Dioxovanadium(V) complexes of Schiff and tetrahydro-Schiff bases encapsulated in zeolite-Y for the aerobic oxidation of styrene

Article abstract of DOI:10.1134/S0023158417030053

A series of dioxovanadium(V) complexes of Schiff and tetrahydro-Schiff bases were encapsulated into the supercages of zeolite-Y and were characterized by X-ray diffraction, SEM, N₂ adsorption/desorption, FT-IR, UV-vis spectroscopy, ICPAES, pair distribution function (PDF) and X-ray absorption near edge structure (XANES) measurements. The encapsulation is achieved by a flexible ligand method in which the transition metal cations were first ion-exchanged into zeolite-Y and then complexed with ligands. The dioxovanadium-exchanged zeolite, dioxovanadium complexes encapsulated in zeolite-Y plus non-encapsulated homogeneous counterparts were all screened as catalysts for the aerobic oxidation of styrene under mild conditions. It was found that the encapsulated complexes showed better activity than their respective nonencapsulated counterparts in most cases. All encapsulated dioxovanadium tetrahydro-Schiff base complexes showed much higher activity in aerobic oxidation of styrene than their corresponding Schiff base complexes.

Full text of DOI:10.1134/S0023158417030053

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 3, pp. 290–299. © Pleiades Publishing, Ltd., 2017.
Published in Russian in Kinetika i Kataliz, 2017, Vol. 58, No. 3, pp. 304–314.
Dioxovanadium(V) Complexes of Schiff and Tetrahydro-Schiff Bases
Encapsulated in Zeolite-Y for the Aerobic Oxidation of Styrene1
Zhongzhen Dinga and Ying Yangb, *
aSchool of Material Science and Engineering, Tianjin Polytechnic University, Xiqing Distrct, Tianjin 300387, China
bState Key Laboratory of Heavy Oil Processing, China University of Petroleum, Changping, Beijing 102249, China
*e-mail: catalyticscience@163.com
Received September 24, 2016
Abstract⎯A series of dioxovanadium(V) complexes of Schiff and tetrahydro-Schiff bases were encapsulated
into the supercages of zeolite-Y and were characterized by X-ray diffraction, SEM, N2 adsorption/desorp-
tion, FT-IR, UV-vis spectroscopy, ICPAES, pair distribution function (PDF) and X-ray absorption near
edge structure (XANES) measurements. The encapsulation is achieved by a flexible ligand method in which
the transition metal cations were first ion-exchanged into zeolite-Y and then complexed with ligands. The
dioxovanadium-exchanged zeolite, dioxovanadium complexes encapsulated in zeolite-Y plus non-encapsu-
lated homogeneous counterparts were all screened as catalysts for the aerobic oxidation of styrene under mild
conditions. It was found that the encapsulated complexes showed better activity than their respective non-
encapsulated counterparts in most cases. All encapsulated dioxovanadium tetrahydro-Schiff base complexes
showed much higher activity in aerobic oxidation of styrene than their corresponding Schiff base complexes.
Keywords: dioxovanadium complexes, Schiff and tetrahydro-Schiff bases, zeolite-Y, XANES, catalytic activ-
ity, aerobic oxidation, styrene
DOI: 10.1134/S0023158417030053
1. INTRODUCTION
tem desirable for easy reactant accessibility and prod-
uct diffusion [10]. Metalloporphyrins and metalloph-
thalocyanines are usually encapsulated into zeolite-Y
for the oxidation of alkanes, alkenes, alcohols and so
on. For example, Balkus and coworkers synthesized a
NaX encapsulated ruthenium perfluorophthalocya-
nine complex (RuF16Pc-X) and it proved to be effi-
cient for the oxidation of cyclohexane with tert-butyl
hydroperoxide as the oxidant [11].
Schiff base transition metal complexes have been
extensively studied because of their potential use as cat-
alysts in a wide range of oxidation reactions [12–15].
However, the number of encapsulated Schiff base
complexes is very limited owing to the presence of rel-
atively rigid CH=N bond. In contrast, hydrogenation
of CH=N to CH2–NH as expected, would increase N
basicity and make the conformation of the metal com-
plex more flexible and hence resulting in active sites
without severe blockage of the channels in zeolitic
matrix [16]. Most importantly, the hydrogenated
ligands are more robust towards leaching than their
Schiff base counterparts, due to immunity from the
CH=N break in the case of acidic conditions, as indi-
rectly revealed by the pioneering works of Luo [17],
Baleizão [18] and Sheldon [19]. Recently, the catalytic
performances of supported metal tetrahydro-salen
complexes have drawn a lot of attention due to their
The oxidation of styrene is of great interest because
styrene oxide and benzaldehyde are important and
versatile synthetic intermediates in chemical industry
[1, 2]. Currently, a wide range of homogeneous metal
complexes for styrene oxidation exist, but most of
them suffer from severe deactivation due to easy for-
mation of dimeric peroxo- and μ-oxo species. Heter-
ogenization of these homogeneous catalysts onto solid
supports has been sought to isolate metal complexes to
prevent their dimerization, increase their ruggedness
and separability [3–8]. In this respect, encapsulation
of transition metal complexes into zeolites gained
much interest in the last decade, since not only the
encapsulation strategy is found to be convenient and
advantageous, because the metal complex, once
formed inside pore cavity, is not easy to diffuse out and
to enter the liquid phase during catalytic reaction, but
also this process can give rise to the materials with
both homogeneous and heterogeneous catalysis char-
acters [9]. Faujasite zeolite including zeolite X and Y,
is frequently chosen as support to encapsulate metal
complexes because of its huge supercage (13 Å) with
smaller pore opening (ca. 8 Å) suitable to accommo-
date metal complexes and three-dimensional pore sys-
1
The article is published in the original.
290

DIOXOVANADIUM(V) COMPLEXES OF SCHIFF
capability to easily activate O2 under very mild condi-
291
tions. Cu(II), Co(II) and Fe(III) tetrahydro-salen
complexes encapsulated in zeolite-Y have been
reported to show improved catalytic activity in oxida-
tion of cycloalkanes with H2O2 in comparison with the
corresponding metal salen complexes [16]. Sakthivel
and coworkers [20] grafted a Cu(II) tetrahydro-salen
complex onto the iodosilane modified surfaces of
MCM-41 and MCM-48 by a direct immobilization
method, and the catalysts could be reused in epoxida-
tion of cyclooctene without significant loss of reactiv-
ity. More recently, our group tethered Cu(II) and
Co(II) tetrahydro-salen complexes onto amino-func-
tionalized SBA-15, and these catalysts displayed
enhanced activity for the aerobic oxidation of styrene
compared to the their respective Cu(II) and Co(II)
salen complexes [21].
f
e
d
c
b
a
15
10
5
0
δ, ppm
Previously, we reported an oxovanadium(IV)-
based hybrid heterogeneous catalyst system, in which
a functionalized oxovanadium(IV) Schiff base com-
plexes of type [VO(N2O2)] bearing chloromethyl
groups were synthesized, directly anchored onto
amino-modified SBA-15 materials [22]. This material
was highly active, selective to epoxide and truly het-
erogeneous for the liquid-phase oxidation of styrene
with air as the oxidant.
As a continuation of this line of research, we firstly
report here the encapsulation of dioxovanadium(V)
complexes of tetrahydro-Schiff bases as well as Schiff
bases into zeolite-Y by a flexible ligand method. These
encapsulated metal complexes are fully characterized
and screened as catalysts for the aerobic oxidation of
styrene in presence of a sacrificial co-reductant isobu-
tyraldehyde.
1
Fig. 1. H NMR spectra of (a) SalenH , (b) [H ]SalenH ,
2
4
2
(c) SalhexenH , (d) [H ]SalhexenH , (e) SalphenH and
(f) [H ]SalphenH .
2
4
2
2
4
2
1
SalhexenH2 and SalphenH2. The H NMR spectra of
SalenH2, SalhexenH2 and SalphenH2 are shown in
Fig. 1. H NMR (CDCl3), δ, ppm: SalenH2 – 13.20
1
(2H, s, OH), 8.35 (2H, s, CH=N), 7.30–6.82 (8H, m,
Ar–H), 3.91 (4H, s, N–CH2CH2–N) (Fig. 1a); Sal-
hexenH2 – 13.68 (2H, s, OH), 8.35 (2H, s, CH=N),
7.29–6.84 (8H, m, Ar–H), 3.62 (4H, t, N–
CH2CH2CH2CH2CH2CH2–N), 1.76–1.59 (4H, m,
N–CH2CH2CH2CH2CH2CH2–N), 1.38–1.22 (4H,
m, N–CH2CH2CH2CH2CH2CH2–N) (Fig. 1c); Sal-
phenH2 – 13.06 (2H, s, OH), 8.65 (2H, s, CH=N),
7.78–6.87 (12H, m, Ar–H) (Fig. 1e).
Schiff base ligand can be easily converted to tetra-
hydro-Schiff base ligand by the hydrogenation of
CH=N to CH2–NH in presence of NaBH4 in MeOH.
Tetrahydro-Schiff base ligands were prepared as fol-
lows: 0.01 mol of SalenH2, SalhexenH2 or SalphenH2
2. EXPERIMENTAL
2.1. Materials
The following chemicals were commercially avail-
able and were used as received: NaOH, ethylenedi-
amine, 1,6-hexylenediamine, o-phenylenediamine, was dissolved in 60 mL of methanol, followed by the
styrene (98%), VOSO4, VO(acac)2 and all organic sol- addition of 0.011 mol of NaBH4 at ambient tempera-
vents are A.R. grade. Salicylaldehyde (99%) is ture. After stirring for 2 h, the solvent was removed
obtained from Acros Organics, New Jersey, USA.
under reduced pressure. The solid product was further
washed with distilled water and recrystallized from
ethanol, and the obtained materials were denoted as
[H4]SalenH2, [H4]SalhexenH2 and [H4]SalphenH2,
2.2. Synthetic Procedures
1
2.2.1. Synthesis of neat dioxovanadium complexes.
First, Schiff base ligands were prepared and followed
the procedures: 30 mmol of ethylenediamine, 1,6-hex-
ylenediamine or o-phenylenediamine was dissolved in
20 mL of ethanol. The obtained solution was then
slowly added to a solution composed of 60 mmol of
salicyladehyde and 40 mL of ethanol under stirring
conditions. The resultant solution was further refluxed
for 1 h, resulting in the precipitation of a yellow or
orange solid, which was filtered and recrystallized
respectively. The H NMR spectra of [H4]SalenH2,
[H4]SalhexenH2 and [H4]SalphenH2 are also shown in
1
Fig. 1. H NMR (CDCl3), δ, ppm: [H4]SalenH2 –
7.25–6.78 (8H, m, Ar–H), 3.99 (4H, s, N–CH2CH2–
N), 2.84 (4H, s, CH2–NH) (Fig. 1b); [H4]SalhexenH2 –
7.18–6.69 (8H, m, Ar–H), 3.99 (4H, t, N–
CH2CH2CH2CH2CH2CH2–N), 1.56–1.49 (4H, m,
N–CH2CH2CH2CH2CH2CH2–N), 1.39–1.34 (4H, m,
N–CH2CH2CH2CH2CH2CH2–N), 2.69 (4H, s,
from ethanol, and denoted respectively as SalenH2, CH2–NH) (Fig. 1d); (DMSO-d6), [H4]SalphenH2 –
KINETICS AND CATALYSIS Vol. 58 No. 3 2017

292
ZHONGZHEN DING, YING YANG
7.58–6.61 (12H, m, Ar–H), 2.55 (4H, s, CH2–NH) remove uncoordinated metal ions, followed by wash-
(Fig. 1f).
The procedures for the preparation of dioxovana-
ing with deionized water until no Cl– anions could be
detected with AgNO3 aqueous solution. The resulting
VO2-L-Y samples were dried at 150°C for several
hours to constant weight. This procedure brought
about brilliant color change, suggesting that the prop-
erties of neat compounds have been dramatically
changed upon encapsulation.
dium(V) complexes of SalenH2, SalhexenH2, Sal-
phenH2 and [H4]SalenH2 were as follows: 2.0 mmol
of Schiff base or tetrahydro-Schiff base ligand was
dissolved in 20 mL of methanol, which was followed
by the drop-wise addition of a solution of 2.0 mmol
VO(acac)2 in 10 mL of methanol at 65°C. The resul-
tant solution was stirred and refluxed for 2 h. After
cooling, the solid product was separated by filtration,
and was further suspended in methanol and oxidized
by passing air while stirring at room temperature for
24 h. The solid obtained was respectively denoted as
VO2-Salen, VO2-Salhexen, VO2-Salphen and VO2-
[H4]SalenH2. The procedures for the preparation of
dioxovanadium(V) complexes of [H4]SalhexenH2
and [H4]SalphenH2 were as follows: 1.0 mmol of tet-
rahydro-Schiff base was mixed with 1.0 mmol of
VO(acac)2 in 6 mL of CH2Cl2, and the mixture was
stirred at room temperature for 2 h. The resulting
material was obtained by filtration, and further oxi-
dized and denoted as VO2-[H4]Salhexen and VO2-
[H4]Salphen.
2.3. Characterization
Powder XRD was collected with a Rigaku X-ray
diffractometer with nickel filtered CuKα radiation
(λ = 1.5418 Å). The samples were scanned in the
range 2θ = 5–40° and in steps of 4°/min. N2 adsorp-
tion/desorption isotherms were recorded at –196°C
with a Micromeritics ASAP 2020. Before measure-
ments, the samples were outgassed at 120°C for 12 h.
BET surface areas (SBET) was calculated from adsorp-
tion branch in the relative pressure range from 0.05 to
0.30. The external surface area and pore volume were
calculated by t-plot method. The infrared spectra (IR)
of samples were recorded in KBr disks using a NICO-
LET impact 410 spectrometer (Melles Griot). UV-vis
spectra were recorded on a Perkin Elmer UV-vis spec-
trophotometer Lambda 20 using barium sulfate as the
standard in the range 200–800 nm. 1H NMR experi-
ments were carried out in sealed NMR tubes on a
Varian Mercury-300 NMR spectrometer. Metal con-
tent was estimated by inductively coupled plasma
atomic emission spectroscopy (ICPAES) analysis
conducted on a Perkin Elmer emission spectrometer.
Scanning electron microscopy (SEM) was performed
on a JSM-6301F scanning microscope FEI instrument.
The samples were coated with gold using a sputter
coater. Total scattering patterns (Bragg and diffuse scat-
tering data) were collected at the high energy beamline
11-ID-C at the Advanced Photo Source (APS) at
Argonne National Laboratory, IL, USA. A wavelength
of 0.10798 Å (115 keV) and 0.6 mm × 0.6 mm beam size
were used to obtain two-dimensional (2D) diffraction
patterns in transmission geometry using a Perkin-
Elmer large area detector placed at 360 mm from the
sample. Samples were packed into the Kapton capil-
laries, and the program Fit2D with a CeO2 calibration
standard was used to integrate the data. PDF patterns
(G(r)) were obtained with the PDFGetX2 software
and the data range used was up to 43 Å–1. V K-edge
XAFS spectra for the vanadium samples and a mixture
of a reference compound and boron nitride which
2.2.2. Preparation of dioxovanadium-exchanged
zeolite, VO2-Y. Firstly, Na-Y was prepared according
to the procedure reported with slight modification
[23]. In a typical synthesis, 1.6 g of sodium hydroxide
and 2.5 g of sodium aluminate (41 wt %) were dispersed
in 25 mL of deionized water under constant stirring, and
then 18 mL of silica sol (25 wt %) was added. The mix-
ture with a molar composition of 13.2 SiO2: 1.0 Al2O3:
16.4 Na2O: 354 H2O was stirred at room temperature
for 1 h to form gel slurry, and then was hydrothermally
crystallized at 100°C for 24 h. Subsequently, the resul-
tant precipitate was separated from the mother liquor
by filtration, washed with deionized water and dried at
150°C for 12 h.
Dioxovanadium-exchanged zeolite-Y (VO2-Y) was
prepared as follows: an amount of 8.0 g Na-Y zeolite
was suspended in 240 mL of distilled water containing
9.48 mmol of VOSO4. The mixture was stirred at 90°C
for 24 h. The obtained materials were filtered, further
oxidized by the procedure above, and washed with hot
distilled water till the filtrate was free from any metal
ion content and dried for 12 h at 150°C in air.
2.2.3. Preparation of encapsulated dioxovanadium(V)
complexes, VO2-L-Y (L = ligand). This process was per-
formed with excess ligands (nligand/nmetal = 3), and
allowed the insertion of ligand in the cavity of the zeo- were pressed into self-supporting disks were measured
lite followed by its complexation with metal ions. The at the 10-BM-B of Advanced Photo Source (APS) at
complexation was carried out in MeOH at 80°C for Argonne National Laboratory. The X-ray absorption
24 h. Uncomplexed ligands and the complexes near edge structures (XANES) presented are fluores-
adsorbed on the exterior surface were removed by full cence data and are average of at least two data sets
Soxhlet-extraction with MeOH till the eluate became from the same sample that were processed using stan-
colorless. The resulting sample was ion-exchanged dard pre-edge background subtraction and edge step
with aqueous 0.01 M NaCl solution at 80°C for 8 h to normalization procedures.
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DIOXOVANADIUM(V) COMPLEXES OF SCHIFF
(b) (c)
293
(а)
3 μm
3 μm
3 μm
e
d
c
b
a
5
10
15
20
25
30
35
40
2θ, deg
Fig. 2. XRD patterns of (a) Na-Y, (b) VO -Y, (c) VO -Salhexen-Y, (d) VO -[H ]Salhexen-Y and (e) VO -Salhexen, and SEM
2
2
2
4
2
images (inset) of (a) VO -Y, (b) VO -Salhexen-Y and (c) VO -[H ]Salhexen-Y.
2
2
2
4
2.4. Catalytic Oxidation
zeolite-Y were recorded at 2θ values between 5° and
40° and are reproduced in Fig. 2. Compared to Na-Y,
no remarkable difference, but only a small reduction in
the intensity of peaks is observed, indicating that the
zeolite framework has not undergone any significant
structural change during the metal exchange and com-
plex encapsulation processes [24], and thus the crystal-
linity of the zeolite is well preserved. No new peaks
assigned to neat dioxovanadium complexes (Fig. 2e) can
be detected in the encapsulated zeolites (Figs. 2c, 2d),
indicating that the metal complexes encapsulated are
highly dispersed in the zeolitic host [25]. The XRD
patterns of other dioxovanadium complexes encapsu-
lated in zeolite-Y are not shown here because their dif-
fractograms are virtually the same and similar to that
of VO2-Salhexen-Y.
SEM micrographs of the modified zeolites exempli-
fied by VO2-Y, VO2-Salhexen-Y and VO2-[H4]Sal-
hexen-Y are depicted in the inset of Fig. 2 and show reg-
ular 2 μm cubic particles similarly to the parent mate-
rial. The SEM images of VO2-Salhexen-Y (Fig. 2b,
inset) and VO2-[H4]Salhexen-Y (Fig. 2c, inset) show
the presence of well-defined zeolite crystals free from
any shadow of metal complexes present on their exter-
nal surface, indicating that the crystallinity of parent
zeolites was kept upon complex entrapment.
The oxidation of styrene with air was carried out in a
batch reactor. In a typical run, 10 mmol of styrene along
with 10 mL of CH3CN, 25 mmol of isobutyraldehyde
and certain amount of catalyst were added into a 100 mL
two-necked flask equipped with a condenser and an air
pump. The reaction was started by bubbling the dry air
with a stable flow rate of 80 mL min–1 controlled by a
flowmeter into the bottom of reactor at reaction tem-
perature. After the reaction was finished, the catalyst
was filtered, washed with CH3CN, dried at 100°C over-
night and reused directly without further purification. A
leaching test was carried out by filtering the catalyst at
the reaction temperature (80°C) after 1.5 h. At this time,
half the volume was filtered and the resulting clear solu-
tion was allowed to react. The percentage of leaching
was estimated by comparing the time-conversion plot of
the twin reactions with and without solid. The liquid
organic products were quantified by using a gas chroma-
tography (Shimadzu, GC-8A) fitted with FID detector
and HP-5 capillary column. The liquid organic prod-
ucts were identified by comparison with authentic sam-
ples and verified by GCMS coupling.
3. RESULTS AND DISCUSSION
3.1. Structural, Morphological
and Textural Properties of Materials
The nitrogen adsorption isotherms for the parent
(Na-Y) and three representative modified zeolites are
typical of microporous materials (not shown). Micro-
pore volumes and external surface areas were calcu-
The X-ray powder diffraction patterns of Na-Y,
VO2-Y and dioxovanadium complexes encapsulated in
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294
ZHONGZHEN DING, YING YANG
Table 1. Textual properties of various materials prepared
Microporous volume*, cm3/g
External surface are, m2/g
Materials
Na-Y
VO2-Y
0.320
0.258
0.170
0.210
17.7
38.4
92.7
85.4
VO2-Salhexen-Y
VO2-[H4]Salhexen-Y
* From the nitrogen adsorption isotherm at –196°C, calculated by the t-method.
lated by fitting the adsorption data in the correspond- compared to Na-Y, all encapsulated vanadium com-
ing theories; the values are given in Table 1. As plexes show nearly unchanged peaks in the region
expected, the micropore volumes of VO2-Y and metal from 1.5 to 5 Å except for a small peak ca. 1.94 Å, and
general intensity decrease consistent with a loss of long
range order beyond 6 Å. The first strong peaks in the
PDFs at ca. 1.63 Å, and the following weaker peaks ca.
2.64 Å are observed in all zeolite based materials, due
to the overlapping T–O bonds within the (Si, Al)O4 tet-
complex encapsulated zeolite-Y show an apparent
reduction when compared with their respective parent
materials. The lowering of the pore volumes indicate
the presence of dioxovanadium complexes within the
cavities of the zeolites, not on the external surface.
Similar observation was also found for inclusion of rahedra (Т) [28], indicating the retention of the basic
Mn(III) Schiff base complexes into zeolite-Y [26]. (Si, Al)O4 tetrahedra building blocks with no measur-
On the other hand, the external surface areas deter-
mined from the t-plot increase with the incorpora-
tion of metal ions by comparing VO2-Y with Na-Y.
able T –O bond breakage during the ion-exchange and
encapsulation processes. The weak peaks at 1.94 Å in
encapsulated samples are due to the V=O pairs [29],
This increase can be explained in terms of the zeolite consistent with the peaks ranged from 1.8 to 2.2 Ǻ for
structural deformation occurring during the cation- three neat dioxovanadium complexes, indicating that
exchanged process [27].
vanadium ions or vanadium complexes have been
introduced into the supercages of zeolite.
Varied information at atomic scale can be obtained
by comparative study of pair distribution function
(PDF) of samples at different preparation stages. As
illustrated in Fig. 3, the PDFs of neat dioxovanadium
complexes (Figs. 3f–3h) are quite different from those
of zeolite based materials (Figs. 3a–3e), indicating the
different interatomic distances present. However,
3.2. Spectroscopic Characterization
1H NMR spectra of all ligands show the aromatic
protons as multiplet in the range 7.78–6.61 ppm (see
1
Fig. 1 and Table 2). H NMR spectra of three tetra-
dentate Schiff base ligands of N2O2 donor sets possess
two phenolic (13.06–13.68 ppm) and azomethine
(8.35–8.65 ppm) groups [30]. The azomethine proton
signals appearing in the spectra of Schiff base ligands
are absent upon hydrogenation, indicating that
CH=N moieties are completely converted to CH2–NH
groups through hydrogenation, which is in accordance
with the newly present signals at 2.55–2.84 ppm due
to NH–CH2 protons in the spectra of tetrahydro-
Schiff base ligands.
G (r)
24
h
g
20
f
16
12
8
e
d
c
b
a
Figure 4 shows the IR spectra of various samples.
The ligand SalhexenH2 exhibits a band at 1646 cm–1
due to azomethine νC=N stretch (Fig. 4a). This band
4
registers a low frequency shift of ca. 29 cm–1 in the
spectra of VO2-Salhexen (Fig. 4c), indicating the
coordination of azomethine nitrogen to the vanadium
[31]. However, the peak attributed to CH=N vibration
disappears in the spectrum of [H4]SalhexenH2, and a
0
0
1
2
3
4
5
6
7
8
9
10
r, Å
new peak at 3288 cm–1 attributed to N–H vibration is
substitutionally observed upon hydrogenation (Fig. 4b).
In the lower frequency region, the bands appeared at
985 and 1040 cm–1 due to νV=O stretches in the spectra
of VO2-Salhexen and VO2-[H4]Salhexen are observed.
Fig. 3. Pair distribution functions for samples of (a) Na-Y,
(b) VO -Y, (c) VO -[H ]Salen-Y, (d) VO -[H ]Salhexen-Y,
2
2
2
4
2
4
(e) VO -Salhexen-Y, (f) VO -[H ]Salen, (g) VO -[H ]Sal-
2
4
2
4
hexen and (h) VO -Salhexen.
2
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DIOXOVANADIUM(V) COMPLEXES OF SCHIFF
Table 2. Structure and 1H NMR chemical shift of various ligands
295
Chemical shift (ppm)
Nomenclature
Structure
N(CH2)xN CH2–NH
OH
CH=N
Ar–H
OH
HO
SalenH2
13.20
8.35
7.30–6.82
3.91
–
N
N
OH
NH NH
OH HO
OH
[H4]SalenH2
SalhexenH2
–
–
7.25–6.78
3.99
2.84
3.62,
13.68
8.35
7.29–6.84 1.76–1.59
1.38–1.22
–
N
N
(CH2)6
OH
OH
3.99
7.18–6.69 1.56–1.49
1.39–1.34
[H4]SalhexenH2
SalphenH2
–
–
2.69
NH
NH
(CH2)6
OH
HO
N
N
13.06
8.65
7.78–6.87
7.58–6.61
–
–
–
OH
HO
NH HN
[H4]SalphenH2
–
–
2.55
at 1200–1600 cm–1 in the spectra of VO2-Salhexen-Y
(Fig. 4g) and VO2-[H4]Salhexen-Y (Fig. 4h), assigned
to C–O, C–N and aromatic ring vibrations of ligands,
are absent in the case of Na-Y and VO2-Y samples. It
Unfortunately, these bands could not be located upon
complex encapsulation due to the appearance of
strong and broad band of the zeolite framework in the
~1000 cm–1 region. It is evident that framework vibra-
tion bands of zeolite-Y dominate the spectra below
1200 cm–1 for all samples. The bands at 462, 570, 699
and 784 as well as 1060 and 1122 cm–1 are attributed to
T–O bending mode, double ring, symmetric stretch-
ing and asymmetric stretching vibrations, respec-
tively [16]. No shift is observed upon introduction of
metal ions and inclusion of metal complexes, further
substantiating that the zeolite framework remains
unchanged. In spite of this, there is obviously a dif-
ference in the range of 1200–1600 cm–1 and 550–
is the same case with the bands at 536 and 513 cm-1 due
to υV-O and υV-N. All these data indicate that dioxova-
nadium complexes have been encapsulated inside zeo-
lite-Y, and similar results can be obtained over other
samples.
The UV-vis spectra of three representative samples
for [H4]SalhexenH2, VO2-[H4]SalhexenH2 and VO2-
[H4]SalhexenH2-Y are reproduced in Fig. 5. The UV-
vis spectrum of [H4]SalhexenH2 exhibits three bands
at 337, 270 and 235 nm, assigned to n–π*, π–π* and
450 cm–1 among zeolite-Y based samples. The bands ϕ–ϕ* transitions, respectively. Appearance of these
KINETICS AND CATALYSIS Vol. 58 No. 3 2017

296
ZHONGZHEN DING, YING YANG
(а)
(b)
h
699
1040
462
d
g
c
b
1060
536
985
f
513
1617
3288
a
e
784
570
1646
1500
1122
1000
4000
1200
900
2000
1500
500
Wavenumbers, cm–1
Fig. 4. FT-IR spectra of (a) SalhexenH , (b) [H ]SalhexenH , (c) VO -Salhexen, (d) VO -[H ]Salhexen, (e) Na-Y, (f) VO -Y,
2
4
2
2
2
4
2
(g) VO -Salhexen-Y and (h) VO -[H ]Salhexen-Y.
2
2
4
bands at lower energy values indicates the coordina- comparison among the spectra of the standards (VO,
V2O3, VO2 and V2O5) shows strong variations in
energy position and intensity of peaks in the pre-edge
tion of ligands to vanadium [32]. Additionally, all diox-
ovanadium complexes are dominated by the presence of
several bands at 370–430 nm, assigned to ligand to region (before ca. 5480 eV) and the edge region (after
5480 eV). The increasing pre-edge intensity with
increasing valence state was observed from V2+ to V5+
(see Figs. 6g–6j). The pre-edge peak is well-known to
be due to a formally forbidden 1s → 3d electronic tran-
sition, which is dipole allowed if the full local Oh sym-
metry is decreased [34]. The pre-edge peak position
and intensity ranged from 5469.4 to 5470.4 eV for VO2-
containng samples is near to that for standard V2O5 at
5468.2 eV, which indicates an oxidation state of V(5+)
and the octahedral symmetry deviation for VO2-con-
tainng samples. This also demonstrates that the recy-
cled VO2-[H4]Salhexen-Y and VO2-Salhexen-Y retain
their oxidation state and distorted octahedral symme-
try, as compared to their fresh ones. However, the pre-
edge feature (mainly peak position) of neat dioxova-
nadium complexes is more close to V2O5 than their
heterogeneous samples, indicating that zeolite matrix
affects the V environment slightly.
metal charge transfer transition (LMCT) [16, 33].
Figure 6 shows different XANES spectra of vari-
ous V-containing samples. The characteristics of the
XANES spectra can be used as a fingerprint method
to describe the V environment in a compound. A
270
235
c
b
337
3.4. Catalytic Properties
a
200
300
400
500
600
700
800
Table 3 summarizes the catalytic results (under
optimized conditions) for the aerobic oxidation of sty-
rene over various catalysts. The VO2-Y catalyst is
almost inactive for the oxidation of styrene (Entry 1),
while VO2-Schiff and VO2-[H4]Schiff base complexes
Wavelength, nm
Fig. 5. UV-vis spectra of (a) SalhexenH , (b) VO -[H ]Sal-
hexen and (c) VO -[H ]Salhexen-Y.
2
2
4
2
4
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DIOXOVANADIUM(V) COMPLEXES OF SCHIFF
297
Table 3. Catalytic properties of various catalysts for the aerobic oxidation of styrene
Product selectivityd (mol%)
V
Styrene
loadinga, conversionb, TOFc, h–1
Entry
Catalysts
Bza
So
others
mmol/g
%
1
2
3
4
5
6
7
8
9
VO2-Y
0.9462
2.9674
2.5445
2.5974
0.2917
0.2688
0.0703
3.0030
2.5707
2.6247
0.3568
0.4397
0.1559
–
6.8
44.2
35.2
50.6
69.1
95.2
65.1
96.2
51.4
68.3
44.1
72.5
95.3
95.0
93.2
90.6
1.8
37.2
34.6
48.7
59.2
88.5
231.5
80.1
50.0
65.0
30.9
41.2
152.8
–
62.5
65.7
50.4
40.2
66.8
57.6
24.6
50.0
66.4
47.3
59.4
56.8
31.0
55.7
58.0
54.2
33.2
34.3
48.6
45.6
30.0
42.4
30.8
35.6
33.6
52.7
29.5
41.2
62.0
44.3
42.0
45.8
4.3
0
VO2-[H4]Salen
VO2-[H4]Salhexen
VO2-[H4]Salphen
VO2-[H4]Salen-Y
VO2-[H4]Salhexen-Y
VO2-[H4]Salphen-Y
VO2-Salen
1.0
14.2
3.2
0
44.6
14.4
0
VO2-Salhexen
10 VO2-Salphen
0
11 VO2-Salen-Y
11.1
2.0
7.0
0
12 VO2-Salhexen-Y
13 VO2-Salphen-Y
14 VO2-[H4]Salhexen-Y (2nd)
15 VO2-[H4]Salhexen-Y (3rd)
–
–
0
16 VO2-[H4]Salhexen-Y (4th)
0.2620
86.4
0
a
b
Estimated by ICPAES. Reaction conditions: supported catalyst 50.0 mg (5.0 mg for the neat catalyst), styrene 1.14 mL (10 mmol),
c
–1
CH CN 10 mL, flow of air 80 mL/min, isobutyraldehyde 2.28 mL (25 mmol), temperature 80°C and duration 8 h. TOF, h : (turn-
3
d
over frequency) moles of substrate converted per mole metal ion per hour. So: styrene oxide, Bza: benzaldehyde and others: including
benzoic acid, phenylacetaldehyde and 1-phenylthane-1,2-diol.
and their encapsulated analogues in zeolite-Y exhibit
activity, which substantiates that the active sites are
dioxovanadium Schiff base or tetrahydro-Schiff base
complexes, not metal ions. It is clear that the catalytic
activities of encapsulated dioxovanadium complexes
are always better over their respective non-encapsu-
lated ones (see Entries 2–7, 9, 10, 12 and 13) except for
VO2-Salen-Y. These results suggest that encapsulation
of neat metal complexes into zeolite-Y successfully
isolated the neat complexes and prevented their
dimerization. The enhanced activity of encapsulated
catalyst may be also due to the synergistic beneficial
catalytic interaction between the metal complexes and
the zeolite support [35]. A leaching test for VO2-Sal-
hexen-Y and VO2-[H4]Salhexen-Y catalysts was per-
formed to verify the heterogeneity of the catalytic pro-
cess. In a duplicate reaction continued after removing
the catalyst by filtration at reaction temperature, it was
found that styrene could just be converted at a very low
rate and this trend was almost parallel to the blank
experiment (not shown). These results suggest that the
large majority of the catalysis is carried out by a truly
heterogeneous dioxovanadium catalyst. VO2-Sal-
hexen-Y and VO2-[H4]Salhexen-Y catalysts can be
recycled three times without significant loss of activity
and variation of their valence state.
Ligand hydrogenation affects catalytic properties
significantly in our studies. VO2-[H4]Salen-Y catalyst
shows higher activity than the corresponding VO2-
Normalised absorption, a.u.
3.0
2.5
j
2.0
i
h
g
1.5
1.0
0.5
0
f
e
d
c
b
a
5440
5460
5480
5500
5520
Energy, eV
Fig. 6. Experimental V K-edge XANES spectra of
(a) VO -Y, (b) VO -Salhexen-Y, (c) VO -[H ]Salhexen-Y,
2
2
2
4
(d) spent VO -[H ]Salhexen-Y, (e) VO -Salhexen,
2
4
2
(f) VO -[H ]Salhexen, (g) V O , (h) VO , (i) V O and
2
4
2
5
2
2 3
(j) VO.
KINETICS AND CATALYSIS Vol. 58 No. 3 2017

298
ZHONGZHEN DING, YING YANG
Salen-Y catalyst (Table 3, Entries 5 and 11). It is the their corresponding Schiff base complexes due to the
same case with VO2-[H4]Salhexen-Y and VO2- improved electronic environment around vanadium
ions. Moreover, the selectivity to styrene oxide was
lower than to benzaldehyde in most cases due to the
acidic nature of zeolite matrix.
[H4]Salphen-Y as compared to VO2-Salhexen-Y and
VO2-Salphen-Y, respectively (Table 3, Entries 6, 7, 12
and 13), as revealed by the higher TOF values. These
results demonstrated by our studies are consistent with
the results reported in the literature [16, 36] that the
improved Schiff base, i.e. tetrahydro-Schiff base
ligands, are beneficial to the enhancement of catalytic
activity. However, this conclusion seems to be false by
comparison of neat dioxovanadium tetrahydro-Schiff
base complexes with their respective dioxovanadium
Schiff base complexes (Table 3, Entries 2–4 and 8–10),
since activity decline was observed after hydrogena-
tion of CH=N to CH2–NH. According to literature
[37], Schiff base ligands become more flexible after
hydrogenation, resulting in easy formation of μ-oxo
and μ-peroxo dimeric and other polymeric species
especially when using oxidant, which may be respon-
sible for the deficiency of neat tetrahydro-Schiff base
complexes.
ACKNOWLEDGMENTS
This work and the use of Advanced Photon Source
are supported by Office of Science, U.S. Department
of Energy under Contract DE-AC02-06CH11357.
Authors also thank J.T. Miller for the sharing standard
vanadium sample data with us.
REFERENCES
1. Grigoropoulou, G., Clark, J.H., and Elings, J.A.,
Green. Chem., 2003, vol. 5, p. 1.
2. Lane, B.S. and Burgess, K., Chem. Rev., 2003, vol. 103,
p. 2457.
3. Corma, A. and Garcia, H., Chem. Rev., 2002, vol. 102,
p. 3837.
Additionally, we found that the acidic nature of
hydrophilic zeolite was reflected in the product distri-
bution in most conditions. The catalytic results over
homogeneous complex catalysts show that benzalde-
hyde (Bza) is the major oxidation product, followed by
styrene oxide (So), similar to literature [38, 39]. The
encapsulated complexes show similar product distri-
butions to the homogeneous counterparts, except for
the presence of phenylacetaldehyde and 1-pheny-
lethane-1,2-diol, formed via ring opening of pheny-
loxirane on acid sites. Lower styrene oxide selectivity
for encapsulated catalysts is also connected with this
ring opening reaction, suggesting that the acidity of
the zeolite support declines the selectivity to pheny-
loxirane. However, the selectivity to styrene oxide was
always higher than to benzaldehyde for VO2-Salphen
and VO2-[H4]Salphen whether encapsulated or not
(Entries 4, 7, 10 and 13), which may be related to the
great steric hindrance of dioxovanadium Salphen or
[H4]Salphen complexes.
4. Sato, T., Dakka, J., and Sheldon, R.A., J. Chem. Soc.,
Chem. Commun., 1994, vol. 16, p. 1887.
5. Camblor, M.A., Corma, A., Martínez, A., and
Pérez Parienta, J., J. Chem. Soc., Chem. Commun.,
1992, vol. 8, p. 589.
6. Tozzola, G., Mantegazza, M.A., Ranghino, G.,
Petrini, G., Bordiga, S., Ricchiardi, G., Lamberti, C.,
Zulian, R., and Zecchina, A., J. Catal., 1998, vol. 179,
p. 64.
7. Singh, A. and Selvam, T., J. Mol. Catal. A: Chem., 1996,
vol. 113, p. 489.
8. Sels, B.F., De Vos, D., and Jacobs, P.A., Tetrahedron
Lett., 1996, vol. 37, p. 8557.
9. Bansal, V.K., Thankachan, P.P., and Prasad, R., Appl.
Catal., A, 2010, vol. 381, p. 8.
10. Sheldon, R.A., Arends, I.W.C.E., and Dijksman, A.,
Catal. Today, 2000, vol. 57, p. 157.
11. Balkus, K.J., Eissa, M., and Levado, R., J. Am. Chem.
Soc., 1995, vol. 117, p. 10753.
12. Canali, L. and Sherrington, D.C., Chem. Soc. Rev.,
1999, vol. 28, p. 85.
13. Kim, G.-J. and Shin, J.-H., Catal. Lett., 1999, vol. 63,
p. 83.
CONCLUSIONS
14. Katsuki, T., Coord. Chem. Rev., 1995, vol. 140, p. 189.
Dioxovanadium(V) complexes of Schiff and tetra-
hydro-Schiff bases were successfully encapsulated in
the supercages of zeolite-Y by a flexible ligand
method, as evidenced by FT-IR, UV-vis spectroscopy,
N2 adsorption/desorption and XANES techniques.
The structural integrity of zeolite throughout the
encapsulation procedure was confirmed by XRD,
SEM and PDF studies. These encapsulated metal
complexes displayed better activity than their corre-
sponding homogeneous analogues due to the site-iso-
lation and synergism between dioxovanadium com-
plexes and zeolite supports. All encapsulated dioxova-
nadium tetrahydro-Schiff base complexes show much
higher activity in aerobic oxidation of styrene than
15. O'Connor, K.J., Wey, S.J., and Burrows, C.J., Tetrahe-
dron Lett., 1992, vol. 33, p. 1001.
16. Jin, C., Fan, W.B., Jia, Y.J., Fan, B.B., Ma, J.H., and
Li, R.F., J. Mol. Catal. A: Chem., 2006, vol. 249, p. 23.
17. Luo, Y. and Lin, J., Microporous Mesoporous Mater.,
2005, vol. 86, p. 23.
18. Baleizao, C., Gigante, B., Das, D., Alvaro, M., Gar-
cia, H., and Corma, A., J. Catal., 2004, vol. 223,
p. 106.
19. Sheldon, R.A., Arends, I.W.C.E., and Lempers, H.E.B.,
Catal. Today, 1998, vol. 41, p. 387.
20. Sakthivel, A., Sun, W., Raudaschl-Sieber, G., Chi-
ang, A.S.T., Hanzlik, M., and Kühn, F.E., Catal.
Commun., 2006, vol. 7, p. 302.
KINETICS AND CATALYSIS
Vol. 58
No. 3
2017

DIOXOVANADIUM(V) COMPLEXES OF SCHIFF
299
21. Yang, Y., Zhang, Y., Hao, S.J., and Kan, Q.B., Chem. 30. Boghaei, D.M. and Mohebi, S., J. Mol. Catal. A:
Eng. J., 2011, vol. 171, p. 1356.
Chem., 2002, vol. 179, p. 41.
31. Murphy, E.F., Schmid, L., Burgi, T., Maciejewski, M.,
22. Yang, Y., Zhang, Y., Hao, S.J., Guan, J.Q., Ding, H.,
Shang, F.P., Qiu, P.P., and Kan, Q.B., Appl. Catal. A:
Gen., 2010, vol. 381, p. 274.
and Baiker, A., Chem. Mater., 2001, vol. 13, p. 1296.
32. Maurya, M.R., Chandrakar, A.K., and Chand, S.,
J. Mol. Catal. A: Chem., 2007, vol. 270, p. 225.
23. Htay, M.M., and Oo, M.M., World Acad. Sci. Eng.,
Technol., 2008, vol. 48, p. 114.
33. Maurya, M.R., Kumar, A., Manikandan, P., and
Chand, S., Appl. Catal. A: Gen., 2004, vol. 277, p. 45.
24. Poltowicz, J., Pamin, K., Tabor, E., Haber, J.,
Adamski, A., and Sojka, Z., Appl. Catal. A: Gen., 2006,
vol. 299, p. 235.
34. Coulston, G.W., Bare, S.R., Kung, H., Birkeland, K.,
Bethke, G.K., Harlow, R., Herron, N., and Lee, P.L.,
Science, 1997, vol. 275, p. 191.
25. Salavati-Niasari, M. and Amiri, A., Appl. Catal. A:
Gen., 2005, vol. 290, p. 46.
35. Gonzalez-Arellano, G., Corma, A., Iglesias, M., and
Sanchez, F., Inorg. Chim. Acta, 2004, vol. 357, p. 3071.
26. Salavati-Niasari, M., Micropor. Mesopor. Mater., 2006,
vol. 95, p. 248.
36. Chen, P., Fan, B.B., Song, M.G., Jin, C., Ma, J.H.,
and Li, R.F., Catal. Commun., 2006, vol. 7, p. 969.
27. Sebastian, J., Mohan, K., and Jasra, R.V., J. Catal.,
2006, vol. 244, p. 208.
37. Wang, X.L., Wu, G.D., Li, J.P., Zhao, N., Wei, W., and
Sun, Y.H., Catal. Lett., 2007, vol. 119, p. 87.
28. Readman, J.E., Forster, P.M., Chapman, K.W.,
Chupas, P.J., Parise, J.B., and Hriljac, J.A., Chem.
Commun., 2009, vol. 23, p. 3383.
38. Indira, V., Halligudi, S.B., Gopinathan, S., and Gopi-
nathan, C., React. Kinet. Catal. Lett., 2001, vol. 73,
p. 99.
29. Keller, D.E., Visser, T., Soulimani, F., Konings-
berger, D.C., and Weckhuysen, B.M., Vib. Spectrosc., 39. Zsigmond, A., Horvath, A., and Notheisz, F., J. Mol.
2007, vol. 43, p. 140.
Catal. A: Chem., 2001, vol. 171, p. 95.
KINETICS AND CATALYSIS Vol. 58 No. 3 2017