Amine templated open-framework vanadium(iii) phosphites with catalytic properties

Article abstract of DOI:10.1039/c2dt32286b

Four novel amine templated open-framework vanadium(iii) phosphites with the formula (C₅N₂H₁₄)_0.5[V(H ₂O)(HPO₃)₂], 1 (C₅N ₂H₁₄ = 2-methylpiperazinium), and (L)_4-x(H ₃O)_x[V₉(H₂O)₆(HPO ₃)_14-y(HPO₄)_y(H₂PO ₃)_3-z(H₂PO₄)_z] ·nH₂O (2, L = cyclopentylammonium, x = 0, y = 3.5, z = 3, n = 0; 3, L = cyclohexylammonium, x = 1, y = 0, z = 0.6, n = 2.33; 4, L = cycloheptylammonium, x = 1, y = 0, z = 0, n = 2.33) were synthesized employing solvothermal reactions and characterized by single-crystal X-ray diffraction, ICP-AES and elemental analyses, thermogravimetric and thermodiffractometric analyses, and IR and UV/vis spectroscopy. Single-crystal data indicate that 1 crystallizes in the triclinic system, space group P1, whereas 2, 3 and 4 crystallize in the hexagonal space group P6₃/m. Compound 1 has a two-dimensional motif with anionic sheets of [V(H₂O)(HPO ₃)₂]^- formula, whose charge is compensated by the 2-methylpiperazinium cations embedded between the layers. In contrast, 2, 3 and 4 present a pillar-layer network giving rise to a three-dimensional framework containing intersecting 16-ring channels with the primary amine templates and the crystallization water molecules enclosed in them. 1, 2, 3 and 4 behave as heterogeneous catalysts for the selective oxidation of alkyl aryl sulfides, with tert-butylhydroperoxide (TBHP) as the oxidizing agent, being active, selective and recyclable for several successive cycles of reaction. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt32286b

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Amine templated open-framework vanadium(III)
phosphites with catalytic properties†
Cite this: Dalton Trans., 2013, 42, 4500
Joseba Orive,a Edurne S. Larrea,a Roberto Fernández de Luis,a Marta Iglesias,b
José L. Mesa,c Teófilo Rojoc and María I. Arriortua*a
Four novel amine templated open-framework vanadium(III) phosphites with the formula (C5N2H14 0.5
)
-
[V(H2O)(HPO3)2], (C5N2H14 2-methylpiperazinium), and (L)4−x(H3O)x[V9(H2O)6(HPO3)14−y(HPO4)y-
1
=
(H2PO3)3−z(H2PO4)z]·nH2O (2, L = cyclopentylammonium, x = 0, y = 3.5, z = 3, n = 0; 3, L = cyclohexylammo-
nium, x = 1, y = 0, z = 0.6, n = 2.33; 4, L = cycloheptylammonium, x = 1, y = 0, z = 0, n = 2.33) were
synthesized employing solvothermal reactions and characterized by single-crystal X-ray diffraction, ICP-AES
and elemental analyses, thermogravimetric and thermodiffractometric analyses, and IR and UV/vis spec-
ˉ
troscopy. Single-crystal data indicate that 1 crystallizes in the triclinic system, space group P1, whereas 2, 3
and 4 crystallize in the hexagonal space group P63/m. Compound 1 has a two-dimensional motif with
anionic sheets of [V(H2O)(HPO3)2]− formula, whose charge is compensated by the 2-methylpiperazinium
cations embedded between the layers. In contrast, 2, 3 and 4 present a pillar-layer network giving rise to a
three-dimensional framework containing intersecting 16-ring channels with the primary amine templates
and the crystallization water molecules enclosed in them. 1, 2, 3 and 4 behave as heterogeneous catalysts
for the selective oxidation of alkyl aryl sulfides, with tert-butylhydroperoxide (TBHP) as the oxidizing agent,
being active, selective and recyclable for several successive cycles of reaction.
Received 28th September 2012,
Accepted 2nd January 2013
DOI: 10.1039/c2dt32286b
www.rsc.org/dalton
phosphates and related compounds with open structures, is
mainly derived from the use of organic compounds that act as
Introduction
The interest in finding suitable catalysts for higher selectivity structure directing agents (SDAs)5 and the substitution of tran-
in chemical reactions has resulted in the ongoing development sition metal ions, which are able to occur in different oxi-
of new synthetic pathways. At first, aluminosilicate zeolites, dation states and/or different coordination numbers.
which show chemical stability and high selectivity for molecu-
In contrast to the four coordinated phosphate oxoanion,
lar shape through their well-defined pores and cages, were the phosphite group has only three oxygen atoms. This feature
extensively studied. Subsequent work has focused on combin- can give rise to new topologies often with lower connectivities.6
ing the well-known catalytic properties of transition elements Since the first organically templated phosphite was syn-
with the shape-selective capacity of zeolites.1
thesized,7 this family of compounds has rapidly grown,
Among microporous solids, open-framework metal phos- highlighting those with a Zn transition metal cation.8 Open--
phates also constitute an important class of materials due to framework phosphites of V,9 Mn,10 Fe,11 Co,12 Ga13 and even
their potential applications in catalysis, gas separation and of Cr,14 Be,15 In16 and U17 have been prepared, but they are
storage.2–4 The wide variety of structural architectures, one-, still less numerous than those reported with Zn.
two- and three-dimensional, shown by the family of
Vanadium is an interesting reactive element that can easily
adopt different coordination environments due to its numer-
ous oxidation states, in minerals 3+, 4+, and 5+. The most
dominant non-metallurgical use of vanadium is in catalysis.18
Synthesis of compounds containing vanadium increased sub-
stantially after the work of Centi et al.19 Nowadays, one of the
most well studied heterogeneous mixed oxide catalyst systems
are vanadium phosphates (V–P–O) which are commercially
used for the selective oxidation of n-butane to maleic anhy-
dride (MA).20–26
aDepartamento de Mineralogía y Petrología and Universidad del País Vasco/Euskal
Herriko Unibertsitatea, UPV/EHU, Apdo. 644, 48080 Bilbao, Spain.
E-mail: maribel.arriortua@ehu.es; Fax: +34-946013500; Tel: +34-946012162
bInstituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de la Cruz 3,
Cantoblanco, 28049 Madrid, Spain
cDepartamento de Química Inorgánica, Facultad de Ciencia y Tecnología,
Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, Apdo. 644,
48080 Bilbao, Spain
†Electronic supplementary information (ESI) available. CCDC 880621, 880622,
880623 and 880624. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt32286b
On the other hand, organic sulfoxides and sulfones are
useful synthetic intermediates for the construction of various
4500 | Dalton Trans., 2013, 42, 4500–4512
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chemically and biologically active molecules.27 The sulfoxides
are obtained by oxidation of thioethers by peracids, peroxides
and alkyl peroxides using transition metal catalysts.28 Depend-
ing on the catalyst selectivity and the reaction conditions, that
is, time, temperature and the relative amount of oxidants,
different proportions of sulfoxide and sulfone are pro-
duced.29,30 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 the oxidizing agent.31 The use
of heterogeneous catalysts in the liquid phase has the inherent
advantages of easier handling, separation, recovery and recy-
cling, and enhanced stability. In addition, from an environ-
mental and economic standpoint, classical stoichiometric
oxidants, such as dichromate or permanganate, should be
replaced by new environmentally friendly catalytic processes
Fig. 1 (a) Polyhedral view of the two-dimensional crystal structure along the
[100] direction. (b) Topological simplification of the inorganic sheets.
using clean oxidants.32 Oxidants such as hydrogen peroxide through oxygen vertices. Each [V(1)O6] octahedron is linked to
and tert-butyl hydroperoxide are ideal, as they are environ- four [HP(1)O3] and two [HP(2)O3] units, while [VO4(H2O)2]
ment-friendly reagents with water and tert-butyl alcohol as the octahedra are exclusively coordinated to [HP(2)O3] units. The
water molecule coordinated to the [VO4(H2O)2] polyhedron
respective by-products.
According to the results of our literature research, catalytic establishes the intralayer H-bonding scheme given in ESI,
studies on phosphite group containing materials are scarce. A Table S2.1 and Fig. S2.2.† The topological analysis of the in-
organic sheets of compound 1 was performed using the TOPOS
purely inorganic layered zirconium phosphite was evaluated as
an acidic catalyst in dehydration of ethanol reactions.33 4.037 program. Considering the two P and two V crystallogra-
Recently, Knoevenagel condensation of benzaldehyde and phically independent atoms as nodes of a simplified network,
a tetranodal net (2-c)2(3-c)2(4-c)(6-c) is obtained. The complex-
ethyl cyanoacetate was carried out with open-framework zinc
phosphites as the base catalytic support.34 As far as we are con- ity of this net is the result of coordination of four types of
cerned, this is the first report of the catalytic properties of structural subunits: hexa-connected [V(1)O6], tetra-connected
[V(2)O4(H2O)], tri-connected [HP(2)O3] and di-connected
vanadium phosphites. There are some studies of related
materials, such as amine-templated vanadyl arsenates35 or [HP(1)O3] (Fig. 1(b)). In order to construct the underlying net of
hybrid vanadates36 as well as of vanadium containing the sheets and to relate this structure to other ones registered
heteropolycompounds.30
in the topological databases, it is necessary to replace 2-con-
In the present work we report on the catalytic properties of nected nodes by edges. After this simplification a trinodal net
four new amine templated open-framework vanadium(III) with known topology type 3,4,4L1338,39 is obtained (ESI,
Fig. S2.3†).
The 2-methylpiperazinium cations are located in the inter-
−y(HPO4)y(H2PO3)3−z(H2PO4)z]·nH2O (2, L = cyclopentylammo- layer space stabilizing the inorganic framework through hydro-
gen bonding interactions (ESI, Table S2.1†) with the three
phosphites, (C5N2H14)0.5[V(H2O)(HPO3)2], 1 (C5N2H14
= 2-
methylpiperazinium), and (L)4−x(H3O)x[V9(H2O)6(HPO3)14
nium, x = 0, y = 3.5, z = 3, n = 0; 3, L = cyclohexylammonium,
x = 1, y = 0, z = 0.6, n = 2.33; 4, L = cycloheptylammonium, x = 1, oxygen atoms belonging to the [HP(1)O3] group (ESI,
y = 0, z = 0, n = 2.33). Moreover, the hydrothermal synthesis, Fig. S2.4†). This organic molecule, which adopts a chair con-
formation, is located over a symmetry centre causing a posi-
tional disorder of the methyl group with an occupation factor
of 0.5. Therefore, the organic molecule looks like a 2,4-
dimethylpiperazine molecule due to the statistical distribution
of the methyl group on two sites. This type of disorder was
observed in the P21/n and C2/c polymorphs of the mono-
dimensional [C5N2H14][Co(HPO4)2]40,41 hydrogen phosphate.
crystal structure determination, as well as the thermal and
spectroscopic behaviour of these phases are described.
Results and discussion
Structures description
The asymmetric unit of (C5N2H14)0.5[V(H2O)(HPO3)2], 1, con-
The asymmetric units of (L)4−x(H3O)x[V9(H2O)6(HPO3)14−y-
tains 15 non-hydrogen atoms, of which two vanadium atoms (HPO4)y(H2PO3)3−z(H2PO4)z]·nH2O (2, L = cyclopentylammo-
nium, x = 0, y = 3.5, z = 3, n = 0; 3, L = cyclohexylammonium,
pying general positions are crystallographically independent x = 1, y = 0, z = 0.6, n = 2.33; 4, L = cycloheptylammonium, x = 1,
situated in inversion centres and two phosphorus atoms occu-
(ESI, Fig. S2.1†).
y = 0, z = 0, n = 2.33) contain three crystallographically inde-
pendent V atoms and four crystallographically distinct P atoms
The two-dimensional crystal structure of compound 1 is
composed of anionic sheets of [V(H2O)(HPO3)2]− formula (ESI, Fig. S2.5†).
placed in the ab plane (Fig. 1(a)).
The structural analyses show that compounds 2, 3 and 4 are
isostructural crystallizing in the hexagonal space group P63/m,
The inorganic sheets consist of octahedral [VO6] and
[VO4(H2O)2] units joined by bridging HPO3 pseudopyramids with very similar cell parameters. These compounds present
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factor of 4/6 balancing the total negative charge of the in-
organic framework but they are disordered in two positions in
order to occupy more interlayer space (ESI, Fig. S2.10†).
The amount of potential-free volume was estimated by the
program PLATON44 assuming that the guest molecules could
be removed from the channels, being 39.8, 45.0 and 46.0% of
the total volume for phases 2, 3 and 4, respectively. The analy-
sis of these voids was performed using the TOPOS 4.0
program37 by means of Voronoi–Dirichlet polyhedra (VDP)45–47
(ESI, Fig. S2.12†). Taking into account the real occupancy
factors of the solvent species, we have estimated the real per-
centages of free voids volumes, obtaining 13.7, 17.8 and 19.2%
for phases 2, 3 and 4, respectively. These data suggest an
increase of free space associated with the increase of the
organic guests size.
Fig. 2 Polyhedral view of the three-dimensional crystal structures of phases 2,
3 and 4 along the a2 axis.
differences in the phosphorous anionic group types and in the
Discussions about the bond distances and angles and poly-
organic templates, as well as in the crystallization water mol- hedral distortion study are included in the ESI (S2(a)†).
ecules content. Their inorganic buildings are isostructural
Thermal study
with the compounds (Hcha)3(H3O)1+x[V9(H2O)6(HPO3)14−x
(BO3)x(H2PO3)3]·2H2O (cha cyclohexylamine and
0.1–1.2)64
and [In9(H2O)6(HPO3)13(H2PO3)2(PO4)(HPO4)]- initial mass losses attributed to the elimination of the water
[(C6N2H18)3] (C6N2H18
2-methyl-1,5-pentanediamine).42
However in the last two compounds, the extra-framework and 285 °C, there are two slopes in the TG curve (6.44%),
-
=
x
=
The thermogravimetric curves of the four phases show small
=
adsorbed by the compounds. For compound 1, between 100
species were not located by single-crystal X-ray diffraction.
Compounds 2, 3 and 4 are based on a pillar-layer network
the first of them very soft and possibly related to the break-
ing of some bonds of water molecules coordinated to the
giving rise to a three dimensional framework containing inter- V(2)O4(H2O)2 polyhedron. And the second one, rather more
secting 16-membered ring channels along the [100], [010] and sharply, from about 210 °C, assigned to the total elimination
[110] directions with the primary amine templates and the
of all coordinated water molecules (6.42% calculated) and
crystallization water molecules enclosed in them (Fig. 2). The coincident with the beginning of the destruction of the crystal
approximate sizes of the channel windows are 13 × 7 Å2 (ESI, structure of the compound observed by thermodiffractometry.
Fig. S2.6†). A detailed structural description of the inorganic
buildings of compounds 2, 3 and 4 is given as ESI (S2†).
In the 330–525 °C temperature range there is a weight loss of
9.35%, which corresponds to a half of the 2-methylpiperazi-
The interlayer spaces of compounds 3 and 4 are compli- nium molecule present in the compound ((C5N2H14)0.25
=
cated by disordered water molecules located in positions
9.125%). The remaining organic matter is lost in two stages
similar to those of the organic cations (ESI, Fig. S2.10†). from 525 °C to 900 °C (ESI, Fig. S3.1†).
Taking into account the general positions for the organic
cations and the O(W1) and O(W2) oxygen atoms, the total
For compounds 2, 3 and 4, the total weight losses observed
in the TG curves are 21.2%, 25.1% and 20.7% respectively (ESI,
occupancy factors for cyclohexylamine or cycloheptylamine + Fig. S3.2†). The decomposition of the phases takes place in
O(W1) + O(W2) must be equal to 1. Based on the elemental several overlapped steps beginning from nearly room tempera-
analyses, for the organic cations an occupancy factor of 0.5
ture, with the removal of the corresponding crystallization and
was estimated. For compound 4, the occupancy factor of the coordinated water molecules. Subsequently, from approxi-
cycloheptylammonium cation is imposed by symmetry. For the mately 100 °C for 3 and 150 °C for 2 and 4 the beginning of
O(W1) and O(W2) occupancy factors calculations, electronic
density values and similar isotropic thermal parameters after that are related to two exothermic peaks around 260 °C and
the organic molecules calcination takes place, in two processes
the final refinement were considered (Occ. O(W1) = 0.30; Occ.
O(W2) = 0.20). Besides, for charge balance an additional
350 °C in the DTA and DSC curves due to the breakage of C–C
and C–N bonds. Finally, at around 750 °C–800 °C of tempera-
proton should be distributed over one crystallization water ture, DTA and DSC curves show an exothermic peak associated
molecule with a site occupancy of 1/6. Although hydronium
cations and water molecules cannot be distinguished in the
with the crystallization of the inorganic residue.
The thermal behaviour of the compounds was also studied
X-ray refinement, short contacts between O(W1) and O(10) atoms by time-resolved X-ray thermodiffractometry in air. The diffrac-
of 2.46(2) and 2.52(2) Å for compounds 3 and 4, respectively,
tion maxima of phase 1 remain unchanged up to 210 °C, with
could indicate the location of the protonated water mol-
temperature considered as the thermal stability limit of the
ecules.43 The O(W3) water molecule is located at logical dis- compound (ESI, Fig. S3.3†). The thermal evolution of the
tances of the organic cations, coexisting with them. For
compound 2, there are no extra framework water molecules, so
cell parameters was studied by Pattern Matching refinement
using the FULLPROF48 program (ESI, Fig. S3.4†). A trend
the cyclopentylammonium cations present an occupancy change in all of the cell parameters evolution is observed at
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Fig. 3 Thermal evolution of the (013), (004), (110) and (111) reflections of phases 2, 3 and 4.
approximately 195 °C, related to the onset of degradation of higher 2θ angles with increasing temperature, suggesting a
the initial phase structure. The parameters b, c and α show a decrease of the c parameter. For compound 2, a very slight
continuous increase with increasing temperature up to 195 °C, shift towards higher angles takes place only until 120 °C. From
attributed to an increase in interlayer space and a slight displa- this temperature to 165 °C, this shift tends towards smaller 2θ
cement between the inorganic layers. The phase volume exhi- angles (see ESI, Fig. S3.6†).
bits a constant increase in the whole temperature range. The
Note that, in spite of the similarities between the three in-
thermal expansion coefficients of the b, c and volume cell par- organic buildings, they behave differently with the heating. In
ameters were qualitatively calculated using the expression compounds 3 and 4, the removal of the crystallization and
V(T) = VTr exp[α0(T − Tr)],49 where VTr is the parameter value at coordinated water molecules causes the positive thermal
room temperature and α0 the thermal expansion coefficient.
expansion of the a axis, being negative in the case of the c axis
For compounds 2, 3 and 4, the diffraction maxima are until the collapse of the structures. Compound 2 behaves in a
observed in the thermodiffractograms until 165 °C, 120 °C and similar way to compounds 3 and 4 until 120 °C and 135 °C.
105 °C, respectively. At these temperatures, the compounds However, from these temperatures, the thermal evolution of
become amorphous, marking the limits of thermal stability. the parameters reverses, resulting in the contraction and
For compounds 2 and 3, at 630 °C and 675 °C, respectively, the expansion of the a and c axis, respectively. Therefore, it seems
50
ˉ
vanadyl phosphate, VO(PO3)2 (I42d (122), a = 10.9900 Å, c = that the removal of the protonated water molecules, that com-
4.2580 Å) crystallizes. This phase undergoes a structural trans- pensate partially the negative charge of the inorganic building,
formation to the space group C2/c (15)51 (a = 15.1400 Å, b = involves the direct destruction of the structures of compounds
4.1950 Å, c = 9.5730 Å, β = 126.54°) with increasing tempera- 3 and 4. In contrast, for compound 2 the beginning of the
ture (ESI, Fig. S3.5†).
destruction of the cyclopentylammonium cations at 150 °C
The thermal evolution of the cell parameters was also gives rise to an unsuccessful attempt of reorganization of the
studied by Pattern Matching refinements. For compounds 2, 3 crystal structure to maintain the stability.
and 4, the a parameter increases with the temperature until
Infrared and UV-Vis spectroscopy
their stability limit, except for compound 2, whose a parameter
increases until 135 °C and then decreases until 165 °C (ESI,
Fig. S3.6†). Fig. 3 shows the thermal evolution of the (013),
(004), (110) and (111) reflections. The (004) reflection of com-
pounds 3 and 4 shifts continuously and progressively towards
In the infrared spectra of the four compounds (ESI, Fig. S4.1†),
from 3100 to 2500 cm−1 a group of overlapped bands corre-
sponding to stretching vibrations (ν) of N–H and C–H bonds of
the different organic molecules appears. The deformation (δ)
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Table 1 Conversion (CT), selectivity towards sulfoxides formation (SSO) and turnover frequency (TOF) data of oxidation of alkyl aryl sulfides catalyzed by 1, 2, 3 and
4
Catalyst
Substrate
Oxidizing agent
Solvent
Temp.
CT (%) (t, min)
SSO (%)
TOF (min−1
)
1
MeSPh
MeSPh
EtBuSPh
ClPhSMe
MeSMePh
MeSPh
MeSPh
MeSPh
MeSPh
MeSPh
MeSPh
TBHP (2.2 equiv.)
H2O2 (1.5 equiv.)
H2O2 (1.5 equiv.)
H2O2 (1.5 equiv.)
H2O2 (1.5 equiv.)
TBHP (1.5 equiv.)
H2O2 (1.5 equiv.)
TBHP (1.5 equiv.)
H2O2 (1.5 equiv.)
TBHP (1.5 equiv.)
H2O2 (1.5 equiv.)
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
50 °C
r.t.
r.t.
r.t.
r.t.
50 °C
r.t.
50 °C
r.t.
50 °C
r.t.
94(120)
99(60)
14(270)
24(270)
19(270)
92(60)
85(60)
70(60)
60(60)
100(60)
77(60)
100
100
100
100
100
100
100
100
100
92
2
4.9
0.1
0.5
1
3.6
5.5
1.5
4.0
8
2
3
4
100
2.8
mode of (NH3 ) groups, for 2, 3 and 4, and of (NH2 ) groups, sulfones,55 as well as for desulfurization of fuels.56 The scope
for 1, appears split into two close bands at 1630 and of the reaction was studied with various substrates (methyl
1615 cm−1, indicating that the organic molecules are proto- phenyl sulfide, ethylbutyl phenyl sulfide, p-chlorophenyl
nated and not coordinated to the inorganic skeleton. With methyl sulfide and methyl p-tolyl sulfide) and two oxidizing
regard to the inorganic part, around 2400 cm−1 a narrow band agents (H2O2 and TBHP) (Table 1).
split into two ones, corresponding to the stretching vibrational
+
+
mode of the P–H bond from the phosphite, can be observed in
the spectra. At lower frequencies, in the 1100–400 cm−1 range,
the bands corresponding to the P–O bonds vibrations of the
phosphite/phosphate groups are observed.
The diffuse reflectance spectra (ESI, Fig. S4.2(a)†) show two
d–d bands, at 14 700 and 22 800 cm−1 for 1 and at approxi-
ð1Þ
mately 15 010 and 22 600 cm−1 for 2, 3 and 4. These bands are
characteristic of the octahedrally coordinated V3+ cations,
which correspond to transitions from the 3T1g(3F) ground state
In the proposed mechanism for the oxidation of
thioethers57 the first stage consists of the activation of the
hydroperoxide by coordination with the metallic centre in
order to obtain the peroxo species. The oxygen atom coordi-
nated to the metal has high electrophilic character, so it could
be easily attacked by the sulfide, giving rise to a transition
state shown in eqn (2). Once the sulfoxide is formed, the
sulphur atom already has a pair of electrons for another
nucleophilic attack to the peroxidic oxygen, originating the
corresponding sulfone, which is the main reaction by-product.
3
3
to T2g(3F) and T1g(3P) terms and are responsible for the light
green colour observed in these phases. The Dq and Racah par-
ameters have been calculated by fitting the experimental
frequencies obtained from the diffuse reflectance spectra
to an energy level diagram for an octahedral d2 system.52
For 1, Dq = 1620 cm−1, B = 585 cm−1, 68% of the free ion
(B0(V3+) = 861 cm−1 53 and C = 3380 cm−1. For 2, 3 and 4,
)
Dq = 1594 cm−1, B = 623 cm−1, 72% of the free ion53 and
C = 3377 cm−1. These values are in the range usually found for
octahedrally coordinated V(III) compounds52,54 but are lower
than those observed for the free ion, indicating a noticeable
degree of covalency between the V3+ cations and the surround-
ing oxygen ions.52
ð2Þ
Catalytic behaviour
Vanadium peroxides are known as very effective oxidants of
different organic and inorganic substrates such as sulfides,
Catalytic reactions with H2O2 as the oxidant. The catalysts
alkenes, alcohols, aromatic and aliphatic hydrocarbons, were first tested for the oxidation of methyl phenyl sulfide
halides and sulfur dioxide.55 These reactions, either stoichio- (MeSPh) with H2O2 as the oxidizing agent. The catalysts over-
metric or catalytic, where the terminal oxidant is normally come a transformation due to the effect of the oxidizing agent.
hydrogen peroxide or an alkylhydroperoxide, are carried out, Compound 1, with a 2D crystal structure, suffers a colour
on average, in mild conditions and associated with good change, from light green to orange-yellowish getting partially
product yield and selectivity.
solved, as can be seen by the colour change of the solution.
The activity of the vanadium catalysts 1–4 was probed Catalysts with 3D crystal structure, 2, 3 and 4, get totally solved
towards oxidation of sulfides (eqn (1)). To note, this reaction is by the action of H2O2, giving rise to orange solutions. There-
interesting both for the preparation of sulfoxides and fore, the reactions take place in homogeneous media. If the
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Fig. 5 Kinetic profiles of 2 for the oxidation reaction of methyl phenyl sulfide
Fig. 4 Conversion of methyl phenyl sulfide (MeSPh) over the four catalysts with
with H2O2 with catalyst : substrate ratios of 1 : 1000 and 1 : 5000.
H2O2 as the oxidizing agent after 15 and 60 minutes of reaction.
Table 2 Reutilization data for oxidation of MeSPh with H2O2 over catalyst 1.
Conversion (CT) and selectivity towards the sulfoxide formation (SSO) results at
60 min of reaction at each cycle (T = 298 K)
catalysts are not treated with the oxidant previously (activation
stage) the reaction runs out of oxidant because of the trans-
formation of the catalysts, and very low conversions are
obtained. The colour changes indicate the oxidation of
vanadium from V(III) to V(V).58
Total conversion was achieved in less than 60 minutes with
a selectivity towards sulfoxide formation of 100% (Fig. 4).
In order to study the effect of the temperature we per-
formed the reaction of 2 at 0 °C. The conversion and selectivity
rates did not show differences with those of the reactions at
r.t. On the other hand, we have reduced the catalyst : substrate
ratio to study the effect of the catalyst concentration. When a
1 : 1000 ratio is used, the conversion rate of 80% was reached
at 20 minutes of the reaction, so there is no activity loss.
However, when a 1 : 5000 ratio is used the conversion rate is a
little bit lower, 68%, reaching it in 60 minutes of the reaction
(Fig. 5).
Catalyst
Cycle
Cat/substr
CT (%)
SSO (%)
1
1
2
3
1/100
1/100
1/100
99
100
56
100
97
100
produces an increase in the interlayer space in the reaction
with MeSPh and a contraction when reacting with the substi-
tuted substrates, explaining the loss of activity observed (ESI,
Fig. S5.2†). The IR spectra of the recovered solids show the
same vibration modes of the fresh catalyst, with a broadening
of the bands due to the loss of crystallinity (ESI, Fig. S5.3†).
This indicates that the reactive species has a similar chemical
composition as 1.
The scope of the reaction with various alkyl aryl sulfides
was studied over catalyst 1. The oxidation reactions were
carried out with ethylbutyl phenyl sulfide (EtBuSPh), p-chloro-
phenyl methyl sulfide (ClPhSMe) and methyl p-tolyl sulfide
(MeSMePh) at room temperature. The kinetic profiles of these
reactions are shown in ESI, Fig. S5.1.† A drastic decrease of the
catalytic activity is observed when electron donor or acceptor
groups are introduced or when the steric hindrance is
increased, with conversions between 24 and 14%, and TOF
between 1 and 0.1 min−1. All the reactions are 100% selective
towards sulfoxides formation.
After the oxidation reactions, compound 1 recovers a green
colour, but darker than the original one. The powder X-ray
diffraction patterns of this dark-green compound show only
one maximum, corresponding to the (001) reflection, dis-
placed from 9.1° in 2θ before the reaction to 8.9° after the reac-
tion with MeSPh, and to 9.3° in the case of the other
substrates. This peak is associated with the layer stacking
direction, therefore, the transformation that takes place
Recycling and heterogeneity test. Recycling tests were
carried out over 1 in the MeSPh-oxidation, because it is the
only reaction where the catalyst can be recovered. During the
successive cycles a decrease in the solid amount is observed,
due to the partial decomposition of the catalyst by the effect of
H2O2. After the second cycle a decrease in the activity is
observed (Table 2).
Leaching tests were made for the same reaction after the
third cycle. Thus, at a conversion of about 15%, the solid was
separated from the reaction media by centrifugation. The
supernatant was then allowed to react, detecting a slight
increase in the conversion, up to 35% after 60 minutes.
Although the effect of introducing different substituents in the
substrates and the X-ray powder diffraction analysis of the
recovered catalyst indicate that the reaction is catalyzed by
the solid phase, there seems to be some leaching. This may
be because some of the solid gets dissolved by the action of
H2O2, this solution also being able to catalyze the oxidation
reaction.
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Catalytic reactions with TBHP as the oxidant. When TBHP the active centres on the surface get blocked when an excess of
is the oxidizing agent, all the materials behave as hetero- TBHP is added, obstructing the substrate to get them.
geneous catalysts, and the reactions take place at 50 °C, achiev- However, if the oxidizing agent is added gradually the sub-
ing 100% conversion rates in short times. The kinetic profiles strate could reach the active centres and the reaction takes
of these reactions are shown in Fig. 6.
place more gradually.
The most remarkable feature of these kinetic profiles is that
After the reactions, the catalysts were filtered and character-
when using 1 with TBHP (2.2 equiv.), the kinetic profile shows ized by X-ray powder diffraction and IR spectroscopy. The
a long induction period. Another test was made adding the powder patterns of the catalysts 2–4 after the reactions with
same quantity of TBHP gradually, 1.5 equiv. at the beginning TBHP did not show significant changes with respect to those
and 0.35 equiv. twice at 40 and at 90 minutes. In this case, the obtained before them (Fig. 7a). However, the pattern of catalyst
kinetic profile is almost linear (ESI, Fig. S5.4†). It seems that 1 after the reaction with TBHP added in one go shows the split-
ting of some maxima. For these reasons, the reactions have
been monitored by means of X-ray powder diffraction (Fig. 7b).
Comparing the X-ray diffractograms of as-synthesized 1, 1 after
the activation stage, 1 after the reaction with TBHP added in
one go and 1 after the reaction with TBHP added in three goes,
a evolution on the (001) reflection can be observed. It seems
that this maximum suffers a gradual displacement to higher
2θ angles in the activation stage, gaining intensity after the
reaction with TBHP added in one go. When TBHP is added in
three times, the maximum displacement is smaller, observing
it as a shoulder. The same behaviour is observed for some less
intense diffraction maxima in all cases.
These maxima shifts should be the result of the oxidation
of 1 not only at the particle surface, but also at the structural
level. The unit cell of the oxidized compound is very similar to
the as-synthesized 1, with a decrease of the c parameter,
associated with a reduction of the interlayer space. Judging by
the powder X-ray diffraction, catalysts 2–4 are only oxidized
Fig. 6 Kinetic profiles of 1, 2, 3 and 4 for the oxidation reaction of methyl
superficially by the action of TBHP.
phenyl sulfide with TBHP.
Fig. 7 Powder X-ray diffraction patterns of (a) catalysts 2–4 before and after the oxidation reaction of MeSPh with TBHP and (b) catalyst 1 before the reaction, after
the activation stage and after the reaction adding TBHP in one go and in three goes.
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Table 3 Reutilization data for oxidation of MeSPh with TBHP over catalyst 4.
Conversion (CT) and selectivity towards the sulfoxide formation (SSO) results at
60 min of reaction at each cycle (T = 323 K)
Catalyst
Cycle
Cat/substr
CT
SSO
4
1
2
3
1/100
1/100
1/100
100
100
96
92
95
93
The IR spectra obtained after the reactions are very similar
to those of the fresh catalysts. However, there are some bands
that are less intense. These bands are related to the torsional
vibration modes (γ) of the (NHn+) and (CH2) groups.
Taking into account that the inorganic frameworks of 1–4
contain no residual solvent accessible voids because their
pores are occupied by guest molecules (organic amine cations
and water molecules), the catalytic activity of these materials
should be attributed to the vanadium centres on the surface.
In order to corroborate this hypothesis, the oxidation reactions
have been reproduced using crystals instead of powder. The
kinetic profiles of these reactions are summarized in ESI,
Fig. S5.5.† For catalysts 2–4, the conversion rates are almost
zero until a certain reaction time when the conversion
increases abruptly, almost to a 100%. After the reaction, the
particle size of the recovered catalysts has been reduced drasti-
cally, probably due to the action of the magnetic stirrer (ESI,
Fig. S5.5†). This fact suggests that it is necessary to increase
the surface area in order for the catalyst to have enough active
centres for the reaction to take place. The kinetic profiles of
catalyst 1 both with powder and crystals are very similar. The
catalyst has been recovered as a powder and the DRX pattern
is the same as that observed when using powder of as-
synthesized 1.
In all cases the phases start to catalyze approximately at
80 minutes, probably because this is the time when there is
enough catalyst in the reaction media with the appropriate
particle size.
Recycling and heterogeneity test. Reutilization is one of the
greatest advantages of heterogeneous catalysts, and can also
provide useful information about the anchoring process and
catalyst stability along the catalytic cycle. Recycling tests were
carried out over 4. The catalyst was recycled for at least 3 runs
maintaining its activity and selectivity (Table 3).
Fig. 8 Kinetic profile of oxidation of MeSPh over 2, and after hot filtering, with
TBHP.
15 mmol for 2, 3 and 4) and VCl3 (0.3 mmol for 1 and
0.6 mmol for 2, 3 and 4) were dissolved in different mixtures
of distilled water and organic solvents (5 ml H2O–25 ml
ethanol for 1, 10 ml H2O–20 ml 1-butanol for 2, 10 ml H2O–
20 ml methanol for 3 and 4). The pH of the resulting solutions
was increased to the desired value with the corresponding
organic amines (pH 4.5 with 5.09 mmol of 2-methylpiperazine
for 1, pH 2–2.5 with 13.94 mmol of cyclopentylamine for 2,
11.40 mmol of cyclohexylamine for 3 and 10.06 mmol of cyclo-
heptylamine for 4). These reaction mixtures were sealed in
PTFE-lined stainless steel pressure vessels. After 3 days at
200 °C for 1 and 2 and at 170 °C for 3 and 4, the vessels were
slowly cooled to room temperature. Fibrous radial crystalline
growth and prismatic crystals for 1 and hexagonal tabular and
plate crystals for 2, 3 and 4 were obtained, and then they were
isolated by filtration, washed with water and acetone and dried
at ambient temperature.
The pH value range in which 2, 3 and 4 are obtained as
single phases in the synthesis is very limited, such that below
59
pH 2 V2(HPO3)3
crystallizes.
appears and above pH 2.5 V(PO4)·H2O60
According to the literature, until now, there were six organi-
cally templated phosphites with V(III) as the single-valence
cation centre. Among them, [C4N2H14][VF(HPO3)2]·H2O,61
(C4H8N2H4)0.5(C4H8N2H3)[V4(HPO3)7(H2O)3]·1.5H2O62
and
The heterogeneity of the reaction was probed by a leaching
test with catalyst 2. When the reaction reached a conversion
rate of 5%, the solid was separated from the liquid by fil-
tration. The supernatant was then allowed to react. After
1 hour without the catalyst the conversion rate was 12%
(Fig. 8). This fact confirms the heterogeneity of the reactions.
(C2H10N2)[V(HPO3)F3]63 compounds were typically prepared
from a metal source containing pentavalent vanadium, V2O5.
However,
a tetravalent vanadium reactant, VOSO4, was
employed to achieve (Hcha)3(H3O)1+x[V9(H2O)6(HPO3)14−x
-
(BO3)x(H2PO3)3]·2H2O (x = 0.1–1.2, cha = cyclohexylamine)64
and [C3N2H5]2[V4(H2O)3(HPO3)4(HPO4)3]61 compounds. In this
research,65 as in the obtaining of the (C2H10N2)0.5[M(HPO3)2]66
phase, a direct V3+ source, VCl3, was used.
Huang and Wang64 proposed the VOSO4–H3BO3–
Experimental section
H3PO3–cyclohexylamine–H2O reaction system, with
1 : 1.5 : 6 : 5 : 278 molar ratio, heated at 180 °C for 3 days, in
order to obtain the compound with (Hcha)3(H3O)1+x
a
Synthesis and characterization
1, 2, 3, and 4 were synthesized under mild hydrothermal con-
ditions and autogenous pressure. H3PO3 (7.50 mmol for 1 and
-
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[V9(H2O)6(HPO3)14−x(BO3)x(H2PO3)3]·2H2O (cha
=
C6H11NH2
Details of crystal data, data measurement and reduction,
and x = 0.1–1.2) formula. The inorganic framework of this structure solution and refinement of the phases 1, 2, 3 and 4
phase is very similar to that presented by compounds 2, 3 and are reported in Table 4. The selected bond distances and
4. The authors suggest that the boric acid is essential for the angles are reported in ESI, Tables S1.1 to S1.4.† All structure
formation of this compound. However, we have proved that it drawings were made using the ATOMS program.74
is possible to achieve compounds 2, 3 and 4 without borate
Physicochemical characterization techniques
incorporation, using a direct V3+ source instead of a V4+
reactant.
Thermogravimetric analyses were performed on an SDT 2960
The amounts of vanadium in compounds 1, 2, 3 and 4 were simultaneous DSC-TGA TA instrument for 1, 2 and 3 and on a
measured by ICP-AES spectroscopy and the amounts of C, N Netzsch STA 449C one for 4. Crucibles containing 20 mg of
and H by elemental analysis. Calculated for 1 (%): V, 18.2; each sample were heated at a rate of 5 °C min−1 from room
C, 10.7; N, 5.0; H, 4.0. Found (%): V, 18.4(3); C, 10.7(4); temperature to 900 °C. Temperature dependence X-ray diffrac-
N, 4.3(5); H, 4.0(2). Calculated for 2 (%): V, 19.3; C, 10.1; tion experiments for all these compounds were carried out in
N, 2.4; H, 3.4. Found (%): V, 19.5(5); C, 9.9(5); N, 2.2(5); H, air on a Bruker D8 Advance diffractometer (Cu Kα radiation)
3.2(4). Calculated for 3 (%): V, 19.9; C, 9.4; N, 1.8; H, 3.7. equipped with a variable-temperature stage (HTK2000), a Pt
Found (%): V, 19.7(4); C, 9.1(4); N, 1.8(4); H, 3.8(5). Calculated sample heater and a Vantec high-speed one dimensional
for 4 (%): V, 19.6; C, 10.8; N, 1.8; H, 3.9. Found (%): V, 19.4(4); detector with six degrees of angular aperture. The powder pat-
C, 10.3(5); N, 1.7(4); H, 3.7(4). The densities of the four terns were recorded in the 8 ≤ 2θ ≤ 30° range (step size =
phases were measured by flotation67 using a mixture of CH2I2 0.033° and time per step = 0.2 s) for 1 and in the 5 ≤ 2θ ≤ 38°
and CHCl3, being 1.931(2) g cm−3 for 1, 1.81(3) g cm−3 for 2, range (step size = 0.033° and time per step = 0.4 s) for 2, 3 and
1.77(4) g cm−3 for 3 and 1.79(3) for 4.
4, in all cases at intervals of 15 °C, increasing the temperature
at a 9 °C min−1 rate from room temperature to 795 °C, except
for 4, which was heated until 300 °C. The IR spectra
Single crystal X-ray diffraction study
An elongated prismatic single-crystal with dimensions 0.55 × (KBr pellets) were obtained with a JASCO FT/IR-6100 spectro-
0.11 × 0.08 mm of 1 and hexagonal plates for 2, 3 and 4 with photometer in the 400–4000 cm−1 range. UV-vis diffuse reflec-
dimensions 0.48 × 0.34 × 0.06 mm, 0.42 × 0.35 × 0.07 mm and tance spectra were registered at room temperature on a Varian
0.13 × 0.10 × 0.03 mm, respectively, were selected under a Cary 5000 (version 1.12) spectrometer in the 250–2500 nm
polarizing microscope and mounted on a glass fibre.
range for all the compounds.
Intensity data were collected at 100 K on an Oxford Diffrac-
tion XCALIBUR 2 diffractometer for 1 and 2, and on a STOE
Catalytic activity
IPDS 2T diffractometer for 3, fitted with a Sapphire 2 CCD Oxidation of organic sulfides was carried out in a batch reactor
detector and an image plate detector (34 cm diameter), at atmospheric pressure, using acetonitrile or dichloro-
respectively, using in both cases graphite monochromated methane as solvents (2 ml). Before the reactions, the materials
Mo-Kα radiation = 0.71073 Å. Data collection for 4 was performed (0.02 mmol) were activated by stirring them with the oxidizing
at 100 K on an AGILENT SUPERNOVA diffractometer (Cu-Kα agent, H2O2 or tert-butyl hydroperoxide (TBHP), in the corre-
radiation = 1.5418 Å; omega scan mode) using a CCD (Atlas) sponding solvent, acetonitrile or dichloromethane, for 30 min
detector. Data frames were processed (unit cell determination, at r.t. or 50 °C, respectively. After this activation stage, the cata-
intensity data integration, correction for Lorentz and polariz- lysts were separated from the liquid media. The reactor with
ation effects,68 and analytical absorption correction69 taking the activated catalyst was then charged with 2 mmol of the cor-
into account the size and shape of the crystals) using the corre- responding sulfide (methyl phenyl sulfide, 4-chlorophenyl
sponding diffractometer software package.70
methyl sulfide, methyl p-tolyl sulfide and 1-ethylbutyl phenyl
The structures were solved by direct methods, the SHELXS sulfide) (ratio V : substrate = 1 : 100, 1 : 1000 and 1 : 5000) in
97 computer program,71 in the triclinic space group P1 for 1 2 ml of solvent. The oxidizing agent was added dropwise and
ˉ
and, in the hexagonal space group P63/m for 2, 3 and 4, and then the suspension was maintained at r.t. or 0 °C, when
then refined by the full matrix least-squares procedure based using H2O2, and heated to 50 °C, with TBHP. Experiments
on F2, using the SHELXL 97 computer program72 belonging to under the same reaction conditions without a catalyst have
the WINGX software package.73 This procedure allowed us to been also conducted and no appreciable amount of product
find the positions of the vanadium and phosphorus atoms, was detected. Reaction samples were taken at regular times
and all the other non-hydrogen atoms (O, C, N) were placed and analyzed on a Hewlett-Packard 5890 II GC-MS. After the
from subsequent Fourier-difference map calculations.
reaction, the catalysts were filtered and characterized by X-ray
During the structure refinement, two types of disorder, posi- powder diffraction and IR spectroscopy.
tional and chemical, were found. In the ESI (S1†), the study of
Blank experiments were carried out in order to determine
the different positional disorders of the phosphorous anionic the extension of the uncatalyzed reaction. At 315 K, using
groups and the chemical disorders between the crystallization acetonitrile as the solvent (10 ml), 1 mmol of methyl phenyl
water molecules and the organic cations is presented. Other sulfide and 3 equiv. of 30% H2O2, the reaction accounted for a
features about the refinement are also included there.
conversion of 5 and 15% after 20 min and 1 h of reaction time
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Table 4 Crystallographic data and structure refinement parameters for phases 1, 2, 3 and 4 obtained by single crystal X-ray diffraction
Phase
1
2
3
4
Molecular weight (g mol−1
Crystal system
Space group
)
280.0
Triclinic
ˉ
P1
2377.9
Hexagonal
P63/m
13.4107(3)
13.4107(3)
27.5678(7)
90
2300.4
Hexagonal
P63/m
13.4578(2)
13.4578(2)
27.6928(7)
90
2332.9
Hexagonal
P63/m
13.41250(10)
13.41250(10)
27.8003(3)
90
a, b, c (Å)
5.2403(1)
9.2240(3)
10.8060(4)
112.901(4)
95.506(2)
90.995(2)
478.08(3)
2
α, β, γ (°)
90
120
90
120
90
120
V (Å3)
4293.74(17)
12
1.81(3), 1.838
0.48 × 0.34 × 0.06
2395
4343.56(14)
12
1.77(4), 1.753
0.42 × 0.35 × 0.07
2305
4331.11(7)
12
1.79(3), 1.778
0.13 × 0.10 × 0.03
2331
Z
ρobs, ρcalc (g cm−3
Crystal size
)
1.931(2), 1.942
0.55 × 0.11 × 0.08
283
F (000)
Temperature (K)
Diffractometer
298(2)
Xcalibur 2
1.378
100(2)
Xcalibur 2
1.357
100(2)
Stoe IPDS 2T
1.338
100(2)
Supernova
11.695
μ (mm−1
)
Tmin./Tmáx.
Radiation (Å)
Limiting indices h, k, l
0.679/0.913
λ(Mo Kα) = 0.71073
0.633/0.926
λ(Mo Kα) = 0.71073
0.654/0.913
λ(Mo Kα) = 0.71073
−18 ≤ h ≤ 17
−16 ≤ k ≤ 17
0.371/0.694
λ(Cu Kα) = 1.54184
−15 ≤ h ≤ 16
−15 ≤ k ≤ 16
h
k
l
6
h
k
l
16
16
33
11
13
l
37
l
33
Theta range (°); Completeness (%)
No. of reflections (measured/independent/observed)
R(int)/R(σ)
3.73–26.73; 98.7
13 363/2012/1730
0.0287/0.0171
146/4
2.83–25.68; 99.2
32 685/2769/2368
0.0289/0.0188
207/44
1.90–28.50; 99.7
54 238/3741/3184
0.0441/0.0185
192/16
3.81–70.04; 99.7
24 685/2810/2740
0.0582/0.0274
188/14
Parameters/restrictions
R [I > 2σ(I)]
R1 = 0.0327
wR2 = 0.0919
R1 = 0.0382
wR2 = 0.0968
x = 0.0671
y = 0.1791
1.087
R1 = 0.0479
wR2 = 0.1355
R1 = 0.0555
wR2 = 0.1393
x = 0.0747
y = 18.9021
1.057
R1 = 0.0613
wR2 = 0.1749
R1 = 0.0705
wR2 = 0.1816
x = 0.1060
y = 10.0529
1.077
R1 = 0.0438
wR2 = 0.1278
R1 = 0.0449
wR2 = 0.1296
x = 0.0835
y = 7.9489
1.096
R [all data]
Weight factor
G.O.F.
Max. and min. e. density (e Å−3
)
0.902, −0.366
1.09, −0.898
1.454, −1.901
1.302, −0.704
Formulas: 1: (C5N2H14)0.5[V(H2O)(HPO3)2]; 2: (C5NH12)4[V9(H2O)6(HPO3)10.5(HPO4)3.5(H2PO4)3]; 3: (C6NH14)3(H3O)[V9(H2O)6(HPO3)14(H2PO3)2.4-
(H2PO4)0.6]·2.33H2O; 4: (C7NH16)3(H3O)[V9(H2O)6(HPO3)14(H2PO3)3]·2.33H2O.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
P
2
2
2
jjFoj ꢀ jFcjj
wðjjFoj ꢀ jFcjjÞ
1
jFoj þ 2jFoj
P
R1
¼
;
wR2
¼
;
w ¼
À
Á
;
p ¼
:
P
2
2
2
jFoj
3
wjFoj
σ2 jFoj þ ðxpÞ þyp
respectively. Under the same conditions but using 1.5 mmol of alkyl aryl sulfides with TBHP as the oxidizing agent, and
TBHP instead of H2O2, the conversion after 5 h was 10%.
can be recovered and reutilised after the reaction showing
no important change in their chemical composition or
crystal structure. However, the use of H2O2 induces the dissolu-
tion of the catalysts, partial for 1 and total for 2, 3 and 4,
giving rise to active species. Catalyst 1 is affected not only by
the size of the substrate, but also by the size of the oxidizing
agent.
Concluding remarks
The mild hydrothermal technique was used for the synthesis
of four novel amine templated open-framework vanadium(III)
phosphites. The crystal structure of 1 consists of a two-dimen-
sional network formed by anionic sheets of [V(H2O)(HPO3)2]−
formula, compensated by the 2-methylpiperazinium cations
embedded between the layers. Phases 2, 3 and 4 present a
Acknowledgements
pillar-layer network giving rise to a three-dimensional frame- This work has been financially supported by the “Ministerio
work containing intersecting 16-ring channels with the de Ciencia e Innovación” (MAT2010-15375, MAT2011-29020-
primary amine templates and the crystallization water mole- C02-02) and the “Gobierno Vasco” (IT-177-07), which we grate-
cules enclosed in them. Phases 2, 3 and 4 behave differently fully acknowledge. The authors thank the technicians of
from the heating due to the presence of crystallization water SGIker (UPV/EHU), Drs F.J. Sangüesa, A. Larrañaga,
molecules in compounds 3 and 4 and the absence of them in J.C. Raposo, L. Bartolomé and P. Vitoria, financed by the National
compound 2. The catalytic tests show that the phases are het- Program for the Promotion of Human Resources within the
erogeneous and active catalysts for the selective oxidation of National Plan of Scientific Research, Development and
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Innovation, “Ministerio de Ciencia y Educación” and “Fondo 11 J. Qiao, L. Zhang, Y. Yu, G. Li, T. Jiang, Q. Huo and Y. Liu,
Social Europeo” (FSE), for the X-ray diffraction, chemical and
spectroscopic measurements. J. Orive and E.S. Larrea wish to
thank the Universidad del País Vasco, UPV/EHU for funding.
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