of spent
CuO-CeO2 nanospheres (c) XRD pattern of reoxidized CuO-CeO2 with isotope labeled
oxygen (18O).
performance of the porous structured CuO-CeO2 nanospheres was
evaluated in the oxidation of cellobiose to gluconic acid in terms
of cellobiose conversion (%) and gluconic acid yield (%) by varying
the catalyst loading. The optimum catalyst loading was observed
to be 25 wt.% of CuO-CeO2 related to the substrate (Fig. 8). When
lower amounts of catalyst were used in the reaction, lower conver-
sion and yields of gluconic acid were obtained, due to the lower
number of oxygen in the catalyst than the number of substrate
molecules. However, with the gradual increase in the relative num-
ber of CuO oxygen (with increasing CuO-CeO2 amount, keeping the
amount of cellobiose constant), the conversion and gluconic acid
yield increase. At a catalyst loading of above 25 wt.%, oxidative by-
products (glycolic acid) was observed, leading to a decrease in the
yield of gluconic acid. This is due to the over-oxidation of gluconic
acid that once formed, competes with the substrate for oxidizing
species available in the reaction media.
After the catalyst loading optimization, the CuO-CeO2
nanospheres were employed directly for the catalytic oxidation
of cellobiose to gluconic acid without any in situ functionalization
or post-modification. The conversions of cellobiose at different
reaction conditions are shown in Table 1. Noteworthy, a blank
reaction was performed at 160 ◦C for 180 min in the absence of
any catalyst, a cellobiose conversion of 5.7% and glucose yield of
5.7% (100% selectivity) was observed, with no traces of gluconic
Fig. 4. XPS spectra of the as-synthesized CuO-CeO2 nanospheres (a) Survey scan (b)
Ce 3d (c) Cu 2p (d) O 1s.
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Fig. 4. (Continued)
Fig. 5. UV-Raman spectra of (a) CuO-CeO2 fresh catalyst (b) CuO-CeO2 spent cata-
lyst.
cellobiose increased sharply at the initial stage, reaching 66.7%
in 120 min, followed by a gradual increase slowly and reaching
73.3% after 240 min. Gluconic acid yield was only observed after
60 min (∼20%) and increased steadily to ∼51% at 180 min. The
spent CuO-CeO2 catalyst without regeneration only afforded the
formation of glycolic acid and formic acid as major products, with
small amounts of gluconic acid yield (8.1%). A dramatic increase
in the formation of gluconic acid (48.3% yield) was observed after
the oxidative regeneration of the catalyst, with a conversion of
69%, comparable to the results obtained over the fresh catalyst. An
increase in the conversion of cellobiose with no significant increase
in the yield of gluconic acid was also observed at 240 min. The
conversion of cellobiose and yield of gluconic acid did not change
significantly, irrespective of the reactor being purged by nitrogen
or not, suggesting that dissolved oxygen did not play any role in
the reaction and that CuO-CeO2 was the source of oxygen supply
in the oxidation reaction. At reaction temperatures higher than
160 ◦C, complete conversion of cellobiose occurred, with lower
yield of gluconic acid (14.3%) and significant amounts of smaller
acids (glycolic acid yield of 65.1%), suggesting the cleavage of C
C
bonds. Compared to reactions performed at 160 ◦C, the formed
gluconic acid at 180 ◦C suffers from oxidative decomposition
and catalytic C C cleavage due to the presence of the reduced
Fig. 6. H2-TPR profile of the as-synthesized CuO-CeO2 nanospheres.
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Fig. 7. Effect of various metal oxides on the yield of gluconic acid. Reaction con-
ditions: 0.050 g of catalyst, 0.205 g of cellobiose, 15 mL of H2O, 160 ◦C reaction
temperature.
Fig. 8. Influence of catalyst loading on the oxidation of cellobiose to gluconic acid.
Reaction conditions: 0.205 g of cellobiose, 15 mL of H2O, 160 ◦C reaction tempera-
ture, 180 min reaction time.
To obtain more insights into the role of solvent in the reaction,
deuterium oxide (D2O) was used as solvent instead of H2O. Glu-
cose (the reaction intermediate) was used as the substrate for the
oxidative conversion over fresh CuO-CeO2 nanospheres for easy
elucidation of the reaction mechanism. Fig. 10 showed the ESI–MS
spectrum of the reaction product. The major reaction product was
observed to be gluconic acid with “H”, without the insertion of
deuterium (D) into the reaction product in the presence of D2O
solvent. An isotopic effect was not observed when the reaction
was performed in D2O, suggesting that H-D exchange processes
are not involved in the oxidative conversion of glucose over CuO-
CeO2 catalyst. The results excluded the oxidative dehydrogenation
mechanism for the formation of gluconic acid, where glucose is con-
verted into the hydrate form in aqueous solution and adsorbed on
the catalyst surface, the hydrate is dehydrogenated and eventually
[21].
To gain further insight into the oxidation reaction mechanism,
isotopically labeled glucose was reacted with the CuO-CeO2 cata-
lyst and investigated using 13C nuclear magnetic resonance (NMR).
Before reaction, 13-C NMR analysis the initial d-(1-13-C) glucose
solution generated resonances at ␦ = 95.88 and 92.06 ppm, which
corresponded to the 13C at C1 position of the ␤-pyranose and ␣-
pyranose configurations of glucose, respectively. Their locations
were consistent with NMR features in previous reports for d-(1-13C)
glucose [50,51]. After the oxidative reaction, a new additional set
of peaks at ␦ = 63.85 and 62.63 ppm and another at ␦ = 180.50 ppm
were observed (Fig. 11b). Previous reports have demonstrated that
these new peaks, ␦ = 180.50 ppm corresponded to the presence of
C1 position of gluconic acid [52]. This confirms the transforma-
tion of the aldehyde group of glucose to a carboxylic group. The
CuO-CeO2 catalyst at elevated temperatures. This explains the
lower yield of gluconic acid at temperatures higher than 160 ◦C
3.3. Oxygen and hydrogen labeling
The CuO18-CeO2 catalyzed cellobiose to gluconic acid oxidation
reaction was performed in water, with the reactor being purged
with high purity nitrogen several times to ensure complete removal
of any oxygen in the reactor. Product analysis using mass spec-
troscopy (Fig. 9) revealed the insertion of O18 lattice oxygen from
the CuO in CuO18-CeO2 nanospheres composite in gluconic acid,
suggesting that the lattice oxygen in CuO and not CeO2 in the
CuO-CeO2 were directly involved in the oxidation reaction. This
further confirmed the theoretical prediction with CuO catalyst for
the direct oxidation of glucose [6]. The behavior of cellobiose oxida-
tion observed in Scheme 1 suggested that the reaction was carried
out following the Mars-van Krevelen mechanism [47]. Surface lat-
tice oxygen atoms from the catalyst were consumed by the C6H12O6
(reaction intermediate) molecules, and oxygen vacancies were cre-
ated. These vacancies were then filled by the oxygen atoms that
diffused from the bulk of the catalyst to the surface. Oxygen label-
ing results suggested that, CeO2 was added to CuO to increase the
surface areas, thermal stability, to improve the oxygen mobility
within the catalyst lattice [48] and also enhance the redox behav-
ior of copper ions by showing more stable activity compared to pure
CeO2 and CuO catalysts. The promotional effect of ceria on copper
oxide in the total oxidation of CO has also been reported by Park
and Ledford [49].
Scheme 1. Simplified reaction scheme for the direct oxidation of cellobiose catalyzed by CuO-CeO2 nanospheres.
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Fig. 9. ESI-MS Spectra of the reaction performed with glucose. The gluconic acid (with 18O oxygen) peak is highlighted in red.(For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
Fig. 10. ESI-MS Spectra of the reaction performed in D2O solvent. The gluconic acid (with 16O oxygen) peak is highlighted in red.(For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11. 13C NMR spectra of d-(1-13C) glucose solution (a) before reaction (b) after reaction. Peaks corresponding to d-(1-13C) glucose are highlighted by a red dotted rectangle.
Peaks corresponding to gluconic acid are highlighted by a black dashed rectangle. Peaks corresponding to isomerization product of d-(1-13C) glucose (d-(1-13C) fructose) are
highlighted by a blue dotted line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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peaks at ␦ = 63.85 and 62.63 ppm corresponded to the presence
of ␣-furanose and ␤-pyranose conformations of d-(1-13C) fructose
[50].
copper ions. The oxidation of cellobiose to gluconic acid on CuO-
CeO2 nanospheres proceeds by the hydrolysis of cellobiose via the
Lewis acid sites of the catalyst, perpendicular end-on adsorption
of glucose (cellobiose hydrolysis product) on surface lattice oxy-
gen atoms of CuO-CeO2 catalyst, abstraction of H-atoms from the
formyl group, formation of gluconate and the hydrogenation of the
gluconate to gluconic acid. 18O-oxgen isotope labeled experiments
coupled with XRD, HR-TEM, XPS, H2-TPR and UV-Raman spectra
analysis revealed that the lattice oxygen of the CuO framework, in
the CuO-CeO2 nanospheres composites, and not the lattice oxygen
from CeO2, was consumed during the oxidation reaction to form
gluconic acid. ESI-MS and 13C NMR analyses revealed that, indeed
surface lattice oxygen was incorporated into glucose to form glu-
conic acid. The Cu+ species finely dispersed on the surface of CeO2,
oxygen vacancies and lattice oxygen are advantageous for promot-
ing the catalytic performance of the CuO-CeO2 catalysts for the
selective oxidation of cellobiose to gluconic acid.
4. Discussion
The results of this study show that the total oxidation of cel-
lobiose over the CuO-CeO2 catalyst was carried out by a redox, i.e.,
Mars-van Krevelen mechanism. Our characterization results clearly
revealed that, only the lattice oxygen of the CuO in the CuO-CeO2
nanospheres composite was consumed in the oxidation reaction
and not the lattice oxygen of CeO2. The XRD patterns of the as-
prepared and spent catalysts were in perfect agreement with our
TEM and HR-TEM analysis, suggesting the reduction of CuO to Cu
in CuO-CeO2 catalyst during the direct oxidation of cellobiose to
gluconic acid. It is generally accepted that the activity of a cat-
alyst is closely related to its structure and morphology [11], in
which porous structure and high specific surface area was particu-
larly effective to enhanced its activity. This phenomenon was also
observed for the direct oxidation of cellobiose over CuO-CeO2 cat-
alyst. Although characterization results prove that CeO2 was not
directly involved in the oxidation reaction, the addition of ceria to
CuO exhibit an enhanced surface area, and the reduction temper-
ature of CuO was lower due to the presence of the CeO2 support.
This suggest that, oxygen is bound loosely in CuO in the presence
of the CeO2 support and a loosely bound oxygen is more active for
cleaving the C H bond [53] of glucose, which is a crucial step for
the formation of gluconic acid. The enhanced formation of gluconic
acid (∼51%) over the as-synthesized catalyst at 180 min, compared
to the formation of gluconic acid (∼8%) over the spent CuO-CeO2
catalyst, further explain that it was indeed the surface lattice oxy-
gen of the CuO in the CuO-CeO2 catalyst that oxidizes the reactant
to form gluconic acid and not the surface lattice oxygen of the CeO2
in CuO-CeO2 catalyst or even both. However, upon the oxidative
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
We acknowledge the financial support from the National
Research Foundation (NRF), Prime Minister’s Office, Singapore
under its Campus for Research Excellence and Technological Enter-
prise (CREATE) program and AcRF Tier 1 grant (RG129/14), Ministry
of Education, Singapore.
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cellobiose and glucose to gluconic acid, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2017.01.009