Selective oxidation reactions over tri- and tetradentate oxovanadium(IV) complexes encapsulated in zeolite-Y

Article abstract of DOI:10.1016/j.cattod.2013.09.015

The preparation and characterization of VO(IV) complexes of 7-amino-5-aza-4-methyl-hept-3-en-2-one and 4,4′-(ethane-1,2-diyldinitrilo) dipentan-2-one inside the super cages of zeolite-Y using the flexible ligand method is described. The structures of these encapsulated complexes were established on the basis of various physico-chemical (XRD, BET and TGA) and spectroscopic studies (UV-Vis, FTIR). The results indicate that zeolite-Y can accommodate these complexes in its super cages without hindering or modifying the framework or structure of the zeolite confirming successful encapsulation of the Schiff-bases throughout the voids of zeolite-Y. These encapsulated complexes were screened as heterogeneous catalysts for various oxidation reactions, viz., phenol, benzene, styrene and cyclohexene using H ₂O₂ as oxidant. For comparison, the corresponding neat complexes were screened as potential homogeneous catalysts for these oxidation reactions. The homogeneous complexes were found to be more active than the corresponding encapsulated VO(IV) complexes for these oxidation reactions. The results also proved that the heterogeneous systems described here represent an efficient and friendly environmental for these oxidation reactions, having advantages over homogeneous catalysts. In addition they have high turnover frequency values (TOF) and better selectivity than the corresponding homogeneous catalysts.

Full text of DOI:10.1016/j.cattod.2013.09.015

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Selective oxidation reactions over tri- and tetradentate
oxovanadium(IV) complexes encapsulated in zeolite-Y
∗
Gavin von Willingh, Hanna S. Abbo, Salam J.J. Titinchi
Department of Chemistry, University of Western Cape, Private Bag X17, Bellville 7535, Cape Town, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2013
Received in revised form 13 August 2013
Accepted 17 September 2013
Available online xxx
The preparation and characterization of VO(IV) complexes of 7-amino-5-aza-4-methyl-hept-3-en-2-one
and 4,4 -(ethane-1,2-diyldinitrilo)dipentan-2-one inside the super cages of zeolite-Y using the flexible
ꢀ
ligand method is described. The structures of these encapsulated complexes were established on the
basis of various physico-chemical (XRD, BET and TGA) and spectroscopic studies (UV–Vis, FTIR). The
results indicate that zeolite-Y can accommodate these complexes in its super cages without hindering or
modifying the framework or structure of the zeolite confirming successful encapsulation of the Schiff-
bases throughout the voids of zeolite-Y. These encapsulated complexes were screened as heterogeneous
catalysts for various oxidation reactions, viz., phenol, benzene, styrene and cyclohexene using H2O2 as
oxidant. For comparison, the corresponding neat complexes were screened as potential homogeneous
catalysts for these oxidation reactions. The homogeneous complexes were found to be more active than
the corresponding encapsulated VO(IV) complexes for these oxidation reactions. The results also proved
that the heterogeneous systems described here represent an efficient and friendly environmental for
these oxidation reactions, having advantages over homogeneous catalysts. In addition they have high
turnover frequency values (TOF) and better selectivity than the corresponding homogeneous catalysts.
Keywords:
Homogenous catalysis
Heterogeneous catalysis
Zeolite-Y
Tri- and tetradentate Schiff base complexes
Oxovanadium(IV)
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
[9]. Although polymer and inorganic oxide-anchored complexes
have been designed and their catalytic activities are as good as
homogeneous catalysts, they suffer from leaching of metal com-
plexes from the solid surface [10]. This disadvantage has restricted
their commercial use.
The coordination chemistry of vanadium is of great current
interest due to its wide application in medicinal, biological and cat-
alytic systems [11]. Oxovanadium complexes have also attracted
considerable attention due to their remarkable catalytic activity
and displayed good selectivity for the oxidation of various sub-
Transition metal complexes of Schiff base ligands have been
employed in homogeneous catalysis for various oxidation reactions
due to their high activity, homogeneity, reproducibility, selectivity
under mild conditions and thus affirm their investigation in organic
synthesis [1]. The problems encountered in homogeneous cat-
alytic systems are the catalyst, reactants and products which are in
one phase, making catalyst–product separation, catalyst recovery,
recycling and reuse difficult [2]. Formation of oxo-dimers and other
polymeric species are always possible in homogeneous catalytic
systems leading to irreversible catalyst deactivation [3]. These
drawbacks have hampered their use by industry. In order to mini-
mize these drawbacks, heterogenization of homogeneous catalysts
was developed to overcome these difficulties and has emerged as a
focus of research [4–7]. Encapsulation of these homogeneous cat-
alysts in microporous materials such as zeolites is an attractive
technique for heterogenization, since no leaching of metal ions or
metal complexes were detected since the complex is trapped in
the zeolite pores [8]. In addition, zeolite-encapsulated complexes
promise better control of selectivity of the reaction and higher sta-
bility as a result of reduced dimerization of complexes in the cavity
strates using H O2 as a green oxidant [12,13]. Encapsulation of
2
oxovanadium(IV) complexes inside the super cages of zeolite-Y and
oxovanadium-exchanged zeolite has also been investigated which
indicated their potential catalytic activity in oxidation reactions
[14–16].
As part of our continuing interest in oxidation reactions by vana-
dium complexes [15,17,18] and considering the demand of more
efficient heterogeneous catalytic systems [19].
We herein report the synthesis and characterization of
new heterogeneous oxovanadium catalytic systems, i.e. zeolite-
Y encapsulated OV(IV) complexes. These complexes are derived
ꢀ
from 7-amino-5-aza-4-methyl-hept-3-en-2-one and 4,4 -(ethane-
1
,2-diyldinitrilo)-dipentan-2-one (Scheme 1).
Catalytic activities of these complexes have been studied
towards the oxidation of phenol, benzene, cyclohexene and
styrene. Neat complexes of the above metal ions have also been
∗ Corresponding author. Tel.: +27 21 959 2217; fax: +27 21 959 3055.
E-mail address: stitinchi@uwc.ac.za (S.J.J. Titinchi).
0
920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.09.015
Please cite this article in press as: G. von Willingh, et al., Selective oxidation reactions over tri- and tetradentate oxovanadium(IV) complexes
encapsulated in zeolite-Y, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.015

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G. von Willingh et al. / Catalysis Today xxx (2013) xxx–xxx
H C
CH
3
H C
H C
CH
H C
3
3
3
3
3
3
N
NH
N
O
NH2
N
N
2
N
O
N
O
O
V
V
O
O
O
O
H C
CH
H C
O
H C
O
CH
H C
3
3
3
3
3
1
2
1
1
HL
HL
VO(L )(acac)
VO(L )
Scheme 1. Structures of the ligands and oxovanadium (IV) complexes.
prepared and their catalytic activities have been compared with
the encapsulated ones.
analyzed using Agilent 7890 gas chromatograph fitted with flame
ionization detector and HP-5 phenylmethylsilicon capillary col-
umn (30 m × 330 pm × 0.25 ␮m). The retention time of all the peaks
were compared with authentic samples.
2
. Experimental
2
2
.3. Preparations
2
.1. Materials
.3.1. 7-Amino-5-aza-4-methyl-hept-3-en-2-one, HL1
The chemicals were used as received without any further purifi-
7
-Amino-5-aza-4-methyl-hept-3-en-one was synthesized
cation. Absolute ethanol (99%) was purchased from Saarchem.
Glacial acetic acid and hydrochloric acid (32%) were purchased
from Merck. H-Y zeolite was purchased from Zeolyst. Carbon tetra-
chloride was purchased from Riedel-de Hahen. Dichloromethane
according to a method described by Styring et al. with slight
modifications [20]. A cooled solution of ethylenediamine (0.3 g,
0
of acetylacetone (0.5 g, 0.005 mol) in (25 ml) DCM at 0 C under
stirring conditions at ambient temperature. The solution was
stirred for 5 min and refluxed for an additional 5 min at 40 C.
.005 mol) in (25 ml) DCM was added dropwise to a cooled solution
◦
(
DCM), phenol (99%), hydrogen peroxide (30%), cyclohexene,
vanadyl acetylacetonate, ethylenediamine, ethyl acetate, acetoni-
trile, acetylacetone were purchased from Sigma–Aldrich.
◦
Afterwards, the solvent was removed in vacuo to give viscous oil.
Yield = 0.97 g (68%).
2.2. Physical methods and analysis
1
H NMR ı ppm (CDCl , 200 MHz): 1.48 (s, 6H, CH ), 1.56 (s, 2H,
3
3
NH ), 2.46 (m, 2H, CH ), 3.42 (m, 2H, CH ), 4.58 (s, 1H,
C H), 10.45
2
2
2
ATR-IR measurements were carried out on a Perkin-Elmer Spec-
(br, 1H, OH).
trum 100 FTIR spectrometer. Electronic spectra were recorded on a
GBC UV/VIS 920 UV–Visible spectrophotometer in absolute ethanol
or in Nujol (by layering the mull of the sample to the inside of one
of the cuvettes while keeping another one layered with Nujol as
reference) as well as using diffuse reflectance under ambient con-
ꢀ
2
2
.3.2. 4,4 -(Ethane-1,2-diyldinitrilo)dipentan-2-one, HL
4
ꢀ
,4 -(Ethane-1,2-diyldinitrilo)dipentan-2-one was synthesized
according to literature procedure [21]. A solution of acetylacetone
2 g, 0.02 mol) in 25 ml DCM was added dropwise to a solution of
(
1
13
ditions. H and C NMR spectra of ligands were recorded in CDCl
3
ethylenediamine (0.6 g, 0.01 mol) in 25 ml DCM under stirring con-
ditions. After addition, the pH of the solution was adjusted to 6 with
1
solution using a Varian Gemini 2000 spectrometer ( H at 200 MHz,
1
3
C at 50.3 MHz) and chemical shifts are indicated in ppm. Sample
a few drops glacial acetic acid. The yellow solution was refluxed for
signals are relative to the resonance of residual protons on car-
bons in the solvent. Electrochemical studies were carried out on
a BAS 100 B electrochemical analyzer. Three-electrode assembly
with Ag/AgCl/KCl (saturated) reference electrode, Pt wire counter
electrodes were used. Working electrode consisted of glassy carbon
microelectrode (2 mm diameter). All investigations were made of
◦
3
–4 h at 40 C. Afterwards, the solvent was removed in vacuo and
gave a yellow solid. The solid product was recrystallized by dis-
solving it in a 1:1 mixture of ethylacetate and DCM by heating.
After recrystallization from the same solvent and twice from CCl4
the product was filtered and air dried to give straw like crystals.
◦
1
Yield = 2.07 g (55.6%); m.p. = 111–113 C. H NMR ı ppm (CDCl3,
−
2
1
0
M of sample solutions in dry acetonitrile solution in presence
2
4
00 MHz): 1.88 (s, 6H, CH ), 1.95 (s, 6H, CH ), 3.43 (m, 4H, CH ),
3
3
2
of lithium perchlorate as supporting electrolyte. Each solution was
degassed with ultra-pure N2 gas for 5 min before each measure-
ment was made.
.97 (s, 2H,
C
H), 10.86 (br, 2H, OH).
2.3.3. Oxovanadium(IV) complexes
Thermal analysis was measured using Perkin Elmer TGA
Q500 Thermobalance. The nitrogen adsorption/desorption and BET
A hot solution of VO(acac)2 (0.265 g, 0.001 mol) in ethanol
(10 ml) was added dropwise to ethanolic solution (10 ml) of the
appropriate ligand (HL1 or HL ) (0.001 mol). The reaction mixture
was heated and stirred under reflux for 5 h. The complex slowly
separated out from filtered, and the solid was washed with hot
◦
2
surface area was determined at −196 C using a Tristar 3000
Micromeritics. All samples were degassed prior to the mea-
◦
surement at 120 C for 12 h. The percentage metal content was
◦
determined using ICP-OES Varian 710-ES spectrophotometer. Scan-
ning Electron Micrographs (SEM) of the encapsulated catalysts
were recorded on field-emission scanning electron microscopy
water followed by ethanol and dried at 100 C. Recrystallization
from MeCN gave analytically pure products.
1
◦
(
Auriga Zeiss SEM) with accelerating voltage: 5 keV. The sam-
VO(L )(acac): light green solid, yield 0.20 g (65.5%); m.p > 300 C
ples were coated with Au-Pd for 30 s using a Quorum Q150T
ES sputter coater to prevent surface changes and to protect the
surface material from thermal damage by the electron beam.
The powder X-ray diffraction was recorded on a Bruker AXS D8
Advance, High-Resolution diffractometer with Cu K␣ Radiation
and
2
◦
VO(L ): green powder, yield 0.18 g; (61.7%;) m.p 235–237 C (Lit.
236 C [22]).
◦
2.3.4. Oxovanadium(IV) exchanged zeolite, VO-H-Y
(
ꢀ = 1.5406 A˚ ) fitted with a PSD Vantec gas detector at Ithemba
labs, Cape Town, South Africa. All catalysis reaction products were
To 1.0 g H-Y suspended in 50 ml deionized water was added
◦
VO(acac) (0.53 g, 2 mmol) The reaction mixture was stirred at 90 C
2
Please cite this article in press as: G. von Willingh, et al., Selective oxidation reactions over tri- and tetradentate oxovanadium(IV) complexes
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3
Table 1
Table 2
FT-IR spectral data of the ligands and VO(IV) complexes.
Electronic spectral data of ligands and VO(IV) complexes.
Compound
ꢁ (cm 1
−
)
Ligand/complex
ꢀm/nm
1
sh
(
C
N)
ꢁ(M O)/(M N)
ꢁ(V O)
HL
202, 302 , 316
2
sh
HL
202, 303 322
HL1
1602(s)
1599(s)
1570(s)
1559(s)
–
–
–
–
978
970
1
sh
VO(L )(acac)
VO(L )(acac)-Y
VO(L )
VO(L )-Y
202, 241 , 308, 347, 773
2
HL
1
201, 319, 373, 607
1
[
[
VO(L )(acac)]
386, 408, 485
418, 481
2
202, 266sh, 302, 320sh, 562, 750
2
VO(L )]
2
204, 318, 618
for 24 h, filtered, washed with copious amounts of hot deionised
water followed by Soxhlet extraction with acetonitrile for 3 h. The
resulting precipitate was dried at 150 C in air for 24 h.
the ꢁ(M O) and ꢁ(M N) modes, respectively [23]. Neat complexes
−
1
◦
exhibit a medium-sharp band around 978–970 cm due to ꢁ(V O)
stretch [17,18,24]. The FTIR patterns of all guest–host catalysts are
very similar and are dominated by the strong bands due to vibra-
tions of the zeolite. These characteristic bands corresponding to the
zeolite framework in all samples are found around at 1050, 450, 780
2.3.5. Synthesis of zeolite-Y encapsulated metal complexes
OV-H-Y zeolite (0.3 g) and ligand (0.7 g) were mixed in MeCN
(
30 ml) and the reaction mixture was refluxed with stirring
−
1
and 394 cm
.
overnight. The resulting material was collected by filtration and
was subsequently Soxhlet extracted in MeCN for 48 h. The solid
No shift or broadening in the structure sensitive band
−
1
◦
at 1050 cm
(due to an asymmetric T O stretch) occurred
residue obtained was dried at 120 C in air for several hours.
demonstrating that little change in the zeolite framework upon
encapsulation or ion exchange occurred indicating that there is no
significant expansion of the zeolite cavity nor dealumination thus
proving that the metal complex fits in the cavity of the zeolite and
that the zeolite matrix remains unchanged [25–27].
The intensity of IR bands in both zeolite encapsulated complexes
is weak in comparison to those of the free metal complexes due to
low concentration of the complexes in zeolite cavities.
2
.4. Catalytic activity
2.4.1. Catalytic liquid phase oxidation reaction
Catalytic liquid phase oxidation reactions were carried out in
a 50 ml round-bottom flask fitted to a water condenser. In a typi-
cal reaction, equimolar amounts of substrate and 30% H O were
mixed in 3 ml MeCN and the reaction mixture was refluxed and
2
2
◦
stirred in an oil bath at 70 C after which the catalyst (0.01 g) to be
tested was added and this was assumed to be the starting point of
the reaction. Progress of the reaction was monitored as a function
of time by withdrawing small aliquots after certain time intervals
and analysing them quantitatively by gas chromatograph.
3.1.2. UV–Vis spectral studies
Table 2 presents the electronic spectral data of ligands, com-
plexes and the corresponding encapsulated complexes. Both
ligands exhibit bands at 202, 302 and ∼320 nm are assigned to
ꢂ → ꢂ*, ␲ → ␲* and n → ␲* transitions, respectively. These bands
shifted to lower energy on complexation indicating the coordina-
tion of the ligands to metal ion [15].
2.4.1.1. Hydroxylation of phenol. Phenol (2.35 g, 0.025 mol), 30%
H O (2.83 g, 0.025 mol) and catalyst (0.01 g) were mixed in 3 ml
2
2
◦
of MeCN at 70 C.
This coordination is further supported by the appearance of a
1
2
lower intensity band at 347 and 320 nm in VO(L )(acac) and VO(L ),
respectively, due to ligand to metal charge transfer (LMCT) transi-
tion. In addition, both neat complexes display new broad bands in
the region 562–773 nm due to the expected d–d transitions (Fig. 1).
The spectra of oxovanadium(IV) complexes are characterized
by three d–d transitions according to the energy level scheme by
Ballhausen and Gray [28].
The encapsulation of the oxovanadium complexes is further
supported by the appearance of d–d transitions. Both encap-
sulated complexes exhibit d–d bands in the visible region at
2.4.1.2. Oxidation of benzene. Benzene (1.95 g, 0.025 mol), 30%
H O (2.83 g, 0.025 mol) and catalyst (0.01 g) were mixed in 3 ml
2
2
◦
MeCN at 70 C.
2.4.1.3. Oxidation of cyclohexene. Cyclohexene (2.05 g, 0.025 mol),
3
0% H O (2.83 g, 0.025 mol) and catalyst (0.01 g) were mixed in
2
2
◦
MeCN (3 ml) at 70 C.
2
.4.1.4. Oxidation of styrene. Styrene (2.60 g, 0.025 mol), 30% H O2
2
(
2.83 g, 0.025 g) and catalyst (0.01 g) were mixed in 3 ml of MeCN
◦
at 70 C.
3
3
3
. Results and discussion
.1. Characterization of catalysts
.1.1. IR spectral studies
The major IR spectral data of the ligands and complexes are pre-
sented in Table 1. Comparisons of the spectra of these complexes
with their respective ligands provide evidence for involvement
of the ligand in complex formation. The ꢁ(C N) band is shifted
to lower wavenumbers indicating the involvement of azomethine
nitrogen in coordination to V(IV) metal centre. Disappearance of
the weak broad band, ꢁ(OH), in both neat complexes confirmed
participation of the OH group indicating its deprotonation followed
by coordination to the metal ion. Bonding of metal ions to the lig-
ands through the nitrogen and oxygen atoms is further supported
−
1
by the presence of new bands in the 400–600 cm region due to
Fig. 1. Electronic spectra of the encapsulated complexes in different concentration.
Please cite this article in press as: G. von Willingh, et al., Selective oxidation reactions over tri- and tetradentate oxovanadium(IV) complexes
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Fig. 2. High resolution scanning electron mircographs of VO-Y (left) and VO(L1)(acac) (right).
6
07–618 nm which are assigned to band I (dxy → dxz,dyz) and band
of metal complex in the cavity of the zeolite. This is in agreement
with the low percent metal content estimated by ICP.
2
2
II (dxy → dx −y ).
2
The third expected transition (dxy → dz ) usually merged with
the strong CT band [23,29].
3.1.5. X-ray powder diffraction studies
The X-ray powder diffraction (XRD) patterns of H-Y, OV-Y and
3
.1.3. Scanning electron microscopy of OV(VI) encapsulated
complexes
The Scanning Electron Micrograph (SEM) of the metal-
their encapsulated complexes in zeolite are presented in Fig. 4. XRD
patterns of the zeolitic host before and after complex formation
indicates no remarkable differences, although a slight change in
intensity of these typical lines in the encapsulated complexes was
noticed. This confirms that metal exchange and encapsulation of
complexes have little influence on the crystallinity and that the zeo-
lite host can accommodate these complexes. The diffraction pattern
for the metal or complexes could not be detected in the encapsu-
lated complexes. This could possibly be due to low loading of metal
complexes present in the zeolite.
exchanged zeolite and their respective encapsulated complexes
indicate the presence of well-defined zeolite crystals without any
shadow of metal ions or complexes present on their external sur-
face (Fig. 2). This confirms that the zeolite preserves its morphology
and structure upon encapsulation of complexes. This indicates that
Soxhlet extraction is an excellent way for the removal of uncom-
plexed ligands and complexes from the zeolite surface.
3
.1.4. Thermal analysis
Thermogravimetric analyses (TGA) data of the encapsulated
3
.1.6. Surface textural studies
The surface textural properties for the zeolite encapsulated
complexes along with the percent mass loss at different steps are
presented in Fig. 3. [VOL (acac)]-Y shows four stage losses. The first
step occurs at temperature 150 C with 2 wt% loss due to the pres-
ence of trapped water or physically adsorbed water. The second
decomposition step at 150–220 C refers to the loss of intra-zeolite
water molecules (1%), i.e. chemisorbed water in the form of OH
groups in zeolite-Y [30].
complexes are presented in Table 3. Clearly, inclusion of VO(IV)
Schiff base complex in the zeolite cavities considerably reduced the
pore volume and the surface area of the zeolite host. This confirms
the existence of VO(IV) complexes in the zeolite cavities rather than
on the external surface [31]. The decreasing values in the surface
area, pore volume and adsorption capacity depends on the amount
of incorporated complexes as well as their molecular size and geo-
metrical conformation inside the zeolitic host.
1
◦
◦
◦
The third decomposition step (250–330 C) with 1.5% weight
loss is due to the loss of coordinated acetyl acetone molecule. The
rest of the organic ligand decomposes in the final step in a wide
range (330–650 C). The thermal decomposition of the encapsu-
Fig. 5 shows the N2 adsorption/desorption isotherms and pore
size distribution for VO-Y and their respective encapsulated cata-
lysts, which are typical type I according to the IUPAC classification
◦
2
lated complex [VOL ]-Y shows a three-stage loss in the TGA trace.
A very small percent weight loss (2%) in the final step which took
place over a wide range of temperature is assigned to the loss of
the chelating ligand indicating the presence of only small amounts
1
[VOL (acac)]-Y
2
[
VOL ]-Y
OV-Y
H-Y
0
10
20
30
40
50
60
70
80
2
theta
1
2
Fig. 3. TGA thermograms for the encapsulated complexes.
Fig. 4. XRD patterns of H-Y, OV-Y, [VO(L )(acac)]-Y and [VO(L )]-Y.
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Table 3
Analytical data and surface area and pore volume data for the catalysts and their parents’ analogues.
2
˚
Catalyst
H-Y
%V content (% wt)
BET surface area (m /g)
Pore volume (ml/g)
Average pore size (A)
–
780
535
518
524
0.48
0.41
0.40
0.40
42.50
31.01
30.47
30.46
OV-H-Y
0.28
0.11
0.12
1
[
[
VO(L )(acac)]-Y
2
VO(L )]-Y
[
[
VO(L1)]
VO(L2)]
280
260
240
220
200
180
160
140
120
12
10
8
[VO(L1)]-Y
[VO(L2)]-Y
VO-Y
6
4
H-Y
2
Blank
0
VO-Y
1
0
1
2
3
4
5
6
24
[VO(L )(acac)]-Y
2
Time (h)
[VO(L )]-Y
Fig. 7. Kinetic plot for benzene conversion over various catalysts. Reaction condi-
tions: Benzene 1.95 g, H O 2.83 g, catalyst 0.01 g, CH CN 3 ml, 70 ◦C.
0.0
0.2
0.4
0.6
0.8
1.0
2
2
3
Relative pressure (P/Po)
Two products catechol and hydroquinone were observed with a
mass balance of >95%.
1
2
Fig. 5. N2 adsorption/desorption isotherms VO-Y, [VO(L )(acac)]-Y and [VO(L )]-Y
and pore size distribution pore diameter (inset).
Blank experiments performed without the catalyst show <1%
conversion. The use of H-Y zeolite as catalyst resulted in a 3.5% phe-
nol conversion showing the ineffectiveness of zeolite framework to
catalyze the reaction. However, in the case of V-exchanged zeolite
the conversion of phenol was higher than the parent H-Y zeolite.
Our results in the oxidation of phenol demonstrate the catalytic
potential of neat and encapsulated VO(IV) complexes, demonstrat-
ing the higher conversion in comparison to VO-Y. This indicates that
the presence of the ligand in the complex significantly increase its
catalytic activity in comparison to VO-Y. It also indicates that the
changes in electronic environment about the central vanadium ions
influence the catalytic performance by 10%. This also serves as evi-
40
35
30
25
20
15
10
5
0
[VO(L1)]
[
[
[
VO(L2)]
VO(L1)]-Y
VO(L2)]-Y
VO-Y
H-Y
1
dence for the successful complexation of VO(IV) ions with HL or
2
HL ligands inside the zeolitic host.
Blank
Fig. 6 shows that % phenol conversion for the encapsulated cata-
lysts increases considerably within 2 h reaching a steady state after
1
2
3
4
5
6
7
8
Time (h)
3
h. The encapsulated catalysts exhibit similar conversion to the
neat complexes. However, the TOF values for the heterogeneous
catalysts were much higher than the homogenous systems by more
than 100-fold.
Fig. 6. Kinetic plot for phenol conversion over various catalysts. Reaction condi-
tions: Phenol 2.35 g, H2O2 2.83 g, catalyst 10 mg, CH3CN 3 ml, 70 C.
◦
In terms of selectivity the heterogeneous catalysts were more
selective for CAT formation compared to the homogeneous reac-
tion. In both homogeneous and heterogeneous catalysts, the
selectivity towards catechol predominated.
and are characteristics of the microporous nature of the materi-
als. This supports the observation that the complexes are present
within the zeolite cages and not on the external surface since the
zeolite crystallinity was retained.
3
.2.2. Oxidation of benzene
3
.2. Catalytic activity studies
The results of the conversion of benzene over time for various
◦
catalysts at 70 C and in the absence of catalyst are illustrated in
Fig. 7. The reaction proceeds selectively for formation of phenol,
since no other products were detected. Using [VO(L )(acac)]-Y and
The synthesized catalysts were screened for various oxidation
reactions under optimized reaction conditions [19,32].
1
2
[
VO(L )]-Y, benzene conversion is 7.4 and 6.5% with TOF value of
3
.2.1. Oxidation of phenol
The catalytic activities and product selectivity of encapsulated
1413 and 1138, respectively. On the other hand, compared with
the corresponding heterogeneous catalysts, the neat complexes,
[VO(L )(acac)] and [VO(L )] showed higher conversion of 12.2 and
10.8% with TOF values of 16 and 13, respectively.
Oxidation of benzene catalyzed by VO-Y and H-Y was also
attempted to investigate the active site for heterogeneous catalysts
1
2
VO(IV) complexes and their neat complexes along with it parent
compounds for the oxidation of phenol are represented in Fig. 6
and Table 4. The liquid-phase hydroxylation of phenol using these
catalysts and H O as an oxidant was studied as a function of time.
2
2
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Table 4
%
Phenol conversion,a product selectivity and TOF values.
TOF (h 1
−
)
Catalyst
% Phenol conversion
% Product selectivity
CAT
CAT/HQ
HQ
1
[
[
[
[
VO(L )(acac)]
34.7
30.4
23.6
20.8
8.5
50.7
53.0
65.2
59.6
81.0
95.8
94.2
49.3
47.0
34.8
40.4
19.0
4.2
1.0
1.1
1.9
1.5
4.3
45
37
4554
5377
644
–
2
VO(L )]
1
VO(L )(acac)]-Y
2
VO(L )]-Y
VO-Y
H-Y
Blank
3.5
0.6
22.8
16.2
5.8
–
a
◦
Reaction conditions: phenol 2.35 g, H2O2 2.83 g, catalyst 10 mg, CH3CN 3 ml, 70 C, 6 h.
6
5
4
3
2
1
0
0
0
0
0
0
100
80
[
[
[
VO(L1)]
VO(L2)]
VO(L1)]-Y
[
[
[
[
VO(L1)]
VO(L2)]
VO(L1)]-Y
VO(L2)]-Y
6
4
2
0
0
0
VO(L2)-Y
VO-Y
VO-Y
H-Y
H-Y
Blank
0
0
1
2
3
4
5
6
24
Blank
0
Time(h)
0
1
2
3
4
5
6
24
Fig. 8. Kinetic plot for styrene conversion over various catalysts. Reaction condi-
tions: Styrene 2.60 g, H2O2 2.83 g, 0.010 g catalyst, CH3CN 3 ml, 70 C.
Time (h)
◦
Fig. 9. Kinetic plots for cyclohexene conversion over various catalysts. Reaction
◦
conditions: Cyclohexene 2.06 g, H2O2 2.83 g, catalyst 10 mg, CH3CN 3 ml, 70 C.
and gave phenol (1.0–4.3%) that was lower than when encapsu-
lated complexes was utilized. The TOF value for the OV-Y was 323.
Less than 1% phenol was detected in the absence of catalyst (blank
reaction). This also substantiates the presence of transition metal
complexes that are the active site and not the vanadium ions in
zeolite.
in low yield is more likely. Blank experiment in the absence of
catalyst and in the presence of oxidant under the same experimen-
tal conditions was negligible for the oxidation of styrene suggesting
that the activity of the encapsulated catalysts is mainly due to the
presence of the metal complexes in the zeolite cavities.
Significant increases in the performance of these catalysts were
observed on increasing the reaction time and level-off after 3 h.
Further increase in reaction time slightly enhanced the catalysts’
performance.
3.2.3. Oxidation of styrene
The catalytic oxidation of styrene using the neat and the cor-
responding encapsulated complexes was studied and results are
1
2
presented in Table 5 and Fig. 8. [VO(L )(acac)]-Y and [VO(L )]-Y
catalysts showed 15.4 and 12.6% conversion of styrene after 6 h,
respectively. As shown in Table 5, oxidation of styrene was more
prominent with neat metal complexes in comparison to encapsu-
lated metal complexes. This could be due to diffusion resistance of
styrene molecules through the zeolite windows [33].
Although the styrene conversion was higher with the neat com-
plexes, the TOF values for the heterogeneous catalysts were much
higher that than of the homogenous analogues.
3.2.4. Oxidation of cyclohexene
Although conversion of styrene using encapsulated catalysts is
distinctly low, the selectivity for benzaldehyde is much higher than
that of the neat complexes. This may be due to the acidic nature
of the zeolite matrix that leads to styrene epoxide ring opening
Fig. 9 presents the % cyclohexene conversion details as a func-
tion of time in the presence of various catalysts as well as in
the absence of catalyst. Table 6 summarizes the % conversion of
cyclohexene for these catalysts and the selectivities for the various
reaction products. As depicted in Fig. 9, both the homogeneous and
heterogeneous catalysts were efficient in cyclohexene oxidation
providing good yields. Encapsulated catalysts led to high conver-
sions reaching values up to 75–77% whereas, the neat complexes
showed a higher conversion rate of cyclohexene into products than
of their encapsulated analogues (83–87%).
[
34]. In general, styrene oxidation gave benzaldehyde as the pre-
dominant product which may be due to over oxidation of styrene
oxide by nucleophilic attack by H O [35] or through direct oxida-
2
2
tive cleavage of the styrene side chain double bond by a radical
reaction mechanism [36]. Formation of 1-phenylethane-1,2-diol is
possibly due to hydrolysis of styrene oxide and to the oxidation of
styreneoxide by a nucleophilic attack of hydroperoxy species on
styreneoxide, followed by cleavage of the intermediate hydroper-
oxystyrene. Formation of other products, e.g. phenylacetaldehyde
could be through isomerization of styrene oxide and is a much
slower process. Other products such as phenylacetaldehyde may
form through further oxidation of benzaldehyde or through isomer-
ization of styreneoxide. Therefore, the presence of styrene oxide
In terms of selectivity, the % yield for cyclohexene oxide is
quite satisfactory (25–39%). It has been reported that H O2 gives
2
much better epoxidation results than tert-butyl hydroperoxide
(TBHP). With H O , the oxidation occurs mostly on the double
2
2
bond giving cyclohexene oxide in good yields, while TBHP promotes
the allylic pathway [37]. The formation of the allylic oxidation
products 2-cyclohexene-1-one and 2-cylohexene-1-ol shows the
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Table 5
Styrene conversion,a product selectivity and TOF values.
TOF (h 1
−
)
Catalyst
% Styrene conversion
% Product selectivity
Bzald
Phacetal
StyO
PhEtdiol
Bzacid
1
[
[
[
[
VO(L )(acac)]
51.8
47.5
15.4
12.6
8.0
51.7
51.0
75.9
78.3
89.6
86.8
89.7
7.2
4.9
6.1
3.6
–
1.5
0.1
6.7
6.2
10.4
6.9
1.0
1.7
6.2
2.3
–
38.6
42.3
5.1
9.6
–
67
57
2972
2229
606
–
2
VO(L )]
1
VO(L )(acac)]-Y
2
VO(L )]-Y
VO-Y
H-Y
Blank
3.2
2.0
3.1
3.1
3.2
3.4
–
–
3.8
–
a
◦
Reaction conditions: styrene 2.60 g, H2O2 2.83 g, 0.010 g catalyst, CH3CN 3 ml, 70 C, 6 h.
Table 6
Cyclohexene conversion,a product selectivity and TOF values.
TOF (h 1
−
)
Catalyst
% Cyclohexene conversion
% Product selectivity
CyOx
CyOl
CyOne
CydiOl
Other
1
[
[
[
[
VO(L )(acac)]
87.2
82.6
74.6
76.6
16.0
8.5
28.4
29.0
25.4
41.2
25.4
49.9
53.1
46.0
34.6
24.8
26.2
24.8
12.9
28.1
21.7
33.4
48.3
35.6
48.3
16.5
12.9
2.5
1.3
1.7
–
1.7
6.1
2.9
1.3
1.8
0.2
–
0.2
4.7
3.1
112
100
14,465
13,615
1219
–
2
VO(L )]
1
VO(L )(acac)]-Y
2
VO(L )]-Y
VO-Y
H-Y
Blank
4.9
–
a
◦
Reaction conditions: cyclohexene 2.06 g, H2O2 2.83 g, catalyst 10 mg, CH3CN 3 ml, 70 C.
preferential attack of the activated C H single bond over the C
C
3.3. Possible reaction pathway of catalysts
double bond. It seems that the diol product resulted from the epox-
ide ring opening under the aqueous acidic conditions. Furthermore,
the cyclohexene conversion was relatively low around 4.9–8.5% in
the absence of the catalyst or H-Y. These results have clearly indi-
cated that an enhancement in the conversion percentages in the
presence of the neat and the encapsulated catalysts confirm the
determining role played by metal complexes encapsulated in the
zeolite [38].
In order to better understand the reaction pathway and inter-
mediate species formed during oxidation of the substrates, we
monitored the progress of the reaction using electronic absorption
spectroscopy by treating a methanolic solution of the neat VO(IV)
complexes with a methanolic solution of H O . As shown in Fig. 10a,
2
2
−
4
addition of a methanolic solution of H O dropwise to a 10
M
2
2
1
methanolic solution of VO(L )(acac) resulted in slight increase in
the intensities for the bands at 202, 241, and 308 nm, while the
intensity of the band at 374 nm remained constant without chang-
ing their positions. On the other hand, the dropwise addition of
3.2.5. Stability and recyclability of the catalyst
The stability and reusability of the encapsulated catalysts were
investigated for the oxidation of phenol. The catalyst was recovered
−
3
1
methanolic H O2 to 10 M methanol solution of VO(L )(acac)
2
by filtration and regenerated by Soxhlet extraction (MeCN), dried at
resulted in spectral changes in the visible region and is depicted
in Fig. 10a (inset). The d–d band in VO(L )(acac) at 773 and 573 nm
◦
1
1
50 C for 6 h and reused for two consecutive runs. The recycled cat-
alyst was found to exhibit comparable activity and selectivity with
that for the fresh catalysts under the same reaction conditions. The
were decreased in intensity and the band 573 nm was disappeared
with further addition of H O . These changes are due to the gen-
2
2
%
conversion was lowered by 2–3% but reserved its selectivity. On
eration of oxoperoxovanadium(V) species [39], where the peroxo
complex, thus formed, transfers one of its oxygens to the substrate
during oxidation. Disappearance of the d–d band and the appear-
ance of the isosbestic point suggests the transformation of OV(IV)
complex into a peroxo species.
the other hand, the oxidation did not continue after removal of the
catalyst from the reaction mixture, thus confirming the necessity
of the catalyst for the reaction to proceed and that no leaching of
vanadium species from the catalyst had occurred.
1
2
Fig. 10. UV–Visible spectral studies of (a) VO(L )(acac) and (b) VO(L ) after the successive dropwise addition of methanolic solution of H2O2.
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0
0
0
.4
.2
.0
a
0
.2
.0
b
0
-
-
-
-
-
0.2
0.4
0.6
0.8
1.0
-
-
-
-
-
0.2
0.4
0.6
0.8
1.0
L2
VO(L1)(acac) + Phenol
VO(L1)(acac) + Phenoxide
VO(L1)(acac)
L1
Phenoxide
PhOH
3
2
1
0
-1
2
.0
1.5
1.0
0.5
0.0
-0.5
-1.0
Potential / V
Potential / V
1
2
1
1
Fig. 11. Cyclic voltammograms of (a) HL , HL , phenol and phenoxide ion (b) VO(L )(acac) (0.01 M) in MeCN, VO(L ) in presence of phenoxide and phenol in dry acetonitrile
in 0.1 M LiClO4 solution (scan rate 100 mV/s).
Similarly, drop wise addition of H O dissolved in MeOH to a
M methanolic solution of VO(L ) resulted in an immediate
loading of metal complexes, and for this reason a carbon paste
electrode was used.
2
2
2
−
4
1
0
decrease in intensity of the strong band at 303 nm which disap-
peared after the second addition of H O . The peak at 270 nm
becomes more distinguishable from the other bands after the
2
2
4. Conclusion
immediate addition of H O2 with a considerable increase in the
2
In this research, oxovanadium(IV) Schiff-base complexes are
intensity. Thereafter it decreased with further addition of H O .
2
2
2
encapsulated in the nanopores of H-Y zeolites using the flexible
ligand method. Encapsulated complexes exhibit fairly clear evi-
dence in the physico-chemical (XRD, BET) and spectral studies
(FTIR, UV) for the well-defined inclusion and distribution of com-
plexes in the nanopores of the zeolite matrix. It was found that
the encapsulated oxovanadium(IV) catalysts are efficient catalysts
The changes in the electronic spectrum of VO(L ) upon reaction
with H O in methanol, is shown in Fig. 10b.
2
2
The same pattern in the visible region was also observed with
2
VO(L ) (inset Fig. 10b). From the evidence of these kinetic studies,
suggests the oxidation of oxovanadium(IV) complex to give oxoper-
oxovanadium(V) species [39]. Oxoperoxovanadium (V) complexes
are very active intermediate species and participate in oxida-
tion reactions by transferring one of its oxygens to the substrate
for oxidation of cyclohexene with H O2 and reasonably good for
2
oxidation of phenol and benzene, but weakly active for oxidation
of styrene. This can be ascribed to a large extent due to shape
selective effects in zeolite pores not allowing the relatively large
substrates to access catalytic centres easily and for products exit
from the channels freely. Oxidation of benzene (<12% conversion)
gives phenol with 100% selectivity. For all reactions tested, a high
selectivity in competitive reactions is observed, which is correlated
to molecular sieving effects (reactants size, selectivity effect). It
can be unambiguously seen from the controlled experiments that
oxovanadium(IV) species are indeed the active centre and transi-
tion metal ions do not play any major role. Results show that the
homogeneous catalytic system is more active in the case of phe-
nol, benzene, cyclohexene and styrene having advantages of higher
conversions and shorter reaction times than the heterogeneous cat-
alysts. Encapsulated complexes, however, can be recovered and
reused without loss of catalytic activity making them better than
their neat analogues. The greater stability is due to the suppression
of dimeric and other polymeric oxo complexes of VO(IV) attributed
to geometric constraints on their formation on encapsulation in
zeolites. These results, together with spectral analyses, suggest that
interaction between the catalyst and hydrogen peroxide gives rise
to peroxovanadium(V) intermediate species which are responsible
for transfer of oxygen atoms to the substrates.
[
19,40].
3.4. Electrochemistry and electrocatalysis
Catalyzed electrochemical-oxidation of the oxovanadium com-
plexes were investigated using the cyclic voltammetry technique.
The current voltage characteristics were recorded in blank runs for
phenol and sodium phenoxide (0.01 M) dissolved in water. Mod-
ification in the nature and position of the oxidation waves of the
substrate in the presence of the catalyst were carefully recorded.
The electrochemical behaviour of the neat complexes was studied
in the presence of phenoxide ion (Fig. 11). Behaviour of the phe-
noxide complexes is unlike that of sodium phenoxide which show
irreversible oxidation at Ep,a 0.46 versus Ag/AgCl, under identical
1
2
conditions. The cyclic voltammogram for VO(L )(acac) and VO(L )
in acetonitrile exhibit an irreversible peak oxidation wave at 1.62
and 0.73 V, respectively (Fig. 11).
From Fig. 11, it is clear that ligand voltammograms did not
shown any change while marked shift in oxidation peak were
observed for oxovanadium complexes with coordinating phenox-
ide ions. Furthermore, phenol did not display any electrode process
up to a potential of 2.0 V over the glassy carbon electrode in dry
acetonitrile, nor even in the presence of the complexes. Hence it
was concluded that the presence of the phenoxide ion was essen-
tial for the catalysis process. It is likely that the oxidative electron
transfer occurs through phenoxide coordinated species. It could be
established that the presence of the complexed species facilitated
the oxidation process [41].
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Please cite this article in press as: G. von Willingh, et al., Selective oxidation reactions over tri- and tetradentate oxovanadium(IV) complexes
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Please cite this article in press as: G. von Willingh, et al., Selective oxidation reactions over tri- and tetradentate oxovanadium(IV) complexes
encapsulated in zeolite-Y, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.015