Vanadyl arsenates as catalysts for selective oxidation of organic sulfides and alkenes

Article abstract of DOI:10.1016/j.molcata.2010.11.031

Two vanadyl arsenates templated with ethylendiamonium (EnVAs) and piperazonium (PipVAs) were evaluated as catalysts for the oxidation of thioethers and alkenes, using H₂O₂ and t-butyl hydroperoxide (TBHP) as oxidants. The intrinsic activity of EnVAs was higher than that of PipVAs for the oxidation of sulfides. Similar results were obtained when using either H₂O₂ or TBHP as oxidants. However, the sterical effects were enhanced when TBHP was used and higher selectivities towards sulfoxides were achieved with this oxidant. The catalytic activity of the V-based materials in the epoxidation of simple alkenes and allylic alcohols was assessed. Upon reuse, both materials show no significant decrease in their catalytic properties.

Full text of DOI:10.1016/j.molcata.2010.11.031

Journal of Molecular Catalysis A: Chemical 335 (2011) 176–182
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Vanadyl arsenates as catalysts for selective oxidation of organic sulfides and
alkenes
a
a
b
a,∗
Teresa Berrocal , Edurne S. Larrea , Marta Iglesias , Maria I. Arriortua
a
Dpto. de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV/EHU), Apdo. 644, 48080 Bilbao, Spain
Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2010
Received in revised form 5 November 2010
Accepted 26 November 2010
Available online 9 December 2010
Two vanadyl arsenates templated with ethylendiamonium (EnVAs) and piperazonium (PipVAs) were
evaluated as catalysts for the oxidation of thioethers and alkenes, using H2O2 and t-butyl hydroperoxide
(TBHP) as oxidants. The intrinsic activity of EnVAs was higher than that of PipVAs for the oxidation of
sulfides. Similar results were obtained when using either H2O2 or TBHP as oxidants. However, the sterical
effects were enhanced when TBHP was used and higher selectivities towards sulfoxides were achieved
with this oxidant. The catalytic activity of the V-based materials in the epoxidation of simple alkenes and
allylic alcohols was assessed. Upon reuse, both materials show no significant decrease in their catalytic
properties.
Keywords:
Sulfides oxidation
Alkene epoxidation
Vanadyl arsenate
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
The incorporation of transition metals, which are able to
occur in different oxidation states and/or different coordination
numbers within an open-framework structure, would offer the
possibility of combining the size and shape selectivity showed by
open-framework materials with the catalytic, magnetic, and pho-
tochemical properties associated with d-block elements. Some of
the transition metals that have been incorporated into microporous
frameworks through hydrothermal synthesis in the presence of
organic templating agents include Zn [3], Co [4], Fe [5], Mo [6] and
V [7].
Open-framework metal phosphates are of considerable tech-
nological importance as shape-selective catalysts, ion-exchange
materials and molecular sieves [1]. Since the discovery of the micro-
porous crystalline aluminophosphates AlPO -n in 1982 [2], a large
4
number of new metal phosphates with open-framework structures
have been synthesized [3–7]. These structures contain not only
tetrahedrally coordinated atoms but also square pyramidal and
octahedral moieties. Among the vast family of open-framework
metal phosphates, the transition metal phosphates constitute an
important group due to their potential activity as redox catalysts.
Out of all the open-framework compounds known, those based on
the phosphate oxoanion appear to be the predominant class [8–10].
In addition to the use of tetrahedral phosphate groups as build-
ing units, other anionic moieties such as borates [11], arsenates
Vanadium is of great interest because it is one of the few
transition metal cations that can easily adopt different coordina-
tion environments. This multicoordinative property of vanadium
suggests that it is possible to synthesize microporous vana-
dium arsenates with various structures, as demonstrated by the
recent success in hydrothermal assembly of a series of vana-
dium arsenates with open framework structure [20]. Moreover,
vanadium has been tested as a good catalyst in an inorganic
vanadyl phosphate [21]. Likewise, a revision of the literature
reveals that, although the catalytic studies of inorganic vanadyl
phosphates are numerous, there are many few references of tem-
plated phosphates with open framework. Furthermore, as far
as we are concerned, there is not any vanadyl arsenate tem-
plated with an organic molecule with catalytic properties reported,
except for the (C5N H )[(VO) (AsO )(HAsO ) OH]·3H O [22], and
[
12], sulfates [13] and selenites [14] have been used successfully
in the preparation of novel open framework structures, some of
them with catalytic properties. Among these, the arsenates are
interesting because, although arsenic belongs to the same group
as phosphorous, the larger size of the AsO43− anion can give rise
to different structures and/or physical properties. Consequently,
many arsenate containing frameworks with different structure and
compositions, have been prepared and characterized [12,15–20].
2
14
3
4
4
2
2
(
C N H16)0.5[(VO)(HAsO )F] [23] obtained and studied by our
6 2 4
research group.
∗ Corresponding author. Tel.: +34 94 601 59 84; fax: +34 94 601 35 00.
E-mail addresses: maribel.arriortua@ehu.es, teresa.berrocal@ehu.es
M.I. Arriortua).
On the other hand, sulfoxide derivatives are known to have
interesting and useful biological and pharmacodynamic proper-
(
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.11.031

T. Berrocal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 176–182
177
ties [24]. The sulfoxides are obtained by oxidation of thioethers by
peracids, peroxides and alkyl peroxides using transition metal cat-
alysts [25]. Depending on the catalyst selectivity and the method
used, different proportions of sulfoxide and sulfone are produced
they were analyzed by GCMS using a Hewlett-Packard 5890 II.
Since thioethers can be oxidized by 30 wt% H O2 to sulfoxides,
2
blank experiments were carried out under the reaction conditions
in order to determine the extension of the uncatalyzed reaction. At
315 K, using acetonitrile as solvent (10 ml), 1 mmol of the methyl
phenyl sulfide and 3 equiv. of 30% H O , the homogeneous reac-
[
26]. In addition, epoxides, produced by the oxidation of olefins,
are extremely useful building blocks in the synthesis of organic
compounds as they act as excellent intermediates that can yield
a great variety of products [27]. When using vanadyl compounds,
the catalytically active oxo-peroxo intermediate is formed in situ
by oxidation of V(IV) to V(V) with an excess of tert-butyl hydroper-
oxide, yielding a tert-butyl hydroperoxovanadium(V) complex.
In most of the methods described in the literature, pollutants are
generated, because of the use of corrosive acids, toxic or dangerous
substances [25]. For this reason it is important to develop clean
methods which allow obtaining the sulfoxide derivatives and the
epoxides without the generation of these undesired substances. In
this sense, the use of hydrogen peroxide instead of other oxidants
for the oxidation of organic substrates is an interesting alternative
because the unique subproduct of the reaction is water. In addition,
hydrogen peroxide is highly efficient in its oxygen content and is a
cheap reagent.
2
2
tion accounted for a conversion of 5 and 15% after 20 min and 1 h
of reaction time respectively. Under the same conditions but using
1.1 mmol of TBHP instead of H O , the conversion after 5 h was 10%.
2
2
After the end of the catalyzed reactions, the catalysts were filtered
and characterized by X ray diffraction and IR spectroscopy.
3. Results and discussion
3.1. Synthesis
The reagents, V O3 (2.335 and 2.001 mmol for EnVAs and Pip-
2
VAs, respectively) and As O ·3H O (3.690 mmol in both cases)
5
2
2
were solved in a mixture of 20 ml of water and 10 ml of ethanol for
EnVAs and 30 ml of water for PipVAs, then 1 ml of HF (57.5 mmol)
was added in both cases. Finally, the ethylenediamine and piper-
azine molecules were used to fix the pH at, approximately, 3 and 1,
respectively. These reaction mixtures were stirred to assure homo-
geneity, sealed in a PTFE-lined stainless steel pressure vessel (filling
The use of open framework phases templated with amines as
catalysts in these reactions is especially appealing due to their
redox properties, including their acidity [22,23,28,29].
◦
During the course of our research, we have synthesized
two new catalytic compounds, fluoro-vanadyl-hydrogenarsenate
with ethylendiamonium and piperazonium as templating agents,
with formulae (C N H12)0.5[(VO)(HAsO )F] and (C N H14)0.5
factor 60%) and then heated at 170 C for 5 days. After reaction, slow
cooling, at 30 K per hour, until room temperature was carried out.
The pH did not show any appreciable change during the hydrother-
mal treatment. The compounds were obtained as prismatic light
and intense green single-crystals, for EnVAs and PipVAs, respec-
tively. The size of the crystals is 0.1 × 0.08 × 0.04 mm for EnVAs and
0.01 × 0.03 × 0.01 mm for PipVAs (Figure SM.1, in the supplemen-
tary material). The yield was, approximately, 80% in both cases.
2
2
4
4
2
[
(VO)(HAsO )F] [30]. In this work the selective oxidation of differ-
4
ent thioethers and alkenes, using H O2 and t-butyl hydroperoxide
2
(
TBHP) as oxidants have been carried out, with the aim of exploring
the possibilities of these recently synthesized materials.
3.2. Structure
2
. Materials and methods
Single-crystal X-ray diffraction data were collected at room
2
.1. Materials
temperature on an Oxford Diffraction XCALIBUR2 automatic
diffractometer equipped with a CCD detector for EnVAs and on a
STOE IPDS (Imaging Plate Difracction System) automatic diffrac-
tometer for PipVAs. The structures were solved by direct methods
with the SHELXS97 computer program [32]. SHELXL97 [33] was
used to refine the structure by the least-squares method based on
F . The final atomic coordinates and thermal parameters have been
deposited at the Cambridge Crystallographic Data Centre (CCDC
68020 and 668019 for EnVAs and PipVAs, respectively). Crystallo-
(
C N H10) [(VO)(HAsO )F]
(EnVAs)
and
(C N H12)
4 2 0.5
2
2
0.5
4
[
(VO)(HAsO )F] (PipVAs) were synthesized by mild hydrothermal
4
conditions under autogenous pressure. The compounds were
obtained as prismatic light and intense green single-crystals, for
EnVAs and PipVAs, respectively. The chemical composition of both
compounds was calculated by atomic absorption spectroscopy
2
(
AAS) and C, H, N-elemental analysis. The amount of the fluoride
6
anions was calculated using a selective electrode. The densities,
measured by the floatation method [31] in a mixture of bromoform
graphic data, atomic coordinates and selected bond distances and
angles are listed in supplementary material, Tables SM.1, SM.2 and
SM.3, respectively.
− −3
Br CH, ꢀ = 2.82 g cm ) and chloroform (Cl CH, ꢀ = 1.476 g cm ),
3 3
3
(
−
3
are 2.69(2) and 2.70(1) g cm , respectively.
Both phases are layered compounds [30]. The crystal struc-
The characterization of EnVAs and PipVAs is carefully described
in Ref. [30] and also, it is summed up at the supplementary material
of the present article.
ture consists of a two-dimensional inorganic skeleton, with
−
[
(VO)(HAsO )F] composition. The sheets are extended along
4
the [1 0 0] direction (Figure SM.3). The ethylenediammonium
and piperazonium cations are located in the interlayer space
(Figure SM.4). These cations stabilize the crystal structure form-
ing, both, ionic interactions and hydrogen bonds with the inorganic
framework. Although the connectivity in the inorganic skeleton is
the same in both structures, there are differences between them.
Due to the bigger size of the piperazonium cation the holes gener-
ated into the inorganic layers are wider in PipVAs (Figure SM.3). On
2.2. Procedure
The catalytic experiments were carried out in a batch reac-
tor at atmospheric pressure, at 323 K, and using acetonitrile
and dichloromethane as solvent (3 ml). In a typical synthesis,
7
tively were stirred in a suspension containing the solvent and 0.778
and 0.740 mol of the corresponding thioethers [methyl phenyl
sulfide, methyl p-tolyl sulfide, 4-clorothioanisol and 1-ethylbutyl
phenyl sulfide] or olefins [styrene, cyclooctene, linalool and geran-
−
3
−3
.78 × 10
and 7.40 × 10 mmol of EnVAs and PipVAs, respec-
the other hand this different size generates different reticular space
between the inorganic layers, 4.5 A˚ for EnVAs and 3 A˚ for PipVAs.
3.3. Catalytic properties
iol]. The oxidant, either TBHP (1.1 equiv.) or H O2 (3 equiv., 30%)
2
was added dropwise, while the overall suspension was heated at
The results given in Fig. 1(a and b), and in Table 1 show that for
the case of methyl phenyl sulfide, which can penetrate in EnVAs, the
3
23 K. Samples were taken at regular times, and after filtration,

1
78
T. Berrocal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 176–182
a 100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
MeSPh
MeSPhCH3
MeSPhCl
EthylbutylSPh
0
1
2
3
4
5
6
t (h)
b 100
Fig. 2. Selectivity curves towards sulfoxide in the oxidation of alkyl aryl sulfides in
the presence of H2O2 on EnVAs.
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
EnVAs H O2
2
EnVAs t-BuOOH
PipVAs H O2
2
PipVAS t-BuOOH
0
1
2
3
4
5
6
t (h)
Fig. 1. Kinetic profile for the catalytic oxidation (a) of alkyl aryl sulfides in the pres-
ence of H2O2, on EnVAs catalyst; (b) of methyl phenyl sulfide on EnVAs and PipVAs
catalysts with H2O2 and t-BuOOH as oxidants.
Fig. 3. Influence of the oxidant on the oxidation of methyl phenyl sulfide with EnVAS
as catalyst.
activity of EnVAs is higher than the activity of PipVAs, in agreement
with the higher intrinsic activity expected for EnVAs. Moreover,
the selectivity curves, given in Fig. 2, show that the sulfoxide is
a primary and unstable product, while the corresponding sulfone
appears as a secondary and stable product. Selectivity depends on
the oxidant, with H O , EnVAs results also most selective towards
the formation of sulfoxide than PipVAs. This would also be consis-
tent with the presence of stronger oxidation sites on EnVAs, taking
into account that the oxidation of sulfoxide to sulfone is a more
demanding reaction than the oxidation of thioether to sulfoxide.
The substrate has a significant influence on the activity and selec-
tivity. The oxidation of the ethylbutyl phenyl sulfide is much slower
than that of methyl phenyl sulfide as a consequence of the large size
of the reactant. The substituents in the aromatic ring and the more
bulky alkyl group imply a decrease in reactivity accompanied by a
significant increase in selectivity.
The oxidant plays an important role in the activity and selectiv-
ity. When the reactions were carried out using TBHP as oxidizing
agent, analogous results were obtained (Fig. 3), i.e. EnVAs is more
active than PipVAs for methyl phenyl sulfide and less active for
2
2
Table 1
Conversion (CT), selectivity towards the sulfoxide formation (SSO) and turnover frequency (TOF) data of oxidation reactions of alkyl aryl sulfides catalyzed by EnVAs and
◦
PipVAs (all the reactions were carried out between 50 and 60 C and with a 1% of catalysts).
Sulfide
MeSPh
Oxidant
CT (%) (time, h)
SSO (%)
TOF (h−1)
EnVAS
EnVAS
PipVAS
EnVAS
PipVAS
PipVAS
40
37
–
38
27
72
71
–
H2O2
TBHP
PhC(CH3)2-OOH
H2O2
TBHP
H2O2
TBHP
H2O2
99 (2)
100 (4)
48 (24)
100 (4)
–
100 (4)
–
100 (7)
96 (5)
100(5.5)
–
100 (4)
100(6.5)
94 (3)
100 (24)
100 (24)
68
84
92
81
–
10
–
100
28
100
–
53
100
23
50
7
4
68
–
52
–
MeSPhCH3
MeSPhCl
EtBuSPh
28
100
47

T. Berrocal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 176–182
179
Table 2
Conversion (CT), products percentage and selectivity data for the oxidation of olefins catalyzed by EnVAs and PipVAs. All the reactions were carried out on a 1% of catalysts
and at r.t., except for the epoxidation of linalool, carried out above 373 K.
(a) Epoxidation of styrene
Oxidant
EnVAs
PipVAs
CT (%)
Epoxide (%)
11
Other (%)
Selectivity (%)
69
CT (%)
Epoxide (%)
Other (%)
Selectivity (%)
20
TBHP
H2O2
16
77
5
74
25
83
5
0
20
83
3
4
0
(b) Epoxidation of cyclooctene
Oxidant
EnVAs
PipVAs
CT (%)
Epoxide (%)
Other (%)
Selectivity (%)
CT (%)
Epoxide (%)
Other (%)
Selectivity (%)
TBHP
H2O2
4
10
4
10
–
–
100
100
77
25
77
25
–
–
100
100
(c) Epoxidation of linalool
Oxidant
EnVAs
PipVAs
CT (%)
F (%)
P (%)
F/P
CT (%)
F (%)
P (%)
F/P
TBHP
H2O2
100
55
67
40
33
15
2
2
5
28
3
18
2
10
1.5
1.8
(d) Epoxidation of geraniol
Oxidant
EnVAs
PipVAs
CT (%)
Epoxide (2,3) (%)
Epoxide (6,7) (%)
Selectivity (6,7) (%)
CT (%)
Epoxide (2,3) (%)
Epoxide (6,7) (%)
Selectivity (6,7) (%)
TBHP
H2O2
51
51
7
25
44
25
86
50
33
47
9.5
23
24.5
19
72
40
100
ethylbutyl phenyl sulfide oxidation. The activity follows the order:
H O > TBHP > PhC(CH ) OOH (Fig. 3), which is accompanied by an
increase in selectivity (Fig. 4).
9
8
7
6
5
4
0
0
0
0
0
0
2
2
3 2
The results obtained in the epoxidation of different alkenes
(
styrene, cyclooctene, linalool and geraniol) using EnVAs and Pip-
VAs based materials as heterogeneous catalysts and H O and tert-
2
2
butyl hydroperoxide as the oxygen source, in dichloromethane,
are summarised in Table 2. The reaction yields without cata-
lysts run under identical conditions are less than 10%.Oxidation
of styrene, catalyzed by EnVAs and PipVAs using H O2 as an
2
30
oxidant gave styrene oxide, benzaldehyde and 1-phenylethane-
H O
2
t-BuOOH
PhC(CH3)2-OOH
2
1
0
0
0
2
1
,2-diol as main products, when the reactions were carried out
Table 3
Reutilisation data. Conversion (CT) and selectivity towards the sulfoxide formation
0
10
20
30
40
50
60
70
80
90
100
(SSO) results at 2 h of reaction at each cycle (T = 323 K).
Ct (SO+SO2
)
Cycle
Cat/Sust
CT (%)
SSO (%)
EnVAs
99
100
98
PipVAs
EnVAs
PipVAs
Fig. 4. Influence of the oxidant on the selectivity for sulfoxide with EnVAS as cata-
lyst.
1
2
3
1/100
1/100
1/500
83
100
100
68
10
95
41
36
92

1
80
T. Berrocal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 176–182
Fig. 5. X-ray powder diffraction patterns of EnVAs and of PipVAs before (a and c, respectively) and after the catalytic processes (b and d, respectively).
using TBHP as oxidizing agent, analogous results were obtained
Table 2a). The oxidation of cyclooctene by H O2 and TBHP; gave
mainly cyclooctene oxide. PipVAs is the most efficient catalyst
nor PipVAs lose their catalytic efficiency significantly after two
catalytic cycles. In addition to these reutilisation tests, leaching
tests were carried out. At a conversion value of about 15%, in an
oxidation reaction of methyl phenyl sulfide with H O as oxidiz-
(
2
(
Table 2b).
2
2
The title compounds also act as catalyst for the oxidation of
ing agent and EnVAs as catalyst, the solid was separated from
the reaction media by centrifugation. The supernatant was then
allowed to react, and any increase of conversion was observed.
The residue containing the catalyst was washed twice with ace-
tone and diethyl ether (3 mL), and each time, after centrifugation,
the organic phase was removed. Finally, fresh reagents were added
to the remaining solid and the mixture was allowed to react
for, reaching the total conversion after 3 h of reaction. This is an
important result for heterogeneous vanadium-based catalysis and
indicates that no deactivation or leaching occurs at least up to
three cycles. After the end, the catalyst was filtered and char-
acterized by X ray diffraction and IR spectroscopy. The powder
patterns and the infrared spectra did not show any appreciable
linalool (3,7-dimethylocta-1,6-dien-3-ol) at temperatures higher
than 373 K, to give rise to furans and pyrans, with a greater degree
of conversion when increasing the reaction time (Table 2c). In this
case the catalyst acts as a bifunctional system (redox-acid).
The catalysts were also evaluated for the oxidation of geraniol
(E)-3,7-dimethylocta-2,6-dien-1-ol). In this case selective oxida-
tion at 6,7-position was achieved with both systems and TBHP as
oxidant (Table 2d).
Reutilisation is one of the greatest advantages of heteroge-
neous catalysts, and can also provide useful information about
the anchoring process and catalyst stability along the catalytic
cycle. As can be seen in Table 3, it is clear that neither EnVAs
(

T. Berrocal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 176–182
181
(IT-177-07). The authors thank the technicians of SGIker, Drs. J.C.
Raposo, F.J. Sangüesa and A. Larra n˜ aga, financed by the National
Program for the Promotion of Human Resources within the National
Plan of Scientific Research, Development and Innovation, “Minis-
terio de Ciencia y Tecnología” and “Fondo Social Europeo” (FSE),
for the chemical analyses and the X-ray diffraction measurement
respectively. T. Berrocal wishes to thank the Universidad del País
Vasco, UPV/EHU, for funding.
(
(
a)
b)
δ (CH2)
δ(As-O-H)
Appendix A. Supplementary data
ν (V-O)
+
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.11.031.
δ (NH2)
νs(As-O)
+
δas(O-As-O)
ν (NH ) , ν (CH )
2
2
νas(As-O)
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ν(cm )
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(
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[
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The different catalytic behaviours between EnVAs and PipVAs,
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4
. Conclusions
[
These new vanadium based materials show high catalytic activ-
(
b) H. Imai, Y. Kamiya, T. Okuhara, J. Catal. 255 (2008) 213;
ity in the oxidation of organic sulfides and epoxidation of simple
alkenes and allylic alcohols. Material EnVAs proved to be a more
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materials were used three times with no significant loss of activ-
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leaching of the active phase and its deactivation.
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Acknowledgements
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This work has been financially supported by the “Ministerio de
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