N-Doped graphene as a metal-free catalyst for glucose oxidation to succinic acid

Article abstract of DOI:10.1039/c7gc00473g

N-Containing graphenes obtained either by simultaneous amination and reduction of graphene oxide or by pyrolysis of chitosan under an inert atmosphere have been found to act as catalysts for the selective wet oxidation of glucose to succinic acid. Selectivity values over 60% at complete glucose conversion have been achieved by performing the reaction at 160 °C and 18 atm O₂ pressure for 20 h. This activity has been attributed to graphenic-type N atoms on graphene. The active N-containing graphene catalysts were used four times without observing a decrease in conversion and selectivity of the process. A mechanism having tartaric and fumaric acids as key intermediates is proposed.

Full text of DOI:10.1039/c7gc00473g

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ARTICLE
N-doped graphene as metal-free catalyst for glucose oxidation to
succinic acid
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
Cristina Rizescu, Iunia Podolean, Josep Albero, Vasile I. Parvulescu, Simona M. Coman, Cristina
b
c
c
Bucur, Marta Puche and Hermenegildo Garcia
DOI: 10.1039/x0xx00000x
www.rsc.org/
N-containing graphenes obtained either by simultaneous amination and reduction of graphene oxide or by pyrolysis of
chitosan under inert atmosphere have been found to act as catalysts for the selective wet oxidation of glucose to succinic
o
acid. Selectity values over 60 % at complete glucose conversion have been achieved by performing the reaction at 160
and 18 atm O
C
2
pressure for 20 h. This activity has been attributed to graphenic-type N atoms on graphene. The active N-
containing graphene catalysts were used four times without observing a decrease in conversion and selectivity of the
process. A mechanism having as key intermediates tartaric and fumaric acids is proposed.
from glucose by other established products derived from
glucose is not as straightforward as the direct CWO.
Introduction
Although the use of ruthenium catalysts for the formation
For the sake of sustainability of resources, there is much
current interest in the use of biomass as feedstock for the
of SA through CWO is a breakthrough in the valorization of
glucose, there is still several drawbacks associated to
ruthenium deactivation by complexation with carboxylates
formed in the process, requiring addition of sacrificial amines
as ligands. A step forward would be the use of a metal-free
catalyst to promote the CWO of glucose to SA.
1
-5
chemical industry.
Considering that cellulose and
hemicelluloses are the largest biomass components, their
chemical or enzymatic hydrolysis makes available large
6
quantities of glucose as starting material. A general type of
transformation that would be highly important for the
chemical industry is the conversion of glucose into monomers.
In this context wet oxidation (WO) of glucose can be, in
principle, a very appealing process since oxygen is the only
reagent used and the reaction medium is water. However, WO
of glucose is a very unselective process and a large variety of
C2-C4 products are formed in unsatisfactorily low individual
The use of defective graphenes (Gs) as metal-free catalysts
1
4-16
is currently under intense investigation,
activity to promote aerobic oxidations of a wide range of
due to their
17-21
organic compounds,
including benzyl alcohols and amines,
benzylic and cycloaliphatic hydrocarbons and styrenes,
benzene oxidation to phenol among others. More recently, it
has been shown that Gs can also act as promoters in the
decomposition of hydroperoxides and ozone towards reactive
7
-10
yields.
To improve the selectivity of WO, it is possible to
imagine that the use of catalysts can control this unselective
process by favouring the formation of certain reactive oxygen
species and promoting some pathways that would not take
2
2-27
oxygen species.
Considering the general activity of
graphenes as oxidation catalysts and their ability to generate
oxygen-centred radicals, it would be of interest to evaluate
how the presence of Gs can alter the product distribution in
the CWO of glucose.
1
1, 12
place in the absence of a catalyst.
In this context, we have recently reported that ruthenium-
amine complexes alter the product distribution of the catalytic
wet oxidation (CWO) of glucose, resulting in the formation of
Gs as catalysts allow a certain control of their catalytic
properties by varying the content of heteroatoms and the
density of defects. Since the presence of amines was found
beneficial in the activity of ruthenium catalysts, we speculated
that N-doped Gs could be suited as catalysts for CWO of
glucose, particularly considering the general oxidation activity
of N-doped Gs. The data obtained confirm the activity of N-
doped Gs as catalysts for the selective CWO of glucose to SA.
1
3
succinic acid (SA) in selectivity values above 80 %. SA is a
highly wanted compound considering its use in the
preparation of polyesters and polyamides and its preparation
a
Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of
Chemistry, University of Bucharest, Bdul Regina Elisabeta 4-12, Bucharest 030016,
Romania
National Institute of Materials Physics, 405 Atomiștilor Str., Măgurele, Ilfov,
b
Romania
Instituto de Tecnología Química CSIC-UPV, Universitat Politecnica de Valencia, Av.
de los Naranjos s/n, 46022 Valencia, Spain
c
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Results and discussion
Table 1. Summary of the N content and the distribution of the N atoms based on the
XPS analysis of the different samples.
Two different types of N-doped Gs were prepared in the
present study for their use as catalysts in the CWO of glucose.
One of the types uses graphene oxide (GO) as starting material
that is reacted with ammonia at three different
N content
%)
Distribution N atoms (%)
Sample
2
8-30
(
concentrations,
while undergoing
a
simultaneous
Quaternary
75
Pyridinic
25
Pyrrolic
-
N-O
-
reduction to form aminated reduced GO [NH
2
-rGO(x), where x
NH
rGO(3.8)
NH
2
-
corresponds to the percentage of N]. For the sake of
comparison samples of rGO was prepared under the same
3.8
5.3
2
-
conditions as NH
2
-rGO(x), but in the absence or low (2%) NH
3
64
24
12
-
rGO(5.3)
concentration, lacking catalytic activity. The second type N-
o
containing G was prepared by pyrolysis at 900 C of chitosan
NH2-
8
.5
51
71
35
28
10
-
-
3
1-33
rGO(8.5)
(N-G).
Chitosan is a polysaccharide of aminoglucose that
N-G
4.8
1
undergoes thermal graphitization and acts simultaneously as a
source of C and N during the pyrolysis. Scheme 1 illustrates the
preparation procedure of the set of samples used.
Scheme 2. Structure of various possible types of N atoms on G. The repored binding
energies in XPS of each different class of N atom are the following: quaternary N (in
green, 400.4 eV), pyridinic N (in red, 398.1 eV), pyrrolic N (in blue, 399.8 eV) and N-
oxide (in orange, 402.8 eV).
Scheme 1. Preparation procedures of the i) NH
rGO samples are prepared two steps: (I) sonication of GO in EG and subsequent
addition of different amounts of aqueous NH , and (II) final hydrothermal treatment in
2 2
-rGO(x) and ii) N-G samples. The NH -
3
o
Teflon-lined autoclave at 175 C for 16 h. The N-G sample was obtained upon pyrolysis
o
of chitosan at 900 C for 6 h under Ar atmosphere.
2
Both types of samples NH -rGO(x) and N-G have been
previously reported and characterized. Table 1 summarizes the
G samples under study, their preparation procedure and their
N
content. N content was determined by combustion
elemental analyses and XPS. It is known that N atoms present
in G samples can be distributed among different families that
2
9,
can be differentiated based on their binding energy in XPS.
4-36
Scheme 2 illustrates the expected different types of N
3
atoms and their reported binding energies. To determine the
variation of the distribution of N atoms in the four samples
under study, experimental high resolution XPS peaks
corresponding to N atoms at binding energy about 400 eV
were deconvoluted among their individual components.
Figure 1 shows the experimental peaks and the best
deconvolution to their individual components. A summary of
the results is also listed in Table 1.
Figure 1. High resolution XPS peaks for N1s recorded for the different N-containing G
samples and the best deconvolution to their individual components.
2
The characteristic 2D morphology of the NH -rGO(8.5) and
N-G samples was observed by TEM. Figure 2 provides some
representative images to illustrate their morphology. The
2
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single layer and few layer structure of the N containing G the weak sites were attributed to the interaction of CO
2
with
O was established by AFM with pyridinic N and the desorption peak at 270 C was correlated
subnanometric vertical resolution. It was observed that most with quaternary N. As it can be seen in Figure 4 and in
of the particles present in the NH -rGO(x) samples present a agreement with XPS characterization, NH -rGO(3.8) and N-G
thickness about 0.5 nm corresponding to single layer G. while samples exhibit a similar TPD profile that is simpler and better
N-G has particles of a thickness of 2-4 nm corresponding to defined than those of NH -rGO(5.3) and NH -rGO(8.5) that
o
samples after dispersion in H
2
2
2
2
2
incomplete exfoliation of the sample or to the re-stacking of have a broader distribution of N atoms.
the sheets during the measurement. Figure 3 presents some
representative images and measurements, showing the typical
structure of the NH
2
-rGO(x) and N-G catalysts present during
the CWO of glucose.
2
Figure 4. CO -TPD profiles of the N-containing Gs.
2
Figure 2. Representative HRTEM images of N-G (a) and NH -rGO(8.5) (b) samples.
As commented in the introduction, the purpose of the
study was to evaluate the catalytic activity of NH -rGO(x) and
2
N-G samples for the selective CWO of glucose to SA. Initial
screening experiments varying the experimental conditions
were unsuccessful and SA was formed in very complex
reaction mixtures with percentages generally about 3 % and
always below 15 %. Table
2 presents some of the
unsatisfactory results obtained under non-optimal conditions.
It is known that the product distribution of WO remarkably
depends on the experimental conditions and the reaction time
as consequence of the instability of the primary products, their
evolution over the time and the simultaneous concurrency of
different reaction pathways. Fortunately, experiments carried
o
out at 160 C under 18 atm of O
2
pressure at 20 h using
2
Figure 3. Frontal views of N-G (a) and NH -rGO(8.5) samples obtained by AFM. The
NH2-rGO(3.8) and N-G led to reaction mixtures in which SA was
present in significant percentages at complete glucose
conversions. Other products that were identified in significant
height of two representative measurements marked on blue and red are presented in
the bottom part with coincident colours.
o
percentages when the reaction is carried out at 160 C under
It is expected that the presence of N atoms on the G sheet 18 atm O
7, 38
2
at 20 h were C2 (lactic and glycolic acids) and C3
3
should introduce some basicity in the materials.
This was (glyceric acid) acids. Blank controls under these optimized
confirmed by thermoprogrammed desorption (TPD) conditions in the absence of any catalyst or in the presence of
measurements after CO2 adsorption. Figure 4 presents the rGO that does not contain N atoms result in a selectivity
temperature-CO2 desorption plots for NH -rGO(x) and N-G toward SA below 3 %, showing the influence of the use of the
2
sample. It was observed that the samples exhibit different appropriate N-containing G catalyst. Table 2 lists some of the
behaviour depending on the preparation procedure and N results of glucose CWO to SA obtained using different
content. Coarsely two broad CO2 desorption peaks at metal-free catalysts.
temperatures about 95 and 270°C were recorded and they
Concerning the catalytic activity of the different N-
were present in variable intensity and proportion depending containing samples, Table 2 data clearly shows that there is a
on the sample. The assignment of the CO2 desorption peaks remarkable influence of the N content and its distribution on
was done based on the different types of nitrogen species the performance of the N-doped Gs. It seems that the absence
2
evidenced by N1s XP spectra (see Figure 1 and Table 1). Thus, or low N content (case of rGO or a NH -rGO(0.5) sample as
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catalyst) or high N loadings with broad distribution of N atoms glucose solution, observing consistently complete glucose
case of NH -rGO(5.3) and NH -rGO(8.5)) is detrimental for the conversions and the same SA selectivity. As an example, Table
catalytic activity. In fact the two active samples have as main 3 presents the results obtained in the reuse as catalyst of the
characteristic the presence of a high population of graphenic N same NH -rGO(3.8) sample. Note that recycling of reported Ru
atoms, suggesting that these atoms are the active sites of the catalyst required the addition of amines as co-catalyst in each
process. It is worth noting that in the case of NH -rGO(3.8) the new run and that the Ru catalyst exhibits some decrease in the
reaction mixture is remarkably simple and that the combined catalytic activity upon reuse. In the present case, reuse of NH
selectivity of SA and lactic acid accounted for over 75 % rGO(3.8) and N-G does not require any sacrificial amine and
(
2
2
2
2
2
-
selectivity, this making the process very attractive.
the catalytic activity is notably maintained.
Table 2. Summary of the results of the catalytic test and their conditions.
Table 3. Recycling experiments of the same NH
Conv
2
-rGO(3.8) sample.
o
T( C)
Selectivity (%)
Selectivity (%)
P
O2
Conv
(%)
Sample
[Time
SA
LA
GlyA
(C3)
GA
Entry
Catalytic run
SA
LA
GlyA
(C3)
GA
(
atm)
(%)
(
h)]
(C4)
(C3)
(C2)
(C4)
(C3)
(C2)
st
NH
rGO(3.8)
NH
rGO(3.8)
NH
rGO(3.8)
NH
rGO(3.8)
NH
rGO(5.3)
NH
rGO(5.3)
NH
2
-
180
[1.5]
180
[20]
180
[20]
160
[20]
180
[1.5]
160
[20]
180
[1.5]
Fresh catalyst (1
1
1
1
1
1
1
1
0
0
8
8
0
8
0
10.9
100
100
100
17.3
100
8.7
2.8
13.5
0
0
71.0
76.7
8.1
0
7.0
0
17.1
15.5
23.3
3.7
1
100
68
8.1
0
3.7
cycle)
nd
-
2
3
4
2
3
4
cycle
cycle
cycle
99.8
100
68
68
68
8.2
8.0
8.1
0
0
0
3.8
3.5
3.8
2
rd
th
-
99.9
2
0
Reaction conditions: 0.5 mmols glucose, 0.025 g NH
2
-rGO(3.8), 10 ml H
2
O, 160°C,
2
-
1
8 atm O , 20 h. The difference until 100% in selectivity corresponds to lactones,
2
67.9
3.7
4.8
4.9
3.1
60.8
<5
0
hexoses and glucose isomers. SA: succinic acid; LA: lactic acid; GlyA: glyceric acid;
GA: glycolic acid.
2
-
8.6
17.8
2.3
7.0
-
5.6
2
-
4.5
0
29.3
3.7
Although the mechanism of WO is very complex with
simultaneous pathways, oxygen reduction with the generation
of oxygen-centred radicals is one of the necessary elementary
2
-
rGO(8.5)
1
80
1.5]
60
20]
60
20]
60
20]
N-G
10
18
18
18
11.3
100
12.0
10.5
0
17.1 processes taking place. Quaternary (graphitic) N atoms appear
15, 28
[
2
among the most general active sites on G to activate O .
1
N-G
rGO
6.1
<5
<5
24.0 Since the reaction takes place in an aqueous phase and in the
presence of strong oxidation radicals the initial distribution of
[
1
<5
<5
13
N atoms is affected as evidenced by the XPS measurements.
[
1
Specifically, the population of the quaternary-like N sites
Blank
<5
10
[
increases at the expense of the pyridine-like N sites due to the
easily protonation of the latter. This process also explains the
Reaction conditions: 0.5 mmols glucose, 0.025 g N-containing G, 10 ml H
2
O. The
longer induction times of the reactions upon reuse. In
addition, the oxidation of pyridine-like N to pyridine-oxide
species is also possible and not be neglected under harsh
reaction conditions (Scheme 3).
difference until 100% in selectivity corresponds to lactones, hexoses and glucose
isomers. SA: succinic acid; LA: lactic acid; GlyA: glyceric acid; GA: glycolic acid.
The proposal of graphenic N atoms as active sites is in
agreement with earlier reports on electrochemical oxygen
reduction reaction that have shown that graphenic N atoms
However, formation of SA also requires other steps,
particularly
those related
to
deoxygenation
and
hydrogenation. In the case of Ru-catalyzed CWO of glucose,
formation of SA was proposed to occur through a mechanistic
pathway involving tartaric and fumaric acids as precursors of
2
8
2
are the most efficient to reduce O by two or four electrons.
Comparison of the catalytic activity of N-containing Gs at
0 h with those previously reported by us using Ru catalysts at
.5 h show that, in general terms, longer reaction times are
1
3
2
1
SA. This proposal was supported by control experiments
showing the feasibility of fumaric acid hydrogenation by
polyols acting as hydrogen source and also deoxygenation of
tartaric acid, presumably taking place by transferring oxygen
atoms to Ru. Based on the above considerations, it can be
proposed a similar reaction mechanism in which the pyridine-
oxide along pyridine-like N species are favourable for the
glycolic acid formation, while quaternary-like N sites are
favourable for the fumaric acid intermediate formation.
Scheme 3 illustrates this proposal. This mechanism would be in
agreement with prior reports in the literature that have shown
required in the present case to achieve complete glucose
1
3
conversion for the same reaction temperatures.
This
comparison also shows that the N-containing G samples tested
in the present study exhibit somewhat lower selectivity to SA
than those reached for Ru catalysts under optimal conditions
1
3
that were above 80 %.
The catalytic stability of NH
2
-rGO(3.8) and N-G was
established by performing four consecutive uses of these
samples, recovering the catalyst at the end of the reaction by
filtration, washing with distilled water and dispersing in a fresh
4
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the ability to rGO to gain oxygen from organic substrates and
ARTICLE
a
2
Reaction conditions: 0.5 mmol substrate, 25 mg N-doped G catalyst, 10 ml H O,
o
b
c
3
9
2
160 C, 18 atm O , 800 rpm, 10 h; GA: glycolic acid, LA: lactic acid; Fumaric acid
also to act as hydrogenation catalysts.
d
e
was formed in 73%; Fumaric acid was also formed in 24 %; Fumaric acid was
f
formed in 7 %; Glucose isomers were also present.
In order to evaluate the potential of the process at a larger
scale, a large scale batch of glucose CWO (5 mmols) was
performed under optimised conditions. After 20 h GC-FID and
GC-MS analyses of the reaction mixture revealed a total
conversion of glucose with a 67 % selectivity in SA in the 0.8 g
(
88.8%) of recovered white solid.
In conclusion, in the present article it has been shown that
N-doped graphenes can promote the CWO of glucose with a
notable selectivity to SA. The available data suggest that this
catalytic activity is related to the presence of graphenic N
atoms. N—doped Gs can be used four times without observing
a decay in the catalytic activity.
Scheme 3. Proposed mechanism for the CWO of glucose to SA (path A) and glycolic acid
path B) using N-containing Gs as catalysts. The evolution of the type of N atoms during
the reaction as a function of the reaction conditions is also indicated.
(
Experimental
To provide some support to our mechanistic proposal, Catalyst preparation.
several reactions with some proposed intermediates were
Synthesis of doped G. Low molecular weight chitosan (Sigma
carried out. Thus, it was observed that tartaric acid under the
conditions of the CWO in the presence of NH -rGO(3.8)
Aldrich) was the precursor of N-G. This polysaccharide was
pyrolyzed as powder under argon atmosphere using the
following program for the oven temperature: annealing at
2
undergoes complete disappearance to a mixture containing
fumaric acid (73 %) as the major product, accompanied by
glycolic acid (27 %). This result lends support to path A in
Scheme 3 of the mechanism and shows that G can act as
deoxygenating catalysts. When tartaric acid was reacted in the
presence of glucose as hydrogen donor in a 1-to-1 mixture
under CWO conditions in the presence of N-G, then, complete
disappearance of tartaric acid accompanied with the formation
of SA (17 %) and glycolic acid (38 %) as major products was
observed. A reaction in which N-G was able to promote
hydrogenation in isopropanol as hydrogen source of fumaric
acid to SA was performed. On the other hand, fumaric acid in
water does not react in the presence of N-containing G
catalysts, showing that in the absence of hydrogen donors, no
hydrogenation occurs. Table 4 summarizes the results of these
controls.
o
o
o
2
00 C for 2 h and, then, heating at 10 C/min up to 900 C,
maintaining this temperature for 6 h. The system was then
cooled at room temperature under argon.
2
The NH -rGO(x) samples were prepared by dispersing 100 mg
of GO obtained by Hummers method in 40 ml of ethylene
glycol upon sonication at 700 W for 3 h and subsequent
addition of increasing aqueous ammonia concentrations (2, 5,
1
0, 25 %) as amine precursor. The dark brown solutions were
transferred to teflon-lined autoclave and the dispersion heated
o
at 175 C under autogeneous pressure for 16 h. The resulting
precipitate was filtered and washed with distilled water until a
neutral pH value was measured for the washing. Finally, the
o
dark solid was dried at 60 C for 24 h.
Suspensions of the corresponding G were obtained by
sonicating at 700 W for 1 h in the aqueous solution the black
solid graphitic powders.
Table 4. Results of the CWO of proposed intermediates in the presence of N-doped Gs
a
as metal-free catalysts.
Catalysts characterization.
Substrate
Catalyst
Conv.
%)
Selectivity (%)
b
b
(
SA
-
GA
LA
XPS measurements were performed at normal angle emission
c
Tartaric acid
NH
2
-rGO (3.8)
-rGO (5.3)
N-G
100
27
69
91
38
-
α
in a Specs setup, by using Al K monochromated radiation (hν
d
e
NH
2
100
-
7
1
4
=
1486.7 eV) of an X-ray gun, operating at 300 W (12 kV/25
mA) power. A flood gun with electron acceleration at 1 eV and
electron current of 100 A was used to avoid charge effects.
100
f
Tartaric
N-G
94
17
ꢀ
acid/Glucose
Photoelectron energy was recorded in normal emission by
1
/1
using a Phoibos 150 analyzer, operating with pass energy of
Fumaric acid
NH
2
-rGO (3.8)
N-G
< 2
< 2
-
-
-
-
-
-
3
0 eV. The XP spectra were fitted by using Voigt profiles
combined with their primitive functions, for inelastic
backgrounds. The Gaussian width of all lines and thresholds do
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not differ considerably from one spectrum to another, being
always in the expected range of 2 eV.
Acknowledgements.
Financial support by the Spanish Ministry of Economy and
Competitiveness (Severo Ochoa, Grapas and CTQ2015-69153-
CO2-R1) and Generalitat Valenciana (Prometeo 2013-014) is
gratefully acknowledged. Prof. Simona M. Coman kindly
acknowledges UEFISCDI for financial support (project PN-II-PT-
PCCA-2013-4-1090, Nr. 44/2014). Cristina Bucur thanks Core
Programme, Project PN-480103/2016.
2
CO -TPD measurements were carried out using the AutoChem
II 2920 station. The samples (30-50 mg) were placed in a U-
shaped quartz reactor of an inner diameter of 0.5 cm and,
o
then, they were pre-treated at 100 C for 1 h with high purity
He (Purity 5.0, from Linde), and then exposed to a flow of CO
from SIAD) for 1 h. After that, the samples were flushed with
2
(
-1
o
a flow of He (50 mL min ) for 20 min at 25 C in order to
remove the weakly adsorbed CO species. TPD measurements
were then carried out heating at a rate of 10 C min till
2
o
-1
o
7
00 C. The CO
using a calibration curve. The results are expressed as mmoles
of CO per gram of catalyst.
2
desorbed was quantified with a TC detector
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