Oxovanadium(iv)-catalysed oxidation of dibenzothiophene and 4,6-dimethyldibenzothiophene

Article abstract of DOI:10.1039/c2dt31433a

The reaction between [V^IVOSO₄] and the tetradentate N₂O₂-donor Schiff base ligand, N,N-bis(o- hydroxybenzaldehyde)phenylenediamine (sal-HBPD), obtained by the condensation of salicylaldehyde and o-phenylenediami

Full text of DOI:10.1039/c2dt31433a

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PAPER
Oxovanadium(IV)-catalysed oxidation of dibenzothiophene and
4,6-dimethyldibenzothiophene†
Adeniyi S. Ogunlaja,a Wadzanai Chidawanyika,a Edith Antunes,b Manuel A. Fernandes,a Tebello Nyokong,a
Nelson Tortoa and Zenixole R. Tshentu*a
Received 2nd July 2012, Accepted 11th September 2012
DOI: 10.1039/c2dt31433a
The reaction between [VIVOSO4] and the tetradentate N2O2-donor Schiff base ligand, N,N-bis-
(o-hydroxybenzaldehyde)phenylenediamine (sal-HBPD), obtained by the condensation of salicylaldehyde
and o-phenylenediamine in a molar ratio of 2 : 1 respectively, resulted in the formation of
[VIVO(sal-HBPD)]. The molecular structure of [VIVO(sal-HBPD)] was determined by single crystal X-ray
diffraction, and confirmed the distorted square pyramidal geometry of the complex with the N2O2 binding
mode of the tetradentate ligand. The formation of the polymer-supported p[VIVO(sal-AHBPD)]
proceeded via the nitrosation of sal-HBPD, followed by the reduction with hydrogen to form an amine
group that was then linked to Merrifield beads followed by the reaction with [VIVOSO4]. XPS and EPR
were used to confirm the presence of oxovanadium(IV) within the beads. The BET surface area and
porosity of the heterogeneous catalyst p[VIVO(sal-AHBPD)] were found to be 6.9 m2 g−1 and 180.8 Å
respectively. Microanalysis, TG, UV-Vis and FT-IR were used for further characterization of both
[VIVO(sal-HBPD)] and p[VIVO(sal-AHBPD)]. Oxidation of dibenzothiophene (DBT) and
4,6-dimethyldibenzothiophene (4,6-DMDBT) was investigated using [VIVO(sal-HBPD)] and
p[VIVO(sal-AHBPD)] as catalysts. Progress for oxidation of these model compounds was monitored with
a gas chromatograph fitted with a flame ionization detector. The oxidation products were characterized
using gas chromatography-mass spectrometry, microanalysis and NMR. Dibenzothiophene sulfone
(DBTO2) and 4,6-dimethyldibenzothiophene sulfone (4,6-DMDBTO2) were found to be the main
products of oxidation. Oxovanadium(IV) Schiff base microspherical beads, p[VIVO(sal-AHBPD)], were
able to catalyse the oxidation of sulfur in dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene
(4,6-DMDBT) to a tune of 88.0% and 71.8% respectively after 3 h at 40 °C. These oxidation results show
promise for potential application of this catalyst in the oxidative desulfurization of crude oils.
own disadvantages in that it usually requires high temperatures
and the use of hydrogen which makes the end product expensive.
Introduction
The Environmental Protection Agency (EPA) requirement to
produce diesel fuels with a very low sulfur content has stimu-
lated much work in the area of desulfurization.1 Organosulfur
containing compounds present in diesel fuels such as benzothio-
phene, dibenzothiophene and 4,6-dimethyldibenzothiophene can
poison the catalysts used to remove the residue of hydrocarbons
and nitrogen oxides derived from combustion reactions.1
The oxidative desulfurization (ODS) technique is now seen as
the alternative for removing sulfur from fuels. The ODS method
involves two steps: (1) oxidation of the sulfur compounds and
(2) liquid extraction of the oxidized products. Oxidative desul-
furization (ODS) has several advantages compared to the latter
such as mild reaction conditions (low temperatures), high selec-
tivity, and the potential ability to oxidize sterically hindered
sulfur compounds. The oxidation of sulfur to sulfoxides and sul-
fones can be achieved by a variety of biological and chemical
methods.1,2 Several research groups have explored the possibility
of using the ODS methods to remove organosulfur compounds
from diesel fuels using tert-butylhydroperoxide (an organic
oxidant) as well as hydrogen peroxide (H2O2).3–10
Hydrodesulfurization (HDS) has been a technique used for
desulfurization in the energy industries, but this process has its
aDepartment of Chemistry, Rhodes University, P.O. Box 94,
Grahamstown, 6140, South Africa. E-mail: z.tshentu@ru.ac.za;
Fax: +27 46622 5109; Tel: +27 46603 8846
bMolecular Science Institute, School of Chemistry, University of
Witwatersrand, Private Bag 3, Wits, Johannesburg 2050, South Africa
†Electronic supplementary information (ESI) available. CCDC 872749
contains the crystallographic data of [VIVO(sal-HBPD)]. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt31433a
Vanadium metal complexes have been employed as catalysts
in the oxidation of sulfur-containing compounds over the last
few years.11–19 The use of oxovanadium(IV) complexes as cata-
lysts in the oxidation of sulfur-containing compounds has been
very promising. When oxovanadium(IV) complexes are oxidized
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in the presence of either H2O2 or tert-butylhydroperoxide, an
oxoperoxovanadium(V) compound is formed, which is the active
species.20 In the present work we have employed a tetradentate
Schiff base ligand as the anchor for vanadyl in the polymer
matrix for the stabilization of this metal centre. It has been
reported that the leaching of the metal ion from polymer supports
can be improved if the denticity of the ligand could be
increased.9,21 The aim of the present work was to carry out an
oxidation study of 4,6-dimethyldibenzothiophene and dibenzo-
ionization detector (FID) and a Zebron Phenomenex DB-5MS:
340 °C capillary column (30 m × 320 μm × 1 μm).
The polymeric microspherical beads were imaged using a
TESCAN Vega TS 5136LM scanning electron microscope
(SEM). Nitrogen adsorption/desorption isotherms were measured
at (350 °C) 77 K using a Micromeritics ASAP 2020 Surface
Area and Porosity Analyzer. Prior to the measurement the beads
(p[(sal-AHBPD)] and p[VIVO(sal-AHBPD)]) were degassed at
150 °C for 10 days after which they were free of moisture.
Atomic Force Microscopy (AFM) imaging was performed
employing a CP-11 Scanning Probe Microscope from Veeco
Instruments in the non-contact mode at a scan rate of 2 Hz using
an MP11123 cantilever. EPR spectra were recorded with a
Bruker ESP 300E X-band spectrometer, and the spin Hamil-
tonian parameters were obtained by simulation of the spectra
with the computer program of Rockenbauer and Korecz.22 X-ray
photoelectron spectroscopy (XPS) measurements on the beads
were performed with a Kratos Axis Ultra X-ray Photoelectron
Spectrometer equipped with a monochromatic Al Kα source
(1486.6 eV). The base pressure of the system was below
3 × 10−7 Pa. XPS experiments were recorded with a 75 W power
source using a hybrid-slot spectral acquisition mode and an
angular acceptance angle of +/− 20°. XPS data analysis was
performed with the Kratos version 2 program.
thiophene (model compounds) using
a polymer-anchored
oxovanadium(IV) metal complex as the catalyst precursor. Leach-
ing studies were carried out to confirm the stabilization of
vanadium(IV) by the Schiff base ligand. The activity of the
heterogeneous catalyst p[VIVO(sal-AHBPD)] was also compared
to its homogeneous [VIVO(sal-HBPD)] counterpart.
Experimental
Material
Dibenzothiophene (Merck Chemicals, USA), 4,6-dimethyldi-
benzothiophene (Sigma-Aldrich, South Africa), Merrifield beads
(5% crosslinked) (Sigma-Aldrich, South Africa), o-phenylene-
diamine (Sigma-Aldrich, South Africa), tert-butylhydroperoxide
(Sigma-Aldrich, Germany) and salicylaldehyde (Merck Chemi-
cals, USA) were used in this study. Vanadium(IV)oxide sulfate
trihydrate [VIVOSO4·3H2O] was obtained from Sigma-
Aldrich (Germany). Hexane (HPLC grade), dimethylformamide
(DMF) and toluene (HPLC grade) were procured from Merck
Chemicals (South Africa). All other chemicals and solvents were
of analytical grade (>99%) and were used without further
purification.
Metal content determination
The vanadium content on both the beads, p[VIVO(sal-AHBPD)],
and the unsupported complex, [VIVO(sal-HBPD)], was deter-
mined by weighing out 0.025 g of each into different vials, and
10 mL of TraceSelect HNO3 (69%) was added to each vial. This
mixture was heated up to a temperature of 40 °C for 12 h to
leach out the vanadium. The acid-leached solution was then
diluted with deionized-distilled water to 100 mL and analysed
by ICP-OES. Vanadium leached during the oxidation reaction
was also determined in the same manner. However, the reaction
products were evaporated to dryness before the acids were added
for digestion.
Instrumentation
FT-IR spectra (4000–650 cm−1 and 700–30 cm−1) were run on
both the Perkin Elmer 100 ATR-FTIR and the Perkin Elmer 400
FT-IR spectrometers respectively. The solid reflectance spectra
of the metal complexes were recorded on a Shimadzu UV-VIS-
NIR Spectrophotometer UV-3100 with an MPCF-3100 sample
compartment with samples mounted between two quartz discs
which fit into a sample holder coated with barium sulfate. The
spectra were recorded over the wavelength range of
2000–250 nm, and the scans were conducted at medium speed
using a 20 nm slit width. A Gallenkamp melting point apparatus
(temperature range, 0–350 °C) was used to determine the ligand
melting point. Microanalysis was carried out using an Elementar
Analysen Systeme Vario® MICRO VI 6.2 GmbH. The vanadium
content on the beads was determined by using a Thermo
Electron (iCAP 6000 Series) inductively coupled plasma (ICP)
spectrometer equipped with an OES detector. The 1H NMR
spectra were recorded on a Bruker 400 MHz spectrometer in
DMSO-d6. Thermal stability of the materials was studied using a
Perkin-Elmer TGA 7 Thermogravimetric Analyzer. The heating
rate was suitably maintained at 10 °C min−1 under a nitrogen
atmosphere/static air from 50 °C temperature up to 750 °C. The
progress of the catalysed reactions was monitored using an
Agilent 7890A gas chromatograph (GC) fitted with a flame
GC-FID/MS study
To determine the initial and residual concentration of the selected
sulfur compound in the organic phase, approximately 0.2 mL
aliquots of liquid samples were withdrawn from the reactor at
fixed time intervals and analyzed by using a gas chromatograph.
Prior to analysis, GC conditions were optimised to efficiently
separate the products from the reactants in the Zebron Pheno-
menex DB-5 column. Helium was used as the carrier gas at a flow
rate of 1.63 mL min−1 with an average velocity of 30.16 cm s−1
and a pressure of 63.73 kPa. The analysis run was started with an
oven temperature of 80 °C ramping to 100 °C for 2 min, and then
increasing the temperature to 320 °C at a rate of 30 °C min−1
.
The oxidation products were confirmed by using a Bruker gas
chromatograph equipped with a mass spectrometer (Bruker
GC-MS-Unit II), and fitted with a non-polar DB-1 column.
Helium gas was also used as the carrier gas, and the same GC
condition employed for the GC-FID was also used in the
GC-MS. For the Mass Spectrometer (MS) the following
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conditions were used: an MS ion source temperature of 200 °C
with an interface temperature of 200 °C and a scan range of m/z
60 to 500.
ethanol, 26.6 mL of salicylaldehyde (0.25 mol) was added and
the mixture stirred while heating at 60 °C for 30 min. After
cooling to room temperature, the yellow precipitate of sal-HBPD
was filtered, washed with methanol to remove impurities and
then dried in the oven. Yield: 94%; mp: 154–157 °C (yellow
crystals). FT-IR (cm−1): 1609 ν(CvN), 1158 ν(C–O), 3357
ν(O–H). 1H NMR (δ, ppm): 13.10 (s, 2H, OH), 8.92 (s, 2H,
–HCvN–), 7.44 (d, 2H, aromatic), 7.41–7.48 (m, J = 8.2, 6H,
aromatic), and 6.81–6.98 (m, J = 7.4, 4H, aromatic). Anal. Calcd
(found) for C20H16N2O2 (%): C, 75.93(75.00); H, 5.10(5.78);
N, 8.86(8.94).
Catalytic oxidation studies
The catalytic oxidation reactions of dibenzothiophene and 4,6-
dimethyldibenzothiophene were carried out using the polymer-
anchored oxovanadium(IV) complex p[VIVO(sal-AHBPD)] as
well as the unsupported complex [VIVO(sal-HBPD)] in a 25 mL
flask respectively. In the reaction vessel, an aqueous solution of
tert-butylhydroperoxide (TBHP), dibenzothiophene or 4,6-
dimethyldibenzothiophene was mixed in 10 mL of a mixture of
toluene–hexane (1 : 4) and the reaction mixture was heated at
40 °C with continuous stirring at 500 rpm in an oil bath. The
oxidation reaction was considered to begin after addition of the
required amount of the TBHP. The oxidation reactions for
dibenzothiophene and 4,6-dimethyldibenzothiophene were
carried out separately. Progress of the reaction was monitored by
using GC-FID.
Nitrosation of N,N-bis(o-hydroxybenzaldehyde)phenylene-
diamine (sal-HBPD) Schiff base. The nitrosation of sal-HBPD
Schiff base was carried out by reacting 3.88 g (0.02 mol) of sal-
HBPD Schiff base with 3.40 g (0.05 mol) of sodium nitrite in
1 M HCl (40 mL) at a temperature of approximately 0 °C for 12 h.
The N,N-bis(4-nitroso-o-hydroxybenzaldehyde)phenylenediamine
(sal-NHBPD) produced was filtered and washed thoroughly with
hot distilled water to remove reaction impurities. Yield: 86.7%;
mp: 108–110 °C (yellow powder). FT-IR (cm−1): 1603 ν(CvN),
1
1154 ν(C–O), 3357 ν(O–H), 1704 ν(NO). H NMR (δ, ppm):
10.40, 9.41 (s, 2H, OH), 8.12 (s, 2H, –HCvN–), 7.44 (d, 2H,
aromatic), 7.41–7.48 (m, 6H, aromatic), and 6.81–6.98 (m, J =
10.4, 4H, aromatic). Anal. Calcd (found) for C20H15N4O4 (%):
C, 64.17(64.22); H, 3.77(4.01); N, 14.97(15.14).
Catalyst preparation
Ligand preparation was adopted from the synthetic route
(Scheme 1) reported by Gupta and Sutar23 with a few modifi-
cations which are noted below.
Reduction of N,N-bis(4-nitroso-o-hydroxybenzaldehyde)-
phenylenediamine (sal-NHBPD) Schiff base. The reduction of
N,N-bis(4-nitroso-o-hydroxybenzaldehyde)phenylenediamine
(sal-NHBPD) was carried out using 3.74 g (0.01 mol) of
Preparation of N,N-bis(o-hydroxybenzaldehyde)phenylenedi-
amine (sal-HBPD) Schiff base. To a solution of o-phenylene-
diamine 10.81 g (0.1 mol) dissolved in 100 mL of absolute
Scheme 1 Syntheses scheme for p[VIVO(sal-AHBPD)] and VIVO(sal-HBPD).23
13910 | Dalton Trans., 2012, 41, 13908–13918
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nitrosated sal-NHBPD Schiff base in 1 M HCl (30 mL) in the
presence of metallic iron (2 g), which produced N,N-bis-
(4-amino-o-hydroxybenzaldehyde)phenylenediamine (sal-AHBPD)
Schiff base. Yield: 80.2%; mp: 101–105 °C (brown powder).
1
FT-IR (cm−1): 1610 ν(CvN), 1161 ν(C–O), 3352 ν(O–H). H
NMR (δ, ppm): 10.20, 9.41 (s, 2H, OH), 8.12 (s, J = 8.2, 2H,
–HCvN–), 7.52 (d, J = 6.1, 2H, aromatic), 7.72–7.31 (m, J =
7.7, 8H, aromatic), and 7.11–6.98 (m, J = 7.9, 31.7, 4H, aro-
matic). Anal. Calcd (found) for C20H17N4O2 (%): C, 68.35
(68.34); H, 5.24(4.90); N, 16.17(16.32).
Fig. 1 Scanning electron microscopy (SEM) micrograph of (A) chloro-
methylated polystyrene beads, (B) ligand-anchored beads and (C)
vanadium incorporated beads p[VIVO(sal-AHBPD)].
[VIVO(sal-HBPD)]·1.5H2O. The
unsupported
vanadium
complex [VIVO(sal-HBPD)] was synthesized by dissolving
[VIVOSO4] 0.16 g (0.001 mol) into a solution of sal-HBPD
Schiff base 1.28 g (0.004 mol) in 25 mL of methanol. The reac-
tion was allowed to proceed for 6 h at 50 °C. The precipitate
formed was filtered and washed with excess methanol and
ethanol. The product [VIVO(sal-HBPD)] was then dried in a
desiccator over silica gel. Yield: 93.1%; mp: >300 °C. FT-IR
(cm−1): 1627 ν(CvN), 1161 ν(C–O), 990 ν(VvO). Anal. Calcd
(found) for C40H34N4O9V2 (%): C, 58.83(60.28); H, 4.20(3.93);
N, 6.86(6.87).
Fig. 2 AFM images of (a) p[sal-AHBPD], (b) p[VIVO(sal-AHBPD)]
before use and (c) p[VIVO(sal-AHBPD)] after use. Scale: 2 × 2 μM.
p[VIVO(sal-AHBPD)]. 1.2 g of sal-AHBPD Schiff base and
1.5 g of chloromethylated polystyrene beads were mixed in
dimethylformamide (DMF) and the reaction mixture was heated
in an oil bath with stirring under reflux at 70 °C for 48 h.
The resulting beads were filtered and then Soxhlet extracted
with methanol to remove the unreacted ligand and further
washed with hot DMF followed by hot methanol. The resulting
beads were further suspended in a DMF solution containing 1 g
(0.0061 mol) of [VIVOSO4] while stirring at 60 °C for 6 h.
The resulting beads were filtered and washed with methanol
using Soxhlet extraction to remove excess [VIVOSO4]. A light
green colouration was observed indicating the presence of
vanadium(IV) on the beads. FT-IR (cm−1): 1627 ν(CvN), 1161
ν(C–O), 990 ν(VvO). Found (%): C, 71.22; H, 7.231; N, 4.99;
V, 5.64.
Results and discussion
Characterisation of the catalysts
SEM, AFM, BET and XPS. The scanning electron micro-
graphs (SEM) of Merrifield beads (5% crosslinked), ligand-
anchored beads and p[VIVO(sal-AHBPD)] are presented in
Fig. 1. A significant change in average diameter and surface
morphology was observed before and after functionalization.
The BET surface area and porosity of the Merrifield beads were
found to be 10.44 m2 g−1 and 197.36 Å respectively. Upon func-
tionalization to p[VIVO(sal-AHBPD)], the surface area and poro-
sity were 6.93 m2 g−1 and 180.75 Å respectively. The drop in
surface area and porosity can be attributed to the ligand and
vanadium(IV) occupying the pores thereby reducing the surface
area and porosity. The nitrogen and vanadium composition
obtained from elemental analysis suggested a V: sal-AHBPD
ratio of 1 : 0.8 within the beads which is close to the expected
1 : 1 ratio for the tetradentate coordination of sal-AHBPD.
Surface roughness as measured by AFM (Fig. 2a–c) shows
the surface morphology of the beads p[(sal-AHBPD)], p[VIVO-
(sal-AHBPD)] and p[VIVO(sal-AHBPD)] after use. The surface
roughness and mean height for p[(sal-AHBPD)] were observed
to be 4.18 Ra nm−1 and 21.31 nm respectively. On reacting
p[(sal-AHBPD)] with vanadyl sulfate [VIVOSO4] to form
p[VIVO(sal-AHBPD)], the surface roughness and mean height
decreased to 1.29 Ra nm−1 and 8.63 nm respectively. This
shows that oxovanadium(IV) has been locked within the ligands
in the beads pores thereby reducing its porosity, and this
phenomenon was in agreement with BET surface measurements.
However, after one cycle of the oxidation studies the surface
roughness and mean height of p[VIVO(sal-HBPD)] (Fig. 2c)
increased to 20.57 Ra nm−1 and 58.66 nm respectively, and this
is in agreement with the morphological changes observed by
SEM.
Structure determination and refinement. Single crystals of
[VIVO(sal-HBPD)] suitable for X-ray analysis were grown by
slow evaporation of the complex in an acetone–methanol (1 : 1)
mixture. Intensity data were collected on a Bruker APEX II
CCD area detector diffractometer with graphite monochromated
Mo Kα radiation (50 kV, 30 mA) using the APEX 2 data collec-
tion software.24a The collection method involved ω-scans of
width 0.5° and 512 × 512 bit data frames. Data reduction was
carried out using the program SAINT+24b and face indexed
absorption corrections were made using XPREP.24b The crystal
structure was solved by direct methods using SHELXTL.25
Non-hydrogen atoms were first refined isotropically followed by
anisotropic refinement by full matrix least-squares calculations
based on F2 using SHELXTL. Hydrogen atoms were first
located in the difference map then positioned geometrically and
allowed to ride on their respective parent atoms. Diagrams and
publication material were generated using SHELXTL and
PLATON.26
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Fig. 4 TG curves of p[(sal-AHBPD)], [VIVO(sal-AHBPD)] and
p[VIVO(sal-AHBPD)].
Fig. 3 High resolution wide scan XPS spectra of (a) p[(sal-AHBPD)],
(b) p[VIVO(sal-AHBPD)] and (c) p[VIVO(sal-AHBPD)]. Inset shows the
expanded V 2p and N 1s signals.
Table 1 Elemental binding energies of p[(sal-AHBPD)] and p[VIVO-
(sal-AHBPD)] as determined by XPS
Binding energy (eV)
Polymer
O 1s N 1s C 1s
V 2p3/2 V 2p1/2
p[(sal-AHBPD)]
531.2 398.2 283.0
531.4 398.0 283.2 514.5
—
—
522.1
522.1
p[VIVO(sal-AHBPD)]
p[VIVO(sal-AHBPD)] used 531.3 399.0 283.0 515.0
XPS was used to investigate the surface chemistry (expected
elements) of p[(sal-AHBPD)], p[VIVO(sal-AHBPD)] and
p[VIVO(sal-AHBPD)] after use. The XPS spectra (Fig. 3) com-
plemented the microanalysis results, showing all the expected
atoms. After functionalization with [VIVOSO4], the O 1s peak
intensity increased significantly, due to the VvO as well as
possibly the presence of some residual solvent (methanol and
ethanol). The V 2p peaks although low in intensity were present
at 522.1 eV (V 2p1/2) and 514.5 eV (V 2p3/2) (Table 1), confirm-
ing the +4 oxidation state of vanadium. There were two N 1s
peaks for p[(sal-AHBPD)], appearing at 398 and 396 eV (Fig. 3
insert) corresponding to the CvN (imine) and NH nitrogen. It
was noticed that the catalyst retained its compounds after it had
been used for one cycle of oxidation. Only qualitative charac-
terization was possible due to the low intensity of the peaks.
Fig. 5 Diffuse reflectance electronic spectra of [VIVO(sal-HBPD)] and
p[VIVO(sal-AHBPD)].
temperature range of 350–650 °C. The thermogram of p[sal-
AHBPD] showed three steps: loss of trapped solvents and intra-
particle water in the range 100–350 °C to a total of ca. 4.7%,
and structural collapse at the temperature range 300–680 °C with
a weight loss of ca. 94.2%. For [VIVO(sal-HBPD)], a stepwise
decomposition of the structure was observed. The initial weight
loss of about 4.6% occurred over the temperature range of
130–340 °C due to the loss of hydrated water molecules. An
overall stepwise decomposition of 75.3% in the complex was
observed between 340–530 °C.
Thermogravimetric analysis
UV-Vis electronic studies
The TG curves for p[(sal-AHBPD)], p[VIVO(sal-AHBPD)] and
[VIVO(sal-HBPD)] are represented in Fig. 4. For p[VIVO(sal-
AHBPD)], surface-trapped water/solvents were removed at a
temperature of around 100 °C while the intra-particle waters
were volatilized in the range of 200–300 °C with a total weight
loss of about 9.73%. The rapid collapse of the p[VIVO(sal-
AHBPD)] with a weight loss of ca. 70.55% occurred in a
The diffuse reflectance spectra of [VIVO(sal-HBPD)] and the
polymer anchored vanadium catalyst p[VIVO(sal-AHBPD)]
exhibited relatively well-matched electronic transitions which are
characteristic of a square pyramidal oxovanadium(IV) complex
(Fig. 5). The electronic spectrum of the neat complex [VIVO(sal-
HBPD)] clearly showed three bands at 405 nm (a1* ← b2),
558 nm (b1* ← b2) and 635 nm (eπ* ← b2) typical of the
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Table 2 Summary of crystal data, data collection and structural
refinement parameters for the complex
Parameters
[VIVO(sal-HBPD)]
Empirical formula
Formula weight (M)
C40H34N4O9V2
816.59
Temperature, T (K)
Wavelength, Mo Kα (Å)
173(2)
0.71073
Crystal system
Space group
Monoclinic
P2(1)/c
a (Å)
b (Å)
19.8921(5)
6.8805(2)
c (Å)
α (°)
27.6507(7)
90.00
β (°)
108.493(2)
90.00
3589.06(17)
4
1.511
0.585
γ (°)
Volume, V (Å3)
Z
Calculated density, ρ (Mg m−3
)
Fig. 6 FT-IR spectra of (A) p[VIVO(sal-AHBPD)] and (B) [VIVO(sal-
HBPD)].
Absorption coefficient, μ (mm−1
F(000)
)
1680
Crystal size (mm3)
Index ranges
0.31 × 0.26 × 0.06
−26 ≤ h ≤ 26, −9 ≤ k ≤ 9,
−36 ≤ l ≤ 35
33 940
five-coordinate square pyramidal geometry.10 The catalyst
p[VIVO(sal-AHBPD)] gave weak adsorption bands and slight
shifts were also observed [365 nm (a1* ← b2) and 680 nm
(eπ* ← b2)]. However, the b1* ← b2 transition was not easily
noticed, and this can be attributed to the weak bands from the
polymer material.
Reflections collected
Independent reflections
Completeness to theta = 28.00°
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
8648 [R(int) = 0.0734]
99.9%
0.9657 and 0.8394
8648/0/496
0.914
R1 = 0.0438, wR2 = 0.0871
R1 = 0.0870, wR2 = 0.0993
0.552 and −0.355
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole (e Å−3
)
IR spectral studies
Upon functionalizing the Merrifield beads, the disappearance of
the CH2–Cl (bending) peak at 1260 cm−1 was observed. The IR
spectra of p[(sal-AHBPD)] and p[VIVO(sal-AHBPD)] exhibit
similar bands for ν(CvN) and ν(C–O) frequencies, except for
the appearance of three new bands for ν(VvO), ν(V–N) and
ν(V–O) in the spectrum of p[VIVO(sal-AHBPD)]. The broad
band appearing in the region 3200–3350 cm−1 in both com-
plexes showed the presence of H2O, in agreement with the TG
results. The IR spectrum of [sal-HBPD] and p[sal-AHBPD]
exhibits two sharp bands at 1606 cm−1 and 1636 cm−1 due to
ν(CvN) (azomethine stretch). These bands undergo a batho-
chromic effect in complexes, indicating coordination of the azo-
methine nitrogen to the vanadium ion. The VvO stretching
frequencies of the complex [VIVO(sal-HBPD)] occurred at
976 cm−1, while ν(VvO) for p[VIVO(sal-AHBPD)] appeared at
982 cm−1 (Fig. 6) which suggested the absence of a –VvO–
VvO– chain structure within the polymeric material.27 The two
non-ligand bands in the region 525–538 cm−1 and
423–436 cm−1 could be assigned to the ν(V–O) and ν(V–N),
respectively.27
Table 3 Selected bond length (Å) and bond angles (°) for [VIVO(sal-
HBPD)]
Bond lengths
(Å)
Bond angles
(°)
C(1)–O(1)
C(20)–O(2)
V(1)–O(1)
V(1)–O(2)
V(1)–O(3)
V(1)–N(1)
V(1)–N(2)
C(7)–N(1)
C(8)–N(1)
C(13)–N(2)
C(14)–N(2)
1.325(3)
1.323(3)
1.9198(2)
1.9249(2)
1.5887(2)
2.0707(2)
2.0652(2)
1.301(3)
1.423(3)
1.436(3)
1.303(3)
O(2)–V(1)–N(1)
O(1)–V(1)–N(2)
C(7)–N(1)–V(1)
C(8)–N(1)–V(1)
O(3)–V(1)–N(2)
O(3)–V(1)–N(1)
N(2)–V(1)–N(1)
O(2)–V(1)–N(1)
O(1)–V(1)–O(2)
O(2)–V(1)–N(2)
O(1)–V(1)–N(1)
143.80(8)
145.13(8)
125.97(1)
125.58(1)
104.31(8)
107.98(8)
143.80(8)
77.83(8)
85.74(7)
87.77(7)
87.55(7)
C(40)–O(5)
C(21)–O(4)
V(2)–O(4)
V(2)–O(5)
V(2)–O(6)
V(2)–N(3)
V(2)–N(4)
C(27)–N(3)
C(28)–N(3)
C(33)–N(4)
C(34)–N(4)
1.328(3)
1.327(3)
1.9209(2)
1.9197(2)
1.5939(2)
2.0423(2)
2.0601(2)
1.295(3)
1.426(3)
1.425(3)
1.309(3)
O(4)–V(2)–N(2)
O(5)–V(2)–N(3)
C(34)–N(4)–V(2)
C(33)–N(4)–V(2)
O(6)–V(2)–N(3)
O(5)–V(2)–N(3)
O(4)–V(2)–N(4)
O(5)–V(2)–O(4)
O(4)–V(2)–N(3)
O(5)–V(2)–N(4)
N(3)–V(2)–N(4)
146.99(8)
144.4(8)
113.70(1)
130.18(2)
106.36(8)
144.40(8)
146.99(8)
85.61(7)
88.21(8)
88.03(7)
78.41(7)
X-ray crystal structure of [VOIV(sal-HBPD]
Crystal and structure refinement data for [VIVO(sal-HBPD)] are
contained in Table 2. Selected bond lengths and bond angles are
given in Table 3. There are two crystallographically independent
molecules in the asymmetric unit of the compound (Fig. 7). The
structure of the compound showed a square pyramidal geometry
with the vanadium atom lying ∼0.59 Å above the N2O2 plane in
each molecule. The N(1)N(2)O(1)O(2) and N(3)N(4)O(4)O(5)
planes from the two molecules bisect each other at an angle of
43.35°. The geometric parameter (τ) was found to be 0.31
thereby confirming a slightly distorted square pyramidal geo-
metry.28,29 This slight distortion was evidenced by the following
bond angles. For the first molecule, O(1)–V(1)–O(2) = 85.74(7)°,
O(1)–V(1)–N(1) = 87.55(7)°, O(2)–V(1)–N(2) = 87.77(7)°,
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Fig. 7 An ORTEP view of the [VIVO(sal-HBPD)] Schiff base complex with ellipsoids drawn at 50% probability level.
O(2)–V(1)–N(1) = 77.83(8)°, O(1)–V(1)–N(2) = 145.13(8)° and
O(2)–V(1)–N(1) = 143.80(8)°. For the second molecule, O(5)–
V(2)–O(4) = 85.61(7)°, O(4)–V(2)–N(3) = 88.21(8)°, O(5)–
V(2)–N(4) = 88.03(7)°, N(3)–V(2)–N(4) = 78.41(7)°, O(4)–
V(2)–N(2) = 146.99(8)° and O(5)–V(2)–N(3) = 144.4(8)°. The
V–N and V–O bond lengths are V(1)–N(1) = 2.0707(2) Å,
V(1)–N(2) = 2.0642(2) Å, V(1)–O(1) = 1.9198(2) Å and V(1)–
O(2) = 1.9249(2) Å (first molecule); V(2)–N(3) = 2.0423(2) Å,
V(2)–N(4) = 2.0601(1) Å, V(2)–O(4) = 1.9209(1) Å and V(2)–
O(5) = 1.9197(1) Å (second molecule). This implies slightly
stronger V–O bonds compared to V–N bonds, which could be
expected for the interaction of vanadyl with a phenolate group.
EPR studies
The EPR spectra of [VIVO(sal-HBPD)] in DMF (10−5 M) and
solid p[VIVO(sal-AHBPD)] are depicted in Fig. 8. The EPR
spectra of VIVO(sal-HBPD) and p[VIVO(sal-AHBPD)] showed a
typical eight-line splitting pattern (Fig. 8), as expected for 51V
(I = 7/2; n = 2I + 1).30 The gz and Az (Table 4) obtained were
1.991 and 162.5 × 10−4 cm−1 for p[VIVO(sal-AHBPD)] in close
agreement with Buglyo et al.,31 Paine et al.32 and recently
Maurya et al.33 for [VIVO(acac)(pydx-ambmz)] and [VIVO(acac)-
(pydx-aebmz)] for which a single crystal X-ray diffraction study
also confirmed the (ONN) binding set.
Fig. 8 First derivative EPR spectra of [VIVO(sal-HBPD)] in DMF and
p[VIVO(sal-AHBPD)] in the solid state.
employing a UV-Vis spectrometer. Addition of TBHP to a solu-
tion of [VIVO (sal-HBPD)] in DMF (10−3 M) yielded the oxo-
peroxido species according to the spectral changes observed.
The gradual disappearance of the d–d bands around 405 nm,
568 nm and 635 nm was observed (Fig. 9), while an increase
in the intensity of the band at 324 nm also occurred which
may be assigned the peroxo-vanadium charge transfer band
(π*v → dσ*).34
The EPR signal of solid p[VIVO(sal-AHBPD)] could be said
to have its isotropic tumbling restricted or annealed as compared
to the signal of [VIVO(sal-HBPD)] in DMF solution giving rise
to additional splitting.30
Reactivity of [VIVO(sal-HBPD)] with TBHP
VIVO-complexes react with peroxides such as tert-butylhydro-
peroxide (TBHP) to give the corresponding VVO(O2)-com-
plexes. It is normally assumed that the corresponding
oxoperoxido-complex formed is the active catalyst which con-
veyed oxidation, such as the oxidation of sulfur compounds
to their corresponding sulfoxides and sulfones. Speciation
studies of the complex [VIVO(sal-HBPD)] were monitored by
Oxidation studies
The model compounds used in this study are dibenzothiophene
(DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), and
the expected products are their sulfoxide and sulfone derivatives
(Scheme 2).
13914 | Dalton Trans., 2012, 41, 13908–13918
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Table 4 Spin Hamiltonian parameters of several VIVO-complexes
obtained from the experimental EPR spectra
Catalyst
gz
|Az| × 10−4 cm
[VOIV(sal-HBPD)]
∼1.957
∼1.991
∼1.995
161.2
162.5
160.4
p[VOIV(sal-AHBPD)]
p[VOIV(sal-AHBPD)] used
Fig. 10 Graph of % conversion for the oxidation of dibenzothiophenes
(DBT) and 4,6-dimethyldibenzothiophenes (4,6-DMDBT) with time.
[TBHP] = 0.5 g (5.5 mmol), [4,6-dimethyldibenzothiophenes] = 0.15 g
(0.707 mmol), [dibenzothiophenes] = 0.15 g (0.814 mmol), p[VIVO(sal-
AHBPD)] = 0.015 g (0.0135 mmol). Temp. = 40 °C. Toluene–hexane
(1 : 4) = 10 mL. Blank means in the absence of p[VIVO(sal-AHBPD)].
homogeneous catalyst [VIVO(sal-HBPD)] under the same con-
ditions applied in the heterogeneous catalyst system. The study
was aimed at comparing the catalytic activity of both systems in
terms of the overall conversion with respect to time. Oxidation
of sulfur in 4,6-DMDBT using [VIVO(sal-HBPD)] was investi-
gated by keeping the moles of TBHP to 4,6-DMDBT constant
and also keeping the moles of vanadium content on the catalyst
constant as employed above for the heterogeneous system
(Table 5). A maximum overall yield of 63.6% (53.2% of 4,6-
dimethyldibenzothiophene sulfones and 10.4% of 4,6-dimethyl-
dibenzothiophene sulfoxides) was observed after 6 h (Fig. 11).
However, the reaction produced an overall conversion of less
than 17% in the absence of the oxovanadium(IV) catalyst
(Fig. 10). The results suggest that the heterogeneous catalyst was
better than its homogeneous counterpart, due to the stability the
Schiff base tetradentate ligand (sal-HBPD) imparts on vanadium(IV)
which further enhances the activity of the catalyst over time.
Justification for the high conversion yield of DBT to DBTO2
could be attributed to the less sterically hindered nature of DBT
as compared to 4,6-DMDBT.
Fig. 9 Spectrophotometric titration of [VIVO(sal-HBPD)] with one
drop portions of TBHP (5 × 10−2 M) in DMF. Inset shows the d–d tran-
sition which disappears upon addition of oxidant.
Scheme 2 The proposed oxidation reaction scheme of 4,6-dimethyl-
dibenzothiophene and dibenzothiophene using tert-butylhydroperoxide
(TBHP).
Oxidation of 4,6-dimethyldibenzothiophene (4,6-DMDBT). In
the oxidation of 4,6-dimethyldibenzothiophene to sulfone, a
p[VIVO(sal-AHBPD)] catalyst with a vanadium content of
4.6 wt% was employed while a Schiff base vanadium complex
catalyst with a vanadium content of 14.6 wt% was used as the
homogeneous catalyst. Stoichiometrically, 2 moles of oxygen are
required to oxidize 1 mole of sulfur to sulfone. The mole ratio of
TBHP to 4,6-DMDBT (7.8 : 1) was employed in order to avoid
limitations of the oxidant reagent and to achieve conversion of
sulfur compounds. Aliquots were withdrawn at different time
intervals and analyzed using the GC-FID.
The oxidation reaction progressed steadily on addition of the
oxidant (TBHP) to give a yield of 70.4% 4,6-dimethyldibenzo-
thiophene sulfones after 120 min. A maximum overall yield of
72.5% (71.8% of 4,6-dimethyldibenzothiophene sulfones and
0.7% of 4,6-dimethyldibenzothiophene sulfoxides) was observed
on further oxidation to 180 min using p[VIVO(sal-AHBPD)]
(Fig. 10). A similar oxidation study was performed using the
Oxidation of dibenzothiophene (DBT). For the oxidation of
dibenzothiophene (DBT), the same conditions used for 4,6-
dimethyldibenzothiophenes were employed. 6.8 moles of TBHP
reacted with 1 mole of DBT, excess TBHP was maintained in
this system so as to avoid deficiency of oxygen in the oxidation
system. The reaction of DBT (Fig. 10) in the absence of a cata-
lyst produced an overall conversion of 23.3%.
The reaction gave an overall yield of 88% (86.8% of dibenzo-
thiophene sulfones and 1.2% of dibenzothiophene sulfoxides)
within 3 h (Fig. 10) of oxidation using the heterogeneous cata-
lyst. However, it was noticed that the oxidation of DBT using the
homogeneous catalyst [VIVO(sal-HBPD)] produced an overall
yield of 87% (85.8% of dibenzothiophene sulfones and 1.2% of
dibenzothiophene sulfoxides) but only after 6 h (Fig. 11). The
selectivity for dibenzothiophene sulfone was observed when
This journal is © The Royal Society of Chemistry 2012
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Table 5 Oxidation of 4,6-DMDBT and DBT using TBHP in the presence of [VIVO(sal-HBPD)] and p[VIVO(sal-AHBPD)]
TBHP
(g)
TBHP
(mmol)
Temp
(°C)
Catalyst
(wt)
Time
(h)
Sulfoxide
(%)
Sulfone
(%)
Total conversion
(%)
TOFa
Substrate
(h−1
)
Heterogeneous catalyst p[VIVO(sal-AHBPD)]
DBT
0.50
0.50
0.50
0.50
5.50
5.50
5.50
5.50
40
40
40
40
—
14.6 mg
—
5
3
5
3
3.2
1.2
9.6
0.7
20.1
86.8
7.4
23.3
88.0
17.0
72.5
—
17.69
—
DBT
4,6-DMDBT
4,6-DMDBT
14.6 mg
71.8
15.36
Homogeneous catalyst [VIVO(sal-HBPD)]
DBT
4,6-DMDBT
0.50
0.50
5.50
5.50
40
40
4.7 mg
4.7 mg
6
6
1.3
10.4
85.7
53.2
87.0
63.6
4.77
2.75
Moles of catalyst (vanadium content) employed = 1.35 × 10−5 moles.a TOF, h−1 (turnover frequency): moles of substrate converted per mole of metal
ion (in the solid state catalyst) per hour. DBT: dibenzothiophene; 4,6-DMDBT: 4,6-dimethyldibenzothiophene.
Fig. 11 Graph of
%
conversion for the oxidation of 4,6-
dimethyldibenzothiophenes and dibenzothiophenes with time using the
homogeneous catalyst [VIVO(sal-HBPD)]. [TBHP] = 0.5 g (5.5 mmol),
[dibenzothiophenes] = 0.15 g (0.814 mmol), [VIVO(sal-HBPD)] =
0.0047 g (0.0135 mmol). Temp. = 40 °C.
Fig. 12 1H NMR spectra of (A) dibenzothiophene sulfone (DBTO2)
and (B) dibenzothiophene (DBT).
Dibenzothiophene (DBT): 1H NMR (400 MHz, DMSO-d6)
(δ, ppm): 8.39 (m, J = 3.0, 2 H), 8.05 (m, 2 H), 7.53 (m, 2 H).
Dibenzothiophene sulfone (DBTO2): 1H NMR (400 MHz,
DMSO-d6) (δ, ppm): 8.22 (d, J = 7.7, 2 H), 8.00 (d, J = 7.6,
2 H), 7.82 (t, J = 7.6, 2 H), 7.67 (t, J = 7.6, 2 H).
[VIVO(sal-HBPD)] and p[VIVO(sal-AHBPD)] were employed as
catalysts. Table 5 shows the better selectivity of the vanadium
anchored polymer catalyst for sulfones within a short time as
compared to [VIVO(sal-HBPD)]. Active sites isolation on the
beads catalyst can also contribute to the better selectivity during
the oxidation reaction. The oxidation results obtained were in
close agreement with the work of Iglesia et al.35 on the oxidation
of organosulfur compounds in which tert-butylhydroperoxide
was also employed.
4,6-Dimethyldibenzothiophene sulfone (4,6-DMDBTO2): 1H
NMR (400 MHz, DMSO-d6) (δ, ppm): 7.98 (d, J = 7.6, 2 H),
7.66 (t, 2 H), 7.44 (d, J = 7.5, 2 H), 2.48 (s, 6 H). 4,6-Dimethyl-
dibenzothiophene (4,6-DMDBT) was insoluble in DMSO-d6.
From the GC-MS data it was observed that an m/z of 216 and
246 for DBTO2 and 4,6-DMDBTO2 respectively can be ident-
ified in the mass spectra.
Characterization of products: microanalysis, NMR, GC-MS
Recyclability studies
Microanalysis of 4,6-dimethyldibenzothiophene sulfone (4,6-
DMDBTO2): Anal. Calcd (found) for C14H12O2S (%): C, 68.38-
(68.38); H, 4.95(5.25); S, 13.12(12.83). For dibenzothiophene
sulfone (DBTO2): Anal. Calcd (found) for C12H10O2S (%): C,
The recyclability of p[VIVO(sal-AHBPD)] was investigated by
further re-using the spent catalyst. The p[VIVO(sal-AHBPD)]
activity was retained even after the first two catalytic cycles with
each oxidation cycle taking approximately 3 h (Fig. 13). An
overall oxidation yield of 80.1% and 68.2% was observed for
both dibenzothiophene (DBT) and 4,6-dimethyldibenzothio-
phene (4,6-DMDBT) respectively after the fourth oxidation
1
66.65(66.02); H, 3.73(4.04); S, 14.83(14.83). H NMR spectra
of DBT and DBTO2 in DMSO-d6 (Fig. 12) show a shift in the
peaks of DBT and DBTO2. The spectral data for the sulfur
compounds and their oxidized products are reported below.
13916 | Dalton Trans., 2012, 41, 13908–13918
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oxovanadium(IV) compound [VIVO(sal-HBPD)] and its polymer-
anchored counterpart p[VIVO(sal-AHBPD)] were synthesized. A
distorted square pyramidal geometry with the N2O2 equatorial
binding mode of the tetradentate ligand was confirmed by
single-crystal X-ray diffraction. The formation of an oxoperoxo-
compound in the catalysed oxidation reaction was confirmed by
the UV-Vis studies. EPR studies confirmed that most of the V(V)
recycled back to V(IV) after a reaction cycle. The polymer-
anchored complex p[VIVO(sal-AHBPD)] proved to be a very
effective catalyst for the oxidation of DBT and 4,6-DMDBT
using tert-butylhydroperoxide (TBHP), producing mostly sul-
fones under the reaction conditions. The low overall oxidation
was observed for both DBT and 4,6-DMDBT in the absence of a
catalyst. The low level of vanadium leaching from p[VIVO(sal-
AHBPD)] after several oxidation cycles further confirmed the
stability of the vanadium Schiff base complex. The oxidation
results obtained with this polymer-anchored vanadium(IV) metal
centre give an insight into its potential as a catalyst in an oxi-
dative desulfurization process due to its ability to retain its
activity and selectivity as compared to the homogeneous
catalyst.
Fig. 13 Recyclability studies in the oxidation of dibenzothiophene
(DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT); n (number of
runs for each oxidation cycle) = 3.
Acknowledgements
We would like to thank the Electron Microscopy Unit (Rhodes
University) for use of the SEM. We are also thankful for
financial support provided by SASOL and the National Research
Foundation (NRF) South Africa.
Fig. 14 (a) First derivative EPR spectrum of (A) p[VIVO(sal-AHBPD]
References
before use; (B) p[VIVO(sal-AHBPD)] after use.
1 B. Pawelec, R. M. Navarro, J. M. Campos-Martin and J. L. G. Fierro,
Catal. Sci. Technol., 2011, 1, 23.
2 R. H. Holm and J. M. Berg, Acc. Chem. Res., 1986, 19, 363.
3 N. d’Alessandro, L. Tonucci, M. Bonetti, M. Di Deo, M. Bressan and
A. Morvillo, New J. Chem., 2003, 27, 989.
4 K. Yazu, Y. Yamamoto, T. Furuya, K. Miki and K. Ukegawa, Energy
Fuels, 2001, 15, 1535.
5 K. Yazu, T. Furuya, K. Miki and K. Ukegawa, Chem. Lett., 2003, 32,
920.
6 F. M. Collins, A. R. Lucy and C. Sharp, J. Mol. Catal. A: Chem., 1997,
117, 397.
7 A. Ishihara, D. Wang, F. Dumeignil, H. Amano, E. W. Qian and T. Kabe,
Appl. Catal., A, 2005, 279, 279.
8 M. Te, C. Fairbridge and Z. Ring, Appl. Catal., A, 2001, 219, 267.
9 R. S. Walmsley, A. S. Ogunlaja, M. J. Coombes, W. Chidawanyika,
C. Litwinski, N. Torto, T. Nyokong and Z. R. Tshentu, J. Mater. Chem.,
2012, 22, 5792.
10 Z. R. Tshentu, C. Togo and R. S. Walmsley, J. Mol. Catal. A: Chem.,
2010, 318, 30.
11 J. A. L. da Silva, J. J. R. F. da Silva and A. J. L. Pombeiro, Coord. Chem.
Rev., 2011, 255, 2232.
12 L. Cedeno-Caero, H. Gomez-Bernal, A. Fraustro-Cuevas, H. D. Guerra-
Gomez and R. Cuevas-Garcia, Catal. Today, 2008, 133–135, 244.
13 F. Gregori, I. Nobili, F. Bigi, R. Maggi, G. Predieri and G. Sartori,
J. Mol. Catal. A: Chem., 2008, 286, 124.
14 V. Conte and B. Floris, Inorg. Chim. Acta, 2010, 363, 1935.
15 O. González-García and L. Cedeño-Caero, Catal. Today, 2009, 148, 42.
16 D. Xu, W. Zhu, H. Li, J. Zhang, F. Zou, H. Shi and Y. Yan, Energy Fuels,
2009, 23, 5929.
17 H. Gómez-Bernal, L. Cedeño-Caero and A. Gutiérrez-Alejandre, Catal.
Today, 2009, 142, 227.
18 L. C. Caero, E. Hernandez, F. Pedraza and F. Murrieta, Catal. Today,
2005, 107–108, 564.
cycle. To determine the quantity of vanadium leached, the
product fractions were combined after filtering, concentrated and
digested in nitric acid. After appropriate dilutions, the vanadium
content was determined by ICP-OES. 0.189(0.001)% and 0.196-
(0.001)% vanadium was observed to have leached out in the
reaction involving DBT and 4,6-DMDBT respectively after the
first reaction. Subsequent cumulative leaching of 0.221(0.001)%,
0.832(0.003)% and 2.529(0.001)% for the second to the fourth
oxidation cycle respectively were observed for DBT. For 4,6-
DMDBT oxidation 0.271(0.0004)%, 1.023(0.006)% and 3.152-
(0.002)% were observed in the same order of cycles mentioned
above. Furthermore, this corresponded well to the slight drop in
activity observed during the recycling studies.
The less intense EPR signals of the used solid p[VIVO(sal-
AHBPD)] indicate that some of the V(V) was not recycled to
V(IV) when compared to its initial amount (Fig. 14). These simi-
larities in the signals indicate that the same vanadium(IV) species
was retained on the beads after oxidation cycles.
Conclusions
The tetradentate N2O2-donor ligand was obtained by the conden-
sation of salicylaldehyde and o-phenylenediamine. The
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 13908–13918 | 13917

View Article Online
19 J. M. Bregeault, M. Vennat, L. Salles, J. I. Piquemal, I. Mahha, E. Briot,
P. C. Bakala, A. Atlamsani and R. Thouvenot, J. Mol. Catal. A: Chem.,
2006, 250, 177.
27 K. Nakamoto, Coordination Compounds in Infrared and Raman Spectra
of Inorganic and Coordination Compounds, John Wiley & Sons, Inc.,
New York, 4th edn, 1986.
20 D. J. Hagrman, R. C. Finn and J. Zubieta, Solid State Sci., 2001, 3,
745.
28 A. W. Addison and T. N. Rao, J. Chem. Soc., Dalton Trans., 1984, 1349.
29 R. S. Walmsley, Z. R. Tshentu, M. A. Fernandes and C. L. Frost, Inorg.
Chim. Acta, 2010, 363, 2215.
30 D. Rehder, Bioinorganic Vanadium Chemistry, John Wiley & Sons, Ltd,
UK, 2008, 68.
31 P. Buglyo, A. Dessi, T. Kiss, G. Micera and D. Sanna, J. Chem. Soc.,
Dalton Trans., 1993, 2057.
32 T. K. Paine, T. Weyhermüller, L. D. Slep, F. Neese, E. Bill, E. Bothe,
K. Weighardt and P. Chaudhuri, Inorg. Chem., 2004, 43, 7324.
33 M. R. Maurya, M. Bisht, A. Kumar, M. L. Kuznetsov, F. Avecilla and
J. C. Pessoa, Dalton Trans., 2011, 40, 6968.
21 N. V. Maksimchuk, M. S. Melgunov, J. Mrowiec-Białon, A. B. Jarzebski
and O. A. Kholdeeva, J. Catal., 2005, 235, 175.
22 A. Rockenbauer and L. Korecz, Appl. Magn. Reson., 1996, 10,
29.
23 K. C. Gupta and A. K. Sutar, J. Mol. Catal. A: Chem., 2007, 272,
64.
24 (a) Bruker,APEX2.Version 1-0, Bruker AXS Inc., Madison, Wisconsin,
USA, 2009; (b) Bruker SAINT+, Version 7.60A, (includes XPREP and
SADABS) Bruker AXS Inc., Madison, Wisconsin, USA, 2005.
25 Bruker SHELXTL Version 5.1, (includes XS, XL, XP, XSHELL) Bruker
AXS Inc., Madison, Wisconsin, USA, 1999.
34 R. S. Walmsley and Z. R. Tshentu, S. Afr. J. Chem., 2010, 63, 95.
35 A. Chica, G. Gatti, B. Moden, L. Marchese and E. Iglesia, Chem.–Eur.
J., 2006, 12, 1960.
26 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
13918 | Dalton Trans., 2012, 41, 13908–13918
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