Nb-based zeolites: Efficient bi-functional catalysts for the one-pot synthesis of succinic acid from glucose

Article abstract of DOI:10.3390/molecules22122218

The one-pot production of succinic acid from glucose was investigated in pure hot water as solvent using Nb (0.02 and 0.05 moles%)-Beta zeolites obtained by a post-synthesis methodology. Structurally, they are comprised of residual framework Al-acid sites, extra-framework isolated Nb (V) and Nb₂O₅ pore-encapsulated clusters. The Nb-modified Beta-zeolites acted as bi-functional catalysts in which glucose is dehydrated to levulinic acid (LA) which, further, suffers an oxidation process to succinic acid (SA). After the optimization of the reaction conditions, that is, at 180 °C, 18 bar O₂, and 12 h reaction time, the oxidation of glucose occurred with a selectivity to succinic acid as high as 84% for a total conversion.

Full text of DOI:10.3390/molecules22122218

molecules
Article
Nb-Based Zeolites: Efficient bi-Functional Catalysts
for the One-Pot Synthesis of Succinic Acid
from Glucose
Magdi El Fergani, Natalia Candu, Simona M. Coman * and Vasile I. Parvulescu *
Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest,
Bdul Regina Elisabeta 4-12, Bucharest 030016, Romania; magdi.belkassim@yahoo.com (M.E.F.);
natalia.candu@chimie.unibuc.ro (N.C.)
* Correspondence: simona.coman@chimie.unibuc.ro (S.M.C.); vasile.parvulescu@chimie.unibuc.ro (V.I.P.);
Tel.: +40-201-305-3764 (S.M.C.); +40-214-100-241 (V.I.P.)
Received: 14 November 2017; Accepted: 12 December 2017; Published: 14 December 2017
Abstract: The one-pot production of succinic acid from glucose was investigated in pure hot water
as solvent using Nb (0.02 and 0.05 moles%)-Beta zeolites obtained by a post-synthesis methodology.
Structurally, they are comprised of residual framework Al-acid sites, extra-framework isolated Nb (V)
and Nb2O5 pore-encapsulated clusters. The Nb-modified Beta-zeolites acted as bi-functional catalysts
in which glucose is dehydrated to levulinic acid (LA) which, further, suffers an oxidation process
to succinic acid (SA). After the optimization of the reaction conditions, that is, at 180 ◦C, 18 bar O2,
and 12 h reaction time, the oxidation of glucose occurred with a selectivity to succinic acid as high as
84% for a total conversion.
Keywords: Beta-zeolite; niobium; dealumination; post-synthesis insertion; glucose; oxidation;
succinic acid
1. Introduction
The use of the biomass as feedstock ensures the sustainability of resources and, on this basis,
it is of a high current interest for the chemical industry [
into monomers emerged as very important. Therefore it is necessary to make it available in large
quantities via either chemical or enzymatic hydrolysis of both cellulose and hemicelluloses [ ].
1–5]. Accordingly, the conversion of glucose
6
However, to answer the complexity and diverse reactivity of the various biomass substrates,
it is necessary to develop novel catalytic systems able to afford with high reaction rates the required
conversions and selectivities of the feedstocks [7,8].
The wet oxidation (WO) of glucose can provide, in principle, a very appealing green route for the
synthesis of platform molecules usable for the monomers preparation, where the solvent is water and
molecular oxygen is the only used oxidation reagent. However, in the absence of a catalyst, the WO of
glucose is a very unselective process [9–12].
In this context, we reported not long ago that ruthenium-amine (i.e., n-butylamine) complexes
improve the selectivity of glucose WO by favoring the formation of certain reactive oxygen species
and promoting some pathways that would not take place in its absence. The catalytic wet oxidation
(CWO) takes place, in this case, with the total conversion of glucose and the selective formation of
succinic acid (SA, selectivity values above 85%), highly used in the preparation of polyesters and
polyamides [13]. The beneficial influence of n-butylamine was explained by several effects, such as:
(i) it favors the desorption of the generated carboxylic acids from the Ru(III) species; (ii) it increases the
stability of the Ru nanoparticles (NPs), and; (iii) it stabilizes the catalytically active species, namely,
Ru(IV) species.
Molecules 2017, 22, 2218; doi:10.3390/molecules22122218
www.mdpi.com/journal/molecules

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An important issue to ensure the economic attractiveness of converting glucose directly to SA
by oxygen, in aqueous medium, is to improve the stability of the Ru(III) catalyst, particularly to find
alternatives for the continuous addition of an excess of amine. Therefore, a step forward represented
the development of ruthenium oxyhydroxide nanoparticles supported on N-doped graphene catalysts.
They are able to convert glucose to SA (selectivity of 87%) in the absence of an excess of amine [14].
Although the use of ruthenium catalysts is considered as a breakthrough in the valorization of
glucose, there are still several drawbacks associated to its use, and the most important is connected
to its critical element status [15]. Its utilization in a nanoparticulate form may offer an efficient use
potential, but the metal-free catalytic alternatives or the use of the non-noble metals-based catalysts is
more challenging to improve the desired sustainability. In this context, we shown that N-containing
graphenes obtained either by simultaneous amination and reduction of graphene oxide or by pyrolysis
of chitosan under inert atmosphere may act as metal-free catalysts for the selective WO of glucose
to SA [16]. However, the process is less efficient compared to corresponding Ru-based graphene
catalysts [14]: at 160 ◦C, under 18 atm O2 and after 20 h the selectivities to SA were in the range of
60–68% at a complete glucose conversion.
Due to their remarkably high acidity (Ho =
were extensively investigated by different research groups, in various transformations of biomass
derivatives [17 22]. The enhancement of both the hydrothermal stability and acidity in aqueous phase
−
8.2), also preserved in water, niobia-based catalysts
–
reactions through the deposition of niobia on oxide supports (e.g., silica, titania, alumina) is well
documented in the literature [23]. Moreover, in the presence of Nb-based carbon nanotubes (CN)
catalysts, levulinic acid and glucose were successfully oxidized to SA, with selectivities of 98–99% and
80%, respectively, for a total conversion of the raw material at 180 ◦C, 10 bar O2, and after 6 h [21].
However, the catalytic performances, expressed in terms of succinic acid yields, were directly correlated
to the nature of the catalytic sites which, in turn, has been dictated by the niobia content and the
preparation route. Therefore, the lower the niobia content in the catalyst, the higher the selectivity to SA.
All the above described catalytic materials require quite laborious and time spending preparation
methodologies. In this respect, a simpler and faster alternative might afford an easier transfer
of the already known structural and chemical properties of the catalysts utilized in the oil-based
industry [24,25] to the new biomass-based industry. Thus, for the particular case of the production of
succinic acid through the one-pot transformation of glucose, synthetic zeolites are potential candidates,
while nanoparticles of niobium can offer a viable alternative to ruthenium since the use of ruthenium
as catalytic active specie is inconvenient from the elemental sustainability point of view. To obtain this,
the low stability of zeolites in hot water should be improved through a heteroatom insertion. In this
respect, the generation of heteroatomic Lewis acid- [26,27] or bifunctional-zeolite catalysts [28] for the
production of the platform molecules is an expanding field.
Based on this state of the art, in this work we focused our interest on a post-synthesis strategy
for the production of highly dispersed Nb species (0.02–0.05 moles %) into the BEA-zeolite matrix.
This strategy supposes a successive zeolite dealumination followed by its impregnation with Nb
etoxide. The new catalysts demonstrated a high catalytic efficiency for the direct synthesis of succinic
acid from glucose and a very good stability against water. The reactions carried out with these catalysts
in water, as solvent, and molecular oxygen, as solely oxidation reagent, afforded the direct oxidation
of glucose to succinic acid with selectivities up to 84% for a total conversion of glucose.
2. Results and Discussion
2.1. Catalysts Characterisation
The X-ray diffraction patterns of the studied H-
β
samples are shown in Figure 1. The characteristic
sharp and intensive reflections of H- zeolites, at 2
β
θ
values of 7.6◦, 21.2◦ and 22.4◦, were consistent
with the typical diffraction lines of the referenced Beta zeolite, indicating a well-developed crystalline
structure of Beta topology, formed by the intergrowth of polymorphs A and B [29].

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Figure 1. X-ray diffraction patterns of H-β samples.
The crystallinity of the samples was defined on the basis of the three characteristics diffraction lines
◦
◦
◦
of Beta zeolite at Bragg angles of 7.5 , 21.3 and 22.4 , and it was calculated by using Equation (1) [30],
considering as reference the Beta zeolite sample with the highest intensity of the diffraction of these
three lines.
3
Intensityi
∑
i = 1
3
%Crystallinity = ꢀ
ꢁ
× 100
(1)
Intensityi
∑
i = 1
Ref
Beta zeolites with a higher Si/Al ratio are characterized by a higher crystallinity (Table 1), that is,
in a good concordance with previous studies [30,31].
Table 1. Crystallinity of the H-Beta sample, calculated with Equation (1).
H-Beta Sample
Si/Al Molar Ratio
Crystalline Phase
Crystallinity (%)
H-β6
H-β12
H-(β18 and β37)
6
12
Beta zeolite
Beta zeolite
Beta zeolite
83.1
99.5
100
18 and 37
Sharper and more intense reflections of H-
slightly larger crystals that could be associated with the higher Al content (Si/Al = 6 for H-
Si/Al = 12 for H- 12). Moreover, all the reflection lines of H- 6 sample were shifted to lower 2
than for H- 12, indicating an increase in the unit cell size that could be associated to an increased
β
6 (Figure 1) indicate highly regular structures and
6 versus
values
β
β
β
θ
β
concentration of longer Al-O bonds compared to Si-O bonds.
In contrast to other zeolites, the structure of the hydrated Beta zeolite shows a reversible change
in the coordination of the framework Al atoms. Depending on the counter-ion it may balance the
framework into a negative charge. Thus, it is generally accepted that in the hydrated protonic form of
the Beta zeolite part of the framework Al atoms, with tetrahedral coordination in Na form, changes the
coordination to octahedral [32].
On the other hand, in an aggressive environment such as HNO3 acid, a severe structural collapse
of the Beta zeolite may occur. Under these conditions the produced mesoporosity is not useful
because the resulted pores collapse and, therefore, the direct connection with the external surface
of the zeolite particles is hindered. However, in the present case, the presence and the intensity of
the typical diffraction lines of the Beta zeolites at 2
dealumination conditions utilized in this work affected only in part the crystallinity of the parent
θ
of 7.6◦, 21.2◦ and 22.5◦ (Figure 2) certify that the

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zeolite, but no obvious broad peak due to the presence of an amorphous (silica-alumina) phase has
been observed.
Figure 2. X-ray diffraction patterns of H-β18 and the corresponding dealuminated samples (DeAl-Beta).
Treatment in nitric (blue) and oxalic acid (red), respectively.
The X-ray diffraction (XRD) pattern of the DeAl-Beta sample prepared through the dealumination
with oxalic acid (Figure 2, red pattern) shows a change in the position of the narrow line from 2
θ
22.48◦ to 22.75◦, corresponding to a decrease of the d302 spacing from 3.952 Å (H-
β18) to 3.906 Å
(DeAl-Beta). The dealumination with nitric acid changed the position of this diffraction line to 22.67◦
that correspond to a d302 spacing of 3.917 Å. These shifts, in fact, confirm a larger contraction of
the zeolite matrix [33] by the treatment with oxalic acid. In conclusion, it emerges that the oxalic
acid is more effective for the extraction of lattice aluminum than nitric acid. The extraction occurred
without a collapse of the zeolite framework structure. This behavior should be associated with the
dual nature of the oxalic acid. In relation to the framework aluminum it may act both as a hydrolyzing
and a chelating agent. The dealumination process takes place via the formation of trioxalato aluminum
complexes with a high complexation constant (logβ3 = 15.1) [34].
Further, the treatment of the DeAl-beta samples with a niobium salt led to an increase in the
d302 spacing. Figure 3 presents the XRD patterns of the Nb-treated samples, while the changes in the
position of the diffraction line from 2θ
22.48◦ and the corresponding d302 spacing are listed in Table 2.
Figure 3. X-ray diffraction patterns of Nb-β samples.
As Table 2 shows, upon the incorporation of Nb into the DeAl-
spacing compared to the DeAl- reference depends on the dealumination route and loading of niobium.
Finally, this corresponds to different expansions of the Nb- zeolite matrix. However, the size of the
β samples, the increase of d302
β
β

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niobium ions and the bond length of Nb-O (1.89 Å for tetracoordinated Nb (V) species in Beta zeolite)
compared to that of Si-O (typically 1.60–1.65 Å in zeolites) [35] do not support an insertion of Nb in the
tetrahedral positions of the Beta matrix. The d302 spacing values measured in this work are too small
to sustain such an effect. Larger d302 spacing values were measured for the samples dealuminated
with oxalic acid (Nb(0.05)-
spacing values increased from 3.906 Å (DeAl-
3.912 Å (DeAl- 37 (oxalic acid)) to 3.969 Å (Nb(0.05)-
β
18O and Nb(0.05)-
β
37O, Table 2, entries 8 and 10). In these cases the d302
18 (oxalic acid)) to 3.936 Å (Nb(0.05)- 18O) and from
37O) and the shifts are close to those reported
β
β
β
β
by Dzwigaj et al. [36] (i.e., an increase of the d302 spacing from 3.912 Å (SiBeta) to 3.954 Å (NbSiBeta,
2θ = 22.45◦) for a 0.02 mo % Nb-SiBeta material prepared by the post synthesis methodology.
Table 2. The changes in diffraction line from 2
θ
22.48◦ and the corresponding d302 spacing for the
Nb-treated catalysts.
◦
Entry
Sample
θ,
d302, Å
1
2
3
4
5
6
7
8
9
DeAl-β12 (nitric acid)
Nb(0.02)-β12
Nb(0.05)-β12
DeAl-β18 (nitric acid)
Nb(0.02)-β18
Nb(0.05)-β18
DeAl-β18 (oxalic acid)
Nb(0.05)-β18O
DeAl-β37 (oxalic acid)
Nb(0.05)-β37O
22.68
22.57
22.66
22.67
22.61
22.63
22.75
22.57
22.71
22.38
3.917
3.936
3.921
3.917
3.929
3.926
3.906
3.936
3.912
3.969
10
The inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of
the liquid phase separated after the Nb insertion step did not evidence the presence of niobium
species suggesting, indeed, an entrapment of all niobia in the Beta zeolite structure. Therefore, it can
be stated that for the samples dealuminated with nitric acid the low expansion of the Beta zeolite
matrix is most probably the effect of its distortion by the non-framework pore encapsulation of Nb(V)
nano-oxides. This process takes place without a real replacement of the tetrahedral aluminum in the
zeolite framework. On the contrary, the dealumination with oxalic acid allows the insertion of Nb in
the tetrahedral positions of the Beta matrix, at least partially.
The structural modifications of the zeolite matrix after the insertion of Nb were also evidenced by
the FTIR study, confirming the XRD analysis results. In the 4000–2500 cm−1 range, the FTIR spectra
of all H-β and Nb(0.02)-β samples (Figures 4 and 5) show a broad band assigned to the stretching
vibrations of the -OH groups from H2O adsorbed on the zeolite surface and/or existing in the zeolite
channels (surface silanol groups, Si-OH). The sharp bands from this region can be associated with the
asymmetric vibrations of some M-OH isolated groups. The FTIR spectra of H-β18 and H-β12 show
typical -OH vibrations in Beta zeolites with bands of non-interacting bridging hydroxyls and the broad
band ranging from 3620 to 3200 cm−1 of mutually interacting Si(OH)Al groups. The OH stretching
modes at wave numbers at 3642 cm−1 (H- 12) and 3647 cm−1 (H-
β β18) are due to the Si-O(H)-Al
groups, at 3743 cm−1 is assigned to terminal silanol (Si-OH) groups in small crystallites and high
external surface area materials, while that at 3667 cm−1 is attributed by some authors to AlO-H groups
of perturbed and extra-framework Al atoms [37].
The incorporation of Nb induced a reduction of all the above IR bands’ intensity and new
bands centered at 3736 and 3671 cm−1 were evidenced, most probably related to the pres−e1nce of
Nb(V)O-H and isolated SiO-H groups. At the same time, no absorption bands at 3555–3602 cm were
evidenced, confirming the lack of Nb(IV), in accordance with a previous report of Tielens et al. [36].
In 1500–500 cm−1 range, corresponding to the framework vibration region, the presence of a new
absorption band centered at 952 cm−1 has also been evidenced (Figures 6 and 7) for the Nb-based
catalyst. According to literature, this band is generally taken as an indication of the metal incorporation

Molecules 2017, 22, 2218
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into the zeolite framework and based on the reports of Corma et al. [38] it can be attributed to
the Si-O-Nb vibrations. However, it is more probable that the presence of such bonds correspond
to the extra-framework isolated Nb(V)O-H species. The absence of a band located at 884 cm−1
indicates the absence of the niobyl (-Nb=O bonds) species that may also account for the presence of
the Nb2O5 clusters [36].
Figure 4. FTIR spectra of H-β18 and Nb(0.02)-β18.
Figure 5. FTIR spectra of H-β12 and Nb(0.02)-β12.
Figure 6. FTIR spectra of H-β18 and Nb(0.02)-β18.

Molecules 2017, 22, 2218
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Figure 7. FTIR spectra of H-β12 and Nb(0.02)-β12.
A sharp peak around 1100 cm−1 is usually noticed in zeolites and assigned to the asymmetric
stretching of the SiO4 tetrahedra. The shift to higher wave numbers is due to the presence of large
amounts of other cations, since the Si-O bond distance is shorter than the Al-O one. Indeed, in the case
of both the H-
β
rich-aluminum samples this characteristic band was evidenced at 1197 cm−1 (H-
12), respectively (Figures 6 and 7). These bands can be associated with the remnant
β18)
and 1202 cm−1 (H-
β
AlO-H group after the dealumination and Nb insertion, indicating either a partially replacement of the
framework Al during the dealumination or the presence of the extra framework AlOx(OH) species.
Indeed, it is already well known and documented that during the zeolite framework
dealumination the Si-O-Al bonds are hydrolyzed, and tetrahedrally coordinated Al is extracted from
the zeolite framework and deposited mostly as octahedrally coordinated mono- and even oligomeric
Al-O extra-framework species [39,40]. Moreover, such residual extra-framework aluminum species
adds Lewis acidity to the synthesized material [41].
The geometry of Nb is dependent on the synthesis method [36]. Indeed, the present procedure led
to extra framework Nb(V)O-H species, while in zeolites prepared through a direct synthesis procedure
tetrahedral species presenting a niobyl bond (-Nb=O) have been identified [38]. However, in the case
of the dealuminated samples with oxalic acid the presence of penta-coordinated framework Nb(V)O-H
species should not be excluded.
2.2. Catalytic Behaviour
The aim of this study was to evaluate the catalytic activity of Nb-based zeolites samples for the
selective CWO of glucose to SA. It is well 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. On the other hand, the catalytic performances are highly influenced by different
catalytic system features as reaction conditions, catalytic active phase characteristics and the support
nature. Therefore, it has been investigated the influence of the oxygen pressure, reaction temperature
and reaction time upon the reaction products distribution.
Figure 8 presents the influence of the oxygen pressure and reaction time upon the distribution of
the most important products, in the presence of the Nb(0.05)-β18 catalyst. As Figure 8 shows, both the
glucose conversion and selectivity to succinic acid increased with the oxygen pressure and reaction
time. Practically, a total conversion of glucose was reached after 6 h, with a selectivity to succinic acid
of 46.8%, at 18 bar O2. However, the selectivity to succinic acid continued to increase in time and,
after 12 h, reached 72.3%.

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Figure 8. The influence of the oxygen pressure and reaction time upon the reaction product distribution
◦
(Reaction conditions: 90 mg glucose; 50 mg Nb(0.05)-β18 catalyst; 10 mL water, 180 C. LA— lactic acid;
GA—glycolic acid, LevA—levulinic acid; SA—succinic acid). Note that the values from graph are
averaged ± %RSD (n = 3).
Lower loadings of niobium (i.e., 0.02 mo %) and reaction temperatures (i.e., 160 ◦C) led to lower
selectivities to succinic acid (Table 3). Interesting enough, the selectivity to succinic acid is favored by
the presence of catalysts with higher Si/Al ratios. In accordance, the highest selectivity to SA (83.6%)
for a total conversion of glucose was obtained in the presence of Nb(0.05)-β37.5, dealuminated with
nitric acid (Table 3). The corresponding Nb(0.05)- 37.5O sample, produced in the presence of oxalic
β
acid, led to a selectivity of succinic acid of only 70% for a total conversion of glucose.
It is likely that lactic and glycolic acids are formed via the already proposed mechanism of the
glucose degradation [20]. On bifunctional catalysts, levulinic acid (LevA) can be also produced from
glucose, usually in two steps involving the glucose isomerization to fructose on Lewis acid centers,
followed by the fructose dehydration to LevA through 5-hydroxymethylfurfural (HMF) intermediate
on Brønsted acid centers [42,43]. However, the dehydration of glucose to levulinic acid through
a levoglucosane intermediate is also possible on materials presenting exclusively Brønsted acidity [44].
◦
In the absence of oxygen, at 180 C and in the presence of Nb(0.05)-
β18 catalyst, glucose is dehydrated
to HMF (5-hydroxymethylfurfural) and levulinic acid with selectivities of 22.0% and 8.0%, respectively.
However, comparing with the CWO reaction, in the presence of the same catalyst and under 18 atm

Molecules 2017, 22, 2218
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O2 (Table 3, entry 3), the dehydration of glucose takes place with a much lower conversion (47.4%)
and after 24 h. Unfortunately, during the glucose dehydration, besides HMF and levulinic acid, a large
spectrum of other water soluble side products and unwanted HMF aldol-condensation byproducts,
generically called “humins”, are obtained in high amounts. These humans block the active sites of
the catalysts, leading to lower catalytic performances. The presence of molecular oxygen seems to
be beneficial in avoiding the formation of such unwanted products (i.e., humins). In addition, for all
CWO reactions the conversion of glucose is faster and orientated to the targeted reaction product,
namely, succinic acid.
Table 3. Reaction products distribution in catalytic wet oxidation (CWO) tests.
S, %
Entry
Catalyst
X, %
LA
GA
LevA
SA
1
2
3
4
5 *
6
Nb(0.02)-
Nb(0.05)-
Nb(0.05)-
Nb(0.05)-
Nb(0.05)-
Nb(0.05)-
β
β
β
β
β
β
12
12
18
37.5
37.5
37.5O
86.2
100
100
100
98.7
100
3.1
1.0
3.1
0.6
2.8
0
5.9
3.1
2.1
0.9
4.3
1.3
19.0
5.3
16.6
3.5
22.7
10.2
52.0
62.3
72.3
83.6
58.3
70.1
Reaction conditions: 90 mg glucose; 50 mg catalyst; 10 mL water; 18 bar O2, 180 ◦C, 12 h; *
GA—glycolic acid, LevA—levulinic acid; SA—succinic acid.
−
160 ◦C; LA—lactic acid;
However, on Nb-zeolite catalysts the presence of levulinic acid suggests a different reaction
pathway to SA compared to Ru-based catalysts [13]. Using Ru-based catalysts only tartaric and
fumaric acids have been detected in the reaction products. Under the WO conditions, hydroxy and
hydroperoxy radicals are produced from the dissociation and oxidation of water (i.e., H2O =
•
OH + H;
•
H2O + O2 =
•
OH + HO2−−). Hydrogen peroxide can also be formed from the hydroperoxy radicals
recombination (i.e., 2HO2 = H2O2 + O2) [45]. Then, these radicals 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. In this case, a plausible pathway of the CWO of glucose to the SA product
might involve a two-step mechanism: (A) a concerted heterogeneous-homogeneous free radical step
leading to tartaric and oxalic acids, followed by the disproportionation of tartaric acid to fumaric and
2,3-dioxosuccinic acids; (B) the catalytic conversion of fumaric acid into SA.
Taking into consideration the literature reports and the experimental results obtained in this
study, the formation of succinic acid can also be reasonably explained in the presence of the Nb-zeolite
catalysts. According to Ziolek et al. [46] the oxidation of cyclohexene, for instance, with hydrogen
peroxide and in the presence of Nb-based catalysts, takes place through superoxo and peroxo species
generated through the interaction of NbOH species with hydrogen peroxide:
H2O2 + Nb − OH = HO2− + Nb − OH2+
(2)
Once formed, the hydroxyperoxy radicals may interact with hydrogen peroxide hydroxyl with
the generation of superoxide radicals:
H2O2 + HO2− = •OH + O2−• + H2O
(3)
The hydroxyl radicals were identified as active species in the formation of cyclohexenediol [47],
while the concurrently produced superoxide radicals were considered to be trapped by the amorphous
niobium oxide. The reaction equilibrium is in this way efficiently shifted towards the
•
OH
radical formation:
O2−• + Nb(V) = Nb(V)O2−•
(4)

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Similar species can also be formed under the CWO conditions used in this study.
Therefore, a two-step mechanism for the production of succinic acid can be proposed, with the
synergetic participation of two kinds of active sites: (i) the extra framework AlOx(OH) species which
dehydrate the glucose to levulinic acid, followed by; (ii) its subsequent oxidation, onto the nano-oxide
Nb2O5 particles, located in the zeolite channels (as extra framework species and/or partly framework
penta-coordinated species), in accordance to the above mechanisms (Scheme 1).
Scheme 1. The synthesis of succinic acid (SA) from glucose in two steps.
These catalysts demonstrated stability under the harsh reaction conditions and can be recycled three
times without noticeable variation of the glucose conversion and selectivity to succinic acid (Table 4).
Table 4. Activity and selectivity of Nb(0.05)-β18 catalyst in recycling tests.
Entry
Catalyst
X, %
SSA, %
1
2 *
3
4
5
Fresh catalyst (1st cycle)
After 2 h from the catalyst separation
86.4
86.5
86.0
86.2
85.8
26.1
26.2
26.0
26.1
25.4
2nd cycle
3rd cycle
4th cycle
Reaction conditions: 90 mg glucose; 50 mg catalyst; 10 mL water; 14 bar O2, 180 ◦C, 6 h; * —the catalyst from entry 1
was separated and the reaction was conducted 3 h longer.
This remarkable stability in water can be due to the presence of the extra-framework aluminum
species. When located on the external surface, they prevent the solubilization of the zeolite framework,
as recently reported by Sels and co-workers [48].
3. Materials and Methods
3.1. Catalysts Preparation
Table 5 lists the Beta zeolites subjected to the postsynthetic modification.
Table 5. Commercial Beta zeolites used for the postsynthetic modification.
The Type of the Zeolite Samples
Cation
Si/Al Ratio
The Notation (Symbols)
Beta
Beta
Beta
Beta
Na+
6
12
18
Na+-β6
+
NH4+
NH4+-β12
NH4+-β18
NH4+-β37
NH4+
NH4
37.5

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3.1.1. Conventional Activation Protocol: Na+/H+ Ion-Exchange
+
Except for
The ion-exchange protocol was applied only for Na+-
Na+-
6 zeolite in water, 100 mL of NH4NO3 0.5 M solution was added. The mixture was stirred
β6 zeolite, all other zeolite materials were purchased in the NH4 form.
β6 zeolite, as follows: to a slurry of 1 g of
β
for 12 h at room temperature. After this time, the solid was separated by filtration, washed with
deionized water, dried and subjected to a novel treatment with 100 mL of NH4NO3 0.5 M solution.
+
The treatment was repeated four times, yielding a NH4
-
β
6 zeolite. T◦he final solid was separated by
filtration, washed with deionized water, and dried under vacuum at 80 C, for 6 h. Finally, the obtained
+
NH4
-
β
6 zeolite was calcined in an oven at 450 ◦C, static air atmosphere, for 10 h, when NH4+ was
decomposed to NH3(g) and H+, generating the H+-
β6 zeolite. The calcination temperature was
raised with a temperature ramp of 2 ◦C/min. The same calcination protocol was also applied to all
zeolites [49]. The obtained samples were denoted as H-β6, H-β12, H-β18 and H-β37.
3.1.2. Synthesis of Nb-β Zeolites through the Dealumination of β Zeolites and Nb Insertion
Nb-β zeolites (1.6 wt % Nb, 0.02 mo %) were prepared through the two-step post-synthesis
method [36]. To obtain a sample with 1.6 wt % Nb, the siliceous (dealuminated)
β zeolites were
+
prepared by treating the NH4
-
β
zeolite materials (Si/Al = 6, 12, 18 and 37.5) with nitric acid as
+
follows: 2.3 g NH4
-β
zeolite was poured in a solution of HNO3 0.25 N. Then, the mixture was stirred
+
at 80 ◦C, for 4 h. In parallel, part of NH4
-
β
zeolites were treated with oxalic acid 0.1 N using 2.5 g
of solid and 50 mL oxalic acid solution. The suspension solution was kept at room temperature,
for 8 h [34]. After separation the solid samples were washed with deionized water and dried in an
oven at 120 ◦C overnight. The obtained samples were denoted De-Al-β.
Then, 2 g of the De-Al-β sample were poured in 100 mL of isopropanol solution containing
3.5
×
10−3 mol L−1 niobium ethoxide (Nb(OC2H5)5) solution and stirred for 3 h, at 80 ◦C, in an inert
◦
atmosphere. Then, the mixture (pH = 5.6) was stirred at 80 C, for 1 h, in air, until there was a complete
removal of isopropanol. The resulted solid was then washed three times with distilled water, dried in
air, at 80 ◦C, for 24 h, and calcined at 450 ◦C in a static air atmosphere for 10 h [36].
In parallel, similar samples with a final concentration of 0.05 mo % Nb were prepared through
similar procedures but adjusting the amounts of raw materials. Table 6 lists all the prepared
catalytic samples.
Table 6. The prepared catalytic samples.
Zeolite Type
Si/Al Ratio
Dealumination Treatment Acid
Nb Content, mo %
Denoted Nb-Based Materials
Beta
Beta
Beta
Beta
Beta
Beta
12
12
18
18
18
Nitric
Nitric
Nitric
Nitric
Oxalic
Oxalic
0.02
0.05
0.02
0.05
0.05
0.05
Nb(0.02)-β12
Nb(0.05)-β12
Nb(0.02)-β18
Nb(0.05)-β18
Nb(0.05)-β18O
Nb(0.05)-β37O
37.5
3.2. Characterization Methods
The obtained solid catalysts were characterized using powder X-ray diffraction (XRD), diffuse
reflectance infrared Fourier transform spectroscopy (DRIFT) and ICP-OES.
Powder X-ray diffraction (XRD) patterns were recorded using a Schimadzu XRD-7000
diffractometer with Cu Kα radiation (
speed of 2 degrees min−1 in the 5–90 degrees 2θ range.
IR diffuse reflectance spectroscopy with Fourier transform (DRIFT) spectra were recorded with
a Thermo spectrometer 4700 (400 scans with a resolution of 4 cm−1) in the range of 600–4000 cm−1
λ = 1.5418 Å, 40 kV, 40 mA) at a step of 0.2θ and a scanning
.

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The contents of niobium and aluminum in the liquid phases were determined by ICP-OES
(Agilent Technologies, Santa Clara, CA, USA, 700 Series) after calibrating the instrument with
standard solutions.
3.3. Catalytic Tests
The activity tests in batch mode were carried out by adding 50 mg of Nb-based zeolite catalyst to
a solution of 90 mg (0.5 mmol) glucose in 10 mL of water. After closing, the reactor was pressured at
10–18 bars with molecular oxygen and heated up to 150–180 ◦C, under stirring (1200 rpm), for 2–24 h.
After reaction, the oxygen was released and the catalyst was recovered by centrifugation and the
products separated by distillation under vacuum. The recovered products were silylated with 60
µL
pyridine, 60 L BSTFA (N,O-bis(trimethylsilyl) trifluoroacetamide) and TMCS (trimethylchlorosilane)
µ
silane agent, diluted with 1 mL of toluene and analyzed by gas chromatography with flame-ionisation
detection (GC-FID) (GC-Shimadzu apparatus, Kyoto, Japan) equipped with a CPSIL8 (commercial
name of a GC column with a stationary phase—5% phenyl, 95% dimethylpolysiloxane) column
◦
(0.53 mm
×
5
µm
×
25 m) in following conditions: detector temperature
−
270 C; column temperature
◦
◦
◦
increased from 45 C to 250 C with a ramp rate of 10 C/min. All samples were analyzed in triplicates.
The identification of the products was made by using a GC-MS Carlo Erba Instruments (Waltham,
MA, USA) QMD 1000 equipped with a Factor Four VF-5HT column (0.32 mm × 0.1 µm × 15 m).
The glucose conversion (X) and selectivities (S) to the reaction products were calculated from
GC-FID chromatographic analysis by using the follow equations:
ni − n t
X =
× 100
(5)
ni
where ni—initial moles of glucose used in reaction; nt—moles of untransformed glucose at time “t”,
determined from gas chromatography (GC) analysis:
Yieldi
X
Si
=
× 100
(6)
The recovered catalyst was dispersed in distilled water and centrifuged three times, then dried at
ambient temperature and used in a consecutive reaction of glucose.
4. Conclusions
The insertion of Nb into the Beta zeolite framework is not replacing the tetrahedral positions
occupied by aluminum, but following a post synthesis methodology it is possible to afford a very high
dispersion of linked Nb(V) species. In accordance to the characterization results, the most probable
state of Nb(V) corresponds to Nb(V)O-H where niobium is linked by Nb-OSi bonds to the zeolitic walls.
In the preparation process, the tetrahedrally coordinated Al is extracted from the zeolite framework
and deposited on the zeolite surface mostly as Al-O extra-framework species. These residual
extra-framework aluminum species exhibit a stronger acidity and also prevent the solubilization
of the zeolite framework in hot water. Accordingly, bi-functional materials were produced after the
post-synthetic insertion of Nb in the zeolite matrix. These new catalysts showed a high efficiency for the
one-pot oxidation of glucose to succinic acid. Most probably, the extra-framework aluminum species
dehydrate glucose to levulinic acid (LA) which, in turn, is efficiently oxidized to succinic acid (SA) by
the Nb species. The novel catalysts can serve as an attractive alternative to the previously developed
catalysts based on the Ru critical element. In the presence of Nb(0.05)-β37.5 catalyst, succinic acid is
produced in water with a selectivity of 84%, for a total conversion of glucose, after 12 h, at 180 ◦C and
18 bar of oxygen.

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Acknowledgments: Simona M. Coman kindly acknowledges UEFISCDI for financial support (project
PN-III-P4-ID-PCE-2016-0533, Nr. 116/2017).
Author Contributions: Magdi El Fergani synthesized the solid catalysts and performed the catalytic experiments,
Natalia Candu characterized the catalysts and analyzed the data, Simona M. Coman and Vasile I. Parvulescu
conceived and designed the experiments and also prepared the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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(CC BY) license (http://creativecommons.org/licenses/by/4.0/).