RuCl3 Supported on N-Doped Graphene as a Reusable Catalyst for the One-Step Glucose Oxidation to Succinic Acid

Article abstract of DOI:10.1002/cctc.201700383

Impregnation of RuCl₃ on N-doped graphenes results in the formation of well-dispersed, small ruthenium oxyhydroxide nanoparticles supported on N-doped graphene that may exhibit high selectivity (87 %) for the conversion of glucose into succinic acid under wet oxidation conditions (160 °C, 18 atm O₂ pressure). Ruthenium loading and N-atom distribution on graphene influence the catalytic activity, the best performing catalyst having 1 wt. % Ru loading on a graphene having a large population of graphenic N atoms. The high catalytic selectivity to succinic acid was correlated with the presence of small ruthenium nanoparticles. The present catalyst improves the best one previously reported because it does not require the continuous addition of an excess of amine to reach high succinic acid selectivity and reusability.

Full text of DOI:10.1002/cctc.201700383

Accepted Article
Title: RuCl3 supported on N-doped graphene as reusable catalyst for
one-step glucose oxidation to succinic acid
Authors: Cristina Rizescu, Iunia Podolean, Bodgan Cojocaru,
Vasile I Parvulescu, Simona Coman, Josep Albero, and
Hermenegildo García
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RuCl
3
supported on N-doped graphene as reusable catalyst for
one-step glucose oxidation to succinic acid
[a]
[a]
[a]
[a]
[a]
Cristina Rizescu, Iunia Podolean, Bogdan Cojocaru, Vasile I. Parvulescu,* Simona M. Coman,
[b]
[b]
Josep Albero, Hermenegildo Garcia*
Abstract: Impregnation of RuCl
3
on N-doped graphenes results in
(ie, glucose, fructose) to valuable dicarboxylic acids appears to
be more challenging and appealing due to the higher availability
of C6 sugars as starting materials.[
the formation of well-dispersed, small ruthenium oxyhydroxide
nanoparticles supported on N-doped graphene that may exhibit high
7]
selectivity (87 %) for the conversion of glucose into succinic acid
In general oxidation of glucose leads to complex mixtures. Thus,
wet air oxidation (WO) of glucose, at 110-140 °C in neutral
solution yields a wide range of reaction products in inadequate
low selectivity, including gluconic and glucaric acids, glucosone,
5-ketogluconic and arabonic acids, and various degradation
products with four or less number of carbon atoms.[8]
Interestingly enough, the WO of glucose, with an unexpected
yield of 62.7% in SA, was achieved at 180 °C in neutral solution
using a supported ruthenium (III) catalyst to enhance the
selectivity.[9] In the presence of Ru(III), the catalytic WO (CWO)
of glucose affords SA, along lesser amounts of carboxylic acids
with a lower molecular mass, such as lactic, glycolic and glyceric
acid.[9] Addition of an excess of amines, such as n-butylamine, to
the catalytic system (n-butylamine/Ru molar ratio of 12.5)
improved the catalytic performance of the supported ruthenium
o
under wet oxidation conditions (160 C, 18 atm O
2
pressure).
Ruthenium loading and N atom distribution on graphene influence
the catalytic activity, the best performing catalyst having 1 wt% Ru
loading on a graphene having a large population of graphenic N
atoms. The high catalytic selectivity to succinic acid has been
correlated with the presence of small ruthenium nanoparticles. The
present catalyst improves the best one previously reported, since it
does not require the continuous addition of an excess of amine to
reach high succinic acid selectivity and reusability.
Introduction
Green chemistry has a continued interest in developing highly
efficient, easily recoverable and recyclable catalysts.[1] In this
respect, due to the high specific surface area, high dispersability
in liquid media, strong adsorption capacity and excellent
mechanical and electronic properties, graphenes (Gs) are
currently attracting considerable interest as supports for active
metals.[2] Besides their intrinsic properties, the physical and
chemical properties of Gs can be tailored in a significant extent
by doping with heteroatoms such as N, P, B, S, or functional
(
III) catalyst (yield to SA of 87.5% for a total conversion of
glucose) and continued addition of n-butylamine in each run
allowed reuse of Ru(III) catalyst for several cycles.[9] This
remarkable and unexpected influence of the presence of n-
butylamine was explained taking into consideration several
effects caused by the presence of amine, such as: i) favoring the
desorption of the carboxylic acids generated in the oxidation that
otherwise would be strongly adsorbed to the Ru(III) species, ii)
increasing the stability of the Ru nanoparticles (NPs) minimizing
their growth by acting as ligands, and iii) stabilizing Ru(IV)
species that are the catalytically active species.
moieties like NH
2
-groups, etc.[3] The presence of these
heteroatoms modulates the strength and mode of interaction
between G and the metal NPs.[
2b, 4]
Considering the current importance of SA as monomer of
polyesters and polyamides, as well as the economic
attractiveness of converting glucose directly to SA by oxygen in
aqueous medium, it would be important to improve the stability
of Ru (III) catalyst, particularly to find alternatives to the
continued addition of an excess of amine. With the aim to further
improve the Ru (III) system for this valuable one-pot synthesis of
SA from glucose, it will be shown in this work that N-containing
Gs as supports of Ru species is an efficient catalyst for the
selective CWO of glucose to SA and it constitutes a green
alternative to the use of molecular amines that become
decomposed under the conditions of CWO. In the new
heterogeneous catalytic system described here, the N atoms
coordinating to Ru(III) are not a molecular additive, but a
constitutive part of the N-doped G acting as support of Ru(III)
complexes, allowing easy separation and recycling of the
catalyst without the need of continued addition of an excess of
ligand to maintain the catalytic activity. Nitrogen is directly
incorporated on the G sheet during the preparation of the
material. In addition, the use of such Ru/N-doped G catalysts
makes also possible decreasing the temperature of the CWO in
On the other hand, nowadays, there is also a high interest in the
use of biomass as renewable feedstock and one promising
strategy is the catalytic aerobic oxidation of raw materials to
valuable organic molecules.[ In this context, not long ago we
developed an efficient heterogeneous Ru(III)-based catalyst for
the oxidation of levulinic acid to succinic acid (SA) (selectivity to
SA 96-98% for levulinic acid conversion of 59-79%).[6] However,
the one-pot, selective catalytic oxidation of C6 sugar molecules
5]
[
a]
C. Rizescu, I. Podolean, B. Cojocaru, V. I. Parvulescu, S. M. Coman
Department of Organic Chemistry, Biochemistry and Catalysis
Faculty of Chemistry, University of Bucharest
Bdul Regina Elisabeta 4-12, Bucharest 030016, Romania
E-mail: vasile.parvulescu@chimie.unibuc.ro
J. Albero, H. Garcia
[b]
Instituto Universitario de Tecnología Química CSIC-UPV
Universitat Politecnica de Valencia
Av. De los Naranjos s/n, 46022 Valencia, Spain
E-mail: hgarcia@qim.upv.es
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201700383
FULL PAPER
comparison with the previously reported n-butylamine modified
Ru-catalytic system.[9] Thus, considering that Ru (III) supported
on N-doped Gs is a truly reusable catalyst, the system described
here is advantageous respect to the previously reported
homogeneous Ru(III) complex.
correspond to the G support, without appearing any peak
attributable to any Ru phase.
Results and Discussion
Catalysts Characterization
Two types of N-doped Gs were prepared and used as supports
of RuCl
3
. In one N-doped G materials, graphene oxide (GO) was
the starting material and it was submitted to treatment with NH
3
solutions of different concentrations. It is known that this
solvothermal process leads to a simultaneous incorporation of N
atoms on the G sheet while GO reduction is taking place,[
10]
2
resulting in three NH -rGO(x) (x indicates the weight percentage
of N) samples. A second type of N-doped G was obtained by
o
pyrolysis of chitosan at 900 C under inert atmosphere. Chitosan
is
a natural polysaccharide of glucosamine obtained by
Figure 1. XRD patterns of the series of RuCl
modified Gs at 5 % loading. From top to bottom: Ru/N-G (black), Ru/NH
3.8) (red), Ru/NH -rGO (5.3) (green), and Ru/NH -rGO (8.5) (blue).
3
catalysts supported on various
deacetylation of chitin and acts as source of carbon and nitrogen.
It has been reported that pyrolysis of this biopolymer at
2
-rGO
(
2
2
o
[11]
temperatures above 900 C results in N-doped G (N-G). Table
lists the N-doped Gs used in the present study as supports
1
and their N content.
Thus, the two broad lines at 2θ angle values of about 25° and
3° are attributable to the (002) and (100) plane reflections of Gs
Figure 1). Apart of typical lines at 25.2 and 43.2°, the XRD
pattern of Ru/NH -rGO(8.5) catalyst displays plane reflections
centered at 18.7 and 52.4°, respectively. The last peak has been
indexed as the γ-band and, although its origin has not been
4
(
2
Table 1. List of N-doped Gs used in this study with their corresponding N-
content and its distribution among different N types.
totally explained, it has been previously observed and attributed
Samples
N
N distribution (%), XPS
by Hirsch et al. to short-range order of aromatic lamellae. [12]
content
In accordance with reports of Knights and co-workers[13] the
pronounced (002) lines at around 23-25.0° could be assigned to
the layer-to-layer distances of the powdered G material with
loose π stacking of the layers, while the very broad (100) line at
around 43° could be attributed to the main dimension in the
plane of a two-dimensional layer structure. Therefore, the d-
spacing was calculated by applying the Bragg’s law: d = λ/2 sinθ
(%)
Quaternary
399.8 eV)
Pyridinic
(398.1 eV)
Amine
(399.8 eV)
(
4
.6
.8
72
28
-
-
-
N-G
3
100
NH
2
-rGO(3.8)
(
where λ is the X-ray wavelength, 1.542 Å, and θ is the Bragg
angle), while the main size of the crystallite along c-axis
evaluated from the width of the (002) diffraction line, denoted
τ002) and the main dimension in the plane of a two-dimensional
NH
NH
2
-rGO(5.3)
-rGO(8.5)
5.3
.5
64
14
36
36
-
(
8
50
2
layer structure (evaluated from the width of the (100) diffraction
line, denoted τ100) were estimated using the Scherrer equation: τ
=Kλ/βcosθ (where K is the dimensionless shape factor that is
considered as 1.84 for τ100 and 0.89 for τ002 calculation; λ is the
X-ray wavelength, 1.542 Å; β is full width at half maximum
intensity in radians, and θ is the Bragg angle).[ If each parallel
layer consists of N layers, τ002 for a parallel layer group is
defined as:[
After support preparation, they were impregnated with RuCl
3
14]
solutions in order to obtain 1 or 5 wt% Ru supported N-
containing G catalysts (Ru/N-containing G) and the materials
were characterized as dry powders by XRD, XPS, Raman and
microscopy techniques.
15]
Except for one sample as it will be commented below, all the τ002 = (N −1)d002 or N = (τ002 + d )/ d
002
002
diffraction peaks recorded in the XRD patterns presented in
Figure 1 for Ru/N-containing G samples as dry powders
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The detailed X-ray crystalline parameters obtained as indicated
above are collected in Table 2. Although it should be remarked
that the information provided by XRD refers to Ru/N-containing
Gs as solid powders and not dispersed in aqueous phase, they
serve to illustrate that even under dry conditions, the stacking is
limited to a few layers. It can be assumed that this stacking will
decrease or will be maintained upon sonication in the aqueous
reaction medium.
Table 2. Structural parameters estimated from X-ray diffraction for the series
of G-supported RuCl at 5 wt% samples as powders.
3
Sample
XRD position of the (002) plane, d-spacing and number of
layers
θ, plane
002)
d-Spacing
Å
τ100, Å
τ002, Å
Number of
layers
(
Ru/N-G
11.95
11.85
3.72
3.75
24.6
30.1
7.8
9.1
3.1
3.4
Ru/NH
2
-
rGO (3.8)
Figure 2. XPS core-level spectra for the N1s region of the G-based Ru
samples at 5 wt% loading.
Ru/NH
rGO (5.3)
2
-
11.55
3.85
26.6
94.9
9.1
3.4
5.1
Ru/NH
rGO (8.5)
2
-
12.0
(9.35)
3.71
(4.74)
15.1
While quaternary N (pyridinium or graphitic sp2 hybridized N
replacing C atoms in the hexagonal network of G) was present
3
in all analyzed samples (Figure 2 and Table 1), amine N (sp N
hybridized),[
17]
2
was identified only for the Ru/NH -rGO (8.5)
sample (ie the sample containing the largest N content).
3
Ru/NH
2
-rGO (8.5) should contain 50% of sp hybridized N atoms
2
Only in the case of the Ru/NH -rGO (8.5) sample at 5 wt% Ru
(
Table 1). The large difference of the quaternary N peak position
loading, a diffraction line at 52.4° was additionally recorded in
the XRD pattern. This peak can be attributed to an amorphous
ruthenium oxide phase, giving valuable information about the
state of Ru on the materials. For all the other samples, no peaks
attributable to ruthenium species could be detected, indicating
that for most of the Ru/N-doped Gs ruthenium particles should
be present as very small, well dispersed crystallites on the
modified G sheets. This will agree with TEM images as
commented below.
in Ru/NH -rGO (8.5) sample (401.3 eV versus 400.4 eV in the
other samples, Figure 2) should correspond to a different
nitrogen loading and different neighbor atoms.[17]
Quite interesting, the higher the amount of N atoms in G, the
smaller the proportion of quaternary N was. Accordingly, in
Ru/NH
quaternary configuration, while in Ru/NH
from the total N exhibit a quaternary configuration and the
majority are sp N atoms corresponding to the amino groups.
2
2
-rGO (3.8) most of the N present in the sample exists in
-rGO (8.5) only 14 %
2
3
XPS afforded information about the electronic state of the
constitutive elements of the G-based Ru samples. Figure 2
presents the experimental high-resolution N1s peaks and the
best deconvolution to its individual components, while Table 1
summarizes the results. Each N type has a characteristic
binding energy (BE) value in XPS: pyridinic N (398.1 eV),
pyrrolic N (399.8 eV) and quaternary N (400.4 -401.3 eV) [
and, accordingly, it is possible to estimate the percentage of
each type of N atom from the best deconvolution of the
experimental XPS N1s peak to individual components. In the
present case, deconvolution of the XPS N1s peaks indicates that
no component with BE corresponding to pyridine N-oxide (∼
2
Except for Ru/NH -rGO (3.8), pyridinic N (N atoms bonded to
two C atoms at the edges or defects of the G sheet, contributing
with one p electron to the π system) should be present
according to XPS N1s deconvolution in all samples in quite
similar proportion (28-36%), irrespective of the
concentration.
N
atom
16]
Doping with N also shifts the BE of the C1s level (not shown).[
For investigated G-based Ru samples at 5 wt% Ru loading, a
sharp peak at around 284.5 eV corresponds to the graphene
sp2 carbons (Figure SI1 in the supporting information). Higher
16b]
2
3
energy components correspond to sp and sp carbon atoms in
different C-O bonding configurations, including simple C-O
bonds, carbonyls C=O bonds and carboxylates (O=C-O) at
about 286, 287 and 289 eV, respectively.[18] Since, in the present
case the samples contain also N atoms, its presence will also
influence the BE of N-coordinated C atoms. Sheng et al.[
402.8 eV) is present in significant proportion in the investigated
samples.
18]
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ChemCatChem
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attributed the BE at 285.8 and 287.5 eV to the sp (C=N) and sp3
C-N) carbon atoms, respectively.
For Ru/N-G and Ru/NH -rGO (5.3) catalysts, analysis of the
Ru3p level indicated a dominant peak centered at 463.3 eV
Figure 3 a), which can be attributed to the RuO
phase.[19] The
Ru3p XPS spectrum of 5wt%Ru/NH -rGO (3.8) showed a BE
centered at 462.2 eV, and its deconvolution revealed as well the
existence of the RuO phase. Notably, the component at 462.2
2
peak at 100 °C, a middle-temperature peak in the range 230-250
°C and a high-temperature peak between 340 and 370 °C.
(
2
(
2
2
2
eV suggests that the incorporated N atoms on the G framework
transfer electron density to the Ru active sites, decreasing the
BE value. Analysis of the O1s level (Figure 3 b) clearly shows a
component at 531.5 eV that corresponds to RuO . The XPS data
2
of Ru3p and N1s support the conclusion that Ru is present on the
surface of N-containing Gs as small, well dispersed Ru(IV)
oxyhydroxide and/or RuO
agreement with the XRD of Ru/NH
2
particles, the latter being also in
-rGO(8.5) that is the only
2
case in which a peak for Ru species was recorded.
2
Figure 4. TEM images at two different magnifications of Ru/NH -rGO(3.8)
before (a and b) and after its use in four consecutive (c and d) reactions.
Images (a) and (c) correspond to a general view of the sample before and
after reaction, respectively. (b) and (d) corresponds to high-resolution images
of (a) and (c) images, respectively, where some Ru NPs have been
highlighted with red arrows.
2
The assignment of the CO desorption peaks was done based
on the different types of nitrogen species determined by N1s
peak deconvolution in the XPS spectra (see Figure 2 and Table
Figure 3. XPS core-level spectra for the Ru3p region of the Ru/N-containing G
samples (a) and best deconvolution XPS core-level spectra for the O1s region
2
of the Ru/NH -rGO (3.8) sample (b).
1
). Thus, the weak sites were attributed to the interaction of CO
with pyridinic N, medium strength sites with quaternary N and
strong sites to the interaction of CO with ruthenium
2
2
2
TEM images of Ru/NH -rGO(3.8) show the expected 2D
oxyhydroxide species. This assignment was also supported by
comparison of the TPD profiles of the samples containing Ru
with those of the N-doped G supports lacking Ru that did not
exhibit the high temperature desorption peak, indicating that the
sites of stronger basicity are associated to the presence of large
ruthenium particles in the samples (Table 3).
TPD measurements by CO2 provide evidence that the basic
strength and density of Ru oxyhydroxy NPs depends on the
interaction with the N-doped G support, the strong basic sites on
Ru being particularly notable for the samples containing the
highest N contents and being absent for the Ru/N-G and
morphology for graphene with light contrast and the presence of
very small Ru NPs of size below 1 nm. Statistical determination
of the particle size indicates that the average diameter of the
fresh sample is Ru/NH -rGO(3.8) 0.74±0.15 nm. Figure 4
2
presents selected images of this sample before and after
reaction. The small size for Ru NPs is in agreement with the
previous comments about the absence in XRD of Ru/NH
rGO(3.8) of any diffraction peak attributable to Ru species.
2
-
Thermo-programmed desorption (TPD) of CO2 after pre-
adsorption of this gas at room temperature can provide a
quantitative indication of the number and strength distribution of
Ru/NH -rGO (3.8) that will be catalytically the most active
2
2
basic sites for both Ru/N-G and Ru/NH -rGO samples. Figure 5
samples. Thus, Ru-N interaction disfavors Ru oxyhydroxy basic
sites that are associated to the presence of large ruthenium
particles with lower catalytic activity.
There are examples in the literature showing that the presence
of graphenic N atoms on G can influence the charge distribution
of the neighbor carbon atoms resulting in a zone on G that is
shows the CO2 desorption profiles as a function of temperature.
The total basic site densities (CO2 mmol/g) were measured by
integration of TPD curves and are summarized in Table 3. The
TPD profiles of modified G-containing Ru samples were
deconvoluted taking three desorption peaks: a low temperature
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especially suited for binding of active metal atoms and metal
NPs.[20]
In addition, it has been calculated and determined
experimentally that N-doped carbon can donate electrons to the
anchored metallic species, favoring the reduced states of the
metals.[4b, 21]
Thus, in line with these precedents it is proposed that in the
present case graphenic nitrogen atoms grafted on G should
contribute, through the creation of an activation region in which a
strong interaction of G with the ruthenium precursor occurs, to
the stabilization of small, well dispersed Ru(IV) oxyhydroxide
2
and/or RuO particles that do not exhibit basicity.
Catalytic activity
As commented in the introduction, the aim of the present study
is to develop a heterogeneous, reusable and selective catalytic
system for the valuable one-pot synthesis of SA from glucose
based on supporting of Ru species on N-doped Gs. This
strategy may provide a greener alternative to the reported
cationic Ru (III) anchored on aminopropylsilica coated magnetic
NP (Ru@MNP), whose main limitation was the requirement of
continued addition of an excess of primary amines in order to
reuse the catalyst.[9]
Under the WO conditions, hydroxy and hydroperoxy radicals are
formed via reduction of oxygen and dissociation and oxidation of
water, resulting also in the generation of a concentration of
hydrogen peroxide.[22] These radicals, together with the free
oxygen, can attack at the reducing end group of glucose (ie,
aldehyde group), resulting in the opening of the glycosidic ring
and the formation of carboxylic acids. Indeed, in the absence of
any catalytic species, experiments proved that the WO of
2
glucose at 180 °C and 10 atm O using water as solvent led to a
complex mixture of various low molecular acids (as lactic,
glyceric and glycolic acids among others) along significant
amounts of aldonic acids. Under these conditions in the absence
of catalyst, glucose conversion was below 12 % and SA
selectivity was below 3 %. Control experiments using N-doped
Gs in the absence of Ru showed that the selectivity to SA at 1.5
h under the previous conditions is also below 5 %.
Figure 5. CO
loading: a. NH
Ru/NH -rGO (5.3) sample; e. Ru/NH
Dotted lines correspond to the low temperature (LT), middle temperature (MT)
and high temperature (HT) regions.
2
-TPD profiles of the N-containing Gs without and at 5 wt% Ru
-rGO (3.8); b. NH -rGO (5.3); c. Ru/NH -rGO (3.8) sample; d.
-rGO (8.5) sample, and f. Ru/N-G sample.
2
2
2
2
2
Glucose conversion and SA selectivity increased dramatically
when the WO was carried out in the presence of Ru/N-doped G
as catalysts. Table 4 provides a summary of the activity data
under various conditions for the samples under study. As it can
be seen there, the presence of Ru at 5 wt% loading on N-
containing G improved both the conversion of glucose (from 20
to 71.4%) and the selectivity to SA (from 3.6 to 21.8%) with
respect to those of the blank control or in the presence of N-
doped Gs. However, the performance of G supported Ru
catalysts was still poor compared to the activity and selectivity of
other heterogeneous Ru catalysts.[9] Decreasing the reaction
Table 3. Basic strength distribution for Ru/N-containing Gs at 5 wt% Ru
loading.
Sample
Base sites density, mmoles/g
Weak
pyridinic)
Medium
(quaternary)
Strong
(Ru(IV)
Total
(
oxyhydroxide)
temperature to 160 oC while increasing the O
pressure to 18
2
Ru/N-G
0.05
-
0.20
0.17
-
-
0.25
0.17
atm, resulted at longer time also in complete glucose
conversions and the selectivity to SA values were much higher
(52.8-62.1%), although still far from 87 % reported for Ru@MNP
in the presence of an excess of butylamine.[9]
Ru/NH
2
-
rGO (3.8)
Ru/NH
rGO (5.3)
2
-
0.02
0.005
0.15
0.175
0.180
0.20
0.21
To further optimize the performance of Ru NPs deposited on N-
doped G, the catalytic activity of samples with lower Ru loading
(1 wt%) was also evaluated. While at 160 oC and 18 atm O
,
Ru/NH
2
-
0.015
2
rGO (8.5)
glucose conversion was complete irrespective of Ru 1 or 5 wt%
loading, the highest selectivity in SA was achieved using
catalysts with smaller Ru loadings, reaching 81-87 % for 1 wt%
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ChemCatChem
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in comparison to the 62-69 % SA selectivity values for 5 wt% Ru
performing catalyst of all the set of samples. In contrast, it is
expected that small Ru particles as those shown in Figure 4 for
Ru/NH -rGO (3.8) should exhibit a larger catalytic activity,
2
(
Table 4, entries 1-8). Other products observed were C3 and C2
carboxylic acids, particularly hydroxypropionic and lactic acids.
In the presence of Ru/NH -rGO (3.8) 1 wt% sample the
combined selectivity to SA and hydroxypropionic acid was over
5% at total glucose conversion what is really a remarkable
2
influencing in a larger extent the course of glucose WO. As
result of the high catalytic activity of Ru/doped G, a higher
proportion of SA that is a product not formed in significant
percentage in the non-catalytic oxidation pathways, as deduced
from the results of the blank controls, will be formed. In fact, as
commented below regarding the reaction mechanism proposal,
the formation of SA should involve other steps besides radical
oxidation, like deoxygenation and hydrogenation that do require
the presence of an active catalyst to occur.
As commented above when discussing the influence of Ru
loading on the catalytic acidity, the N-containing G as support
plays a role controlling the catalytic activity of supported
ruthenium oxyhydroxide NPs. To confirm the role of nitrogen-
doping on G in achieving these high performances, independent
tests were carried out with an analogous catalyst having the
same loading of Ru (1 wt%), but deposited on a nitrogen-free
9
value considering typical complex reaction mixtures formed in
WO (Table 4, entry 1). However, irrespective of the Ru loading
(
ie, 1 or 5 wt%) the glucose oxidation needs a long induction
period. The temporal evolution of the main reaction products is
depicted in Figure SI2 in the supplementary information.
The influence of Ru loading on the selectivity of the CWO
reaction can be interpreted considering that higher loadings
should lead to larger, less-active Ru particles that should favor
glucose decomposition to small carboxylic acids. Thus, it
appears that the observed catalytic activity arises from a
synergetic effect between the Ru loading, and the type and
content of N-atoms from GO. Accordingly, for the same Ru
loading (ie, 1 wt%) an increased amount of quaternary N leads
to a higher selectivity in SA (Figure 6), while an increased Ru
loading from 1 wt% to 5 wt% for the same support leads to a
decrease of the selectivity to SA (Table 4) as result of the larger
particle size. The optimal combination corresponded to a lower
Ru loading (ie, 1 wt%, corresponding to a higher metal
dispersion) with a higher concentration of quaternary nitrogen
reduced graphene oxide (rGO) obtained similarly to NH
but without NH in the hydrothermal reduction (Table 4, entry 9.
See Figure SI3 and SI4 for the corresponding XPS).
2
-rGO,
3
(
corresponding to a lower amount of N on the GO support).
This explanation is in agreement with the catalytic activity of
Ru/NH -rGO (8.5) at 5 wt% Ru loading containing larger Ru
oxide NPs even detectable by XRD that is the poorest
2
Table 4. Catalytic performance of N-containing G supporting Ru in the catalytic WO of glucose.a
Entry
Selectivity, %b
Conversion
%)
Catalyst
SA
LA
(C3)
HPA
(C3)
GlyA
(C3)
GA
(C2)
(
(C4)
1
2
3
4
5
6
7
8
9
Ru/NH
Ru/NH
2
-rGO (3.8) 1wt%
-rGO (3.8) 5 wt%
100
100
100
100
100
100
100
71.4
100
11.3
< 12%
87.0
69.1
83.2
65.0
81.3
62.3
52.8
21.8
-
1.9
8.8
2.6
13.8
5.1
10.2
12.8
< 3 %
-
8.4
0
1.7
2
10.2
9.2
0
11.8
3.5
Ru/N-G 1 wt%
Ru/N-G 5 wt%
0
11.4
9.0
5.2
0
5.0
Ru/NH
Ru/NH
Ru/NH
Ru/NH
2
-rGO (5.3) 1 wt%
2
-rGO (5.3) 5 wt%
2
-rGO (8.5) 5 wt%
-rGO (3.8) 5wt%c
2
3.0
15.3
6.8
0
8.4
1.1
0
25.3
< 5%
23.4
17
0
Ru/G 1 wt%
N-Ga
-
3.3
7
1
0
1
< 5%
< 3%
0
< 3%
< 3%
1
Blanka
0
< 3%
< 5%
a
, 20 h. b The difference to 100% in selectivity corresponds to lactones,
Reaction conditions: 0.5 mmoles glucose, 0.025 g catalyst, 10 ml H
2
O, 160 °C, 18 atm O
2
c
hexoses and glucose isomers. SA – succinic acid; LA – lactic acid; HPA – 3-hydroxypropionic acid; GlyA – glyceric acid; GA – glycolic acid. 180 °C, 10 atm O
2
,
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ChemCatChem
10.1002/cctc.201700383
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1.5 h.
understandable considering the conditions of CWO. It was,
however, somehow, surprising the significant decrease revealed
The optimal N-containing G supports are those having the
largest proportion of graphenic N atoms. Figure 6 presents the
correlation between the percentage of graphenic N atoms on G
and SA selectivity. SA selectivity also follows an inverse trend
by XPS in the O/Ru ratio of the four times used 1 wt% Ru/NH
rGO (3.8) respect to the fresh sample (Table SI1). This variation
suggests that the oxygen content of the RuO NPs decreases
2
-
2
with the CO
2
TPD measurements of the catalyst, those samples
along the reactions, something that contrasts with the previously
commented increase in the oxygen coordination of the C atoms.
This oxygen depletion on Ru atoms is probably caused by the
reducing power of glucose and polyols that can act as
hydrogenating reagents through hydrogen transfer mechanism
and is indicative of the role of Ru as CWO catalyst. The binding
energy of the Ru3p peak indicates, however, that the Ru(IV)
oxidation state is preserved in the reaction.
exhibiting higher density of strong basic sites due to large
ruthenium NPs being the less selective towards SA.
Stability of the Ru/NH -rGO (3.8) catalyst at 1 wt% Ru loading
2
was confirmed by recovering the material after the reaction and
reusing in four consecutive runs (Figure SI5).
Figure 6. Variation of the selectivity to SA as a function of the type and
Concerning the reaction pathways involved in the formation of
SA, the previous findings that ruthenium oxide NPs are able to
promote the deoxygenation of tartaric acid to fumaric acid, as
well as hydrogen transfer from polyols to fumaric acid has
allowed us propose the mechanism shown in Scheme 1. In this
mechanism the key intermediates will be tartaric acid formed by
oxidative decarboxylation from glucose and fumaric acids. None
of these C4 acids are detected in significant amount in the
reaction mixtures, probably due their fast conversion into SA.
This reaction mechanism is compatible with the observation that
O/Ru decreases during the CWO, making possible for Ru to
promote the oxidative deoxygenation of tartaric acid into fumaric
acid that is a process that would not occur in the absence of Ru
catalyst (step i in Scheme 1) and that been observed already for
Ru@MNP. Hydrogenation of fumaric acid would occur, on the
other hand, through hydrogen transfer from polyols that is a
process that is generally catalyzed by Ru.
content of N atoms for N-containing G supported Ru at 1 wt% Ru loading.
It was observed that the activity for glucose conversion and the
selectivity towards SA do not change in four consecutive uses.
Moreover, analysis of the four times used Ru/NH -rGO (3.8)
2
sample showed that the Ru content was maintained. Analysis of
the particle size distribution of Ru NPs by HRTEM of the four
2
times reused Ru/NH -rGO (3.8) sample showed some increase
in the average particle size and in the dispersion of the size
distribution (see Figure 4). However, these variations of average
particle size from 0.93±0.26 nm of the four times used with
respect to the 0.74±0.15 nm of the fresh sample were relatively
minor and, apparently at this level they are not reflected in the
Scheme 1. Proposed intermediates in the conversion of glucose into succinic
acid.
catalytic performance. Although the reasons for this moderate Conclusions
particle size increase need further study, it has to be noted that
carboxylic acids formed in the present reactions are known
metal mobilizing/complexing agents favoring particle growth
through the Ostwald ripening mechanism.
It has been found that the conversion of glucose into succinic
o
acid in water by oxygen can be promoted at 160 C and 18 atm.
pressure with selectivity close to 90 % by using ruthenium
O
2
In addition, XPS analysis of the four times used 1 wt% Ru/NH
rGO (3.8) was also performed and compared with the data of the
fresh sample. Table SI1 summarizes the relevant XPS data. The
2
-
supported on N-containing graphene as catalyst without the
addition of an excess of amine as co-catalyst. The Ru/N-doped
G is reusable at least in four consecutive runs without changes
in activity and selectivity of the sample, although
characterization of the used catalysts indicates an increase in
the oxygen coordination on graphene C atoms and a decrease
1
2
wt% Ru/NH -rGO (3.8) catalyst showed notable variations in
the C1s peak that indicate the occurrence of an extensive
oxidation (Figure SI1). These changes in the C1s are
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ChemCatChem
10.1002/cctc.201700383
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in the oxygen coordination around Ru. Characterization data
shows that the predominant Ru species present initially on the
catalyst are ruthenium oxyhydroxide NPs of particle size below 1
nm. It has been observed that the performance of the Ru/N-
doped G catalyst depends on Ru loading and the N atom
distribution, being the best performing samples those having
small size and homogeneously distributed ruthenium NPs as
consequence of the low Ru loading and the presence of
graphenic N atoms on G. The present findings constitute a step
forward towards the development of catalyst to promote the
selective conversion of glucose into succinic acid.
XPS measurements were performed in a Specs instrument using Al K
α
monochromated radiation (hν = 1486.7 eV) of an X-ray gun, operating at
00 W (12 kV/25 mA) power. A flood gun with electron acceleration at 1
3
eV and electron current of 100 µA was used in order to avoid charge
effects. Photoelectron energy was recorded at normal angle emission by
using a Phoibos 150 analyzer, operating with pass energy of 30 eV. The
XP spectra were fitted by using Voigt profiles combined with their
primitive functions, for inelastic backgrounds.[24] The Gaussian width of
all lines and thresholds do not differ considerably from one spectrum to
another, being always in the range of 2 eV.
2
CO -TPD measurements were carried out using the AutoChem II 2920
station. The samples (30-50 mg), placed in a U-shaped quartz reactor
with an inner diameter of 0.5 cm, were pre-treated under He (Purity 5.0,
from Linde) at 100 ºC for 1 h, and then exposed to a flow of CO
2
(from
Experimental Section
SIAD) for 1 h. After that, the sample was purged with a flow of He (50 mL
-1
min ) for 20 min at 25 ºC in order to remove the weakly adsorbed CO
2
Catalysts preparation
species. TPD measurement was then started, with a heating rate of 10
ºC min-1 till 700 ºC. The desorbed products were analyzed with a TC
Two different types of N-doped G were used in the present study. In one
case N-doped G was prepared by pyrolysis of chitosan and subsequent
exfoliation of the carbon residue, as previously reported.[23] In brief, low
molecular weight chitosan (Sigma Aldrich) was pyrolized as powder
under argon atmosphere using the following temperature program for the
2 2
detector. The CO desorbed, expressed as mmols of CO per gram of
catalyst, was determined using a calibration curve.
Catalytic tests
oven: annealing at 200 oC for 2 h and, then, heating at 10 oC/min up to
Activity tests in batch mode were carried out by adding 25 mg of
catalysts to a solution of 90 mg (0.5 mmols) glucose in 10 mL water.
After closing, the reactor was pressurized between 10 and 18 bars with
molecular oxygen and heated up to 160-180 °C, under stirring (1200
rpm), for 1.5-20 h. After reaction, oxygen was released, the catalyst was
separated by centrifugation and the products were recovered from the
o
9
00 C, maintaining this flow.
A second type of N-doped Gs were prepared by dispersing 100 mg of
GO, previously 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 (5, 10, 20 %) as nitrogen
source. The dark brown solutions were transferred to Teflon-lined
residue after removal of H O under reduced pressure. The recovered
2
products were silylated, diluted with 1 mL of toluene and analyzed by
GC-FID chromatography (GC-Shimadzu apparatus). The identification of
the products was made using a GC-MS Carlo Erba Instruments QMD
1000 equipped with a Factor Four VF-5HT column.
o
autoclave and heated at 175 C under autogenous pressure for 16 h. The
resulting black solid was filtered and washed with distilled water until
o
neutral pH was obtained. Finally, the NH2-doped Gs were dried at 60 C
for 24 h.
Suspensions of the corresponding N-doped Gs were obtained by
sonicating the black graphitic powders at 700 W for 1 h in the aqueous
and removing the non-exfoliable particles by centrifugation.
Acknowledgements
Financial support by the Spanish Ministry of Economy and
Competitiveness (Severo Ochoa, Grapas and CTQ2015-69563-
CO2-R1) and by the Generalitat Valenciana (Prometeo 2013-
014) is gratefully acknowledged. J.A. also thanks the Universitat
Politecnica de Valencia for a postdoctoral scholarship. Prof.
Simona M. Coman kindly acknowledges UEFISCDI for financial
support (project PN-II-PT-PCCA-2013-4-1090, Nr. 44/2014).
2 3
Impregnation of the N- and NH -doped Gs with a RuCl solution at basic
pH (pH=13) was performed by adding 2 g of N-containing G to an
aqueous solution (400 mL distilled water) of hydrated ruthenium (III)
chloride (Aldrich) in amounts corresponding to a final concentration of 1
wt% and 5 wt% Ru. The pH was adjusted at 13 with the aid of a 1 M
aqueous solution of NaOH. The slurry was maintained under stirring at
25 °C for 24 h. The solid was then separated by centrifugation, washed
twice with distilled water and acetone, dried under vacuum and calcined
at 400 °C, for 4 h.
Keywords: N-doped graphene • ruthenium • oxidation • glucose
•
succinic acid
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Entry for the Table of Contents
FULL PAPER
Glucose is converted in 87 %
selectivity to succinic acid
under catalytic wet oxidation
conditions using ruthenium
supported on N-containing
graphene as catalyst without
the addition of amines as co-
catalyst.
Cristina Rizescu, Iunia Podolean,
Bogdan Cojocaru, Vasile I. Parvulescu,*
Simona M. Coman, Josep Albero,
Hermenegildo Garcia*
Page No. – Page No.
3
RuCl supported on N-doped
graphene as reusable catalyst for one-
step glucose oxidation to succinic
acid
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