Unprecedented Catalytic Wet Oxidation of Glucose to Succinic Acid Induced by the Addition of n-Butylamine to a RuIII Catalyst

Article abstract of DOI:10.1002/cssc.201600474

A new pathway for the catalytic wet oxidation (CWO) of glucose is described. Employing a cationic Ru@MNP catalyst, succinic acid is obtained in unprecedently high yield (87.5 %) for a >99.9 % conversion of glucose, most probably through a free radical mechanism combined with catalytic didehydroxylation of vicinal diols and hydrogenation of the resulted unsaturated intermediate.

Full text of DOI:10.1002/cssc.201600474

DOI: 10.1002/cssc.201600474
Communications
Unprecedented Catalytic Wet Oxidation of Glucose to
Succinic Acid Induced by the Addition of n-Butylamine to
a RuIII Catalyst
Iunia Podolean,[a] Cristina Rizescu,[a] Camelia Bala,[b] Lucian Rotariu,[b] Vasile I. Parvulescu,[a]
Simona M. Coman,*[a] and Hermenegildo Garcia*[c]
A new pathway for the catalytic wet oxidation (CWO) of glu-
cose is described. Employing a cationic Ru@MNP catalyst, suc-
cinic acid is obtained in unprecedently high yield (87.5%) for
a >99.9% conversion of glucose, most probably through
a free radical mechanism combined with catalytic didehydroxy-
lation of vicinal diols and hydrogenation of the resulted unsa-
turated intermediate.
the presence of this catalyst, the oxidation of LA to SA takes
place not only with high LA conversions (X=59–79%), but also
with high selectivities to SA (S=96–98%).[6] The same catalyst
was proven to be highly efficient for the one-pot production
of sorbitol and glycerol starting from cellulose even in the ab-
sence of an external hydrogen source.[5] Extremely important,
in both cases, the catalyst was easily separated from the prod-
uct mixture by an external magnetic force making possible its
repeated use without any deactivation under hydrothermal or
oxidation conditions.
It is now unanimously accepted that the biomass transforma-
tion to platform molecules could provide a long-term solution
to the industrial dependence on fossil carbon.[1] In this respect,
chemistry is challenging the development of efficient catalytic
methods for such processes and succinic acid (SA) is among
the targets.[2] A potential starting material to obtain this com-
pound is levulinic acid (LA) but most of the reported oxidative
procedures of LA to SA raise immediate environmental con-
cerns related to the use of stoichiometric inorganic oxidation
reagents, high energy consumption, and the use of homoge-
neous catalysts.[3]
Nowadays, the selective oxidation of C6 sugar molecules
(i.e., glucose and fructose) emerged as an extremely important
area in oxidation chemistry.[7] Following this route, it has been
demonstrated that the oxidation of glucose may lead to the
production of important chemicals, such as gluconic, glucuron-
ic, glucaric, and 2-keto-d-gluconic acids.[8] As a general rule,
these liquid-phase oxidation reactions, producing the corre-
sponding carbonyl compounds or carboxylic acids in high
yields, were usually carried out at 20–808C and slightly basic
pH (pH 7–9), mainly with air or oxygen, in aqueous medium
and in the presence of supported Pt-group metals. However,
since bi- and multimetallic catalysts are more efficient and less
prone to deactivation than monometallic catalysts, Pt may be
replaced with these systems.[9] Another alternative might be
the combination of monometallic catalysts with organic modi-
fiers, such as nitrogen- and phosphorus-containing com-
pounds, which may be simply added to the reaction mix-
ture.[10]
In the last years many efforts have been devoted to develop
oxidation systems using solid catalysts and environmentally-
benign molecular oxygen as a sole oxidant. Not less important,
the development of easily recoverable and recyclable hetero-
geneous catalysts has attracted considerable research inter-
est.[4]
Trying to give answers to these issues, not long ago we re-
ported a cationic Ru-based catalyst (4 wt% RuIII@MNP), devel-
oped by combining propylamine-functionalized silica-coated
magnetic nanoparticles (MNP) with grafted cationic Ru.[5,6] In
Encouraged by our previous results, here we report the
simple development of an unprecedented highly active and
selective system for the catalytic wet oxidation (CWO) of glu-
cose to SA by combining 4 wt% RuIII@MNP with n-butylamine
as a strong nitrogenated ligand. This new system enables the
transformation of glucose into 2,3-dihydroxybutanedioic acid
and its forward didehydroxylation into SA, providing evidence
for its practical application to produce oxygen-reduced com-
pounds from biomass-derived materials.
[a] I. Podolean, C. Rizescu, 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: simona.coman@chimie.unibuc.ro
[b] C. Bala, L. Rotariu
Department of Analytic Chemistry, Faculty of Chemistry
University of Bucharest
Bdul Regina Elisabeta 4-12, Bucharest 030016 (Romania)
Developed as a benign method for the mineralization of the
organic pollutants contained in wastewaters, CWO recently
became a routine way to convert lignin into aromatic alde-
hydes[11] or glucose into gluconic acid.[12]
[c] H. Garcia
Instituto Universitario de Tecnologia Quimica (CSIC-UPV)
Universitat Politꢀcnica de Valencia
Avenida de los Naranjos S/N, 46022 Valencia (Spain)
E-mail: hgarcia@qim.upv.es
Blank experiments carried out under CWO conditions (T=
1808C, 10 atm of O2) led to moderate reaction rates (conver-
sion of 12%) and to a non-selective broad distribution of prod-
ucts. Among these, the most important was glycolic acid (se-
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cssc.201600474.
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lectivity of 28.4%), but several lactones, hexoses, and glucose
isomers were also identified as by-products.
Indeed, the analysis of the reaction products indicated an in-
crease in the yields of the low molecular mass carboxylic acids
(i.e., lactic, glycolic, and glyceric acids) in the successive runs
that was accompanied in the elemental analysis of the spent
4 wt% Ru@MNP catalysts by an accumulation of organic com-
pounds (Table 1).
In the presence of 4 wt% RuIII@MNP catalyst, the CWO of
glucose (see the Experimental Section) took place with an un-
expected selectivity of 62.7% to SA, for a conversion of glu-
cose of 100%, in only 1.5 h. Lower molecular mass carboxylic
acids, such as lactic, glycolic, and glyceric acid, constituted the
difference. Small amounts of solid brown products were also
formed, most probably through glucose caramelization.
Unfortunately, during the catalyst recycling tests, even if the
catalyst remained highly active, a decreased selectivity to SA of
about 20% per catalytic cycle was observed. After the third re-
cycle, the selectivity in SA was only 20.8% (Figure 1).
Table 1. Elemental analysis of fresh and spent 4 wt% RuIII@MNP catalyst.
Catalyst
N [%]
C [%]
H [%]
Fresh
Spent[a]
0.303
0.393
1.387
3.538
0.578
1.757
[a] 3rd cycle.
In accordance with literature reports,[14] for an oxidation re-
action, the addition of a base facilitates the alcohol group de-
hydrogenation by H abstraction, favors desorption of produced
carboxylic acids, and protects the catalysts against deactiva-
tion. However, experiments carried out in the presence of
NaOH failed. In the presence of NaOH, only insoluble high-
molecular-weight compounds commonly named “caramels”
were identified, in agreement with reports showing their pro-
duction by controlled heating of carbohydrates with different
reagents, such as sodium hydroxide, sodium sulphite, or am-
monium chloride.[15]
When replacing NaOH by n-butylamine (pKa =10.8) glucose
was almost totally transformed, in only 1.5 h, to a mixture of
succinic, glyceric, glycolic, lactic, and 3-hydroxypropanoic
(3-HPA) acids, with selectivities of 11.5, 15.8, 25.0, 2.6, and
2.8%, respectively. Besides these carboxylic acids, 36% from
the reaction products was constituted by lactones, known to
be highly unstable in the aqueous conditions where they can
undergo further oxidation to the corresponding diacids, pre-
sumably through the intermediacy of the hydroxyacids.[16]
In this way, the formation of SA can be reasonably ex-
plained. It is likely that lactic, glycolic, and glyceric acids are
formed via the already proposed mechanism of the glucose al-
kaline degradation including an enediol intermediate mecha-
nism and successive enolization, b-elimination, and rearrange-
ment steps.[17] Nevertheless, under the CWO conditions, hy-
droxy radicals are produced from the dissociation and oxida-
tion of water (i.e., H2O!OH+H; H2O+O2!OH+HO2). Hydro-
peroxy radicals can also be formed from the oxidation of water
(i.e., H2O+O2!OH+HO2) and hydrogen peroxide by its re-
combination (i.e., 2HO2!H2O2 +O2).[13] Although the hydroper-
oxy radicals are less reactive than the hydroxy radicals, at high
Figure 1. Catalytic performance of 4 wt% RuIII@MNP upon recycling in the
CWO of glucose. Note that, besides SA, the reaction mixture contained
a wide distribution of products, the major ones with selectivity lower than
10% being lactic, glycolic, and glyceric acids.
In LA oxidation, under similar reaction conditions, the same
catalyst was recyclable for several catalytic runs without notice-
able activity or selectivity decay.[6] This behavior suggested
that RuIII species are strongly coordinated and that this interac-
tion may prevent the leaching of the active species. Indeed,
characterization of the fresh and spent catalyst through diffuse
reflectance infrared Fourier transform (DRIFT) and inductively
couled plasma atomic emission spectroscopy (ICP-OES) spec-
troscopy, confirmed the chemical integrity of the catalyst.
During recycling, the amount of leached Ru in the reaction
products was negligible (<0.01 ppm, as detected from ICP-
OES analysis), whereas the DRIFT spectra evidenced the pres-
ence of the characteristic bands assigned to the hydrochloride concentrations they could play an important role. These radi-
ꢀCꢀNH3+Clꢀ species (i.e., 1859, 1557, and 1426 cmꢀ1) also in
cals together with the free oxygen can attack at the reducing
end group, resulting in the opening of the glycosidic ring and
the formation of carboxylic acids. Moreover, in the presence of
the n-butylamine, these strong oxidizing species can cause
carbon–carbon bond cleavages increasing the reaction rate of
the CWO and the concentration of the glucose alkaline degra-
dation products. Such behavior is fully consistent with litera-
ture reports.[18] No oxidation products of amine were found in
the spent catalyst.[6]
These results might suggest that the catalyst deactivation is
caused by the created acidic pH, in which case the non-dissoci-
ated carboxylic acids would be very strongly adsorbed on cata-
lyst. As consequence, the active catalyst sites would be
blocked, the selective reaction would be poisoned, and the
over-oxidation processes would be favored.[13]
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the reaction products. This could be owing to the complete
mineralization of the amine and/or its protonation. It can be
reasonably assumed that protonated amine would be better
protected from oxidation compared to neutral basic amine
(Figure S1 in the Supporting Information).
Surprisingly, the mixture of high amounts of n-butylamine
with the RuIII-based catalyst (i.e., n-butylamine to glucose
molar ratio of 0.5 and n-butylamine to Ru molar ratio of 12.5)
exerted a strong synergic effect in CWO process (Figure 2).
Thus a selectivity of 87.9% to SA was obtained by adding
Scheme 1. Possible reaction pathways for SA formation through a heteroge-
neous–homogeneous free radical mechanism to 2,3-dihydroxybutanedioic
acid (tartaric acid), followed by its disproportionation to fumaric acid.
Figure 2. The effect of n-butylamine on the catalytic performance of the
fresh and spent 4 wt% RuIII@MNP catalyst in the oxidation of glucose.
n-butylamine to the fresh catalyst (by comparing with 62.7%
SA, in the absence of n-butylamine, Figure 1) and a selectivity
of 73.1% to SA by adding in the second run n-butylamine to
a catalyst that was used without n-butylamine in the first run
(in comparison with 43% SA, obtained in the second catalytic
cycle again in the absence of n-butylamine, Figure 1). This se-
lectivity to SA at almost complete glucose conversion
is remarkable and, as far as we know, it has never
been previously reported from glucose.
dioxosuccinic acid is, however, not stable in a strong oxidizing
environment, where it can be transformed in oxalic acid under
the action of hydroxyl radicals. Accordingly, the key step in the
process would be the catalytic conversion of tartaric acid into
SA (Scheme 2). To provide support to this mechanistic proposal
and, particularly to the activity of 4 wt% RuIII@MNP as dehy-
In a moderate alkaline media, glucose is isomerized
to fructose, resulting in an equilibrium mixture of the
two sugars. However, it seems that the mechanism of
the CWO of glucose does not involve its isomeriza-
tion to fructose as the first step of reaction. Neither
Scheme 2. Possible reaction pathway for the reduction of fumaric acid to SA.
the presence of fructose as intermediate nor of its
derivative-products has been evidenced.
Based on these considerations, a plausible pathway
droxylation/reduction catalyst, two control experiments were
of the CWO of glucose to the unexpected SA product might
involve a two-step mechanism: a concerted heterogeneous-ho-
mogeneous free radical mechanism leading to 2,3-dihydroxy-
butanedioic (tartaric acid) and oxalic acids that is also in ac-
cordance to a previous report,[17] followed by disproportiona-
tion of 2,3-dihydroxybutanedioic acid to fumaric and 2,3-dioxo-
succinic acids (Scheme 1) that is quite similar to the diol-dis-
proportionation reported by Arceo et al.[19] The 2,3-
made: i) tartaric acid was transformed under the same reaction
conditions in the presence of 4 wt% RuIII@MNP and n-butyla-
mine, and ii) mixed fumaric acid with water and iso-propanol
(or small amounts of glucose) was treated under the same re-
action conditions in the presence of 4 wt% RuIII@MNP and n-
butylamine. It was observed an almost complete disappear-
ance of tartaric acid (over 98% conversion) in just 30 min reac-
tion and the formation of SA in 48% selectivity, accompanied
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by glycolic acid (38% selectivity, experiment i). Moreover, treat-
ing fumaric acid with aqueous iso-propanol, SA was formed
with a selectivity of 48.4%, along of lactic acid (15.6%), hydrox-
ypropionic acid (2.8%), glycolic acid (5.5%), oxalic acid (5.8%),
maleic acid (6.0%), and methylsuccinic acid (16%, Figure S3).
Interestingly enough, in the presence of small amounts of glu-
cose, 26.4% of SA was found in the reaction products (Fig-
ure S4). Attempts to detect the presence of tartaric and fuma-
ric acids in the reaction mixture at short reaction times failed,
indicating that these proposed intermediates should be highly
reactive under the reaction conditions. The Supporting Infor-
mation provides detailed information of the stability of SA
under CWO conditions in the presence of 4wt% RuIII@MNP as
well as the reaction of tartaric and fumaric acid (Figure S1–S4).
Oxalic acid is a typical reducing agent and its conjugate
base, oxalate (C2O42ꢀ), is a chelating agent for metal cations. Al-
ternatively, RuIII species could catalyze the oxalic acid decom-
position, which produces H2 and CO2, in accordance to previ-
ous reports.[20] However, this approach is less probable under
CWO conditions. It is likely that under the conditions of CWO,
molecular hydrogen is not even formed, but the acceptorless
dehydrogenation of keto-alcohols or hydroxy-acids intermedi-
ates affords di-ketones or keto-acids and causes the fumaric
acid reduction to SA. A closely related approach is the so
called “hydrogen borrowing” strategy[21,22] during which the
hydrogen equivalents borrowed from the substrate (i.e., glu-
cose) are temporarily stored at the RuIV sites, and instead of
being released, are used for reduction of the formed fumaric
acid. Both mechanisms are possible (see the Supporting Infor-
mation), but it is difficult to establish the involvement degree
of each at this stage of research.
Figure 3. CV traces of the 2 mm RuCl3 aqueous solution and 2 mm RuCl3
aqueous solution with different amounts of n-butylamine.
RuCl3. Even larger added volumes (25 mm) shifted the oxida-
tion peak from +1163 to +1244 mV that may correspond to
the formation of a Ru-bis-butylamine adduct. The observed
shift can be assigned to a higher steric hindrance of the RuIII
cation in this new environment. Consecutively, a shift of the
RuIII/RuIV oxidation peak was observed. Higher concentrations
of n-butylamine (e.g., >5 mm) also corresponded to the ab-
sence of any reduction counter-peak on back scanning, also
confirming the high stability of the in situ produced oxidized
RuIV-bis-amine adduct.
Based on these measurements, one may assume that the
role of n-butylamine in this reaction is twofold: it provides
a steric stability of the Ru species and stabilizes the RuIV that
appears to be the active catalytic species.
SA is a refractory acid, having a low reactivity in CWO condi-
tions. Imamura23 already reported that at 2208C and 30 bars O2
SA can be removed in a proportion of only 8% in 2 h of degra-
dation reaction. Accordingly, once formed its additional degra-
dation is difficult to be achieved (Figure S1).
In summary, RuIV-bis-amine adducts afford a new catalytic
system for the wet oxidation (CWO) of glucose to succinic
acid. The pathway follows a two-step mechanism involving
both the free radicals from homogeneous phase and the cata-
lytic active sites from solid phase. The addition of n-butylamine
provides access to unprecedented high selectivities to succinic
acid for a total conversion of glucose by changing the oxida-
tion potential of the Ru species. None of the active intermedi-
ates (tartaric or fumaric acids) could be identified even at low
conversions. However, further studies of the mechanism and
investigation of other substrates and N-additives are in prog-
ress.
Comparative cyclic voltamograms (CV) of RuCl3 and RuCl3–
butylamine systems suggest the role of RuIV in this reaction.
RuIV is a better ꢀOH ions remover (Scheme 1) than RuIII.
Figure 3 shows the CVs for RuCl3 (2.0 mm in aqueous solu-
tion) and RuCl3/n-butylamine (2.0 mm/2.5–25 mm) when a re-
versed potential (El) of 1400 mV was applied. The scan started
in a positive direction with respect to the system potential at
rest (Ei=0 V). For RuCl3 solution, the oxidation peak from
+1163 mV can be attributed to RuIII/RuIV couple. The reduction
process occurs at +796 mV, showing that process is not rever-
sible. Because n-butylamine did not show any electrochemical
activity in the working potential range, the peak recorded for
RuCl3–butylamine has to be assigned to the RuIII/RuIV couple. A
decrease of the oxidation peak intensity of RuIII/RuIV
(+1163 mV) can be observed after adding 2.5–7.5 mm n-butyl-
amine. Interestingly, increasing the added volume of n-butyla-
mine to 12.5 mm led to a secondary oxidation peak at a poten-
tial of +539 mV, which can be assigned to the oxidation of
a RuIII-mono butylamine complex. This shift is generated by
the electron donor ability of n-butylamine ligand facilitating
the oxidation of the RuIII at lower potentials compared to
Experimental Section
Activity tests in batch mode were carried out by adding to a solu-
tion of 90 mg (0.5 mmol) glucose in 10 mL of water, 50 mg of
4 wt% RuIII@MNP (0.02 mmol Ru) catalyst. After closing, the reactor
was pressured at 10 bars with molecular oxygen and heated up to
1808C, under stirring (1200 rpm), for 1–2 h. The same catalytic ex-
periments were done by using 0.25 mmol of n-butylamine as pro-
moter. After reaction, the oxygen was released and the catalyst
was magnetically recovered by placing a permanent magnet on
the reactor wall, and the products were separated by distillation
under vacuum. The recovered products were silylated, diluted with
1 mL of toluene and analyzed by gas chromatography–flame ioni-
zation detector (GC–FID) chromatography (GC-Shimadzu appara-
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Acknowledgements
The work was supported by the strategic grant POSDRU/159/1.5/
S/137750, Project “Postdoctoral programme for training scientific
researchers” co-financed by the European Social Foundation
within the Sectorial Operational Program Human Resources De-
velopment 2007–2013. Prof. Simona Coman kindly acknowledges
UEFISCDI for financial support (project PN-II-PT-PCCA-2013-4-
1090, Nr. 44/2014). Also, we thank Prof. Curt Reimann for critical
reading and helpful suggestions related to manuscript prepara-
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Keywords: biomass conversion · heterogeneous catalysis ·
oxidation · ruthenium · succinic acid
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Revised: May 17, 2016
Published online on && &&, 0000
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COMMUNICATIONS
I. Podolean, C. Rizescu, C. Bala, L. Rotariu,
V. I. Parvulescu, S. M. Coman,* H. Garcia*
SA with Ru+BuNH2: Glucose is con-
verted in 87.5% selectivity to succinic
acid at complete conversion under cata-
lytic wet oxidation conditions using RuIII
supported on aminopropyl-functional-
ized silica-coated magnetic nanoparti-
cles as the catalyst and n-butylamine as
the base. The addition of n-butylamine
provides access to unprecedented high
selectivities to succinic acid for a total
conversion of glucose by changing the
oxidation potential of the Ru species.
&& – &&
Unprecedented Catalytic Wet
Oxidation of Glucose to Succinic Acid
Induced by the Addition of n-
Butylamine to a RuIII Catalyst
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