Vanadium(IV) complexes with picolinic acids in NaY zeolite cages: Synthesis, characterization and catalytic behaviour

Article abstract of DOI:10.1039/a706679a

Encapsulated vanadium picolinic complexes have been synthesized by treatment of a dehydrated form of VO²⁺-NaY zeolite with molten picolinic acids and characterized by X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), EPR, FTIR and UV-VIS spectroscopies, and XRD. It was suggested by XRD and XPS that the complexes were located in the zeolite cavities. Differences in the spectroscopic properties of encapsulated and impregnated samples were explained in terms of coordination of vanadium complexes with zeolite -OH groups. The stability of VO(pic)₂ and its adduct with pyridine depended strongly on the complex location. The encapsulated vanadium picolinate complex retained solution-like activity in the liquid-phase oxidation of hydrocarbons and alcohols with hydrogen peroxide.

Full text of DOI:10.1039/a706679a

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Vanadium(IV) complexes with picolinic acids in NaY zeolite cages
Synthesis, characterization and catalytic behaviour
Alexander Kozlov,a Kiyotaka Asakura,b and Yasuhiro Iwasawaa*¤
a Department of Chemistry, Graduate School of Science, T he University of T okyo, Hongo,
Bunkyo-ku, T okyo 113, Japan
b Research Center for Spectrochemistry, Faculty of Science, T he University of T okyo, Hongo,
Bunkyo-ku, T okyo 113, Japan
Encapsulated vanadium picolinic complexes have been synthesized by treatment of a dehydrated form of VO2`-NaY zeolite with
molten picolinic acids and characterized by X-ray photoelectron spectroscopy (XPS), extended X-ray absorption Ðne structure
(
EXAFS), X-ray absorption near-edge structure (XANES), EPR, FTIR and UVÈVIS spectroscopies, and XRD. It was suggested
by XRD and XPS that the complexes were located in the zeolite cavities. Di†erences in the spectroscopic properties of encapsu-
lated and impregnated samples were explained in terms of coordination of vanadium complexes with zeolite wOH groups. The
stability of VO(pic) and its adduct with pyridine depended strongly on the complex location. The encapsulated vanadium
2
picolinate complex retained solution-like activity in the liquid-phase oxidation of hydrocarbons and alcohols with hydrogen
peroxide.
The great expansion in the study of acidic and bifunctional
catalysis over zeolites1 has led to a variety of technological
applications for petrochemical reactions. However, there is
much less information available for catalytic systems where
zeolite-based materials act as redox catalysts. In the last
decade there has been signiÐcant interest in the design of
redox molecular sieves and considerable progress has been
made in their use in liquid-phase oxidations.1h3 Two routes,
involving isomorphous substitution of transition metal ions in
the lattice of zeolites or encapsulation of transition metal com-
plexes in zeolite voids can be applied to prepare redox zeolite
catalysts.4h7 The Ðrst method dictates occupation of the
regular tetrahedral framework sites by metal cations. The
requirements imposed by coordination chemistry, as well as
those derived from zeolite synthesis procedure, often make it
difficult to modify the metal speciÐcation in these materials,
resulting in a slight change in catalytic behaviour. However,
the second method provides the opportunity of physical
entrapment of metal complexes in an ordered matrix and
allows one to avoid both covalent and ionic attachment to the
solid support. Generally, so called “ship-in-a-bottleÏ catalysts
can have the combined property of solution-like activity and
shape selectivity.8,9
hexane oxidations.14,15 Immobilization of metal complexes
with enhanced homolytic reactivity, such as vanadium com-
plexes with pyridine- and pyrazine-carboxylic acids,16 was not
successful until our recent study17 which reported the synthe-
sis of vanadium complexes with picolinic acid encapsulated in
Y-zeolite supercages.
In the present paper we extend our previous study to vana-
dium complexes with substituted picolinic acids in NaY
zeolite. We also report detailed characterization of the struc-
ture of the vanadium species by XPS, EXAFS, XANES, EPR,
FTIR and UVÈVIS spectroscopies. Location of the complexes
is also discussed based on XPS and XRD data.
Experimental
Materials
The starting vanadium compound VOSO É nH O (n \ 3È4)
4 2
purity 99.9%) and special grade solvents were purchased from
(
Wako Pure Chemical Industries. Picolinic acid (picH) (99%),
quinaldic acid (QpicH) (99%), 5-n-butyl-2-picolinic acid
(BupicH) (95%) were received from Soekawa Chemicals. Urea
hydrogen peroxide addition compound (98%, Aldrich) was
used as an oxidant. The authentic samples for gas chromato-
graphic analysis (all EP grade) were received from Soekawa
Chemicals. NaY zeolite with an Si/Al ratio of 5.6 was supplied
by Tosoh Company (HSZ-320NAA, Lot. 3001) and pretreated
before use by a literature method.18
Since Romanovsky et al. reported the synthesis and cataly-
sis of encapsulated metal phthalocyanate complexes in NaY
zeolite in 1977,10 a variety of encapsulated complexes have
been synthesized and characterized. However, studies have
concentrated on epoxidation catalysis and only a few pub-
lications have been concerned with alkane and benzene
hydroxylation reactions.
Sample preparation
The ability of vanadium complexes to activate reduced
forms of oxygen is well known and numerous applications to
a variety of hydrocarbon oxidations have been reported in
homogeneous systems.11h13 The balance between heterolytic
and homolytic oxygen transfer is strongly inÑuenced by the
type of ligands coordinated to vanadium. Recently, encapsu-
VO(pic) Æ H O. Vanadium(IV) complexes with picolinic
2
2
acids were synthesized in a similar manner to that reported in
ref. 19. 100 ml of 0.4 mol l~1 aqueous solution of VOSO
4
(
8.68 g) was added to 250 ml of 0.16 mol l~1 hot (333 K)
aqueous solution of picH (9.84 g) within 1È2 min. 5% NH OH
4
was used to control the pH of the mixture at ca. 7. The
lated VO(salen)-NaY and VO(bipy) -NaY catalysts have been
mixture was stirred at 333 K for 1 h. A blue precipitate of the
2
reported, which showed activity for cyclohexene and cyclo-
VO(pic) É H O formed and was Ðltered, washed with hot
2
2
water, and dried in vacuum. C
H N O V; Calc.: C 43.76%,
1
2 10 2 6
”
E-mail: iwasawa=utsc.s.u-tokyo.ac.jp
H 3.04%, N 8.51%; found: C 42.88%, H 3.17%, N 8.35%.
J. Chem. Soc., Faraday T rans., 1998, 94(6), 809È816
809

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supporting disks, were measured at the BL-7C station of the
Photon Factory in the National Laboratory for High Energy
Physics (KEK-PF) with a positron energy of 2.5 GeV and a
storage ring current 250È350 mA (Proposal No. 95G199).
Synchrotron radiation was monochromatized by a Si(111)
sagital focusing double crystal. Higher harmonics were
removed by using a total-reÑection double mirror and detun-
ing the parallelism for the intensity to 60% of the maximum.
V K-edge spectra were recorded in a transmission mode using
ionization chambers Ðlled with Ñowing N for I and I as
2
0
X-ray detectors. The EXAFS data were analysed by the
EXAFS analysis program Rigaku EXAFS (REX).
Fig. 1 Calibration plot for quantiÐcation of vanadium in zeolites
QuantiÐcation of vanadium in samples was performed
using energy dispersive X-ray Ñuorescence (XRF) on a Seiko
SII SEA2010, equipped with a rhodium source for X-rays, a
series of secondary targets and Si (Li) drift detector. Typical
counting time was 100 s with dead time of 60È70% under
excitation conditions of 50 kV and 12 mA. The ratio of the
intensities of the vanadium peak to the silicon peak was
plotted against the vanadium loading in Fig. 1 which was used
to determine the vanadium content of the samples. The cali-
bration curve was obtained by using NaY zeolites impreg-
constructed with VO(acac) -NaY impregnated samples
2
VO2‘-NaY. 5 g of a hydrated form of NaY zeolite was ion
exchanged with 100 ml of 0.05 mol l~1 aqueous solution of
VOSO for 10 h.20 The resultant mixture was Ðltered and
4
washed twice with 50 ml of water. All the procedures were
performed under argon (99.9999% purity) atmosphere. The
VO2`-NaY was dried in vacuum at 298 K for 4 h, followed by
evacuation at 503 K for 2 h. The resultant VO2`-NaY zeolite
was used as a precursor for a “ship-in-a-bottleÏ type synthesis
without storage.
nated with given amounts of VO(acac) .
2
XRD patterns were obtained using Ni Ðltered Cu-Ka radi-
ation and the Rigaku computer-controlled instrument in
2HÈH mode.
XPS were registered with a Rigaku XPS 7000 spectrometer
Encapsulated VO(pic) -NaY (SB). 4 g of the VO2`-NaY
2
using a monochromatic aluminum Ka
X-ray excitation
zeolite precursor was treated with 10 g of molten picH at 433
1,2
source (1486.6 eV) and an X-ray power of 200 W (accelerating
voltage 20 keV, current 10 mA) in an analysis chamber with a
base pressure less than 5 ] 10~7 Pa. Samples were pressed
into thin disks and placed in a pre-chamber, outgassed to less
than 1 ] 10~6 Pa at room temperature, and then transferred
to the analysis chamber. The binding energies were referred to
K for 6 h under an argon atmosphere. The sample was then
Soxhlet extracted with pyridine for 60 h and dried in vacuum
for 4 h at 298 K. The sample was further heated at 413 K for
0 h in a He Ñow to remove adsorbed pyridine before use as a
catalyst. The sample thus obtained is denoted as SB.
1
1
54.0 eV for the Si 2s level. The relative concentration (n /n )
Encapsulated VO(Bupic) -NaY (SB) and VO(Qpic) -NaY
1
2
2
2
of two elements (1 and 2) was estimated by the following
equation using ScoÐeldÏs photoelectron cross-sections (p ):22
(
SB). These samples were prepared by a similar procedure to
that described above for encapsulated VO(pic) -NaY (SB)
i
2
sample, except for prolonged time of complexation (10 h at
n /n \ (I /I )(p j S /p j S )
1
2
1 2 2 2 2 1 1 1
4
33 K).
where p , p are the photoionization cross-sections; j , j are
the electron mean free paths and S , S are the analyser trans-
missions.
1
2
1
2
Impregnated VO(pic) -NaY (IM). A pyridine solution of
1
2
2
VO(pic) É H O (0.36 g) was added to a suspension of 4 g
2
2
VO2`-NaY zeolite in pyridine and the mixture was stirred at
3
53 K for 1 h. The pyridine was then allowed to evaporate
Catalytic oxidation
under a fan at room temperature, followed by drying in
vacuum for 4 h at 298 K and heating at 413 K for 10 h in a
He Ñow.
Liquid-phase oxidation reactions were performed in ther-
mostatted cylindrical vessels in air under vigorous stirring at
313 K. Addition of the catalyst was considered to be the start-
ing point of the reaction. Reaction solutions were analysed by
temperature-programmed GC chromatography using a 6 m
column of 15% Unisole 30T on Uniport HP.
Impregnated VO(Bupic) -NaY (IM) and VO(Qpic) -NaY
2
2
(
IM). These samples were prepared by a similar procedure to
that described above for impregnated VO(pic) -NaY (IM).
2
Spectroscopic characterization
Results and Discussion
EPR spectra for powdered samples were obtained at room
temperature on a JES-RE2X spectrometer. The microwave
power was 1.00 mW with a resonance frequency of 9.1 GHz
Preparation of encapsulated vanadium complexes with pico-
linic acids consisted of three steps: (1) preparation of
vanadium-modiÐed zeolite as a precursor, (2) synthesis of
encapsulated vanadium complexes by a Ñexible ligand method
and (3) post-synthesis treatments. In the Ðrst step, NaY zeolite
was ion exchanged with an aqueous solution of vanadyl
sulfate followed by dehydration in vacuum at 368 or 503 K,
resulting in VO2`-NaY zeolite precursor. The VO2`-NaY
samples were partially dehydrated at 368 K and fully dehy-
(
X-band). The g values were determined relative to DPPH
(
g \ 2.0036). Positions of m \ ^7/2 transitions and m \
I
I
^
5/2 transitions were used to estimate the g and A values. As
a general rule the quadrupole term was ignored in interpreta-
tion of the EPR spectra.21
UVÈVIS di†use reÑectance spectra in the region 250È900
nm were recorded on a Hitachi U-3500 spectrometer using a
quartz cell for powdered samples at room temperature in air.
A disk of NaY zeolite was used as a reference. FTIR spectra
drated at 503 K and exhibited EPR parameters g \ 1.937,
A
g \ 1.998, A \ 192.9 G and A \ 79.0 G, and g \ 1.911,
M
M
A
A
M
A
g \ 1.996, A \ 205.8 G and A \ 85.4 G, respectively
M
(
resolution 2 cm~1) were recorded with a JASCO FT/IR-300E
(Table 1). In the present study the fully dehydrated VO2`-
NaY zeolite was used. The choice of vanadium-modiÐed
zeolite is quite important for the synthesis. Obviously, com-
plexation must occur from the outer parts of the zeolite
spectrometer using a self-supporting disk.
V K-edge XAFS spectra for the vanadium samples and a
mixture of a reference compound and boron nitride, as self-
810
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Table 1 EPR parameters for selected NaY zeolite supported vana-
relatively severe synthesis conditions. During the high-
dium complexes
temperature treatment formation of a small amount of CO ,
2
due to ligand decarboxylation, was observed.
samplea
368 K
gM
gA
AM/G
AA/G
In the third step, the encapsulated vanadium samples were
Soxhlet extracted with pyridine for 60 h. This was sufficient to
remove most of the non-complexed ligand as well as vana-
dium picolinates located on the external surfaces of the zeolite
(Table 2). The other solvents such as acetonitrile showed low
efficiency for washing out surface-bound complexes. The
Soxhlet-extracted samples contained much absorbed pyridine
that could signiÐcantly a†ect the spectroscopic and catalytic
properties of the prepared materials. The samples were, there-
fore, further treated at 413 K for 12 h in a Ñow of He to
remove the adsorbed pyridine.
VO2`-NaY
G
1.998
1.996
1.996
1.994
1.995
1.996
1.937
1.911
1.945
1.947
1.942
1.942
79.0
85.4
67.1
66.1
69.3
71.5
192.9
205.8
175.3
174.1
180.1
182.3
dehydrated 503 K
VO(pic) -NaY (SB)
2
VO(pic) -NaY (IM)
2
VO(Bupic) -NaY (SB)
2
VO(Qpic) -NaY (SB)
2
a Except for VO2`-NaY and VO(pic) -NaY (IM) the EPR parameters
2
were taken for samples after Soxhlet extraction.
crystal toward the inner parts, where vanadium ions/species in
zeolite would be required to have low mobility. In practice,
V5`-NaY zeolite synthesized by a solid-state reaction and a
hydrated form of ion exchanged VO2`-NaY zeolite did not
produce encapsulated complexes. In other words, complexes
were formed on the external surfaces of the zeolite and were
removed by Soxhlet extraction similar to the impregnated
materials. Redistribution of vanadium species is probably due
to their high mobility from the inner to the external surface of
the zeolite crystal. In consequence, use of zeolites modiÐed
with the less mobile V4`\O species was necessary. Dehydra-
tion of the ion-exchanged VO2`-NaY zeolite in vacuum at
Location of complexes
The diameters of the VO2`-picolinate complexes are larger
than the zeolite-cage free-aperture (ca. 0.74 nm), so that the
complexes cannot e†ectively penetrate into zeolite, but they
are small enough to be conÐned in the large cavity (id ca. 1.2
nm) as shown in Fig. 3. Hence, the location of the complexes
depends strongly on the preparation procedure. Table 2 shows
XRF data for vanadium bulk concentration in the zeolite
before and after Soxhlet extraction. Surface-bound complexes
in the VO(pic) -NaY (IM) sample prepared by an impregna-
2
503 K produced homogeneously distributed V4` species with
tion method were mainly removed by Soxhlet extraction, as
low mobility, which were located on type III sites inside the
zeolite supercage.20 The dehydrated VO2`-NaY zeolite was
found to be the best precursor for the synthesis of various
encapsulated vanadium picolinate complexes by a Ñexible
ligand method. All synthesis procedures, i.e. preparation of the
VO2`-NaY zeolite precursor and the treatment with picolinic
acids described below were performed under an argon atmo-
sphere (99.9999% purity) to prevent V4`-species being oxi-
dized and to minimize non-uniform vanadium redistribution
by moisture. Moreover, formation of vanadium(V) species is
undesirable because of the ability of vanadium(V) to catalyse
hydrolysis of the NaY framework under the synthesis condi-
tion.23
In the second step (Ñexible ligand synthetic procedure), the
VO2`-NaY was treated with molten picolinic or substituted
picolinic acids. Complexation with picolinic acids was accom-
panied by a colour change from grey to greenish-brown. EPR
spectra showed the gradual disappearance of the VO2`-NaY
signal and the appearance of a new signal with parameters
shown in Table 2. On the other hand, negligible extractability
of vanadium complexes in the VO(pic) -NaY (SB) sample and
the other (SB) samples prepared by the Ñexible ligand method
indicates intrazeolitic location of complexes. Two other reli-
able estimates of the location of the complexes were obtained
by XRD and from comparative analysis of XPS and XRF
data.
2
XRD patterns for (a) NaY zeolite, (b) VO(pic) -NaY (IM),
2
(c) VO2`-NaY and (d) VO(pic) -NaY (SB) samples are shown
2
in Fig. 4. There was no signiÐcant loss in zeolite crystallinity
for impregnated or encapsulated VO(pic) -NaY samples as
compared with the crystallinity of NaY zeolite. Note that the
2
impregnated (IM) sample showed no additional feature which
can be ascribed to a crystalline phase of vanadium com-
pounds. Analysis of XRD patterns can supply information
about the location of extraframework species such as charge-
compensating cations and guest molecules in the zeolite pore
system.24,25 Qualitative information about site occupancies
can be obtained from comparison of the relative intensities of
selected XRD lines.26,27 Quayle and Lunsford suggested that
a relationship exists between the relative intensities of 331, 311
and 220 XRD peaks and cation location in faujasite-type zeo-
lites.26 If I [ I [ I , sodium cations are randomly
similar to those for an impregnated VO(pic) -NaY sample
2
(
2
g \ 1.947, g \ 1.994, A \ 174.1 G, and A \ 66.1 G) (Fig.
A
M
A
M
). Note that the high melting points of the picolinic acids (e.g.
pyridine-2-carboxylic acid (picH) mp \ 412È415 K) demand
3
31
220
311
distributed within the lattice, but the cations are assumed to
be located at sites II and I@ if I [ I [ I . The XRD
331
311
220
patterns (b) and (c) in Fig. 4 are similar to the pattern (a).
Table 2 XRF and XPS data for selected NaY zeolite supported
vanadium complexes
Si/V ratio
XRF
before
Soxhlet
extraction
after
Soxhlet
extraction
sample
XPS
VO2`-NaY
50
119
18
73
17
139
15
14
14
30
14
34
14
30
È
20
300
18
È
18
VO(pic) -NaY (SB)
2
VO(pic) -NaY (IM)
2
VO(Bupic) -NaY (SB)
2
VO(Bupic) -NaY (IM)
VO(Qpic) -NaY (SB)
2
Fig. 2 EPR spectra of (a) VO2`-NaY zeolite, (b) VO(pic) -NaY (SB),
2
VO(Qpic) -NaY (IM)
È
2
and (c) VO(pic) -NaY (IM)
2
2
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
811

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Fig. 4 XRD patterns for (a) NaY, (b) impregnated VO(pic) -NaY
2
(
IM) zeolite, (c) VO2`-NaY and (d) VO(pic) -NaY (SB) zeolite
2
sumed, therefore, that the large molecules of the VO(pic)2
complex displaced the sodium cations, located in the super-
cages, from their original positions. Such intensity variations
have been found for a variety of “ship-in-a-bottleÏ systems,
while complexes located on the external surface inÑuence to a
lesser extent,14,26,28,29 Note that, in XRD patterns, the line
222, that could be ascribed to the antisymmetric character of
the charge distribution with respect to the supercage centre,28
is absent.
The location of the complexes in the zeolite cages is also
suggested from a combination of XPS and XRF analyses.7
XPS provides information about the relative concentrations of
elements in the surface ca. 40È50 Ó-thick layers of the sample
(
ca. 1% of the crystal), while XRF allows one to estimate bulk
concentrations of the sample. It was found, from comparison
of the signal intensities of the V 2p level for the encapsu-
3
@2
Fig. 3 Computer graphic view of (a) VO(pic) and (b) VO(Qpic) in
lated samples (SB) and impregnated samples (IM), that the
2
2
NaY supercage as created by Cerius2 v1.6 with open force Ðeld bur-
VO(pic) -NaY (IM) sample had a higher complex concentra-
chart 1.01-universal 1.01
2
tion at the surface than the VO(pic) -NaY (SB) sample (Table
2
2
). The XPS measurements were carried out for various
VO2`-picolinate samples, and the surface concentration ratios
(Si/V) are listed in Table 2. Obviously, the impregnated
samples (IM) possess much higher vanadium concentrations
at the external surfaces than the encapsulated samples (SB),
despite lower vanadium loading of the impregnated samples.
Thus, retention of the random location of sodium cations
within the zeolite lattice is assumed, upon impregnation of the
NaY zeolite with VO(pic) É H O and ion exchange with
2
2
VO2`. However, analysis of the XRD pattern for the
VO(pic) -NaY (SB) sample reveals that complex formation is
For example, the VO(Qpic) -NaY (SB) sample shows an eight-
2
2
accompanied by signiÐcant cation redistribution. We pre-
fold lower surface concentration (XPS) than the bulk concen-
Table 3 Binding energy (eV) for crystalline and NaY zeolite supported vanadium complexes
state
V 2p
VO(pic) É H O
VO(pic) -NaY (SB)
VO(pic) -NaY (IM)
VO2`-NaY
2
2
2
2
516.5
532.0
400.2
285.1
288.5
È
516.6
532.2
400.2
285.0
288.5
154.0
102.8
74.3
516.7
532.2
400.2
284.9
288.5
154.0
102.9
74.6
516.4
532.2
È
3@2
O 1s
N 1s
C 1s (CwC)
È
(
OwCxO)
Si 2s
154.0
103.1
74.6
Si 2p
Al 2p
È
È
Na 1s
È
1072.8
1073.1
1073.1
812
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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tration (XRF), as shown in Table 2. However, the VO(Qpic) -
NaY (IM) sample possesses surface-enriched vanadium
changes may be caused by distortion of the encapsulated com-
plexes inside the zeolite supercages or by a di†erence in coor-
dination by the wOH groups of the zeolite.
2
species, as reÑected by the much smaller Si/V ratio in XPS
than that in XPS for the SB sample and in XRF for the IM
sample. Interestingly, for all the encapsulated samples, the
surface concentration of vanadium determined by XPS was
lower than that for the VO2`-NaY precursor. This can be
explained by migration of non-complexed vanadyl ions under
the high-temperature synthesis conditions used in the Ñexible
ligand method.
UVÈVIS di†use reÑectance spectra for various VO(pic) -
2
NaY samples are depicted in Fig. 5. The VO(pic) -NaY (SB)
2
sample (b) showed bands at 740, 560 and 370 nm assigned to
the electronic transitions b ] e*, b ] b *, and b ] 1a *,
2
2
1
2
1
respectively, which resembled the bands for VO2`-picolinate
complex in solution (730, 560 and 365 nm).31 The band at ca.
270 nm is attributed to ligand p ] p* transition overlapped
with LMCT of O to V4`. Note that the encapsulated
In addition to the information about location of the com-
plexes, some preliminary information about the states of both
zeolite support and complexes was obtained from the XPS
data. Table 3 lists the binding energies for V 2p3@2 , O 1s, N 1s,
C 1s, Si 2s, Si 2p, Al 2p, and Na 1s in various vanadium
samples. The vanadium samples exhibited similar binding
energies. At least two di†erent kinds of carbon atoms (CwC
VO(pic) -NaY samples dried at room temperature in vacuum
(c) and treated at 413 K for 10 h in a Ñow of He (b) showed
2
similar spectra. However, there is a distinct di†erence in the
spectra (d) and (e) between the impregnated VO(pic) -NaY
2
samples dried at room temperature and treated at 413 K.
Impregnated VO(pic) -NaY sample, after vacuum drying,
2
2
85.1 eV and OwCxO 288.5 eV) and the only kind of nitro-
exhibited bands at 670, 530(sh) and 270 nm [Fig. 5 (e)], while
gen atom (400.2 eV) were observed in the supported VO(pic) -
NaY samples (SB and IM), similar to the result for
the VO(pic) -NaY (IM) sample after heat treatment in a Ñow
2
2
of He showed bands at 745, 550 and 273 nm [Fig. 5(d)]. The
VO(pic) É H O. Thus the chemical states of vanadium atom
and the picolinate ligand in the encapsulated samples (SB) are
not di†erent from that of the unsupported complex.
di†erence in the behaviour of the impregnated and encapsu-
lated samples provides additional evidence about the location
2
2
of the VO(pic) complexes in the samples (SB and IM). The
2
post-synthesis heat treatment aimed to remove adsorbed pyri-
dine and to decompose the adduct of vanadium picolinates
with pyridine which might be formed during Soxhlet extrac-
tion or the impregnation procedure. Indeed, the di†use reÑec-
tance spectra for the impregnated sample changed from Fig. 5
(e) to (d) on heat treatment, which reÑects the decomposition
Structure of complexes
The structure of encapsulated complexes formed by inter-
action of ligands with intrazeolitic transition metal ions is
generally a†ected by speciÐc conditions inside the zeolite
framework, such as high electrostatic Ðeld and spatial
restriction. In the present study the NaY encapsulated vana-
dium picolinate complex, as a representative sample among
the three SB samples synthesized, was characterized by means
of EPR, EXAFS, XANES, FTIR, and UVÈVIS spectroscopies.
FTIR spectra of the encapsulated VO(pic) -NaY (SB),
2
VO(Bupic) -NaY (SB), and VO(Qpic) -NaY (SB) samples
2
2
exhibited a strong band at ca. 1380È1390 cm~1 and doublet
bands at 1440È1490 cm~1, which are characteristic of pico-
linate species.30 The band at ca. 1530 cm~1 for the VO(pic) -
2
NaY (SB) sample before Soxhlet extraction, which is attribut-
able to picolinic acid, almost disappeared after Soxhlet extrac-
tion for 60 h. The FTIR data for a series of the picolinates are
summarized in Table 4. No additional lines that could be
ascribed to coordination modes di†erent from bidentate were
observed. The peak positions for VO(pic) -NaY (SB) are
2
similar to those for crystalline VO(pic) complex and
2
VO(pic) -NaY (IM), which can be considered as indirect evi-
2
dence for the bis-picolinate structure. On the other hand, the
peaks positions for the encapsulated vanadium complexes
with substituted picolinic acids shifted to a larger extent.
Thus, the peak positions are only slightly inÑuenced by loca-
tion of the complexes (SB and IM), but are changed by the
nature of the picolinic ligand. This can be explained by
changes in V-site geometry imposed by trapping of the vana-
dium picolinate complexes in the zeolite supercages. Such
Fig. 5 Di†use reÑectance spectra of VO(pic) -NaY samples
2
(
(
referenced to NaY): (a) physical mixture of NaY and VO(pic) É H O,
2
2
b) VO(pic) -NaY (SB) calcined in a Ñow of He (413 K for 10 h), (c)
2
VO(pic) -NaY (SB) dried in vacuum (298 K for 4 h), (d) VO(pic) -NaY
(IM) calcined in a Ñow of He (413 K for 10 h) and (e) VO(pic) -NaY
2
2
2
(IM) dried in vacuum (298 K for 4 h)
Table 4 UVÈVIS di†use reÑectance and FTIR spectral data for characterization of vanadium species
VO(pic) - VO(pic) - VO(pic) in VO(Bupic) -
VO(Qpic) -
2
2
2
2
2
NaY (SB)
NaY (SB)
NaY (IM)
VO(pic) É H O
solution
NaY (SB)
2
2
FTIR
l/cm~1)
1573
1480
1570
1478
1449
1389
1297
1262
745
1568
1480
1450
1389
1297
1264
720
560
370
266
È
1567
1488
1466
1379
1303
1267
755
(
1488
1468
1381
1304
1263
1
1
1
454
389
299
È
È
UVÈVIS
di†use reÑectance
740
560
370
730
560
365
550
273
È
320
(j/nm)
275
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
813

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of the VO(pic) (py) adduct. On the other hand, the same treat-
ment did not lead to any visible change in the spectra for the
VO(pic) -NaY (SB) was related to the intensity change in the
second shell. The second peak developed more with the refer-
2
2
encapsulated samples [Fig. 5(c) and (b)]. It is, therefore, pos-
ence complexes VO(pic) É H O and VO(pic) (py) which have
2
2
2
sible to suppose that the formation of VO(pic) (py) is sup-
coordination of H O and pyridine in the axial position [Fig. 7
2
2
pressed by spatial restriction, since the VO(pic)
are located inside the zeolite supercage.
2 complexes
(c) and (d)]. The encapsulated VO2`-picolinate complex may
interact with the wall of the zeolite supercage, probably
through a zeolite hydroxy group which coordinates to an
axial position of the vanadium complex.
Fig. 6 shows vanadium K-edge XANES spectra of the
VO(pic) -NaY (IM) and VO(pic) -NaY (SB) samples calcined
2
2
in a Ñow of He (413 K for 10 h) as well as of reference com-
The most useful information about the VO2` coordination
sphere can be obtained from an EPR study.33,34 To elucidate
the change in coordination of the vanadium atom, the EPR
spectra were recorded for the ion-exchanged VO2`-NaY pre-
cursor as well as for the zeolite-supported vanadium pico-
linate complexes (Fig. 2 and Table 1). All the samples
exhibited hyperÐne splitting of two sets of eight lines, which
reÑects an unpaired electron density on the vanadium ion and
an axial symmetry of its ligand Ðeld, and can be treated in
terms of parallel and perpendicular components of the g and
A values.
pounds VO(pic) É H O and VO(pic) (py). The pre-edge and
2
2
2
near-edge features were almost the same in position and inten-
sity for the impregnated and encapsulated VO(pic) -NaY
2
samples and similar to those of VO(pic) É H O. The pre-edge
2
2
peak position was higher by ca. 4.0 eV than that for vanadium
foil (not shown). These results indicate an oxidation state of
V4`. Comparison of the XANES spectra of the VO(pic) -NaY
2
(
SB) sample with the corresponding spectra for well deÐned
vanadium compounds reveals square pyramidal or distorted
octahedral coordination around the vanadium site for the
encapsulated SB sample.32
The Fourier transforms of the EXAFS data for the vana-
dium picolinate species are shown in Fig. 7. The di†erence in
The VO(pic) -NaY (SB) sample showed disappearance of
2
the VO2`-NaY zeolite signal (g \ 1.911, g \ 1.996, A \
A
M
A
205.8 G and A \ 85.4 G) and appearance of a new signal
M
the Fourier transforms between VO(pic) -NaY (IM) and
with g \ 1.945, g \ 1.996, A \ 175.3 G and A \ 67.1 G.
2
A
M
A
M
These g and A values are correlated with distorted octahedral
vanadyl species in the VO(pic) -NaY (SB) sample. On the
2
other hand, the VO(pic) -NaY (IM) sample showed super-
2
position of two EPR signals with di†erent parameters. One of
them is a broad signal with g B 1.95, indicating the formation
of crystalline VO(pic) on the external surface of the zeolite.35
2
The other is an anisotropic 16-line signal similar to that for
the VO(pic) -NaY (SB) sample. The similarity of the EPR
2
parameters for the VO(pic) -NaY (SB) and VO(pic) -NaY
(
2
2
IM) samples suggests the absence of signiÐcant structural dis-
tortion of the zeolite-encapsulated VO(pic) complex. Upon
2
complexation with picolinic acids, a decrease in A values was
observed with a simultaneous increase in g . This is inter-
A
preted in terms of a decrease in the VxO bond covalency
(
*g [ 0) while covalency of the V-ligand bond increases
A
(*A \ 0).20,21
Generally, introduction of substituents on the ligand per-
iphery does not signiÐcantly a†ect the complex geometry and,
in consequence, the EPR parameters. On the other hand, the
correlation between donor number of a solvent and A and g
values has been reported.36 SigniÐcant deviations of the EPR
parameters were observed for VO(Bupic) -NaY (SB) and
Fig. 6 Vanadium K-edges XANES of (a) VO(pic) -NaY (IM), (b)
2
2
VO(Qpic) -NaY (SB) in comparison with those for VO(pic) -
VO(pic) -NaY (SB), (c) VO(pic) É H O, and (d) VO(pic) (py)
2
2
2
2
2
2
NaY (SB) (Table 1). The di†erence is reasonably ascribed to a
di†erence in sixth-site coordination of the encapsulated vana-
dium picolinates. Actually, coordination with the zeolite OH
group may be weakened due to spatial restriction imposed by
encapsulation. A computer modelling study on the location of
the vanadium picolinates in the NaY supercage indicated that
VO(Qpic) preferentially occupies the centre of the supercage,
2
whereas the location of VO(pic) in the supercage is less
2
restricted and the complex can come close to the wall of the
supercage (Fig. 3).
Stability of the VO(pic) -NaY (SB) complex and gas-phase
2
oxidation
Thermal decomposition of the vanadyl bis-picolinate complex
has been demonstrated to occur through dimeric species.37
Hence, we expected increased stability for the encapsulated
complexes dispersed in the supercages in comparison with the
impregnated sample, which could allow us to use such
materials as catalysts under oxidation conditions.
Under non-oxidative conditions, thermal decomposition of
pyridine-2-carboxylate proceeds via decarboxylation to yield
CO2 and the corresponding aromatic amine. Temperature-
Fig. 7 k3-weighted Fourier transforms of EXAFS data for: (a)
VO(pic) -NaY (IM), (b) VO(pic) -NaY (SB), (c) VO(pic) É H O, and
2
2
2
2
(
d) VO(pic) (py)
2
814
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Fig. 8 k3-weighted vanadium K-edge EXAFS and Fourier-transform
data for encapsulated VO(pic) -NaY (SB) samples: (ÈÈÈ) as synthe-
2
sized; (É É É É É É É) calcined in air at 523 K for 3 h
Fig. 10 Product accumulation in catalytic oxidation with encapsu-
lated VO(pic) -NaY (SB) catalyst. Reaction conditions: acetonitrile,
2
2
0 ml; substrate (cyclohexane, isopropyl alcohol, benzene), 6 mmol;
H O É urea adduct, 20 mmol; VO(pic) -NaY (SB), 0.04 g (1 ] 10~5
programmed decomposition (TPD) experiments were carried
2
2
2
mol), 313 K.
out in a Ñow of He at a heating rate of 2 K min~1. The TPD
spectra for CO evolution showed maxima at 633, 633 and
2
(IM) sample through dimeric species that are hardly formed
when the complex is encapsulated.
5
93 K for the encapsulated VO(pic) , VO(Bupic) , and
2
2
VO(Qpic) complexes, respectively. These values were similar
to those for the impregnated samples. However, under oxida-
2
Thus, the encapsulated VO(pic) ,
VO(Bupic)2
and
2
VO(Qpic) complexes were found to be stable up to 573 K in
tive conditions, the stability of the complexes was found to
depend strongly on the complex location. Fig. 8 shows
2
air. However, despite the relatively high stability of the encap-
sulated complexes, attempts to use these materials as catalysts
for gas-phase oxidation reactions such as the selective oxida-
tion of light alkanes and alkenes, below 573 K were unsuc-
cessful.
EXAFS data for VO(pic) -NaY (SB) before and after calcina-
2
tion in oxygen at 523 K for 3 h. The similarity in the Fourier
transforms and the raw EXAFS data for both samples indi-
cates the retention of the bis-picolinate structure of the
complex. In contrast, the VO(pic) -NaY (IM) sample showed
deÐnite changes in EXAFS oscillations and Fourier trans-
2
Catalytic performance for heterogeneous liquid-phase oxidation
forms (Fig. 9). This might have been caused by oxidative
Catalysis of vanadium peroxo complexes containing pyri-
dinecarboxylato ligands in solutions has been studied by
Mimoun et al.14 and ShulÏpin et al.38,39 It was observed that
these compounds can e†ectively catalyse hydroxylation of
aromatic and aliphatic hydrocarbons as well as the oxidation
destruction of the VO(pic) complex to vanadium(V) oxide
2
species. This suggestion agrees with the absence of any band
in the FTIR spectra characteristic of picolinato ligand as well
as the complete disappearance of the V4` signal in the EPR
spectra. The di†erence in the behaviour of the impregnated
and encapsulated materials seems to be due to easier destruc-
complex in the case of VO(pic) -NaY
of alcohols. We found that the encapsulated VO(pic) complex
2
detaches from the zeolite when treated with 30% H O solu-
2
2
tion of the VO(pic)2
tion. Hence, in the present study, the preliminary catalytic oxi-
dations were carried out under anhydrous conditions to
suppress the leaching. It was found that cyclohexane, isopro-
pyl alcohol and benzene were catalytically oxidized by
2
VO(pic) -NaY (SB) in the presence of urea hydrogen peroxide
2
adduct in acetonitrile to cyclohexyl hydroperoxide, acetone
and phenol, respectively. The reaction proÐles are shown in
Fig. 10, and demonstrate that the encapsulated VO(pic) -NaY
2
(
SB) sample is an e†ective catalyst for the selective oxidation
of hydrocarbons. Details at the catalytic performance will be
reported separately.40
Conclusions
Vanadium picolinate complexes VO(pic) -NaY (SB),
2
VO(Bupic) -NaY (SB), and VO(Qpic) -NaY (SB) encapsulated
2
2
inside NaY zeolite supercages were synthesized by the Ñexible
ligand method that involved treatment of a VO2`-NaY
zeolite with molten picolinic acids followed by Soxhlet extrac-
tion to remove excess ligands and complexes located on exter-
nal surfaces of the zeolite. Careful characterization of the
encapsulated species by EXAFS, XANES, FTIR, EPR and
UVÈVIS spectroscopies proved the formation of bis-picolinate
complexes with distorted octahedral coordination around
vanadium. The coordination of zeolite OH groups to the
Fig. 9 k3-weighted vanadium K-edge EXAFS and Fourier-transform
data for impregnated VO(pic) -NaY (IM) samples: (ÈÈÈ) as synthe-
2
sized, (É É É É É É É) calcined in air at 523 K for 3 h
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
815

View Article Online
vanadium complexes was used to explain the di†erence in
spectroscopic properties of the impregnated and encapsulated
samples as well as the signiÐcant size dependence of the EPR
parameters for the encapsulated samples. It was suggested by
XRD, as well as by comparative XPSÈXRF, studies that the
complexes were encapsulated in the zeolite supercages. The
18 K. Maruszewski, D. P. Strommen, K. Handrich and R. Kincaid,
Inorg.Chem., 1991, 30, 4579.
1
2
9
0
R. L. Dutta and S. Ghosh, J. Indian Chem. Soc., 1967, 44, 271.
G. Martini, M. F. Ottaviani and G. L. Seravalli, J. Phys. Chem.,
1
975, 79, 1716.
21 H. J. Stoklosa, J. R. Wasson and B. J. McCormick, Inorg. Chem.,
1974, 13, 592.
2
2
2
2
2
2
3
4
5
6
Handbook of X-ray Photoelectron Spectroscopy, JEOL, Tokyo,
encapsulated VO(pic) complex retained solution-like activity
2
1
991, p. 12.
in the selective oxidation of hydrocarbons.
C. A. Trujillo, U. N. Uribe, P. P. Knops-Gerrits, L. A. A. Oviedo
and P. A. Jacobs, J. Catal., 1997, 168, 1.
C. Kirschhock and H. Fuess, J Chem. Soc., Faraday T rans., 1995,
This work was supported by CREST (Core Research for Evol-
utional Science and Technology) of JST (Japan Science and
Technology Corporation).
9
1, 1813.
Z. A. Kaszkur, R. H. Jones, D. Waller, C. R. A. Catlow and J. M.
Thomas, J. Phys. Chem., 1993, 97, 426.
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816
J. Chem. Soc., Faraday T rans., 1998, V ol. 94