Catalytic activity and structure properties of doped VOHPO4 ·0.5H2O with nanosized Ru, Au, Fe and Mn in benzene hydroxylation

Article abstract of DOI:10.1166/jnn.2013.7575

The promotional effect of nanosized Ru, Fe, Au, and Mn particles on VOHPO₄ ·0.5H₂O (VHP) catalytic properties was investigated in benzene hydroxylation reaction using hydrogen hydroperoxide (H₂O₂) as oxidant. Catalytic results indicated a profound effect of the nanoparticle dopants on VHP catalyst activity and products distribution. Amongst the promoted VHP catalysts, Au/VHP exhibited high catalytic effect with benzene conversion of 76% at a combined 85.5% selectivity toward the formation of phenol and hydroquinone achieved in 6 h under optimised reaction conditions. The extended scope application of nanosized doped Au-VHP showed to provide an effective catalyst for activation of the aromatic hydrocarbons C-H bonds into oxygenate derivatives. The catalyst could be re-used for several cycles with insignificant loss of activity. The doped nanosized Au-VHP catalyst provide a clean promising catalytic route based on heterogeneous catalysis for transformation of aromatics into value-added oxygenates. Copyright

Full text of DOI:10.1166/jnn.2013.7575

Copyright © 2013 American Scientific Publishers
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Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 5053–5060, 2013
Catalytic Activity and Structure Properties of
Doped VOHPO4 ·0ꢀ5H2O with Nanosized Ru, Au,
Fe and Mn in Benzene Hydroxylation
Peter R. Makgwane1ꢀ2ꢀ∗ and Suprakas Sinha Ray1ꢀ2ꢀ∗
1Department of Applied Chemistry, University of Johannesburg, Doornfontein, 2028, South Africa
2DST/CSIR National Center for Nano-Structured Materials (NCNSM), Council for Scientific and
Industrial Research (CSIR), Pretoria, 0001, South Africa
The promotional effect of nanosized Ru, Fe, Au, and Mn particles on VOHPO4 ·0ꢀ5H2O (VHP) cat-
alytic properties was investigated in benzene hydroxylation reaction using hydrogen hydroperoxide
(H2O2ꢁ as oxidant. Catalytic results indicated a profound effect of the nanoparticle dopants on VHP
catalyst activity and products distribution. Amongst the promoted VHP catalysts, Au/VHP exhibited
high catalytic effect with benzene conversion of 76% at a combined 85.5% selectivity toward the
formation of phenol and hydroquinone achieved in 6 h under optimised reaction conditions. The
extended scope application of nanosized doped Au-VHP showed to provide an effective catalyst for
activation of the aromatic hydrocarbons C H bonds into oxygenate derivatives. The catalyst could
be re-used for several cycles with insignificant loss of activity. The doped nanosized Au-VHP cata-
lyst provide a clean promising catalytic route based on heterogeneous catalysis for transformation
of aromatics into value-added oxygenates.
Keywords: Vanadium Phosphorus Oxide, Nanosized Particles, Dopants, Oxidation,
Hydroxylation, Benzene, Phenol.
1. INTRODUCTION
Commercially, phenol is produced by the non-catalysed
cumene liquid-phase oxidation or the Hock process.6 Typ-
ically, the cumene oxidation process is performed in a
series of cascaded bubble-column reactors operated at
Liquid-phase oxidations are important industrial reac-
tions for the functionalization of hydrocarbons into oxy-
genated chemical derivatives.1–2 One of such reactions is
the hydroxylation of benzene, which yield phenol and its
overoxidation products hydroquinone, catechol and benzo-
quinone that are used as intermediates in the production
of other value-added chemicals.3ꢀ4 The high volume of
waste effluents generated by the traditional oxidation pro-
cesses using strong mineral acids as catalysts is an area
of main concern for environmental impact. On the other
hand, the increasing pressure from the stringent environ-
mental law requirements imposed on chemical industries
continues to be the driving force in the development of
green and sustainable catalytic processes.5 In line of these
concerns, the development of effective catalytic oxidation
systems that can use the environmentally benign hydrogen
peroxide (H2O2ꢁ oxidants are particularly desirable. The
advantages of H2O2 as green atom-efficient oxidant is that
it yield only water as by-products.
ꢀ
5–10 bar and 100–130 C in the presence of an aque-
ous basic Na2CO3.7 The cumene conversion to the cumene
hydroperoxide (CHP) content is limited to 30–35% in
order to ensure low formation of by-products such as
dimethylphenylcarbinol and acetophenone.6 The obtained
CHP is concentrated to about 65–90% in vacuum columns
to remove the unreacted cumene which is recycled back to
the reactor, while the obtained CHP is acid-cleavaged
to phenol and acetone usually by 0.1–2% H2SO4 at
ꢀ
60–65 C.7 The Hock process constitute several reaction
stages that include, (1) preparation of cumene from iso-
propylation of benzene, (2) oxidation of cumene to CHP,
and (3) acid cleavage of CHP into phenol and acetone.
This poses serious demands on process economics. On
the other hand, the use of mineral acids such as H2SO4
usually yield harzadous waste metal salts that are diffi-
cult to dispose. Based on the recent demands from the
green chemistry chemical conversions protocols, the devel-
opment of improved catalytic processes that especially use
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 7
1533-4880/2013/13/5053/008
doi:10.1166/jnn.2013.7575
5053

Catalytic Activity and Structure Properties of Doped VOHPO4 ·0ꢂ5H2O with Nanosized Ru, Au, Fe and Mn
Makgwane and Ray
heterogeneous catalytic route to avoid the use of waste
salt generating substrates such as Na2CO3 and H2SO4
are needed.5 Consequently, there has been a considerable
research interest devoted on realising efficient green cat-
alytic systems for the direct synthesis of phenol from
readily available starting materials such benzene without
modification or functionalization. Accordingly, the liquid-
phase hydroxylation of benzene over several studied solid
catalyst using H2O2 presents a promising green synthetic
protocol to phenol and its derivative chemicals.3ꢀ4ꢀ8–12
Vanadium phosphorous oxides (VPOs) on the other
hand present highly active catalyst systems with eas-
ily changeable V4+/V5+ redox properties that give them
superior catalytic properties ideal for the activation of
process of benzene and extended scope of application to
other aromatics.
2. EXPERIMENTAL DETAILS
2.1. Preparation of Different VOHPO4 ꢁ0.5H2O
Based Catalysts
The VOHPO4 · 0ꢂ5H2O (VHP) catalyst was prepared by
refluxing V2O5 (12.0 g, Aldrich, 99%) and o-H3PO4
(116 g, 85% Aldrich) in deionised water (300 mL) for
8 h. After the reaction, the yellow VOPO4 ·2H2O solid was
recovered by vacuum filtration, washed with cold water
ꢀ
(100 mL) and acetone (100 mL) and dried in air (110 C,
12 h). 4 g of VOPO4 ·2H2O was refluxed with isobutanol
(80 mL) for 21 h, and the resulting blue VOHPO4 ·0ꢂ5H2O
solid was recovered by filtration, refluxed further at 100 ꢀC
in deionised water (9 mL H2O/g solid) for 2 h to remove
VO(H2PO4ꢁ2 impurity phase,29 filtered hot, and dried in
C
H and OH bonds into alcohols, ketones, carboxylic
acids, etc.13–15 The enhanced VPOs activity stems from the
excellent vanadium metal redox properties and the acidic
nature due to intercalated phosphorous element in the cat-
alyst structure. With recent attention directed towards the
development of green chemical processing technologies,
the VPOs derived catalysts present the desired catalytic
properties such as redox and medium acidic nature that
are amenable to perform oxidations reactions of hydro-
carbons into oxygenates based on their earlier success-
ful application.16–18 Amongst the VPOs catalyst phases,
the studies on catalytic performance of vanadyl hydrogen
phosphate hemihydrate, VOHPO4 ·0ꢂ5H2O (VHP) catalyst
phase in liquid-phase oxidations are few.19ꢀ20 It has been
recently reported that VHP catalyst phase is catalytically
active as its calcined phase, (VO)2P2O7 in the liquid-phase
oxidations of aromatic hydrocarbons into oxygenates.21ꢀ22
Moreover, several groups demonstrated that the activity
and selectivity of VPOs catalyst systems can be tailored
by the dopants promotional effect.23–25 Since the excel-
lent vanadium based catalysts catalytic activity is mainly
due to the balanced V+3/V+4/V+5 redox properties, it can
be anticipated that the addition of small amount of metal
nanoparticle would influence this oxidation states ratios,
thus overall catalytic performance of the catalysts. This
can be especially interesting considering the unique struc-
tural properties offered by metal catalyst nanoparticles of
< 10 nm size with properties that are inaccessible in their
bulk form.26 With that in mind, we are in the present con-
tribution reporting on the catalytic activity of (VOHPO4 ·
0ꢂ5H2O) or VHP catalyst doped with nanosized parti-
cles of ruthenium (Ru), manganese (Mn), gold (Au) and
iron (Fe) metals to improve on catalytic properties of the
VOHPO4 · 0ꢂ5H2O catalyst for hydroxylation reaction of
benzene into phenol and its derived products. The enhaced
catalytic activity of Ru, Au, Mn and Fe nanoparticles
in oxidation reactions have been reported previously in
organic synthesis.27ꢀ28 We show that the combined effect
of excellent VHP redox properties and nanosized metal
dopants structural properties effect provide a highly reac-
tive VHP catalyst system with efficient conversion rates
and satisfying selectivity in the hydroxylation reaction
ꢀ
air (110 C, 16 h).
The promoted VHP nano-catalysts were prepared using
RuCl3 · xH2O, 99.98%; HAuCl4 · 3H2O, 51.0%; Fe(III) ·
9H2O, 99.98%; and Mn4 · 3H2O, 51.0% metal precursors
all from Sigma-Aldrich Chemicals. Initially, promoters
nanoparticles were prepared before deposition onto VHP
catalyst. The polyvinyl alcohol (PVA) (Aldrich, MW ∼
10000, 80% hydrolysed) was used as a protecting agent.
In a typical preparation, the 2 ml of aqueous PVA (2 wt.%)
was added to 300 ml of deionised H2O and the amount of
respective promoter was adjusted based on the total w/w%
of VHP to be added. The resulting promoter solution was
stirred vigorously at 50 ꢀC for 1 h and followed by a
rapid injection of few drops (∼ 2 mL) of 0.5 M aqueous
NaBH4 (Aldrich Chemicals, 97% purity). The solution was
allowed to age for 24 h at room temperature under contin-
uous stirring, thereafter the deposition of VHP was then
added under stirring and kept in contact with promoters
for 8 h. The nominal amount of respective promoter metals
was kept constant at 2 wt.%. The solids were collected by
filtration, washed thoroughly with distilled water anꢀd ace-
tone, respectively, and dried in an air oven at 120 C for
24 h. The catalysts so-prepared were denoted as Au/VHP,
Ru/VHP, Fe/VHP, and Mn/VHP, respectively.
2.2. Characterisation Procedures of Catalyst
Total specific surface area and pore volume of VHP based
catalysts were measured by nitrogen physisorption at 77 K,
using a Micromeritics TRISTAR 3000 instrument. Before
ꢀ
N2 physisorption, the samples were degassed at 120 C
for 12 h under a continuous flow of N2 gas to remove
adsorbed contaminants. The spectra were recorded with a
resolution of 2 cm−1. X-ray diffraction (XRD) experiments
were recorded on a PAnalytical XPERT-PRO diffractome-
ter measurement, using Ni filtered CuKꢃ radiation (ꢄ =
1ꢂ5406 Å) at 40 kV/50 mA. The diffraction measurements
5054
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Makgwane and Ray
Catalytic Activity and Structure Properties of Doped VOHPO4 ·0ꢂ5H2O with Nanosized Ru, Au, Fe and Mn
Table I. Catalytic activity performance of different VHP based catalysts
were collected at room temperature in a Bragg-Brentano
in benzene hydroxylation.
geometry with scan range, 2ꢅ = 10–70ꢀ using continuous
ꢀ
scanning at a rate of 0.02 /s. The nanoparticles dimen-
Selectivity (%)
Catalyst Benzene conversion (%) Phenol aHQ Others Phenol/HQ
sions of the catalysts were measured by High Resolution-
Transmission Electron Microscopy (HR-TEM) JEOL JEM
2100, operated at an accelerating voltage of 200 kV. Before
TEM analysis, the samples were prepared by suspending
the solid in ethanol and submitting the suspension to ultra-
sonication for 30 min. Afterward a drop was extracted
placed on holly carbon-coated copper grid for TEM mea-
surements. The Raman spectra were recorded with a
T64000 Raman spectrometer from HORIBA Scientific,
Jobin Yvon Technology (Villeneuve d’Ascq, France). The
Raman spectra were excited with either the 514.5 nm lines
of a Coherent Innova® 70C Series Ion Laser System and
the 100× or 50× objectives of an Olympus microscope
was used to focus the laser beam (spot size ∼ 2–12 ꢆm)
on the samples and also collected the backscattered Raman
signal. An integrated triple spectrometer was used in the
double subtractive mode to reject Rayleigh scattering and
dispersed the light onto a liquid nitrogen-cooled Sym-
phony CCD detector.
Blank
VHP
Fe/VHP
Mn/VHP
Au/VHP
Ru/VHP
< 1
14.2
22.6
18.8
48.3
32.8
100.0
−
−
55.3 16.6 28.1
59.2 23.9 16.9
51.3 18.7 30.0
65.2 23.3 11.5
71.9
83.1
70.0
88.5
91.8
70.2 21.6
8.2
Notes: Reaction conditions: benzene (10 mmol) and 50% H2O2 (30 mmol), 0.10 g
catalyst, CH3CN (6 ml), T = 70 ꢀC and t = 3 h. aHQ = hydroquinone.
the blank reaction of benzene and H2O2 oxidant in the
absence of catalyst in order to establish the base perfor-
mance of the non-catalytic reaction. The reaction gave
only less than 1% benzene conversion after 3 h (Table I).
This indicated that the free radicals chain-initiation rates
are slow in the absence of the catalyst irrespective of the
added reactive H2O2 oxidant. When the same reaction was
performed with pure VHP and different metal nanoparti-
cles doped-VHP catalysts, there was a substantial increase
of conversion from less than 1% in the non-catalysed reac-
tion to 14% or more indicating the significant catalytic
effect under similar reaction conditions (Table I). This
demonstrated the active participation of VHP catalyst in
the activation of benzene C H bond via homolytic cat-
alytic cleavage of O O bond of H2O2 into highly reactive
free radicals capable to initiate the chain oxidation mech-
anism. Moreover, the slow chain initiation rates for the
non-catalysed reaction are in agreement with the reported
findings, that self-thermal decomposition of H2O2 into
reactive oxygen radicals species to initiate the hydrocar-
bons oxidations chain reactions is inadequate.30 The cat-
alytic decomposition of HOOH by vanadium is assumed
to occur via the formation of either coordinated vanadium
hydroperoxo species or vanadium peroxo species that are
oxygen-donors to facilitates the transfer of electrophilic
oxygen to the formed free radicals.2
2.3. Catalytic Activity Testing Procedure
The catalytic activity testing was done in a 50 ml two
necked round bottom flask equipped with the magnetic
stirrer and reflux condenser. The reactor was charged
with benzene (10 mmol), 50% H2O2 (30 mmol) catalyst
(0.10 g) and 6 mL acetonitrile (CH3CN) solvent. The tem-
perature of the reaction was controlled by immersing the
reactor in an oil bath. Samples withdrawn from the reactor
containing the benzene oxidation products were analyzed
using a Supelco SPB-20 column (30 m × 0ꢂ25 mm i.d. ×
0ꢂ25 mm) on Agilent 7890A Gas Chromatograph (GC)
equipped with flame ionization detector (FID). GC aꢀnal-
ysis program was as follow: FID temperature (300 C),
ꢀ
injector temperature (250 C), and oven column teꢀmpera-
ꢀ
ꢀ
ture, 80 C (3 min) then ramped to 220 C at 10 C/min
and split was 1:100 at constant flow rate. To study the
possible metal catalyst leaching, we used the followiꢀng
reaction conditions (catalyst = 0ꢂ1 g, temperature = 70 C
and time = 1 h). After the 1 h reaction time, the catalyst
was separated by filtration while the reaction liquid mix-
ture was hot and the reaction was allowed to react further
for a total reaction time of 3 h. For catalyst recyclability
testing experiments, after each reaction cycle the catalyst
was filtered from the liquid solution and washed with ace-
tone followed by drying in the air for 2 h at 110 ꢀC before
use in the next cycle.
The effect of nanosized Au, Ru, Mn, and Fe particles
dopants on VHP catalyst performance was significantly
pronounced on both activity and products distribution pat-
tern. For example, Au and Ru as VHP dopants gave better
activities with conversions in a range of 32–50%, which
were higher than the one achieved with the pure VHP
catalyst of 14.2%. On the other hand, the Mn/VHP and
Fe/VHP catalysts performance was poor showing low con-
versions of 18.8.5% and 22.6%, respectively. In addition
to the low catalytic activity displayed by Mn/VHP, the
catalyst also showed poorly combined selectivity towards
phenol and hydroquinone formation of 70.0% compared
to Au/VHP of 88.5%. However, Ru/VHP catalyst showed
slightly high phenol/hydroquinone selectivity of 91.8%
although its conversion was lower than the one obtained
with Au/VHP catalyst. The low benzene conversion and
3. RESULTS AND DISCUSSION
3.1. Catalytic Activity Measurements
Initially, in the catalytic activity testing of different pro-
moted VHP and pure VHP catalysts, we first performed
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Catalytic Activity and Structure Properties of Doped VOHPO4 ·0ꢂ5H2O with Nanosized Ru, Au, Fe and Mn
Makgwane and Ray
Table II. Effect H2O2 concentration on benzene hydroxylation rates
using doped-Au/VHP catalyst.
100
90
80
70
60
50
40
30
20
10
0
Selectivity (%)
Benzene/H2O2
molar ratio
Benzene
Conversion
Phenol
HQ
conversion (%) Phenol HQ Others Phenol/HQ
1
2
3
4
5
14.3
36.8
48.3
53.6
56.8
83.2
73.5
72.8
62.4
61.4
10.6
18.3
15.7
12.4
10.6
6ꢂ2
8ꢂ2
11ꢂ5
25ꢂ2
28ꢂ0
94.8
91.8
88.5
74.8
72.0
Others
Notes: Reaction conditions: benzene (10 mmol), Au/VHP catalyst (0.10 g), CH3CN
(6 ml), T = 70 ꢀC and t = 3 h. HQ = hydroquinone.
poor products selectivity toward phenol/hydroquinone dis-
played by the Mn/VHP and Fe/VHP catalysts can be
attributed to the possible coverage of VHP catalyst active
sites probably due to the formation of large particles size
and particles clustering as confirmed later by TEM results
(Fig. 5).
0
1
2
3
4
5
6
7
8
Time/h
Fig. 1. Time-courses dependence of benzene conversion and products
selectivity using nanosized doped-Au/VHP catalyst. Reaction conditions:
benzene (10 mmol), 50% H2O2 (30 mmol), catalyst (0.10 g), CH3CN
(6 ml), T = 70 ꢀC.
Based on the catalytic activity results obtained for dif-
ferent promoted VHP catalysts and their product selectiv-
ity (Table I), the nanosized doped-Au/VHP was chosen as
the best performing catalyst to further investigate the effect
of other reaction parameters on the oxidation rates. The
effect of the molar ratio of benzene and H2O2 oxidant on
hydroxylation rates and products selectivity was studied in
a range of 1:1 to 1:5 using Au/VHP nanosized-doped cata-
lyst. The results obtained are listed in Table II. The conver-
sion of benzene increased significantly with the increasing
H2O2 oxidant molar ratio to benzene from 14.3% at 1:1 to
48.3% at 1:3 and thereafter started to show raising slowly
up to 56.8%. On the other hand, the selectivity towards
combined phenol/Hydroquinone dropped relatively from
94.8% at 1:1 molar ratio to 72.0% at 1:5. The overoxida-
tion of benzene into other products apart from the phenol
and hydroquinone formation was observed to be serve at
higher H2O2 concentrations above 1:3 ration with respect
benzene. Consequently, the 1:3 ratio seems to give the
acceptable conversion rates of 48.3% at the preserved com-
bined selectivity of 88.5% toward the formation of phenol
and hydroquinone.
Figure 1 illustrates the time-dependence evolution of
benzene conversion rate and different products selectiv-
ity using the nanosized doped-Au/VHP catalyst. The con-
version rate of benzene increased gradually up to about
62% conversion at 4 h, and then thereafter started to show
reaching the plateau with the increasing reaction time to
the maximum conversion of up to 76% after 8 h. At 76%
benzene conversion, phenol and hydroquinone were the
major products with respective selectivities of 61% and
24.5% or combined of 85.5%.
catalyst was separated by filtration followed by washiꢀng
with acetone and drying in an oven for 2 h at 110 C
before use in the next reaction cycle. Although it was dif-
ficult to recover the catalyst in the lab-scale due to a small
amount of few milligrams usually used, we maintained
the catalyst recovery of 93–96% based on the initial mass
used for each reaction cycle. The catalyst did not show any
significant change in activity after 5 reuse testing cycles
with the benzene conversion stabilized at around 73–77%.
Inversely, the selectivity of phenol changed slightly in a
range of 63–66%. The short window period recyclability
testing of nanosized doped-Au/VHP catalyst in the present
study give only the rough estimation of its performance but
not the true long-term performance deactivation that usu-
ally requires several hundred hours of on-stream testing.22
This would require the use of a well-controlled reactor
such as continuous-flow system (e.g., micro-channels or
fixed-bed reactor) that will ensure uniform contact of reac-
tants with the catalyst material for a prolonged usage
period without the significant limitations imposed by the
loss of the catalyst mass.
Based on the succesfull application of nanosized doped-
Au/VHP as effective hydroxylation catalyst for benzene
reaction to phenol, we expanded the catalyst scope of appli-
cation to other industrially important aromatics and cyclic
ring hydrocarbons such as toluene, styrene and cyclohex-
ane under the similar optimized reaction conditions. The
toluene hydroxylation is an important reaction route for
the synthesis of benzyl alcohol, benzaldehyde, benzoic acid
and cresols,31 while styrene hydroxylation leads to the for-
mation of flavor and fragrance intermediate, benzaldehyde
isolated in high yields.32 Similarly, the hydroxylation of
cyclohexane is an industrially important reaction process
for the synthesis of cyclohexanol and cyclohexanone as
intermediates to adipic acid for the production of nylon
To evaluate the stability of the catalyst activity with time
on-stream, the nanosized doped-Au/VHP catalyst was re-
used for five consecutive batch oxidation cycles (Fig. 2).
Each reaction cycle was performed under similar reaction
conditions for 8 h. After each reaction cycle, the used
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Makgwane and Ray
Catalytic Activity and Structure Properties of Doped VOHPO4 ·0ꢂ5H2O with Nanosized Ru, Au, Fe and Mn
Table III. Scope application of nanosized doped-Au/VHP in hydroxylation of different aromatics.
Entry
1
Substrate
Conversion (%)
71.0
Selectivity to major products (%)
Hydroquinone (24.5)
Phenol (61.0)
–
–
–
–
2
68.3
Cyclohexanol (58.4)
Cyclohexanone (25.8)
3
4
73.7
36.0
Benzaldehyde (48.0)
Benzaldehyde (60.1)
Benzyl alcohol (10.2)
Benzyl alcohol (12.7)
Benzoic acid (23.6)
Benzoic (9.2)
–
CH2
Cresols (10.8)
CH3
Note: Reaction conditions: benzene (10 mmol) and 50% H2O2 (30 mmol), 0.10 g catalyst, CH3CN (6 ml), T = 70 ꢀC and t = 8 h.
relater polymer products.33 It can be seen in Table III that
the nanosized doped-Au/VHP catalyst showed catalytic
effect in all tested hydroxylation reactions giving efficient
substrates conversions. Interestingly, was the high catalytic
activity displayed by nanosized doped-Au/VHP on cyclo-
hexane oxidation with much improved substrate conversion
at preserved high selectivity toward the formation cyclo-
hexanol and cyclohexanone products. Commercially, the
cyclohexane oxidation process is limited to 5–10% con-
version in order to preserve selectivity which is in a range
of 85–90%. By comparison, our present catalysed cyclo-
hexane liquid-phase oxidation reaction performed better
than that of the industrial cyclohexane oxidation process.
A cyclohexane conversion of 68.3% at cyclohexanol and
cyclohexanone combined selectity of 84.2% was achieved
using the nanosized doped Au-VHP catalyst. The results
obtained for nanosized doped-Au/VHP catalyst in Table III
present a facile catalytic synthetic protocol of converting
various aromatics or cyclic ring hydrocarbons into func-
tionalized oxygenates of industrial importance that can be
extended to other typical reactions.
3.2. Characterisation of Catalyst Materials
To understand the correlation between the structure prop-
erties of the different nanosized particle doped-VHP
catalysts and their catalytic performance in benzene liquid-
phase oxidation, the catalysts were characterized using
XRD, HR-TEM, Raman and N2 physisorption. The XRD
structural patterns of the pure and promoted VHP catalysts
are presented in Figure 3. The results confirmed the forma-
tion of the XRD diffraction patterns assignable to the VHP
catalyst phase (PDF file JCP2 00-047-0953) (Fig. 3). For
different promoted VHP catalysts, there were detectable
presence of XRD peaks assigned to some of the promoter
metals, otherwise others showed mainly VHP diffraction
patterns. For example, the broader peaks of metallic Au
nanocatalyst at 2ꢅ = 38ꢀ and 44ꢀ are visible in the XRD
patterns, Figure 3 marked with Au.34 The diffraction peak
of RuO2 nanocrystalline that usually appear at 2ꢅ = 35ꢀ
could not be observed. This is probably due to either well-
dispersion or low loading of RuO2 (2 wt.%). Similarly, for
Fe/VHP and Mn/VHP promoted catalysts it was difficult
to recognise XRD diffraction peaks assignable to them.
The XRD peaks of Mn phases of MnO and MnO2 usually
100
Benzene
Phenol
Others
90
80
70
60
50
40
30
20
10
0
Hydroquinone
1
2
3
4
5
Recycling cycles
Fig. 2. Recyclability testing of nanosized doped-Au/VHP catalyst in
benzene hydroxylation reaction. Reaction conditions: benzene (10 mmol),
50% H2O2 (30 mmol), catalyst (0.10 g), CH3CN (6 ml), T = 70 ꢀC,
t = 8 h.
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Catalytic Activity and Structure Properties of Doped VOHPO4 ·0ꢂ5H2O with Nanosized Ru, Au, Fe and Mn
Makgwane and Ray
Fig. 3. XRD patterns of pure VHP and different nanosized doped-VHP
catalysts.
Fig. 4. Raman analysis of pure VHP and different nanosized doped-
VHP catalysts.
appear at approximately, 2ꢅ = 31ꢀ, 41ꢀ and 48ꢀ lines but
these lines seems to be overlapping with the diffraction
patterns of the VHP catalyst. Nevertheless, the presence
of the different metals on the VHP surface although some
were not detectable by the XRD analysis was confirmed
by the EDX mesurements (Table IV) and TEM images
(Fig. 5).
and Mn/VHP catalysts are related to the PVA.36 These
different structural phases or bands for nanosized metal
doped-VHP enunciate the possible structural difference of
VHP induced by the nature of the promoter metals which
can have significant influence on their catalytic activity as
observed in Table I.
Figure 4 illustrates the Raman analysis of different VHP
based catalysts. The pure VHP showed similar Raman
bands to the literature reported VOHPO4 · 0ꢂ5H2O struc-
Figure 5 illustrates the TEM images of different pro-
moted VHP catalysts, including the reference pure VHP
catalyst phase. The TEM images of all promoted VHP
catalysts showed plate-like layers structural morphology
typical of vandium phosphorous oxide catalyst systems
(Figs. 5(a)–(d)).29 These platelets layers showed a stacked
together arrangement on top of each other with the average
mean particles size in a range of 30–50 nm as illustrated
for the pure VHP catalyst in Figure 5(e). For both nano-
sized doped-Au/VHP and Ru/VHP catalysts, they showed
the formation of well-dispersed particles with a mean size
in the range of 5–9 nm which could be related to their
better observed catalytic activity in table. The Fe/VHP
catalyst indicated possible particles aggregates clusters
although it showed the formation of small nanosize parti-
cles comparable to those of Ru/VHP and Au/VHP doped
catalysts (Fig. 5(d)). Inversely, the Mn/VHP catalyst indi-
cated the formation of large clustered particles with size
greater than 10 nm or equivalent to the VHP platelets in
some instances (Fig. 5(b)). The large particles size for-
mation of Mn on VHP, thus showing to be covering its
surface can be assumed to be responsible for the poor cat-
alytic activity performance observed Mn/VHP catalyst in
Table I.
ture dominated by an intense band at 985.76 cm−1 37
.
The band at 982 cm−1 had been assigned to V
vibrational mode while the additional two minor bands
appearing at 1110 and 1151 cm−1 belongs to the P
O
P
O
stretching bonds of the VHP.35 Other low intense bands
assignable to VHP are noticeable at lower Raman region
of 200–600 cm−1. Some of these bands are also visible
in the doped-VHP catalyst with different nanosized metal
particles. The Ru/VHP, Au/VHP and Mn/VHP catalyst
also showed the formation of new intense band at around
600 cm−1 that was not observed in the pure VHP and
Fe/VHP doped catalysts Raman bands. The peak belong-
ing to VHP (982 cm−1ꢁ was more visible in the Fe/VHP
doped catalysts, while other doped indicated possible shift
or covered by the additional two high intense peaks
appearing at 926 and 1076 cm−1 for Ru/AHP, Au/VHP
Table IV. Physico-chemical properties of promoted VHP catalysts.
Catalyst aLoadings (wt.%) Surface area (m2/g) Pore volume (cm3/g)
VHP
–
14.3
19.3
15.1
9.6
0.23
0.17
0.19
0.22
0.20
Ru/VHP
Au/VHP
Mn/VHP
Fe/VHP
1.25
1.18
1.36
1.09
The textural and physical data of the pure and doped
VHP catalysts are summarised in Table IV. The BET sur-
face areas of the pure VHP catalyst was found to be
13 m2/g while VHP doped with nanosize particles of
13.4
Note: aMeasured by EDX.
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Makgwane and Ray
Catalytic Activity and Structure Properties of Doped VOHPO4 ·0ꢂ5H2O with Nanosized Ru, Au, Fe and Mn
Fig. 5. TEM images of different nanosized doped-VHP based catalysts. (a) Au/VHP, (b) Mn/VHP, (c) Ru/VHP, (d) Fe/VHP, (e) layered platelets of
pure VHP, (f) nanoparticles of Ru on VHP single layer and (g) overlapping VHP platelets with visible dark-spot nanoparticles in-between the platelets
layers.
different metals showed varying surface areas depending
on the nature of the dopant. The Ru, Fe, and Au doped
VHP catalysts showed slightly higher surface areas than
the pure VHP catalyst. Such an observation may be due to
small nanosize particles as indicated by TEM results. On
the other hand, the Mn/VHP promoted catalyst displayed
a drop in surface area, which could be attributed to Mn
large particles size formation on the surface or interact-
ing with VHP (Fig. 5). Similarly, the pore volume of the
promoted VHP catalysts indicated no noticeable variation
with a significant trend. These structural differences can
have a profound effect on VHP catalytic performance as
observed in Table I, probably due to electronic effect on
+4/V+5 ratio of the synthesized VHP catalysts.
V
4. CONCLUSION
In summary, the catalytic results obtained showed that
the catalytic properties of VHP catalyst in the hydrox-
ylation reaction of benzene using H2O2 as oxidant can
enhanced siginificantly by the used of nanosized metal par-
ticles effect. The most interesting observation was that the
catalysts were highly active without being calcined or acti-
vated as usually done for VPO catalyst system. Amongst
J. Nanosci. Nanotechnol. 13, 5053–5060, 2013
5059

Catalytic Activity and Structure Properties of Doped VOHPO4 ·0ꢂ5H2O with Nanosized Ru, Au, Fe and Mn
Makgwane and Ray
the doped-VHP catalysts, Ru/VHP exhibited excellent cat-
alytic performance and reusability after several consec-
utive testing. A benzene conversion of up to 76% at
85.5% total selectivity towards the formation of phenol and
hydroquinone was obtained using nanaosized doped Au-
VHP catalyst. Inversely, the Mn/VHP catalyst exhibited
low activity that indicated that the Mn promotion inhib-
ited the catalytic active sites of VHP catalyst. Based on
characterisation results, the influence induced by the dif-
ferent promoter metals electronic and structural properties
are proposed to be of importance to improve the VHP cata-
lyst catalytic performance probably related to the enhanced
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Acknowledgments: Authors are grateful for the finan-
cial support from the University of Johannesburg and the
National Research Foundation (NRF) of South Africa. The
DST/CSIR National Centre for Nano-Structured Materials
is thanked for providing with research equipment to per-
form this work. Dr. Linda Prinsloo (University of Pretoria)
is thanked for helping Raman analysis.
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Received: 31 October 2012. Accepted: 15 January 2013.
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