Biomass Oxidation: Formyl C-H Bond Activation by the Surface Lattice Oxygen of Regenerative CuO Nanoleaves

Article abstract of DOI:10.1002/anie.201503916

An integrated experimental and computational investigation reveals that surface lattice oxygen of copper oxide (CuO) nanoleaves activates the formyl C-H bond in glucose and incorporates itself into the glucose molecule to oxidize it to gluconic acid. The reduced CuO catalyst regains its structure, morphology, and activity upon reoxidation. The activity of lattice oxygen is shown to be superior to that of the chemisorbed oxygen on the metal surface and the hydrogen abstraction ability of the catalyst is correlated with the adsorption energy. Based on the present investigation, it is suggested that surface lattice oxygen is critical for the oxidation of glucose to gluconic acid, without further breaking down the glucose molecule into smaller fragments, because of C-C cleavage. Using CuO nanoleaves as catalyst, an excellent yield of gluconic acid is also obtained for the direct oxidation of cellobiose and polymeric cellulose, as biomass substrates.

Full text of DOI:10.1002/anie.201503916

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Angewandte
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International Edition: DOI: 10.1002/anie.201503916
German Edition: DOI: 10.1002/ange.201503916
Sustainable Chemistry
À
Biomass Oxidation: Formyl C H Bond Activation by the
Surface Lattice Oxygen of Regenerative CuO
Nanoleaves**
Prince N. Amaniampong, Quang Thang Trinh, Bo Wang, Armando Borgna,
Yanhui Yang,* and Samir H. Mushrif*
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Abstract: An integrated experimental and computational
the role of lattice oxygen in catalyzing the oxidation reaction.
investigation reveals that surface lattice oxygen of copper
In alignment with the extensive recent research in converting
lignocellulosic biomass to fuels and chemicals, significant
interest is now generated in oxidizing cellulosic sugars into
sugar acids and its derivatives,[6] using metal oxides as
catalysts.[7]
Hence, herein 1) we perform the oxidation of glucose,
cellobiose and cellulose on a CuO catalyst, in the form of
nanoleaves, with an excellent yield of gluconic acid, 2) using
scanning electron microscopy (SEM), high-resolution trans-
mission electron microscopy (HRTEM), X-ray diffraction
(XRD), and isotope labeling experiments we demonstrate
that the lattice oxygen in the catalyst is consumed in the
reaction and that the catalyst regains its (chemical) structure,
morphology, and activity upon oxygen treatment, and 3) using
DFT calculations we reveal the role of surface lattice oxygen
À
oxide (CuO) nanoleaves activates the formyl C H bond in
glucose and incorporates itself into the glucose molecule to
oxidize it to gluconic acid. The reduced CuO catalyst regains its
structure, morphology, and activity upon reoxidation. The
activity of lattice oxygen is shown to be superior to that of the
chemisorbed oxygen on the metal surface and the hydrogen
abstraction ability of the catalyst is correlated with the
adsorption energy. Based on the present investigation, it is
suggested that surface lattice oxygen is critical for the oxidation
of glucose to gluconic acid, without further breaking down the
À
glucose molecule into smaller fragments, because of C C
cleavage. Using CuO nanoleaves as catalyst, an excellent yield
of gluconic acid is also obtained for the direct oxidation of
cellobiose and polymeric cellulose, as biomass substrates.
À
in activating the formyl C H bond in sugars. The complete
reaction mechanism, involving the insertion of the surface
lattice oxygen into the sugar molecule, in perfect agreement
M
etal oxides are well-known oxidation catalysts.[1] Oxida-
tion of hydrocarbons on metal oxides is believed to occur by
a Mars-van-Krevelen-type mechanism, using the lattice
oxygen.[2] However, surface-ad-
À
with the experimental findings, is revealed. The C H bond
sorbed oxygen is also reported to
Table 1: Reactant conversion and gluconic acid yields for the CuO-catalyzed oxidation of glucose,
cellobiose, and cellulose. Reaction temperature=1508C. The optimum glucose/catalyst ratio of 1:1 is
used (refer to Table S1 in the Supporting Information for details).
participate in the reaction.[2a,f, 3] It is
proposed that surface oxygen leads
to “electrophilic oxidation”; whereas
the lattice oxygen leads to “nucleo-
philic oxidation” of the hydrocar-
bons.[2a,d] The nature and role of
these different oxygen species and
the underlying bond activation
mechanism[4] are not yet fully under-
stood. Similarly, in the case of mol-
ecules with oxygen-containing func-
tional groups, exchange of surface
lattice oxygen with the reactant is
reported,[1e, 5] with little insight into
Biomass Substrate Catalystc
Reaction time [min] Conversion [%] Gluconic acid yield [%]
glucose[a]
CuO fresh
CuO spent
CuO regenerated 30
CuO fresh 30
CuO regenerated 30
CuO fresh 180
30
30
100
100
100
98.5
100
96.8
86.8Æ1.4
2.0Æ1.8
glucose
glucose[a]
85.4Æ0.8
65.7Æ0.5
71.7Æ1.1
59.0Æ2.5
cellobiose[a,b]
cellobiose[a,b]
cellulose[a,b]
[a] Reactor purged with nitrogen. [b] Reaction temperature=2008C. [c] The fresh catalyst refers to the
as-synthesized catalyst, spent catalyst refers to the catalyst which has undergone one reaction cycle, and
regenerated catalyst refers to the catalyst which, after one reaction cycle, is reoxidized for 120 minutes
under oxygen flow.
[*] P. N. Amaniampong,[+] Dr. Q. T. Trinh,[+] Prof. Y. Yang,
Prof. S. H. Mushrif
activation by surface lattice oxygen is compared with that of
chemisorbed oxygen on the surface. The crucial role of
surface lattice oxygen in the oxidation of glucose to gluconic
School of Chemical and Biomedical Engineering
Nanyang Technological University
62 Nanyang Drive, Singapore 637459 (Singapore)
E-mail: YHYang@ntu.edu.sg
SHMushrif@ntu.edu.sg
P. N. Amaniampong,[+] Dr. B. Wang, Dr. A. Borgna
Institute of Chemical and Engineering Sciences
A*STAR (Agency for Science, Technology and Research)
1 Pesek Road, Jurong Island, Singapore, 627833 (Singapore)
À
acid, with minimum C C cleavage, is explained.
The conversions of the oxidation of glucose, cellobiose,
and cellulose on CuO nanoleaves and the yields of gluconic
acid are shown in Table 1. The simplified reaction scheme for
glucose oxidation on CuO is shown in Scheme 1. The fresh
catalyst gives more than 85% yield of gluconic acid from
glucose; whereas, the spent catalyst results in the formation of
only smaller polyols like ethylene glycol, erythritol, and
[+] These authors contributed equally to this work.
[**] Financial support from the Singapore Agency for Science, Tech-
nology and Research, A*STAR (P.N.A., B.W., and A.B.), Nanyang
Technological University Singapore (S.H.M. and Q.T.T.), the
National Research Foundation (NRF), Prime Minister’s Office,
Singapore under its Campus for Research Excellence and Techno-
logical Enterprise (CREATE) program and AcRF Tier 1 (grant
number RG129/14), MOE, Singapore (Y.Y.) is acknowledged. We
thank Prof. Xu Rong and Mrs. Christine Veras for their help in
isotope-labeling experiments and in the design of the frontispiece
art, respectively.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503916.
Scheme 1. Simplified reaction scheme for the oxidation of glucose
catalyzed by CuO.
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smaller acids like glycolic, lactic, and formic acids. However,
after the oxidative regeneration, the catalyst regains its
selectivity towards gluconic acid. The conversion of glucose
and the yield of gluconic acid did not change significantly,
irrespective of the reactor being purged by nitrogen or not.
This suggests that dissolved oxygen does not play any role in
the reaction and that CuO is the source of oxygen in the
oxidation reaction. At reaction temperatures higher than
1508C, gluconic acid yield decreases and the yield of C1–C4
the regeneration of the catalyst and explains the excellent
yields of gluconic acid, using the reoxidized catalyst.
The morphology and the structure of CuO nanoleaves
were investigated using SEM and HRTEM. The images of the
freshly prepared catalyst, the spent catalyst and the regen-
erated/reoxidized catalyst are shown in Figure 2. The leaf-like
morphology of the fresh catalyst can be seen in Figure 2a,
À
acids increases, suggesting the cleavage of C C bonds. The
CuO catalyst also gives excellent yield of gluconic acid, with
cellobiose and cellulose as biomass substrates. However,
higher temperatures are required for cellobiose and cellulose,
since H3O+ ions, which are reversibly generated in hot water
at higher temperatures,[8] catalyze the hydrolysis of cellobiose
and cellulose to glucose.
The XRD pattern of as-synthesized CuO nanoleaves
(compare Figure 1a) shows signature peaks of copper(II)
oxide, dominated by CuO(111) and CuO(À111) surfaces
(2q = 38.6 and 35.58, respectively).[9] Additionally, a control
Figure 2. SEM and HRTEM images of (a,b) as-synthesized CuO nano-
leaves, (c,d) CuO spent samples, and e,f) CuO samples re-oxidized
(regenerated) for 120 minutes in flowing oxygen of 37 mLminÀ1
.
Figure 1. X-ray diffractograms of CuO nanoleaves samples. a) Fresh
catalyst sample. b) Catalyst sample under reaction conditions, but
without any reaction. c) Spent catalyst after the glucose oxidation
reaction. d) Regenerated/reoxidized catalyst.
with sizes less than a micron and thickness of about 30 nm.
The nanoleaf is an assembly of 1D nanowires, in agreement
with the proposed formation mechanism of 2D CuO by
transformation and assembly of 1D Cu(OH)2.[10] The spent
catalyst, upon removal of oxygen from the CuO lattice and
after being reduced to Cu, however, acquires sphere-like
morphology[11] (Figure 2c). After post-treating the spent CuO
residues in oxygen flow for 120 minutes, almost complete
transformation of sphere-like Cu particles back to leaf-like
CuO morphology is observed (Figure 2e). BET surface area
of the as-synthesized CuO was found to be 23 m2gÀ1, whereas
the sphere-like spent catalyst had a surface area of 4 m2gÀ1,
comparable to the surface area (5.64 m2gÀ1) of the sphere-like
CuO reported earlier.[12] To better understand the effect of the
nanoleaf morphology on the reaction, CuO nanocubes were
also synthesized and characterized (Figure S6 in the Support-
ing Information). The XRD pattern of nanocubes is very
similar to that of nanoleaves and their BET surface area is
only 4.07 m2gÀ1. When tested for the reaction, nanocubes
experiment (denoted as blank reaction in Figure 1b) was
performed where the catalyst and deionized water, without
the addition of any biomass substrate, were stirred in the
batch reactor at 473 K for 30 minutes. The XRD pattern of
the dried catalyst is identical to that of the fresh catalyst,
strongly suggesting that the structural integrity of the catalyst
is maintained under reaction conditions. The XRD pattern of
the spent CuO catalyst reveals diffraction peaks at 2q = 43.4,
50.6, and 74.18, attributed to the (111), (200), and (220) facets
of pure copper (Cu) face-centered cubic structure (fcc),
respectively. This shows that CuO is reduced to Cu during the
oxidation of glucose to gluconic acid, suggesting that lattice
oxygen in CuO is consumed in the oxidation reaction.
However, the reoxidized catalyst (Figure 1d) reveals XRD
peaks, similar to those of fresh CuO nanoleaves. This confirms
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resulted in 94.2% conversion and 52.8% gluconic acid yield.
These results suggest that the higher surface-to-volume ratio
of nanoleaves results in a higher active Cu(111) surface area
(as explained later), and thus, higher yield of gluconic acid.
The HRTEM images of fresh, spent and regenerated CuO
samples are shown in Figure 2b, 2d, and 2 f, respectively. The
lattice fringe of the CuO(111) fresh sample is regular, with
a distance of 0.234 nm, and that of the spent catalyst is
0.206 nm, in agreement with existing literature.[13,14]
The regenerated CuO samples show a lattice fringe of
0.234 nm, similar to that of the fresh CuO sample. These
results are in perfect agreement with our XRD and SEM
analysis, suggesting the reduction of CuO to Cu during the
glucose to gluconic acid reaction and its regeneration upon
oxidation. CuO catalyzed glucose to gluconic acid oxidation
reaction was also performed in O18-labeled water (H2O18),
with the reactor being purged by nitrogen several times to
ensure complete removal of any oxygen in the reactor.
Analysis of reaction products using mass spectrometry did not
reveal any O18 in gluconic acid, further suggesting CuO as the
source of oxygen for the reaction. Details of isotope studies
are provided in section 6 of the Supporting Information.
To confirm the insertion of surface lattice oxygen of CuO
into glucose, to reveal the underlying reaction mechanism and
to investigate the role of lattice oxygen in enhancing the
catalytic activity of CuO, we performed density functional
theory (DFT) calculations. Computations were performed
using the ab initio total-energy and molecular-dynamics
program VASP (Vienna ab-initio simulation program)[15]
and details are provided in section 2 of the Supporting
Information. Simulations were performed on CuO(111) and
Cu(111) surfaces to comprehensively evaluate the catalytic
conversion of glucose to gluconic acid on the fresh/regen-
erated catalyst and on the spent catalyst (with and without
chemisorbed oxygen), respectively. There are two different
Cu sites on a CuO(111) surface; three coordinated Cu
referred to as Cu3 and four coordinated Cu referred to as
Cu4. Similarly, there are O3 and O4 oxygen sites (refer to
Figure S1 in the Supporting Information for details). The
most stable structure (adsorption energy of 91 kJmolÀ1) of the
adsorbed glucose molecule on CuO(111) surface consists of
the carbonyl carbon atom attached to the O3 site and the
carbonyl oxygen attached to two Cu3 sites (compare
Figure 3). Upon glucose adsorption, the surface lattice O3
atom is pushed 0.33 Š above the original position. All the
possible adsorption conformations are shown in Figure S7 of
the Supporting Information.
Figure 3. Energy profile for the oxidation of glucose to gluconic acid
on CuO(111) surface. Reaction intermediates and transition states are
shown. Activation energy barriers (Ea) shown with the transition-state
structures are in kJmolÀ1. Copper color balls represent Cu atoms, red
color balls represent O atoms, gray color balls represent C atoms, and
white color balls represent H atoms.
À1
À
H dissociation is only 87 kJmol . After the C H dissociation,
the surface lattice oxygen atom that binds with the carbonyl
carbon gets further pushed 0.11 Š higher from the surface.
À
After C H dissociation, formation of gluconate is downhill by
18 kJmolÀ1. The strong binding of gluconate to the surface, by
the bidentate configuration, could be the driving force to push
the lattice oxygen out from the surface and generate the
oxygen vacancy on the surface (Figure 3). This is similar to the
generation of an oxygen vacancy on the CuO(111) surface,
driven by H2O formation from adsorbed H and lattice
oxygen.[16] In the structure of chemisorbed gluconate, the
surface oxygen has completely come out from the lattice of
CuO. Gluconate gets converted to adsorbed gluconic acid,
with an activation barrier for the hydrogenation of
97 kJmolÀ1. The transfer of H from the adjacent surface O3
atom to the gluconate is endothermic by 92 kJmolÀ1. Finally,
the energy costs to desorb gluconic acid from the surface is
À1
À
83 kJmol . The activation of formyl C H bond and the
hydrogenation of adsorbed gluconate by the water-mediated
hydrogen-shuttling pathway[17] were also evaluated by DFT
calculations and by experiments, using deuterated water as
the reaction medium (details provided in section 10 of the
Supporting Information). Based on the results, it is suggested
that the incorporation of water into the reaction mechanism is
unlikely and the most preferred pathway is the one shown in
Figure 3.
The mechanism of glucose oxidation to gluconic acid
À
involves the dissociation of the formyl C H bond, formation
of chemisorbed gluconate, hydrogenation of gluconate, and
the desorption of gluconic acid. The reaction energy profile
and the structures of transition states and intermediates for
the oxidation of glucose to gluconic acid on CuO(111) surface
are shown in Figure 3. In agreement with the experimental
data, suggesting the incorporation of the lattice oxygen into
glucose, the energy profile computed using DFT further
corroborates the viability of the extraction of surface lattice
The complete mechanism of glucose oxidation to gluconic
acid on Cu(111) is shown in Figure S9 of the Supporting
Information. The adsorption energy of glucose on Cu(111) is
only 21 kJmolÀ1 (as compared to 91 kJmolÀ1 on CuO).
Because of the weaker binding of glucose on the Cu(111)
À
À
oxygen from the CuO catalyst. The C H bond is activated by
surface, the activation energy barrier for C H dissociation is
two adjacent surface O3 sites and the activation barrier for C
increased to 141 kJmolÀ1 (TS1, Figure 4a). Compared to the
À
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acid selectivity when CuO nanoleaves are reduced to Cu
nanospheres.
To characterize the ability of surface-adsorbed oxygen
À
activating the formyl C H bond, two different scenarios were
tested; one with glucose adsorbed on a single chemisorbed
oxygen atom on Cu(111), where the carbonyl carbon is
coordinated with the oxygen; and the other where glucose is
adsorbed with the carbonyl carbon and the formyl hydrogen
on two chemisorbed (dissociated) oxygen atoms, which are in
close proximity to each other. The most preferred reaction
pathways are reported here, while other competitive path-
ways were also considered and evaluated and details are
provided in section 10 of the Supporting Information. The
adsorption of glucose by chemisorbed oxygen is energetically
more favorable than that on pure Cu (DEads on pure Cu is
21 kJmolÀ1). The adsorption energy increases from 38 to
62 kJmolÀ1 when glucose is coordinated by two chemisorbed
oxygen atoms (compare Figure 5). The activation energy
Figure 4. Transition states and activation energy (Ea) barriers (in
kJmolÀ1) for the oxidation of glucose on Cu(111) surface. Glucose is
adsorbed on the metal surface and the chemisorbed oxygen on the
surface oxidizes it. The atom coloring scheme is the same as in
Figure 3.
À
C H dissociation on CuO(111), assisted by surface lattice
À
oxygen atoms, the C H scission on the Cu(111) surface has
a much higher activation barrier. This explains the lower
selectivity towards gluconic acid on the spent Cu catalyst. In
the absence of lattice oxygen, molecular oxygen needs to be
activated on the Cu(111) surface to oxidize the dehydrogen-
ated glucose to form gluconate. The structure TS2 (Figure 4b)
is the transition state for the combination of the chemisorbed
oxygen and the dehydrogenated glucose to form gluconate.
Hydrogenation of gluconate to adsorbed gluconic acid has an
activation barrier of 96 kJmolÀ1 (Figure 4c), in excellent
agreement with literature.[18]
À
barrier for C H dissociation systematically decreases with
The activation barrier for O2 dissociation on Cu(111) is
only 28 kJmolÀ1, consistent with the results of Mavrikakis.[19]
This low barrier can facilitate the formation of chemisorbed
oxygen on Cu(111) surface from the dissolved oxygen in
water and it could possibly be a promising oxidant to oxidize
glucose. However, the O2 dissociation reaction is extremely
exothermic (À213 kJmolÀ1), and hence because of the strong
À
Figure 5. Transition states of the C H bond dissociation on a) a single
chemisorbed oxygen atom on Cu(111), b) cooperatively by two neigh-
boring chemisorbed oxygen atoms on Cu(111), and c) by the surface
lattice oxygen atoms in CuO(111). Oxygen atoms involved in the
process are labelled. Activation energy barriers (Ea) and adsorption
energies (DEads) are in kJmolÀ1. The atom coloring scheme is same as
in Figure 3.
À
Cu O bond between the chemisorbed oxygen and the
Cu(111) surface on the hollow fcc site,[19] the recombination
of this oxygen with the dehydrogenated glucose could be
challenging. This is consistent with our calculations, since the
activation energy barrier for gluconate formation is as high as
138 kJmolÀ1. Additionally, the computed activation barrier
for H2O splitting on Cu(111) is 132 kJmolÀ1, in agreement
with the barrier reported by the research groups of Li[20] and
Mavrikakis.[21] This high activation barrier possibly hinders
the formation of surface hydroxy groups to oxidize glucose.
This further corroborates that water is not involved in the
oxidation reaction.
increase in the adsorption energy of glucose. The barrier is
highest for pure Cu(111) (141 kJmolÀ1), followed by
129 kJmolÀ1 for glucose adsorbed on a single chemisorbed
oxygen on Cu(111), 98 kJmolÀ1 for glucose adsorbed on two
adjacent chemisorbed oxygen atoms on Cu(111) and
87 kJmolÀ1 for CuO(111). The decrease in the C H bond
À
activation barrier is consistent with the findings of Norskov
and co-workers.[3c] When glucose adsorbs on the oxygen
À
The key steps in the oxidation of glucose to gluconic acid
species on (or within) the surface, the C H dissociation
À
are the dissociation of the formyl C H bond in glucose and
directly forms gluconate, and hence the mechanism is slightly
different from that on pure Cu(111). However, the possibility
of having two chemisorbed oxygen atoms on a Cu surface at
such a close proximity that both carbon and hydrogen atoms
of the formyl group can be coordinated, as shown in
Figure 5b, is remote. Literature on oxygen–copper systems
suggests that chemisorbed oxygen could only be present at
extremely low coverage (metastable) and that thin surface
oxide structures are more stable than chemisorbed oxygen.
Additionally, there is a strong repulsion between partially
negatively charged oxygens.[16,22] This is in excellent agree-
ment with our experimental data, which shows excellent
the formation of chemisorbed gluconate. In the case of
À
CuO(111), surface lattice oxygen atoms assist in the C H
dissociation and bring down the activation energy barrier to
87 kJmolÀ1 (from 141 kJmolÀ1 on Cu(111)). Besides, the
formation of gluconate on CuO is easy because of the
consumption of the surface lattice oxygen as oxidant; whereas
the combination of chemisorbed oxygen with dehydrogenated
glucose on Cu(111) is difficult. These results explain why CuO
is a superior catalyst than Cu for the reaction. The lower
activity of Cu(111) towards the oxidation of glucose to
gluconic acid also explains the significant drop in gluconic
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oxygen atoms of CuO(111), dissociation of the formyl C H
bond in glucose, the formation of gluconate, and the hydro-
genation of gluconate to gluconic acid. The reduced CuO
catalyst, in the form of nanoleaves, can be regenerated
effectively upon oxidation. Implementing a combined exper-
imental and theoretical approach, we reveal that the surface
À
lattice oxygen in CuO(111) activates the formyl C H bond in
glucose by the virtue of strong adsorption of glucose and it
further moves out of the lattice to get inserted into the glucose
molecule for its oxidation to gluconic acid. Energy barriers for
À
C H dissociation and gluconate formation on pure Cu are
significantly higher. The presence of chemisorbed oxygen
atoms on Cu increases the adsorption energy of glucose and
À
also activates the C H bond (to a lesser extent though than
surface lattice oxygen); however, chemisorbed oxygen atoms
on Cu may not exist under experimental conditions. The CuO
nanoleaves also give excellent yields of gluconic acid, with
cellobiose and cellulose as starting materials.
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À
Keywords: biomass · catalysis · C H bond activation ·
lattice oxygen · sustainable chemistry
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Received: April 29, 2015
Published online: June 26, 2015
Angew. Chem. Int. Ed. 2015, 54, 8928 –8933
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