β-pinene oxidation by hydrogen peroxide catalyzed by modified niobium-MCM

Article abstract of DOI:10.1016/j.apcata.2012.01.032

Although solid niobium has a high surface area, its high Lewis and Br?nsted acidity is a drawback that always compromises its catalytic performance in oxidation reactions. In this work, treating the niobium catalyst with hydrogen peroxide circumvents this disadvantage and results in a significant increase in the selectivity of the oxidation of β-pinene. In addition, the efficiency of the niobium catalysts supported on MCM was investigated. Nb-MCM//H₂O₂ and Nb-MCM were the most active catalysts. Good selectivity of up to 85% at a 93-97% substrate conversion has been achieved. The catalyst can easily be recovered and reused several times without loss in activity.

Full text of DOI:10.1016/j.apcata.2012.01.032

Applied Catalysis A: General 419–420 (2012) 215–220
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
␤-pinene oxidation by hydrogen peroxide catalyzed by modified niobium-MCM
Jakelyne V. Coelhoa, Luiz C.A. Oliveiraa,∗, Flavia C.C. Mouraa, Patterson P. de Souzab,
Cesar Augusto Silvac, Kenia Barros Batistac, Márcio José da Silvac
a Department of Chemistry, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais State, zip code: 31270-901, Brazil
b Department of Chemistry, Federal Center of Technological Education of Minas Gerais, Belo Horizonte, Minas Gerais State, zip code: 30.421-169, Brazil
c Department of Chemistry, Federal University of Vic¸ osa, Vic¸ osa, Minas Gerais State, zip code: 36570-000, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Although solid niobium has a high surface area, its high Lewis and Brønsted acidity is a drawback that
always compromises its catalytic performance in oxidation reactions. In this work, treating the niobium
catalyst with hydrogen peroxide circumvents this disadvantage and results in a significant increase in
the selectivity of the oxidation of ␤-pinene. In addition, the efficiency of the niobium catalysts supported
on MCM was investigated. Nb-MCM//H2O2 and Nb-MCM were the most active catalysts. Good selectivity
of up to 85% at a 93–97% substrate conversion has been achieved. The catalyst can easily be recovered
and reused several times without loss in activity.
Received 21 September 2011
Received in revised form 16 January 2012
Accepted 28 January 2012
Available online 5 February 2012
Keywords:
Niobium
MCM
© 2012 Elsevier B.V. All rights reserved.
␤-pinene
Hydrogen peroxide
1. Introduction
containing niobium and niobium oxide supported on other oxides
also show this effect. Nb-zeolite, mesoporous Nb-MCM-41 and Nb-
The oxidation of monoterpenes is an attractive route for the val-
orization of these inexpensive and renewable natural olefins [1].
Oxygenated terpenes serve as raw materials for pharmaceutical
industries [2], fragrances [3] and fine chemistry and are used in
organic synthesis as chiral building blocks [4]. However, in olefins,
oxidation reactions generally use toxic and expensive metallic
oxidants required in stoichiometric quantities [5]. Thus, the devel-
opment of processes based on a clean oxidant such as hydrogen
peroxide is environmentally and economically welcomed [6–8].
Heterogeneous catalysts with a high surface area can be an attrac-
tive option. Mesoporous materials, such as MCM-41, are inorganic
layer compounds have redox properties, acidic properties and/or
photosensitivities that give them interesting catalytic behaviors
[11–13]. Over the past several years, increasing interest has been
shown with regard to niobium-containing mesoporous molecular
sieves. Niobosilicate mesoporous molecular sieves were first syn-
thesized in 1997 [14–16]. The niobium species in this material can
play various functions in heterogeneous catalysis and can exhibit
a very high activity in the oxidation of organic compounds with
hydrogen peroxide [13,14]. Our group proposed a modification in
the Nb-MCM material by H2O2-treatment as a means to increase
the number of oxidizer groups on the surface through the formation
of the peroxo species [12,17].
˚
solid materials that have uniform pores ranging from 20–500 A.
They are synthesized in the presence of a surfactant, such as
cetyltrimethylammonium halide, that arranges itself into cylindri-
cal micelles with the polar groups pointing out towards the surface
of the cylindrical arrangement [9]. The main problems in using
MCM-41 for catalysis are its poor acidity and low thermal stability.
However, silicon substitution for metal could make the material a
very active catalyst for some reactions [10]. Niobium compounds
are important catalysts for various reactions. Niobium oxides sig-
nificantly enhance the catalytic activity and prolong the catalyst life
when small amounts are added to known catalysts. Mixed oxides
Following up on our previous study, we report in this paper, for
the first time, the results obtained for the oxidation of ␤-pinene by
H2O2 in CH3CN solutions catalyzed by niobium-containing MCM-
41. After treatment with hydrogen peroxide, we assessed the
catalytic performance of niobium oxide alone and niobium oxide
supported by MCM in ␤-pinene oxidation reactions.
2. Experimental
2.1. Chemicals
All chemicals were purchased from commercial sources. ␤-
pinene (99% w/w) was purchased from Sigma–Aldrich. The
ammonium niobium oxalate NH4[NbO(C2O4)2(H2O)](H2O)3 was
∗
Corresponding author. Tel.: +55 31 3409 6384; fax: +55 31 3409 5700.
E-mail address: luizoliveira@qui.ufmg.br (L.C.A. Oliveira).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2012.01.032

216
J.V. Coelho et al. / Applied Catalysis A: General 419–420 (2012) 215–220
2.3. Catalytic tests
The reactions were carried out in a stirred 50 mL three-necked
glass reactor equipped with a reflux condenser under atmospheric
pressure. Typically, the catalyst (25.0 mg) and the required amount
of aqueous H2O2 were dissolved in CH3CN (15.0 mL), stirred and
then heated to 60 ◦C. Finally, ␤-pinene (3.75 mmol) was added to
the solution and the reactions proceeded for 8 h. Dodecane was
used as an internal standard (1.0 mmol). Blank reactions that were
catalyst-free were carried out under the same conditions.
2.4. GC and GC–MS analysis
The reactions were monitored by GC analysis in a Varian 450
chromatograph equipped with a FID and fitted with a Carbowax
20 M capillary column (30 m length, 0.25 mm i.d., 0.25 mm film
thickness). The gas chromatography conditions were 80 ◦C for
3 min, followed by heating rate at 10 ◦C/min until the final temper-
ature of 250 ◦C was reached and maintained for 2 min. The injector
temperature was 250 ◦C, and the detector temperature was 280 ◦C.
The reaction products were quantified from the corresponding
chromatographic peak areas compared to the calibration curves,
which were built with (−) myrtenal (≥98.5% w/w, Fluka) and (−)
myrtenol (≥98.5% w/w, Fluka) as the GC standards.
Fig. 1. Synthesis of the Nb-MCM//H2O2 catalyst.
The reaction products were identified by GC/MS analysis using a
Shimadzu MS-QP 5050A mass spectrometer operating in the elec-
tronic impact mode at 70 eV and coupled with a Shimadzu 17A GC
and comparison to authentic samples.
provided by CBMM. Acetonitrile (Merck, HPLC grade) was used
as received without prior treatment. The 34% w/w aqueous H2O2
(Vetec, Brazil) was the oxidant employed in all reactions and had
its concentration determined by titration against a KMnO4 solution
[18].
3. Results and discussion
3.1. Characterization of the catalysts
2.2. Synthesis and characterization
To investigate the morphology of the materials, scanning elec-
tron microscopy was carried out before and after the incorporation
of niobium into the MCM structure (Fig. 2). The micrographs of the
materials displayed similar morphologies before and after modifi-
cation with niobium and the H2O2 treatment. In addition, the EDS
analysis showed the presence of niobium in the material after the
treatment with hydrogen peroxide (Nb-MCM//H2O2), showing that
there was no leaching of niobium.
Fig. 3 displays the Raman spectra of MCM catalysts (Fig. 3a)
and after magnification of band around 994 cm−1 in the catalysts
(Fig. 3b). In the modified niobia, spectra bands are observed at 994,
670 and 549 cm−1, which can be assigned to stretching modes of
metal coordinated to peroxo ligands [20]. In niobia without treat-
ment these spectra bands are not observed. The band observed at
990 cm−1 is attributed to peroxo stretching [ꢀ(O O)] and theoreti-
cal calculations [21] indicated that different stoichiometries in the
peroxo complex present different modes for ꢀ(O O). In tetra and
triperoxo species, three modes for ꢀ(O O) are expected, although
in di (cis and trans) and monoperoxo complexes two and one modes
are predicted, respectively. In Fig. 3b a symmetric band at 994 cm−1
is observed, suggesting the presence of diperoxo species [22].
The determination of peroxo group amounts was carried out by
titration of the H2O2 excess during the catalyst preparation with
sodium vanadate (NaVO3; 0.01 mol/L). The results show that there
was a formation of approximately 1.80 × 1021 peroxo sites/g of the
catalyst during the treatment of Nb-MCM material to obtain the
Nb-MCM//H2O2 catalyst.
The Nb-containing MCM was prepared as described in literature
[19]. In the synthesis of Nb-MCM-41, 1.63 g of cetyltrimethylam-
monium bromide (4.47 mmol; Vetec) was dissolved in a mixture
of 10.0 ml of sodium hydroxide (1 mol/L; Aldrich), 100.0 ml of
water and 0.0059 g of ammonium niobium oxalate (18.40 mmol).
Next, 3.70 mL of tetraethyl orthosilicate (17.00 mmol; Aldrich)
was added dropwise. The mixtures were stirred at 25 ◦C for
2 h, and the pH was maintained at 11 through the addition of
sodium hydroxide. The solids obtained were filtered, washed with
water, dried and calcined at 600 ◦C for 4 h under a flow of air
(100 mL min−1).
The Nb-doped MCM was submitted to hydrogen peroxide treat-
ment in order to generate more active surface sites according
to the methods reported in recent works [11]. This treat-
ment was prepared by adding 0.30 g of Nb-MCM, 4 mL of H2O2
(34% w/w) and 80 mL of water and mixing them for 30 min.
The material was vacuum-filtered, washed and oven-dried for
12 h at 90 ◦C. A simplified scheme of the MCM modification
by doping with Nb and treating with H2O2 is illustrated in
Fig. 1.
The Nb-MCM and Nb-MCM//H2O2 catalysts were characterized
by nitrogen adsorption at 77 K (−196 ◦C) and CO2 adsorption at
273 K (0 ◦C), in a Coulter Omnisorb 610 system and in an AUTOSORB
1-Quantachrome system, respectively. The surface area was calcu-
lated using the BET model; the total pore volume was estimated
from the amount of nitrogen adsorbed at P/P0 = 0.95; the micropore
volume was derived from the DR plot; and the pore size distribu-
tion (PSD) was calculated based on the density functional theory
(DFT). Energy dispersive X-ray data were collected using EDS/INCA
350 equipment.
To confirm this data we performed the titration of acid groups
before (Nb-MCM) and after (Nb-MCM//H2O2) modification of
Nb-doped catalyst. Since the peroxo groups replace the acid
group on the surface [23] it is possible obtain the peroxo group
amount considering the decreasing in the acid site after the H2O2

J.V. Coelho et al. / Applied Catalysis A: General 419–420 (2012) 215–220
217
Fig. 2. SEM Micrographs of the (a) MCM support, (b) Nb-MCM, (c) Nb-MCM//H2O2 catalysts, and (d) EDS analysis obtained for the Nb-MCM//H2O2 catalyst after MCM
modification with niobium and H2O2 treatment.
i.e., 1.25 × 1021 peroxo sites/g, which is according to data from titra-
treatment to obtain the Nb-MCM//H2O2 catalyst. The titration of
NaOH (0.0097 mol/L) excess with HCl (0.0084 mol/L) showed that
the acid sites amount was 6.72 × 1021 and 5.47 × 1021 H+ sites/g
for Nb-MCM and Nb-MCM//H2O2, respectively. The peroxo group
amounts can be obtained by the difference between these values,
tion with NaVO3.
Fig. 4 shows the textural properties of the materials. A typical
N2-adsorption desorption for MCM-41 [24] was obtained show-
ing a specific surface area of 1406 m2 g−1 (Fig. 4a). These types of
Fig. 3. Raman spectra of MCM catalysts (a) and magnification of band around 994 cm−1 in the catalyst (b).

218
J.V. Coelho et al. / Applied Catalysis A: General 419–420 (2012) 215–220
a
40
100
80
60
40
20
0
500
400
300
200
100
0
MCM-41
35
30
25
20
15
10
5
Nb-MCM//H2O2
Nb-MCM
Pinocarvone
Pinocarveol
Myrtenol
Myrtenoic acid
Conversion
0
0
2
4
6
8
0.0
0.1
0.2
0.3
0.4
0.5
P/P
0.6
0.7
0.8
0.9
1.0
Time/ h
0
Fig. 6. The selectivity for the various oxidation products from the reactions of ␤-
pinene with H2O2 in CH3CN solution catalyzed by Nb-MCM//H2O2.
b
Nb-MCM
MCM-41
Nb-MCM//H2O2
3.0
2.5
2.0
1.5
1.0
0.5
0.0
␤-pinene oxidation in presence of recyclable, heterogeneous cata-
lysts.
Expectedly, the allylic oxidation appeared to be the main route
for the ␤-pinene oxidation. The main oxidation products identi-
fied were pinocarvone (1a), pinocarveol (1b), myrtenol (1c) and
myrtenoic acid (1d) (Fig. 5) [26]. The epoxy-␤-pinene was detected
in all catalyzed reactions but in lower amounts.
Table 1 shows the results obtained from the ␤-pinene oxidation
catalyzed by niobium oxide before and after it was treated with
hydrogen peroxide. In absence of any catalyst, a low conversion
into ␤-pinene epoxide was observed (Table 1). In the presence of
the untreated niobium oxide, the high Brønsted acidity provokes
a skeletal rearrangement allowing for isomerization. The isomers
formed were limonene, ␥-terpinene and ␥-terpinolene. However, a
very poor conversion into the oxidation products, 29%, was reached
(Table 1).
After introducing the peroxo group into the niobium catalyst
and lowering its acidity, a noticeable improvement in the oxidation
selectivity was achieved. This improvement resulted in a drastic
reduction of undesirable reactions, including isomerization and
skeletal rearrangements, due to the increase in oxidation selectivity
(ca. 71%). The reactions carried out in the absence of H2O2 reagent
displayed a very poor conversion into the oxidation products was
obtained (Table 1).
The change in reaction selectivity over time for the Nb-
MCM//H2O2-catalyzed oxidation reactions of ␤-pinene is displayed
in Fig. 6. The allylic oxidation products in the endocyclic posi-
tions, pinocarvone and pinocarveol, were preferentially formed at
the beginning of the reaction. This result suggests that the forma-
tion of the hydroperoxo radical, which is probably the intermediate
favored in these reactions at the allylic position. High conversions
were reached after 8 h of reaction (Fig. 6). The amount of reaction
conversion increased steadily, reached the maximum values (ca.
10
20
30
40
50
60
70
80
90
100
Pore diameter/Å
Fig. 4. Adsorption/desorption of nitrogen at 77 K of the catalysts.
isotherms are usually classified as a mixture of type IV and type I
according to IUPAC classification. According to Biz and Occelli [25],
the decrease in P/P0 where the inflection point is found is directly
proportional to pore narrowing. The Nb-containing MCM showed
a strong change in textural properties and a decrease in the surface
areas, 267 m2 g−1 for Nb-MCM and 204 m2 g−1 for Nb-MCM//H2O2.
The strong decreases in the specific surface area and pore volume
(Fig. 4b) were likely due to the differences between the ionic radii of
Nb5+ (64 pm) and Si4+ (40 pm). Furthermore, the large difference in
surface areas between MCM-41 and Nb-MCM can be due to poorer
structural integrity of Nb-MCM after niobium incorporation.
3.2. The niobium-catalyzed oxidation of ˇ-pinene by hydrogen
peroxide
Recently, an efficient method for ␤-pinene oxidation that
was based on the homogeneous catalyst PdCl2/H2O2/CH3CN was
described by our group [26]. Herein, we have performed the
O
OH
OH
O
OH
Nb-MCM//H2O2
H2O2 / 60 ºC
+
+
+
(1)
(1a)
(1b)
(1c)
(1d)
Fig. 5. ␤-pinene oxidation products obtained in reactions catalyzed by Nb-MCM//H2O2 or Nb-H2O2 (niobium oxide treated with hydrogen peroxide).

J.V. Coelho et al. / Applied Catalysis A: General 419–420 (2012) 215–220
219
Table 1
Effects of the niobium catalyst on the conversion and selectivity of the ␤-pinene oxidation by H2O2 in CH3CN solution.a
Exp.
Catalyst (25 mg)
Conversionc (%)
Products distributionc (%)
Others productsd
Oxidation selectivityc (%)
Isomerse
Oxidation products
1a
1b
1c
1d
1b
2
–
Nb
Nb-H2O2
Nb-H2O2
<5
90
97
<5
0
20
24
15
5
51
5
95
29
71
80
–
31
32
22
–
30
32
25
–
17
15
20
–
22
21
33
3
4f
5
a
Reaction conditions: ␤-pinene (3.75 mmol), H2O2 (15.9 mmol); 60 ◦C, 8 h reaction.
␤-pinene-epoxide was the main oxidation product.
Determined by GC analysis.
Complex mixture of minority products: water addition products and non-identified products.
The main isomers formed were limonene, ␥-terpinene and ␥-terpinolene.
Reaction was carried out in the absence of H2O2 in the solution.
b
c
d
e
f
Table 2
Effects of the Nb-MCM//H2O2 catalyst on the conversion and selectivity of the ␤-pinene oxidation by H2O2 in CH3CN solution.a
Exp.
Catalyst (25 mg)
Conversionb (%)
Products distribution (%)
Oxidation selectivity (%)c
Other productsc
Isomersd
Oxidation products
1a
1b
1c
1d
1
2
MCM
Nb-MCM
Nb-MCM//H2O2
Nb-MCM//H2O2
70
96
93
<5
37
28
26
29
39
28
12
10
24
44
62
61
27
21
24
21
24
27
24
25
18
20
18
15
31
32
34
39
3
4e
a
Reaction conditions: ␤-pinene (3.75 mmol), H2O2 (15.9 mmol); 60 ◦C; 8 h reaction.
Determined by GC analysis.
Complex mixture of minor products.
The main isomers formed were limonene, ␥-terpinene and ␥-terpinolene.
Reaction was carried out in the absence of H2O2 in the solution.
b
c
d
e
90%) after a reaction time of 6 h and remained at the same level.
The selectivities displayed a similar pattern.
note that even though the niobium catalyst had been previously
treated with hydrogen peroxide, a minimum conversion into the
oxidation products was obtained when the reactions were done
without the addition of this oxidant (Table 2). Based on the num-
ber of peroxo sites and considering the ␤-pinene conversion to
oxidation products turnover frequency (defined as TOF = (moles
of ␤-pinene converted to oxidation products)/[(moles of peroxo
sites) × time of reaction]) values of 0.90 × 10−3, 1.66 × 10−3 and
2.33 × 10−3 s−1 were obtained for the reaction with MCM, Nb-MCM,
MCM-Nb//H2O2 catalysts, respectively.
Fig. 6 shows the ␤-pinene oxidation reaction selectivity when
Nb-MCM//H2O2 was used as the catalyst used. Different from
what was previously observed, myrtenoic acid is the major prod-
uct formed in these reactions. Similar to the results described for
the Nb-H2O2-catalyzed reactions, a high conversion was achieved
after six hours in presence of the Nb-MCM//H2O2 catalyst. These
results were probably due to the presence of the oxidizing groups
generated on the Nb-MCM//H2O2 by pretreatment with hydrogen
peroxide, replacing some of the acidic groups on the surface with
peroxo groups [13].
Although a good conversion was achieved in presence of only the
MCM support (ca. 70%), only a low conversion into the oxidation
products was reached (Table 2). For these reactions, the formation
of isomers was predominant, probably due to both the Lewis and
Brønsted acidity of the MCM support. We verified that ␤-pinene
can be efficiently oxidized by H2O2 using both niobium catalysts,
Nb-MCM or Nb-MCM//H2O2. High conversions were reached, and
a significant improvement in the selectivity for the oxidation prod-
ucts was achieved. The incorporation of the niobium into the MCM
resulted in a significant increase in the conversion to the oxida-
tion products. The reaction selectivity was slightly altered, and an
increase in the formation of myrtenoic acid was observed (Fig. 7
and Table 2). Moreover, the undesirable competitive isomeriza-
tion reactions were drastically reduced (Table 2). It is important
40
100
35
3.3. The efficiency of the Nb catalysts after successive
recovery/reutilization
30
25
20
15
10
5
80
60
40
20
0
Both of the Nb catalysts, Nb-H2O2 and Nb-MCM//H2O2, were
recovered at the end of the reaction process and repeatedly
utilized to catalyze the oxidation ␤-pinene oxidation reaction
(Figs. 8 and 9).
Very high Nb recovery yields were achieved by extracting
it from the reaction medium through decantation and simple
filtration of the solid catalyst. Remarkably, the Nb-MCM//H2O2
catalytic activity remained almost unaltered even after four
recovery/reutilization cycles. Although high recovery yields were
obtained for Nb-H2O2, a significant decrease in the efficiency was
observed (Fig. 9). This noticeable trend for a loss in the catalytic
activity was verified and was likely due to the deactivation peroxo
groups on the catalyst surfaces.
Pinocarvone
Pinocarveol
Myrtenol
Myrtenoic acid
Total conversion
0
0
2
4
6
8
Time/ h
Fig. 7. Oxidation product selectivity of the Nb-MCM//H2O2-catalyzed reactions of
␤-pinene with H2O2 in CH3CN solution.

220
J.V. Coelho et al. / Applied Catalysis A: General 419–420 (2012) 215–220
In this work, a significant advance was achieved for the
Recovery yield
Conversion
development methods free of chloride ions and metal oxidants.
␤-pinene was efficiently converted into the allylic oxidation
products by H2O2 in all reactions. Furthermore, it was also demon-
strated that the catalytic activity of Nb-MCM//H2O2 remained
unaltered even after several recovery/reutilization cycles. There-
fore, the use of Nb in combination with hydrogen peroxide
represents a significant improvement compared with the tradi-
tional catalysts used in oxidation reactions, such as Pd and Au.
A catalyst, like the one reported here, can quickly become a
viable alternative to produce oxygenated products from natural
olefins at lower costs and under environmentally friendly reaction
conditions.
100
80
60
40
20
0
Acknowledgment
1
2
3
4
The authors are grateful for the financial support from CAPES,
CNPq, FAPEMIG, CEFET-BH and FUNARBE (Brazil).
Reuses
Fig. 8. Recovery yield and conversion percentages obtained after successive reuses
of Nb-MCM//H2O2 catalyst in the ␤-pinene oxidation by H2O2 in CH3CN solution.
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4. Conclusions
The innovative aspect of this work is that the highly active
peroxo species can be formed on solid surfaces for use in hetero-
geneous catalysis. Furthermore, new niobium compound synthesis
routes and modifications with H2O2 that promote an increase in the
catalytic activity of the catalysts are reported.