Hydroxylation of phenol over molybdovanadophosphoric acid modified zirconia

Article abstract of DOI:10.1016/j.molcata.2007.10.005

Molybdovanadophosphoric acid (MVPA) supported zirconia was synthesized by an incipient wetness impregnation method taking zirconium hydroxide and MVPA as the precursors. The neat and supported catalysts were calcined at 500 °C and characterized by nitroge

Full text of DOI:10.1016/j.molcata.2007.10.005

Available online at www.sciencedirect.com
Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
Hydroxylation of phenol over molybdovanadophosphoric
acid modified zirconia
∗
K.M. Parida , Sujata Mallick
Colloids and Material Chemistry Department, Institute of Minerals and Materials Technology (CSIR),
Bhubaneswar 751013, Orissa, India
Received 20 August 2007; received in revised form 1 October 2007; accepted 3 October 2007
Available online 9 October 2007
Abstract
Molybdovanadophosphoric acid (MVPA) supported zirconia was synthesized by an incipient wetness impregnation method taking zir-
◦
conium hydroxide and MVPA as the precursors. The neat and supported catalysts were calcined at 500 C and characterized by nitrogen
adsorption–desorption, X-ray powder diffraction, FTIR, UV–vis DRS, temperature programmed reduction (TPR) and Raman spectroscopy tech-
niques. The 15 wt.% MVPA stabilizes the tetragonal phase of zirconia, which leads to an increase in surface area. The Keggin structure of MVPA
◦
remains unaltered up to 500 C. The catalytic activities were evaluated for hydroxylation of phenol using H
2
O
2
as the oxidant. Among all the
promoted catalysts, 15 wt.% molybdovanadophosphoric acid supported zirconia exhibits highest catalytic activity in phenol hydroxylation, giving
9% conversion having 61% para selectivity.
2007 Elsevier B.V. All rights reserved.
4
©
Keywords: Molybdovanadophosphoric acid; Phenol hydroxylation; Hydroquinone; Catechol
1
. Introduction
TiMMS [5], TS-1 [7], TiMMM [8], metal complexes [9] and
polyoxometalates (POMs) [10–12]. Takehire and co-workers
[11,12] reported the hydroxylation of phenols with 30% aqueous
H2O2 to produce hydroquinone and catechol using vanadium-
containing POMs with Keggin structure.
Oxidation of phenol to hydroquinone (HQ) and catechol
(
CL) is an industrially interesting reaction, as the products have
several important large-scale uses such as photographic film
developer, antioxidant, polymerization inhibitor, medicines and
organic synthesis, perfumes, etc. [1,2]. In addition, the quinone
derivatives play an important role in bio-systems and find many
miscellaneous industrial applications. Several soluble oxidizing
agents either in stoichiometric quantity or catalytic amount have
been used for the conversion of phenol into hydroquinone (para-
dihydroxybenzene) and catechol (ortho-dihydroxybenzene) [3].
But the disadvantage found in most cases is the evolution of
undesired toxic gases as well as liquid and solid wastes besides
the process being highly expensive. Also in case of homoge-
neous catalytic process, hydroxylation of phenol gives very low
para to ortho ratio. But the use of heterogeneous catalyst is a
promising alternative to over come this problem. The different
solid acid tested so far include TiHMS [4,5], TiMCM-41 [4–6],
The acid–base, redox behavior and well-known Keggin
structure of POMs play an important role in synthesis and
applications, especially homogeneous and heterogeneous
catalysis. Their properties are dependent on the nature and
the relative position of the metals in the framework because
the acid and redox properties of POMs are controlled at the
atomic and molecular levels by changing the constituent
elements. Molybdenum and tungsten are the main con-
stitutive metals in POMs. The general trends for the acid
strengths of these Keggin acids are W > Mo (for polyatom) and
5+
4+
P
> Si (for the heteroatom) [13]. The oxidation potential
(or oxidizing ability) has been reported to be in the order:
[PMo12O40]3 > [SiMo12O40] > [PW12O40] > [SiW12O40]
−
4−
3−
4−
[14]. In addition, the introduction of vanadium (V) into the
Keggin framework [PMo12O40]3 is beneficial for redox
catalysis [15], shifting its reactivity from acid dominated to
redox dominated, as demonstrated by the selective oxidation
of methanol to either dimethyl ether or formaldehyde [16] and
−
∗
Corresponding author.
E-mail address: kmparida@yahoo.com (K.M. Parida).
1
381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.10.005

K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
105
◦
by other selective oxidation of alkanes and aldehydes [17–20].
However, the disadvantage of POMs as catalysts is their low
surface area and low thermal stability. To minimize these
disadvantages, the POMs were supported on different carriers
and also, impregnating solutions in different solvents were
employed [21].
It should be noted that in the literature, practically there are
many papers on heteropoly acid as catalyst for hydroxylation
of phenol but no data is available on molybdovanadophospho-
ric acid supported on zirconia. Our paper is the first attempt to
evaluate the catalytic activity of molybdovanadophosphoric acid
supported on zirconia for hydroxylation of phenol.
stirring followed by heating at 100 C till complete evapora-
◦
tion (4 h). Then it was dried in oven at 110 C for 24 h. Catalysts
with different MVPA loading from 3 to 20 wt.% were then cal-
cined at 500 C. The catalysts will be herein after referred to
xZMVPA (x = 3–20 wt.%). Similarly, 15 wt.% PMA supported
zirconia was also prepared for comparison purpose with 15 wt.%
ZMVPA.
◦
2.4. Physico-chemical characterization
The BET surface area and pore size distribution were deter-
mined by multipoint N adsorption–desorption method at liquid
2
◦
N2 temperature (−196 C) by a Sorptomatic 1990 (Thermo-
Quest, Italy). Prior to analyses, all the samples were subjected
2
. Experimental
◦
to vacuum at 200 C to ensure a clean surface.
The X-ray powdered diffraction pattern was recorded on a
Philips PW 1710 diffractometer with automatic control. The
patterns were run with a monochromatic Cu K␣ radiation with
2
.1. Preparation of zirconia
Zirconium hydroxide gel was prepared from zirconium oxy-
◦
−1
.
a scan rate of 2 min
The FTIR spectra were taken using Jasco FTIR 5300 in KBr
chloride aqueous solution (S. D. Fine Chemicals Ltd.) by drop
wise addition of ammonium hydroxide solution (25% ammonia)
−
1
matrix in the range of 400–4000 cm
.
(Merck Specialities Private Limited) up to pH 9.5. The hydrogel
The UV–vis spectra of the samples were recorded in a Varian
UV–vis DRS spectrophotometer fitted with Carry 1E software.
The spectra were recorded against the boric acid as reference.
The H2-TPR of all the samples was carried out in a
CHEMBET-3000 (Quantachrome, USA) instrument. About
was refluxed for 24 h, then filtered and washed with deionized
◦
water and dried in an oven at 120 C for 24 h and crushed to fine
◦
powders. Then it was calcined at 500 C for 5 h (named Z herein
after) and kept in desiccator for further use.
0
.1 g sample was taken inside quartz ‘U’ tube and degassed at
2
.2. Preparation of phosphomolybdic acid and
◦
350 C for 1 h with helium gas flow. The sample was then cooled
molybdovanadophosphoric acid
◦
to 30 C and at this temperature the gas flow was changed to 5%
H2 in nitrogen. It was then heated at a heating rate of 10 C/min
up to 800 C and the spectra were recorded.
◦
2
.2.1. Preparation of phosphomolybdic acid
Ammonium heptamolybdate (20 g) (Merck Specialities Pri-
◦
Raman spectra were obtained on inVia Raman microscope
vate Limited) was dissolved in dilute H3PO4. Then it was
acidified with concentrated HCl till complete formation of yel-
low precipitation. It was then filtered and washed with distilled
(Renishaw, UK) equipped with a counter current detector
(CCD). The spectra were recorded using 514.5 nm excitation
−1
◦
lines from an argon ion laser with a spectral resolution of 1 cm
.
water and dried in an oven at 70 C.
2.5. Catalytic activity
2
.2.2. Preparation of molybdovanadophosphoric acid
First 22.3 g of MoO3 (Aldrich, 99.5% pure) and 1.3 g V2O5
Aldrich 99.6% pure) were added to 350 ml of distilled water.
The mixture was refluxed for 1 h. Then 1.7 g of 85% phosphoric
acid (Aldrich) was added in three equal aliquots, approximately
0 min apart. After addition of phosphoric acid, another 150 ml
of distilled water was added to the flask. The mixture was
allowed to reflux for 16 h with vigorous stirring and a bright
orange solution was formed. The liquid in the flask was evapo-
rated and a bright orange solid was obtained.
The oxidation of phenol was carried out in a round bottom
(
flask equipped with a reflux condenser. About 5 mmol of the
substrate was dissolved in 15 ml of deionized water and 0.05 g of
the catalyst was added to it. To this mixture, 5 mmol of aqueous
H2O2 (30%) was added drop wise. The reaction mixture was
1
◦
stirred for 2 h at 60 C (Scheme 1). The reaction products were
analyzed off-line by gas chromatography (Shimadzu GC-17A)
X-ray fluorescence analysis showed that Mo to V ratio in
molybdovanado phosphoric acid sample is 11:1.
2
.3. Preparation of molybdovanadophosphoric acid and
phosphomolybdic acid supported zirconia
A series of catalysts having different loading from 3 to
2
0 wt.% were synthesized by impregnating 2 g of Z with an
aqueous solution of molybdovanadophosphoric acid (MVPA)
0.06–0.30 g/10–50 ml of conductivity water) under constant
(
Scheme 1.

1
06
K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
through capillary column (ZB-1, 30 m length, 0.53 mm I.D. and
3
3
3
.0 ␮m film thickness) using Flame ionization Detector (FID).
. Results and discussion
.1. Characterization
Table 1 shows the BET surface area of neat and different
weight percentage of ZMVPA samples. Zirconium oxyhydrox-
◦
2
ide dried at 120 C showed a surface area 412 m /g. After
◦
calcinations at 500 C, the surface area drastically decreased
to 142 m /g. Addition of MVPA on zirconia results in an
2
increase of the surface area. The 15 wt.% MVPA loaded zirco-
2
nia show maximum surface area 229 m /g. However, on further
loading, the surface area decreases. This implies that the pres-
ence of MVPA plays a prominent role in making the material
porous. The added MVPA strongly interact with zirconia reduc-
ing surface diffusion of zirconia and so inhibits sintering. It
stabilized the tetragonal phase of zirconia, which leads to an
increase in surface area. However, when the MVPA content
increases beyond 15 wt.%, pore blocking takes place due to
the presence of an excess amount of molybdovanadophosphoric
acid.
The nitrogen adsorption–desorption isotherms of parent zir-
conia and 15 wt.% ZMVPA were shown in Fig. 1a and b,
respectively. The isotherms are very much similar to the type
IV isotherm according to the IUPAC classification of adsorption
isotherms [22], which is a typical characteristic of mesoporous
solids. Inflections at P/P0 0.4–0.6 for 15 wt.% ZMVPA and at
0
.45–0.65 for parent zirconia were seen in the figure, which
are due to the spontaneous filling of mesopores due to capillary
condensation.
Fig. 2 shows the BJH pore size distribution of zirconia and
1
5 wt.% ZMVPA. From the figure one can see that there is no
appreciable change in pore diameter by incorporation of 15 wt.%
ZMVPA on zirconia.
The XRD patterns of zirconia and ZMVPA samples are pre-
sented in Fig. 3. It was observed that pure zirconia contains
mixture of tetragonal and monoclinic phases with the latter as
the major constituents. However, XRD of low MVPA loading
samples is similar to that of the support. As percentage loading
Fig. 1. (a) N2 adsorption (ꢀ) and desorption (᭹) isotherms of zirconia. (b) N2
adsorption (ꢀ) and desorption (᭹) isotherms of 15 wt.% ZMVPA.
Table 1
of MVPA increases, it shows increase in tetragonal phase while
15 wt.% ZMVPA is fully tetragonal. But on further increase in
MVPA loading, it shows monoclinic along with tetragonal.
Fig. 4 shows the UV–vis DRS spectra of (a) zirconia, (b)
Surface area, conversion and selectivity of various catalysts towards hydroxyla-
tion of phenol
2
Catalysts
Surface area (m /g) Conversion (%) Selectivity (%)
HQ
CL
1
5 wt.% ZPMA and (c) 15 wt.% ZMVPA samples calcined at
◦
Z
142
154
165
190
5
12
22
35
41
49
46
5
45.3 54.6
49.5 50.3
50.2 49.6
52.9 46.5
56.3 43.1
60.6 39.3
62.8 37.1
500 C. The zirconia (Fig. 4a) exhibits a strong absorption band
at 230 nm, which may be attributed to the charge transfer from
3
6
9
1
1
2
1
wt.% ZMVPA
wt.% ZMVPA
wt.% ZMVPA
2 wt.% ZMVPA 199
5 wt.% ZMVPA 229
0 wt.% ZMVPA 213
−
4+
oxide species to zirconium cation (O → Zr ). In contrast, the
spectraofboth15 wt.%ZPMAand15 wt.%ZMVPAshowbroad
band at 250 nm which match well with the literature values [23],
suggesting thereby the presence of undegraded H3PM12O40
species. An additional band is found at 308 nm in compared
with ZPMA, indicating the incorporation of vanadium into the
Keggin ion [24].
5 wt.% ZPMA
211
24
75
Phenol (5 mmol), H2O2 (5 mmol), catalyst (0.05 g) and water (15 ml), time 2 h,
HQ: hydroquinone and CL: catechol.

K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
107
Fig. 2. Distribution of pore size as a function of pore radius of (a) zirconia and
(
b) 15 wt.% of ZMVPA.
Fig. 4. UV–vis spectra of (a) Z, (b) 15 wt.% ZPMA, and (c) 15 wt.% ZMVPA.
The FTIR spectra of zirconia, 15 wt.% ZPMA and 15 wt.%
1
–
621 and 1386 cm 1 which are due to bending vibration of
−
(H–O–H)– and –(O–H–O)– bond. The band at 600 cm is
◦
ZMVPA calcined at 500 C are presented in Fig. 5. The FTIR
−1
−
1
spectrum of Z shows broad band in the region of 3410 cm
attributed to the presence of Zr–O–H bond (Fig. 5a). In addition
to these bands, the FTIR spectra of 15 wt.% ZPMA (Fig. 5b)
show bands 1059, 888, 789 cm assigned to the symmetric
stretching of P–O, Mo–Oc–Mo, Mo–Oe–Mo respectively and a
weak shoulder at 954 cm due to Mo–Ot, confirmed the pres-
ence of the undegraded phosphomolybdic acid. Here, ‘t’ tends
for the terminal oxygen bonding to one Mo atom, ‘c’ for the
corner sharing oxygen atom connecting Mo3O13 units and ‘e’
for the edge sharing oxygen-connecting Mo [25]. There is slight
shifting of bands in FTIR spectra of 15 wt.% ZMVPA sample
due to asymmetric stretching of OH group and two bands at
−
1
−
1
(
Fig. 5c) which indicates that the Keggin phase remains unal-
tered after the incorporation of vanadium into the Keggin ion
26].
Fig. 6 gives the H2-TPR profiles obtained from the various
[
catalysts. It was observed that no H2 consumption was recorded
for the zirconia support. The support reducibility seems to have
Fig. 3. XRD patterns of (a) Z, (b) 6 wt.% ZMVPA, (c) 9 wt.% ZMVPA, (d)
1
5
2 wt.% ZMVPA, (e) 15 wt.% ZMVPA and (f) 20 wt.% ZMVPA (calcined at
00 C).
◦
Fig. 5. FTIR spectra of (a) Z, (b) 15 wt.% ZPMA, and (c) 15 wt.% ZMVPA.

1
08
K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
The spectra of MVPA supported zirconia sample reveals rela-
tively intense Raman bands pertaining to the tetragonal phase.
−
1
Further, a broad band observed at 1059 cm could be attributed
to the anti-symmetric PO4 stretching mode. In addition, the
Raman bands due to the Mo–Oxygen stretching vibration can
−
1
−1
be seen at 987, 970 and 830 cm . The band at 250 cm is
assigned to the Mo–O–Mo bending modes of the intact Keggin
structure.
3.2. Hydroxylation of phenol
The conversion of phenol and selectivity to hydroquinone
◦
and catechol over modified zirconia systems at 60 C in pres-
ence of hydrogen peroxide in water medium is shown in
Table 1. Vanadium free phosphomolybdic acid supported on
zirconia (15 wt.% ZPMA) showed very low activity (5%) com-
pared to vanadium-containing phosphomolybdic acid (15 wt.%
ZMVPA), however, the former show remarkably high ortho
selectivity. ThisindicatesthatvanadiumatomlocatedintheKeg-
gin structure enhanced the oxidative activity. It is observed that
among different loading of molybdovanadophosphoric acid on
zirconia, the 3 wt.% ZMVPA showed 12% conversion having
49% para, 50% ortho selectivity. The conversion increased to
a maximum of 49% conversion having 61% para, 39% ortho
selectivity at 15 wt.% loading. On further increasing the weight
percentage to 20 wt.%, the conversion decreases to 46%. For
Fig. 6. TPR spectra of (a) zirconia, (b) 15 wt.%ZPMA and (c) 15 wt.%ZMVPA.
been modified in the presence of PMA and MVPA on zirconia.
TPR pattern of ZPMA and ZMVPA catalysts are similar, since
they consist of two main reduction peaks observed at tempera-
15 wt.% loading there is monolayer coverage of MVPA on the
support. But above 15 wt.% there is over loading of MVPA and
some pores of zirconia are blocked.
The reaction mechanism for the oxidation of aromatic com-
pounds employing transition metals has been studied by others
◦
ture around 450 and 695 C. Introduction of a vanadium atom
in anion or in countercation position of heteropoly molybdates
makes reduction of molybdenum species easier. The reduction
[
21,27]. Thereactionpathforthepresentstudyinvolvestheinter-
◦
up to 450 C could also include reduction of vanadium contain-
•
•
action of ZMVPA catalysts with H2O2 yields OH and HO2
◦
ing species. The second main peak (at about 695 C) is probably
•
species via a redox mechanism. Then the OH radical attacks
the phenol ring and yields hydroquinone and catechol as the
major product. Oxygen and water are formed as the side prod-
ucts by the decomposition of the hydrogenperoxi radical and
H2O2, respectively. So the reaction mainly follows free radical
mechanism (Scheme 2).
connected with a deeper reduction of Mo species (HPMo12)
and/or with reduction of a new phase containing V and P.
Raman spectra of zirconia and 15 wt.% ZMVPA promoted
zirconia samples are shown in Fig. 7. Normally, crystalline zir-
−
1
conia exhibits characteristic Raman bands in the 500–700 cm
region. From Fig. 7, the spectra of zirconia reveal the Raman
bands pertaining to a mixture of monoclinic (221, 331, 380, 478
Generally, thecatalyticactivityandproductselectivityinphe-
nol hydroxylation by hydrogen peroxide are strongly influenced
by surface area of the catalyst, solvents, reaction time, reac-
tion temperature, molar ratio of phenol to hydrogen peroxide,
and catalyst amount, which are investigated systematically as
follows on 15 wt.% ZMVPA.
−1
−1
and 639 cm ) and tetragonal (290, 312, 455, 645 cm ) phases.
3.2.1. Influence of the solvents
The solvents used in the catalytic reaction are known to have a
profound influence on phenol conversion and the ratio of hydro-
quinone to catechol over titanium silicates [28,29] and vanadium
silicates-2 [30]. The influence of solvents on conversion and
selectivity of the reaction over 15 wt.% ZMVPA is illustrated
in Table 2. When 1,2-dichloroethane is used as a solvent, the
catalytic reaction does not take place. In case of acetone or in
ethanol low conversion is observed, while in case of acetoni-
trile the conversion was found to be 22%. In water, both phenol
Fig. 7. Raman spectra of the catalyst (a) zirconia and (b) 15 wt.% ZMVPA.
2 2
and H O dissolved easily and active hydroxyl radicals can be

K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
109
Scheme 2.
Table 2
sion increases from 12 to 49% with increase in reaction time
from 0.5 to 2 h and then remains almost constant with further
rise of reaction time up to 3 h. This indicated that the optimized
reaction conditions are more favorable than the reported ones
as much longer reaction time was employed while less hydro-
quinone and catechol yield were obtained. In case of selectivity
ratio (HQ/CL), it increases from 1.27 to 1.98.
Influence of solvent on hydroxylation of phenol reaction
Solvent
Conversion (%)
Selectivity (%)
Hydroquinone
Catechol
1
,2-Dichloroethane
2
22
8
6
49
20.3
48.2
60.4
49.2
60.6
78.9
50.6
39.3
50.0
39.4
Acetonitrile
Acetone
Ethanol
Water
3.2.3. Influence of reaction temperature
Phenol (5 mmol), H2O2 (5 mmol), catalyst (0.05 g) and water (15 ml), time 2 h.
The influence of reaction temperature on the hydroxylation
of phenol is shown in Fig. 9. The reaction was carried out
◦
in the temperature region 40–90 C over 15 wt.% ZMVPA as
formed. This effect of the solvents strongly supports that radi-
cal reaction mechanism is in operation in phenol hydroxylation.
Radicals generated are more stable in polar solvents. As ethanol
is a well-known scavenger of hydroxyl radical, negligible reac-
tion conversion took place.
catalyst while keeping the other parameters fixed. The percent-
age of conversion of phenol increased from 38 to 51 with the
increase in temperature from 40 to 80 C. This is due to the fact
that H2O2 decomposition increases with the increase in tem-
perature. This indicates that higher temperature up to 80 C is
beneficial for the hydroxylation of phenol. But above 80 C,
the conversion of phenol is decreased due to enhanced H2O2
decomposition. The same trend was also reported by Jang and
◦
◦
◦
3
.2.2. Influence of reaction time
The dependence of catalytic activity on reaction time was
shown in Fig. 8. It was found that the percentage of conver-
Fig. 8. Effect of time on conversion of phenol using 15 wt.% ZMVPA as catalyst
Fig. 9. Effect of temperature on conversion of phenol using 15 wt.% ZMVPA
as catalyst (0.05 g), phenol (5 mmol), H2O2 (5 mmol), and acetic acid (5 ml).
(
0.05 g) phenol (5 mmol), H2O2 (5 mmol), and acetic acid (5 ml).

1
10
K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
Table 3
Hydroxylation of phenol over 15 wt.% ZMVPA and comparison of the result with other reported methods
Catalyst used
Time (min)
Conversion (%)
Selectivity (%)
HQ
Reference
CL
ZMVPA
Fe-MCM-41
TS-1
120
10
720
360
49
60
27
43
60
32
50
12
39
68
50
88
[This method]
[31]
[33]
[34]
Modified zeolite
co-workers [31] over Fe-MCM-41 as catalyst. Table 3 repre-
sents comparative feature between the performance of ZMVPA
and other reported catalysts for hydroxylation of phenol. Cat-
alytic activity of our catalyst is comparable to other reported
catalysts in terms of conversion and hydroquinone, catechol
selectivity.
3
.2.4. Influence of molar ratio of phenol/H2O2
The activity of hydroxylation of phenol in different
phenol/H2O2 molar ratio is shown in Fig. 10. It is observed
from the figure that the conversion of phenol is higher at lower
phenol/H2O2 ratio. On the other hand increase in selectivity of
catechol and decrease in phenol conversion is observed with
the increase in phenol/H2O2 molar ratio. Similar results also
reported by Neuman et al. [32]. The coordination of H2O2 on
Fig. 11. Effect of catalyst amount on conversion of phenol using 15 wt.%
ZMVPA as catalyst (0.05 g) phenol (5 mmol), H2O2 (5 mmol), and acetic acid
(
5 ml).
•
the metal, and its subsequent decomposition to OH radical were
the necessary steps in this reaction as described above. At the
molar ratio of 1, H2O2 could freely coordinate to metal and
decompose to form OH radical. Hence, high phenol conver-
3
.3. Recyclability of the catalyst
•
The catalyst with 15 wt.% loading was used for recycling
sion was attained. At the molar ratio of 2, low amount of H2O2
must compete with excess phenol for coordination, and hence
experiments. In order to regenerate the catalyst after 2 h of reac-
tion, it was separated by filtration, washed with conductivity
water several times, dried at 110 C and calcined at 500 C and
usedinthehydroxylationofphenolwithafreshreactionmixture.
In the regenerated sample after two cycles, the yield decreases
by 4%.
•
formation of OH radicals might be significantly reduced. Thus,
◦
◦
the conversion decreased.
3
.2.5. Influence of catalyst amount
The variation of the catalytic activity with the amount of
catalyst is shown in Fig. 11. It is observed that with increasing
theamountofcatalystfrom0.025to0.05 gthephenolconversion
enhanced from 32 to 49%. Further increment of catalyst amount
to 0.1 g resulted in only a marginal increase in phenol conversion
4
. Conclusions
XRD and Raman spectra results indicated that the presence
of MVPA retarded the crystallization of zirconia and stabi-
lized ZrO2 in tetragonal phase. Nitrogen adsorption–desorption
studies revealed that the modified MVPA samples retain the
mesoporosity, where as there is no appreciable change in
pore diameter after impregnation of MVPA on zirconia. It
is confirmed from FTIR and Raman spectra that the molyb-
dovanadophosphoric acid keeps its Keggin type structure when
supported on zirconia. TPR spectra revealed that the introduc-
tion of a vanadium atom in anion or in countercation position
of heteropoly molybdates makes reduction of molybdenum
species easier. Vanadium atom located in the Keggin structure
enhanced the catalysts oxidative activity towards hydroxylation
of phenol. Among all the promoted catalyst, 15 wt.% of molyb-
dovanadophosphoric acid impregnated on zirconia showed
highercatalyticactivityandhaving61%paraselectivity. Molyb-
dovanadophosphoric acid supported on zirconia acts as an effi-
cient and stable solid acid catalyst for hydroxylation of phenol.
(
51%).
Fig. 10. Effect of molar ratio of phenol/H2O2 on conversion of phenol using
5 wt.% ZMVPA as catalyst (0.05 g), and acetic acid (5 ml).
1

K.M. Parida, S. Mallick / Journal of Molecular Catalysis A: Chemical 279 (2008) 104–111
111
Acknowledgements
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The authors are thankful to Prof. B.K. Mishra Director,
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constant encouragement and permission to publish this paper.
Mrs. Sujata Mallick is obliged to CSIR for the award of SRF.
DST is gratefully acknowledged for the financial support.
1
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